Copilot Chat: agent request crashes with "Cannot read properties of undefined (reading 'type')" in countToolTokens when a tool input schema uses top-level JSON-Schema if/then (BYOK / custom model)

Type: Bug

Summary

In agent mode with a custom / BYOK OpenAI-compatible model, a chat request aborts before sending whenever the active tool set includes an MCP tool whose input JSON Schema uses top-level if / then conditional keywords. The extension's local token counter (countToolTokens → countObjectTokens → countMessageObjectTokens → tokenLength) dereferences .type on a schema node it doesn't recognise and throws TypeError: Cannot read properties of undefined (reading 'type'). "Try Again" replays the same tool set and fails identically.

This only reproduces with custom/BYOK models, where the extension counts tool tokens locally. First-party (Copilot-hosted) models offload token counting and are unaffected.

Environment

• VS Code: 1.126.0 (commit 7e7950df89d055b5a378379db9ee14290772148a, 2026-06-23, stable)
• Bundled Copilot extension: 0.54.0
• OS: Linux (WSL2 remote)
• Model: a custom OpenAI-compatible endpoint registered via BYOK (provider type "custom OpenAI-compatible" / "custom endpoint")

Steps to reproduce

1. Register a custom OpenAI-compatible model via BYOK and select it in agent mode.
2. Register an MCP server that exposes a tool whose input schema uses top-level if/then, e.g.:

   {
     "name": "run_query",
     "inputSchema": {
       "type": "object",
       "properties": { "query": { "type": "string" }, "mode": { "type": "string" } },
       "if":   { "properties": { "mode": { "const": "nl" } } },
       "then": { "required": ["query"] }
     }
   }
   

3. Send any agent-mode request with that tool in scope.

Actual behaviour

The extension first logs (repeatedly, per request):

[warning] Tool <tool-name> failed validation: object has unsupported top-level schema keyword 'if'
[warning] Tool <tool-name> failed validation: object has unsupported top-level schema keyword 'then'

…then the request aborts with:

TypeError: Cannot read properties of undefined (reading 'type')
    at gre.tokenLength (<vscode-server>/bin/7e7950df.../extensions/copilot/dist/extension.js:1662:34964)
    at gre.countMessageObjectTokens (...:1662:36231)
    at gre.countMessageObjectTokens (...:1662:36504)
    at process.processTicksAndRejections (node:internal/process/task_queues:104:5)
    at async gre.countMessageObjectTokens (...:1662:36493)
    at async gre.countObjectTokens (...:1662:36795)
    at async gre.countToolTokens (...:1662:35927)
    at async LG.countToolTokens (...:1690:16849)
    at async LG._provideLanguageModelResponse (...:1690:11650)
    at async LG.provideLanguageModelResponse (...:1690:15320)

Surfaced in the UI as: "Sorry, your request failed. Please try again."

Expected behaviour

Unsupported/unknown JSON-Schema keywords (if/then/else, etc.) should be ignored or stripped during token counting, not crash the request. tokenLength/countObjectTokens should be null-safe when walking schema nodes that have no type. At worst the tool should be skipped (as the validation warning implies), not abort the whole request.

Notes / workaround

• Deselecting the offending tool in the agent-mode tools picker avoids the crash.
• Top-level if/then is valid JSON Schema (Draft-07+); the failure is in how the token counter handles a schema node it chose not to support.

Impact

Any third-party MCP tool that ships a Draft-07 conditional schema makes custom/BYOK models unusable whenever that tool is in scope. Client request IDs from affected sessions available on request for telemetry correlation.