feat: add support for Anthropic's strict tool use

[ping-Toven]

Description

Adds support for Anthropic's strict tool use feature, which validates tool parameters to ensure Claude calls functions with correctly-typed arguments. This is useful for building reliable agentic workflows where type-safe function calls are critical.

When strictToolUse: true is set in chat settings:

• Adds strict: true to each tool definition in the request body
• Includes the required anthropic-beta: structured-outputs-2025-11-13 header

Usage:

const model = provider.chat('anthropic/claude-3.5-sonnet', {
  strictToolUse: true,
});

Reference: https://docs.anthropic.com/en/docs/build-with-claude/tool-use/strict-tool-use

Implementation Details

• Added strictToolUse?: boolean setting to OpenRouterChatSettings
• Added getStrictToolUseHeaders() helper method for beta header injection
• Modified tool mapping in getArgs() to conditionally include strict: true
• Updated both doGenerate() and doStream() to include headers when enabled
• Added 9 new unit tests covering both streaming and non-streaming scenarios

Notes

• The feature is opt-in (defaults to false), so existing behavior is unchanged
• The anthropic-beta header is sent whenever strictToolUse is enabled, even if no tools are in the request

Checklist

• I have run pnpm stylecheck and pnpm typecheck
• I have run pnpm test and all tests pass
• I have added tests for my changes (if applicable)
• I have updated documentation (if applicable)

Changeset

• I have run pnpm changeset to create a changeset file

Note: A changeset is required for your changes to trigger a release. If your PR only contains docs, tests, or CI changes that don't need a release, run pnpm changeset --empty instead.

Review & Testing Checklist for Human

• Verify strict: true placement: The property is added at the tool level (alongside type: 'function'), not inside the function object. Confirm this matches OpenRouter's expected format for Anthropic models.
• E2E test with actual API: Unit tests use mocked servers. Test with a real Anthropic model through OpenRouter to verify the feature works end-to-end.
• Verify beta header is current: The hardcoded header value structured-outputs-2025-11-13 should be checked against current Anthropic docs.

Recommended test plan:

1. Create a simple script using the provider with strictToolUse: true
2. Call an Anthropic model (e.g., anthropic/claude-3.5-sonnet) with a tool that has strict schema requirements
3. Verify the model returns correctly-typed arguments matching the schema

---

Requested by: Tomas Oliva (@ping-Toven)
Devin session: https://app.devin.ai/sessions/b201995bb0ef4d1f8fa8a6531de0f6fb

[devin-ai-integration]

Original prompt from Tomas

@Devin add support for anthropic's strict tool use with full unit test coverage.
Thread URL: https://openrouter.slack.com/archives/C07UF9XLTFF/p1765928745422299

[devin-ai-integration]

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