fix(gemini): drop sampling parameters and candidate_count for Gemini 3 models

[SantiagoDePolonia]

Summary

Google's Gemini 3 migration guides say to remove temperature, top_p and top_k from every request ("Gemini 3's reasoning capabilities are optimized for the default settings", "Remove these parameters from all requests") and list candidate_count as unsupported on Gemini 3.x. In native mode GoModel builds the generationConfig itself, so it now leaves those four fields out for Gemini 3 and later and keeps forwarding them for Gemini 2.5, Gemma and other model IDs.

• New geminiGeneration parser reads the <major>.<minor> generation from gemini-* IDs, including Vertex google/ prefixes, and dropsSamplingParameters gates on major >= 3.
• max_tokens, stop, penalties, response_format and thinking settings are unchanged.
• OpenAI-compatible mode is untouched; Google's own compatibility layer receives the body as-is.
• Table-driven tests for the parser and the config builder, plus a docs note.

Caveat for reviewers: Google documents these parameters as "no longer recommended" and unsupported, not as returning HTTP 400 today. This is a proactive alignment with the migration guidance (mirrors #856 for Anthropic), and it changes behavior for callers that set temperature on a Gemini 3 model. Independent of #862, which fixes an actual 400 on reasoning_effort: minimal.

References: What's new in Gemini 3.5 Flash, What's new in Gemini 3.8 Flash.

Summary by CodeRabbit

• Bug Fixes

  • Gemini 3 and later models no longer receive unsupported sampling parameters such as temperature, top-p, top-k, and candidate count.
  • Gemini 2.5, Gemma, and other supported models continue to honor these parameters.
  • Maximum output tokens, stop sequences, penalties, response formats, and thinking settings remain unaffected.

• Documentation

  • Added guidance describing sampling-parameter behavior across Gemini model versions.

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ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: ASSERTIVE

Plan: Team

Run ID: 72922997-ba06-4b3c-b0fd-b6381f0968fe

📥 Commits

Reviewing files that changed from the base of the PR and between a8fe362 and 5d60722.

📒 Files selected for processing (3)

• docs/providers/gemini.mdx
• internal/providers/gemini/native.go
• internal/providers/gemini/native_sampling_test.go

Included review availability: Your plan provides up to 4 included reviews per hour; 1 remains after this review.

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📝 Walkthrough

Walkthrough

The Gemini provider now omits selected sampling parameters for Gemini 3 and later models. Gemini 2.5 and non-Gemini models retain the previous behavior. Tests and provider documentation cover the model-specific behavior.

Changes

Gemini sampling parameters

Layer / File(s)	Summary
Model generation detection and request filtering internal/providers/gemini/native.go	The provider parses Gemini model generations and omits temperature, top_p, top_k, and candidate_count for Gemini 3 and later models.
Sampling behavior validation and documentation internal/providers/gemini/native_sampling_test.go, docs/providers/gemini.mdx	Tests cover parsing and generation configuration behavior. The documentation describes the model-specific sampling rules and unaffected settings.

Estimated code review effort: 2 (Simple) | ~10 minutes

Merge Risk: ⚪ Minimal · up to 5d607

The PR removes unsupported sampling fields from native Gemini 3 and later requests while preserving existing behavior for other models; no actionable merge-blocking risk remains after normal checks and review.

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The patch grows clear beneath the moon,
Small changes hop in place,
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🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 33.33% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 6 functions across 2 files. (1 skipped: 1…	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly describes the main change: removing sampling parameters and candidate_count for Gemini 3 models.
Description check	✅ Passed	The description is detailed, relevant, and covers the change, rationale, affected modes, preserved settings, tests, documentation, caveats, and references. It uses a "## Summary" heading instead of th…
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

Full details: Description check

Explanation

The description is detailed, relevant, and covers the change, rationale, affected modes, preserved settings, tests, documentation, caveats, and references. It uses a "## Summary" heading instead of the template's required "## Description" heading, but the required information is present.

Full details: Docstring Coverage

Explanation

Docstring coverage is 33.33% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 6 functions across 2 files. (1 skipped: 1 unsupported.)

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Codecov Report

❌ Patch coverage is 96.00000% with 1 line in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
internal/providers/gemini/native.go	96.00%	1 Missing ⚠️

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[greptile-apps]

Confidence Score: 5/5

The native Gemini request-generation behavior is safe to merge based on direct execution across relevant Gemini and control model identifiers.

Direct request-generation checks confirmed that Gemini 3+ identifiers omit the four intended fields while Gemini 2.5 and non-Gemini controls preserve them; focused package tests also passed.

Files Needing Attention: No files need follow-up attention.

T-Rex Logs

What T-Rex did

• Ran an authored Go test through the JSON core.ChatRequest to the native generation-config path for public, publisher-prefixed, full Vertex resource-path, and mixed-case Gemini 3+/4 identifiers, with Gemini 2.5, Gemma, and Imagen controls.
• The harness showed that before the PR commit, temperature, topP, topK, and candidateCount were present for Gemini 3+/4, but after the PR commit those fields were absent for Gemini 3+/4 and retained for the controls.
• Focused package tests for Gemini ran and passed, confirming the request-generation behavior and parser coverage.
• No production files were changed; the authored test source was saved as a review artifact and the temporary in-package copy was removed after execution.

_{Ran code and verified through T-Rex}

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