fix: use Anthropic's native output_format for structured output

[03-CiprianoG]

Fixes #25

Problem

When using Output.object({ schema }) or generateObject() with Anthropic models via the Helicone gateway, the model returns prose instead of JSON because response_format (OpenAI's parameter) is sent but Anthropic ignores it.

See #25 for full details, reproduction steps, and request logs.

Solution

Detect Anthropic models (this.modelId.startsWith('anthropic/')) in buildRequestBody() and use Anthropic's native output_format parameter instead of OpenAI's response_format.

Before (broken for Anthropic)

{
  "response_format": {
    "type": "json_schema",
    "json_schema": { "schema": {...}, "strict": true, "name": "response" }
  }
}

Anthropic ignores response_format → model returns prose → NoObjectGeneratedError.

After (works for all providers)

Anthropic models → uses output_format (native Anthropic API):

{
  "output_format": {
    "type": "json_schema",
    "json_schema": { "schema": {...}, "name": "response" }
  }
}

All other models → continues using response_format (OpenAI-compatible), unchanged.

Changes

• src/helicone-language-model.ts: Updated buildRequestBody() to branch on this.modelId.startsWith('anthropic/') when handling responseFormat. Anthropic models get output_format, others get response_format as before.

Notes

• Anthropic's output_format does not support strict: true (OpenAI-specific), so it's omitted for Anthropic models.
• The Helicone gateway passes unknown body fields through to the underlying provider, so output_format reaches Anthropic's API correctly.
• This approach is consistent with how @ai-sdk/anthropic handles structured output natively (via the outputFormat structured output mode).

[03-CiprianoG]

Closing this PR — the root cause is in the AI Gateway, not the AI SDK provider.

The gateway's toAnthropic() translation in packages/llm-mapper/transform/providers/openai/request/toAnthropic.ts ignores response_format entirely when translating OpenAI requests to Anthropic format. It should map response_format: { type: "json_schema", json_schema: { schema } } to Anthropic's native output_config: { format: { type: "json_schema", schema } }.

Filed a proper issue and PR on the gateway repo instead:

• Issue: (will link below)
• PR: (will link below)

The output_format approach in this PR also doesn't work because the gateway rejects unknown body fields (Unrecognized key: "output_format").

[03-CiprianoG]

AI Gateway (the real fix):

• Issue: AI Gateway: Anthropic structured output (response_format → output_config) not translated helicone#5639
• PR: feat: translate response_format to Anthropic output_config for structured output helicone#5640
• Adds response_format → output_config.format translation in toAnthropic()