fix(genai): Support cached_content for Google context caching

[b-antosik-marcura]

Summary

When using Google's explicit context caching, the API requires that system_instruction, tools, and tool_config are NOT set in the request - they should already be part of the cached content.

This PR fixes cached_content propagation for Google GenAI by:

• Extracting cached_content from user-provided GenerateContentConfig
• Skipping system_instruction, tools, tool_config when cached_content is present
• Adding cached_content to the list of config fields that get merged

Related Issues

• Similar to Bug (Google GenAI): Config Labels Not Preserved #1759 (labels not preserved in config)

Documentation

• Google AI Context Caching

Test plan

• Added unit tests for cached_content extraction
• Added unit tests verifying system_instruction/tools/tool_config are skipped when cached_content is present
• All existing tests pass
• Manual test and example summary from LangSmith:

---

Important

Fix handling of cached_content in Google GenAI context caching by skipping certain fields and adding tests.

• Behavior:
  • In handle_genai_structured_outputs() and handle_genai_tools(), skip setting system_instruction, tools, and tool_config if cached_content is present.
  • Extract cached_content from GenerateContentConfig in update_genai_kwargs() and add it to config_fields_to_merge.
• Tests:
  • Add tests in test_genai_config_merging.py to verify cached_content extraction and behavior when present.
  • Ensure system_instruction, tools, and tool_config are not set when cached_content is provided.
  • Verify that cached_content does not override existing base config values.

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1. instructor/providers/gemini/utils.py:350

• Draft comment:
  Good addition: 'cached_content' is now included in the config fields merge. This ensures that any user-provided cached content is propagated correctly.
• Reason this comment was not posted:
  Confidence changes required: 0% <= threshold 85% None

2. instructor/providers/gemini/utils.py:887

• Draft comment:
  The extraction of 'cached_content' (along with 'thinking_config') from the user config is implemented clearly. Consider refactoring this repeated extraction logic into a helper function to adhere to DRY principles.
• Reason this comment was not posted:
  Confidence changes required: 33% <= threshold 85% None

3. instructor/providers/gemini/utils.py:1000

• Draft comment:
  The conditional block that omits tools, tool_config, and system_instruction when 'cached_content' is present is well-commented and clear. Consider consolidating similar extraction logic between the structured outputs and tools handlers for maintainability.
• Reason this comment was not posted:
  Confidence changes required: 33% <= threshold 85% None

4. tests/test_genai_config_merging.py:240

• Draft comment:
  Test 'test_update_genai_kwargs_config_object_cached_content' effectively verifies that 'cached_content' is extracted from the config object. Good coverage for this new feature.
• Reason this comment was not posted:
  Confidence changes required: 0% <= threshold 85% None

5. tests/test_genai_config_merging.py:285

• Draft comment:
  The tests for 'handle_genai_structured_outputs' and 'handle_genai_tools' properly check that attributes like system_instruction, tools, and tool_config are omitted when 'cached_content' is provided. This comprehensive test coverage reinforces the intended behavior.
• Reason this comment was not posted:
  Confidence changes required: 0% <= threshold 85% None

Workflow ID: wflow_22PHFL1GIzlh6bW5

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