diff --git a/packages/server/src/server/mcp.ts b/packages/server/src/server/mcp.ts
index 05136f5b6..dc803939d 100644
--- a/packages/server/src/server/mcp.ts
+++ b/packages/server/src/server/mcp.ts
@@ -213,8 +213,8 @@ export class McpServer {
                 await this.validateToolOutput(tool, result, request.params.name);
                 return result;
             } catch (error) {
-                if (error instanceof ProtocolError && error.code === ProtocolErrorCode.UrlElicitationRequired) {
-                    throw error; // Return the error to the caller without wrapping in CallToolResult
+                if (error instanceof ProtocolError) {
+                    throw error; // Keep protocol-level failures as JSON-RPC errors.
                 }
                 return this.createToolError(error instanceof Error ? error.message : String(error));
             }
diff --git a/packages/server/src/server/streamableHttp.ts b/packages/server/src/server/streamableHttp.ts
index 74e689892..090fc953e 100644
--- a/packages/server/src/server/streamableHttp.ts
+++ b/packages/server/src/server/streamableHttp.ts
@@ -635,7 +635,8 @@ export class WebStandardStreamableHTTPServerTransport implements Transport {
             if (options?.parsedBody === undefined) {
                 try {
                     rawMessage = await req.json();
-                } catch {
+                } catch (error) {
+                    this.onerror?.(error instanceof Error ? error : new Error(String(error)));
                     return this.createJsonErrorResponse(400, -32_700, 'Parse error: Invalid JSON');
                 }
             } else {
@@ -649,7 +650,8 @@ export class WebStandardStreamableHTTPServerTransport implements Transport {
                 messages = Array.isArray(rawMessage)
                     ? rawMessage.map(msg => JSONRPCMessageSchema.parse(msg))
                     : [JSONRPCMessageSchema.parse(rawMessage)];
-            } catch {
+            } catch (error) {
+                this.onerror?.(error instanceof Error ? error : new Error(String(error)));
                 return this.createJsonErrorResponse(400, -32_700, 'Parse error: Invalid JSON-RPC message');
             }
 
diff --git a/packages/server/test/server/streamableHttp.test.ts b/packages/server/test/server/streamableHttp.test.ts
index ab6f22342..d07a2cb75 100644
--- a/packages/server/test/server/streamableHttp.test.ts
+++ b/packages/server/test/server/streamableHttp.test.ts
@@ -209,6 +209,54 @@ describe('Zod v4', () => {
         });
 
         describe('POST Requests', () => {
+            it('should call onerror when request JSON is invalid', async () => {
+                const errorSpy = vi.fn();
+                transport.onerror = errorSpy;
+
+                const request = new Request('http://localhost/mcp', {
+                    method: 'POST',
+                    headers: {
+                        Accept: 'application/json, text/event-stream',
+                        'Content-Type': 'application/json'
+                    },
+                    body: '{invalid-json'
+                });
+
+                const response = await transport.handleRequest(request);
+
+                expect(response.status).toBe(400);
+                const errorData = await response.json();
+                expectErrorResponse(errorData, -32_700, /Invalid JSON/);
+                expect(errorSpy).toHaveBeenCalledTimes(1);
+                expect(errorSpy.mock.calls[0]?.[0]).toBeInstanceOf(Error);
+            });
+
+            it('should call onerror when request JSON-RPC message is invalid', async () => {
+                const errorSpy = vi.fn();
+                transport.onerror = errorSpy;
+
+                const request = new Request('http://localhost/mcp', {
+                    method: 'POST',
+                    headers: {
+                        Accept: 'application/json, text/event-stream',
+                        'Content-Type': 'application/json'
+                    },
+                    body: JSON.stringify({
+                        jsonrpc: '2.0',
+                        method: 123,
+                        params: {}
+                    })
+                });
+
+                const response = await transport.handleRequest(request);
+
+                expect(response.status).toBe(400);
+                const errorData = await response.json();
+                expectErrorResponse(errorData, -32_700, /Invalid JSON-RPC message/);
+                expect(errorSpy).toHaveBeenCalledTimes(1);
+                expect(errorSpy.mock.calls[0]?.[0]).toBeInstanceOf(Error);
+            });
+
             it('should handle post requests via SSE response correctly', async () => {
                 sessionId = await initializeServer();
 
@@ -271,6 +319,31 @@ describe('Zod v4', () => {
                 });
             });
 
+            it('should return a JSON-RPC error when calling an unknown tool', async () => {
+                sessionId = await initializeServer();
+
+                const toolCallMessage: JSONRPCMessage = {
+                    jsonrpc: '2.0',
+                    method: 'tools/call',
+                    params: {
+                        name: 'missing-tool',
+                        arguments: {}
+                    },
+                    id: 'call-missing'
+                };
+
+                const request = createRequest('POST', toolCallMessage, { sessionId });
+                const response = await transport.handleRequest(request);
+
+                expect(response.status).toBe(200);
+
+                const text = await readSSEEvent(response);
+                const eventData = parseSSEData(text);
+
+                expectErrorResponse(eventData, -32_602, /Tool missing-tool not found/);
+                expect((eventData as JSONRPCErrorResponse).id).toBe('call-missing');
+            });
+
             it('should reject requests without a valid session ID', async () => {
                 const request = createRequest('POST', TEST_MESSAGES.toolsList);
                 const response = await transport.handleRequest(request);