fix(mcp): preserve repeated HTTP host allowlists (#8164)

src/main/mcpHttpArguments.ts

@@ -29,7 +29,7 @@ const MCP_HTTP_OPTIONS = z.object({
     .array(z.string())
     .optional()
     .describe(
-      "Host/Origin names a non-loopback bind may serve; default MCP_ALLOWED_HOSTS, else the bind is refused",
+      "Host/Origin names a non-loopback bind may serve; repeat or separate names with commas; default MCP_ALLOWED_HOSTS, else the bind is refused",
     ),
   help: z.boolean().optional().describe("Show help"),
 });
@@ -85,6 +85,7 @@ export const parseMcpHttpArguments = (
   arguments_: readonly string[],
 ): McpHttpArguments => {
   const values = new Map<string, string>();
+  const allowedHostValues: string[] = [];
   let http = false;
   for (let index = 0; index < arguments_.length; index += 1) {
     const argument = arguments_[index] ?? "";
@@ -100,7 +101,8 @@ export const parseMcpHttpArguments = (
     if (inline === undefined) index += 1;
     if (value === undefined || (inline === undefined && value.startsWith("-")))
       return invalid(`Missing value for ${flag}`);
-    values.set(flag, value);
+    if (flag === "--allowed-hosts") allowedHostValues.push(value);
+    else values.set(flag, value);
   }
   if (!http) return invalid("--host and --port require --http");
   const host = values.get("--host")?.trim();
@@ -111,7 +113,8 @@ export const parseMcpHttpArguments = (
     portText === undefined ? undefined : mcpHttpPortSchema.safeParse(portText);
   if (port !== undefined && !port.success)
     return invalid(`Invalid --port: ${MCP_HTTP_PORT_CONSTRAINT}`);
-  const allowedHostsText = values.get("--allowed-hosts");
+  const allowedHostsText =
+    allowedHostValues.length === 0 ? undefined : allowedHostValues.join(",");
   const allowedHosts =
     allowedHostsText === undefined
       ? undefined

src/server/http/serveStreamableHttp.test.ts

@@ -21,6 +21,7 @@ import {
 } from "./serveStreamableHttp.js";
 import { StreamableHttpListenError } from "./streamableHttpListenError.js";
 import { mcpAllowedHostsTextSchema } from "../../config/mcpHttpEndpoint.js";
+import { parseMcpHttpArguments } from "../../main/mcpHttpArguments.js";
 import { RequestTrackingMcpServer } from "../RequestTrackingMcpServer.js";
 
 const servers: StreamableHttpServer[] = [];
@@ -346,6 +347,49 @@ it.each([
   expect(JSON.parse(response.body)).toEqual({ allowed: true });
 });
 
+it("serves every declared repeated host while refusing undeclared hosts and origins", async () => {
+  const parsed = parseMcpHttpArguments([
+    "--http",
+    "--allowed-hosts",
+    "a.example",
+    "--allowed-hosts=b.example",
+  ]);
+  if (!parsed.ok || parsed.allowedHosts === undefined)
+    throw new Error("Expected valid repeated host lists");
+  const { served } = await start({
+    host: "0.0.0.0",
+    allowedHosts: parsed.allowedHosts,
+    routes: [
+      {
+        method: "GET",
+        path: "/probe",
+        handle: () => Response.json({ allowed: true }),
+      },
+    ],
+  });
+  const url = new URL(served.url);
+  url.hostname = "127.0.0.1";
+  url.pathname = "/probe";
+  for (const host of ["a.example", "b.example"]) {
+    const response = await rawHttpRequest(url, {
+      headers: { host, origin: `http://${host}` },
+    });
+    expect(response.status, host).toBe(200);
+    expect(JSON.parse(response.body)).toEqual({ allowed: true });
+  }
+  expect(
+    (await rawHttpRequest(url, { headers: { host: "undeclared.example" } }))
+      .status,
+  ).toBe(403);
+  expect(
+    (
+      await rawHttpRequest(url, {
+        headers: { host: "a.example", origin: "http://undeclared.example" },
+      })
+    ).status,
+  ).toBe(403);
+});
+
 it("announces an admitted non-loopback URL and gates it on the operator allowlist", async () => {
   const { served } = await start({
     host: "0.0.0.0",