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Title: MCP tools with dots in names cause 400 Bad Request — should match MCP spec #2581
Description
When an MCP server returns tools with dots (.) in their names, Copilot CLI sends them to the API which rejects them with a 400 error:
✗ Execution failed: CAPIError: 400 tools.19.custom.name: String should match pattern
'^[a-zA-Z0-9_-]{1,128}$'
The MCP specification explicitly allows dots in tool names (section 5.1.1). VS Code Copilot handles this by remapping tool names internally (dots → underscores), but the CLI passes them through raw and the API rejects them.
Steps to Reproduce
- Configure an MCP server that returns tools with dots in names (e.g., Backstage's @backstage/plugin-mcp-actions-backend v0.1.10+ which generates names like catalog.get-entity-facets)
- Start Copilot CLI
- Send any prompt → 400 Bad Request
Expected Behavior
API should accept dots per the MCP spec.
Actual Behavior
Hard failure — CLI becomes completely unusable when any MCP server has dotted tool names.
Environment
- Copilot CLI: latest
- OS: macOS / Windows / Linux (all affected)
Related Issues
- VS Code (warning, not error): Warning about invalid MCP tool name microsoft/vscode#308463
- IntelliJ (same hard failure): Tool name validation gives unnecessary warning microsoft/copilot-intellij-feedback#1583
- MCP spec discussion on tool name pattern: spec/docs: specify tool name format SEP-986 modelcontextprotocol/modelcontextprotocol#1603
- MCP Spec — Tool Names: https://modelcontextprotocol.io/specification/2025-11-25/server/tools
Reacted by Noah Pendleton, Marc Jessome and Florian Magin
Activity
neilhughesp commentedon Apr 8, 2026
if using a gpt model
✗ Execution failed: CAPIError: 400 Invalid 'tools[50].name': string does not match pattern. Expected a string that match
the pattern '^[a-zA-Z0-9_-]+$'. (Request ID: ABF5:A4EA9:227CEAA:258497D:69D6691C)
if using a Claude model i get a generic 400
✗ Execution failed: CAPIError: 400 400 Bad Request
(Request ID: 3601:AECAE:23B320B:26D6666:69D66A8B)
matt-gribben commentedon Apr 13, 2026
I'm also seeing this issue
copilot-cli-bot commentedon Sep 29, 2026
This appears to be resolved as of GitHub Copilot CLI v1.0.89. Please update to that release or a newer stable version. We're closing this as completed. If it still happens, please comment with your version and reproduction steps.