diff --git a/.changeset/fix-silent-errors.md b/.changeset/fix-silent-errors.md
new file mode 100644
index 000000000..7e4ef5bf3
--- /dev/null
+++ b/.changeset/fix-silent-errors.md
@@ -0,0 +1,5 @@
+---
+"@modelcontextprotocol/sdk": patch
+---
+
+Fix silent transport errors by ensuring onerror callback is invoked for all error responses in StreamableHTTPServerTransport
diff --git a/packages/server/src/server/streamableHttp.ts b/packages/server/src/server/streamableHttp.ts
index 74e689892..298f1296b 100644
--- a/packages/server/src/server/streamableHttp.ts
+++ b/packages/server/src/server/streamableHttp.ts
@@ -284,9 +284,10 @@ export class WebStandardStreamableHTTPServerTransport implements Transport {
         status: number,
         code: number,
         message: string,
-        options?: { headers?: Record<string, string>; data?: string }
+        options?: { headers?: Record<string, string>; data?: string; cause?: Error }
     ): Response {
         const error: { code: number; message: string; data?: string } = { code, message };
+        this.onerror?.(options?.cause ?? new Error(message));
         if (options?.data !== undefined) {
             error.data = options.data;
         }
@@ -321,7 +322,6 @@ export class WebStandardStreamableHTTPServerTransport implements Transport {
             const hostHeader = req.headers.get('host');
             if (!hostHeader || !this._allowedHosts.includes(hostHeader)) {
                 const error = `Invalid Host header: ${hostHeader}`;
-                this.onerror?.(new Error(error));
                 return this.createJsonErrorResponse(403, -32_000, error);
             }
         }
@@ -331,7 +331,6 @@ export class WebStandardStreamableHTTPServerTransport implements Transport {
             const originHeader = req.headers.get('origin');
             if (originHeader && !this._allowedOrigins.includes(originHeader)) {
                 const error = `Invalid Origin header: ${originHeader}`;
-                this.onerror?.(new Error(error));
                 return this.createJsonErrorResponse(403, -32_000, error);
             }
         }
@@ -553,8 +552,7 @@ export class WebStandardStreamableHTTPServerTransport implements Transport {
             });
 
             return new Response(readable, { headers });
-        } catch (error) {
-            this.onerror?.(error as Error);
+        } catch {
             return this.createJsonErrorResponse(500, -32_000, 'Error replaying events');
         }
     }
@@ -586,23 +584,9 @@ export class WebStandardStreamableHTTPServerTransport implements Transport {
      * Handles unsupported requests (`PUT`, `PATCH`, etc.)
      */
     private handleUnsupportedRequest(): Response {
-        return Response.json(
-            {
-                jsonrpc: '2.0',
-                error: {
-                    code: -32_000,
-                    message: 'Method not allowed.'
-                },
-                id: null
-            },
-            {
-                status: 405,
-                headers: {
-                    Allow: 'GET, POST, DELETE',
-                    'Content-Type': 'application/json'
-                }
-            }
-        );
+        return this.createJsonErrorResponse(405, -32_000, 'Method not allowed.', {
+            headers: { Allow: 'GET, POST, DELETE' }
+        });
     }
 
     /**
@@ -807,8 +791,10 @@ export class WebStandardStreamableHTTPServerTransport implements Transport {
             return new Response(readable, { status: 200, headers });
         } catch (error) {
             // return JSON-RPC formatted error
-            this.onerror?.(error as Error);
-            return this.createJsonErrorResponse(400, -32_700, 'Parse error', { data: String(error) });
+            return this.createJsonErrorResponse(400, -32_700, 'Parse error', {
+                data: String(error),
+                cause: error instanceof Error ? error : new Error(String(error))
+            });
         }
     }
 
diff --git a/packages/server/test/server/streamableHttp.test.ts b/packages/server/test/server/streamableHttp.test.ts
index ab6f22342..81c25acf2 100644
--- a/packages/server/test/server/streamableHttp.test.ts
+++ b/packages/server/test/server/streamableHttp.test.ts
@@ -765,4 +765,107 @@ describe('Zod v4', () => {
             await expect(transport.start()).rejects.toThrow('Transport already started');
         });
     });
+
+    describe('HTTPServerTransport - onerror callback', () => {
+        let transport: WebStandardStreamableHTTPServerTransport;
+        let errors: Error[];
+
+        beforeEach(async () => {
+            transport = new WebStandardStreamableHTTPServerTransport({
+                sessionIdGenerator: () => randomUUID()
+            });
+            errors = [];
+            transport.onerror = error => errors.push(error);
+            await transport.start();
+        });
+
+        it('should call onerror for invalid Accept header on POST', async () => {
+            const request = createRequest('POST', TEST_MESSAGES.initialize, { accept: 'application/json' });
+            const response = await transport.handleRequest(request);
+
+            expect(response.status).toBe(406);
+            expect(errors.length).toBe(1);
+            expect(errors[0]!.message).toMatch(/Not Acceptable/);
+        });
+
+        it('should call onerror for invalid Content-Type on POST', async () => {
+            const request = new Request('http://localhost/mcp', {
+                method: 'POST',
+                headers: {
+                    Accept: 'application/json, text/event-stream',
+                    'Content-Type': 'text/plain'
+                },
+                body: JSON.stringify(TEST_MESSAGES.initialize)
+            });
+            const response = await transport.handleRequest(request);
+
+            expect(response.status).toBe(415);
+            expect(errors.length).toBe(1);
+            expect(errors[0]!.message).toMatch(/Unsupported Media Type/);
+        });
+
+        it('should call onerror for invalid JSON on POST', async () => {
+            const request = new Request('http://localhost/mcp', {
+                method: 'POST',
+                headers: {
+                    Accept: 'application/json, text/event-stream',
+                    'Content-Type': 'application/json'
+                },
+                body: 'not valid json'
+            });
+            const response = await transport.handleRequest(request);
+
+            expect(response.status).toBe(400);
+            expect(errors.length).toBe(1);
+            expect(errors[0]!.message).toMatch(/Parse error/);
+        });
+
+        it('should call onerror for invalid JSON-RPC message on POST', async () => {
+            const request = new Request('http://localhost/mcp', {
+                method: 'POST',
+                headers: {
+                    Accept: 'application/json, text/event-stream',
+                    'Content-Type': 'application/json'
+                },
+                body: JSON.stringify({ not: 'a jsonrpc message' })
+            });
+            const response = await transport.handleRequest(request);
+
+            expect(response.status).toBe(400);
+            expect(errors.length).toBe(1);
+            expect(errors[0]!.message).toMatch(/Parse error.*Invalid JSON-RPC/);
+        });
+
+        it('should call onerror for invalid Accept header on GET', async () => {
+            const request = createRequest('GET', undefined, { accept: 'application/json' });
+            const response = await transport.handleRequest(request);
+
+            expect(response.status).toBe(406);
+            expect(errors.length).toBe(1);
+            expect(errors[0]!.message).toMatch(/Not Acceptable/);
+        });
+
+        it('should call onerror for unsupported HTTP methods', async () => {
+            const request = new Request('http://localhost/mcp', { method: 'PUT' });
+            const response = await transport.handleRequest(request);
+
+            expect(response.status).toBe(405);
+            expect(errors.length).toBe(1);
+            expect(errors[0]!.message).toBe('Method not allowed.');
+        });
+
+        it('should preserve original error object when createJsonErrorResponse receives cause', async () => {
+            const thrown = new Error('boom-from-onmessage');
+            transport.onmessage = () => {
+                throw thrown;
+            };
+
+            const request = createRequest('POST', TEST_MESSAGES.initialize);
+            const response = await transport.handleRequest(request);
+
+            expect(response.status).toBe(400);
+            expect(errors.length).toBe(1);
+            expect(errors[0]).toBe(thrown);
+        });
+    });
 });