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954954 messages.5.content.0: Invalid `signature` in `thinking` block. The block is bound to a different conversation. Remove the block, or set `thinking.block_binding.prefix_mismatch_behavior` to "drop_block". That setting requires the `thinking-binding-controls-2026-08-01` value in the `anthropic-beta` header.
955955 ```
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957 The API enforces the check for new accounts created on or after August 31, 2026. For accounts created earlier, the API records the mismatch but doesn't act on it unless the request sets `thinking.block_binding.prefix_mismatch_behavior`, which opts into enforcement. On those accounts, if you send the `thinking-binding-controls-2026-08-01` beta header and leave that field unset, the response lists each block that failed the check in `input_transformations` as a `thinking_mismatch_allowed` entry. Anthropic plans to enforce the check for every account on future models, so make your application compatible now: the same patterns keep the prompt cache warm, and you can test against the check from any account by sending `prefix_mismatch_behavior`. If you ship a tool or framework that people run with their own API key, test that way before launch: your key is probably on an older account, and your users on new ones hit the check before you do. To see whether your own account is enforced by default, send a request that edits history without the beta header: a 400 that names the header means it is.
957 The API enforces the check for new accounts created on or after August 31, 2026. For accounts created earlier, the API records the mismatch but doesn't act on it unless the request sets `thinking.block_binding.prefix_mismatch_behavior`, which opts into enforcement. On those accounts, if you send the `thinking-binding-controls-2026-08-01` beta header and leave that field unset, the response lists each block that failed the check in `input_transformations` as a `thinking_mismatch_allowed` entry. Make your application compatible with the check regardless of your account's age: the same patterns keep the prompt cache warm, and you can test against the check from any account by sending `prefix_mismatch_behavior`. If you ship a tool or framework that people run with their own API key, test that way before launch: your key is probably on an older account, and your users on new ones hit the check before you do. To see whether your own account is enforced by default, send a request that edits history without the beta header: a 400 that names the header means it is.
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959959 The error is permanent for that request body: an automatic retry loop won't clear it. To continue without the invalidated reasoning instead of failing, strip the `thinking` blocks from the history and retry once, or send the `thinking-binding-controls-2026-08-01` [beta header](https://platform.claude.com/docs/en/api/beta-headers) and set `prefix_mismatch_behavior` to `"drop_block"` (the default is `"error"`). With `"drop_block"`, the API drops the mismatched block and every thinking block after it in the conversation, and reports each with `reason: "prefix_binding_mismatch"` in the response's `input_transformations` array:
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12231223 * Editing, reordering, or removing earlier turns. This includes deleting old tool results, snipping turns out of the middle of the transcript, and client-side compaction that keeps recent turns and their thinking blocks verbatim behind a summary (including background compaction that swaps its summary in a few turns later). Instead, use server-side [compaction](https://platform.claude.com/docs/en/build-with-claude/compaction) or [context editing](https://platform.claude.com/docs/en/build-with-claude/context-editing) ([tool result clearing](https://platform.claude.com/docs/en/build-with-claude/context-editing#tool-result-clearing) for old tool results), or one of the client-side compaction shapes in [Trim context on the server](https://platform.claude.com/docs/en/models/fable-5-1/migration-guide#fable-5-1-trim-context).
12241224 * Injecting content you don't persist, for example a per-turn reminder appended after the `tool_result` blocks and removed on the next request. Instead, send the reminder as a [turn-scoped system message](https://platform.claude.com/docs/en/build-with-claude/mid-conversation-system-messages#turn-scoped-system-messages) and leave it in the history.
1225 * Rebuilding the top-level `system` prompt or the `tools` array between requests in the same conversation, for example to update the current date or to add or remove a tool. Instead, append a [mid-conversation system message](https://platform.claude.com/docs/en/build-with-claude/mid-conversation-system-messages) that carries the new instruction ("The current date is 2026-09-14.") or `tool_addition` and `tool_removal` blocks.
1225 * Rebuilding the top-level `system` prompt or the `tools` array between requests in the same conversation, for example to update the current date or to add or remove a tool. Instead, append a [mid-conversation system message](https://platform.claude.com/docs/en/build-with-claude/mid-conversation-system-messages) that carries the new instruction ("The current date is 2026-09-14.") or `tool_addition` and `tool_removal` blocks. A tool that wasn't declared in `tools` at the start can be [defined inside the `tool_addition` block](https://platform.claude.com/docs/en/build-with-claude/mid-conversation-system-messages#define-tools-in-a-message-beta) (beta header `inline-tools-2026-09-15`).
12261226 * An image or document URL that serves different bytes on a later request. The check covers the bytes, not the URL string, so a rotating signed URL for the same file is fine. For content you reference across turns, upload it once with the [Files API](https://platform.claude.com/docs/en/build-with-claude/files) and send the `file_id`, or send base64.
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12281228 Each replacement also keeps earlier turns byte-identical and preserves the [prompt cache](https://platform.claude.com/docs/en/build-with-claude/prompt-caching) hits that editing the history, `system` prompt, or `tools` array would lose.
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15361536 The value applies to the following user turn and every later turn until another `role: "system"` message changes it. Only the named levels are accepted (`low`, `medium`, `high`, `xhigh`, `max`), and the `mid-conversation-output-config-2026-07-01` beta header is required. See [Per-message effort](https://platform.claude.com/docs/en/build-with-claude/effort#change-effort-mid-conversation-beta).
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15382. **Change instructions and tools with mid-conversation system messages:** To change instructions or tools partway through a session, append a [`role: "system"` message](https://platform.claude.com/docs/en/build-with-claude/mid-conversation-system-messages), with `tool_addition` and `tool_removal` blocks for tool changes (beta header `mid-conversation-tool-changes-2026-07-01`, with the full tool set declared in `tools` at session start). This preserves prompt cache hits on earlier turns and keeps the conversation history append-only. The same message replaces forced `tool_choice` when a specific tool must run on the current turn (see [Breaking changes](https://platform.claude.com/docs/en/models/fable-5-1/migration-guide#fable-5-1-breaking-changes)). For a reminder that applies to one turn only, send it as a separate text-only `role: "system"` message with `clear_at: "next_user_message"` ([turn-scoped system messages](https://platform.claude.com/docs/en/build-with-claude/mid-conversation-system-messages#turn-scoped-system-messages), beta header `mid-conversation-system-clear-at-2026-08-21`) and leave it in the history: it stops rendering after the next user message and costs no tokens once cleared. A message that carries `tool_addition` or `tool_removal` blocks can't be turn-scoped.
15382. **Change instructions and tools with mid-conversation system messages:** To change instructions or tools partway through a session, append a [`role: "system"` message](https://platform.claude.com/docs/en/build-with-claude/mid-conversation-system-messages), with `tool_addition` and `tool_removal` blocks for tool changes (beta header `mid-conversation-tool-changes-2026-07-01`, with the full tool set declared in `tools` at session start). With the `inline-tools-2026-09-15` beta header instead, a `tool_addition` block can [carry the tool's full definition](https://platform.claude.com/docs/en/build-with-claude/mid-conversation-system-messages#define-tools-in-a-message-beta), so a tool that is unknown at session start doesn't need to be in `tools`. This preserves prompt cache hits on earlier turns and keeps the conversation history append-only. The same message replaces forced `tool_choice` when a specific tool must run on the current turn (see [Breaking changes](https://platform.claude.com/docs/en/models/fable-5-1/migration-guide#fable-5-1-breaking-changes)). For a reminder that applies to one turn only, send it as a separate text-only `role: "system"` message with `clear_at: "next_user_message"` ([turn-scoped system messages](https://platform.claude.com/docs/en/build-with-claude/mid-conversation-system-messages#turn-scoped-system-messages), beta header `mid-conversation-system-clear-at-2026-08-21`) and leave it in the history: it stops rendering after the next user message and costs no tokens once cleared. A message that carries `tool_addition` or `tool_removal` blocks can't be turn-scoped.
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154015403. **Use `fallbacks: "default"` for refusals:** Keep handling `stop_reason: "refusal"` and reading `stop_details.category` before response content. To re-run refused requests on another model automatically, set `fallbacks: "default"` (beta, `server-side-fallback-2026-07-01` header). `"default"` retries a declined request on the model Anthropic recommends for that category. The permitted fallback targets for Claude Fable 5.1 are Claude Opus 4.8 (`claude-opus-4-8`) and Claude Opus 5 (`claude-opus-5`). An explicit `fallbacks` list may name either. The fallback model doesn't receive Claude Fable 5.1's thinking blocks. If you build the retry yourself, [fallback credit](https://platform.claude.com/docs/en/build-with-claude/fallback-credit) applies on the same terms as Claude Fable 5. See [Refusals and fallback](https://platform.claude.com/docs/en/build-with-claude/refusals-and-fallback).
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