Codex on AWS bedrock bug causing 10x charges

## Summary Native Codex CLI requests to Amazon Bedrock Mantle cannot opt into GPT-5.6 Sol explicit prompt caching. On an agentic coding workload, this has produced a large volume of cache-write tokens and materially higher cost. This is related to #35300, but adds independent production usage evidence from the native `amazon-bedrock` provider. ## Environment - Codex CLI: `0.147.0` - Provider: native `amazon-bedrock` - Endpoint: Bedrock Mantle Responses API, `us-east-1` - Model: `openai.gpt-5.6-sol` ## Observed production usage For the completed days 2026-08-05 through 2026-08-08, Cost Explorer usage quantities and the Bedrock rate card produced the following cache-aware estimate for Sol: | Requests | Cache-write tokens | Estimated cache-write cost | Estimated total cost | |---:|---:|---:|---:| | 3,656 | 171.94M | $1,182.09 | $1,386.46 | Cache writes were about 85% of the model's estimated spend. A local Codex session also reported 76 Sol requests with 6.709M `cache_write_input_tokens`, zero `cached_input_tokens`, and an average of about 88K cache-write tokens per request. There were no client errors in the corresponding CloudWatch metrics. These are usage-derived estimates, not finalized AWS invoice amounts. ## Investigation Codex already emits a session-scoped `prompt_cache_key`, but the request types for both HTTP and WebSocket Responses requests do not include either: - `prompt_cache_options` - `prompt_cache_breakpoint` The built-in Amazon Bedrock provider config exposes t…