api/beta/messages / documentation change

### Beta System Message Output Config ### Beta Thinking Block Binding ### Beta Thinking Dropped Input Transformation ### Beta Thinking Prefix Mismatch Behavior

The whole hunk

from line 18, old and new numbered

lines

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from line 18

1818

1919 - `string`

2020

21 - `"message-batches-2024-09-24" or "prompt-caching-2024-07-31" or "computer-use-2024-10-22" or 38 more`

21 - `"message-batches-2024-09-24" or "prompt-caching-2024-07-31" or "computer-use-2024-10-22" or 41 more`

2222

2323 - `"message-batches-2024-09-24"`

2424

from line 101

101101 - `"thinking-display-updates-2026-08-18"`

102102

103103 - `"ce-user-management-2026-07-13"`

104

105 - `"mid-conversation-output-config-2026-07-01"`

106

107 - `"thinking-binding-controls-2026-08-01"`

108

109 - `"mid-conversation-system-clear-at-2026-08-21"`

104110

105111- `"anthropic-user-profile-id": optional string`

106112

from line 1400

13941400

13951401 See [models](https://docs.anthropic.com/en/docs/models-overview) for additional details and options.

13961402

1397 - `"claude-sonnet-5" or "claude-fable-5" or "claude-mythos-5" or 12 more`

1403 - `"claude-fable-5-1" or "claude-mythos-5-1" or "claude-sonnet-5" or 14 more`

13981404

13991405 The model that will complete your prompt.

14001406

14011407 See [models](https://docs.anthropic.com/en/docs/models-overview) for additional details and options.

14021408

1409 - `"claude-fable-5-1"`

1410

1411 Frontier intelligence for ambitious tasks across coding, scientific discovery, and enterprise workflows

1412

1413 - `"claude-mythos-5-1"`

1414

1415 Our most capable model for cybersecurity and biology research, available through trusted access programs

1416

14031417 - `"claude-sonnet-5"`

14041418

14051419 High-performance model for coding and agents

from line 1493

14791493 - `"assistant"`

14801494

14811495 - `"system"`

1496

1497 - `clear_at: optional "next_user_message" or "never" or null`

1498

1499 How long this system message's text stays in front of the model. `"never"` (the default) renders it on every request that includes it. `"next_user_message"` renders it only for the user turn it follows: once a later `role: "user"` message exists in `messages` the message stays in the array (send it unchanged) but is no longer shown to the model. Only permitted on `role: "system"` messages.

1500

1501 - `"next_user_message"`

1502

1503 - `"never"`

1504

1505 - `output_config: optional BetaSystemMessageOutputConfig or null`

1506

1507 Per-message output configuration on a role:"system" input message.

1508

1509 Fields here apply per-turn; `format` remains top-level only. An

1510 empty `{}` is accepted on a message that carries content; a message

1511 with neither content nor output_config fields is rejected.

1512

1513 - `effort: optional "low" or "medium" or "high" or 2 more or null`

1514

1515 All possible effort levels.

1516

1517 - `"low"`

1518

1519 - `"medium"`

1520

1521 - `"high"`

1522

1523 - `"xhigh"`

1524

1525 - `"max"`

14821526

14831527- `model: Model`

14841528

from line 1826

17821826

17831827 - `type: "enabled"`

17841828

1829 - `block_binding: optional BetaThinkingBlockBinding or null`

1830

1831 Controls for block binding: what happens when a thinking block this

1832 request sends back fails the conversation check. Every field is optional;

1833 an empty object means every default.

1834

1835 - `prefix_mismatch_behavior: optional BetaThinkingPrefixMismatchBehavior or null`

1836

1837 What happens when a thinking block in `messages` fails the conversation

1838 check: it was created in a different conversation, or the messages before

1839 it have changed since. `"error"` (the default) fails the request with a

1840 400 error. `"drop_block"` removes the failing blocks and the request

1841 proceeds; the model no longer sees the dropped reasoning.

1842

1843 - `"error"`

1844

1845 - `"drop_block"`

1846

17851847 - `display: optional "summarized" or "omitted" or "updates" or null`

17861848

17871849 Controls how thinking content appears in the response. When set to `summarized`, thinking is returned normally. When set to `omitted`, thinking content is redacted but a signature is returned for multi-turn continuity. Defaults to `summarized`.

from line 1861

17991861 - `BetaThinkingConfigAdaptive object`

18001862

18011863 - `type: "adaptive"`

1864

1865 - `block_binding: optional BetaThinkingBlockBinding or null`

1866

1867 Controls for block binding: what happens when a thinking block this

1868 request sends back fails the conversation check. Every field is optional;

1869 an empty object means every default.

18021870

18031871 - `display: optional "summarized" or "omitted" or "updates" or null`

18041872

from line 2369

23012369

23022370 - `type: "browser_toolset_20260801"`

23032371

2304 - `allowed_callers: optional array of "direct" or "code_execution_20250825" or "code_execution_20260120" or "code_execution_20260521"`

2305

2306 - `"direct"`

2307

2308 - `"code_execution_20250825"`

2309

2310 - `"code_execution_20260120"`

2311

2312 - `"code_execution_20260521"`

2313

23142372 - `cache_control: optional BetaCacheControlEphemeral or null`

23152373

23162374 Create a cache control breakpoint at this content block.

from line 2995

29372995

29382996 - `type: "computer_toolset_20260801"`

29392997

2940 - `allowed_callers: optional array of "direct" or "code_execution_20250825" or "code_execution_20260120" or "code_execution_20260521"`

2941

2942 - `"direct"`

2943

2944 - `"code_execution_20250825"`

2945

2946 - `"code_execution_20260120"`

2947

2948 - `"code_execution_20260521"`

2949

29502998 - `cache_control: optional BetaCacheControlEphemeral or null`

29512999

29523000 Create a cache control breakpoint at this content block.

from line 4876

48284876

48294877 See [models](https://docs.anthropic.com/en/docs/models-overview) for additional details and options.

48304878

4831 - `"claude-sonnet-5" or "claude-fable-5" or "claude-mythos-5" or 12 more`

4879 - `"claude-fable-5-1" or "claude-mythos-5-1" or "claude-sonnet-5" or 14 more`

48324880

48334881 The model that will complete your prompt.

48344882

48354883 See [models](https://docs.anthropic.com/en/docs/models-overview) for additional details and options.

4884

4885 - `"claude-fable-5-1"`

4886

4887 Frontier intelligence for ambitious tasks across coding, scientific discovery, and enterprise workflows

4888

4889 - `"claude-mythos-5-1"`

4890

4891 Our most capable model for cybersecurity and biology research, available through trusted access programs

48364892

48374893 - `"claude-sonnet-5"`

48384894

from line 5621

55655621

55665622 - `"fast"`

55675623

5624 - `input_transformations: optional array of BetaThinkingDroppedInputTransformation or null`

5625

5626 Changes the API made to the request's input before showing it to the model:

5627 one entry per change, in request order. Today the only entry type is

5628 `thinking_dropped` — a `thinking`, `redacted_thinking` or `connector_text`

5629 block from the request's `messages` that was removed from the prompt instead

5630 of being shown to the model because it failed a binding check. More entry

5631 types may be added over time; ignore types you do not recognize.

5632

5633 Requires `anthropic-beta: thinking-binding-controls-2026-08-01`. Present on

5634 every such response from a model that supports extended thinking, as `[]`

5635 when nothing was changed; without the beta, blocks are removed all the same

5636 but nothing is reported. Removed blocks contribute nothing to

5637 `usage.input_tokens`. When streaming, the array is final in `message_start`;

5638 the final `message_delta` event carries it only when a server-side model

5639 fallback happened mid-stream, in which case it holds the serving model's

5640 entries and replaces the one in `message_start`.

5641

5642 - `path: string`

5643

5644 Where the removed block was in your request, as `messages.{i}.content.{j}`:

5645 `i` indexes the `messages` array you sent and `j` that message's `content`

5646 array — the same form error messages use.

5647

5648 - `reason: "model_binding_mismatch" or "prefix_binding_mismatch" or "organization_binding_mismatch" or "end_user_binding_mismatch"`

5649

5650 Which binding check removed the block: `model_binding_mismatch` — it was

5651 created by a model whose reasoning the requested model may not read;

5652 `prefix_binding_mismatch` — the conversation before it differs from the

5653 conversation it was created in (the rest of that turn's consecutive thinking

5654 blocks are removed with it, each with this reason);

5655 `organization_binding_mismatch` — it was created under a different

5656 organization (an Anthropic organization, AWS account or Google Cloud project)

5657 and this organization is not one of its additional organizations;

5658 `end_user_binding_mismatch` — it was created for a different end user, or

5659 was removed by the consumer-organization binding. A block that would fail

5660 several checks reports one reason, in this order of precedence:

5661 `organization_binding_mismatch`, `end_user_binding_mismatch`,

5662 `model_binding_mismatch`, `prefix_binding_mismatch`.

5663

5664 - `"model_binding_mismatch"`

5665

5666 - `"prefix_binding_mismatch"`

5667

5668 - `"organization_binding_mismatch"`

5669

5670 - `"end_user_binding_mismatch"`

5671

5672 - `type: "thinking_dropped"`

5673

5674 Always `thinking_dropped` for this entry type.

5675

5676 default: thinking_dropped

5677

55685678- `BetaRawMessageStreamEvent = BetaRawMessageStartEvent or BetaRawMessageDeltaEvent or BetaRawMessageStopEvent or 3 more`

55695679

55705680 - `BetaRawMessageStartEvent object`

from line 5771

56615771 - `server_tool_use: BetaServerToolUsage or null`

56625772

56635773 The number of server tool requests.

5774

5775 - `input_transformations: optional array of BetaThinkingDroppedInputTransformation or null`

5776

5777 Changes the API made to the request's input before showing it to the model:

5778 one entry per change, in request order. Today the only entry type is

5779 `thinking_dropped` — a `thinking`, `redacted_thinking` or `connector_text`

5780 block from the request's `messages` that was removed from the prompt instead

5781 of being shown to the model because it failed a binding check. More entry

5782 types may be added over time; ignore types you do not recognize.

5783

5784 Requires `anthropic-beta: thinking-binding-controls-2026-08-01`. Present on

5785 every such response from a model that supports extended thinking, as `[]`

5786 when nothing was changed; without the beta, blocks are removed all the same

5787 but nothing is reported. Removed blocks contribute nothing to

5788 `usage.input_tokens`. When streaming, the array is final in `message_start`;

5789 the final `message_delta` event carries it only when a server-side model

5790 fallback happened mid-stream, in which case it holds the serving model's

5791 entries and replaces the one in `message_start`.

5792

5793 - `path: string`

5794

5795 Where the removed block was in your request, as `messages.{i}.content.{j}`:

5796 `i` indexes the `messages` array you sent and `j` that message's `content`

5797 array — the same form error messages use.

5798

5799 - `reason: "model_binding_mismatch" or "prefix_binding_mismatch" or "organization_binding_mismatch" or "end_user_binding_mismatch"`

5800

5801 Which binding check removed the block: `model_binding_mismatch` — it was

5802 created by a model whose reasoning the requested model may not read;

5803 `prefix_binding_mismatch` — the conversation before it differs from the

5804 conversation it was created in (the rest of that turn's consecutive thinking

5805 blocks are removed with it, each with this reason);

5806 `organization_binding_mismatch` — it was created under a different

5807 organization (an Anthropic organization, AWS account or Google Cloud project)

5808 and this organization is not one of its additional organizations;

5809 `end_user_binding_mismatch` — it was created for a different end user, or

5810 was removed by the consumer-organization binding. A block that would fail

5811 several checks reports one reason, in this order of precedence:

5812 `organization_binding_mismatch`, `end_user_binding_mismatch`,

5813 `model_binding_mismatch`, `prefix_binding_mismatch`.

5814

5815 - `type: "thinking_dropped"`

5816

5817 Always `thinking_dropped` for this entry type.

5818

5819 default: thinking_dropped

56645820

56655821 - `BetaRawMessageStopEvent object`

56665822

from line 6110

59546110 "cache_creation_input_tokens": 0,

59556111 "cache_read_input_tokens": 0,

59566112 "input_tokens": 0,

5957 "model": "claude-sonnet-5",

6113 "model": "claude-fable-5-1",

59586114 "output_tokens": 0,

59596115 "type": "message"

59606116 }

from line 6125

59696125 },

59706126 "service_tier": "standard",

59716127 "speed": "standard"

5972 }

6128 },

6129 "input_transformations": [

6130 {

6131 "path": "path",

6132 "reason": "model_binding_mismatch",

6133 "type": "thinking_dropped"

6134 }

6135 ]

59736136}

59746137```

59756138

from line 6154

59916154

59926155 - `string`

59936156

5994 - `"message-batches-2024-09-24" or "prompt-caching-2024-07-31" or "computer-use-2024-10-22" or 38 more`

6157 - `"message-batches-2024-09-24" or "prompt-caching-2024-07-31" or "computer-use-2024-10-22" or 41 more`

59956158

59966159 - `"message-batches-2024-09-24"`

59976160

from line 6237

60746237 - `"thinking-display-updates-2026-08-18"`

60756238

60766239 - `"ce-user-management-2026-07-13"`

6240

6241 - `"mid-conversation-output-config-2026-07-01"`

6242

6243 - `"thinking-binding-controls-2026-08-01"`

6244

6245 - `"mid-conversation-system-clear-at-2026-08-21"`

60776246

60786247- `"anthropic-user-profile-id": optional string`

60796248

from line 7524

73557524

73567525 See [models](https://docs.anthropic.com/en/docs/models-overview) for additional details and options.

73577526

7358 - `"claude-sonnet-5" or "claude-fable-5" or "claude-mythos-5" or 12 more`

7527 - `"claude-fable-5-1" or "claude-mythos-5-1" or "claude-sonnet-5" or 14 more`

73597528

73607529 The model that will complete your prompt.

73617530

73627531 See [models](https://docs.anthropic.com/en/docs/models-overview) for additional details and options.

73637532

7533 - `"claude-fable-5-1"`

7534

7535 Frontier intelligence for ambitious tasks across coding, scientific discovery, and enterprise workflows

7536

7537 - `"claude-mythos-5-1"`

7538

7539 Our most capable model for cybersecurity and biology research, available through trusted access programs

7540

73647541 - `"claude-sonnet-5"`

73657542

73667543 High-performance model for coding and agents

from line 7618

74417618

74427619 - `"system"`

74437620

7621 - `clear_at: optional "next_user_message" or "never" or null`

7622

7623 How long this system message's text stays in front of the model. `"never"` (the default) renders it on every request that includes it. `"next_user_message"` renders it only for the user turn it follows: once a later `role: "user"` message exists in `messages` the message stays in the array (send it unchanged) but is no longer shown to the model. Only permitted on `role: "system"` messages.

7624

7625 - `"next_user_message"`

7626

7627 - `"never"`

7628

7629 - `output_config: optional BetaSystemMessageOutputConfig or null`

7630

7631 Per-message output configuration on a role:"system" input message.

7632

7633 Fields here apply per-turn; `format` remains top-level only. An

7634 empty `{}` is accepted on a message that carries content; a message

7635 with neither content nor output_config fields is rejected.

7636

7637 - `effort: optional "low" or "medium" or "high" or 2 more or null`

7638

7639 All possible effort levels.

7640

7641 - `"low"`

7642

7643 - `"medium"`

7644

7645 - `"high"`

7646

7647 - `"xhigh"`

7648

7649 - `"max"`

7650

74447651- `model: Model`

74457652

74467653 The model that will complete your prompt.

from line 7886

76797886

76807887 - `type: "enabled"`

76817888

7889 - `block_binding: optional BetaThinkingBlockBinding or null`

7890

7891 Controls for block binding: what happens when a thinking block this

7892 request sends back fails the conversation check. Every field is optional;

7893 an empty object means every default.

7894

7895 - `prefix_mismatch_behavior: optional BetaThinkingPrefixMismatchBehavior or null`

7896

7897 What happens when a thinking block in `messages` fails the conversation

7898 check: it was created in a different conversation, or the messages before

7899 it have changed since. `"error"` (the default) fails the request with a

7900 400 error. `"drop_block"` removes the failing blocks and the request

7901 proceeds; the model no longer sees the dropped reasoning.

7902

7903 - `"error"`

7904

7905 - `"drop_block"`

7906

76827907 - `display: optional "summarized" or "omitted" or "updates" or null`

76837908

76847909 Controls how thinking content appears in the response. When set to `summarized`, thinking is returned normally. When set to `omitted`, thinking content is redacted but a signature is returned for multi-turn continuity. Defaults to `summarized`.

from line 7921

76967921 - `BetaThinkingConfigAdaptive object`

76977922

76987923 - `type: "adaptive"`

7924

7925 - `block_binding: optional BetaThinkingBlockBinding or null`

7926

7927 Controls for block binding: what happens when a thinking block this

7928 request sends back fails the conversation check. Every field is optional;

7929 an empty object means every default.

76997930

77007931 - `display: optional "summarized" or "omitted" or "updates" or null`

77017932

from line 8319

80888319

80898320 - `type: "browser_toolset_20260801"`

80908321

8091 - `allowed_callers: optional array of "direct" or "code_execution_20250825" or "code_execution_20260120" or "code_execution_20260521"`

8092

8093 - `"direct"`

8094

8095 - `"code_execution_20250825"`

8096

8097 - `"code_execution_20260120"`

8098

8099 - `"code_execution_20260521"`

8100

81018322 - `cache_control: optional BetaCacheControlEphemeral or null`

81028323

81038324 Create a cache control breakpoint at this content block.

from line 8944

87238944 via `configs.zoom.enabled`.

87248945

87258946 - `type: "computer_toolset_20260801"`

8726

8727 - `allowed_callers: optional array of "direct" or "code_execution_20250825" or "code_execution_20260120" or "code_execution_20260521"`

8728

8729 - `"direct"`

8730

8731 - `"code_execution_20250825"`

8732

8733 - `"code_execution_20260120"`

8734

8735 - `"code_execution_20260521"`

87368947

87378948 - `cache_control: optional BetaCacheControlEphemeral or null`

87388949

from line 9982

97719982

97729983 See [models](https://docs.anthropic.com/en/docs/models-overview) for additional details and options.

97739984

9774 - `"claude-sonnet-5" or "claude-fable-5" or "claude-mythos-5" or 12 more`

9985 - `"claude-fable-5-1" or "claude-mythos-5-1" or "claude-sonnet-5" or 14 more`

97759986

97769987 The model that will complete your prompt.

97779988

97789989 See [models](https://docs.anthropic.com/en/docs/models-overview) for additional details and options.

97799990

9991 - `"claude-fable-5-1"`

9992

9993 Frontier intelligence for ambitious tasks across coding, scientific discovery, and enterprise workflows

9994

9995 - `"claude-mythos-5-1"`

9996

9997 Our most capable model for cybersecurity and biology research, available through trusted access programs

9998

97809999 - `"claude-sonnet-5"`

978110000

978210001 High-performance model for coding and agents

from line 10132

991310132

991410133 See [models](https://docs.anthropic.com/en/docs/models-overview) for additional details and options.

991510134

9916 - `"claude-sonnet-5" or "claude-fable-5" or "claude-mythos-5" or 12 more`

10135 - `"claude-fable-5-1" or "claude-mythos-5-1" or "claude-sonnet-5" or 14 more`

991710136

991810137 The model that will complete your prompt.

991910138

992010139 See [models](https://docs.anthropic.com/en/docs/models-overview) for additional details and options.

10140

10141 - `"claude-fable-5-1"`

10142

10143 Frontier intelligence for ambitious tasks across coding, scientific discovery, and enterprise workflows

10144

10145 - `"claude-mythos-5-1"`

10146

10147 Our most capable model for cybersecurity and biology research, available through trusted access programs

992110148

992210149 - `"claude-sonnet-5"`

992310150

from line 11492

1126511492 from its schema.

1126611493

1126711494 - `type: "browser_toolset_20260801"`

11268

11269 - `allowed_callers: optional array of "direct" or "code_execution_20250825" or "code_execution_20260120" or "code_execution_20260521"`

11270

11271 - `"direct"`

11272

11273 - `"code_execution_20250825"`

11274

11275 - `"code_execution_20260120"`

11276

11277 - `"code_execution_20260521"`

1127811495

1127911496 - `cache_control: optional BetaCacheControlEphemeral or null`

1128011497

from line 13922

1370513922

1370613923 - `type: "computer_toolset_20260801"`

1370713924

13708 - `allowed_callers: optional array of "direct" or "code_execution_20250825" or "code_execution_20260120" or "code_execution_20260521"`

13709

13710 - `"direct"`

13711

13712 - `"code_execution_20250825"`

13713

13714 - `"code_execution_20260120"`

13715

13716 - `"code_execution_20260521"`

13717

1371813925 - `cache_control: optional BetaCacheControlEphemeral or null`

1371913926

1372013927 Create a cache control breakpoint at this content block.

from line 15427

1522015427

1522115428 See [models](https://docs.anthropic.com/en/docs/models-overview) for additional details and options.

1522215429

15223 - `"claude-sonnet-5" or "claude-fable-5" or "claude-mythos-5" or 12 more`

15430 - `"claude-fable-5-1" or "claude-mythos-5-1" or "claude-sonnet-5" or 14 more`

1522415431

1522515432 The model that will complete your prompt.

1522615433

1522715434 See [models](https://docs.anthropic.com/en/docs/models-overview) for additional details and options.

15435

15436 - `"claude-fable-5-1"`

15437

15438 Frontier intelligence for ambitious tasks across coding, scientific discovery, and enterprise workflows

15439

15440 - `"claude-mythos-5-1"`

15441

15442 Our most capable model for cybersecurity and biology research, available through trusted access programs

1522815443

1522915444 - `"claude-sonnet-5"`

1523015445

from line 16766

1655116766

1655216767 See [models](https://docs.anthropic.com/en/docs/models-overview) for additional details and options.

1655316768

16554 - `"claude-sonnet-5" or "claude-fable-5" or "claude-mythos-5" or 12 more`

16769 - `"claude-fable-5-1" or "claude-mythos-5-1" or "claude-sonnet-5" or 14 more`

1655516770

1655616771 The model that will complete your prompt.

1655716772

1655816773 See [models](https://docs.anthropic.com/en/docs/models-overview) for additional details and options.

16774

16775 - `"claude-fable-5-1"`

16776

16777 Frontier intelligence for ambitious tasks across coding, scientific discovery, and enterprise workflows

16778

16779 - `"claude-mythos-5-1"`

16780

16781 Our most capable model for cybersecurity and biology research, available through trusted access programs

1655916782

1656016783 - `"claude-sonnet-5"`

1656116784

from line 17630

1740717630

1740817631 See [models](https://docs.anthropic.com/en/docs/models-overview) for additional details and options.

1740917632

17410 - `"claude-sonnet-5" or "claude-fable-5" or "claude-mythos-5" or 12 more`

17633 - `"claude-fable-5-1" or "claude-mythos-5-1" or "claude-sonnet-5" or 14 more`

1741117634

1741217635 The model that will complete your prompt.

1741317636

1741417637 See [models](https://docs.anthropic.com/en/docs/models-overview) for additional details and options.

17638

17639 - `"claude-fable-5-1"`

17640

17641 Frontier intelligence for ambitious tasks across coding, scientific discovery, and enterprise workflows

17642

17643 - `"claude-mythos-5-1"`

17644

17645 Our most capable model for cybersecurity and biology research, available through trusted access programs

1741517646

1741617647 - `"claude-sonnet-5"`

1741717648

from line 17774

1754317774

1754417775 See [models](https://docs.anthropic.com/en/docs/models-overview) for additional details and options.

1754517776

17546 - `"claude-sonnet-5" or "claude-fable-5" or "claude-mythos-5" or 12 more`

17777 - `"claude-fable-5-1" or "claude-mythos-5-1" or "claude-sonnet-5" or 14 more`

1754717778

1754817779 The model that will complete your prompt.

1754917780

1755017781 See [models](https://docs.anthropic.com/en/docs/models-overview) for additional details and options.

17782

17783 - `"claude-fable-5-1"`

17784

17785 Frontier intelligence for ambitious tasks across coding, scientific discovery, and enterprise workflows

17786

17787 - `"claude-mythos-5-1"`

17788

17789 Our most capable model for cybersecurity and biology research, available through trusted access programs

1755117790

1755217791 - `"claude-sonnet-5"`

1755317792

from line 18035

1779618035

1779718036 See [models](https://docs.anthropic.com/en/docs/models-overview) for additional details and options.

1779818037

17799 - `"claude-sonnet-5" or "claude-fable-5" or "claude-mythos-5" or 12 more`

18038 - `"claude-fable-5-1" or "claude-mythos-5-1" or "claude-sonnet-5" or 14 more`

1780018039

1780118040 The model that will complete your prompt.

1780218041

1780318042 See [models](https://docs.anthropic.com/en/docs/models-overview) for additional details and options.

1780418043

18044 - `"claude-fable-5-1"`

18045

18046 Frontier intelligence for ambitious tasks across coding, scientific discovery, and enterprise workflows

18047

18048 - `"claude-mythos-5-1"`

18049

18050 Our most capable model for cybersecurity and biology research, available through trusted access programs

18051

1780518052 - `"claude-sonnet-5"`

1780618053

1780718054 High-performance model for coding and agents

from line 18123

1787618123

1787718124 See [models](https://docs.anthropic.com/en/docs/models-overview) for additional details and options.

1787818125

17879 - `"claude-sonnet-5" or "claude-fable-5" or "claude-mythos-5" or 12 more`

18126 - `"claude-fable-5-1" or "claude-mythos-5-1" or "claude-sonnet-5" or 14 more`

1788018127

1788118128 The model that will complete your prompt.

1788218129

1788318130 See [models](https://docs.anthropic.com/en/docs/models-overview) for additional details and options.

18131

18132 - `"claude-fable-5-1"`

18133

18134 Frontier intelligence for ambitious tasks across coding, scientific discovery, and enterprise workflows

18135

18136 - `"claude-mythos-5-1"`

18137

18138 Our most capable model for cybersecurity and biology research, available through trusted access programs

1788418139

1788518140 - `"claude-sonnet-5"`

1788618141

from line 18250

1799518250

1799618251 See [models](https://docs.anthropic.com/en/docs/models-overview) for additional details and options.

1799718252

17998 - `"claude-sonnet-5" or "claude-fable-5" or "claude-mythos-5" or 12 more`

18253 - `"claude-fable-5-1" or "claude-mythos-5-1" or "claude-sonnet-5" or 14 more`

1799918254

1800018255 The model that will complete your prompt.

1800118256

1800218257 See [models](https://docs.anthropic.com/en/docs/models-overview) for additional details and options.

18258

18259 - `"claude-fable-5-1"`

18260

18261 Frontier intelligence for ambitious tasks across coding, scientific discovery, and enterprise workflows

18262

18263 - `"claude-mythos-5-1"`

18264

18265 Our most capable model for cybersecurity and biology research, available through trusted access programs

1800318266

1800418267 - `"claude-sonnet-5"`

1800518268

from line 18355

1809218355

1809318356 See [models](https://docs.anthropic.com/en/docs/models-overview) for additional details and options.

1809418357

18095 - `"claude-sonnet-5" or "claude-fable-5" or "claude-mythos-5" or 12 more`

18358 - `"claude-fable-5-1" or "claude-mythos-5-1" or "claude-sonnet-5" or 14 more`

1809618359

1809718360 The model that will complete your prompt.

1809818361

1809918362 See [models](https://docs.anthropic.com/en/docs/models-overview) for additional details and options.

1810018363

18364 - `"claude-fable-5-1"`

18365

18366 Frontier intelligence for ambitious tasks across coding, scientific discovery, and enterprise workflows

18367

18368 - `"claude-mythos-5-1"`

18369

18370 Our most capable model for cybersecurity and biology research, available through trusted access programs

18371

1810118372 - `"claude-sonnet-5"`

1810218373

1810318374 High-performance model for coding and agents

from line 18503

1823218503

1823318504 - `type: "enabled"`

1823418505

18506 - `block_binding: optional BetaThinkingBlockBinding or null`

18507

18508 Controls for block binding: what happens when a thinking block this

18509 request sends back fails the conversation check. Every field is optional;

18510 an empty object means every default.

18511

18512 - `prefix_mismatch_behavior: optional BetaThinkingPrefixMismatchBehavior or null`

18513

18514 What happens when a thinking block in `messages` fails the conversation

18515 check: it was created in a different conversation, or the messages before

18516 it have changed since. `"error"` (the default) fails the request with a

18517 400 error. `"drop_block"` removes the failing blocks and the request

18518 proceeds; the model no longer sees the dropped reasoning.

18519

18520 - `"error"`

18521

18522 - `"drop_block"`

18523

1823518524 - `display: optional "summarized" or "omitted" or "updates" or null`

1823618525

1823718526 Controls how thinking content appears in the response. When set to `summarized`, thinking is returned normally. When set to `omitted`, thinking content is redacted but a signature is returned for multi-turn continuity. Defaults to `summarized`.

from line 18539

1825018539

1825118540 - `type: "adaptive"`

1825218541

18542 - `block_binding: optional BetaThinkingBlockBinding or null`

18543

18544 Controls for block binding: what happens when a thinking block this

18545 request sends back fails the conversation check. Every field is optional;

18546 an empty object means every default.

18547

1825318548 - `display: optional "summarized" or "omitted" or "updates" or null`

1825418549

1825518550 Controls how thinking content appears in the response. When set to `summarized`, thinking is returned normally. When set to `omitted`, thinking content is redacted but a signature is returned for multi-turn continuity. Defaults to `summarized`.

from line 18603

1830818603

1830918604 See [models](https://docs.anthropic.com/en/docs/models-overview) for additional details and options.

1831018605

18311 - `"claude-sonnet-5" or "claude-fable-5" or "claude-mythos-5" or 12 more`

18606 - `"claude-fable-5-1" or "claude-mythos-5-1" or "claude-sonnet-5" or 14 more`

1831218607

1831318608 The model that will complete your prompt.

1831418609

1831518610 See [models](https://docs.anthropic.com/en/docs/models-overview) for additional details and options.

1831618611

18612 - `"claude-fable-5-1"`

18613

18614 Frontier intelligence for ambitious tasks across coding, scientific discovery, and enterprise workflows

18615

18616 - `"claude-mythos-5-1"`

18617

18618 Our most capable model for cybersecurity and biology research, available through trusted access programs

18619

1831718620 - `"claude-sonnet-5"`

1831818621

1831918622 High-performance model for coding and agents

from line 18751

1844818751

1844918752 - `type: "enabled"`

1845018753

18754 - `block_binding: optional BetaThinkingBlockBinding or null`

18755

18756 Controls for block binding: what happens when a thinking block this

18757 request sends back fails the conversation check. Every field is optional;

18758 an empty object means every default.

18759

18760 - `prefix_mismatch_behavior: optional BetaThinkingPrefixMismatchBehavior or null`

18761

18762 What happens when a thinking block in `messages` fails the conversation

18763 check: it was created in a different conversation, or the messages before

18764 it have changed since. `"error"` (the default) fails the request with a

18765 400 error. `"drop_block"` removes the failing blocks and the request

18766 proceeds; the model no longer sees the dropped reasoning.

18767

18768 - `"error"`

18769

18770 - `"drop_block"`

18771

1845118772 - `display: optional "summarized" or "omitted" or "updates" or null`

1845218773

1845318774 Controls how thinking content appears in the response. When set to `summarized`, thinking is returned normally. When set to `omitted`, thinking content is redacted but a signature is returned for multi-turn continuity. Defaults to `summarized`.

from line 18786

1846518786 - `BetaThinkingConfigAdaptive object`

1846618787

1846718788 - `type: "adaptive"`

18789

18790 - `block_binding: optional BetaThinkingBlockBinding or null`

18791

18792 Controls for block binding: what happens when a thinking block this

18793 request sends back fails the conversation check. Every field is optional;

18794 an empty object means every default.

1846818795

1846918796 - `display: optional "summarized" or "omitted" or "updates" or null`

1847018797

from line 18994

1866718994

1866818995 See [models](https://docs.anthropic.com/en/docs/models-overview) for additional details and options.

1866918996

18670 - `"claude-sonnet-5" or "claude-fable-5" or "claude-mythos-5" or 12 more`

18997 - `"claude-fable-5-1" or "claude-mythos-5-1" or "claude-sonnet-5" or 14 more`

1867118998

1867218999 The model that will complete your prompt.

1867319000

1867419001 See [models](https://docs.anthropic.com/en/docs/models-overview) for additional details and options.

19002

19003 - `"claude-fable-5-1"`

19004

19005 Frontier intelligence for ambitious tasks across coding, scientific discovery, and enterprise workflows

19006

19007 - `"claude-mythos-5-1"`

19008

19009 Our most capable model for cybersecurity and biology research, available through trusted access programs

1867519010

1867619011 - `"claude-sonnet-5"`

1867719012

from line 20724

2038920724

2039020725 See [models](https://docs.anthropic.com/en/docs/models-overview) for additional details and options.

2039120726

20392 - `"claude-sonnet-5" or "claude-fable-5" or "claude-mythos-5" or 12 more`

20727 - `"claude-fable-5-1" or "claude-mythos-5-1" or "claude-sonnet-5" or 14 more`

2039320728

2039420729 The model that will complete your prompt.

2039520730

2039620731 See [models](https://docs.anthropic.com/en/docs/models-overview) for additional details and options.

20732

20733 - `"claude-fable-5-1"`

20734

20735 Frontier intelligence for ambitious tasks across coding, scientific discovery, and enterprise workflows

20736

20737 - `"claude-mythos-5-1"`

20738

20739 Our most capable model for cybersecurity and biology research, available through trusted access programs

2039720740

2039820741 - `"claude-sonnet-5"`

2039920742

from line 21469

2112621469

2112721470 - `"fast"`

2112821471

21472 - `input_transformations: optional array of BetaThinkingDroppedInputTransformation or null`

21473

21474 Changes the API made to the request's input before showing it to the model:

21475 one entry per change, in request order. Today the only entry type is

21476 `thinking_dropped` — a `thinking`, `redacted_thinking` or `connector_text`

21477 block from the request's `messages` that was removed from the prompt instead

21478 of being shown to the model because it failed a binding check. More entry

21479 types may be added over time; ignore types you do not recognize.

21480

21481 Requires `anthropic-beta: thinking-binding-controls-2026-08-01`. Present on

21482 every such response from a model that supports extended thinking, as `[]`

21483 when nothing was changed; without the beta, blocks are removed all the same

21484 but nothing is reported. Removed blocks contribute nothing to

21485 `usage.input_tokens`. When streaming, the array is final in `message_start`;

21486 the final `message_delta` event carries it only when a server-side model

21487 fallback happened mid-stream, in which case it holds the serving model's

21488 entries and replaces the one in `message_start`.

21489

21490 - `path: string`

21491

21492 Where the removed block was in your request, as `messages.{i}.content.{j}`:

21493 `i` indexes the `messages` array you sent and `j` that message's `content`

21494 array — the same form error messages use.

21495

21496 - `reason: "model_binding_mismatch" or "prefix_binding_mismatch" or "organization_binding_mismatch" or "end_user_binding_mismatch"`

21497

21498 Which binding check removed the block: `model_binding_mismatch` — it was

21499 created by a model whose reasoning the requested model may not read;

21500 `prefix_binding_mismatch` — the conversation before it differs from the

21501 conversation it was created in (the rest of that turn's consecutive thinking

21502 blocks are removed with it, each with this reason);

21503 `organization_binding_mismatch` — it was created under a different

21504 organization (an Anthropic organization, AWS account or Google Cloud project)

21505 and this organization is not one of its additional organizations;

21506 `end_user_binding_mismatch` — it was created for a different end user, or

21507 was removed by the consumer-organization binding. A block that would fail

21508 several checks reports one reason, in this order of precedence:

21509 `organization_binding_mismatch`, `end_user_binding_mismatch`,

21510 `model_binding_mismatch`, `prefix_binding_mismatch`.

21511

21512 - `"model_binding_mismatch"`

21513

21514 - `"prefix_binding_mismatch"`

21515

21516 - `"organization_binding_mismatch"`

21517

21518 - `"end_user_binding_mismatch"`

21519

21520 - `type: "thinking_dropped"`

21521

21522 Always `thinking_dropped` for this entry type.

21523

21524 default: thinking_dropped

21525

2112921526### Beta Message Delta Usage

2113021527

2113121528- `BetaMessageDeltaUsage object`

from line 21673

2127621673

2127721674 See [models](https://docs.anthropic.com/en/docs/models-overview) for additional details and options.

2127821675

21279 - `"claude-sonnet-5" or "claude-fable-5" or "claude-mythos-5" or 12 more`

21676 - `"claude-fable-5-1" or "claude-mythos-5-1" or "claude-sonnet-5" or 14 more`

2128021677

2128121678 The model that will complete your prompt.

2128221679

2128321680 See [models](https://docs.anthropic.com/en/docs/models-overview) for additional details and options.

21681

21682 - `"claude-fable-5-1"`

21683

21684 Frontier intelligence for ambitious tasks across coding, scientific discovery, and enterprise workflows

21685

21686 - `"claude-mythos-5-1"`

21687

21688 Our most capable model for cybersecurity and biology research, available through trusted access programs

2128421689

2128521690 - `"claude-sonnet-5"`

2128621691

from line 21980

2157521980

2157621981 See [models](https://docs.anthropic.com/en/docs/models-overview) for additional details and options.

2157721982

21578 - `"claude-sonnet-5" or "claude-fable-5" or "claude-mythos-5" or 12 more`

21983 - `"claude-fable-5-1" or "claude-mythos-5-1" or "claude-sonnet-5" or 14 more`

2157921984

2158021985 The model that will complete your prompt.

2158121986

2158221987 See [models](https://docs.anthropic.com/en/docs/models-overview) for additional details and options.

21988

21989 - `"claude-fable-5-1"`

21990

21991 Frontier intelligence for ambitious tasks across coding, scientific discovery, and enterprise workflows

21992

21993 - `"claude-mythos-5-1"`

21994

21995 Our most capable model for cybersecurity and biology research, available through trusted access programs

2158321996

2158421997 - `"claude-sonnet-5"`

2158521998

from line 23295

2288223295

2288323296 See [models](https://docs.anthropic.com/en/docs/models-overview) for additional details and options.

2288423297

22885 - `"claude-sonnet-5" or "claude-fable-5" or "claude-mythos-5" or 12 more`

23298 - `"claude-fable-5-1" or "claude-mythos-5-1" or "claude-sonnet-5" or 14 more`

2288623299

2288723300 The model that will complete your prompt.

2288823301

2288923302 See [models](https://docs.anthropic.com/en/docs/models-overview) for additional details and options.

2289023303

23304 - `"claude-fable-5-1"`

23305

23306 Frontier intelligence for ambitious tasks across coding, scientific discovery, and enterprise workflows

23307

23308 - `"claude-mythos-5-1"`

23309

23310 Our most capable model for cybersecurity and biology research, available through trusted access programs

23311

2289123312 - `"claude-sonnet-5"`

2289223313

2289323314 High-performance model for coding and agents

from line 23388

2296723388 - `"assistant"`

2296823389

2296923390 - `"system"`

23391

23392 - `clear_at: optional "next_user_message" or "never" or null`

23393

23394 How long this system message's text stays in front of the model. `"never"` (the default) renders it on every request that includes it. `"next_user_message"` renders it only for the user turn it follows: once a later `role: "user"` message exists in `messages` the message stays in the array (send it unchanged) but is no longer shown to the model. Only permitted on `role: "system"` messages.

23395

23396 - `"next_user_message"`

23397

23398 - `"never"`

23399

23400 - `output_config: optional BetaSystemMessageOutputConfig or null`

23401

23402 Per-message output configuration on a role:"system" input message.

23403

23404 Fields here apply per-turn; `format` remains top-level only. An

23405 empty `{}` is accepted on a message that carries content; a message

23406 with neither content nor output_config fields is rejected.

23407

23408 - `effort: optional "low" or "medium" or "high" or 2 more or null`

23409

23410 All possible effort levels.

23411

23412 - `"low"`

23413

23414 - `"medium"`

23415

23416 - `"high"`

23417

23418 - `"xhigh"`

23419

23420 - `"max"`

2297023421

2297123422### Beta Message Tokens Count

2297223423

from line 24770

2431924770

2432024771 See [models](https://docs.anthropic.com/en/docs/models-overview) for additional details and options.

2432124772

24322 - `"claude-sonnet-5" or "claude-fable-5" or "claude-mythos-5" or 12 more`

24773 - `"claude-fable-5-1" or "claude-mythos-5-1" or "claude-sonnet-5" or 14 more`

2432324774

2432424775 The model that will complete your prompt.

2432524776

2432624777 See [models](https://docs.anthropic.com/en/docs/models-overview) for additional details and options.

24778

24779 - `"claude-fable-5-1"`

24780

24781 Frontier intelligence for ambitious tasks across coding, scientific discovery, and enterprise workflows

24782

24783 - `"claude-mythos-5-1"`

24784

24785 Our most capable model for cybersecurity and biology research, available through trusted access programs

2432724786

2432824787 - `"claude-sonnet-5"`

2432924788

from line 25263

2480425263

2480525264 See [models](https://docs.anthropic.com/en/docs/models-overview) for additional details and options.

2480625265

24807 - `"claude-sonnet-5" or "claude-fable-5" or "claude-mythos-5" or 12 more`

25266 - `"claude-fable-5-1" or "claude-mythos-5-1" or "claude-sonnet-5" or 14 more`

2480825267

2480925268 The model that will complete your prompt.

2481025269

2481125270 See [models](https://docs.anthropic.com/en/docs/models-overview) for additional details and options.

25271

25272 - `"claude-fable-5-1"`

25273

25274 Frontier intelligence for ambitious tasks across coding, scientific discovery, and enterprise workflows

25275

25276 - `"claude-mythos-5-1"`

25277

25278 Our most capable model for cybersecurity and biology research, available through trusted access programs

2481225279

2481325280 - `"claude-sonnet-5"`

2481425281

from line 25523

2505625523 The number of web search tool requests.

2505725524

2505825525 default: 0, minimum: 0

25526

25527 - `input_transformations: optional array of BetaThinkingDroppedInputTransformation or null`

25528

25529 Changes the API made to the request's input before showing it to the model:

25530 one entry per change, in request order. Today the only entry type is

25531 `thinking_dropped` — a `thinking`, `redacted_thinking` or `connector_text`

25532 block from the request's `messages` that was removed from the prompt instead

25533 of being shown to the model because it failed a binding check. More entry

25534 types may be added over time; ignore types you do not recognize.

25535

25536 Requires `anthropic-beta: thinking-binding-controls-2026-08-01`. Present on

25537 every such response from a model that supports extended thinking, as `[]`

25538 when nothing was changed; without the beta, blocks are removed all the same

25539 but nothing is reported. Removed blocks contribute nothing to

25540 `usage.input_tokens`. When streaming, the array is final in `message_start`;

25541 the final `message_delta` event carries it only when a server-side model

25542 fallback happened mid-stream, in which case it holds the serving model's

25543 entries and replaces the one in `message_start`.

25544

25545 - `path: string`

25546

25547 Where the removed block was in your request, as `messages.{i}.content.{j}`:

25548 `i` indexes the `messages` array you sent and `j` that message's `content`

25549 array — the same form error messages use.

25550

25551 - `reason: "model_binding_mismatch" or "prefix_binding_mismatch" or "organization_binding_mismatch" or "end_user_binding_mismatch"`

25552

25553 Which binding check removed the block: `model_binding_mismatch` — it was

25554 created by a model whose reasoning the requested model may not read;

25555 `prefix_binding_mismatch` — the conversation before it differs from the

25556 conversation it was created in (the rest of that turn's consecutive thinking

25557 blocks are removed with it, each with this reason);

25558 `organization_binding_mismatch` — it was created under a different

25559 organization (an Anthropic organization, AWS account or Google Cloud project)

25560 and this organization is not one of its additional organizations;

25561 `end_user_binding_mismatch` — it was created for a different end user, or

25562 was removed by the consumer-organization binding. A block that would fail

25563 several checks reports one reason, in this order of precedence:

25564 `organization_binding_mismatch`, `end_user_binding_mismatch`,

25565 `model_binding_mismatch`, `prefix_binding_mismatch`.

25566

25567 - `"model_binding_mismatch"`

25568

25569 - `"prefix_binding_mismatch"`

25570

25571 - `"organization_binding_mismatch"`

25572

25573 - `"end_user_binding_mismatch"`

25574

25575 - `type: "thinking_dropped"`

25576

25577 Always `thinking_dropped` for this entry type.

25578

25579 default: thinking_dropped

2505925580

2506025581### Beta Raw Message Start Event

2506125582

from line 26505

2598426505

2598526506 See [models](https://docs.anthropic.com/en/docs/models-overview) for additional details and options.

2598626507

25987 - `"claude-sonnet-5" or "claude-fable-5" or "claude-mythos-5" or 12 more`

26508 - `"claude-fable-5-1" or "claude-mythos-5-1" or "claude-sonnet-5" or 14 more`

2598826509

2598926510 The model that will complete your prompt.

2599026511

2599126512 See [models](https://docs.anthropic.com/en/docs/models-overview) for additional details and options.

26513

26514 - `"claude-fable-5-1"`

26515

26516 Frontier intelligence for ambitious tasks across coding, scientific discovery, and enterprise workflows

26517

26518 - `"claude-mythos-5-1"`

26519

26520 Our most capable model for cybersecurity and biology research, available through trusted access programs

2599226521

2599326522 - `"claude-sonnet-5"`

2599426523

from line 27250

2672127250

2672227251 - `"fast"`

2672327252

27253 - `input_transformations: optional array of BetaThinkingDroppedInputTransformation or null`

27254

27255 Changes the API made to the request's input before showing it to the model:

27256 one entry per change, in request order. Today the only entry type is

27257 `thinking_dropped` — a `thinking`, `redacted_thinking` or `connector_text`

27258 block from the request's `messages` that was removed from the prompt instead

27259 of being shown to the model because it failed a binding check. More entry

27260 types may be added over time; ignore types you do not recognize.

27261

27262 Requires `anthropic-beta: thinking-binding-controls-2026-08-01`. Present on

27263 every such response from a model that supports extended thinking, as `[]`

27264 when nothing was changed; without the beta, blocks are removed all the same

27265 but nothing is reported. Removed blocks contribute nothing to

27266 `usage.input_tokens`. When streaming, the array is final in `message_start`;

27267 the final `message_delta` event carries it only when a server-side model

27268 fallback happened mid-stream, in which case it holds the serving model's

27269 entries and replaces the one in `message_start`.

27270

27271 - `path: string`

27272

27273 Where the removed block was in your request, as `messages.{i}.content.{j}`:

27274 `i` indexes the `messages` array you sent and `j` that message's `content`

27275 array — the same form error messages use.

27276

27277 - `reason: "model_binding_mismatch" or "prefix_binding_mismatch" or "organization_binding_mismatch" or "end_user_binding_mismatch"`

27278

27279 Which binding check removed the block: `model_binding_mismatch` — it was

27280 created by a model whose reasoning the requested model may not read;

27281 `prefix_binding_mismatch` — the conversation before it differs from the

27282 conversation it was created in (the rest of that turn's consecutive thinking

27283 blocks are removed with it, each with this reason);

27284 `organization_binding_mismatch` — it was created under a different

27285 organization (an Anthropic organization, AWS account or Google Cloud project)

27286 and this organization is not one of its additional organizations;

27287 `end_user_binding_mismatch` — it was created for a different end user, or

27288 was removed by the consumer-organization binding. A block that would fail

27289 several checks reports one reason, in this order of precedence:

27290 `organization_binding_mismatch`, `end_user_binding_mismatch`,

27291 `model_binding_mismatch`, `prefix_binding_mismatch`.

27292

27293 - `"model_binding_mismatch"`

27294

27295 - `"prefix_binding_mismatch"`

27296

27297 - `"organization_binding_mismatch"`

27298

27299 - `"end_user_binding_mismatch"`

27300

27301 - `type: "thinking_dropped"`

27302

27303 Always `thinking_dropped` for this entry type.

27304

27305 default: thinking_dropped

27306

2672427307 - `type: "message_start"`

2672527308

2672627309 default: message_start

from line 28245

2766228245

2766328246 See [models](https://docs.anthropic.com/en/docs/models-overview) for additional details and options.

2766428247

27665 - `"claude-sonnet-5" or "claude-fable-5" or "claude-mythos-5" or 12 more`

28248 - `"claude-fable-5-1" or "claude-mythos-5-1" or "claude-sonnet-5" or 14 more`

2766628249

2766728250 The model that will complete your prompt.

2766828251

2766928252 See [models](https://docs.anthropic.com/en/docs/models-overview) for additional details and options.

28253

28254 - `"claude-fable-5-1"`

28255

28256 Frontier intelligence for ambitious tasks across coding, scientific discovery, and enterprise workflows

28257

28258 - `"claude-mythos-5-1"`

28259

28260 Our most capable model for cybersecurity and biology research, available through trusted access programs

2767028261

2767128262 - `"claude-sonnet-5"`

2767228263

from line 28990

2839928990

2840028991 - `"fast"`

2840128992

28993 - `input_transformations: optional array of BetaThinkingDroppedInputTransformation or null`

28994

28995 Changes the API made to the request's input before showing it to the model:

28996 one entry per change, in request order. Today the only entry type is

28997 `thinking_dropped` — a `thinking`, `redacted_thinking` or `connector_text`

28998 block from the request's `messages` that was removed from the prompt instead

28999 of being shown to the model because it failed a binding check. More entry

29000 types may be added over time; ignore types you do not recognize.

29001

29002 Requires `anthropic-beta: thinking-binding-controls-2026-08-01`. Present on

29003 every such response from a model that supports extended thinking, as `[]`

29004 when nothing was changed; without the beta, blocks are removed all the same

29005 but nothing is reported. Removed blocks contribute nothing to

29006 `usage.input_tokens`. When streaming, the array is final in `message_start`;

29007 the final `message_delta` event carries it only when a server-side model

29008 fallback happened mid-stream, in which case it holds the serving model's

29009 entries and replaces the one in `message_start`.

29010

29011 - `path: string`

29012

29013 Where the removed block was in your request, as `messages.{i}.content.{j}`:

29014 `i` indexes the `messages` array you sent and `j` that message's `content`

29015 array — the same form error messages use.

29016

29017 - `reason: "model_binding_mismatch" or "prefix_binding_mismatch" or "organization_binding_mismatch" or "end_user_binding_mismatch"`

29018

29019 Which binding check removed the block: `model_binding_mismatch` — it was

29020 created by a model whose reasoning the requested model may not read;

29021 `prefix_binding_mismatch` — the conversation before it differs from the

29022 conversation it was created in (the rest of that turn's consecutive thinking

29023 blocks are removed with it, each with this reason);

29024 `organization_binding_mismatch` — it was created under a different

29025 organization (an Anthropic organization, AWS account or Google Cloud project)

29026 and this organization is not one of its additional organizations;

29027 `end_user_binding_mismatch` — it was created for a different end user, or

29028 was removed by the consumer-organization binding. A block that would fail

29029 several checks reports one reason, in this order of precedence:

29030 `organization_binding_mismatch`, `end_user_binding_mismatch`,

29031 `model_binding_mismatch`, `prefix_binding_mismatch`.

29032

29033 - `"model_binding_mismatch"`

29034

29035 - `"prefix_binding_mismatch"`

29036

29037 - `"organization_binding_mismatch"`

29038

29039 - `"end_user_binding_mismatch"`

29040

29041 - `type: "thinking_dropped"`

29042

29043 Always `thinking_dropped` for this entry type.

29044

29045 default: thinking_dropped

29046

2840229047 - `type: "message_start"`

2840329048

2840429049 default: message_start

from line 29134

2848929134 - `server_tool_use: BetaServerToolUsage or null`

2849029135

2849129136 The number of server tool requests.

29137

29138 - `input_transformations: optional array of BetaThinkingDroppedInputTransformation or null`

29139

29140 Changes the API made to the request's input before showing it to the model:

29141 one entry per change, in request order. Today the only entry type is

29142 `thinking_dropped` — a `thinking`, `redacted_thinking` or `connector_text`

29143 block from the request's `messages` that was removed from the prompt instead

29144 of being shown to the model because it failed a binding check. More entry

29145 types may be added over time; ignore types you do not recognize.

29146

29147 Requires `anthropic-beta: thinking-binding-controls-2026-08-01`. Present on

29148 every such response from a model that supports extended thinking, as `[]`

29149 when nothing was changed; without the beta, blocks are removed all the same

29150 but nothing is reported. Removed blocks contribute nothing to

29151 `usage.input_tokens`. When streaming, the array is final in `message_start`;

29152 the final `message_delta` event carries it only when a server-side model

29153 fallback happened mid-stream, in which case it holds the serving model's

29154 entries and replaces the one in `message_start`.

29155

29156 - `path: string`

29157

29158 Where the removed block was in your request, as `messages.{i}.content.{j}`:

29159 `i` indexes the `messages` array you sent and `j` that message's `content`

29160 array — the same form error messages use.

29161

29162 - `reason: "model_binding_mismatch" or "prefix_binding_mismatch" or "organization_binding_mismatch" or "end_user_binding_mismatch"`

29163

29164 Which binding check removed the block: `model_binding_mismatch` — it was

29165 created by a model whose reasoning the requested model may not read;

29166 `prefix_binding_mismatch` — the conversation before it differs from the

29167 conversation it was created in (the rest of that turn's consecutive thinking

29168 blocks are removed with it, each with this reason);

29169 `organization_binding_mismatch` — it was created under a different

29170 organization (an Anthropic organization, AWS account or Google Cloud project)

29171 and this organization is not one of its additional organizations;

29172 `end_user_binding_mismatch` — it was created for a different end user, or

29173 was removed by the consumer-organization binding. A block that would fail

29174 several checks reports one reason, in this order of precedence:

29175 `organization_binding_mismatch`, `end_user_binding_mismatch`,

29176 `model_binding_mismatch`, `prefix_binding_mismatch`.

29177

29178 - `type: "thinking_dropped"`

29179

29180 Always `thinking_dropped` for this entry type.

29181

29182 default: thinking_dropped

2849229183

2849329184 - `BetaRawMessageStopEvent object`

2849429185

from line 30468

2977730468

2977830469 - `"model_context_window_exceeded"`

2977930470

30471### Beta System Message Output Config

30472

30473- `BetaSystemMessageOutputConfig object`

30474

30475 Per-message output configuration on a role:"system" input message.

30476

30477 Fields here apply per-turn; `format` remains top-level only. An

30478 empty `{}` is accepted on a message that carries content; a message

30479 with neither content nor output_config fields is rejected.

30480

30481 - `effort: optional "low" or "medium" or "high" or 2 more or null`

30482

30483 All possible effort levels.

30484

30485 - `"low"`

30486

30487 - `"medium"`

30488

30489 - `"high"`

30490

30491 - `"xhigh"`

30492

30493 - `"max"`

30494

2978030495### Beta Text Block

2978130496

2978230497- `BetaTextBlock object`

from line 31388

3067331388

3067431389 default: thinking

3067531390

31391### Beta Thinking Block Binding

31392

31393- `BetaThinkingBlockBinding object`

31394

31395 Controls for block binding: what happens when a thinking block this

31396 request sends back fails the conversation check. Every field is optional;

31397 an empty object means every default.

31398

31399 - `prefix_mismatch_behavior: optional BetaThinkingPrefixMismatchBehavior or null`

31400

31401 What happens when a thinking block in `messages` fails the conversation

31402 check: it was created in a different conversation, or the messages before

31403 it have changed since. `"error"` (the default) fails the request with a

31404 400 error. `"drop_block"` removes the failing blocks and the request

31405 proceeds; the model no longer sees the dropped reasoning.

31406

31407 - `"error"`

31408

31409 - `"drop_block"`

31410

3067631411### Beta Thinking Block Param

3067731412

3067831413- `BetaThinkingBlockParam object`

from line 31430

3069531430

3069631431 - `type: "adaptive"`

3069731432

31433 - `block_binding: optional BetaThinkingBlockBinding or null`

31434

31435 Controls for block binding: what happens when a thinking block this

31436 request sends back fails the conversation check. Every field is optional;

31437 an empty object means every default.

31438

31439 - `prefix_mismatch_behavior: optional BetaThinkingPrefixMismatchBehavior or null`

31440

31441 What happens when a thinking block in `messages` fails the conversation

31442 check: it was created in a different conversation, or the messages before

31443 it have changed since. `"error"` (the default) fails the request with a

31444 400 error. `"drop_block"` removes the failing blocks and the request

31445 proceeds; the model no longer sees the dropped reasoning.

31446

31447 - `"error"`

31448

31449 - `"drop_block"`

31450

3069831451 - `display: optional "summarized" or "omitted" or "updates" or null`

3069931452

3070031453 Controls how thinking content appears in the response. When set to `summarized`, thinking is returned normally. When set to `omitted`, thinking content is redacted but a signature is returned for multi-turn continuity. Defaults to `summarized`.

from line 31480

3072731480

3072831481 - `type: "enabled"`

3072931482

31483 - `block_binding: optional BetaThinkingBlockBinding or null`

31484

31485 Controls for block binding: what happens when a thinking block this

31486 request sends back fails the conversation check. Every field is optional;

31487 an empty object means every default.

31488

31489 - `prefix_mismatch_behavior: optional BetaThinkingPrefixMismatchBehavior or null`

31490

31491 What happens when a thinking block in `messages` fails the conversation

31492 check: it was created in a different conversation, or the messages before

31493 it have changed since. `"error"` (the default) fails the request with a

31494 400 error. `"drop_block"` removes the failing blocks and the request

31495 proceeds; the model no longer sees the dropped reasoning.

31496

31497 - `"error"`

31498

31499 - `"drop_block"`

31500

3073031501 - `display: optional "summarized" or "omitted" or "updates" or null`

3073131502

3073231503 Controls how thinking content appears in the response. When set to `summarized`, thinking is returned normally. When set to `omitted`, thinking content is redacted but a signature is returned for multi-turn continuity. Defaults to `summarized`.

from line 31532

3076131532

3076231533 - `type: "enabled"`

3076331534

31535 - `block_binding: optional BetaThinkingBlockBinding or null`

31536

31537 Controls for block binding: what happens when a thinking block this

31538 request sends back fails the conversation check. Every field is optional;

31539 an empty object means every default.

31540

31541 - `prefix_mismatch_behavior: optional BetaThinkingPrefixMismatchBehavior or null`

31542

31543 What happens when a thinking block in `messages` fails the conversation

31544 check: it was created in a different conversation, or the messages before

31545 it have changed since. `"error"` (the default) fails the request with a

31546 400 error. `"drop_block"` removes the failing blocks and the request

31547 proceeds; the model no longer sees the dropped reasoning.

31548

31549 - `"error"`

31550

31551 - `"drop_block"`

31552

3076431553 - `display: optional "summarized" or "omitted" or "updates" or null`

3076531554

3076631555 Controls how thinking content appears in the response. When set to `summarized`, thinking is returned normally. When set to `omitted`, thinking content is redacted but a signature is returned for multi-turn continuity. Defaults to `summarized`.

from line 31568

3077931568

3078031569 - `type: "adaptive"`

3078131570

31571 - `block_binding: optional BetaThinkingBlockBinding or null`

31572

31573 Controls for block binding: what happens when a thinking block this

31574 request sends back fails the conversation check. Every field is optional;

31575 an empty object means every default.

31576

3078231577 - `display: optional "summarized" or "omitted" or "updates" or null`

3078331578

3078431579 Controls how thinking content appears in the response. When set to `summarized`, thinking is returned normally. When set to `omitted`, thinking content is redacted but a signature is returned for multi-turn continuity. Defaults to `summarized`.

from line 31599

3080431599 - `type: "thinking_delta"`

3080531600

3080631601 default: thinking_delta

31602

31603### Beta Thinking Dropped Input Transformation

31604

31605- `BetaThinkingDroppedInputTransformation object`

31606

31607 - `path: string`

31608

31609 Where the removed block was in your request, as `messages.{i}.content.{j}`:

31610 `i` indexes the `messages` array you sent and `j` that message's `content`

31611 array — the same form error messages use.

31612

31613 - `reason: "model_binding_mismatch" or "prefix_binding_mismatch" or "organization_binding_mismatch" or "end_user_binding_mismatch"`

31614

31615 Which binding check removed the block: `model_binding_mismatch` — it was

31616 created by a model whose reasoning the requested model may not read;

31617 `prefix_binding_mismatch` — the conversation before it differs from the

31618 conversation it was created in (the rest of that turn's consecutive thinking

31619 blocks are removed with it, each with this reason);

31620 `organization_binding_mismatch` — it was created under a different

31621 organization (an Anthropic organization, AWS account or Google Cloud project)

31622 and this organization is not one of its additional organizations;

31623 `end_user_binding_mismatch` — it was created for a different end user, or

31624 was removed by the consumer-organization binding. A block that would fail

31625 several checks reports one reason, in this order of precedence:

31626 `organization_binding_mismatch`, `end_user_binding_mismatch`,

31627 `model_binding_mismatch`, `prefix_binding_mismatch`.

31628

31629 - `"model_binding_mismatch"`

31630

31631 - `"prefix_binding_mismatch"`

31632

31633 - `"organization_binding_mismatch"`

31634

31635 - `"end_user_binding_mismatch"`

31636

31637 - `type: "thinking_dropped"`

31638

31639 Always `thinking_dropped` for this entry type.

31640

31641 default: thinking_dropped

31642

31643### Beta Thinking Prefix Mismatch Behavior

31644

31645- `BetaThinkingPrefixMismatchBehavior = "error" or "drop_block"`

31646

31647 What happens when a thinking block in `messages` fails the conversation

31648 check: it was created in a different conversation, or the messages before

31649 it have changed since. `"error"` (the default) fails the request with a

31650 400 error. `"drop_block"` removes the failing blocks and the request

31651 proceeds; the model no longer sees the dropped reasoning.

31652

31653 - `"error"`

31654

31655 - `"drop_block"`

3080731656

3080831657### Beta Thinking Turns

3080931658

from line 33578

3272933578 from its schema.

3273033579

3273133580 - `type: "browser_toolset_20260801"`

32732

32733 - `allowed_callers: optional array of "direct" or "code_execution_20250825" or "code_execution_20260120" or "code_execution_20260521"`

32734

32735 - `"direct"`

32736

32737 - `"code_execution_20250825"`

32738

32739 - `"code_execution_20260120"`

32740

32741 - `"code_execution_20260521"`

3274233581

3274333582 - `cache_control: optional BetaCacheControlEphemeral or null`

3274433583

from line 34205

3336634205

3336734206 - `type: "computer_toolset_20260801"`

3336834207

33369 - `allowed_callers: optional array of "direct" or "code_execution_20250825" or "code_execution_20260120" or "code_execution_20260521"`

33370

33371 - `"direct"`

33372

33373 - `"code_execution_20250825"`

33374

33375 - `"code_execution_20260120"`

33376

33377 - `"code_execution_20260521"`

33378

3337934208 - `cache_control: optional BetaCacheControlEphemeral or null`

3338034209

3338134210 Create a cache control breakpoint at this content block.

from line 34966

3413734966

3413834967 See [models](https://docs.anthropic.com/en/docs/models-overview) for additional details and options.

3413934968

34140 - `"claude-sonnet-5" or "claude-fable-5" or "claude-mythos-5" or 12 more`

34969 - `"claude-fable-5-1" or "claude-mythos-5-1" or "claude-sonnet-5" or 14 more`

3414134970

3414234971 The model that will complete your prompt.

3414334972

3414434973 See [models](https://docs.anthropic.com/en/docs/models-overview) for additional details and options.

34974

34975 - `"claude-fable-5-1"`

34976

34977 Frontier intelligence for ambitious tasks across coding, scientific discovery, and enterprise workflows

34978

34979 - `"claude-mythos-5-1"`

34980

34981 Our most capable model for cybersecurity and biology research, available through trusted access programs

3414534982

3414634983 - `"claude-sonnet-5"`

3414734984

from line 35512

3467535512

3467635513 See [models](https://docs.anthropic.com/en/docs/models-overview) for additional details and options.

3467735514

34678 - `"claude-sonnet-5" or "claude-fable-5" or "claude-mythos-5" or 12 more`

35515 - `"claude-fable-5-1" or "claude-mythos-5-1" or "claude-sonnet-5" or 14 more`

3467935516

3468035517 The model that will complete your prompt.

3468135518

3468235519 See [models](https://docs.anthropic.com/en/docs/models-overview) for additional details and options.

35520

35521 - `"claude-fable-5-1"`

35522

35523 Frontier intelligence for ambitious tasks across coding, scientific discovery, and enterprise workflows

35524

35525 - `"claude-mythos-5-1"`

35526

35527 Our most capable model for cybersecurity and biology research, available through trusted access programs

3468335528

3468435529 - `"claude-sonnet-5"`

3468535530

from line 37663

3681837663

3681937664 - `string`

3682037665

36821 - `"message-batches-2024-09-24" or "prompt-caching-2024-07-31" or "computer-use-2024-10-22" or 38 more`

37666 - `"message-batches-2024-09-24" or "prompt-caching-2024-07-31" or "computer-use-2024-10-22" or 41 more`

3682237667

3682337668 - `"message-batches-2024-09-24"`

3682437669

from line 37746

3690137746 - `"thinking-display-updates-2026-08-18"`

3690237747

3690337748 - `"ce-user-management-2026-07-13"`

37749

37750 - `"mid-conversation-output-config-2026-07-01"`

37751

37752 - `"thinking-binding-controls-2026-08-01"`

37753

37754 - `"mid-conversation-system-clear-at-2026-08-21"`

3690437755

3690537756- `"anthropic-user-profile-id": optional string`

3690637757

from line 39065

3821439065

3821539066 See [models](https://docs.anthropic.com/en/docs/models-overview) for additional details and options.

3821639067

38217 - `"claude-sonnet-5" or "claude-fable-5" or "claude-mythos-5" or 12 more`

39068 - `"claude-fable-5-1" or "claude-mythos-5-1" or "claude-sonnet-5" or 14 more`

3821839069

3821939070 The model that will complete your prompt.

3822039071

3822139072 See [models](https://docs.anthropic.com/en/docs/models-overview) for additional details and options.

3822239073

39074 - `"claude-fable-5-1"`

39075

39076 Frontier intelligence for ambitious tasks across coding, scientific discovery, and enterprise workflows

39077

39078 - `"claude-mythos-5-1"`

39079

39080 Our most capable model for cybersecurity and biology research, available through trusted access programs

39081

3822339082 - `"claude-sonnet-5"`

3822439083

3822539084 High-performance model for coding and agents

from line 39158

3829939158 - `"assistant"`

3830039159

3830139160 - `"system"`

39161

39162 - `clear_at: optional "next_user_message" or "never" or null`

39163

39164 How long this system message's text stays in front of the model. `"never"` (the default) renders it on every request that includes it. `"next_user_message"` renders it only for the user turn it follows: once a later `role: "user"` message exists in `messages` the message stays in the array (send it unchanged) but is no longer shown to the model. Only permitted on `role: "system"` messages.

39165

39166 - `"next_user_message"`

39167

39168 - `"never"`

39169

39170 - `output_config: optional BetaSystemMessageOutputConfig or null`

39171

39172 Per-message output configuration on a role:"system" input message.

39173

39174 Fields here apply per-turn; `format` remains top-level only. An

39175 empty `{}` is accepted on a message that carries content; a message

39176 with neither content nor output_config fields is rejected.

39177

39178 - `effort: optional "low" or "medium" or "high" or 2 more or null`

39179

39180 All possible effort levels.

39181

39182 - `"low"`

39183

39184 - `"medium"`

39185

39186 - `"high"`

39187

39188 - `"xhigh"`

39189

39190 - `"max"`

3830239191

3830339192 - `model: Model`

3830439193

from line 39491

3860239491

3860339492 - `type: "enabled"`

3860439493

39494 - `block_binding: optional BetaThinkingBlockBinding or null`

39495

39496 Controls for block binding: what happens when a thinking block this

39497 request sends back fails the conversation check. Every field is optional;

39498 an empty object means every default.

39499

39500 - `prefix_mismatch_behavior: optional BetaThinkingPrefixMismatchBehavior or null`

39501

39502 What happens when a thinking block in `messages` fails the conversation

39503 check: it was created in a different conversation, or the messages before

39504 it have changed since. `"error"` (the default) fails the request with a

39505 400 error. `"drop_block"` removes the failing blocks and the request

39506 proceeds; the model no longer sees the dropped reasoning.

39507

39508 - `"error"`

39509

39510 - `"drop_block"`

39511

3860539512 - `display: optional "summarized" or "omitted" or "updates" or null`

3860639513

3860739514 Controls how thinking content appears in the response. When set to `summarized`, thinking is returned normally. When set to `omitted`, thinking content is redacted but a signature is returned for multi-turn continuity. Defaults to `summarized`.

from line 39526

3861939526 - `BetaThinkingConfigAdaptive object`

3862039527

3862139528 - `type: "adaptive"`

39529

39530 - `block_binding: optional BetaThinkingBlockBinding or null`

39531

39532 Controls for block binding: what happens when a thinking block this

39533 request sends back fails the conversation check. Every field is optional;

39534 an empty object means every default.

3862239535

3862339536 - `display: optional "summarized" or "omitted" or "updates" or null`

3862439537

from line 40034

3912140034

3912240035 - `type: "browser_toolset_20260801"`

3912340036

39124 - `allowed_callers: optional array of "direct" or "code_execution_20250825" or "code_execution_20260120" or "code_execution_20260521"`

39125

39126 - `"direct"`

39127

39128 - `"code_execution_20250825"`

39129

39130 - `"code_execution_20260120"`

39131

39132 - `"code_execution_20260521"`

39133

3913440037 - `cache_control: optional BetaCacheControlEphemeral or null`

3913540038

3913640039 Create a cache control breakpoint at this content block.

from line 40659

3975640659 via `configs.zoom.enabled`.

3975740660

3975840661 - `type: "computer_toolset_20260801"`

39759

39760 - `allowed_callers: optional array of "direct" or "code_execution_20250825" or "code_execution_20260120" or "code_execution_20260521"`

39761

39762 - `"direct"`

39763

39764 - `"code_execution_20250825"`

39765

39766 - `"code_execution_20260120"`

39767

39768 - `"code_execution_20260521"`

3976940662

3977040663 - `cache_control: optional BetaCacheControlEphemeral or null`

3977140664

from line 41798

4090541798

4090641799 - `string`

4090741800

40908 - `"message-batches-2024-09-24" or "prompt-caching-2024-07-31" or "computer-use-2024-10-22" or 38 more`

41801 - `"message-batches-2024-09-24" or "prompt-caching-2024-07-31" or "computer-use-2024-10-22" or 41 more`

4090941802

4091041803 - `"message-batches-2024-09-24"`

4091141804

from line 41881

4098841881 - `"thinking-display-updates-2026-08-18"`

4098941882

4099041883 - `"ce-user-management-2026-07-13"`

41884

41885 - `"mid-conversation-output-config-2026-07-01"`

41886

41887 - `"thinking-binding-controls-2026-08-01"`

41888

41889 - `"mid-conversation-system-clear-at-2026-08-21"`

4099141890

4099241891#### Returns

4099341892

from line 42064

4116542064

4116642065 - `string`

4116742066

41168 - `"message-batches-2024-09-24" or "prompt-caching-2024-07-31" or "computer-use-2024-10-22" or 38 more`

42067 - `"message-batches-2024-09-24" or "prompt-caching-2024-07-31" or "computer-use-2024-10-22" or 41 more`

4116942068

4117042069 - `"message-batches-2024-09-24"`

4117142070

from line 42147

4124842147 - `"thinking-display-updates-2026-08-18"`

4124942148

4125042149 - `"ce-user-management-2026-07-13"`

42150

42151 - `"mid-conversation-output-config-2026-07-01"`

42152

42153 - `"thinking-binding-controls-2026-08-01"`

42154

42155 - `"mid-conversation-system-clear-at-2026-08-21"`

4125142156

4125242157#### Returns

4125342158

from line 42339

4143442339

4143542340 - `string`

4143642341

41437 - `"message-batches-2024-09-24" or "prompt-caching-2024-07-31" or "computer-use-2024-10-22" or 38 more`

42342 - `"message-batches-2024-09-24" or "prompt-caching-2024-07-31" or "computer-use-2024-10-22" or 41 more`

4143842343

4143942344 - `"message-batches-2024-09-24"`

4144042345

from line 42422

4151742422 - `"thinking-display-updates-2026-08-18"`

4151842423

4151942424 - `"ce-user-management-2026-07-13"`

42425

42426 - `"mid-conversation-output-config-2026-07-01"`

42427

42428 - `"thinking-binding-controls-2026-08-01"`

42429

42430 - `"mid-conversation-system-clear-at-2026-08-21"`

4152042431

4152142432#### Returns

4152242433

from line 42596

4168542596

4168642597 - `string`

4168742598

41688 - `"message-batches-2024-09-24" or "prompt-caching-2024-07-31" or "computer-use-2024-10-22" or 38 more`

42599 - `"message-batches-2024-09-24" or "prompt-caching-2024-07-31" or "computer-use-2024-10-22" or 41 more`

4168942600

4169042601 - `"message-batches-2024-09-24"`

4169142602

from line 42679

4176842679 - `"thinking-display-updates-2026-08-18"`

4176942680

4177042681 - `"ce-user-management-2026-07-13"`

42682

42683 - `"mid-conversation-output-config-2026-07-01"`

42684

42685 - `"thinking-binding-controls-2026-08-01"`

42686

42687 - `"mid-conversation-system-clear-at-2026-08-21"`

4177142688

4177242689#### Returns

4177342690

from line 42745

4182842745

4182942746 - `string`

4183042747

41831 - `"message-batches-2024-09-24" or "prompt-caching-2024-07-31" or "computer-use-2024-10-22" or 38 more`

42748 - `"message-batches-2024-09-24" or "prompt-caching-2024-07-31" or "computer-use-2024-10-22" or 41 more`

4183242749

4183342750 - `"message-batches-2024-09-24"`

4183442751

from line 42828

4191142828 - `"thinking-display-updates-2026-08-18"`

4191242829

4191342830 - `"ce-user-management-2026-07-13"`

42831

42832 - `"mid-conversation-output-config-2026-07-01"`

42833

42834 - `"thinking-binding-controls-2026-08-01"`

42835

42836 - `"mid-conversation-system-clear-at-2026-08-21"`

4191442837

4191542838#### Returns

4191642839

from line 43778

4285543778

4285643779 See [models](https://docs.anthropic.com/en/docs/models-overview) for additional details and options.

4285743780

42858 - `"claude-sonnet-5" or "claude-fable-5" or "claude-mythos-5" or 12 more`

43781 - `"claude-fable-5-1" or "claude-mythos-5-1" or "claude-sonnet-5" or 14 more`

4285943782

4286043783 The model that will complete your prompt.

4286143784

4286243785 See [models](https://docs.anthropic.com/en/docs/models-overview) for additional details and options.

43786

43787 - `"claude-fable-5-1"`

43788

43789 Frontier intelligence for ambitious tasks across coding, scientific discovery, and enterprise workflows

43790

43791 - `"claude-mythos-5-1"`

43792

43793 Our most capable model for cybersecurity and biology research, available through trusted access programs

4286343794

4286443795 - `"claude-sonnet-5"`

4286543796

from line 44523

4359244523

4359344524 - `"fast"`

4359444525

44526 - `input_transformations: optional array of BetaThinkingDroppedInputTransformation or null`

44527

44528 Changes the API made to the request's input before showing it to the model:

44529 one entry per change, in request order. Today the only entry type is

44530 `thinking_dropped` — a `thinking`, `redacted_thinking` or `connector_text`

44531 block from the request's `messages` that was removed from the prompt instead

44532 of being shown to the model because it failed a binding check. More entry

44533 types may be added over time; ignore types you do not recognize.

44534

44535 Requires `anthropic-beta: thinking-binding-controls-2026-08-01`. Present on

44536 every such response from a model that supports extended thinking, as `[]`

44537 when nothing was changed; without the beta, blocks are removed all the same

44538 but nothing is reported. Removed blocks contribute nothing to

44539 `usage.input_tokens`. When streaming, the array is final in `message_start`;

44540 the final `message_delta` event carries it only when a server-side model

44541 fallback happened mid-stream, in which case it holds the serving model's

44542 entries and replaces the one in `message_start`.

44543

44544 - `path: string`

44545

44546 Where the removed block was in your request, as `messages.{i}.content.{j}`:

44547 `i` indexes the `messages` array you sent and `j` that message's `content`

44548 array — the same form error messages use.

44549

44550 - `reason: "model_binding_mismatch" or "prefix_binding_mismatch" or "organization_binding_mismatch" or "end_user_binding_mismatch"`

44551

44552 Which binding check removed the block: `model_binding_mismatch` — it was

44553 created by a model whose reasoning the requested model may not read;

44554 `prefix_binding_mismatch` — the conversation before it differs from the

44555 conversation it was created in (the rest of that turn's consecutive thinking

44556 blocks are removed with it, each with this reason);

44557 `organization_binding_mismatch` — it was created under a different

44558 organization (an Anthropic organization, AWS account or Google Cloud project)

44559 and this organization is not one of its additional organizations;

44560 `end_user_binding_mismatch` — it was created for a different end user, or

44561 was removed by the consumer-organization binding. A block that would fail

44562 several checks reports one reason, in this order of precedence:

44563 `organization_binding_mismatch`, `end_user_binding_mismatch`,

44564 `model_binding_mismatch`, `prefix_binding_mismatch`.

44565

44566 - `"model_binding_mismatch"`

44567

44568 - `"prefix_binding_mismatch"`

44569

44570 - `"organization_binding_mismatch"`

44571

44572 - `"end_user_binding_mismatch"`

44573

44574 - `type: "thinking_dropped"`

44575

44576 Always `thinking_dropped` for this entry type.

44577

44578 default: thinking_dropped

44579

4359544580 - `type: "succeeded"`

4359644581

4359744582 default: succeeded

4359844583

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