fix: cancel a Streamable HTTP call by closing its stream (#2140)

The Cancel button POSTed `notifications/cancelled` on every transport. On
a 2026-07-28 Streamable HTTP connection that is the wrong signal — the
spec makes closing the request's SSE response stream the cancellation
signal there and says the notification is neither required nor expected —
so a spec-compliant server acknowledged it 202 and dropped it while the
tool ran to completion.

The SDK already implements the fork, off `transport.hasPerRequestStream`.
Every Inspector connection is wrapped in `MessageTrackingTransport`, which
did not forward that flag, so the SDK read `undefined` and took the stdio
branch on every client — the CLI and TUI included, whose real
`StreamableHTTPClientTransport` advertises it correctly.

The web client needed three more links: its browser transport answers for
the upstream transport that lives on the backend, so it advertises the
flag for `streamable-http` and applies the SDK's `requestSignal` to its
`POST /api/mcp/send`; and `/api/mcp/send`, held open for the whole call,
forwards the browser's disconnect to the upstream `transport.send`.

The test fixture's `toWebRequest` built a Web Request with no `signal`, so
no fixture server could observe a disconnect at all — without that, a test
asserting the cancel reaches the server passes vacuously.

stdio and SSE keep `notifications/cancelled`: they multiplex every request
over one shared channel and have no per-request stream to close. So does
the legacy era, which predates the mechanism.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_013hzwzS8UvBsr4yZm7o7sev
Signed-off-by: cliffhall <cliff@futurescale.com>

README.md

@@ -163,6 +163,42 @@ Each config below is a ready-made server for exercising one feature by hand. Loa
 | `subscriptions-{legacy,modern}-http.json` | Resource subscriptions, both eras                   | [#1630](https://github.com/modelcontextprotocol/inspector/issues/1630) |
 | `subscriptions-never-acknowledged-http.json` | A `subscriptions/listen` answered with a bare result  | [#2097](https://github.com/modelcontextprotocol/inspector/issues/2097) |
 | `tasks-{legacy,modern}-http.json`         | Tasks, both eras                                    | [#1631](https://github.com/modelcontextprotocol/inspector/issues/1631) |
+| `cancellation-modern-http.json`           | Cancelling a call by closing its response stream    | [#2140](https://github.com/modelcontextprotocol/inspector/issues/2140) |
+
+#### Cancelling a call
+
+`cancellation-modern-http.json` serves `slow_task`, which reports progress once
+a second for up to 60 seconds and returns early — saying how far it got — if it
+is cancelled. Connect with **Protocol Era = Modern**.
+
+Run `slow_task` from the Tools tab and click **Cancel** after a few seconds. The
+progress must stop immediately, and the result must read `Cancelled after Ns`.
+On the broken build the progress kept arriving until the tool completed all 60
+seconds, because the Inspector was sending the wrong cancellation signal
+([#2140](https://github.com/modelcontextprotocol/inspector/issues/2140)).
+
+The 2026-07-28 spec makes this
+[transport-specific](https://modelcontextprotocol.io/specification/2026-07-28/basic/patterns/cancellation#transport-specific-cancellation):
+for Streamable HTTP, **closing the request's SSE response stream is the
+cancellation signal**, and a `notifications/cancelled` is "neither required nor
+expected"; stdio, which has no per-request stream to close, keeps the
+notification. A spec-compliant server therefore answers the notification `202
+Accepted` and drops it — which is precisely what the reporter observed, with the
+task running on to completion while the Inspector reported it cancelled.
+
+The SDK already implements that fork, off `transport.hasPerRequestStream`. Every
+Inspector connection is wrapped in `MessageTrackingTransport` (it feeds the
+Protocol and Network tabs), which did not forward the flag — so the SDK saw
+`undefined` and took the stdio branch on **every** client, CLI and TUI included.
+The web client needed two more links in the chain: its browser-side transport
+answers for the real upstream one that lives on the Node backend, and the abort
+has to survive the `POST /api/mcp/send` hop to reach it.
+
+Watch it on the wire in the **Protocol** tab: cancelling now emits no
+`notifications/cancelled` frame at all, and the `tools/call` entry ends as an
+aborted request rather than a completed one. Switch the same server to
+**Legacy** and the notification comes back — that era has no per-stream
+mechanism, so it is still the correct signal there.
 
 #### MCP Apps
 

clients/web/src/test/core/mcp/messageTrackingTransport.test.ts

@@ -12,6 +12,8 @@ class FakeTransport implements Transport {
   sessionId?: string;
   // Optional on the SDK Transport interface; stdio omits it, HTTP defines it.
   setProtocolVersion?: (version: string) => void;
+  // Optional too: Streamable HTTP advertises it, stdio and SSE say nothing.
+  hasPerRequestStream?: boolean;
   async start(): Promise<void> {}
   async send(message: JSONRPCMessage): Promise<void> {
     this.sent.push(message);
@@ -272,4 +274,19 @@ describe("MessageTrackingTransport.setProtocolVersion", () => {
     expect(() => tracked.setProtocolVersion("2025-06-18")).not.toThrow();
     expect(tracked.protocolVersion).toBe("2025-06-18");
   });
+
+  // #2140: the SDK reads `hasPerRequestStream` off the transport it was handed
+  // — always this wrapper — to choose between aborting a request's own stream
+  // and POSTing `notifications/cancelled`. A wrapper that answers for the base
+  // transport instead of forwarding its answer breaks Cancel on every client.
+  it("forwards the base transport's per-request-stream capability", () => {
+    const { tracked, base } = makeTracked();
+    expect(tracked.hasPerRequestStream).toBeUndefined();
+
+    base.hasPerRequestStream = true;
+    expect(tracked.hasPerRequestStream).toBe(true);
+
+    base.hasPerRequestStream = false;
+    expect(tracked.hasPerRequestStream).toBe(false);
+  });
 });

clients/web/src/test/core/mcp/remote/remoteClientTransport.test.ts

@@ -569,4 +569,123 @@ describe("RemoteClientTransport", () => {
 
     await transport.close();
   });
+
+  // #2140: the Cancel button's wire signal is chosen by the SDK from the
+  // transport's `hasPerRequestStream`. The real upstream transport lives on the
+  // backend, so this transport answers for it — and must answer per server
+  // type, since only Streamable HTTP gives a request a stream of its own to
+  // close.
+  describe("per-request-stream cancellation (#2140)", () => {
+    it("advertises hasPerRequestStream only for streamable-http", () => {
+      const forType = (c: MCPServerConfig) =>
+        new RemoteClientTransport(
+          { baseUrl, fetchFn: vi.fn() as unknown as typeof fetch },
+          c,
+        ).hasPerRequestStream;
+
+      expect(
+        forType({ type: "streamable-http", url: "http://server.example/mcp" }),
+      ).toBe(true);
+      // stdio and SSE multiplex every request over one shared channel, so
+      // aborting anything would cancel the whole session rather than the call.
+      // The SDK must keep sending `notifications/cancelled` for them.
+      expect(forType(config)).toBe(false);
+      expect(forType({ type: "sse", url: "http://server.example/sse" })).toBe(
+        false,
+      );
+    });
+
+    it("applies the SDK's requestSignal to the browser-to-backend send fetch", async () => {
+      const seenInits: (RequestInit | undefined)[] = [];
+      let sse = createPushableSseStream();
+      const fetchFn = vi
+        .fn<typeof fetch>()
+        .mockImplementation(async (input, init) => {
+          const url = String(input);
+          if (url.endsWith("/api/mcp/connect")) {
+            return new Response(JSON.stringify({ sessionId: "abc" }), {
+              status: 200,
+            });
+          }
+          if (url.includes("/api/mcp/events")) {
+            sse = createPushableSseStream();
+            return sse.response;
+          }
+          if (url.endsWith("/api/mcp/send")) {
+            seenInits.push(init);
+            return new Response(JSON.stringify({ ok: true }), { status: 200 });
+          }
+          return new Response("not found", { status: 404 });
+        });
+
+      const transport = new RemoteClientTransport({ baseUrl, fetchFn }, config);
+      await transport.start();
+
+      const controller = new AbortController();
+      await transport.send(
+        { jsonrpc: "2.0", method: "notifications/initialized" },
+        { requestSignal: controller.signal },
+      );
+      expect(seenInits[0]?.signal).toBe(controller.signal);
+
+      // No signal supplied (stdio/SSE, where the SDK never creates one) must
+      // leave the fetch unaborted rather than passing `undefined` through as a
+      // deliberate value.
+      await transport.send({
+        jsonrpc: "2.0",
+        method: "notifications/roots/list_changed",
+      });
+      expect(seenInits[1]).not.toHaveProperty("signal");
+
+      await transport.close();
+    });
+
+    it("aborting the requestSignal rejects the in-flight send and releases its response wait", async () => {
+      let sse = createPushableSseStream();
+      const fetchFn = vi
+        .fn<typeof fetch>()
+        .mockImplementation(async (input, init) => {
+          const url = String(input);
+          if (url.endsWith("/api/mcp/connect")) {
+            return new Response(JSON.stringify({ sessionId: "abc" }), {
+              status: 200,
+            });
+          }
+          if (url.includes("/api/mcp/events")) {
+            sse = createPushableSseStream();
+            return sse.response;
+          }
+          if (url.endsWith("/api/mcp/send")) {
+            // The backend holds this route open for the whole call, so model it
+            // as a fetch that only settles when the caller aborts.
+            return new Promise<Response>((_resolve, reject) => {
+              init?.signal?.addEventListener("abort", () =>
+                reject(new DOMException("Aborted", "AbortError")),
+              );
+            });
+          }
+          return new Response("not found", { status: 404 });
+        });
+
+      const transport = new RemoteClientTransport({ baseUrl, fetchFn }, config);
+      await transport.start();
+
+      const controller = new AbortController();
+      const sent = transport.send(
+        { jsonrpc: "2.0", id: 7, method: "tools/call" },
+        { requestSignal: controller.signal },
+      );
+      const settled = vi.fn();
+      void sent.then(settled, settled);
+
+      controller.abort();
+      await expect(sent).rejects.toThrow(/Aborted/);
+      // The response wait registered for id 7 must have been cancelled too —
+      // otherwise this send would hang past the abort waiting for a JSON-RPC
+      // response the aborted request can never produce.
+      expect(settled).toHaveBeenCalledTimes(1);
+
+      await transport.close();
+    });
+  });
 });

clients/web/src/test/integration/mcp/inspectorClient-modern-era.test.ts

@@ -437,6 +437,63 @@ describe("modern-era negotiation (2026-07-28)", () => {
     expect(result.result!.isError).toBeFalsy();
   });
 
+  // #2140: on the 2026-07-28 era, closing the request's own response stream IS
+  // the cancellation signal for Streamable HTTP; `notifications/cancelled` is
+  // the stdio mechanism and the spec says it is neither required nor expected
+  // here. The SDK implements that fork off `transport.hasPerRequestStream` —
+  // but every Inspector connection is wrapped in `MessageTrackingTransport`,
+  // which was not forwarding it, so all three clients POSTed the notification
+  // and a spec-compliant server acknowledged it `202` and kept running.
+  //
+  // This asserts at the far end, on the server's own request signal: nothing on
+  // the client side distinguishes a cancel that reached the server from one
+  // that was dropped, which is exactly how this shipped.
+  it("cancels by aborting the request's stream, which the server observes", async () => {
+    let sawAbort!: (value: boolean) => void;
+    const aborted = new Promise<boolean>((resolve) => {
+      sawAbort = resolve;
+    });
+    let started!: () => void;
+    const running = new Promise<void>((resolve) => {
+      started = resolve;
+    });
+
+    const slowServer = createTestServerHttp({
+      serverInfo: createTestServerInfo("cancel-era-test", "1.0.0"),
+      tools: [
+        {
+          name: "slow_task",
+          description: "Runs until the client cancels it",
+          handler: async (_params, _context, extra) => {
+            extra?.signal?.addEventListener("abort", () => sawAbort(true), {
+              once: true,
+            });
+            started();
+            // Never settles on its own, so a regression is a timeout rather
+            // than a pass.
+            return new Promise(() => {});
+          },
+        },
+      ],
+      modern: {},
+    });
+    await slowServer.start();
+    server = slowServer;
+
+    const connected = await connectWithEra(slowServer.url, "modern");
+    const { tools } = await connected.listTools();
+    const tool = tools.find((t) => t.name === "slow_task");
+
+    // Hold the rejection immediately so it is never seen as unhandled while we
+    // wait on the server side.
+    const settled = connected.callTool(tool!, {}).catch((err: unknown) => err);
+    await running;
+    expect(connected.cancelToolCall()).toBe(true);
+
+    await expect(settled).resolves.toBeInstanceOf(ToolCallCancelledError);
+    await expect(aborted).resolves.toBe(true);
+  }, 30_000);
+
   it("cancels an in-flight MRTR call while its embedded request is pending", async () => {
     const started = await startMrtrServer(createMrtrTool());
     const connected = await connectWithEra(started.url, "modern");

clients/web/src/test/integration/mcp/remote/cancel-per-request-stream.test.ts

@@ -0,0 +1,199 @@
+/**
+ * #2140 — the Cancel button must send the transport's cancellation signal.
+ *
+ * On a 2026-07-28 Streamable HTTP connection the spec makes *closing the
+ * request's SSE response stream* the cancellation signal, and says a
+ * `notifications/cancelled` is neither required nor expected. The SDK
+ * implements that fork itself: `Protocol.request` aborts a per-request
+ * `AbortController` — handed to the transport as
+ * `TransportSendOptions.requestSignal` — when the connection is modern **and**
+ * the transport advertises `hasPerRequestStream`; otherwise it POSTs the
+ * notification.
+ *
+ * The web client lost it at the proxy boundary. The real upstream transport
+ * lives on the Node backend, and the browser's `RemoteClientTransport` neither
+ * advertised the flag nor applied a `requestSignal` to its
+ * `POST /api/mcp/send` — so every web cancel took the stdio branch, the server
+ * answered the notification `202 Accepted` and dropped it, and the tool kept
+ * running to completion. Only a manual disconnect actually cancelled anything.
+ *
+ * This drives the whole chain the browser drives — InspectorClient ->
+ * RemoteClientTransport -> the Hono backend -> a real upstream Streamable HTTP
+ * transport -> a modern test server — and asserts at the far end, on the
+ * server's own request abort signal. Nothing shallower reaches it: each of the
+ * three hops can drop the signal on its own, and a unit test of any one of them
+ * passes while the chain stays broken.
+ */
+
+import { describe, it, expect, afterEach } from "vitest";
+import { serve } from "@hono/node-server";
+import type { ServerType } from "@hono/node-server";
+import { InspectorClient } from "@inspector/core/mcp/inspectorClient.js";
+import { createRemoteTransport } from "@inspector/core/mcp/remote/createRemoteTransport.js";
+import { createRemoteApp } from "@inspector/core/mcp/remote/node/server.js";
+import { ToolCallCancelledError } from "@inspector/core/mcp/toolCallCancelledError.js";
+import { eraToVersionNegotiation } from "@inspector/core/mcp/types.js";
+import {
+  createTestServerHttp,
+  createTestServerInfo,
+  type TestServerHttp,
+  type ToolDefinition,
+} from "@modelcontextprotocol/inspector-test-server";
+
+function deferred<T>(): { promise: Promise<T>; resolve: (value: T) => void } {
+  let resolve!: (value: T) => void;
+  const promise = new Promise<T>((r) => {
+    resolve = r;
+  });
+  return { promise, resolve };
+}
+
+/**
+ * A tool that never returns on its own. It reports when it started, and whether
+ * the server-side request signal — which the SDK aborts when the client closes
+ * this request's response stream — ever fired.
+ */
+function createSlowTool(): {
+  tool: ToolDefinition;
+  started: Promise<void>;
+  aborted: Promise<boolean>;
+} {
+  const started = deferred<void>();
+  const aborted = deferred<boolean>();
+  const tool: ToolDefinition = {
+    name: "slow_task",
+    description: "Runs until the client cancels it",
+    handler: async (_params, _context, extra) => {
+      extra?.signal?.addEventListener("abort", () => aborted.resolve(true), {
+        once: true,
+      });
+      started.resolve();
+      // Never settles: this call ends by cancellation or not at all, so a
+      // regression shows up as the test's own timeout rather than as a pass.
+      return new Promise(() => {});
+    },
+  };
+  return { tool, started: started.promise, aborted: aborted.promise };
+}
+
+async function startRemoteBackend(): Promise<{
+  baseUrl: string;
+  server: ServerType;
+  authToken: string;
+}> {
+  const { app, authToken } = createRemoteApp({
+    initialConfig: { defaultEnvironment: {} },
+  });
+  return new Promise((resolve, reject) => {
+    const server = serve(
+      { fetch: app.fetch, port: 0, hostname: "127.0.0.1" },
+      (info) => {
+        const port =
+          info && typeof info === "object" && "port" in info
+            ? (info as { port: number }).port
+            : 0;
+        resolve({ baseUrl: `http://127.0.0.1:${port}`, server, authToken });
+      },
+    );
+    server.on("error", reject);
+  });
+}
+
+describe("web-client cancellation over a per-request stream (#2140)", () => {
+  let client: InspectorClient | null = null;
+  let backend: ServerType | null = null;
+  let upstream: TestServerHttp | null = null;
+
+  afterEach(async () => {
+    if (client) {
+      try {
+        await client.disconnect();
+      } catch {
+        // Ignore disconnect errors.
+      }
+      client = null;
+    }
+    if (backend) {
+      const server = backend;
+      backend = null;
+      await new Promise<void>((resolve, reject) => {
+        server.close((err) => (err ? reject(err) : resolve()));
+      });
+    }
+    if (upstream) {
+      try {
+        await upstream.stop();
+      } catch {
+        // Ignore stop errors.
+      }
+      upstream = null;
+    }
+  });
+
+  it("aborts the upstream request stream, which the server observes as cancellation", async () => {
+    const slow = createSlowTool();
+    upstream = createTestServerHttp({
+      serverInfo: createTestServerInfo("cancel-test", "1.0.0"),
+      tools: [slow.tool],
+      modern: {},
+    });
+    await upstream.start();
+
+    const { baseUrl, server, authToken } = await startRemoteBackend();
+    backend = server;
+
+    const connected = new InspectorClient(
+      { type: "streamable-http", url: upstream.url },
+      {
+        environment: {
+          transport: createRemoteTransport({ baseUrl, authToken }),
+        },
+        versionNegotiation: eraToVersionNegotiation("modern"),
+      },
+    );
+    await connected.connect();
+    client = connected;
+    expect(connected.getProtocolEra()).toBe("modern");
+
+    const { tools } = await connected.listTools();
+    const tool = tools.find((t) => t.name === "slow_task");
+    expect(tool).toBeDefined();
+
+    // Hold the rejection from the first turn so the runner never sees it as
+    // unhandled while we wait on the server side.
+    const settled = connected.callTool(tool!, {}).catch((err: unknown) => err);
+
+    await slow.started;
+    expect(connected.cancelToolCall()).toBe(true);
+
+    // The user-facing contract is unchanged: a deliberate cancel still surfaces
+    // as ToolCallCancelledError, whichever wire signal carried it.
+    await expect(settled).resolves.toBeInstanceOf(ToolCallCancelledError);
+
+    // The assertion that matters. Before the fix the server saw only a
+    // `notifications/cancelled` it was free to ignore (and did), its request
+    // signal never fired, and this promise never resolved.
+    await expect(slow.aborted).resolves.toBe(true);
+  }, 30_000);
+
+  it("advertises the per-request stream only for streamable-http", () => {
+    // The flag is what the SDK forks on, and the factory is where a server's
+    // configured type reaches the transport. stdio and SSE multiplex every
+    // request over one shared channel: there is no per-request stream to close,
+    // and aborting anything would take the whole session down — so they must
+    // keep the `notifications/cancelled` mechanism.
+    const create = createRemoteTransport({ baseUrl: "http://unused.example" });
+    expect(
+      create({ type: "stdio", command: "echo", args: [] }, {}).transport
+        .hasPerRequestStream,
+    ).toBe(false);
+    expect(
+      create({ type: "sse", url: "http://unused.example/sse" }, {}).transport
+        .hasPerRequestStream,
+    ).toBe(false);
+    expect(
+      create({ type: "streamable-http", url: "http://unused.example/mcp" }, {})
+        .transport.hasPerRequestStream,
+    ).toBe(true);
+  });
+});

core/mcp/inspectorClient.ts

@@ -648,9 +648,12 @@ export class InspectorClient extends InspectorClientEventTarget {
   >();
   private rawWireRequestCounter = 0;
   // Abort controller for the in-flight ordinary (non-task) tool call. Aborting
-  // it makes the SDK send a `notifications/cancelled` for that request (the MCP
+  // it hands the SDK the MCP cancellation flow for that request and rejects the
-  // cancellation flow) and reject the pending call, which `callTool` surfaces as
+  // pending call, which `callTool` surfaces as a `ToolCallCancelledError`. Which
-  // a `ToolCallCancelledError`. Undefined when no ordinary call is in flight.
+  // signal reaches the server is the transport's business, not ours: a
+  // per-request-stream transport on a 2026-era connection has that request's
+  // stream aborted, and everything else gets `notifications/cancelled` (#2140).
+  // Undefined when no ordinary call is in flight.
   // Task-augmented calls have a server-side task and are cancelled via
   // `cancelRequestorTask` instead, so they don't use this (#1458).
   private activeToolCallAbortController?: AbortController;
@@ -1169,8 +1172,10 @@ export class InspectorClient extends InspectorClientEventTarget {
     if (this.requestTimeout !== undefined) {
       opts.timeout = this.requestTimeout;
     }
-    // When provided, aborting this signal makes the SDK send a
+    // When provided, aborting this signal cancels the request and rejects it
-    // `notifications/cancelled` for the request and reject it (#1458).
+    // (#1458). The SDK picks the wire signal from the transport: an aborted
+    // per-request SSE stream on a 2026-era Streamable HTTP connection, and
+    // `notifications/cancelled` everywhere else (#2140).
     if (signal) {
       opts.signal = signal;
     }
@@ -2976,10 +2981,13 @@ export class InspectorClient extends InspectorClientEventTarget {
 
   /**
    * Cancel the in-flight ordinary (non-task) tool call started by
-   * {@link callTool}. Aborting its request makes the SDK send a
+   * {@link callTool}. Aborting its request runs the MCP cancellation flow and
-   * `notifications/cancelled` to the server (the MCP cancellation flow) and
+   * rejects the pending call, which `callTool` surfaces as a
-   * reject the pending call, which `callTool` surfaces as a
+   * {@link ToolCallCancelledError}. The SDK chooses the wire signal from the
-   * {@link ToolCallCancelledError}.
+   * transport: on a 2026-era Streamable HTTP connection it aborts that
+   * request's own SSE response stream — the spec's cancellation signal —
+   * rather than sending `notifications/cancelled`, which is the stdio
+   * mechanism and remains in use there (#2140).
    *
    * Task-augmented calls have a server-side task and are cancelled via
    * {@link cancelRequestorTask} instead — this is a no-op for them (and whenever

core/mcp/messageTrackingTransport.ts

@@ -203,6 +203,30 @@ export class MessageTrackingTransport implements Transport {
     return this.baseTransport.sessionId;
   }
 
+  /**
+   * Forward the base transport's per-request-stream capability (#2140).
+   *
+   * The SDK's `Protocol.request` reads this off the transport it was handed —
+   * which is always this wrapper — to decide how a cancelled request is
+   * signalled: abort that request's own stream (the 2026-07-28 signal for
+   * Streamable HTTP) or POST a `notifications/cancelled` (the stdio one).
+   *
+   * Not forwarding it made the wrapper *answer* the question, `undefined`, for
+   * every client. So the Cancel button POSTed a notification even on a modern
+   * Streamable HTTP connection, where a spec-compliant server acknowledges it
+   * `202` and drops it — the tool kept running and only a disconnect really
+   * cancelled anything. That hit the CLI and TUI as much as the web client:
+   * their real `StreamableHTTPClientTransport` advertises the flag correctly
+   * and this wrapper hid it.
+   *
+   * A base transport that says nothing stays `undefined` rather than becoming
+   * `false`, since the SDK's check is `=== true` either way and inventing a
+   * value here would misreport what the transport claimed.
+   */
+  get hasPerRequestStream(): boolean | undefined {
+    return this.baseTransport.hasPerRequestStream;
+  }
+
   // Implemented as a concrete method (rather than delegating the base
   // transport's optional `setProtocolVersion`) so the SDK Client always
   // invokes it after the initialize handshake — including for stdio, whose

core/mcp/remote/node/server.ts

@@ -1022,6 +1022,36 @@ export function createRemoteApp(
       // Auth errors may reject the wait via onerror while send() also throws.
       void responseWait.catch(() => {});
     }
+
+    // #2140: this route is held open for the whole call, so the browser
+    // aborting its `POST /api/mcp/send` mid-flight *is* the client's
+    // cancellation signal on a per-request-stream upstream. Forward it as
+    // `requestSignal` so the upstream StreamableHTTP transport closes that
+    // request's SSE response stream, which is what the 2026-07-28 spec makes
+    // the cancellation signal. A transport with no per-request stream (stdio,
+    // SSE) ignores the option, and a *cancel* never reaches it as an abort
+    // anyway: `RemoteClientTransport` withholds `hasPerRequestStream` there, so
+    // the SDK keeps sending `notifications/cancelled`. Forwarding the
+    // disconnect unconditionally is still right — a browser that went away
+    // mid-call is not waiting for the answer whatever the transport is.
+    const upstreamAbort = new AbortController();
+    const clientSignal = c.req.raw.signal;
+    const onClientDisconnect = () => {
+      upstreamAbort.abort(clientSignal.reason);
+      // The response will never arrive now; release the wait so this handler
+      // unwinds instead of sitting on a promise nothing can settle.
+      if (requestId !== undefined) {
+        session.cancelRequestWait(requestId);
+      }
+    };
+    if (clientSignal.aborted) {
+      onClientDisconnect();
+    } else {
+      clientSignal.addEventListener("abort", onClientDisconnect, {
+        once: true,
+      });
+    }
+
     try {
       // SEP-2243: apply the client's mirrored headers to the upstream request,
       // restricted to the `Mcp-Param-` prefix so a client can't inject other
@@ -1031,6 +1061,7 @@ export function createRemoteApp(
       await session.transport.send(message, {
         relatedRequestId: relatedRequestId as string | number | undefined,
         ...(paramHeaders && { headers: paramHeaders }),
+        requestSignal: upstreamAbort.signal,
       });
       if (responseWait) {
         await responseWait;
@@ -1051,6 +1082,7 @@ export function createRemoteApp(
       const msg = err instanceof Error ? err.message : String(err);
       return c.json({ ok: false, kind: "transport_error", error: msg });
     } finally {
+      clientSignal.removeEventListener("abort", onClientDisconnect);
       session.endSend();
     }
   });

core/mcp/remote/remoteClientTransport.ts

@@ -261,6 +261,25 @@ export class RemoteClientTransport implements Transport {
   private readonly options: RemoteTransportOptions;
   private readonly config: import("../types.js").MCPServerConfig;
 
+  /**
+   * Whether the upstream connection gives each request its own response stream
+   * (#2140). The SDK's `Protocol.request` reads this: on a 2026-era connection
+   * a transport advertising it has its per-request stream **aborted** as the
+   * spec's cancellation signal, and no `notifications/cancelled` is sent; a
+   * transport that does not gets the stdio mechanism instead.
+   *
+   * The real upstream transport lives on the backend, so this transport has to
+   * answer for it. Streamable HTTP opens one POST — and one SSE response
+   * stream — per request, so it qualifies; stdio and SSE multiplex every
+   * request over one shared channel and must keep sending the notification.
+   *
+   * Advertising it is only half the fix: `requestSend` applies the SDK's
+   * `requestSignal` to the browser-to-backend fetch, and `/api/mcp/send`
+   * forwards that disconnect to the upstream `transport.send`, which is what
+   * actually closes the stream the server is watching.
+   */
+  readonly hasPerRequestStream: boolean;
+
   /**
    * Intentionally returns undefined. The MCP Client checks transport.sessionId to detect
    * reconnects and skip initialize. Our _sessionId is the remote server's session ID, not
@@ -312,6 +331,7 @@ export class RemoteClientTransport implements Transport {
   ) {
     this.options = options;
     this.config = config;
+    this.hasPerRequestStream = config.type === "streamable-http";
   }
 
   setAuthRecovery(handlers: AuthRecoveryHandlers | undefined): void {
@@ -757,10 +777,17 @@ export class RemoteClientTransport implements Transport {
       }),
     };
 
+    // #2140: aborting this fetch is the cancellation signal for a
+    // per-request-stream upstream. The backend holds `/api/mcp/send` open for
+    // the whole call (it awaits the JSON-RPC response), so the disconnect
+    // reaches it mid-flight and it aborts the upstream request's SSE stream.
+    // For stdio/SSE `hasPerRequestStream` is false, so the SDK never supplies
+    // a `requestSignal` and this is undefined.
     const res = await this.fetchFn(`${this.baseUrl}/api/mcp/send`, {
       method: "POST",
       headers: this.headers,
       body: JSON.stringify(body),
+      ...(options?.requestSignal && { signal: options.requestSignal }),
     });
 
     if (!res.ok) {

specification/v2_ux.md

@@ -390,7 +390,11 @@ When a tool execution triggers sampling or elicitation requests, they appear inl
   - Step description if provided
   - Elapsed time display
 - Execute button with loading state
-- **Cancel button** sends `notifications/cancelled`
+- **Cancel button** sends the transport-appropriate cancellation signal (#2140):
+  on a 2026-07-28 Streamable HTTP connection it aborts the request's own SSE
+  response stream, which the spec makes the cancellation signal; on stdio (and
+  on any pre-2026 connection, where the per-stream mechanism does not exist) it
+  sends `notifications/cancelled`
 - **Inline Client Request Queue** - When tool triggers sampling/elicitation:
   - Pending requests shown inline (not as separate modal)
   - Queue counter shows total pending requests

specification/v2_ux_features.md

@@ -110,7 +110,11 @@ When a tool execution triggers sampling or elicitation requests, they appear inl
   - Step description if provided
   - Elapsed time display
 - Execute button with loading state
-- **Cancel button** sends `notifications/cancelled`
+- **Cancel button** sends the transport-appropriate cancellation signal (#2140):
+  on a 2026-07-28 Streamable HTTP connection it aborts the request's own SSE
+  response stream, which the spec makes the cancellation signal; on stdio (and
+  on any pre-2026 connection, where the per-stream mechanism does not exist) it
+  sends `notifications/cancelled`
 - **Inline Client Request Queue** - When tool triggers sampling/elicitation:
   - Pending requests shown inline (not as separate modal)
   - Queue counter shows total pending requests

test-servers/configs/cancellation-modern-http.json

@@ -0,0 +1,12 @@
+{
+  "serverInfo": {
+    "name": "cancellation-showcase",
+    "version": "1.0.0"
+  },
+  "tools": [{ "preset": "slow_task" }, { "preset": "echo" }],
+  "transport": {
+    "type": "streamable-http",
+    "port": 6606,
+    "modern": true
+  }
+}

test-servers/src/preset-registry.ts

@@ -33,6 +33,7 @@ import {
   createCollectUrlElicitationTool,
   createUrlElicitationFormTool,
   createSendNotificationTool,
+  createSlowTaskTool,
   createGetAnnotatedMessageTool,
   createGetTempTool,
   createGetTempExtraTool,
@@ -165,6 +166,8 @@ function resolveToolPreset(
       return createUrlElicitationFormTool();
     case "send_notification":
       return createSendNotificationTool();
+    case "slow_task":
+      return createSlowTaskTool();
     case "get_annotated_message":
       return createGetAnnotatedMessageTool();
     case "get_temp":

test-servers/src/test-server-fixtures.ts

@@ -1219,6 +1219,71 @@ export function createCollectUrlElicitationTool(): ToolDefinition {
   };
 }
 
+/**
+ * Create a "slow_task" tool that runs until the client cancels it (#2140).
+ *
+ * It emits a progress notification every second up to `seconds` (default 60)
+ * and completes only if it is never cancelled, so there is a long, visible
+ * window in which to click Cancel.
+ *
+ * The point is what it does on the way out. On the 2026-07-28 era a client
+ * cancels a Streamable HTTP request by closing that request's response stream,
+ * and the SDK surfaces the disconnect to this handler as `extra.signal`. So the
+ * tool reports which signal it actually received: cancelled through the stream,
+ * the progress stops immediately; cancelled by a `notifications/cancelled` the
+ * server is free to ignore, the progress keeps arriving until the tool
+ * completes on its own — the exact symptom reported.
+ */
+export function createSlowTaskTool(): ToolDefinition {
+  return {
+    name: "slow_task",
+    description:
+      "Run for up to `seconds`, reporting progress each second, until cancelled",
+    inputSchema: {
+      seconds: z
+        .number()
+        .int()
+        .min(1)
+        .max(600)
+        .optional()
+        .describe("How long to run before completing on its own (default 60)"),
+    },
+    handler: async (
+      params: Record<string, unknown>,
+      _context?: TestServerContext,
+      extra?: HandlerExtra,
+    ) => {
+      const total = typeof params.seconds === "number" ? params.seconds : 60;
+      const progressToken = extra?._meta?.progressToken;
+      for (let step = 1; step <= total; step++) {
+        const slept = await new Promise<boolean>((resolve) => {
+          const timer = setTimeout(() => resolve(true), 1000);
+          extra?.signal?.addEventListener(
+            "abort",
+            () => {
+              clearTimeout(timer);
+              resolve(false);
+            },
+            { once: true },
+          );
+        });
+        if (!slept || extra?.signal?.aborted) {
+          // Cancelled. Return rather than keep working — the whole point of
+          // the demo is that the work actually stops.
+          return toToolResult(`Cancelled after ${step - 1}s`);
+        }
+        if (progressToken !== undefined) {
+          await extra?.sendNotification?.({
+            method: "notifications/progress",
+            params: { progressToken, progress: step, total },
+          });
+        }
+      }
+      return toToolResult(`Completed all ${total}s without being cancelled`);
+    },
+  };
+}
+
 /**
  * Create a "send_notification" tool that sends a notification message from the server
  */

test-servers/src/test-server-http.ts

@@ -38,9 +38,21 @@ import {
  * SDK v2's {@link WebStandardStreamableHTTPServerTransport} speaks the Fetch API
  * (`Request`/`Response`) rather than Node `req`/`res`. The JSON body is passed
  * to `handleRequest` via `parsedBody` (Express already parsed it), so the Web
- * request carries only method, URL, and headers.
+ * request carries only method, URL, headers — and a `signal`.
+ *
+ * The signal is load-bearing, not decoration (#2140). On the 2026-07-28 era a
+ * client cancels a request by closing that request's response stream, and the
+ * SDK server surfaces the disconnect to a tool handler as `ctx.mcpReq.signal`
+ * — reading it off this Request. Building one without a signal therefore makes
+ * every fixture server deaf to cancellation, which is indistinguishable from a
+ * client that never sent the signal: a test asserting that a cancel reaches the
+ * server passes only if this is wired.
+ *
+ * `res`'s `close` is the disconnect, but only when the response had not already
+ * finished — Express emits it on every completed response too, and aborting
+ * then would cancel handlers that had already succeeded.
  */
-function toWebRequest(req: Request): globalThis.Request {
+function toWebRequest(req: Request, res: Response): globalThis.Request {
   const headers = new Headers();
   for (const [key, value] of Object.entries(req.headers)) {
     if (Array.isArray(value)) {
@@ -50,7 +62,17 @@ function toWebRequest(req: Request): globalThis.Request {
     }
   }
   const url = `http://localhost${req.originalUrl || req.url}`;
-  return new globalThis.Request(url, { method: req.method, headers });
+  const controller = new AbortController();
+  res.on("close", () => {
+    if (!res.writableEnded) {
+      controller.abort();
+    }
+  });
+  return new globalThis.Request(url, {
+    method: req.method,
+    headers,
+    signal: controller.signal,
+  });
 }
 
 /**
@@ -436,7 +458,7 @@ export class TestServerHttp {
       }
       this.currentRequestHeaders = extractHeaders(req);
       try {
-        const webResponse = await handler.fetch(toWebRequest(req), {
+        const webResponse = await handler.fetch(toWebRequest(req, res), {
           parsedBody: req.body,
         });
         await writeWebResponse(res, webResponse);
@@ -540,9 +562,12 @@ export class TestServerHttp {
         }
 
         try {
-          const webResponse = await transport.handleRequest(toWebRequest(req), {
+          const webResponse = await transport.handleRequest(
-            parsedBody: req.body,
+            toWebRequest(req, res),
-          });
+            {
+              parsedBody: req.body,
+            },
+          );
           await writeWebResponse(res, webResponse);
         } catch (error) {
           // If response already sent (e.g., by OAuth middleware), don't send another
@@ -577,7 +602,7 @@ export class TestServerHttp {
 
         try {
           const webResponse = await newTransport.handleRequest(
-            toWebRequest(req),
+            toWebRequest(req, res),
             { parsedBody: req.body },
           );
           await writeWebResponse(res, webResponse);
@@ -615,7 +640,9 @@ export class TestServerHttp {
       // Let the transport handle the GET request
       this.currentRequestHeaders = extractHeaders(req);
       try {
-        const webResponse = await transport.handleRequest(toWebRequest(req));
+        const webResponse = await transport.handleRequest(
+          toWebRequest(req, res),
+        );
         await writeWebResponse(res, webResponse);
       } catch (error) {
         if (!res.headersSent) {