fix: transformer function being undefined and not receiving output (#18)

Author: iboughtbed

.changeset/wild-insects-tell.md

@@ -0,0 +1,5 @@
+---
+"trpc-to-mcp": patch
+---
+
+fix: transformer function being undefined and not receiving output

src/server.ts

@@ -15,7 +15,6 @@ import type {
 } from "@trpc/server/unstable-core-do-not-import";
 
 import { extractToolsFromProcedures, type McpTool } from "./tools";
-import type { McpMeta } from "./types";
 
 export function setRequestHandler<TRecord extends RouterRecord>(
   server: Server,
@@ -42,12 +41,10 @@ export function setRequestHandler<TRecord extends RouterRecord>(
       trpcCaller,
     );
 
-    const meta = procedure.meta as McpMeta | undefined;
-    const transformMcpProcedure = meta?.mcp?.transformMcpProcedure;
-
-    if (typeof transformMcpProcedure === "function") {
+    if (typeof tool.transformMcpProcedure === "function") {
       // @ts-expect-error path in router
-      const result = await transformMcpProcedure(procedure(args));
+      const output = await procedure(args);
+      const result = await tool.transformMcpProcedure(output);
 
       return {
         content: result,

src/tools.ts

@@ -7,13 +7,14 @@ import type {
 } from "@trpc/server/unstable-core-do-not-import";
 import { z } from "zod/v4";
 
-import type { McpMeta } from "./types";
+import type { McpMeta, TransformMcpProcedureFunction } from "./types";
 
 export type McpTool = {
   name: string;
   description: string;
   pathInRouter: string[];
   inputSchema?: z.core.JSONSchema.JSONSchema;
+  transformMcpProcedure?: TransformMcpProcedureFunction;
 };
 
 export function mergeInputs(inputs: z.ZodObject[]) {
@@ -44,6 +45,7 @@ export function extractToolsFromProcedures<
         name: meta?.mcp?.name ?? name.replace(/\./g, "_"),
         description: meta?.mcp?.description ?? "",
         pathInRouter,
+        transformMcpProcedure: meta?.mcp?.transformMcpProcedure,
       };
 
       if (schema) {

src/types.ts

@@ -10,8 +10,10 @@ export type McpMeta = {
     enabled?: boolean;
     description?: string;
     name?: string;
-    transformMcpProcedure?: (
-      output: inferProcedureOutput<AnyProcedure>,
-    ) => MaybePromise<ContentBlock[]>;
+    transformMcpProcedure?: TransformMcpProcedureFunction;
   };
 };
+
+export type TransformMcpProcedureFunction = (
+  output: inferProcedureOutput<AnyProcedure>,
+) => MaybePromise<ContentBlock[]>;