MCP should return empty instead of error if tools / prompts / resources are not defined

[theodormarcu]

Describe the bug
Currently, if you set up a server with the tools defined, but no prompts / resources, attempting to get tools/resources will throw an error instead of returning empty.

To Reproduce
Steps to reproduce the behavior:

1. Set up an MCP server + client
2. Define a test tool
3. Try calling listPrompts or listResources

Expected behavior

All list* methods (that return collections) should return empty arrays when there is nothing to return.

get* should throw an error.

Logs

Additional context

Returning an empty array is a idiomatic pattern when a collection of items is empty.

Express.js

When you query routes that don't match anything, Express returns a 404 rather than crashing
app.get('/api/items') will return an empty array [] when no items exist, not an error

React

When mapping over an empty array of items, React doesn't throw errors - it renders nothing
{items.map(item => <Component key={item.id} {...item} />)} works fine when items is empty

PostgreSQL / Sequelize

User.findAll() returns an empty array if no users exist
It only throws if the table itself doesn't exist (which would be a structural issue)

Next.js

getStaticPaths() can return an empty array if there are no dynamic routes
It doesn't error out because an empty set of paths is valid

Redux

Selectors return empty arrays or default values when querying state that doesn't exist
const todos = useSelector(state => state.todos || []) handles missing state gracefully

[TheHogginator]

The problem here is the McpServer only enables the capabilities once you register the first prompt/resource etc. So the behaviour is correct I'd say.

If you want to advertise the capability without necessarily registering a handler you can pass capabilities in the constructor

const server = new McpServer({
  name: "Demo",
  version: "1.0.0"
}, {
  capabilities: {
    prompts: {},
    resources: {}
  }
});

[rikhuijzer]

Relatedly, a dynamic resource (note the ResourceTemplate) like

server.resource(
  "user-profile",
  new ResourceTemplate("users://{userId}/profile", { list: undefined }),
  async (uri, { userId }) => ({
    contents: [{
      uri: uri.href,
      text: `Profile data for user ${userId}`
    }]
  })
);

will not be listed when for example requesting the resources with npx @modelcontextprotocol/inspector. But a static one will:

server.resource(
  "greeting",
  "test://greeting",
  async (uri, { name }) => ({
    contents: [{
      uri: uri.href,
      text: `Hello, ${name}`
    }]
  })
);

[Toavina23]

Relatedly, a dynamic resource (note the ResourceTemplate) like

server.resource(
"user-profile",
new ResourceTemplate("users://{userId}/profile", { list: undefined }),
async (uri, { userId }) => ({
contents: [{
uri: uri.href,
text: Profile data for user ${userId}
}]
})
);
will not be listed when for example requesting the resources with npx @modelcontextprotocol/inspector. But a static one will:

server.resource(
"greeting",
"test://greeting",
async (uri, { name }) => ({
contents: [{
uri: uri.href,
text: Hello, ${name}
}]
})
);

I was confused by this behavior today while trying to create a dymanic resource, and did not see it in the inspector. Can I have a indication of how to test my dynamic resources with the inspector please ?

[lamemind]

The problem here is the McpServer only enables the capabilities once you register the first prompt/resource etc. So the behaviour is correct I'd say.

If you want to advertise the capability without necessarily registering a handler you can pass capabilities in the constructor

const server = new McpServer({
name: "Demo",
version: "1.0.0"
}, {
capabilities: {
prompts: {},
resources: {}
}
});

Hello,
Claude Desktop floods log file with "Method not found" errors.
This should not be considered the correct behavior IMO

Thanks