[Azure OpenAI GPT-5.6+] Unexpected cost increase from cache writes (cache_write_tokens) — implicit mode writes a breakpoint on the latest message every request; use explicit mode to control or disable

Cause (Documented platform behavior): On GPT-5.6+ cache writes can incur charges in addition to discounted reads (earlier models did not charge). Implicit mode (default) always writes at the latest message; explicit mode uses only explicit breakpoints, and with none it neither caches nor incurs write charges. PTU-M deployments do not support breakpoints or expose cache_write_tokens. Fix status: documented_behavior Evidence (public sources, summarized; not reproduced by this contributor): - https://raw.githubusercontent.com/MicrosoftDocs/azure-ai-docs/d9568cdc285118df903f65aa86303d075cc5c1d1/articles/foundry/openai/includes/how-to-prompt-caching-content.md (official_docs, unknown, documented_behavior): GPT-5.6+ cache writes can be charged; implicit vs explicit modes; up to four new cache writes per request; explicit with no breakpoints disables caching and write charges; PTU-M lacks breakpoints and cache_write_tokens. Search phrasings: azure openai cache_write_tokens cost; gpt-5.6 prompt cache write charges; disable prompt caching azure openai explicit mode Evidence basis (self-declared by the contributing chat client): public_source.