Title: MCP tools with dots in names cause 400 Bad Request — should match MCP spec

[chauhansachinkr]

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

1. 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)
2. Start Copilot CLI
3. 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

[neilhughesp]

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]

I'm also seeing this issue

Execution failed: CAPIError: 400 Invalid 'tools[21].name': string does not match pattern. Expected a string that matches the pattern '^[a-zA-Z0-9_-]+$'. 
(Request ID: C128:2D65C6:67D1174:82940F6:69DC4D15)