[joaorufino]

Description

Claude on AWS Bedrock (InvokeModel) and GCP Vertex AI rejects the top-level output_format field of the Messages API:

400 output_format: This field is deprecated. Use 'output_config.format' instead.

Structured outputs are generally available as output_config.format, but some Anthropic-native clients still send the beta field. For example, LiteLLM (1.105 through 1.108) sets output_format for a JSON-schema response_format on models it marks as supporting native structured outputs. The AWSAnthropic and GCPAnthropic translators forward output_format unchanged, so these requests fail on both clouds.

This moves output_format to output_config.format in the AWSAnthropic and GCPAnthropic messages and count_tokens translators:

• other output_config fields, such as effort, are kept;
• an output_config.format already in the request takes precedence;
• output_format: null is dropped;
• the schema is moved verbatim, so its property order is preserved;
• a malformed output_config (not an object) is left for the backend to reject.

The Anthropic passthrough translator is unchanged.

Each request shape, sent through the gateway (before this change) with claude-sonnet-4-6:

request body	AWSAnthropic messages	GCPAnthropic messages	GCPAnthropic count_tokens	with this change, sent as
output_format	400 deprecated	400 deprecated	400 extra inputs	output_config.format
output_format + output_config.format	400 deprecated	400 deprecated		output_config.format
output_format: null	400 extra inputs	400 extra inputs		field dropped
output_config.format (+ effort)	200, schema applied	200, schema applied	200	unchanged

Related Issues/PRs (if applicable)

Related PR: #2040 (stripped structured outputs on GCP; this keeps them instead).

Special notes for reviewers (if applicable)

• AWSAnthropic count_tokens is changed for consistency but not verified live (our credentials lack bedrock:CountTokens). Bedrock validates that body as an InvokeModel request, which rejects output_format.
• The Anthropic Messages → AWS Bedrock Converse translator accepts both fields but drops the schema (200 with unconstrained output). That is a separate gap, not changed here.
• I used an AI assistant (Claude) to help investigate and write this change; I have reviewed and tested it and can revise it as needed.

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