Gemini schema sanitizer doesn't strip contentEncoding from MCP tool schemas

[RaviTharuma]

Bug Description

When using Gemini-based agents (e.g. oh-my-opencode Junior Sisyphus) with MCP servers that define contentEncoding in their tool schemas, the Gemini API rejects the request with a 400 error.

The existing schema sanitizer (introduced in v1.0.137 via #3140) strips properties/required from non-object types, but does not strip contentEncoding or contentMediaType — both valid JSON Schema keywords that Gemini's function calling API doesn't support.

Error

Bad Request: {
  "error": {
    "code": 400,
    "message": "Invalid JSON payload received. Unknown name \"contentEncoding\" at 'request.tools[0].function_declarations[87].parameters.properties[4].value.items.properties[2].value': Cannot find field.\nInvalid JSON payload received. Unknown name \"contentEncoding\" at 'request.tools[0].function_declarations[88].parameters.properties[4].value.items.properties[2].value': Cannot find field.\nInvalid JSON payload received. Unknown name \"contentEncoding\" at 'request.tools[0].function_declarations[89].parameters.properties[4].value.items.properties[2].value': Cannot find field.",
    "status": "INVALID_ARGUMENT"
  }
}

Reproduction

1. Configure an MCP server that uses contentEncoding in tool schemas (e.g. chrome-devtools-mcp — tools like take_screenshot, take_snapshot, upload_file use contentEncoding: "base64")
2. Start a session with a Gemini model
3. The API call fails with the above error

Expected Behavior

The Gemini schema sanitizer should strip unsupported JSON Schema keywords before sending function declarations. At minimum:

• contentEncoding
• contentMediaType

Environment

• opencode v1.2.15
• MCP server: chrome-devtools-mcp (mcpm)
• Gemini model via oh-my-opencode Junior Sisyphus agent

Related

• AI_APICallError: GenerateContentRequest.tools[0].function_declarations[32].parameters. required[0]: property is not defined with Gemini API #3140 (original schema sanitization fix)

[Br1an67]

I'd like to work on this. The fix adds contentEncoding and contentMediaType stripping to the existing sanitizeGemini function in transform.ts.

PR: #15593