Messages changedapi/beta/messages
Nearest release: v2.1.296, published 4 hours before this site recorded the change. Shown because the two are within 24 hours of each other. Nothing here says the release caused the edit.
Recorded here
Lines+35added
Lines−182removed
From line
161
where the diff opens
First seen
14 Aug 2026
this site's first read of the page
Recorded edits23to this page, all time
The whole hunk
from line 161, old and new numbered
/
This page is larger than the 256 KiB this site keeps, so one side of the diff below stops where the stored text does.
from line 161
161161
162162 Each input message must be an object with a `role` and `content`. You can specify a single `user`-role message, or you can include multiple `user` and `assistant` messages.
163163
164 If the final message uses the `assistant` role, the response content will continue immediately from the content in that message. This can be used to constrain part of the model's response.
164 If the final message uses the `assistant` role, the response content will continue immediately from the content in that message. This can be used to constrain part of the model's response. This is called prefill. On models that don't support prefill, creating a message that ends with a partial `assistant` response returns a 400 error. See [Prefill not supported](https://platform.claude.com/docs/en/api/errors#prefill-not-supported).
165165
166166 Example with a single `user` message:
167167
from line 179
179179 ]
180180 ```
181181
182 Example with a partially-filled response from Claude:
182 Example with a partially-filled response from Claude, for models that support prefill:
183183
184184 ```json
185185 [
from line 3166
31663166
31673167 - `"claude-haiku-4-5"`
31683168
3169 Fastest model with near-frontier intelligence
3170
31713169 - `"claude-haiku-4-5-20251001"`
31723170
3173 Fastest model with near-frontier intelligence
3174
31753171 - `"claude-opus-4-5"`
31763172
31773173 Powerful intelligence for long-running agents and coding
from line 3762
37663762
37673763- `fallbacks: optional BetaFallbacksParam or null`
37683764
3769 Opt-in server-side retry on one or more substitute models when the requested model declines for policy reasons. Tried in order: if the first entry also declines, the second is tried, and so on. The string "default" requests the requested model's server-defined default fallback configuration.
3765 Opt-in server-side retry on one or more substitute models when the requested model declines for policy reasons. Tried in order: if the first entry also declines, the second is tried, and so on. Some models don't support fallbacks; on those models, a list of fallback models returns a 400 error. See [Server-side fallback](https://platform.claude.com/docs/en/build-with-claude/refusals-and-fallback#server-side-fallback). The string "default" requests the requested model's server-defined default fallback configuration. On a model that doesn't support fallbacks, the request runs on the requested model alone, so a declined request stays declined.
37703766
37713767 - `array of BetaFallbackParam`
37723768
from line 3860
38643860
38653861 - `display: optional "summarized" or "omitted" or "updates" or null`
38663862
3867 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`.
3863 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. The default depends on the model; see [Controlling thinking display](https://platform.claude.com/docs/en/build-with-claude/thinking#controlling-thinking-display).
38683864
38693865 - `"summarized"`
38703866
from line 3886
38903886
38913887 - `display: optional "summarized" or "omitted" or "updates" or null`
38923888
3893 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`.
3889 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. The default depends on the model; see [Controlling thinking display](https://platform.claude.com/docs/en/build-with-claude/thinking#controlling-thinking-display).
38943890
38953891 - `"summarized"`
38963892
from line 3992
39963992
39973993- `thinking: optional BetaThinkingConfigParam`
39983994
3999 Configuration for enabling Claude's extended thinking.
3995 Configuration for Claude's thinking.
40003996
4001 When enabled, responses include `thinking` content blocks showing Claude's thinking process before the final answer. Requires a minimum budget of 1,024 tokens and counts towards your `max_tokens` limit.
3997 With `{"type": "adaptive"}`, Claude decides when and how much to think. With `{"type": "enabled"}` (manual extended thinking), you set a `budget_tokens` of at least 1,024. Thinking tokens count toward your `max_tokens` limit.
40023998
4003 See [extended thinking](https://platform.claude.com/docs/en/build-with-claude/extended-thinking) for details.
3999 Which `type` values are accepted, and what happens when you omit `thinking`, depend on the model. See [thinking](https://platform.claude.com/docs/en/build-with-claude/thinking#configuring-thinking) for each model's behavior.
40044000
40054001 - `BetaThinkingConfigEnabled object`
40064002
from line 7032
70367032
70377033 - `"claude-haiku-4-5"`
70387034
7039 Fastest model with near-frontier intelligence
7040
70417035 - `"claude-haiku-4-5-20251001"`
70427036
7043 Fastest model with near-frontier intelligence
7044
70457037 - `"claude-opus-4-5"`
70467038
70477039 Powerful intelligence for long-running agents and coding
from line 7174
71827174
71837175 If true, tool will not be included in initial system prompt. Only loaded when returned via tool_reference from tool search.
71847176
7185 - `strict: optional boolean`
7186
7187 When true, guarantees schema validation on tool names and inputs
7188
7189 - `BetaMCPToolset object`
7190
7191 Configuration for a group of tools from an MCP server.
7192
7193 Allows configuring enabled status and defer_loading for all tools
7194 from an MCP server, with optional per-tool overrides.
7195
7196 - `type: "mcp_toolset"`
7197
7198 - `mcp_server_name: string`
7199
7200 Name of the MCP server to configure tools for
7201
7202 minLength: 1, maxLength: 255
7203
7204 - `cache_control: optional BetaCacheControlEphemeral or null`
7205
7206 Create a cache control breakpoint at this content block.
7207
7208
7177 - `strict
No line in this hunk matches that.