Codex cannot use Amazon Bedrock via the Databricks AI Gateway (Converse-only provider)

[bjanssen11]

Summary

Routing Codex to an Amazon Bedrock Model Provider Service through the Databricks AI Gateway does not work. The gateway serves the amazon_bedrock provider type only over Bedrock's native Converse API. Codex speaks the OpenAI Responses API, which the gateway rejects for Bedrock, so ucode codex --provider <bedrock-mps> cannot reach the provider and Codex falls back to its own picker (system-hosted OpenAI models).

Evidence

Tested against an amazon_bedrock MPS with the Databricks-Model-Provider-Service header set:

• POST /ai-gateway/codex/v1/responses → INVALID_PARAMETER_VALUE: Native_api_type=openai/v1/responses is not supported for provider Amazon Bedrock.
• POST /ai-gateway/mlflow/v1/chat/completions → Native_api_type=mlflow/v1/chat/completions is not supported for provider Amazon Bedrock.
• POST /ai-gateway/model/{modelId}/converse and /converse-stream → work, returning real Bedrock output.

Declaring native_api_types on the MPS targets (for example openai/v1/responses) does not change this. The runtime still refuses non-Converse dialects for the amazon_bedrock provider, so that field is catalog metadata rather than a runtime switch.

Impact

• Codex cannot use Amazon Bedrock through the gateway today.
• Pi and OpenCode are unaffected because they speak Converse. Pi support is in the follow-up PR.

Next steps

• This is blocked on the gateway adding OpenAI Responses (or Chat) translation for the amazon_bedrock provider.
• Until that lands, ucode should not present Bedrock as a Codex provider, or should surface a clear "not supported" message instead of silently falling back.