feat: Add Gemini 3 support with ThinkingLevel and thought signatures

[gunpal5]

Summary

• Add ThinkingLevel enum (LOW, HIGH) to ThinkingConfig for Gemini 3 models
• Add Gemini3ProPreview and Gemini3FlashPreview model constants
• Fix Schema.AdditionalProperties causing API errors by adding [JsonIgnore]
• Add comprehensive integration tests for Gemini 3 thinking features

Changes

New Features

• ThinkingLevel: Control the depth of model reasoning with ThinkingLevel.LOW or ThinkingLevel.HIGH
• Gemini 3 Model Constants: Easy access to new Gemini 3 preview models
• Thought Signature Support: Automatic preservation in function calling workflows

Bug Fixes

• Fixed additionalProperties causing INVALID_ARGUMENT errors when using function calling

Closes #94, Closes #87

Summary by CodeRabbit

Release Notes

• New Features

  • Added support for Gemini 3 Pro Preview and Gemini 3 Flash Preview models with advanced thinking capabilities for enhanced reasoning and function calling.
  • Introduced ThinkingLevel configuration (LOW, HIGH, UNSPECIFIED) for models supporting thinking-enabled generation.

• Documentation

  • Updated MCP integration guidance with expanded reference materials.

• Tests

  • Added comprehensive integration and unit tests for Gemini 3 thinking and function-calling workflows.

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[coderabbitai]

Caution

Review failed

The pull request is closed.

📝 Walkthrough

Walkthrough

This PR adds Gemini 3 support with thinking configuration features. Changes include two new model constants (Gemini3ProPreview and Gemini3FlashPreview), a ThinkingLevel enum and property for ThinkingConfig, modification to Schema serialization behavior, comprehensive unit and integration tests for thinking-enabled generation and function calling, updated README guidance, and removal of the legacy README_old.md file.

Changes

Cohort / File(s)	Summary
Documentation README.md, README_old.md	Updated MCP integration guidance in README with explicit bullet-point links; removed legacy README_old.md containing comprehensive Gemini SDK documentation.
Model Constants src/GenerativeAI/Constants/GoogleAIModels.cs	Added two public string constants: Gemini3ProPreview and Gemini3FlashPreview, each with XML documentation describing advanced thinking features.
Type System Updates src/GenerativeAI/Types/ContentGeneration/Common/Schema.cs	Changed AdditionalProperties from [JsonPropertyName("additionalProperties")] to [JsonIgnore], excluding this field from serialization with clarifying documentation.
Generation Config src/GenerativeAI/Types/ContentGeneration/Config/GenerationConfig.cs	Added public ThinkingLevel enum with values THINKING_LEVEL_UNSPECIFIED, LOW, and HIGH; added ThinkingLevel? property to ThinkingConfig.
Unit Tests tests/GenerativeAI.Tests/Model/Gemini3_ThinkingConfig_Tests.cs	New test class with 12 test methods validating ThinkingConfig serialization/deserialization, ThinkingLevel enum round-tripping, Part and Content thought signature preservation, and Gemini 3 model constant presence.
Integration Tests tests/GenerativeAI.IntegrationTests/Gemini3_ThinkingConfig_Integration_Tests.cs, tests/GenerativeAI.IntegrationTests/Gemini3_FunctionCalling_Tests.cs	Two comprehensive integration test suites exercising thinking-enabled generation, function calling with thought signatures, chat sessions with thinking context, auto function calling, streaming with thinking, and ThinkingLevel behavior against live Gemini 3 API.

Sequence Diagram(s)

sequenceDiagram
    participant Client
    participant GenerativeModel
    participant GoogleAI as Google AI API
    participant Response as Response Handler

    Client->>GenerativeModel: Request with ThinkingConfig<br/>(IncludeThoughts, ThinkingBudget,<br/>ThinkingLevel)
    
    GenerativeModel->>GoogleAI: Send generation request<br/>with thinking configuration
    
    rect rgb(200, 220, 255)
    Note over GoogleAI: API processes with thinking enabled
    GoogleAI->>GoogleAI: Generate response with<br/>thought content
    end
    
    GoogleAI->>Response: Return response with:<br/>- Thought parts<br/>- ThoughtSignature<br/>- Function call metadata
    
    Response->>Response: Extract and preserve<br/>ThoughtSignature from parts
    
    alt Function Call Present
        Response->>Client: Return function call details<br/>with thought signature intact
        Client->>GenerativeModel: Resend with function result<br/>+ preserved ThoughtSignature
        GenerativeModel->>GoogleAI: Continue conversation<br/>maintaining thought context
    else No Function Call
        Response->>Client: Return complete response<br/>with thoughts and content
    end

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Estimated code review effort

🎯 3 (Moderate) | ⏱️ ~20 minutes

Possibly related PRs

• Add thought signature support for Microsoft.Extensions.AI #88 — Implements preservation of ThoughtSignature in ToPart/ToAiContents conversions, complementing this PR's thought signature handling in responses.
• Update the current Gemini Models #68 — Adds model constants (Gemini 2.5 and embedding variants) to GoogleAIModels.cs, same file modified in this PR for Gemini 3 preview models.
• feat: AOT compatibility throughout the SDK #23 — Modifies Schema.cs and GenerationConfig.cs classes, which are updated in this PR for thinking configuration support.

Poem

🐰 Gemini three hops into the light,
With thoughts so deep and thinking bright!
ThinkingLevel stacked, function calls aligned—
Signatures preserved, a feature refined!
The thinking rabbit hops in delight! 🥕✨

✨ Finishing touches

• 📝 Generate docstrings

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment
• Commit unit tests in branch feature/gemini3-thinking-support

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📜 Recent review details

Configuration used: defaults

Review profile: CHILL

Plan: Pro

📥 Commits

Reviewing files that changed from the base of the PR and between b30ef5c and 7f05376.

📒 Files selected for processing (8)

• README.md
• README_old.md
• src/GenerativeAI/Constants/GoogleAIModels.cs
• src/GenerativeAI/Types/ContentGeneration/Common/Schema.cs
• src/GenerativeAI/Types/ContentGeneration/Config/GenerationConfig.cs
• tests/GenerativeAI.IntegrationTests/Gemini3_FunctionCalling_Tests.cs
• tests/GenerativeAI.IntegrationTests/Gemini3_ThinkingConfig_Integration_Tests.cs
• tests/GenerativeAI.Tests/Model/Gemini3_ThinkingConfig_Tests.cs

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