Gemini 3 models fail with "Invalid JSON payload received. Unknown name 'parameters'" error

[lvsao]

OpenCode GitHub Issue Report

Issue Title

Gemini 3 models fail with "Invalid JSON payload received. Unknown name 'parameters'" error when using antigravity-auth plugin

Issue Description

When using google/antigravity-gemini-3-flash or google/antigravity-gemini-3-pro models, OpenCode fails with a 400 error due to invalid tool schema format for Gemini API.

Error Message

Invalid JSON payload received. Unknown name "parameters" at 'request.tools[0]': Cannot find field.
Status: 400
Requested Model: antigravity-gemini-3-flash
Effective Model: gemini-3-flash
Endpoint: https://daily-cloudcode-pa.sandbox.googleapis.com/v1internal:streamGenerateContent?alt=sse

Tool Debug Payload (Problematic)

The tool schema being sent to Gemini contains an incorrectly formatted structure:

[{
  "functionDeclarations": [...],
  "parameters": {
    "type": "OBJECT",
    "properties": {
      "_placeholder": {
        "type": "BOOLEAN",
        "description": "Placeholder. Always pass true."
      }
    },
    "required": ["_placeholder"]
  }
}]

Expected Behavior

Tool schemas should be formatted according to Gemini API specifications, with:

1. Proper type: "object" (lowercase)
2. No _placeholder parameters
3. No unsupported JSON Schema fields like $schema
4. String-only enum values

Environment

• OpenCode Version: v1.1.23 (latest)
• Plugin: [email protected]
• OS: Windows
• Model: google/antigravity-gemini-3-flash (all variants fail)
• Project: J:\apps\max-Translator\max-ai-omni-translator

Related Issues

• OpenCode #3140: "AI_APICallError: GenerateContentRequest.tools[0].function_declarations[32].parameters. required[0]: property is not defined"
• OpenCode #4512: "sanitize tools for Gemini 3 compatibility" (fixed in PR #4538 but issue persists)
• OpenCode #4665: "OpenCode plus Zen Gemini 3 Pro model causes errors"
• Qwen Code #1186: "API Error 400 with Gemini model: Invalid JSON payload received. Unknown name '$schema'"

Reproduction Steps

1. Install opencode-antigravity-auth@beta plugin
2. Configure google/antigravity-gemini-3-flash model
3. Run: opencode run "Hello" --model=google/antigravity-gemini-3-flash
4. Observe 400 error with "Unknown name 'parameters'" message

Workaround

Claude models (antigravity-claude-sonnet-4-5-thinking) work correctly with the same configuration.

Suggested Fix

Implement proper tool schema sanitization for Gemini API:

1. Remove $schema field from tool parameter schemas
2. Convert non-string enum values to strings
3. Ensure type field uses lowercase values
4. Remove placeholder parameters like _placeholder
5. Validate schema against Gemini's expected format

See OpenCode PR #4538 for previous attempts at fixing similar issues.