fix: forward the negotiated protocol version to the remote backend transport

The web client's SDK Client owns the `initialize` handshake, so the
`setProtocolVersion()` call it makes after initialize lands on
RemoteClientTransport — a pass-through to the Node backend, not the real
HTTP transport. The backend's StreamableHTTPClientTransport therefore never
learned the negotiated version and stamped no `Mcp-Protocol-Version` on any
request after initialize, starting with `notifications/initialized`. A
stateful server that requires the header rejects that notification with
HTTP 400.

RemoteClientTransport now records the version and rides it on every
`/api/mcp/send`; the backend applies it to its transport before the send, so
that request and everything the transport issues afterwards (the standalone
SSE GET, the session DELETE) carry the header. The version is validated
against a token pattern on the backend so a client can't push arbitrary
bytes into an upstream header.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_0119LaPQ3v4NyBK3ZTdM9j84

clients/web/src/test/integration/mcp/remote/remote-session.test.ts

@@ -120,6 +120,40 @@ describe("RemoteSession", () => {
     expect(session.transport).toBe(transport);
   });
 
+  // #1935: the browser's Client negotiates the version, so the backend has to
+  // be told before it can stamp `Mcp-Protocol-Version` on upstream requests.
+  it("applyProtocolVersion forwards a new version to the transport once", () => {
+    const session = new RemoteSession("s8a");
+    const setProtocolVersion = vi.fn();
+    session.setTransport({ setProtocolVersion } as unknown as Transport);
+
+    session.applyProtocolVersion("2025-11-25");
+    session.applyProtocolVersion("2025-11-25");
+    expect(setProtocolVersion.mock.calls).toEqual([["2025-11-25"]]);
+
+    // A renegotiated version (e.g. after a reconnect) is re-applied.
+    session.applyProtocolVersion("2026-07-28");
+    expect(setProtocolVersion).toHaveBeenLastCalledWith("2026-07-28");
+  });
+
+  it("applyProtocolVersion ignores an absent or non-token version", () => {
+    const session = new RemoteSession("s8b");
+    const setProtocolVersion = vi.fn();
+    session.setTransport({ setProtocolVersion } as unknown as Transport);
+
+    session.applyProtocolVersion(undefined);
+    // Header injection attempt — must never reach the upstream transport.
+    session.applyProtocolVersion("2025-11-25\r\nX-Evil: 1");
+    session.applyProtocolVersion("");
+    expect(setProtocolVersion).not.toHaveBeenCalled();
+  });
+
+  it("applyProtocolVersion is a no-op on a transport without setProtocolVersion (stdio)", () => {
+    const session = new RemoteSession("s8c");
+    session.setTransport({} as unknown as Transport);
+    expect(() => session.applyProtocolVersion("2025-11-25")).not.toThrow();
+  });
+
   it("hasEventConsumer reflects whether a consumer is attached", () => {
     const session = new RemoteSession("s9");
     expect(session.hasEventConsumer()).toBe(false);

clients/web/src/test/integration/mcp/remote/transport.test.ts

@@ -452,6 +452,58 @@ describe("Remote transport e2e", () => {
       }
     });
 
+    it("end-to-end: the negotiated Mcp-Protocol-Version reaches the upstream server after initialize (#1935)", async () => {
+      // The browser's SDK Client owns the initialize handshake, so its
+      // setProtocolVersion() lands on the *remote* transport. Without the
+      // forward to the backend, the backend's real streamable-HTTP transport
+      // never learns the version and every post-initialize request — starting
+      // with `notifications/initialized` — goes out without the header, which
+      // a stateful server may reject outright.
+      mcpHttpServer = createTestServerHttp({
+        serverInfo: createTestServerInfo(),
+        tools: [createEchoTool()],
+        serverType: "streamable-http",
+      });
+      await mcpHttpServer.start();
+
+      const config: MCPServerConfig = {
+        type: "streamable-http",
+        url: mcpHttpServer.url,
+      };
+
+      const { client, fetchRequestLogState } =
+        await setupRemoteAndConnect(config);
+
+      try {
+        await client.listTools();
+
+        const protocolVersion = client.getProtocolVersion();
+        expect(protocolVersion).toBeDefined();
+
+        const posts = fetchRequestLogState
+          .getFetchRequests()
+          .filter((r) => r.method === "POST");
+        const headerFor = (body: string): string | undefined => {
+          const entry = posts.find((r) => (r.requestBody ?? "").includes(body));
+          expect(entry).toBeDefined();
+          for (const [k, v] of Object.entries(entry?.requestHeaders ?? {})) {
+            if (k.toLowerCase() === "mcp-protocol-version") return v;
+          }
+          return undefined;
+        };
+
+        // The initialize POST itself predates negotiation and carries none.
+        expect(headerFor('"method":"initialize"')).toBeUndefined();
+        // Everything after it does.
+        expect(headerFor('"method":"notifications/initialized"')).toBe(
+          protocolVersion,
+        );
+        expect(headerFor('"method":"tools/list"')).toBe(protocolVersion);
+      } finally {
+        await client.disconnect();
+      }
+    });
+
     it("end-to-end: SEP-2243 Mcp-Param-* mirroring reaches the upstream modern server (#1846)", async () => {
       // A modern (2026-07-28) server whose `get_weather` tool annotates `city`
       // with `x-mcp-header: "City"`. The SDK's modern handler validates the