fix(mcp): harden tool schemas and dispatch (#6815)

apps/docs/docs/management/mcp.mdx

@@ -8,18 +8,17 @@ tags:
 
 # Model Context Protocol (MCP)
 
-Homarr exposes a stateless [Model Context Protocol](https://modelcontextprotocol.io) server at `/api/mcp`. It maps the
-current Homarr API to tools for compatible AI clients.
+Connect an AI client to Homarr to manage boards, apps, integrations, and other services through MCP tools.
+
+Homarr uses MCP v2 (protocol `2026-07-28`) at `/api/mcp`, with compatibility for 2025 Streamable HTTP clients.
+Requests are stateless: each request authenticates independently, without an MCP session ID.
 
 ![The AI / MCP management tab](./img/mcp.png)
 
-The endpoint implements MCP `2026-07-28` and accepts 2025 Streamable HTTP clients through a compatibility path. The old
-`/api/mcp/mcp` route remains as an alias; use `/api/mcp` for new clients.
+## Connect a client
 
-## Configure a client
-
-Create an API key under **Management → Tools → API → Authentication**, then add the endpoint and header to the client's
-MCP configuration:
+Create an API key under **Management → Tools → API → Authentication**. Add this configuration to your client,
+replacing the URL and API key:
 
 ```json
 {
@@ -34,7 +33,10 @@ MCP configuration:
 }
 ```
 
-Clients without Streamable HTTP support can use `mcp-remote` over STDIO:
+Use the complete key, including the ID, dot, and token. Keep it private and use HTTPS for remote connections.
+The **AI / MCP** tab on Homarr's API page provides your endpoint, configuration, and available tools.
+
+For clients that only support STDIO, use `mcp-remote`:
 
 ```json
 {
@@ -47,29 +49,29 @@ Clients without Streamable HTTP support can use `mcp-remote` over STDIO:
 }
 ```
 
-The **AI / MCP** tab on Homarr's API page shows the current endpoint, configuration, and generated tool catalog. Filter
-the catalog by namespace, tool name, or description; matching categories expand automatically.
+### OAuth
 
-## Authentication and permissions
+Clients with OAuth support can authorize through OAuth 2.1 with PKCE instead of an API key.
+Homarr publishes discovery metadata under `/.well-known/`.
 
-MCP supports API keys and OAuth 2.1 with PKCE. API keys are simpler for direct client configuration; OAuth is available
-for clients that support dynamic authorization.
+If your reverse proxy does not preserve the public host and protocol, set `BASE_URL` to your public origin,
+such as `https://homarr.example.com`, without a path.
 
-Copy the complete generated API key, including both parts and the dot: `<id>.<token>`. Sending only the ID or token
-returns `invalid_token`. Keep the full key private when sharing client configuration or diagnostics.
+## Permissions and tools
 
-When Homarr is behind a reverse proxy that does not preserve the public host and protocol, set `BASE_URL` to the exact
-public origin, for example `https://homarr.example.com`. Do not include a path. OAuth clients discover the authorization
-server and protected resource through Homarr's `/.well-known/` routes.
+Tools use the permissions of the API key owner or OAuth user. Queries read data; mutations change it.
+Inspect the **AI / MCP** tab or your client's tool list for available actions.
 
-Tools use the permissions of the API key owner or authorized user. Creating a key does not grant additional access.
-Custom Widget and Workshop authoring tools, prompts, and resources are visible only to administrators.
+Custom Widget and Workshop authoring tools, prompts, and resources require administrator access.
+For Custom Widgets, follow [Connect an agent](./custom-widgets/agent-authoring).
+Secret values are configured in Homarr and are never returned through MCP.
 
-Use HTTPS when the endpoint crosses an untrusted network.
+The built-in [Assistant](./assistant) shares the MCP tool catalog, uses your Homarr session, and asks for approval
+before mutations by default.
 
-## Verify the endpoint
+## Verify the connection
 
-The 2025 compatibility path accepts a compact `tools/list` request:
+This request lists tools through the older-client compatibility path. It checks authentication and tool discovery:
 
 ```sh
 curl -X POST \
@@ -80,25 +82,14 @@ curl -X POST \
   https://homarr.example.com/api/mcp
 ```
 
-The removed HTTP+SSE `/sse` and `/message` transports return `410 Gone`.
+A successful discovery or initialization response alone does not verify that tools can be listed.
 
-Successful `server/discover` or `initialize` responses confirm connectivity, but do not validate the tool catalog.
-Use `tools/list` above to check that the client can discover tools. If it returns
-`Date cannot be represented in JSON Schema`, the server failed to generate a tool schema; changing the API key will
-not resolve that error. Update Homarr to a release containing the invitation date schema fix.
+## Troubleshooting
 
-## Tools and resources
-
-The generated catalog includes available board, app, integration, Docker, Kubernetes, media, DNS, download, and other
-Homarr procedures. Queries are read-only; mutations change state. The exact catalog depends on the release and current
-user permissions, so inspect `tools/list` or the table in Homarr instead of relying on a static list.
-
-The `invite_createInvite` tool accepts `expirationDate` as an ISO 8601 timestamp with a timezone, such as
-`2026-12-01T18:00:00Z` or `2026-12-01T19:00:00+01:00`.
-
-For Custom Widgets, install the official skill and follow the preview-and-evidence workflow described in
-[Connect an agent](./custom-widgets/agent-authoring). Secret values are configured in Homarr and are never returned
-through MCP.
-
-The built-in [Assistant](./assistant) uses the same tool catalog with the current Homarr session and asks for approval
-before mutations by default.
+- **`invalid_token`**: copy the complete API key in `<id>.<token>` format.
+- **Missing tools**: check the user's permissions. Invalid tool schemas and duplicate names are omitted; check server
+  logs for `MCP tool omitted from catalog` and report the diagnostic with your Homarr version. Exclude API keys.
+- **`Date cannot be represented in JSON Schema`**: update Homarr. Older versions could fail to list all tools because
+  of an unsupported date schema.
+- **`410 Gone` on `/sse` or `/message`**: use `/api/mcp` with a Streamable HTTP client. The old `/api/mcp/mcp` URL remains
+  an alias.

apps/nextjs/src/app/api/assistant/chat/route.ts

@@ -7,7 +7,7 @@ import { createUIMessageStream, createUIMessageStreamResponse, jsonSchema, stepC
 import { cookies } from "next/headers";
 import { z } from "zod/v4";
 
-import { createTRPCContext, mcpRouter } from "@homarr/api/mcp";
+import { callMcpTool, createTRPCContext, mcpRouter } from "@homarr/api/mcp";
 import {
   createAssistantGenerationAccessToken,
   getAssistantRequestContextEntitiesAsync,
@@ -547,19 +547,7 @@ export async function POST(request: Request) {
             }
             if (contextRequestKey !== null) loadedCustomWidgetContextRequests.add(contextRequestKey);
             try {
-              const procedure = mcpTool.pathInRouter.reduce<unknown>(
-                (current, segment) =>
-                  (typeof current === "object" || typeof current === "function") && current !== null
-                    ? (current as Record<string, unknown>)[segment]
-                    : undefined,
-                caller,
-              );
-              if (typeof procedure !== "function") {
-                throw new Error("Procedure not callable");
-              }
-              const result = await (procedure as (value: unknown) => Promise<unknown>)(
-                executionInput && Object.keys(executionInput as object).length > 0 ? executionInput : undefined,
-              );
+              const result = await callMcpTool(caller, mcpTool, executionInput);
               customWidgetDiscoveryPhase.observe(mcpTool.name, result);
               return toAssistantToolOutput(result, {
                 maxCharacters: getAssistantToolOutputMaxCharacters(mcpTool.name),

apps/nextjs/src/app/api/mcp/[transport]/route.spec.ts

@@ -2,8 +2,11 @@
 
 import { beforeEach, describe, expect, test, vi } from "vitest";
 
+import type * as CatalogModule from "../_extract-tools";
+
 const mocks = vi.hoisted(() => ({
   authenticate: vi.fn(),
+  extractCatalog: vi.fn(),
   ipAddress: vi.fn(() => "127.0.0.1"),
   toolProcedure: vi.fn(),
   loggerWarn: vi.fn(),
@@ -14,7 +17,9 @@ const mocks = vi.hoisted(() => ({
 }));
 
 vi.mock("next/server", () => ({ userAgent: () => ({ ua: "MCP route test" }) }));
-vi.mock("@homarr/api/mcp", () => ({
+vi.mock("@homarr/api/mcp", async () => ({
+  callMcpTool: (await import("../../../../../../../packages/api/src/mcp-tools")).callMcpTool,
+  extractMcpToolsFromProcedures: mocks.extractCatalog,
   createTRPCContext: vi.fn(() => ({})),
   mcpRouter: { createCaller: vi.fn(() => ({ board: { getAllBoards: mocks.toolProcedure } })) },
 }));
@@ -54,6 +59,7 @@ vi.mock("../_extract-tools", () => ({
       description: "List boards",
       type: "query",
       pathInRouter: ["board", "getAllBoards"],
+      inputMode: "none",
       inputSchema: { type: "object", properties: {} },
     },
     {
@@ -61,6 +67,7 @@ vi.mock("../_extract-tools", () => ({
       description: "List Custom Widgets",
       type: "query",
       pathInRouter: ["customWidget", "list"],
+      inputMode: "none",
       inputSchema: { type: "object", properties: {} },
     },
     {
@@ -68,6 +75,7 @@ vi.mock("../_extract-tools", () => ({
       description: "Search Workshop",
       type: "query",
       pathInRouter: ["customWidget", "workshopSearch"],
+      inputMode: "none",
       inputSchema: { type: "object", properties: {} },
     },
   ],
@@ -305,3 +313,21 @@ describe("MCP protocol errors", () => {
     expect(metadata).not.toHaveProperty("message");
   });
 });
+
+test("logs catalog omissions only when the shared catalog is first built", async () => {
+  const diagnostic = { name: "broken", path: "broken", reason: "invalid_schema" };
+  const tools = [{ name: "valid" }];
+  mocks.extractCatalog.mockReturnValue({ tools, diagnostics: [diagnostic] });
+  const { extractMcpTools } = await vi.importActual<typeof CatalogModule>("../_extract-tools");
+  expect(extractMcpTools()).toBe(tools);
+  expect(extractMcpTools()).toBe(tools);
+  expect(mocks.extractCatalog).toHaveBeenCalledTimes(1);
+  expect(mocks.loggerWarn).toHaveBeenCalledExactlyOnceWith("MCP tool omitted from catalog", diagnostic);
+});
+
+test("rejects unauthenticated tool calls before execution", async () => {
+  mocks.authenticate.mockResolvedValue(null);
+  const { response } = await callMcp("tools/call", { name: "board_getAllBoards", arguments: {} });
+  expect(response.status).toBe(401);
+  expect(mocks.toolProcedure).not.toHaveBeenCalled();
+});

apps/nextjs/src/app/api/mcp/_extract-tools.ts

@@ -1,11 +1,12 @@
 import { extractMcpToolsFromProcedures, mcpRouter } from "@homarr/api/mcp";
+import { createLogger } from "@homarr/core/infrastructure/logs";
 
+const logger = createLogger({ module: "mcp-catalog" });
 let cache: ReturnType<typeof extractMcpToolsFromProcedures> | null = null;
 
 export function extractMcpTools() {
-  if (cache) return cache;
-
+  if (cache) return cache.tools;
   cache = extractMcpToolsFromProcedures(mcpRouter);
-
-  return cache;
+  for (const diagnostic of cache.diagnostics) logger.warn("MCP tool omitted from catalog", diagnostic);
+  return cache.tools;
 }

apps/nextjs/src/app/api/mcp/_protocol.spec.ts

@@ -1,12 +1,18 @@
 // @vitest-environment node
 
+import { initTRPC, TRPCError } from "@trpc/server";
 import { describe, expect, test, vi } from "vitest";
 import { z } from "zod/v4";
 
 import type { McpTool } from "@homarr/api/mcp";
+import { appManageSchema } from "@homarr/validation/app";
+import { extractMcpToolsFromProcedures } from "@homarr/api/mcp";
+import type { McpMeta } from "../../../../../../packages/api/src/mcp-tools";
 
 import { createMcpProtocolHandler } from "./_protocol";
 
+vi.mock("@homarr/api/mcp", async () => import("../../../../../../packages/api/src/mcp-tools"));
+
 const modernMetadata = {
   "io.modelcontextprotocol/protocolVersion": "2026-07-28",
   "io.modelcontextprotocol/clientInfo": { name: "homarr-test", version: "1.0.0" },
@@ -52,7 +58,7 @@ const echoTool: McpTool = {
   description: "Echo a message",
   type: "query",
   pathInRouter: ["echo"],
-  inputValidator: echoValidator,
+  inputMode: "object",
   inputSchema: z.toJSONSchema(echoValidator),
 };
 
@@ -163,3 +169,197 @@ describe("MCP v2 protocol", () => {
     expect((await parseResponse(listResponse)).result.tools).toEqual([expect.objectContaining({ name: "echo" })]);
   });
 });
+
+describe.each([false, true])("MCP argument dispatch (modern=%s)", (modern) => {
+  const trpc = initTRPC.context<{ user: string }>().meta<McpMeta>().create();
+  const exposed = trpc.procedure.meta({ mcp: { enabled: true } });
+
+  const call = async (
+    handler: ReturnType<typeof createMcpProtocolHandler>,
+    name: string,
+    argumentsValue: Record<string, unknown>,
+  ) => {
+    let request = createRequest("tools/call", { name, arguments: argumentsValue }, name);
+    if (!modern) {
+      request = new Request("http://homarr.test/api/mcp", {
+        method: "POST",
+        headers: { "Content-Type": "application/json", Accept: "application/json, text/event-stream" },
+        body: JSON.stringify({
+          jsonrpc: "2.0",
+          id: 1,
+          method: "tools/call",
+          params: { name, arguments: argumentsValue },
+        }),
+      });
+    }
+    const response = await handler.fetch(request);
+    expect(response.headers.has("Mcp-Session-Id")).toBe(false);
+    return (await parseResponse(response)).result;
+  };
+
+  test("transforms once and does not coerce, insert defaults, or remove properties before tRPC", async () => {
+    const transform = vi.fn((value: string) => `${value}!`);
+    const execution = vi.fn((input: unknown) => input);
+    const router = trpc.router({
+      update: exposed
+        .input(
+          z.looseObject({
+            name: z.string().transform(transform),
+            limit: z.number().default(7),
+          }),
+        )
+        .mutation(({ input }) => execution(input)),
+    });
+    const catalog = extractMcpToolsFromProcedures(router);
+    expect(catalog.diagnostics).toEqual([]);
+    const realCaller = router.createCaller({ user: "one" });
+    const dispatch = vi.fn(realCaller.update);
+    const handler = createMcpProtocolHandler({
+      caller: { update: dispatch },
+      tools: catalog.tools,
+      version: "test",
+      instructions: "test",
+      formatToolError: () => "Invalid input",
+    });
+    const argumentsValue = { name: "hello", extra: "preserved" };
+    const result = await call(handler, "update", argumentsValue);
+    expect(result.isError).not.toBe(true);
+    expect(dispatch).toHaveBeenCalledWith(argumentsValue);
+    expect(transform).toHaveBeenCalledTimes(1);
+    expect(execution).toHaveBeenCalledWith({ name: "hello!", limit: 7, extra: "preserved" });
+    await call(handler, "update", { name: "hello", limit: "7" });
+    expect(dispatch).toHaveBeenCalledTimes(1);
+    expect(execution).toHaveBeenCalledTimes(1);
+  });
+
+  test("retains overlapping chained constraints, refinements, and empty object defaults", async () => {
+    const execution = vi.fn((input: unknown) => input);
+    const router = trpc.router({
+      bounded: exposed
+        .input(z.object({ count: z.number().min(2) }))
+        .input(z.object({ count: z.number().max(5) }))
+        .mutation(({ input }) => execution(input)),
+      refined: exposed
+        .input(z.object({ value: z.string().refine((value) => value !== "blocked") }))
+        .mutation(({ input }) => execution(input)),
+      defaults: exposed.input(z.object({ count: z.number().default(3) })).query(({ input }) => input),
+      none: exposed.query(({ input }) => ({ absent: input === undefined })),
+    });
+    const catalog = extractMcpToolsFromProcedures(router);
+    expect(catalog.diagnostics).toEqual([]);
+    const handler = createMcpProtocolHandler({
+      caller: router.createCaller({ user: "one" }),
+      tools: catalog.tools,
+      version: "test",
+      instructions: "test",
+      formatToolError: () => "Invalid input",
+    });
+    for (const count of [1, 6]) expect((await call(handler, "bounded", { count })).isError).toBe(true);
+    expect((await call(handler, "refined", { value: "blocked" })).isError).toBe(true);
+    expect(execution).not.toHaveBeenCalled();
+    expect((await call(handler, "bounded", { count: 3 })).isError).not.toBe(true);
+    expect(execution).toHaveBeenCalledWith({ count: 3 });
+    expect((await call(handler, "defaults", {})).content).toEqual([{ type: "text", text: '{"count":3}' }]);
+    expect((await call(handler, "none", {})).content).toEqual([{ type: "text", text: '{"absent":true}' }]);
+  });
+
+  test("accepts app inputs that tRPC normalizes before checking constraints", async () => {
+    const mutation = vi.fn((input: unknown) => input);
+    const router = trpc.router({ create: exposed.input(appManageSchema).mutation(({ input }) => mutation(input)) });
+    const { tools, diagnostics } = extractMcpToolsFromProcedures(router);
+    expect(diagnostics).toEqual([]);
+    const handler = createMcpProtocolHandler({
+      caller: router.createCaller({ user: "admin" }),
+      tools,
+      version: "test",
+      instructions: "test",
+      formatToolError: () => "Invalid input",
+    });
+    const argumentsValue = {
+      name: `${" ".repeat(64)}App `,
+      description: " description ",
+      iconUrl: " icon ",
+      href: null,
+      pingUrl: " https://example.com ",
+    };
+    expect((await call(handler, "create", argumentsValue)).isError).not.toBe(true);
+    expect(mutation).toHaveBeenCalledExactlyOnceWith({
+      name: "App",
+      description: "description",
+      iconUrl: "icon",
+      href: null,
+      pingUrl: "https://example.com",
+    });
+    expect((await call(handler, "create", { ...argumentsValue, pingUrl: " javascript:alert(1) " })).isError).toBe(true);
+    expect(mutation).toHaveBeenCalledTimes(1);
+  });
+
+  test("accepts Zod ISO timestamps without seconds while rejecting invalid dates", async () => {
+    const mutation = vi.fn((input: unknown) => input);
+    const router = trpc.router({
+      invite: exposed
+        .input(z.object({ expirationDate: z.iso.datetime({ offset: true }) }))
+        .mutation(({ input }) => mutation(input)),
+    });
+    const { tools, diagnostics } = extractMcpToolsFromProcedures(router);
+    expect(diagnostics).toEqual([]);
+    const handler = createMcpProtocolHandler({
+      caller: router.createCaller({ user: "admin" }),
+      tools,
+      version: "test",
+      instructions: "test",
+      formatToolError: () => "Invalid input",
+    });
+    for (const expirationDate of ["2026-12-01T18:00Z", "2026-12-01T19:00+01:00"]) {
+      expect((await call(handler, "invite", { expirationDate })).isError).not.toBe(true);
+      expect(mutation).toHaveBeenLastCalledWith({ expirationDate });
+    }
+    for (const expirationDate of ["2026-02-30T18:00Z", "not-a-date"]) {
+      expect((await call(handler, "invite", { expirationDate })).isError).toBe(true);
+    }
+    expect(mutation).toHaveBeenCalledTimes(2);
+  });
+
+  test("enforces tRPC permissions before executing mutations", async () => {
+    const mutation = vi.fn(() => "changed");
+    const router = trpc.router({
+      restricted: exposed
+        .use(({ ctx, next }) => {
+          if (ctx.user !== "admin") throw new TRPCError({ code: "FORBIDDEN" });
+          return next();
+        })
+        .input(z.object({}))
+        .mutation(mutation),
+    });
+    const { tools, diagnostics } = extractMcpToolsFromProcedures(router);
+    expect(diagnostics).toEqual([]);
+    const handler = createMcpProtocolHandler({
+      caller: router.createCaller({ user: "guest" }),
+      tools,
+      version: "test",
+      instructions: "test",
+      formatToolError: () => "Forbidden",
+    });
+    expect((await call(handler, "restricted", {})).isError).toBe(true);
+    expect(mutation).not.toHaveBeenCalled();
+  });
+
+  test("isolates authenticated callers when reusing catalog schemas", async () => {
+    const router = trpc.router({ who: exposed.query(({ ctx }) => ctx.user) });
+    const { tools } = extractMcpToolsFromProcedures(router);
+    const handlers = ["alice", "bob"].map((user) =>
+      createMcpProtocolHandler({
+        caller: router.createCaller({ user }),
+        tools,
+        version: "test",
+        instructions: "test",
+        formatToolError: () => "Failed",
+      }),
+    );
+    const results = await Promise.all(handlers.map((handler) => call(handler, "who", {})));
+    expect(results.map((result) => result.content)).toEqual([
+      [{ type: "text", text: '"alice"' }],
+      [{ type: "text", text: '"bob"' }],
+    ]);
+  });
+});

apps/nextjs/src/app/api/mcp/_protocol.ts

@@ -1,5 +1,6 @@
-import { createMcpHandler, McpServer } from "@modelcontextprotocol/server";
+import { createMcpHandler, fromJsonSchema, McpServer } from "@modelcontextprotocol/server";
 
+import { callMcpTool } from "@homarr/api/mcp";
 import type { McpTool } from "@homarr/api/mcp";
 
 import { toAssistantToolOutput } from "../assistant/chat/assistant-tool-output";
@@ -15,15 +16,6 @@ interface CreateMcpProtocolHandlerOptions {
   onProtocolError?: (error: Error) => void;
 }
 
-const resolveProcedure = (caller: unknown, path: string[]) =>
-  path.reduce<unknown>(
-    (current, segment) =>
-      (typeof current === "object" || typeof current === "function") && current !== null
-        ? (current as Record<string, unknown>)[segment]
-        : undefined,
-    caller,
-  );
-
 export const createMcpProtocolHandler = ({
   caller,
   tools,
@@ -52,20 +44,11 @@ export const createMcpProtocolHandler = ({
           tool.name,
           {
             description: tool.description,
-            inputSchema: tool.inputValidator,
+            inputSchema: fromJsonSchema(tool.inputSchema as Parameters<typeof fromJsonSchema>[0]),
           },
           async (args) => {
             try {
-              const procedure = resolveProcedure(caller, tool.pathInRouter);
-              if (typeof procedure !== "function") {
-                return {
-                  content: [{ type: "text" as const, text: `Tool "${tool.name}" is not callable` }],
-                  isError: true,
-                };
-              }
-
-              const input = Object.keys(args).length > 0 ? args : undefined;
-              const result = await (procedure as (value: unknown) => Promise<unknown>)(input);
+              const result = await callMcpTool(caller, tool, args);
               return {
                 content: [
                   {

packages/api/package.json

@@ -51,11 +51,13 @@
     "@homarr/validation": "workspace:^0.1.0",
     "@homarr/workshop": "workspace:^0.1.0",
     "@kubernetes/client-node": "catalog:",
+    "@modelcontextprotocol/server": "catalog:",
     "@tanstack/react-query": "catalog:",
     "@trpc/client": "catalog:",
     "@trpc/react-query": "catalog:",
     "@trpc/server": "catalog:",
     "@trpc/tanstack-react-query": "catalog:",
+    "ajv": "catalog:",
     "lodash.clonedeep": "catalog:",
     "next": "catalog:",
     "react": "catalog:",

packages/api/src/mcp-tools.ts

@@ -1,3 +1,5 @@
+import { Ajv2020 } from "ajv/dist/2020.js";
+import { addFormats, AjvJsonSchemaValidator } from "@modelcontextprotocol/server/validators/ajv";
 import type { AnyProcedure, AnyRootTypes, Router, RouterRecord } from "@trpc/server/unstable-core-do-not-import";
 import { z } from "zod/v4";
 
@@ -15,23 +17,35 @@ export interface McpTool {
   type: "query" | "mutation";
   pathInRouter: string[];
   inputSchema: z.core.JSONSchema.JSONSchema;
-  inputValidator: z.ZodObject;
+  inputMode: "none" | "object";
 }
 
-const emptyInput = z.object({});
+export interface McpToolDiagnostic {
+  name: string;
+  path: string;
+  reason: "unsupported_input" | "invalid_schema" | "duplicate_name" | "invalid_name";
+}
 
+export interface McpToolCatalog {
+  tools: McpTool[];
+  diagnostics: McpToolDiagnostic[];
+}
+
+const emptyInput = z.strictObject({});
+const schemaEngine = new Ajv2020({
+  strict: false,
+  validateSchema: true,
+  coerceTypes: false,
+  useDefaults: false,
+  removeAdditional: false,
+});
+addFormats(schemaEngine);
+const schemaValidator = new AjvJsonSchemaValidator(schemaEngine);
 const isProcedure = (value: AnyProcedure | RouterRecord): value is AnyProcedure => typeof value === "function";
 
-const mergeInputs = (inputs: z.ZodObject[]) => inputs.reduce((merged, input) => merged.extend(input.shape), emptyInput);
-
-const getObjectInput = (input: z.ZodType) => {
+const getObjectInput = (input: unknown): z.ZodObject | null => {
   if (input instanceof z.ZodObject) return input;
-
-  if (input instanceof z.ZodOptional) {
-    const unwrappedInput = input.unwrap();
-    if (unwrappedInput instanceof z.ZodObject) return unwrappedInput;
-  }
-
+  if (input instanceof z.ZodOptional) return getObjectInput(input.unwrap());
   return null;
 };
 
@@ -50,71 +64,110 @@ const getMcpMetadata = (meta: unknown): McpMeta["mcp"] => {
   return result;
 };
 
-const getProcedureInput = (inputs: z.ZodType[]) => {
-  if (inputs.length === 0) return emptyInput;
-  if (inputs.length === 1) {
-    const input = inputs[0] ?? emptyInput;
-    return getObjectInput(input) ?? input;
-  }
-
-  const objectInputs = inputs.map(getObjectInput);
-  if (objectInputs.every((input): input is z.ZodObject => input !== null)) return mergeInputs(objectInputs);
-  return null;
-};
-
-const normalizeJsonSchema = (schema: z.core.JSONSchema.JSONSchema): z.core.JSONSchema.JSONSchema => {
-  const properties = schema.properties ?? {};
-  const required = (schema.required ?? []).filter((key) => {
-    const property = properties[key];
-    return !property || typeof property !== "object" || !("default" in property);
-  });
-
-  return {
-    ...schema,
-    type: "object",
-    properties,
-    required,
-  };
-};
-
 export function extractMcpToolsFromProcedures<TRoot extends AnyRootTypes, TRecord extends RouterRecord>(
   router: Router<TRoot, TRecord>,
-): McpTool[] {
+): McpToolCatalog {
   const tools: McpTool[] = [];
+  const diagnostics: McpToolDiagnostic[] = [];
+  const names = new Map<string, string[]>();
 
   for (const [path, procedure] of Object.entries(router["_def"].procedures)) {
     if (!isProcedure(procedure)) continue;
     const definition = procedure["_def"];
     if (definition.type === "subscription") continue;
-
     const metadata = getMcpMetadata(definition.meta);
     if (!metadata?.enabled) continue;
 
-    const inputs = definition.inputs.filter((input): input is z.ZodType => input instanceof z.ZodType);
-    const procedureInput = getProcedureInput(inputs);
-
-    if (
-      procedureInput === null ||
-      (!(procedureInput instanceof z.ZodObject) && !procedureInput.safeParse(undefined).success)
-    ) {
-      console.warn(`[MCP] Procedure ${path} does not use an object input schema; using an empty schema`);
+    const name = metadata.name ?? path.replaceAll(".", "_");
+    const paths = names.get(name) ?? [];
+    paths.push(path);
+    names.set(name, paths);
+    if (!/^[a-zA-Z0-9_.-]{1,128}$/.test(name)) {
+      diagnostics.push({ name, path, reason: "invalid_name" });
+      continue;
     }
-    const inputValidator = procedureInput instanceof z.ZodObject ? procedureInput : emptyInput;
 
-    tools.push({
-      name: metadata.name ?? path.replaceAll(".", "_"),
-      description: metadata.description ?? "",
-      type: definition.type,
-      pathInRouter: path.split("."),
-      inputValidator,
-      inputSchema: normalizeJsonSchema(
-        z.toJSONSchema(inputValidator, {
-          io: "input",
-          unrepresentable: "any",
-        }),
-      ),
-    });
+    const inputs = definition.inputs;
+    let inputMode: McpTool["inputMode"] = "object";
+    if (inputs.every((input) => input instanceof z.ZodUndefined || input instanceof z.ZodVoid)) inputMode = "none";
+    const objectInputs = inputs.map(getObjectInput);
+    if (inputMode === "object" && !objectInputs.every((input) => input !== null)) {
+      diagnostics.push({ name, path, reason: "unsupported_input" });
+      continue;
+    }
+
+    try {
+      let input: z.ZodType = emptyInput;
+      if (inputMode === "object") {
+        const objects = objectInputs.filter((schema): schema is z.ZodObject => schema !== null);
+        input = objects[0] ?? emptyInput;
+        for (const object of objects.slice(1)) input = z.intersection(input, object);
+      }
+      const inputSchema = z.toJSONSchema(input, {
+        io: "input",
+        override: ({ zodSchema, jsonSchema }) => {
+          if (zodSchema["_zod"].def.type !== "string") return;
+          if (zodSchema["_zod"].def.checks?.some((check) => check["_zod"].def.check === "overwrite")) {
+            // trim/case normalization runs before these checks in tRPC. They cannot constrain raw JSON.
+            delete jsonSchema.minLength;
+            delete jsonSchema.maxLength;
+            delete jsonSchema.pattern;
+            delete jsonSchema.allOf;
+            delete jsonSchema.format;
+          }
+          // Zod's ISO regex permits minute precision; JSON Schema date-time requires seconds.
+          if (jsonSchema.format === "date-time" && typeof jsonSchema.pattern === "string") {
+            delete jsonSchema.format;
+          }
+        },
+      });
+      // Chained tRPC object inputs produce allOf; MCP requires an explicit object root.
+      if (inputSchema.type !== undefined && inputSchema.type !== "object") throw new Error("Invalid root");
+      inputSchema.type = "object";
+      // Zod and the SDK disagree on JSON Schema vocabulary types; compilation checks the actual schema.
+      schemaValidator.getValidator(inputSchema as Parameters<typeof schemaValidator.getValidator>[0]);
+      tools.push({
+        name,
+        description: metadata.description ?? "",
+        type: definition.type,
+        pathInRouter: path.split("."),
+        inputMode,
+        inputSchema,
+      });
+    } catch {
+      // Conversion errors can contain schema metadata. Only expose a stable, safe reason.
+      diagnostics.push({ name, path, reason: "invalid_schema" });
+    }
   }
 
-  return tools.toSorted((left, right) => left.name.localeCompare(right.name));
+  const duplicates = new Set<string>();
+  for (const [name, paths] of names) {
+    if (paths.length < 2) continue;
+    duplicates.add(name);
+    for (const path of paths) diagnostics.push({ name, path, reason: "duplicate_name" });
+  }
+  return {
+    tools: tools
+      .filter((tool) => !duplicates.has(tool.name))
+      .toSorted((left, right) => left.name.localeCompare(right.name)),
+    diagnostics: diagnostics.toSorted(
+      (left, right) => left.path.localeCompare(right.path) || left.reason.localeCompare(right.reason),
+    ),
+  };
+}
+
+/** Dispatch raw JSON through the caller so tRPC owns parsing, transforms, and authorization. */
+export async function callMcpTool(caller: unknown, tool: McpTool, input: unknown) {
+  let procedure = caller;
+  for (const segment of tool.pathInRouter) {
+    if ((typeof procedure !== "object" && typeof procedure !== "function") || procedure === null) {
+      throw new Error("Tool is not callable");
+    }
+    procedure = (procedure as Record<string, unknown>)[segment];
+  }
+  if (typeof procedure !== "function") throw new Error("Tool is not callable");
+  let procedureInput = input;
+  if (tool.inputMode === "none") procedureInput = undefined;
+  else if (input === undefined) procedureInput = {};
+  return await (procedure as (value: unknown) => Promise<unknown>)(procedureInput);
 }

packages/api/src/mcp.ts

@@ -1,5 +1,5 @@
 export { createTRPCContext } from "./trpc";
-export { extractMcpToolsFromProcedures } from "./mcp-tools";
+export { callMcpTool, extractMcpToolsFromProcedures } from "./mcp-tools";
 export type { McpTool } from "./mcp-tools";
 
 import { createTRPCRouter } from "./trpc";

packages/api/src/test/mcp.spec.ts

@@ -1,9 +1,12 @@
+import { initTRPC } from "@trpc/server";
 import type { TRPCError } from "@trpc/server";
 import { describe, expect, test, vi } from "vitest";
 import { z } from "zod/v4";
 
 import { mcpRouter } from "../mcp";
-import { extractMcpToolsFromProcedures } from "../mcp-tools";
+import type { McpMeta } from "../mcp-tools";
+import { createMcpProtocolHandler } from "../../../../apps/nextjs/src/app/api/mcp/_protocol";
+import { callMcpTool, extractMcpToolsFromProcedures } from "../mcp-tools";
 
 vi.mock("@homarr/auth", () => ({}));
 
@@ -157,7 +160,7 @@ function actualToolInventory() {
     { _def?: { type?: "query" | "mutation" } }
   >;
   return extractMcpToolsFromProcedures(mcpRouter)
-    .map((tool) => {
+    .tools.map((tool) => {
       const procedure = procedures[tool.pathInRouter.join(".")];
       const type = procedure?.["_def"]?.type;
       if (type !== "query" && type !== "mutation") throw new Error(`Unable to classify MCP tool '${tool.name}'`);
@@ -175,7 +178,7 @@ describe("production MCP router", () => {
   });
 
   test("keeps every secret-bearing tool behind a mutation procedure", () => {
-    const tools = extractMcpToolsFromProcedures(mcpRouter);
+    const tools = extractMcpToolsFromProcedures(mcpRouter).tools;
     const procedures = mcpRouter["_def"].procedures as unknown as Record<
       string,
       { _def?: { type?: "query" | "mutation" } }
@@ -189,7 +192,7 @@ describe("production MCP router", () => {
   });
 
   test("gives every production tool a description", () => {
-    const tools = extractMcpToolsFromProcedures(mcpRouter);
+    const tools = extractMcpToolsFromProcedures(mcpRouter).tools;
     for (const tool of tools) expect(tool.description, `Tool ${tool.name} should have a description`).toBeTruthy();
     expect(tools.find((tool) => tool.name === "customWidget_getAuthoringPrompt")?.description).toBe(
       "Get the current Custom Widget authoring instructions.",
@@ -197,16 +200,17 @@ describe("production MCP router", () => {
   });
 });
 
-test("MCP tools are deterministically ordered and retain executable schemas", () => {
-  const tools = extractMcpToolsFromProcedures(mcpRouter);
+test("MCP tools are deterministically ordered and have valid production schemas", () => {
+  expect(extractMcpToolsFromProcedures(mcpRouter).diagnostics).toEqual([]);
+  const tools = extractMcpToolsFromProcedures(mcpRouter).tools;
   const toolNames = tools.map((tool) => tool.name);
 
   expect(toolNames).toEqual(toolNames.toSorted((left, right) => left.localeCompare(right)));
-  expect(tools.every((tool) => tool.inputValidator instanceof z.ZodObject)).toBe(true);
+  expect(tools.every((tool) => tool.inputSchema.type === "object")).toBe(true);
 });
 
 test("publishes both Custom Widget template input formats", () => {
-  const tool = extractMcpToolsFromProcedures(mcpRouter).find(
+  const tool = extractMcpToolsFromProcedures(mcpRouter).tools.find(
     (candidate) => candidate.name === "customWidget_validateTemplate",
   );
 
@@ -355,3 +359,119 @@ describe("custom widget authoring procedure access", () => {
     );
   });
 });
+
+describe("MCP catalog diagnostics", () => {
+  const trpc = initTRPC.meta<McpMeta>().create();
+  const exposed = trpc.procedure.meta({ mcp: { enabled: true } });
+
+  test("omits unsupported parsers and malformed schemas without losing valid tools", () => {
+    const router = trpc.router({
+      good: exposed.input(z.object({ name: z.string() })).query(() => "ok"),
+      date: exposed.input(z.object({ date: z.date() })).query(() => "unused"),
+      scalar: exposed.input(z.string()).query(() => "unused"),
+      parser: exposed.input({ parse: (value: unknown) => value }).query(() => "unused"),
+      malformed: exposed
+        .input(z.object({ value: z.string().meta({ type: "invalid-json-type" }) }))
+        .query(() => "unused"),
+    });
+    const catalog = extractMcpToolsFromProcedures(router);
+    expect(catalog.tools.map((tool) => tool.name)).toEqual(["good"]);
+    expect(catalog.diagnostics).toEqual([
+      { name: "date", path: "date", reason: "invalid_schema" },
+      { name: "malformed", path: "malformed", reason: "invalid_schema" },
+      { name: "parser", path: "parser", reason: "unsupported_input" },
+      { name: "scalar", path: "scalar", reason: "unsupported_input" },
+    ]);
+  });
+
+  test("omits all duplicate names even when one procedure has an invalid schema", () => {
+    const shared = trpc.procedure.meta({ mcp: { enabled: true, name: "shared" } });
+    const catalog = extractMcpToolsFromProcedures(
+      trpc.router({
+        first: shared.query(() => "first"),
+        second: shared.input(z.object({ date: z.date() })).query(() => "second"),
+        good: exposed.query(() => "good"),
+        invalidName: trpc.procedure.meta({ mcp: { enabled: true, name: "bad name" } }).query(() => "unused"),
+      }),
+    );
+    expect(catalog.tools.map((tool) => tool.name)).toEqual(["good"]);
+    expect(catalog.diagnostics.filter((diagnostic) => diagnostic.reason === "duplicate_name")).toEqual([
+      { name: "shared", path: "first", reason: "duplicate_name" },
+      { name: "shared", path: "second", reason: "duplicate_name" },
+    ]);
+    expect(catalog.diagnostics).toContainEqual({ name: "bad name", path: "invalidName", reason: "invalid_name" });
+  });
+
+  test("distinguishes empty object input from no input for shared Assistant dispatch", async () => {
+    const router = trpc.router({
+      none: exposed.query(({ input }) => input),
+      explicitNone: exposed.input(z.undefined()).query(({ input }) => input),
+      object: exposed.input(z.object({ value: z.string().default("default") })).query(({ input }) => input),
+      optional: exposed
+        .input(z.object({ value: z.string().default("default") }).optional())
+        .query(({ input }) => input),
+    });
+    const catalog = extractMcpToolsFromProcedures(router);
+    expect(catalog.diagnostics).toEqual([]);
+    for (const tool of catalog.tools) {
+      const result = await callMcpTool(router.createCaller({}), tool, {});
+      if (tool.inputMode === "none") expect(result).toBeUndefined();
+      else expect(result).toEqual({ value: "default" });
+    }
+  });
+});
+
+test.each([false, true])(
+  "production catalog registers through the real protocol handler (modern=%s)",
+  async (modern) => {
+    const { tools, diagnostics } = extractMcpToolsFromProcedures(mcpRouter);
+    expect(diagnostics).toEqual([]);
+    const handler = createMcpProtocolHandler({
+      caller: {},
+      tools,
+      version: "test",
+      instructions: "test",
+      formatToolError: () => "Tool failed",
+    });
+    const headers: Record<string, string> = {
+      "Content-Type": "application/json",
+      Accept: "application/json, text/event-stream",
+    };
+    const params: Record<string, unknown> = {};
+    if (modern) {
+      headers["Mcp-Method"] = "tools/list";
+      headers["Mcp-Name"] = "homarr";
+      params["_meta"] = {
+        "io.modelcontextprotocol/protocolVersion": "2026-07-28",
+        "io.modelcontextprotocol/clientInfo": { name: "catalog-test", version: "1" },
+        "io.modelcontextprotocol/clientCapabilities": {},
+      };
+    }
+    const response = await handler.fetch(
+      new Request("http://homarr.test/api/mcp", {
+        method: "POST",
+        headers,
+        body: JSON.stringify({ jsonrpc: "2.0", id: 1, method: "tools/list", params }),
+      }),
+    );
+    expect(response.status).toBe(200);
+    expect(response.headers.has("Mcp-Session-Id")).toBe(false);
+    const body = await response.text();
+    let payload = body;
+    if (response.headers.get("Content-Type")?.includes("text/event-stream")) {
+      payload =
+        body
+          .split("\n")
+          .find((line) => line.startsWith("data: "))
+          ?.slice(6) ?? "";
+    }
+    const result = JSON.parse(payload) as {
+      error?: unknown;
+      result: { tools: { name: string; inputSchema: unknown }[] };
+    };
+    expect(result.error).toBeUndefined();
+    expect(result.result.tools.map((tool) => ({ name: tool.name, inputSchema: tool.inputSchema }))).toEqual(
+      tools.map((tool) => ({ name: tool.name, inputSchema: tool.inputSchema })),
+    );
+  },
+);

pnpm-lock.yaml

@@ -366,6 +366,9 @@ catalogs:
     ai:
       specifier: ^7.0.56
       version: 7.0.56
+    ajv:
+      specifier: 8.18.0
+      version: 8.18.0
     assistant-stream:
       specifier: 0.3.34
       version: 0.3.34
@@ -1520,6 +1523,9 @@ importers:
       '@kubernetes/client-node':
         specifier: 'catalog:'
         version: 1.4.0(supports-color@10.2.2)
+      '@modelcontextprotocol/server':
+        specifier: 'catalog:'
+        version: 2.0.0
       '@tanstack/react-query':
         specifier: 'catalog:'
         version: 5.101.2(react@19.2.7)
@@ -1535,6 +1541,9 @@ importers:
       '@trpc/tanstack-react-query':
         specifier: 'catalog:'
         version: 11.18.0(@tanstack/react-query@5.101.2(react@19.2.7))(@trpc/client@11.18.0(@trpc/server@11.18.0(typescript@7.0.2))(typescript@7.0.2))(@trpc/server@11.18.0(typescript@7.0.2))(react@19.2.7)(typescript@7.0.2)
+      ajv:
+        specifier: 'catalog:'
+        version: 8.18.0
       lodash.clonedeep:
         specifier: 'catalog:'
         version: 4.5.0

pnpm-workspace.yaml

@@ -123,6 +123,7 @@ catalog:
   adm-zip: 0.6.0
   acorn: ^8.16.0
   acorn-jsx: ^5.3.2
+  ajv: 8.18.0
   jszip: ^3.10.1
   sql.js: ^1.14.1
   babel-plugin-react-compiler: ^1.0.0