AI_APICallError (400 Bad Request) with GitHub Copilot when MCP tool has empty description, affects claude-sonnet-4.6 / claude-opus-4.6 only

[PauloDevelo]

Description

Summary

When an MCP server registers a tool with an empty string description ("description": ""), OpenCode forwards that empty description to the GitHub Copilot API as-is. GitHub Copilot's API accepts this for claude-sonnet-4.5 and older models, but rejects it with 400 Bad Request for claude-sonnet-4.6 and claude-opus-4.6.

Steps to Reproduce

1. Configure an MCP server that exposes at least one tool with "description": "" (e.g. mcp-seq — npx -y mcp-seq — whose seq_get-alertstate and seq_get-signals tools have empty descriptions)
2. Enable the MCP server in opencode.jsonc
3. Select github-copilot/claude-sonnet-4.6 or github-copilot/claude-opus-4.6
4. Send any message — the call fails immediately with AI_APICallError: Bad Request

Root Cause (confirmed via curl)

GitHub Copilot's API returns 400 Bad Request when the request body contains a tool with "description": "" and the model is claude-sonnet-4.6 or claude-opus-4.6. The same request with claude-sonnet-4.5 returns 200 OK.

Minimal reproduction:

Returns 400 ❌ — empty description + claude-sonnet-4.6

curl -X POST https://api.githubcopilot.com/chat/completions
-H "Authorization: Bearer $TOKEN"
-H "Content-Type: application/json"
-d '{
"model": "claude-sonnet-4.6",
"messages": [{"role": "user", "content": "hi"}],
"tools": [{"type":"function","function":{"name":"t","description":"","parameters":{"type":"object","properties":{"x":{"type":"string"}}}}}],
"tool_choice": "auto",
"stream": true
}'

Returns 200 ✅ — non-empty description, same model

Change "description": "" to "description": "Get something"

Returns 200 ✅ — empty description, older model

Change model to "claude-sonnet-4.5"

Proposed Fix

Before sending tool definitions to any provider, OpenCode should sanitize tool descriptions: replace empty string "" with a fallback such as "No description provided" or the tool name. MCP servers are not guaranteed to provide non-empty descriptions, so this defensive normalization is necessary.

Environment

• OpenCode version: 1.2.10
• Provider: github-copilot
• Failing models: claude-sonnet-4.6, claude-opus-4.6
• Working models: claude-sonnet-4.5, GPT-4o, o3, Gemini models
• Platform: Windows native CLI (win32), Linux (confirmed in a Podman container)
• MCP server trigger: mcp-seq (npx -y mcp-seq)

Plugins

MCP server mcp-seq was causing me the issue

OpenCode version

1.2.10

Steps to reproduce

1. Configure an MCP server that exposes at least one tool with "description": "" (e.g. mcp-seq — npx -y mcp-seq — whose seq_get-alertstate and seq_get-signals tools have empty descriptions)
2. Enable the MCP server in opencode.jsonc
3. Select github-copilot/claude-sonnet-4.6 or github-copilot/claude-opus-4.6
4. Send any message — the call fails immediately with AI_APICallError: Bad Request

Screenshot and/or share link

No response

Operating System

Windows native CLI (win32), Linux (confirmed in a Podman container)

Terminal

Powershell

[vikrant82]

Can this be related - #18935 (comment)

But this is only happening for me since a week or so..