feat(core): MCP server instrumentation without breaking Miniflare

[betegon]

Closes #16826, #16654, #16666

Different approach from #16807 .

Using Proxy was causing issues in cloudflare #16182.

Now using fill we shouldn't have those problems as fill doesn't create a new wrapper object with a different identity, so now:

1. fill just replaces the method on the existing object
2. The transport object keeps its original identity
3. When transport.start() runs and accesses private fields, this is still the original transport object
4. The WeakMap recognizes it as the same object that owns the private fields

What's inside

• Support for new MCP SDK methods (mcpServerInstance.tool(), mcpServerInstance.resource(), etc.)
• Tracing instrumentation
• Error handling

Tracing

It follows OTEL semantic conventions for MCP and adds more attributes we thought are useful.

It also handles PII based on user setting of sendDefaultPii.

Tracing flow

1. Transport receives tools/call request (id: 2)
2. Create INACTIVE mcp.server span
3. Store span inrequestIdToSpanMap[2] = { span, method: "tools/call", startTime }
4. Execute handler with span context (handler gets requestId: 2)
5. Handler finds span using requestId: 2
6. Tool execution happens within span context
7. Response sent with tool results
8. completeSpanWithToolResults(2, result) enriches and completes span

Error handling

1. error capture - errorCapture.ts

• Non-invasive error reporting that never interferes with MCP service operation
• Error context with MCP-specific metadata
• PII filtering respects sendDefaultPii settings
• Resilient to Sentry failures (wrapped in try-catch)

2. Tool execution error capturing - handlers.ts

• Captures exceptions thrown during tool execution
• Preserves normal MCP behaviour (errors converted to isError: true)
• Includes tool name, arguments, and request context
• Handles both sync and async tool handlers

3. Transport error instrumentation - transport.ts

• Captures connection errors and network failures
• Intercepts JSON-RPC error responses
• Includes transport type and session information
• Handles error responses being sent back to clients

[AbhiPrasad]

Very nice, all good from my eyes in overall direction!

[betegon]

Thanks @AbhiPrasad! Will keep working on this and close the other PR using Proxy. Will apply your suggestions here!

[cursor]

Bug: Incorrect `this` Context in Handler Calls

The this context is incorrectly handled when calling the originalHandler. The originalHandler.apply(originalHandler, handlerArgs) call sets this to the handler function itself, instead of preserving the original context from the wrapper. This also affects the fallback path in createWrappedHandler. The wrapper's this context must be explicitly passed to createErrorCapturingHandler to ensure the original MCP handler executes with its intended context.

packages/core/src/integrations/mcp-server/handlers.ts#L58-L61

sentry-javascript/packages/core/src/integrations/mcp-server/handlers.ts

Lines 58 to 61 in cdb920b

	): unknown {
	try {
	const result = originalHandler.apply(originalHandler, handlerArgs);

packages/core/src/integrations/mcp-server/handlers.ts#L42-L44

sentry-javascript/packages/core/src/integrations/mcp-server/handlers.ts

Lines 42 to 44 in cdb920b

	} catch (error) {
	DEBUG_BUILD && logger.warn('MCP handler wrapping failed:', error);
	return originalHandler.apply(this, handlerArgs);

Fix in Cursor • Fix in Web

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Bug: Transport Methods Not Instrumented After Initialization

The wrapTransportOnMessage, wrapTransportSend, wrapTransportOnClose, and wrapTransportError functions only instrument transport methods if they are defined at connect time. This fill-based approach fails to instrument methods set after initial connection, unlike the previous Proxy-based implementation which intercepted later assignments. As a result, lazily initialized transport handlers will not be instrumented.

packages/core/src/integrations/mcp-server/transport.ts#L17-L53

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

Lines 17 to 53 in cdb920b

	*/
	export function wrapTransportOnMessage(transport: MCPTransport): void {
	if (transport.onmessage) {
	fill(transport, 'onmessage', originalOnMessage => {
	return function (this: MCPTransport, jsonRpcMessage: unknown, extra?: unknown) {
	if (isJsonRpcRequest(jsonRpcMessage)) {
	const messageTyped = jsonRpcMessage as { method: string; id: string | number };
	// Create isolation scope for this request (standard Sentry pattern)
	const isolationScope = getIsolationScope().clone();
	return withIsolationScope(isolationScope, () => {
	// Create manual span that stays open until response
	const spanConfig = buildMcpServerSpanConfig(jsonRpcMessage, this, extra as ExtraHandlerData);
	const span = startInactiveSpan(spanConfig);
	// Store span context for handler correlation using requestId
	storeSpanForRequest(messageTyped.id, span, messageTyped.method);
	// Execute handler within span context
	return withActiveSpan(span, () => {
	return (originalOnMessage as (...args: unknown[]) => unknown).call(this, jsonRpcMessage, extra);
	});
	});
	}
	if (isJsonRpcNotification(jsonRpcMessage)) {
	return createMcpNotificationSpan(jsonRpcMessage, this, extra as ExtraHandlerData, () => {
	return (originalOnMessage as (...args: unknown[]) => unknown).call(this, jsonRpcMessage, extra);
	});
	}
	return (originalOnMessage as (...args: unknown[]) => unknown).call(this, jsonRpcMessage, extra);
	};
	});
	}
	}

Fix in Cursor • Fix in Web

---

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[AbhiPrasad]

1st pass.

Before I review further, I'd like to see the develop docs PR so that I can make sure all the attributes are being added as we expect. Right now it's a bit hard to know if an attribute is missing or not.

I assume much of this is AI generated, which is fine, but let's make sure we clean the comments up (and expand the jsdoc string appropriately).

[AbhiPrasad]

Are these vendored in from the mcp package? If so we need to put the mcp library license + sha we grabbed the types from in this file.

[AbhiPrasad]

m: Instead of inline comments like this, I would prefer if we added notes in the jsdoc to document behaviour. Right now this comment is a bit redundant with the code.

Ditto with other instances of this.

[AbhiPrasad]

l: I would prefer if we called reduce on Object.entries(spanData) to do this operation instead of calling filter + Object.fromEntries on the constructed array.

[AbhiPrasad]

The sdk should not be setting tags by default. Tags are meant to be only set by users.

What we can instead do is set a mechanism: https://develop.sentry.dev/sdk/data-model/event-payloads/exception/#exception-mechanism.

[AbhiPrasad]

Let's add an example usage snippet to the jsdoc here.