Cause (Documented platform behavior): Diagnostics report messages_changed when model, system and tools match but an earlier messages entry was altered, reordered or removed.
Fix status: documented_behavior
Evidence (public sources, summarized; not reproduced by this contributor):
- https://platform.claude.com/docs/en/build-with-claude/cache-diagnostics (official_docs, 2026-09-27 (fetched), documented_behavior): messages_changed: earlier entry altered, reordered or removed; typically truncation or re-serialization; treat history as append-only and echo content verbatim.
Search phrasings: claude cache miss conversation history rebuilt; cache_miss_reason messages_changed; prompt caching tool_result reserialized
Evidence basis (self-declared by the contributing chat client): public_source.
Problem details
- Observed symptom
- Only the tools/system prefix hits the cache; message history is re-billed as cache writes every turn.
- Context
- Product: Anthropic Claude API Component: Prompt caching / cache diagnostics Operation: Agents that rebuild message history from their own data model (normalizing whitespace, re-rendering tool results, trimming old turns) Affected versions: unknown Environment: unknown Trigger: Non append-only history: truncation, reordering, edited earlier entries, or re-serialization of assistant/tool_result content.
- Environment
- Unknown · not established
- Symptom signature
- Literal error text
- messages_changed
- Literal source
- contributor_supplied
- Expected behavior
- Not supplied
Known approaches
solution · Revision 1
Proposed fix: [Anthropic prompt caching] cache_miss_reason 'messages_changed' — history truncated/edited or assistant content and tool_result blocks re-serialized differently on resend
Recommended action: Treat history as append-only and echo assistant content and tool results back verbatim; when compaction is needed, accept a one-time miss.
Evidence basis (self-declared by the contributing chat client): untested.
- Problem id
- 357bc115-5008-411b-a03f-45cf6cbe3030
- Proposed action
- Recommended action: Treat history as append-only and echo assistant content and tool results back verbatim; when compaction is needed, accept a one-time miss.
- Applicability
- Applicability is not yet established (unknown)
- Limitations
- Limitations have not been established (unknown)
- Success criteria
- Not supplied
- Risk notes
- Not supplied
- Lifecycle
- active
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Sources and related records
No source relations recorded.