copilot --acp doesn't call into MCP servers configured when creating a session

[richardpark-msft]

Describe the bug

MCP servers, passed in session setup, aren't used. It's possible they're also not loaded, but I've confirmed that copilot doesn't call or see any tools, even when given a really obvious name.

Affected version

0.0.389-0 Commit: bbac972

Steps to reproduce the behavior

1. Create an app using @modelcontextprotocol/sdk

2. Create your session, like so:

   // Create a new session
    const sessionResult = await connection.newSession({
        cwd: process.cwd(),
        mcpServers: <acp.McpServer[]>[
            <acp.McpServerStdio>{
                command: "node",
                args: ["./dist/mcp.js"],
                name: "example mcp server",
                env: [],
            },
        ],
    });

   mcp.js is just basically this:

   mcpServer.registerTool(
      // super obvious name to ensure we're calling into this tool
      'the_best_tool_in_the_world_for_everyone',
      {
          description: 'Summarize any text using an LLM',
          inputSchema: {
              text: z.string().describe('Text to summarize')
          }
      },
      async ({ text }: { text: string }) => {
          try {
              return {
                  content: [
                      {
                          type: 'text',
                          text: `You called me with ${text}!`
                      }
                  ]
              };
          } catch (error) {
              stderr.write(`[Tool Error] ${JSON.stringify(error, null, 2)}\n`);
              throw error;
          }
      }

);

3. Run a prompt, through ACP:

```typescript
const promptResult = await connection.prompt({
         sessionId: sessionResult.sessionId,
         prompt: [
             {
                 type: "text",
                 //text: "Hello, agent!",
                 text: "Call the_best_tool_in_the_world_for_everyone with hello",
             },
         ],
     });

4. Copilot will respond that it doesn't know about that tool.

Expected behavior

copilot, in --acp mode, should load and use the MCP servers passed in the session initialization.

As a comparison, running copilot interactively, from the command line, with an additional MCP config that uses the same MCP server, will do the expected thing and ask me for permissions to run the tool and will execute it.

Additional context

• WSL, Ubuntu Linux 24.04
• Using stdio transport for MCP