Tools without inputSchema are not visible in some MCP clients

[artemnovichkov]

Is your feature request related to a problem? Please describe.
I experimented with the SDK and found a strange behavior. Tools without inputSchema are not visible in Cursor and Claude Desktop, but work fine in Visual Studio Code. Here is my fix to make them available everywhere:

let tool = Tool(name: "swift_version",
                description: "Returns the current Swift version",
                inputSchema: .object([
                    "type": .string("object")
                ]))

Here is a full code if my example:
https://github.com/artemnovichkov/swift-version-mcp/blob/master/Sources/SwiftVersionMCP/main.swift

I'm not sure is this problem related to the SDK or to the clients.

Describe the solution you'd like
If you want to use the tool without inputSchema, it may be configured like:

let tool = Tool(name: "swift_version",
                description: "Returns the current Swift version")

[stallent]

In their defense, input schema isn't optional for tools per the spec. So i'm not sure this is something that should be changed. From the spec:

{
  name: string;          // Unique identifier for the tool
  description?: string;  // Human-readable description
  inputSchema: {         // JSON Schema for the tool's parameters
    type: "object",
    properties: { ... }  // Tool-specific parameters
  },
  annotations?: {        // Optional hints about tool behavior
    title?: string;      // Human-readable title for the tool
    readOnlyHint?: boolean;    // If true, the tool does not modify its environment
    destructiveHint?: boolean; // If true, the tool may perform destructive updates
    idempotentHint?: boolean;  // If true, repeated calls with same args have no additional effect
    openWorldHint?: boolean;   // If true, tool interacts with external entities
  }
}

I certainly get how without params whats the point, but spec interpretation is always somewhat subjective which is why you are seeing diff behavior from diff tools. Regardless, the spec does say optional for optional things so pretty reasonable to assume its required which is why it doesn't work in Claude Desktop especially, since they created the spec.

[mattt]

Hi @artemnovichkov. I agree with @stallent. The MCP specification is clear that input schema is required, and I think it makes sense to make that parameter explicit and required in the SDK, too.

I do agree with you that the semantics are awkward; I think things like, for example, an environment's swift version or your current location would probably be better represented as a resource. But Claude Desktop and other clients aren't entirely consistent about how resources are treated, so tools are your best bet (even if it's an empty call).

There is a potential footgun we should disarm, though. Value conforms to ExpressibleByNilLiteral. So if you pass inputSchema: nil, that turns into a null value which isn't valid. In a future version, I intend to relax inputSchema to something like any Encodable so that you could use something like JSONSchema instead of Value.