When the API rejects cache_control on a mid-conversation system turn with an "additional properties", "unknown field", or similar error, Claude now detects this and demotes the cache breakpoint to the trailing message rather than the system block. Logged as: [mid-conv-system] proxy rejected cache_control on the api_system tail — demoting the breakpoint to the trailing message for this conversation.
Evidence
"retry:api-system-cache-demote" (search for "q9i")
Strings lifted out of the shipped bundle, so the claim above can be checked against them.