How to add a Global Callable Class which Implements Callable and Register it as a Tool in MCP Server

A "global callable class" in Apex is a class marked global that implements the Callable interface. That means:

• global: it can be accessed outside your package/namespace (so external callers can use it).
• Callable: it exposes a single entry point, call(String action, Map<String, Object> args), and you route to different methods based on action.

In simple terms: it’s a single front door you can use to call multiple backend methods by name.

Utility

• Lets external systems (or tools like MCP) call Apex methods dynamically.
• You can add new “actions” without changing the interface — just add a new when branch.
• Works well for generic integrations where the caller only knows a method name + arguments.

Benefits

• One endpoint, many operations: no need to build separate REST endpoints per method.
• Flexible: can evolve without breaking callers.
• Tool‑friendly: easy for automation tools to call a single “dispatcher”.
• Centralized control: you can validate, log, and secure everything in one place.

A global class Extension which implements Callable can’t be invoked over REST by itself. You still need an API surface (Apex REST, Tooling, or a custom endpoint) that calls Extension.call().

Typical pattern:

• Create an Apex REST class that receives action + args.

• Inside it, instantiate Extension and call extension.call(action, args).

• Your MCP tool calls that REST endpoint.

Here’s the code for Apex side (Callable + REST wrapper), and the MCP tool call in Node.

Apex: callable class

Extension.apxc
global class Extension implements Callable {
  String concatStrings(String stringValue) {
    return stringValue + stringValue;
  }

  Decimal squareNumbers(Decimal decimalValue) {
    return decimalValue * decimalValue;
  }

  global Object call(String action, Map<String, Object> args) {
    switch on action {
      when 'concatStrings' {
        return this.concatStrings((String)args.get('stringValue'));
      }
      when 'squareNumbers' {
        return this.squareNumbers((Decimal)args.get('decimalValue'));
      }
      when else {
        throw new ExtensionMalformedCallException('Method not implemented');
      }
    }
  }

  global class ExtensionMalformedCallException extends Exception {}
}

Apex: REST wrapper

ExtensionCallApi.apxc
@RestResource(urlMapping='/ExtensionCall/*')
global with sharing class ExtensionCallApi {
  @HttpPost
  global static String callExtension() {
    Map<String, Object> body = (Map<String, Object>)JSON.deserializeUntyped(
      RestContext.request.requestBody.toString()
    );

    String action = (String)body.get('action');
    Map<String, Object> args = (Map<String, Object>)body.get('args');

    Extension ext = new Extension();
    Object result = ext.call(action, args == null ? new Map<String, Object>() : args);

    return JSON.serialize(new Map<String, Object>{ 'result' => result });
  }
}

MCP tool (Node)

server.js
mcpServer.registerTool(
  "extensionCall",
  {
    description: "Dispatches to Apex Extension.call via REST.",
    inputSchema: z.object({
      action: z.string(),
      args: z.record(z.any()).optional()
    })
  },
  async ({ action, args }) => {
    const result = await conn.apex.post("/ExtensionCall/", {
      action,
      args: args ?? {}
    });

    return {
      content: [{ type: "text", text: JSON.stringify(result) }],
    };
  }
);

Now, To call this tool, Use the MCP UI with the tool name extensionCall and pass action + args:

For Concatinate:

call extensionCall {"action":"concatStrings","args":{"stringValue":"abc"}}

For Square:

call extensionCall {"action":"squareNumbers","args":{"decimalValue":7}}

Use it from the CLI like this:

powershell
Invoke-RestMethod http://localhost:3000/api/mcp/call -Method Post -ContentType "application/json" `
  -Body '{"server":"Custom MCP","tool":"extensionCall","args":{"action":"concatStrings","args":{"stringValue":"abc"}}}' |
  ConvertTo-Json -Depth 10

For Square:

powershell
Invoke-RestMethod http://localhost:3000/api/mcp/call -Method Post -ContentType "application/json" `
  -Body '{"server":"Custom MCP","tool":"extensionCall","args":{"action":"squareNumbers","args":{"decimalValue":3.5}}}' |
  ConvertTo-Json -Depth 10

You will see the output like this:

Here’s the end‑to‑end flow for easy understanding for a UI tool call, mapped to the files/classes you have now:

1) UI sends the tool call

• Agentforce Prompt Box sends tools/call with:

{ "name": "extensionCall", "arguments": { "action": "...", "args": {...} } }

2) MCP server receives it

• Custom MCP server (server.js) receives the request over stdio.

3) Tool handler runs

• extensionCall handler executes.

• It calls getSfConnection() to create a jsforce.Connection.

4) MCP server calls Salesforce

• Handler sends POST to Apex REST:

/ExtensionCall/

with { action, args }.

5) Apex REST invokes Callable

• ExtensionCallApi receives the request.

• It instantiates Extension and calls Extension.call(action, args).

6) Result flows back

• Apex returns JSON (e.g., { "result": 16 }).

• Tool returns content + structuredContent. (If the result is a string, structuredContent gives error as it expects object/record only. So in that case we simply removed the structuredContent in return block)

server.js
 return {
  content: [{ type: "text", text: JSON.stringify(result) }],
  structuredContent: result //Removed this line
  };

• UI renders the response.

Now, You have a Global Callable Class which contain Multiple Actions and Implements Callable Registered as a Tool in Your Custom MCP Server!