derive MCP output schema from SDK

Author: RhysSullivan

bun.lock

@@ -66,7 +66,8 @@
     "@executor-js/config": "workspace:*",
     "@executor-js/sdk": "workspace:*",
     "@modelcontextprotocol/sdk": "^1.29.0",
-    "effect": "catalog:"
+    "effect": "catalog:",
+    "zod": "^4.3.6"
   },
   "devDependencies": {
     "@effect/atom-react": "catalog:",
@@ -78,8 +79,7 @@
     "bun-types": "catalog:",
     "react": "catalog:",
     "tsup": "catalog:",
-    "vitest": "catalog:",
-    "zod": "^4.3.6"
+    "vitest": "catalog:"
   },
   "peerDependencies": {
     "@effect/atom-react": "catalog:",

packages/plugins/mcp/package.json

@@ -161,8 +161,30 @@ describe("MCP elicitation (end-to-end)", () => {
         },
         required: ["content"],
       });
-      expect(schema?.outputTypeScript).toContain("content: unknown[]");
-      expect(schema?.outputTypeScript).toContain("structuredContent?: unknown");
+      const outputSchema = schema?.outputSchema as {
+        readonly properties: {
+          readonly content: {
+            readonly items: {
+              readonly anyOf: readonly unknown[];
+            };
+          };
+        };
+      };
+      expect(outputSchema.properties.content.items.anyOf).toEqual(
+        expect.arrayContaining([
+          expect.objectContaining({
+            properties: expect.objectContaining({
+              type: { const: "text", type: "string" },
+              text: { type: "string" },
+            }),
+            required: ["type", "text"],
+          }),
+        ]),
+      );
+      expect(schema?.outputTypeScript).toContain('type: "text"');
+      expect(schema?.outputTypeScript).toContain(
+        "structuredContent?: { [k: string]: unknown; }",
+      );
 
       const result = yield* executor.tools.invoke(
         simpleEcho.id,

packages/plugins/mcp/src/sdk/elicitation.test.ts

@@ -14,6 +14,8 @@ import {
 import type { HttpClient } from "effect/unstable/http";
 
 import type { OAuthClientProvider } from "@modelcontextprotocol/sdk/client/auth.js";
+import { CallToolResultSchema } from "@modelcontextprotocol/sdk/types.js";
+import * as z from "zod/v4";
 
 import {
   type CredentialBindingRef,
@@ -319,25 +321,29 @@ const normalizeNamespace = (config: McpSourceConfig): string =>
     command: config.transport === "stdio" ? config.command : undefined,
   });
 
-const mcpCallToolResultOutputSchema = (
-  structuredContentSchema?: unknown,
-): Record<string, unknown> => ({
-  type: "object",
-  properties: {
-    content: {
-      type: "array",
-      items: {},
-    },
-    structuredContent: structuredContentSchema ?? {},
-    isError: { const: false },
-    _meta: {
-      type: "object",
-      additionalProperties: {},
+type JsonSchemaObject = Record<string, unknown> & {
+  readonly properties?: Record<string, unknown>;
+};
+
+const McpCallToolResultJsonSchema = z.toJSONSchema(CallToolResultSchema) as JsonSchemaObject;
+
+const mcpCallToolResultOutputSchema = (structuredContentSchema?: unknown): JsonSchemaObject => {
+  const defaultStructuredContentSchema =
+    McpCallToolResultJsonSchema.properties?.structuredContent ?? {};
+
+  return {
+    ...McpCallToolResultJsonSchema,
+    properties: {
+      ...McpCallToolResultJsonSchema.properties,
+      structuredContent:
+        structuredContentSchema === undefined
+          ? defaultStructuredContentSchema
+          : structuredContentSchema,
+      isError: { const: false },
     },
-  },
-  required: structuredContentSchema === undefined ? ["content"] : ["content", "structuredContent"],
-  additionalProperties: {},
-});
+    required: structuredContentSchema === undefined ? ["content"] : ["content", "structuredContent"],
+  };
+};
 
 const toBinding = (entry: McpToolManifestEntry): McpToolBinding =>
   McpToolBinding.make({