Gemini 3.1 Pro API returns 400 error due to tool schema incompatibility

[yufan001]

Description

When using Gemini 3.1 Pro (via google/gemini-3.1-pro-preview or google/antigravity-gemini-3.1-pro), all requests fail with a 400 error due to tool schema incompatibility.

Error Details

Status: 400
Tool Debug Missing: 1
Tool Debug Summary: idx=0, hasCustom=true, customSchema=true, hasFunction=false, functionSchema=false

The problematic tool is question (idx=0). The schema for the question tool's nested properties doesn't meet Gemini's strict JSON Schema validation requirements.

Root Cause

Gemini API has stricter JSON Schema validation than other providers. The question tool's schema has nested objects that may be missing required type fields or have structural issues that Gemini rejects.

Related issues:

• [Bug] Gemini models reject edit tool due to anyOf schema violation in parameters.edits #14700 - Gemini models reject edit tool due to anyOf schema violation
• [FEATURE]: Knowledge Harness of SOTA models as blueprint for cheap worker models #23382 (gemini-cli) - MCP tool schemas with missing type:'object' cause provider rejection

Environment

• OpenCode version: 1.3.2
• Model: google/gemini-3.1-pro-preview, google/antigravity-gemini-3.1-pro
• Platform: macOS (darwin)

Workaround

Use other models that don't have this schema validation issue:

• bailian-coding-plan/glm-5
• nvidia/glm5
• nvidia/minimax-m2.1

Expected Behavior

Gemini 3.1 Pro should work without schema errors.

Actual Behavior

All requests to Gemini 3.1 Pro fail with 400 Bad Request.

[github-actions]

This issue might be a duplicate of existing issues. Please check:

• "question" tool schema causes "400 Bad Request" with Gemini/Vertex AI` #11413: Same root cause — question tool schema causes 400 Bad Request with Gemini/Vertex AI
• bug: Vertex AI 400 error - Tool schemas with anyOf are incompatible with Gemini API #14788: Vertex AI 400 error due to tool schemas with anyOf being incompatible with Gemini API
• Gemini: all requests fail due to anyOf schema in github_create_pull_request_review tool #14509: Gemini requests failing due to anyOf schema in tool definitions

Please review these issues and comment there if your problem is the same, so we can consolidate.