Add Anthropic structured output support

[hiasinho]

What this does

Adds native structured output support for Anthropic Claude 4.5+ models using the GA output_config API. When with_schema is called on a chat using a supported Anthropic model, the schema is sent via output_config.format with type json_schema, and the response is automatically parsed as JSON.

Because I can't contribute to PR #528, I opened this one.

Type of change

• Bug fix
• New feature
• Breaking change
• Documentation
• Performance improvement

Scope check

• I read the Contributing Guide
• This aligns with RubyLLM's focus on LLM communication
• This isn't application-specific logic that belongs in user code
• This benefits most users, not just my specific use case

Required for new features

• I opened an issue before writing code and received maintainer approval
• Linked issue: [FEATURE] Support Anthropic's new structured outputs support #501

PRs for new features or enhancements without a prior approved issue will be closed.

Quality check

• I ran overcommit --install and all hooks pass
• I tested my changes thoroughly
  • For provider changes: Re-recorded VCR cassettes with bundle exec rake vcr:record[provider_name]
  • All tests pass: bundle exec rspec
• I updated documentation if needed
• I didn't modify auto-generated files manually (models.json, aliases.json)

AI-generated code

• I used AI tools to help write this code
• I have reviewed and understand all generated code (required if above is checked)

API changes

• Breaking change
• New public methods/classes
• Changed method signatures
• No API changes

[llenodo]

Nice PR I almost implemented the same thing before finding this.

One suggestion to make it more future proof which assumes future versions of claude (5+) will support structured outputs so we wont need to come back to this later

[KieranP]

@hiasinho Any chance this PR could also include structured output for anthropic models through AWS Bedrock? The AWS bedrock integration was recently rewritten to use converse API, which support structured output: https://docs.aws.amazon.com/bedrock/latest/userguide/structured-output.html

[hiasinho]

Nice PR I almost implemented the same thing before finding this.

One suggestion to make it more future proof which assumes future versions of claude (5+) will support structured outputs so we wont need to come back to this later

Done!

[hiasinho]

@hiasinho Any chance this PR could also include structured output for anthropic models through AWS Bedrock? The AWS bedrock integration was recently rewritten to use converse API, which support structured output: https://docs.aws.amazon.com/bedrock/latest/userguide/structured-output.html

Would love to do it, but I don't use AWS. So no creds to create VCRs. Tests will fail. Feel free to build on it.

[llenodo]

@KieranP see #596 for bedrock support

PR is currently closed per community guidelines

[crmne]

This shouldn't be another test but use the test above.

[crmne]

Fixed them myself to keep momentum.

[codecov]

Codecov Report

❌ Patch coverage is 50.00000% with 10 lines in your changes missing coverage. Please review.
✅ Project coverage is 80.82%. Comparing base (80c3373) to head (2d286e0).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
lib/ruby_llm/providers/anthropic/capabilities.rb	9.09%	10 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #608      +/-   ##
==========================================
- Coverage   80.89%   80.82%   -0.07%     
==========================================
  Files         113      113              
  Lines        5099     5112      +13     
  Branches     1310     1313       +3     
==========================================
+ Hits         4125     4132       +7     
- Misses        974      980       +6     

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