Copy page

MCP

ZuploMcpSdk

The ZuploMcpSdk class provides a helper API for custom MCP tool handlers to interact with the MCP runtime. Use it to read metadata from the incoming tools/call request and to override fields of the tool result the gateway sends back to the AI client.

Code
import { ZuploContext, ZuploMcpSdk, ZuploRequest } from "@zuplo/runtime";

export default async function (request: ZuploRequest, context: ZuploContext) {
  const sdk = new ZuploMcpSdk(context);

  // Read the incoming tool call request
  const mcpRequest = sdk.getRawCallToolRequest();
  context.log.info(`Tool called: ${mcpRequest?.params.name}`);

  // Invoke a route on your gateway
  const response = await context.invokeRoute("/todos");

  // Override the content the model sees, keeping auto-derived structuredContent
  sdk.setRawCallToolResult({
    content: [{ type: "text", text: "Fetched the todo list" }],
  });

  return response;
}

ZuploMcpSdk is available in custom tool handlers for both the MCP Server handler and the MCP Gateway. It is exported from @zuplo/runtime.

Methods

setRawCallToolResult(result)

Overrides fields of the MCP tool result the gateway is about to send to the AI client. Every field is independently optional — an absent field keeps the value the gateway derives from the downstream response.

Code
setRawCallToolResult(result: ZuploMcpToolResultOverride): void

Parameters

The result argument is a ZuploMcpToolResultOverride object with the following optional fields:

Field	Type	Description
content	CallToolResult["content"]	Content blocks the model reads. Replaces the gateway's default (the serialized downstream body in a text block).
structuredContent	Record<string, unknown>	The structured form of the tool's output. Must be a JSON object — the MCP 2025-11-25 wire format rejects arrays and scalars.
_meta	Record<string, unknown>	Per-call metadata. Replaces the gateway's default of {}.

Compact summary example

The most useful combination is content alone: a human-readable summary replaces the raw serialized body in the model's context, while structuredContent is still auto-derived from the downstream response. This reduces token consumption without losing data.

Code
import { ZuploContext, ZuploMcpSdk, ZuploRequest } from "@zuplo/runtime";

export default async function (request: ZuploRequest, context: ZuploContext) {
  const response = await context.invokeRoute("/todos");
  new ZuploMcpSdk(context).setRawCallToolResult({
    content: [{ type: "text", text: "Fetched the todo list" }],
  });
  return response;
}

The model sees "Fetched the todo list" instead of the full JSON body, but the structuredContent field still carries the complete payload for spec-compliant clients that read it.

Full override example

Override all available fields at once:

Code
import { ZuploContext, ZuploMcpSdk, ZuploRequest } from "@zuplo/runtime";

export default async function (request: ZuploRequest, context: ZuploContext) {
  const response = await context.invokeRoute("/orders");
  const orders = await response.json();

  new ZuploMcpSdk(context).setRawCallToolResult({
    content: [
      {
        type: "text",
        text: `Found ${orders.length} orders totaling $${orders.total}`,
      },
    ],
    structuredContent: { orders: orders.items, count: orders.length },
    _meta: { source: "order-service", version: "2.0" },
  });

  return response;
}

How context lookup works

setRawCallToolResult writes the override to the context that carried the matching tools/call request. The lookup travels exactly one context.invokeRoute generation up from the context passed to the constructor.

This means:

• In a typical custom tool handler that calls context.invokeRoute, the override is written to the parent context (the one carrying the tools/call request), which is correct.
• If your handler calls invokeRoute to a route that also calls invokeRoute, and you construct ZuploMcpSdk in that grandchild invocation, the override is stored on the immediate parent — not the grandparent that carries the tool call. The gateway never reads it. Construct ZuploMcpSdk in the handler that is one level below the MCP route.

getRawCallToolRequest()

Retrieves the original MCP tools/call request object from the context. Use this to access metadata like the _meta field from the incoming tool call.

Code
getRawCallToolRequest(): CallToolRequest | null

Returns the CallToolRequest object, or null if no MCP tool call is in flight on the current or parent context.

Code
import { ZuploContext, ZuploMcpSdk, ZuploRequest } from "@zuplo/runtime";

export default async function (request: ZuploRequest, context: ZuploContext) {
  const sdk = new ZuploMcpSdk(context);
  const mcpRequest = sdk.getRawCallToolRequest();

  if (mcpRequest) {
    const meta = mcpRequest.params._meta;
    context.log.info(`Incoming _meta: ${JSON.stringify(meta)}`);
  }

  // ... handle the tool call
}

Unlike setRawCallToolResult, this method can be called any number of times — the request object is not consumed.

Output schema enforcement

When a tool advertises an outputSchema (via includeOutputSchema on the handler or route config), the gateway validates the tool result's structuredContent against that schema before sending it to the client. A result whose structuredContent does not conform to the advertised schema fails the call with a diagnostic isError result instead of a raw protocol error.

This enforcement is skipped on the error path (isError: true), matching the MCP SDK's own behavior.

includeOutputSchema implies includeStructuredContent

Advertising an outputSchema while returning no structuredContent makes the advertised schema inaccurate — spec-compliant clients reject the call. When includeOutputSchema resolves to true, the gateway automatically forces includeStructuredContent to true as well, and logs a warning naming the route where the override kicked in.

You can set both explicitly to avoid the warning:

Code
{
  "x-zuplo-route": {
    "handler": {
      "export": "mcpServerHandler",
      "module": "$import(@zuplo/runtime)",
      "options": {
        "includeOutputSchema": true,
        "includeStructuredContent": true
      }
    }
  }
}

Type reference

ZuploMcpToolResultOverride

Code
interface ZuploMcpToolResultOverride {
  content?: CallToolResult["content"];
  structuredContent?: Record<string, unknown>;
  _meta?: Record<string, unknown>;
}

The subset of a tools/call result that a module author may override via setRawCallToolResult. Every field is independently optional.

See also

• MCP Server custom tools — how to build custom MCP tool handlers with TypeScript
• MCP Server handler — handler configuration reference including includeOutputSchema and includeStructuredContent
• MCP Gateway introduction — overview of the MCP Gateway product
• MCP specification — the canonical protocol reference