feat(mcp-server): Add options to capture input and output arguments in spans

This introduces `recordInputs` and `recordOutputs` options to the MCP server wrapper and related functions, allowing for more granular control over the data captured in spans.

Author: betegon

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

@@ -95,12 +95,14 @@ export function getNotificationAttributes(
  * @param type - Span type (request or notification)
  * @param message - JSON-RPC message
  * @param params - Optional parameters for attribute extraction
+ * @param recordInputs - Whether to capture input arguments in spans
  * @returns Type-specific attributes for span instrumentation
  */
 export function buildTypeSpecificAttributes(
   type: McpSpanType,
   message: JsonRpcRequest | JsonRpcNotification,
   params?: Record<string, unknown>,
+  recordInputs?: boolean,
 ): Record<string, string | number> {
   if (type === 'request') {
     const request = message as JsonRpcRequest;
@@ -109,7 +111,7 @@ export function buildTypeSpecificAttributes(
     return {
       ...(request.id !== undefined && { [MCP_REQUEST_ID_ATTRIBUTE]: String(request.id) }),
       ...targetInfo.attributes,
-      ...getRequestArguments(request.method, params || {}),
+      ...(recordInputs ? getRequestArguments(request.method, params || {}) : {}),
     };
   }
 

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

@@ -6,11 +6,9 @@
  * request ID collisions between different MCP sessions.
  */
 
-import { getClient } from '../../currentScopes';
 import { SPAN_STATUS_ERROR } from '../../tracing';
 import type { Span } from '../../types-hoist/span';
 import { MCP_PROTOCOL_VERSION_ATTRIBUTE } from './attributes';
-import { filterMcpPiiFromSpanData } from './piiFiltering';
 import { extractPromptResultAttributes, extractToolResultAttributes } from './resultExtraction';
 import { buildServerAttributesFromInfo, extractSessionDataFromInitializeResponse } from './sessionExtraction';
 import type { MCPTransport, RequestId, RequestSpanMapValue } from './types';
@@ -57,8 +55,14 @@ export function storeSpanForRequest(transport: MCPTransport, requestId: RequestI
  * @param transport - MCP transport instance
  * @param requestId - Request identifier
  * @param result - Execution result for attribute extraction
+ * @param recordOutputs - Whether to capture output results in spans
  */
-export function completeSpanWithResults(transport: MCPTransport, requestId: RequestId, result: unknown): void {
+export function completeSpanWithResults(
+  transport: MCPTransport,
+  requestId: RequestId,
+  result: unknown,
+  recordOutputs: boolean,
+): void {
   const spanMap = getOrCreateSpanMap(transport);
   const spanData = spanMap.get(requestId);
   if (spanData) {
@@ -76,19 +80,11 @@ export function completeSpanWithResults(transport: MCPTransport, requestId: Requ
       }
 
       span.setAttributes(initAttributes);
-    } else if (method === 'tools/call') {
-      const rawToolAttributes = extractToolResultAttributes(result);
-      const client = getClient();
-      const sendDefaultPii = Boolean(client?.getOptions().sendDefaultPii);
-      const toolAttributes = filterMcpPiiFromSpanData(rawToolAttributes, sendDefaultPii);
-
+    } else if (method === 'tools/call' && recordOutputs) {
+      const toolAttributes = extractToolResultAttributes(result);
       span.setAttributes(toolAttributes);
-    } else if (method === 'prompts/get') {
-      const rawPromptAttributes = extractPromptResultAttributes(result);
-      const client = getClient();
-      const sendDefaultPii = Boolean(client?.getOptions().sendDefaultPii);
-      const promptAttributes = filterMcpPiiFromSpanData(rawPromptAttributes, sendDefaultPii);
-
+    } else if (method === 'prompts/get' && recordOutputs) {
+      const promptAttributes = extractPromptResultAttributes(result);
       span.setAttributes(promptAttributes);
     }
 

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

@@ -1,7 +1,7 @@
 import { fill } from '../../utils/object';
 import { wrapAllMCPHandlers } from './handlers';
 import { wrapTransportError, wrapTransportOnClose, wrapTransportOnMessage, wrapTransportSend } from './transport';
-import type { MCPServerInstance, MCPTransport } from './types';
+import type { MCPServerInstance, McpServerWrapperOptions, MCPTransport } from './types';
 import { validateMcpServerInstance } from './validation';
 
 /**
@@ -22,18 +22,26 @@ const wrappedMcpServerInstances = new WeakSet();
  * import { McpServer } from '@modelcontextprotocol/sdk/server/mcp.js';
  * import { StreamableHTTPServerTransport } from '@modelcontextprotocol/sdk/server/streamableHttp.js';
  *
+ * // Default: inputs and outputs are NOT captured
  * const server = Sentry.wrapMcpServerWithSentry(
  *   new McpServer({ name: "my-server", version: "1.0.0" })
  * );
  *
+ * // Capture both inputs and outputs
+ * const server = Sentry.wrapMcpServerWithSentry(
+ *   new McpServer({ name: "my-server", version: "1.0.0" }),
+ *   { recordInputs: true, recordOutputs: true }
+ * );
+ *
  * const transport = new StreamableHTTPServerTransport();
  * await server.connect(transport);
  * ```
  *
  * @param mcpServerInstance - MCP server instance to instrument
+ * @param options - Optional configuration for recording inputs and outputs
  * @returns Instrumented server instance (same reference)
  */
-export function wrapMcpServerWithSentry<S extends object>(mcpServerInstance: S): S {
+export function wrapMcpServerWithSentry<S extends object>(mcpServerInstance: S, options?: McpServerWrapperOptions): S {
   if (wrappedMcpServerInstances.has(mcpServerInstance)) {
     return mcpServerInstance;
   }
@@ -43,6 +51,8 @@ export function wrapMcpServerWithSentry<S extends object>(mcpServerInstance: S):
   }
 
   const serverInstance = mcpServerInstance as MCPServerInstance;
+  const recordInputs = options?.recordInputs ?? false;
+  const recordOutputs = options?.recordOutputs ?? false;
 
   fill(serverInstance, 'connect', originalConnect => {
     return async function (this: MCPServerInstance, transport: MCPTransport, ...restArgs: unknown[]) {
@@ -52,8 +62,8 @@ export function wrapMcpServerWithSentry<S extends object>(mcpServerInstance: S):
         ...restArgs,
       );
 
-      wrapTransportOnMessage(transport);
-      wrapTransportSend(transport);
+      wrapTransportOnMessage(transport, recordInputs);
+      wrapTransportSend(transport, recordOutputs);
       wrapTransportOnClose(transport);
       wrapTransportError(transport);
 

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

@@ -1,71 +1,37 @@
 /**
  * PII filtering for MCP server spans
  *
- * Removes sensitive data when sendDefaultPii is false.
- * Uses configurable attribute filtering to protect user privacy.
+ * Removes network-level sensitive data when sendDefaultPii is false.
+ * Input/output data (request arguments, tool/prompt results) is controlled
+ * separately via recordInputs/recordOutputs options.
  */
 import type { SpanAttributeValue } from '../../types-hoist/span';
-import {
-  CLIENT_ADDRESS_ATTRIBUTE,
-  CLIENT_PORT_ATTRIBUTE,
-  MCP_LOGGING_MESSAGE_ATTRIBUTE,
-  MCP_PROMPT_RESULT_DESCRIPTION_ATTRIBUTE,
-  MCP_PROMPT_RESULT_MESSAGE_CONTENT_ATTRIBUTE,
-  MCP_PROMPT_RESULT_PREFIX,
-  MCP_REQUEST_ARGUMENT,
-  MCP_RESOURCE_URI_ATTRIBUTE,
-  MCP_TOOL_RESULT_CONTENT_ATTRIBUTE,
-  MCP_TOOL_RESULT_PREFIX,
-} from './attributes';
+import { CLIENT_ADDRESS_ATTRIBUTE, CLIENT_PORT_ATTRIBUTE, MCP_RESOURCE_URI_ATTRIBUTE } from './attributes';
 
 /**
- * PII attributes that should be removed when sendDefaultPii is false
+ * Network PII attributes that should be removed when sendDefaultPii is false
  * @internal
  */
-const PII_ATTRIBUTES = new Set([
-  CLIENT_ADDRESS_ATTRIBUTE,
-  CLIENT_PORT_ATTRIBUTE,
-  MCP_LOGGING_MESSAGE_ATTRIBUTE,
-  MCP_PROMPT_RESULT_DESCRIPTION_ATTRIBUTE,
-  MCP_PROMPT_RESULT_MESSAGE_CONTENT_ATTRIBUTE,
-  MCP_RESOURCE_URI_ATTRIBUTE,
-  MCP_TOOL_RESULT_CONTENT_ATTRIBUTE,
-]);
+const NETWORK_PII_ATTRIBUTES = new Set([CLIENT_ADDRESS_ATTRIBUTE, CLIENT_PORT_ATTRIBUTE, MCP_RESOURCE_URI_ATTRIBUTE]);
 
 /**
- * Checks if an attribute key should be considered PII.
+ * Checks if an attribute key should be considered network PII.
  *
  * Returns true for:
- * - Explicit PII attributes (client.address, client.port, mcp.logging.message, etc.)
- * - All request arguments (mcp.request.argument.*)
- * - Tool and prompt result content (mcp.tool.result.*, mcp.prompt.result.*) except metadata
- *
- * Preserves metadata attributes ending with _count, _error, or .is_error as they don't contain sensitive data.
+ * - client.address (IP address)
+ * - client.port (port number)
+ * - mcp.resource.uri (potentially sensitive URIs)
  *
  * @param key - Attribute key to evaluate
- * @returns true if the attribute should be filtered out (is PII), false if it should be preserved
+ * @returns true if the attribute should be filtered out (is network PII), false if it should be preserved
  * @internal
  */
-function isPiiAttribute(key: string): boolean {
-  if (PII_ATTRIBUTES.has(key)) {
-    return true;
-  }
-
-  if (key.startsWith(`${MCP_REQUEST_ARGUMENT}.`)) {
-    return true;
-  }
-
-  if (key.startsWith(`${MCP_TOOL_RESULT_PREFIX}.`) || key.startsWith(`${MCP_PROMPT_RESULT_PREFIX}.`)) {
-    if (!key.endsWith('_count') && !key.endsWith('_error') && !key.endsWith('.is_error')) {
-      return true;
-    }
-  }
-
-  return false;
+function isNetworkPiiAttribute(key: string): boolean {
+  return NETWORK_PII_ATTRIBUTES.has(key);
 }
 
 /**
- * Removes PII attributes from span data when sendDefaultPii is false
+ * Removes network PII attributes from span data when sendDefaultPii is false
  * @param spanData - Raw span attributes
  * @param sendDefaultPii - Whether to include PII data
  * @returns Filtered span attributes
@@ -80,7 +46,7 @@ export function filterMcpPiiFromSpanData(
 
   return Object.entries(spanData).reduce(
     (acc, [key, value]) => {
-      if (!isPiiAttribute(key)) {
+      if (!isNetworkPiiAttribute(key)) {
         acc[key] = value as SpanAttributeValue;
       }
       return acc;

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

@@ -76,7 +76,7 @@ function buildSentryAttributes(type: McpSpanConfig['type']): Record<string, stri
  * @returns Created span
  */
 function createMcpSpan(config: McpSpanConfig): unknown {
-  const { type, message, transport, extra, callback } = config;
+  const { type, message, transport, extra, callback, recordInputs } = config;
   const { method } = message;
   const params = message.params;
 
@@ -93,7 +93,7 @@ function createMcpSpan(config: McpSpanConfig): unknown {
   const rawAttributes: Record<string, string | number> = {
     ...buildTransportAttributes(transport, extra),
     [MCP_METHOD_NAME_ATTRIBUTE]: method,
-    ...buildTypeSpecificAttributes(type, message, params),
+    ...buildTypeSpecificAttributes(type, message, params, recordInputs),
     ...buildSentryAttributes(type),
   };
 
@@ -116,13 +116,15 @@ function createMcpSpan(config: McpSpanConfig): unknown {
  * @param jsonRpcMessage - Notification message
  * @param transport - MCP transport instance
  * @param extra - Extra handler data
+ * @param recordInputs - Whether to capture input arguments in spans
  * @param callback - Span execution callback
  * @returns Span execution result
  */
 export function createMcpNotificationSpan(
   jsonRpcMessage: JsonRpcNotification,
   transport: MCPTransport,
   extra: ExtraHandlerData,
+  recordInputs: boolean,
   callback: () => unknown,
 ): unknown {
   return createMcpSpan({
@@ -131,6 +133,7 @@ export function createMcpNotificationSpan(
     transport,
     extra,
     callback,
+    recordInputs,
   });
 }
 
@@ -159,12 +162,14 @@ export function createMcpOutgoingNotificationSpan(
  * @param jsonRpcMessage - Request message
  * @param transport - MCP transport instance
  * @param extra - Optional extra handler data
+ * @param recordInputs - Whether to capture input arguments in spans
  * @returns Span configuration object
  */
 export function buildMcpServerSpanConfig(
   jsonRpcMessage: JsonRpcRequest,
   transport: MCPTransport,
   extra?: ExtraHandlerData,
+  recordInputs?: boolean,
 ): {
   name: string;
   op: string;
@@ -180,7 +185,7 @@ export function buildMcpServerSpanConfig(
   const rawAttributes: Record<string, string | number> = {
     ...buildTransportAttributes(transport, extra),
     [MCP_METHOD_NAME_ATTRIBUTE]: method,
-    ...buildTypeSpecificAttributes('request', jsonRpcMessage, params),
+    ...buildTypeSpecificAttributes('request', jsonRpcMessage, params, recordInputs),
     ...buildSentryAttributes('request'),
   };
 

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

@@ -30,8 +30,9 @@ import { isJsonRpcNotification, isJsonRpcRequest, isJsonRpcResponse, isValidCont
  * For "initialize" requests, extracts and stores client info and protocol version
  * in the session data for the transport.
  * @param transport - MCP transport instance to wrap
+ * @param recordInputs - Whether to capture input arguments in spans
  */
-export function wrapTransportOnMessage(transport: MCPTransport): void {
+export function wrapTransportOnMessage(transport: MCPTransport, recordInputs: boolean): void {
   if (transport.onmessage) {
     fill(transport, 'onmessage', originalOnMessage => {
       return function (this: MCPTransport, message: unknown, extra?: unknown) {
@@ -51,7 +52,7 @@ export function wrapTransportOnMessage(transport: MCPTransport): void {
           const isolationScope = getIsolationScope().clone();
 
           return withIsolationScope(isolationScope, () => {
-            const spanConfig = buildMcpServerSpanConfig(message, this, extra as ExtraHandlerData);
+            const spanConfig = buildMcpServerSpanConfig(message, this, extra as ExtraHandlerData, recordInputs);
             const span = startInactiveSpan(spanConfig);
 
             // For initialize requests, add client info directly to span (works even for stateless transports)
@@ -73,7 +74,7 @@ export function wrapTransportOnMessage(transport: MCPTransport): void {
         }
 
         if (isJsonRpcNotification(message)) {
-          return createMcpNotificationSpan(message, this, extra as ExtraHandlerData, () => {
+          return createMcpNotificationSpan(message, this, extra as ExtraHandlerData, recordInputs, () => {
             return (originalOnMessage as (...args: unknown[]) => unknown).call(this, message, extra);
           });
         }
@@ -89,8 +90,9 @@ export function wrapTransportOnMessage(transport: MCPTransport): void {
  * For "initialize" responses, extracts and stores protocol version and server info
  * in the session data for the transport.
  * @param transport - MCP transport instance to wrap
+ * @param recordOutputs - Whether to capture output results in spans
  */
-export function wrapTransportSend(transport: MCPTransport): void {
+export function wrapTransportSend(transport: MCPTransport, recordOutputs: boolean): void {
   if (transport.send) {
     fill(transport, 'send', originalSend => {
       return async function (this: MCPTransport, ...args: unknown[]) {
@@ -119,7 +121,7 @@ export function wrapTransportSend(transport: MCPTransport): void {
               }
             }
 
-            completeSpanWithResults(this, message.id, message.result);
+            completeSpanWithResults(this, message.id, message.result, recordOutputs);
           }
         }
 

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

@@ -127,6 +127,7 @@ export interface McpSpanConfig {
   transport: MCPTransport;
   extra?: ExtraHandlerData;
   callback: () => unknown;
+  recordInputs?: boolean;
 }
 
 export type SessionId = string;
@@ -183,3 +184,14 @@ export type SessionData = {
   protocolVersion?: string;
   serverInfo?: PartyInfo;
 };
+
+/**
+ * Options for configuring the MCP server wrapper.
+ * @internal
+ */
+export type McpServerWrapperOptions = {
+  /** Whether to capture tool/prompt input arguments in spans. @default false */
+  recordInputs?: boolean;
+  /** Whether to capture tool/prompt output results in spans. @default false */
+  recordOutputs?: boolean;
+};