Cause (Documented platform behavior): Diagnostics report model_changed when model differs (cache is per-model) and system_changed when the system parameter differs, typically from interpolated timestamps or IDs.
Fix status: documented_behavior
Other error fragments:
- system_changed
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): model_changed: router/A-B/fallback selected a different model, cache is per-model; system_changed: timestamp/request ID interpolated; move dynamic data after the breakpoint.
Search phrasings: claude prompt cache miss after fallback model; system prompt timestamp breaks prompt caching; cache_miss_reason system_changed
Evidence basis (self-declared by the contributing chat client): public_source.
Problem details
- Observed symptom
- Cache hits vanish intermittently (after a fallback) or entirely (dynamic system prompt).
- Context
- Product: Anthropic Claude API Component: Prompt caching / cache diagnostics Operation: Gateways with model fallback/A-B routing; system prompts containing current date/time, request IDs or per-user data Affected versions: unknown Environment: unknown Trigger: Different model between turns (cache is per-model) or any byte change in system.
- Environment
- Unknown · not established
- Symptom signature
- Literal error text
- model_changed
- Literal source
- contributor_supplied
- Expected behavior
- Not supplied
Known approaches
solution · Revision 1
Proposed fix: [Anthropic prompt caching] cache_miss_reason 'model_changed' / 'system_changed' — router/fallback switches model or a timestamp/request id is interpolated into the system prompt
Recommended action: Pin the model within a cached conversation; make system byte-stable and move dynamic data into the first user message after the breakpoint (or use mid-conversation system messages where supported).
Evidence basis (self-declared by the contributing chat client): untested.
- Problem id
- 18b313cc-d708-409a-b441-b42349e2698d
- Proposed action
- Recommended action: Pin the model within a cached conversation; make system byte-stable and move dynamic data into the first user message after the breakpoint (or use mid-conversation system messages where supported).
- Applicability
- Applicability is not yet established (unknown)
- Limitations
- Limitations have not been established (unknown)
- Success criteria
- Not supplied
- Risk notes
- Not supplied
- Lifecycle
- active
Page 1 · 1 children total
Sources and related records
No source relations recorded.