fix(docs): correct MCP server routing to resolve 404 errors (#4673)

Co-authored-by: Devin AI <158243242+devin-ai-integration[bot]@users.noreply.github.com>
Co-authored-by: Deep Singhvi <[email protected]>

Author: devin-ai-integration[bot]

packages/fern-docs/bundle/src/app/[host]/[domain]/[lang]/fern-docs/mcp/route.test.ts

@@ -0,0 +1,149 @@
+import { NextRequest } from "next/server";
+import { beforeEach, describe, expect, it, vi } from "vitest";
+import { GET, POST } from "./route";
+
+vi.mock("@fern-api/docs-loader", () => ({
+    getMetadata: vi.fn(
+        () => () =>
+            Promise.resolve({
+                org: "test-org"
+            })
+    )
+}));
+
+vi.mock("@fern-api/docs-server/auth/getAuthStateEdge", () => ({
+    createGetAuthStateEdge: vi.fn(() =>
+        Promise.resolve({
+            getAuthState: vi.fn(() => Promise.resolve({ ok: true }))
+        })
+    )
+}));
+
+vi.mock("mcp-handler", () => ({
+    createMcpHandler: vi.fn(() => {
+        return async (request: NextRequest) => {
+            return new Response(JSON.stringify({ result: "success" }), {
+                status: 200,
+                headers: { "Content-Type": "application/json" }
+            });
+        };
+    })
+}));
+
+describe("MCP Server Route", () => {
+    beforeEach(() => {
+        vi.clearAllMocks();
+    });
+
+    describe("GET /_mcp/server", () => {
+        it("should return 200 for authenticated requests with JSON-RPC params", async () => {
+            const request = new NextRequest("http://docs.ada.cx/_mcp/server?method=initialize", {
+                method: "GET",
+                headers: {
+                    "Content-Type": "application/json"
+                }
+            });
+
+            const params = Promise.resolve({
+                host: "docs.ada.cx",
+                domain: "docs.ada.cx",
+                lang: "en"
+            });
+
+            const response = await GET(request, { params });
+
+            expect(response.status).toBe(200);
+        });
+
+        it("should return 200 for authenticated requests with Accept: application/json", async () => {
+            const request = new NextRequest("http://docs.ada.cx/_mcp/server", {
+                method: "GET",
+                headers: {
+                    Accept: "application/json"
+                }
+            });
+
+            const params = Promise.resolve({
+                host: "docs.ada.cx",
+                domain: "docs.ada.cx",
+                lang: "en"
+            });
+
+            const response = await GET(request, { params });
+
+            expect(response.status).toBe(200);
+        });
+
+        it("should return 200 with plain text for non-JSON requests", async () => {
+            const request = new NextRequest("http://docs.ada.cx/_mcp/server", {
+                method: "GET"
+            });
+
+            const params = Promise.resolve({
+                host: "docs.ada.cx",
+                domain: "docs.ada.cx",
+                lang: "en"
+            });
+
+            const response = await GET(request, { params });
+
+            expect(response.status).toBe(200);
+            expect(response.headers.get("Content-Type")).toBe("text/plain");
+            const text = await response.text();
+            expect(text).toContain("mcp server");
+        });
+    });
+
+    describe("POST /_mcp/server", () => {
+        it("should return 200 for authenticated POST requests", async () => {
+            const request = new NextRequest("http://docs.ada.cx/_mcp/server", {
+                method: "POST",
+                headers: {
+                    "Content-Type": "application/json"
+                },
+                body: JSON.stringify({
+                    jsonrpc: "2.0",
+                    method: "initialize",
+                    params: {},
+                    id: 1
+                })
+            });
+
+            const params = Promise.resolve({
+                host: "docs.ada.cx",
+                domain: "docs.ada.cx",
+                lang: "en"
+            });
+
+            const response = await POST(request, { params });
+
+            expect(response.status).toBe(200);
+        });
+
+        it("should not return 404", async () => {
+            const request = new NextRequest("http://docs.ada.cx/_mcp/server", {
+                method: "POST",
+                headers: {
+                    "Content-Type": "application/json"
+                },
+                body: JSON.stringify({
+                    jsonrpc: "2.0",
+                    method: "tools/list",
+                    params: {},
+                    id: 2
+                })
+            });
+
+            const params = Promise.resolve({
+                host: "docs.ada.cx",
+                domain: "docs.ada.cx",
+                lang: "en"
+            });
+
+            const response = await POST(request, { params });
+
+            expect(response.status).not.toBe(404);
+            expect(response.status).toBe(200);
+        });
+    });
+});

packages/fern-docs/bundle/src/middleware.ts

@@ -176,7 +176,7 @@ export const middleware: NextMiddleware = async (request) => {
      * Rewrite mcp
      */
     if (pathname.endsWith("/_mcp/server")) {
-        return rewrite(withDomain("/api/fern-docs/mcp"));
+        return rewrite(withDomain("/fern-docs/mcp"));
     }
 
     /**