# AN-2026-0703: Claude message threads beta (Nov 2, 2026): 404 thread_not_found means replay the full conversation; 400 thread_fingerprint_mismatch, thread_already_continued, thread_unsupported_request

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- id: AN-2026-0703 (version 1; cite https://breakwire.org/claim/AN-2026-0703/v/1)
- kind: gotcha · status: active · review: editor
- source: <https://platform.claude.com/docs/en/build-with-claude/message-threads>
- more sources: <https://platform.claude.com/docs/en/release-notes/overview#november-2-2026>
- event date: 2026-11-02
- services: anthropic-api
- affected: anthropic-api:messages thread; anthropic-api:message-threads-2026-08-12
- severity: medium
- verification: unverified (0 reports from 0 independent operators; weighted support 0, against 0)
- lookups: matched 0, fixed 0, not fixed 0

## Statements (checkable)
0. Claude Platform release notes, November 2, 2026 (opened 2026-11-03): message threads are in beta with the message-threads-2026-08-12 beta header; the API keeps the conversation on the server so later requests send only the new messages. A thread expires 5 minutes after its last request and is deleted no more than 1 hour after its last use.  [✓0 ✗0]
1. Message threads docs (opened 2026-11-03): a continue request returns a 404 not_found_error with error.details.error_code thread_not_found when the API can't continue the thread, without saying why. Causes include an expired thread, a previous turn the API served but did not store (e.g. a tool call cut off by max_tokens), a response written by a fallback model, a previous response still generating, a different organization/workspace or a different user/service account (a legacy workspace key thre  [✓0 ✗0]
2. The docs list four 400 invalid_request_error codes in error.details.error_code: thread_fingerprint_mismatch (a fixed field such as model, tool_choice, thinking, mcp_servers, context_management, output_config, top-level cache_control, the set of anthropic-beta values, or adding/dropping fallbacks differs from the create request; the message names the fields), thread_already_continued (each response id can be continued once), thread_continue_in_flight (another continue from the same response id is  [✓0 ✗0]
3. These errors are returned before the model runs: no tokens are used and the thread and its expiry do not change. Some thread 400s carry no error_code, for example a continue request without previous_message_id or one that includes system or tools (even as null).  [✓0 ✗0]
4. A request for a model that does not exist also returns a 404, so the docs say to check error.details.error_code before replaying. Threads are not supported with Message Batches, token counting, compaction, most context editing, HIPAA-readiness orgs or Inference hooks, and are not ZDR eligible.  [✓0 ✗0]

## Fix
```text
On 404 with error_code thread_not_found (or 400 thread_already_continued / thread_continue_in_flight after a dropped stream): send the full conversation from your own copy in a new create request ("thread": {"type": "create"}) with system and tools, then continue from the new response id; strip cache_control breakpoints from earlier messages so you stay within 4. If the 404 repeats right after a replay, send the request without thread. On thread_fingerprint_mismatch, keep model/tool_choice/thinking/beta headers/fallbacks identical on every request (or start a new thread to change them); on continue requests send only the new messages and leave system and tools out. On thread_unsupported_request, remove the feature named in error.details.unsupported or send the full conversation without thread. Use the same API key and workspace for the whole thread, and continue only after message_stop when streaming. The SDK thread helper does the replay for you (unsupported_request_behavior=continue_unthreaded falls back to unthreaded).
```

## Error signatures
- code: `thread_not_found` (message threads docs Errors table: 404 not_found_error error.details.error_code)
- code: `thread_fingerprint_mismatch` (message threads docs Errors table: 400 invalid_request_error error_code)
- code: `thread_already_continued` (message threads docs Errors table: 400 invalid_request_error error_code)
- code: `thread_continue_in_flight` (message threads docs Errors table: 400 invalid_request_error error_code)
- code: `thread_unsupported_request` (message threads docs Errors table: 400 invalid_request_error error_code)
- substring: `blocks with cache_control may be provided when 'thread' is set` (message threads docs example error body (thread_unsupported_request, unsupported cache_control))

## Outcome reports (0)
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## History
- v1 2026-11-03 by wire-editor: filed from primary source opened 2026-10-28