tool_search isn't working for mcp tools on gpt-5.5

[roblourens]

I ask it to run an mcp tool, it uses tool_search, then says the tool is still not available. Sometimes it does a bunch of random things like running terminal commands and searching the workspace for the tool. Sometimes it calls a subagent and the suagent has the tool. I disabled subagents in this run.

722fee01.copilotmd
722fee01.request.json

[FrancoisJULIENNE]

I can reproduce the same issue with Azure DevOps MCP.

Reproduction context

• VS Code: 1.118.1 (user setup)
• Commit: 034f571df509819cc10b0c8129f66ef77a542f0e
• Date: 2026-04-29T17:36:44+03:00
• Electron: 39.8.8
• Chromium: 142.0.7444.265
• Node.js: 22.22.1
• V8: 14.2.231.22-electron.0
• OS: Windows_NT x64 10.0.26200
• MCP server: Azure DevOps MCP (@azure-devops/mcp@latest)
• MCP transport: stdio
• Affected MCP tools: mcp_azure-devops_*
• Example target tool: mcp_azure-devops_wit_get_work_item
• Failing model: GPT-5.5 Xhigh
• Working model in the same environment: Claude Sonnet 4.6

All organization names, project names, work item IDs, and local paths below are anonymized.
No PAT, token, private URL, customer data, or business content is included.

Relevant VS Code settings

{
  "chat.mcp.discovery.enabled": true,
  "chat.mcp.autoStart": "newAndOutdated",
  "github.copilot.chat.responsesApi.toolSearchTool.enabled": true
}

MCP tools are enabled in Copilot Chat through the tools picker / Configure Tools.

Anonymized MCP configuration

{
  "servers": {
    "azure-devops": {
      "type": "stdio",
      "command": "npx",
      "args": [
        "-y",
        "@azure-devops/mcp@latest",
        "ExampleOrg",
        "--authentication",
        "envvar",
        "-d",
        "core",
        "work",
        "work-items",
        "search"
      ],
      "env": {
        "ADO_MCP_AUTH_TOKEN": "${env:ADO_MCP_AUTH_TOKEN}"
      }
    }
  }
}

Expected behavior

When GPT-5.5 needs a deferred MCP tool, the expected sequence is:

tool_search("mcp_azure-devops_wit_get_work_item")
  -> tool_search returns mcp_azure-devops_wit_get_work_item
  -> GPT-5.5 invokes mcp_azure-devops_wit_get_work_item
  -> work item data is returned and used

Actual behavior with GPT-5.5 Xhigh

GPT-5.5 correctly calls tool_search and the Azure DevOps MCP tool is returned.

However, it does not invoke the returned MCP tool. Instead, it falls back to terminal actions such as:

run_in_terminal
Write-Host "..."

Observed sequence:

tool_search("mcp_azure-devops_wit_get_work_item")
  -> MCP Azure DevOps tool is returned
  -> GPT-5.5 does not invoke the returned MCP tool
  -> GPT-5.5 falls back to run_in_terminal / Write-Host

This produces no real Azure DevOps data via MCP and breaks the expected workflow.

Working behavior with Claude Sonnet 4.6

With the same VS Code workspace, the same MCP server, the same Azure DevOps MCP configuration, and the same user request, Claude Sonnet 4.6 behaves correctly:

tool_search("mcp_azure-devops_wit_get_work_item")
  -> tool_search returns mcp_azure-devops_wit_get_work_item
  -> Claude immediately invokes mcp_azure-devops_wit_get_work_item
  -> Claude receives the work item data
  -> Claude produces the expected report

This suggests that the MCP server itself is correctly configured and functional.

Example anonymized prompt

Read Azure DevOps work item 12345 from ExampleProject and summarize its description,
comments, and acceptance criteria. Use the Azure DevOps MCP tools.

Why this looks related to this issue

The MCP tool is successfully discovered through tool_search, but GPT-5.5 does not complete the next step: invoking the returned tool.

The issue does not seem to be caused by:

• Azure DevOps MCP configuration
• missing PAT
• MCP server startup failure
• missing or incorrectly named tool
• VS Code not exposing the MCP server
• MCP tools being disabled in Copilot Chat

It appears to be the same failure mode described in this issue: GPT-5.5 discovers an MCP tool with tool_search, but does not actually call it afterward.