diff --git a/.changeset/read-body-without-request-body-stream.md b/.changeset/read-body-without-request-body-stream.md
new file mode 100644
index 0000000000..2376b64051
--- /dev/null
+++ b/.changeset/read-body-without-request-body-stream.md
@@ -0,0 +1,5 @@
+---
+'@modelcontextprotocol/server': patch
+---
+
+`readRequestBody`, and with it `createMcpHandler` and the Streamable HTTP transport, now reads request bodies in runtimes that have no `Request.body` stream, such as Firefox. Previously every POST there was answered `400` with a JSON-RPC parse error (`-32700`). In those runtimes the body is read whole and then checked against `maxRequestBodySize`, so oversized bodies are still answered `413`.
diff --git a/packages/server/src/server/requestBody.ts b/packages/server/src/server/requestBody.ts
index a11e8d99ed..f583f94f90 100644
--- a/packages/server/src/server/requestBody.ts
+++ b/packages/server/src/server/requestBody.ts
@@ -28,7 +28,9 @@ export function resolveMaxRequestBodySize(value: number | undefined): number {
  * Reads a request body as text, up to `maxBytes` (default
  * {@linkcode DEFAULT_MAX_REQUEST_BODY_SIZE}). A declared `Content-Length` over the
  * limit is refused without reading anything; otherwise the read stops as soon as
- * more than the limit has arrived. Stream failures propagate.
+ * more than the limit has arrived. Where the runtime has no `Request.body` stream
+ * (Firefox), the body is read whole and then checked against the limit. Stream
+ * failures propagate.
  */
 export async function readRequestBody(
     request: Request,
@@ -40,6 +42,11 @@ export async function readRequestBody(
     if (request.body === null) {
         return { tooLarge: false, text: '' };
     }
+    // Firefox does not implement `Request.body` yet, though `arrayBuffer()` works.
+    if (request.body === undefined) {
+        const bytes = await request.arrayBuffer();
+        return bytes.byteLength > maxBytes ? { tooLarge: true } : { tooLarge: false, text: new TextDecoder().decode(bytes) };
+    }
     const reader = request.body.getReader();
     const decoder = new TextDecoder();
     let received = 0;
diff --git a/packages/server/test/server/createMcpHandler.test.ts b/packages/server/test/server/createMcpHandler.test.ts
index a1b9ce7986..5536ef7606 100644
--- a/packages/server/test/server/createMcpHandler.test.ts
+++ b/packages/server/test/server/createMcpHandler.test.ts
@@ -623,6 +623,20 @@ describe('createMcpHandler — stateless legacy fallback (the default)', () => {
         }
     });
 
+    it('reads the body where the runtime has no Request.body stream (Firefox), keeping the size limit', async () => {
+        const { factory, state } = testFactory();
+        const handler = createMcpHandler(factory);
+        const withoutBodyStream = (request: Request): Request => Object.defineProperty(request, 'body', { value: undefined });
+
+        const served = await handler.fetch(withoutBodyStream(postRequest(modernToolsCall('echo', { text: 'no body stream' }))));
+        expect(served.status).toBe(200);
+        expect(await served.text()).toContain('no body stream');
+
+        const refused = await handler.fetch(withoutBodyStream(postRequest('x'.repeat(4 * 1024 * 1024 + 1))));
+        expect(refused.status).toBe(413);
+        expect(state.contexts).toHaveLength(1);
+    });
+
     it('still serves the modern path on the same endpoint (one factory, both legs)', async () => {
         const { factory, state } = testFactory();
         const handler = createMcpHandler(factory);