fix(client): do not wait for the end of an unread POST reply body

.changeset/release-post-reply-body.md

@@ -0,0 +1,5 @@
+---
+'@modelcontextprotocol/client': patch
+---
+
+`StreamableHTTPClientTransport.send()` and `SSEClientTransport.send()` no longer wait for the end of a POST reply with a 2xx status whose body they do not read (a `202`, a reply to a message with no requests, a reply with an unexpected content type). A server or proxy that sends the headers of such a reply and keeps it open no longer stalls `send()` or `Client.connect()`. The unread body is still read to its end, but `send()` does not wait for that; a body that has not ended after 100 ms is cancelled, which closes its HTTP/1.1 connection unless a custom `fetch` holds a clone of the reply. A body with no `getReader()`, as a custom `fetch` may return, is read in the same way but never cancelled. POST replies with any other status and `terminateSession()` still wait for the end of the body.

packages/client/src/client/sse.ts

@@ -5,6 +5,7 @@ import {
     fetchLeavingRedirects,
     fetchWithinOrigin,
     JSONRPCMessageSchema,
+    releaseBody,
     SdkError,
     SdkErrorCode,
     SdkHttpError,
@@ -491,7 +492,7 @@ export class SSEClientTransport implements Transport {
             }
 
             // Release connection - POST responses don't have content we need
-            await response.text?.().catch(() => {});
+            releaseBody(response);
         } catch (error) {
             this.onerror?.(error as Error);
             throw error;

packages/client/src/client/streamableHttp.ts

@@ -16,6 +16,7 @@ import {
     mcpNameSource,
     mediaTypeEssence,
     PROTOCOL_VERSION_META_KEY,
+    releaseBody,
     SdkError,
     SdkErrorCode,
     SdkHttpError,
@@ -1164,7 +1165,7 @@ export class StreamableHTTPClientTransport implements Transport {
 
             // If the response is 202 Accepted, there's no body to process
             if (response.status === 202) {
-                await response.text?.().catch(() => {});
+                releaseBody(response);
                 // if the accepted notification is initialized, we start the SSE stream
                 // if it's supported by the server
                 if (isInitializedNotification(message)) {
@@ -1208,14 +1209,14 @@ export class StreamableHTTPClientTransport implements Transport {
                         this.onmessage?.(msg);
                     }
                 } else {
-                    await response.text?.().catch(() => {});
+                    releaseBody(response);
                     throw new SdkError(SdkErrorCode.ClientHttpUnexpectedContent, `Unexpected content type: ${contentType}`, {
                         contentType
                     });
                 }
             } else {
                 // No requests in message but got 200 OK - still need to release connection
-                await response.text?.().catch(() => {});
+                releaseBody(response);
             }
         } catch (error) {
             // Intentional per-request abort BEFORE response headers (the

packages/core-internal/src/shared/transport.ts

@@ -111,6 +111,26 @@ export function unfollowedRedirect(url: string | URL, response: Response): strin
     return `Redirect to ${target.href} not followed; use that URL as the endpoint if it is the intended server`;
 }
 
+/** How long `releaseBody()` lets a body end by itself. */
+const RELEASE_BODY_DEADLINE_MS = 100;
+
+/** Reads the unread body of `response` to its end without waiting for it, and cancels what has not ended at the deadline. Never throws. */
+export function releaseBody(response: Response): void {
+    const release = async () => {
+        if (typeof response.body?.getReader !== 'function') return void (await response.text());
+        const reader = response.body.getReader();
+        const cancel = async () => reader.cancel();
+        const timer = setTimeout(() => void cancel().catch(() => {}), RELEASE_BODY_DEADLINE_MS);
+        try {
+            let done = false;
+            while (!done) ({ done } = await reader.read());
+        } finally {
+            clearTimeout(timer);
+        }
+    };
+    release().catch(() => {});
+}
+
 /**
  * Options for sending a JSON-RPC message.
  */