feat(core): Enrich MCP SDK spans instead of duplicating them

dev-packages/e2e-tests/test-applications/node-express-mcp-v2/instrument.mjs

@@ -6,4 +6,6 @@ Sentry.init({
   debug: !!process.env.DEBUG,
   tunnel: `http://localhost:3031/`, // proxy server
   tracesSampleRate: 1,
+  // Registers a tracer provider, which turns on the MCP SDK's built-in OpenTelemetry spans.
+  enableOpenTelemetrySetup: process.env.MCP_SDK_OTEL === '1',
 });

dev-packages/e2e-tests/test-applications/node-express-mcp-v2/package.json

@@ -8,7 +8,9 @@
     "test": "playwright test",
     "clean": "npx rimraf node_modules pnpm-lock.yaml",
     "test:build": "pnpm install && pnpm build",
-    "test:assert": "pnpm test"
+    "test:build-mcp-sdk-otel": "pnpm install && pnpm add --config.block-exotic-subdeps=false https://pkg.pr.new/@modelcontextprotocol/server@919bb8e https://pkg.pr.new/@modelcontextprotocol/node@919bb8e https://pkg.pr.new/@modelcontextprotocol/client@919bb8e && pnpm build",
+    "test:assert": "pnpm test",
+    "test:assert-mcp-sdk-otel": "MCP_SDK_OTEL=1 pnpm test"
   },
   "dependencies": {
     "@cfworker/json-schema": "^4.0.0",
@@ -28,6 +30,13 @@
   },
   "type": "module",
   "sentryTest": {
-    "optional": true
+    "optional": true,
+    "optionalVariants": [
+      {
+        "build-command": "pnpm test:build-mcp-sdk-otel",
+        "assert-command": "pnpm test:assert-mcp-sdk-otel",
+        "label": "node-express-mcp-v2 (MCP SDK OpenTelemetry)"
+      }
+    ]
   }
 }

dev-packages/e2e-tests/test-applications/node-express-mcp-v2/tests/mcp-sdk-otel.test.ts

@@ -0,0 +1,49 @@
+import { expect, test } from '@playwright/test';
+import { collectStreamedSpans, getSpanOp, type SerializedStreamedSpan } from '@sentry-internal/test-utils';
+import { Client, StreamableHTTPClientTransport } from '@modelcontextprotocol/client';
+
+test.skip(process.env.MCP_SDK_OTEL !== '1', 'Only runs in the MCP SDK OpenTelemetry variant');
+
+/** Resolves with the single MCP span of the request whose span is named `spanName`, once its trace is complete. */
+async function getMcpSpan(spanName: string): Promise<SerializedStreamedSpan> {
+  const spans = await collectStreamedSpans(
+    'node-express-mcp-v2',
+    spans => spans.some(span => span.is_segment) && spans.some(span => span.name === spanName),
+  );
+  const mcpSpans = spans.filter(span => span.attributes?.['mcp.method.name']);
+  expect(mcpSpans).toHaveLength(1);
+  expect(getSpanOp(spans.find(span => span.is_segment)!)).toBe('http.server');
+
+  const [mcpSpan] = mcpSpans;
+  expect(mcpSpan!.name).toBe(spanName);
+  expect(mcpSpan!.is_segment).toBe(false);
+  expect(mcpSpan!.attributes?.['sentry.kind']?.value).toBe('server');
+  expect(getSpanOp(mcpSpan!)).toBe('mcp.server');
+  return mcpSpan!;
+}
+
+test('enriches the MCP SDK spans instead of duplicating them', async ({ baseURL }) => {
+  const client = new Client({ name: 'test-client-v2', version: '1.0.0' });
+
+  const initializeSpanPromise = getMcpSpan('initialize');
+  await client.connect(new StreamableHTTPClientTransport(new URL(`${baseURL}/mcp`)));
+  const initializeSpan = await initializeSpanPromise;
+  expect(initializeSpan.attributes?.['mcp.client.name']?.value).toBe('test-client-v2');
+  expect(initializeSpan.attributes?.['mcp.server.name']?.value).toBe('Echo-V2');
+
+  const toolSpanPromise = getMcpSpan('tools/call echo');
+  await client.callTool({ name: 'echo', arguments: { message: 'foobar' } });
+  const toolSpan = await toolSpanPromise;
+  expect(toolSpan.attributes?.['mcp.request.argument.message']?.value).toBe('"foobar"');
+  expect(toolSpan.attributes?.['mcp.tool.result.content_count']?.value).toBe(1);
+
+  const promptSpanPromise = getMcpSpan('prompts/get echo');
+  await client.getPrompt({ name: 'echo', arguments: { message: 'foobar' } });
+  expect((await promptSpanPromise).attributes?.['mcp.prompt.name']?.value).toBe('echo');
+
+  const errorSpanPromise = getMcpSpan('tools/call always-error');
+  await client.callTool({ name: 'always-error', arguments: {} }).catch(() => undefined);
+  expect((await errorSpanPromise).status).toBe('error');
+
+  await client.close();
+});

dev-packages/e2e-tests/test-applications/node-express-mcp-v2/tests/mcp.test.ts

@@ -3,6 +3,8 @@ import { waitForStreamedSpan, getSpanOp } from '@sentry-internal/test-utils';
 import { Client } from '@modelcontextprotocol/client';
 import { StreamableHTTPClientTransport } from '@modelcontextprotocol/client';
 
+test.skip(process.env.MCP_SDK_OTEL === '1', 'Covered by mcp-sdk-otel.test.ts');
+
 // The server in this app is never wrapped with `wrapMcpServerWithSentry` — spans are produced
 // solely by the auto-instrumenting `mcpServer` integration.
 test('auto-instruments a stable MCP SDK v2 server (no manual wrap)', async ({ baseURL }) => {

packages/core/src/integrations/mcp-server/correlation.ts

@@ -94,6 +94,53 @@ export function storeSpanForRequest(
   });
 }
 
+/**
+ * Records a request's response on its span: session metadata from `initialize`, the JSON-RPC
+ * error, and tool or prompt results. Does not end the span.
+ * @param transport - MCP transport instance
+ * @param span - The request span
+ * @param method - MCP method name
+ * @param result - Execution result for attribute extraction
+ * @param error - The JSON-RPC error response, if any
+ * @param capturePolicy - Capture policy resolved when the request began
+ */
+export function applyResponseToSpan(
+  transport: MCPTransport,
+  span: Span,
+  method: string,
+  result: unknown,
+  error: JsonRpcError | undefined,
+  capturePolicy: ResolvedMcpOptions,
+): void {
+  const responseSessionData =
+    method === 'initialize' ? extractSessionDataFromInitializeResponse(result) : extractSessionDataFromResponse(result);
+  if (method === 'initialize' && (responseSessionData.protocolVersion || responseSessionData.serverInfo)) {
+    updateSessionDataForTransport(transport, responseSessionData);
+  }
+  span.setAttributes(buildServerAttributesFromInfo(responseSessionData.serverInfo));
+  if (responseSessionData.protocolVersion) {
+    span.setAttribute(MCP_PROTOCOL_VERSION, responseSessionData.protocolVersion);
+  }
+
+  if (error) {
+    span.setAttribute(RPC_RESPONSE_STATUS_CODE, String(error.code));
+    if (isJsonRpcServerError(error.code)) {
+      span.setAttribute(ERROR_TYPE, String(error.code));
+      span.setStatus({ code: SPAN_STATUS_ERROR, message: error.message });
+    }
+  } else if (method === 'tools/call') {
+    const toolAttributes = extractToolResultAttributes(result, capturePolicy.recordOutputs);
+    span.setAttributes(toolAttributes);
+    // oxlint-disable-next-line typescript/no-deprecated -- Preserve classification based on the existing tool result attribute.
+    if (toolAttributes[MCP_TOOL_RESULT_IS_ERROR] === true) {
+      span.setAttribute(ERROR_TYPE, 'tool_error');
+      span.setStatus({ code: SPAN_STATUS_ERROR, message: 'internal_error' });
+    }
+  } else if (method === 'prompts/get') {
+    span.setAttributes(extractPromptResultAttributes(result, capturePolicy.recordOutputs));
+  }
+}
+
 /**
  * Completes span with results and cleans up correlation
  * @param transport - MCP transport instance
@@ -110,44 +157,8 @@ export function completeSpanWithResults(
   const spanMap = getOrCreateSpanMap(transport);
   const spanData = spanMap.get(requestId);
   if (spanData) {
-    const { span, method } = spanData;
-    const responseSessionData =
-      method === 'initialize'
-        ? extractSessionDataFromInitializeResponse(result)
-        : extractSessionDataFromResponse(result);
-    if (method === 'initialize' && (responseSessionData.protocolVersion || responseSessionData.serverInfo)) {
-      updateSessionDataForTransport(transport, responseSessionData);
-    }
-    const responseAttributes: Record<string, string | number> = {
-      ...buildServerAttributesFromInfo(responseSessionData.serverInfo),
-    };
-    if (responseSessionData.protocolVersion) {
-      responseAttributes[MCP_PROTOCOL_VERSION] = responseSessionData.protocolVersion;
-    }
-    if (Object.keys(responseAttributes).length > 0) {
-      span.setAttributes(responseAttributes);
-    }
-
-    if (error) {
-      span.setAttributes({ [RPC_RESPONSE_STATUS_CODE]: String(error.code) });
-      if (isJsonRpcServerError(error.code)) {
-        span.setAttributes({ [ERROR_TYPE]: String(error.code) });
-        span.setStatus({ code: SPAN_STATUS_ERROR, message: error.message });
-      }
-    } else if (method === 'tools/call') {
-      const toolAttributes = extractToolResultAttributes(result, spanData.capturePolicy.recordOutputs);
-      span.setAttributes(toolAttributes);
-      // oxlint-disable-next-line typescript/no-deprecated -- Preserve classification based on the existing tool result attribute.
-      if (toolAttributes[MCP_TOOL_RESULT_IS_ERROR] === true) {
-        span.setAttributes({ [ERROR_TYPE]: 'tool_error' });
-        span.setStatus({ code: SPAN_STATUS_ERROR, message: 'internal_error' });
-      }
-    } else if (method === 'prompts/get') {
-      const promptAttributes = extractPromptResultAttributes(result, spanData.capturePolicy.recordOutputs);
-      span.setAttributes(promptAttributes);
-    }
-
-    span.end();
+    applyResponseToSpan(transport, spanData.span, spanData.method, result, error, spanData.capturePolicy);
+    spanData.span.end();
     spanMap.delete(requestId);
   }
 }

packages/core/src/integrations/mcp-server/index.ts

@@ -2,6 +2,7 @@ import { fill } from '../../utils/object';
 import { wrapAllMCPHandlers, wrapExistingHandlers } from './handlers';
 import { wrapTransportError, wrapTransportOnClose, wrapTransportOnMessage, wrapTransportSend } from './transport';
 import type { MCPServerInstance, McpServerWrapperOptions, MCPTransport } from './types';
+import { decorateUpstreamInstrumentation } from './upstreamInstrumentation';
 import { validateMcpServerInstance } from './validation';
 
 /**
@@ -34,9 +35,13 @@ function applyMissingMcpOptions(target: McpServerWrapperOptions, source: McpServ
   }
 }
 
-function instrumentTransport(transport: MCPTransport, options: McpServerWrapperOptions): void {
-  wrapTransportOnMessage(transport, options);
-  wrapTransportSend(transport, options);
+function instrumentTransport(
+  transport: MCPTransport,
+  options: McpServerWrapperOptions,
+  upstreamInstrumented: boolean,
+): void {
+  wrapTransportOnMessage(transport, options, upstreamInstrumented);
+  wrapTransportSend(transport, options, upstreamInstrumented);
   wrapTransportOnClose(transport);
   wrapTransportError(transport);
 }
@@ -177,6 +182,7 @@ export function wrapMcpServerWithSentry<S extends object>(mcpServerInstance: S,
   const serverInstance = mcpServerInstance as MCPServerInstance;
   const wrapperOptions: McpServerWrapperOptions = { ...options };
   wrappedMcpServerOptions.set(mcpServerInstance, wrapperOptions);
+  const upstreamInstrumented = decorateUpstreamInstrumentation(serverInstance);
 
   fill(serverInstance, 'connect', originalConnect => {
     return async function (this: MCPServerInstance, transport: MCPTransport, ...restArgs: unknown[]) {
@@ -187,7 +193,7 @@ export function wrapMcpServerWithSentry<S extends object>(mcpServerInstance: S,
         }
 
         isTransportInstrumented = true;
-        instrumentTransport(transport, wrapperOptions);
+        instrumentTransport(transport, wrapperOptions, upstreamInstrumented);
       };
       const restoreStart = interceptTransportStart(transport, instrumentTransportOnce);
 

packages/core/src/integrations/mcp-server/spans.ts

@@ -23,7 +23,7 @@ import type {
   ExtraHandlerData,
   JsonRpcNotification,
   JsonRpcRequest,
-  McpSpanConfig,
+  McpSpanType,
   MCPTransport,
   ResolvedMcpOptions,
 } from './types';
@@ -45,7 +45,7 @@ function createSpanName(method: string, target?: string): string {
  * @param type - Span type configuration
  * @returns Sentry-specific attributes
  */
-function buildSentryAttributes(type: McpSpanConfig['type']): Record<string, string> {
+function buildSentryAttributes(type: McpSpanType): Record<string, string> {
   let op: string;
   let origin: string;
 
@@ -72,43 +72,45 @@ function buildSentryAttributes(type: McpSpanConfig['type']): Record<string, stri
 }
 
 /**
- * Unified builder for creating MCP spans
+ * Builds the name and attributes of an MCP notification span
  * @internal
- * @param config - Span configuration
- * @returns Created span
  */
-function createMcpSpan(config: McpSpanConfig): unknown {
-  const { type, message, transport, extra, callback, options } = config;
+export function buildMcpNotificationSpanConfig(
+  type: 'notification-incoming' | 'notification-outgoing',
+  message: JsonRpcNotification,
+  transport: MCPTransport,
+  options: ResolvedMcpOptions,
+  extra?: ExtraHandlerData,
+): { name: string; attributes: Record<string, string | number> } {
   const { method } = message;
   const params = message.params;
   const client = getClient();
   const spanStreamingEnabled = !!client && hasSpanStreamingEnabled(client);
 
-  // Determine span name based on type and OTEL conventions
-  let spanName: string;
-  if (type === 'request') {
-    const targetInfo = extractTargetInfo(method, params || {});
-    const target = spanStreamingEnabled && !targetInfo.targetIsLowCardinality ? undefined : targetInfo.target;
-    spanName = method ? createSpanName(method, target) : MCP_SERVER_SPAN_NAME_FALLBACK;
-  } else {
-    // For notifications, use method name directly per OpenTelemetry conventions.
-    // With span streaming, span names have to be low cardinality, so a message without a method name gets a static name.
-    spanName = method || (spanStreamingEnabled ? MCP_NOTIFICATION_SPAN_NAME_FALLBACK : method);
-  }
+  // For notifications, use method name directly per OpenTelemetry conventions.
+  // With span streaming, span names have to be low cardinality, so a message without a method name gets a static name.
+  const name = method || (spanStreamingEnabled ? MCP_NOTIFICATION_SPAN_NAME_FALLBACK : method);
 
   const rawAttributes: Record<string, string | number> = {
     ...buildTransportAttributes(transport, extra, message),
     [MCP_METHOD_NAME]: method,
-    ...buildTypeSpecificAttributes(type, message, params, options?.recordInputs),
+    ...buildTypeSpecificAttributes(type, message, params, options.recordInputs),
     ...buildSentryAttributes(type),
   };
 
   const userInfo = Boolean(client?.getDataCollectionOptions().userInfo);
   const attributes = filterMcpPiiFromSpanData(rawAttributes, userInfo) as Record<string, string | number>;
 
+  return { name, attributes };
+}
+
+function startMcpNotificationSpan(
+  { name, attributes }: { name: string; attributes: Record<string, string | number> },
+  callback: () => unknown,
+): unknown {
   return startSpan(
     {
-      name: spanName,
+      name,
       // oxlint-disable-next-line typescript/no-deprecated
       forceTransaction: true,
       attributes,
@@ -133,14 +135,10 @@ export function createMcpNotificationSpan(
   options: ResolvedMcpOptions,
   callback: () => unknown,
 ): unknown {
-  return createMcpSpan({
-    type: 'notification-incoming',
-    message: jsonRpcMessage,
-    transport,
-    extra,
+  return startMcpNotificationSpan(
+    buildMcpNotificationSpanConfig('notification-incoming', jsonRpcMessage, transport, options, extra),
     callback,
-    options,
-  });
+  );
 }
 
 /**
@@ -157,13 +155,10 @@ export function createMcpOutgoingNotificationSpan(
   options: ResolvedMcpOptions,
   callback: () => unknown,
 ): unknown {
-  return createMcpSpan({
-    type: 'notification-outgoing',
-    message: jsonRpcMessage,
-    transport,
-    options,
+  return startMcpNotificationSpan(
+    buildMcpNotificationSpanConfig('notification-outgoing', jsonRpcMessage, transport, options),
     callback,
-  });
+  );
 }
 
 /**