fix: support json_object response format for Anthropic models (BRA-3896)

[mklgallegos]

Context

Fixes BRA-3896. When users selected "JSON output" mode in playgrounds with Claude models (including Vertex AI), JSON parsing failed with the error:

invalid JSON: SyntaxError: Unexpected token '`', "```json{ "... is not valid JSON

This was because the proxy only handled json_schema response format for Anthropic, not json_object.

Description

Extended the existing tool-based workaround for structured output to also handle json_object response format. When json_object is specified, the proxy now creates a tool with a generic { type: "object" } schema and forces tool use, same as it does for json_schema. This ensures Claude returns clean JSON instead of markdown-wrapped text.

Changes:

• Modified condition at proxy.ts:2600 to check for both json_schema and json_object
• For json_object, use generic { type: "object" } schema (since no specific schema is provided)

Testing

• Added capture test verifying request transformation for json_object response format
• confirmed test fails without the fix
• Verified locally in playground with Claude 4.5 Sonnet - JSON output mode now returns clean JSON

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Project	Deployment	Review	Updated (UTC)
ai-proxy	Ready	Preview, Comment	Jan 15, 2026 9:33pm

[knjiang]

[nit]: code might be more defensive if we match up-front for the data.type. currently, if someone adds another parsed.data.type, it defaults to type: "object".

i think this is fine since we are migrating away though.

[ibolmo]

brief read is that json_object supersedes json_schema. do we know what happens if a user has a json_object but also passes a json_schema? is it possible in the ui for a user to switch to that new format output and still leave behind the schema?