diff --git a/.github/workflows/cost-calculator-pages.yml b/.github/workflows/cost-calculator-pages.yml
index a0ed978..6de710b 100644
--- a/.github/workflows/cost-calculator-pages.yml
+++ b/.github/workflows/cost-calculator-pages.yml
@@ -27,7 +27,7 @@ jobs:
         working-directory: tools/cost_calculator
         shell: pwsh
         run: |
-          node --test calculator.test.mjs
+          node --test calculator.test.mjs server.test.mjs
           if ($LASTEXITCODE -ne 0) { exit $LASTEXITCODE }
           node --check app.mjs
           if ($LASTEXITCODE -ne 0) { exit $LASTEXITCODE }
diff --git a/samples/gpt_live/js/README.md b/samples/gpt_live/js/README.md
index e076290..1b673dd 100644
--- a/samples/gpt_live/js/README.md
+++ b/samples/gpt_live/js/README.md
@@ -128,6 +128,9 @@ only and answers `POST /token` only when the request's `Host` and `Origin` are i
 `localhost` or `127.0.0.1` address, so other websites, including ones that rebind their DNS name
 to `127.0.0.1`, cannot get a token. This suits a local development sample; a hosted app would
 sign users in with their own identity, for example with MSAL.js.
+Request targets that cannot be parsed as URLs receive HTTP 400 without stopping
+the server or requesting a token. Later valid requests still work, and token
+requests must still pass the same-origin checks.
 
 #### Why the sample supplies its own WebSocket transport
 
@@ -157,6 +160,8 @@ Run the offline tests (no Azure resources, browser, or audio devices). They driv
 Projects SDK client: the server's agent calls go to an in-memory HTTP client, and the page's
 Live sessions talk to a scripted WebSocket transport. The browser transport runs on Node.js's
 built-in WebSocket against a local `ws` server.
+HTTP regressions cover malformed request targets followed by valid page,
+configuration, and same-origin token requests.
 
 ```powershell
 npm test
diff --git a/samples/gpt_live/js/server.mjs b/samples/gpt_live/js/server.mjs
index 1b39598..6c33868 100644
--- a/samples/gpt_live/js/server.mjs
+++ b/samples/gpt_live/js/server.mjs
@@ -74,11 +74,18 @@ function describeTokenFailure(error) {
  */
 export function createServer({ credential, agent, endpoint, keepAgent = true, log = () => {} }) {
   const server = http.createServer(async (request, response) => {
-    const { pathname } = new URL(request.url, "http://localhost");
     const send = (status, type, body) => {
       response.writeHead(status, { "content-type": type, "cache-control": "no-store" });
       response.end(body);
     };
+    let pathname;
+    try {
+      pathname = new URL(request.url, "http://localhost").pathname;
+    } catch (error) {
+      if (error.code !== "ERR_INVALID_URL") throw error;
+      send(400, "text/plain", "Bad request");
+      return;
+    }
     if (request.method === "GET" && pathname === "/config") {
       send(
         200,
diff --git a/samples/gpt_live/js/test/server.test.mjs b/samples/gpt_live/js/test/server.test.mjs
index eb44bb6..f1008e2 100644
--- a/samples/gpt_live/js/test/server.test.mjs
+++ b/samples/gpt_live/js/test/server.test.mjs
@@ -55,8 +55,10 @@ function request(port, path, { method = "GET", headers = {} } = {}) {
       let body = "";
       response.setEncoding("utf8");
       response.on("data", (chunk) => (body += chunk));
+      response.on("error", reject);
       response.on("end", () => resolve({ status: response.statusCode, type: response.headers["content-type"], body }));
     });
+    outgoing.setTimeout(5000, () => outgoing.destroy(new Error("Request timed out")));
     outgoing.on("error", reject);
     outgoing.end();
   });
@@ -70,6 +72,32 @@ const ownPage = (port, host = "localhost") => ({
 });
 
 describe("HTTP surface", () => {
+  it("rejects malformed targets without losing subsequent page, config, or token requests", () =>
+    withServer({}, async ({ running: { port }, scopes }) => {
+      for (const target of ["//[", "//%zz", "http://[", "http://localhost:bad/"]) {
+        for (const method of ["GET", "POST"]) {
+          const bad = await request(port, target, { method });
+          assert.equal(bad.status, 400, `${method} ${target}`);
+          assert.equal(bad.body, "Bad request");
+          assert.equal(bad.type, "text/plain");
+          assert.equal((await request(port, "/?value=%5B%20%25")).status, 200);
+          const config = await request(port, "/config?value=%5B%20%25");
+          assert.equal(config.status, 200);
+          assert.equal(JSON.parse(config.body).endpoint, ENDPOINT);
+        }
+      }
+      assert.deepEqual(scopes, []);
+      for (const path of ["/missing%20file", "/%63onfig"]) {
+        assert.equal((await request(port, path)).status, 404, path);
+      }
+      assert.equal((await request(port, "/token", { method: "POST" })).status, 403);
+      assert.deepEqual(scopes, []);
+      const token = await request(port, "/token?value=%5B%20%25", { method: "POST", headers: ownPage(port) });
+      assert.equal(token.status, 200);
+      assert.equal(JSON.parse(token.body).token, "offline-test-token");
+      assert.deepEqual(scopes, [TOKEN_SCOPE]);
+    }));
+
   it("serves the page, the bundled page script, and the agent configuration", async () => {
     const { running, project } = await start();
     const { port } = running;
diff --git a/tools/cost_calculator/README.md b/tools/cost_calculator/README.md
index 88a8812..07e95fb 100644
--- a/tools/cost_calculator/README.md
+++ b/tools/cost_calculator/README.md
@@ -22,6 +22,8 @@ Open **http://127.0.0.1:8095**. To change the port, set `$env:PORT = "8096"`
 before running the command. The server listens only on the loopback interface.
 Stop it with Ctrl+C. Opening `index.html` as `file://` will not work because
 the page loads modules and the local price snapshot.
+Request targets that cannot be parsed as URLs receive HTTP 400 without stopping
+the local server; later valid requests are still served.
 
 Choose hosting, pipeline, and model; change call assumptions or deployment
 rates. The tool shows per-call, per-minute, call-volume list prices, component costs, and
@@ -210,5 +212,8 @@ captured.
 ## Checks
 
 ```powershell
-node --test tools\cost_calculator\calculator.test.mjs
+node --test tools\cost_calculator\calculator.test.mjs tools\cost_calculator\server.test.mjs
 ```
+
+The server regression test starts the actual local Node process, checks malformed
+request rejection and subsequent asset requests, then stops the process.
diff --git a/tools/cost_calculator/server.mjs b/tools/cost_calculator/server.mjs
index 7f0258f..43c8d1b 100644
--- a/tools/cost_calculator/server.mjs
+++ b/tools/cost_calculator/server.mjs
@@ -7,7 +7,14 @@ const mime = { html: "text/html", css: "text/css", mjs: "text/javascript", json:
 const port = Number(process.env.PORT || 8095);
 if (!Number.isInteger(port) || port < 1 || port > 65535) throw new Error("PORT must be an integer from 1 to 65535.");
 const server = createServer(async (req, res) => {
-  const path = new URL(req.url, "http://localhost").pathname;
+  let path;
+  try {
+    path = new URL(req.url, "http://localhost").pathname;
+  } catch (error) {
+    if (error.code !== "ERR_INVALID_URL") throw error;
+    res.writeHead(400, { "Content-Type": "text/plain; charset=utf-8", "Cache-Control": "no-store" }).end("Bad request");
+    return;
+  }
   const file = path === "/" ? "index.html" : path.slice(1);
   if (!["GET", "HEAD"].includes(req.method) || !allowed.has(file)) {
     res.writeHead(404).end("Not found");
diff --git a/tools/cost_calculator/server.test.mjs b/tools/cost_calculator/server.test.mjs
new file mode 100644
index 0000000..a438481
--- /dev/null
+++ b/tools/cost_calculator/server.test.mjs
@@ -0,0 +1,81 @@
+import assert from "node:assert/strict";
+import { spawn } from "node:child_process";
+import { once } from "node:events";
+import http from "node:http";
+import test from "node:test";
+import { fileURLToPath } from "node:url";
+
+function request(port, path, method = "GET") {
+  return new Promise((resolve, reject) => {
+    const outgoing = http.request({ host: "127.0.0.1", port, path, method, agent: false }, response => {
+      let body = "";
+      response.setEncoding("utf8");
+      response.on("data", chunk => { body += chunk; });
+      response.on("error", reject);
+      response.on("end", () => resolve({ status: response.statusCode, headers: response.headers, body }));
+    });
+    outgoing.setTimeout(5000, () => outgoing.destroy(new Error("Request timed out")));
+    outgoing.on("error", reject);
+    outgoing.end();
+  });
+}
+
+test("the calculator process rejects malformed targets and keeps serving assets", { timeout: 15000 }, async t => {
+  const reservation = http.createServer();
+  reservation.listen(0, "127.0.0.1");
+  await once(reservation, "listening");
+  const { port } = reservation.address();
+  await new Promise((resolve, reject) => reservation.close(error => error ? reject(error) : resolve()));
+
+  const child = spawn(process.execPath, [fileURLToPath(new URL("./server.mjs", import.meta.url))], {
+    env: { ...process.env, PORT: String(port) },
+    stdio: ["ignore", "pipe", "pipe"],
+  });
+  let stderr = "";
+  child.stderr.setEncoding("utf8").on("data", chunk => { stderr += chunk; });
+  const closed = new Promise(resolve => child.once("close", resolve));
+  t.after(async () => {
+    if (child.exitCode === null && child.signalCode === null) child.kill();
+    await closed;
+  });
+  await new Promise((resolve, reject) => {
+    let stdout = "";
+    const timer = setTimeout(() => reject(new Error(`Server startup timed out: ${stderr}`)), 5000);
+    child.once("error", error => { clearTimeout(timer); reject(error); });
+    child.once("exit", code => {
+      clearTimeout(timer);
+      reject(new Error(`Server exited during startup (${code}): ${stderr}`));
+    });
+    child.stdout.setEncoding("utf8").on("data", chunk => {
+      stdout += chunk;
+      if (stdout.includes(`http://127.0.0.1:${port}`)) {
+        clearTimeout(timer);
+        resolve();
+      }
+    });
+  });
+
+  for (const target of ["//[", "//%zz", "http://[", "http://localhost:bad/"]) {
+    for (const method of ["GET", "POST"]) {
+      const bad = await request(port, target, method);
+      assert.equal(bad.status, 400, `${method} ${target}`);
+      assert.equal(bad.body, "Bad request");
+      assert.match(bad.headers["content-type"], /^text\/plain/);
+      const page = await request(port, "/?value=%5B%20%25");
+      assert.equal(page.status, 200);
+      assert.match(page.body, /Voice Agent Cost Lab/);
+    }
+  }
+  for (const file of ["index.html", "styles.css", "app.mjs", "calculator.mjs", "prices.json", "favicon.svg"]) {
+    assert.equal((await request(port, `/${file}`)).status, 200, file);
+    const head = await request(port, `/${file}`, "HEAD");
+    assert.equal(head.status, 200, file);
+    assert.equal(head.body, "");
+  }
+  for (const path of ["/missing%20file", "/%69ndex.html", "/server.mjs"]) {
+    assert.equal((await request(port, path)).status, 404, path);
+  }
+  assert.equal((await request(port, "/", "POST")).status, 404);
+  assert.equal(child.exitCode, null, stderr);
+  assert.equal(stderr, "");
+});