Fix: Respect explicit listChanged: false capability setting

[mattzcarey]

Problem

When instantiating McpServer with capabilities.tools.listChanged set to false, the setting was being overwritten to true when registering a tool via registerTool().

const server = new McpServer(
  { name: 'my-server', version: '1.0' },
  { capabilities: { tools: { listChanged: false } } }
);

server.registerTool('ping', {}, async () => ({ content: [{ type: 'text', text: 'pong' }] }));
// After connect, capabilities.tools.listChanged is incorrectly `true`

This prevents servers from opting out of list change notifications, which may be desirable for stateless server implementations.

Fixes #1488

Solution

Changed the registerCapabilities calls in setToolRequestHandlers(), setResourceRequestHandlers(), and setPromptRequestHandlers() to use nullish coalescing, only defaulting to true when not explicitly set:

// Before
this.server.registerCapabilities({
    tools: { listChanged: true }
});

// After
this.server.registerCapabilities({
    tools: { listChanged: this.server.getCapabilities().tools?.listChanged ?? true }
});

Design Decisions

1. Made Server.getCapabilities() public - This was the minimal change needed to allow McpServer to check existing capability values. The method already existed as private; we simply changed visibility.

2. Defaulting at registration time - We considered moving the default elsewhere (e.g., constructor), but listChanged: true only makes sense when tools/resources/prompts are actually registered. Keeping the default at registration time preserves this semantic.

3. Nullish coalescing (?? true) - This ensures:

   • Explicit false → stays false
   • Explicit true → stays true
   • Not set (undefined) → defaults to true (backwards compatible)

4. No breaking changes - Existing code that doesn't set listChanged continues to get listChanged: true as before.

Test Plan

Added 4 tests:

• should default tools.listChanged to true when not explicitly set
• should respect tools.listChanged: false when explicitly set
• should respect resources.listChanged: false when explicitly set
• should respect prompts.listChanged: false when explicitly set

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Latest commit: 292c34a

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commit: 292c34a

[claude]

Code review

No issues found. Checked for bugs and CLAUDE.md compliance.