[Go] Anthropic: structured output conformance fails — response is not valid JSON

[yesudeep]

Summary

The Go Anthropic plugin fails structured output conformance tests. When JSON output format is requested, the model response is not valid JSON:

model failed to generate output matching expected schema: message is not a valid JSON

This affects all Anthropic models that support structured output (claude-sonnet-4-5, claude-haiku-4-5, claude-opus-4-6) for both streaming and non-streaming requests.

Expected behavior

When output.format = "json" is requested with a JSON schema, the Go Anthropic plugin should pass the output schema to the Anthropic API (e.g. via response_format or tool-based extraction) so the model returns valid JSON matching the schema.

Reproduction

cd py && bin/conform check-model anthropic --runtime go --runner native

Look for "Structured Output Conformance" failures.

Affected models

• anthropic/claude-sonnet-4-5 — Structured Output (streaming + non-streaming)
• anthropic/claude-haiku-4-5 — Structured Output (streaming + non-streaming)
• anthropic/claude-opus-4-6 — Structured Output (streaming + non-streaming)

Notes

The Python Anthropic plugin handles this correctly by passing the JSON schema constraint to the API. The Go plugin likely needs to implement the same schema-passing mechanism.