Forward W3C traceparent to MCP servers via _meta (SEP-414)

Plumb the active execute_tool span's W3C traceparent through
LanguageModelToolInvocationOptions and IMcpToolCallContext, then inject
it into the JSON-RPC tools/call payload's _meta so MCP server-side spans
can be parented to the client trace.

Refs #302301

Author: zhichli

extensions/copilot/src/extension/tools/vscode-node/toolsService.ts

@@ -175,6 +175,17 @@ export class ToolsService extends BaseToolsService {
 
 		const startTime = Date.now();
 
+		// Propagate W3C trace context to MCP tools so server-side spans can be correlated
+		// with this `execute_tool` span (MCP SEP-414, see #302301). Only set if not already
+		// supplied by the caller and OTel produced a real span context.
+		const optionsWithTrace = options as vscode.LanguageModelToolInvocationOptions<Object> & { traceparent?: string };
+		if (!optionsWithTrace.traceparent) {
+			const ctx = span.getSpanContext();
+			if (ctx) {
+				optionsWithTrace.traceparent = `00-${ctx.traceId}-${ctx.spanId}-01`;
+			}
+		}
+
 		return vscode.lm.invokeTool(getContributedToolName(name), options, token).then(
 			result => {
 				span.setStatus(SpanStatusCode.OK);

src/vs/workbench/api/common/extHostLanguageModelTools.ts

@@ -130,6 +130,8 @@ export class ExtHostLanguageModelTools implements ExtHostLanguageModelToolsShape
 				subAgentInvocationId: isProposedApiEnabled(extension, 'chatParticipantPrivate') ? options.subAgentInvocationId : undefined,
 				chatStreamToolCallId: isProposedApiEnabled(extension, 'chatParticipantAdditions') ? options.chatStreamToolCallId : undefined,
 				preToolUseResult: isProposedApiEnabled(extension, 'chatParticipantPrivate') ? options.preToolUseResult : undefined,
+				traceparent: isProposedApiEnabled(extension, 'chatParticipantPrivate') ? options.traceparent : undefined,
+				tracestate: isProposedApiEnabled(extension, 'chatParticipantPrivate') ? options.tracestate : undefined,
 			}, token);
 
 			const dto: Dto<IToolResult> = result instanceof SerializableObjectWithBuffers ? result.value : result;

src/vs/workbench/contrib/chat/common/tools/languageModelToolsService.ts

@@ -197,6 +197,14 @@ export interface IToolInvocation {
 	selectedCustomButton?: string;
 	/** Pre-tool-use hook result passed from the extension, if the hook was already executed externally. */
 	preToolUseResult?: IExternalPreToolUseHookResult;
+	/**
+	 * Optional W3C trace context `traceparent` value identifying the parent distributed
+	 * tracing span for this tool invocation. Forwarded to MCP tool implementations as
+	 * `_meta.traceparent` (MCP SEP-414).
+	 */
+	traceparent?: string;
+	/** Optional W3C trace context `tracestate` value paired with {@link traceparent}. */
+	tracestate?: string;
 }
 
 export interface IToolInvocationContext {

src/vs/workbench/contrib/mcp/common/mcpLanguageModelToolContribution.ts

@@ -266,7 +266,12 @@ class McpToolImplementation implements IToolImpl {
 			content: []
 		};
 
-		const callResult = await this._tool.callWithProgress(invocation.parameters as Record<string, unknown>, progress, { chatRequestId: invocation.chatRequestId, chatSessionResource: invocation.context?.sessionResource }, token);
+		const callResult = await this._tool.callWithProgress(invocation.parameters as Record<string, unknown>, progress, {
+			chatRequestId: invocation.chatRequestId,
+			chatSessionResource: invocation.context?.sessionResource,
+			traceparent: invocation.traceparent,
+			tracestate: invocation.tracestate,
+		}, token);
 		const details: Mutable<IToolResultInputOutputDetails> = {
 			input: JSON.stringify(invocation.parameters, undefined, 2),
 			output: [],

src/vs/workbench/contrib/mcp/common/mcpServer.ts

@@ -1199,6 +1199,14 @@ export class McpTool implements IMcpTool {
 			if (context?.chatRequestId) {
 				meta['vscode.requestId'] = context.chatRequestId;
 			}
+			// Propagate W3C trace context to the MCP server (MCP SEP-414) so server-side
+			// spans can be correlated with the client trace.
+			if (context?.traceparent) {
+				meta['traceparent'] = context.traceparent;
+				if (context.tracestate) {
+					meta['tracestate'] = context.tracestate;
+				}
+			}
 
 			const taskHint = this._definition.execution?.taskSupport;
 			const serverSupportsTasksForTools = h.capabilities.tasks?.requests?.tools?.call !== undefined;

src/vs/workbench/contrib/mcp/common/mcpTypes.ts

@@ -453,6 +453,13 @@ export interface IMcpPromptMessage extends MCP.PromptMessage { }
 export interface IMcpToolCallContext {
 	chatSessionResource: URI | undefined;
 	chatRequestId?: string;
+	/**
+	 * Optional W3C trace context `traceparent` value to forward to the MCP server
+	 * via `_meta.traceparent` on the JSON-RPC `tools/call` request (MCP SEP-414).
+	 */
+	traceparent?: string;
+	/** Optional W3C trace context `tracestate` value paired with {@link traceparent}. */
+	tracestate?: string;
 }
 
 /**

src/vs/workbench/contrib/mcp/test/common/mcpServerRequestHandler.test.ts

@@ -381,6 +381,34 @@ suite('Workbench - MCP - ServerRequestHandler', () => {
 			assert.strictEqual(e.name, 'Canceled');
 		}
 	});
+
+	test('callTool forwards _meta.traceparent to the JSON-RPC payload (MCP SEP-414)', async () => {
+		const traceparent = '00-0af7651916cd43dd8448eb211c80319c-b7ad6b7169203331-01';
+		const tracestate = 'rojo=00f067aa0ba902b7';
+
+		const callPromise = handler.callTool({
+			name: 'echo',
+			arguments: { hello: 'world' },
+			_meta: { traceparent, tracestate, progressToken: 'tok-1' },
+		});
+
+		const sentMessages = transport.getSentMessages();
+		const callRequest = sentMessages[2] as MCP.JSONRPCRequest & MCP.CallToolRequest;
+		assert.strictEqual(callRequest.method, 'tools/call');
+		assert.deepStrictEqual(callRequest.params._meta, {
+			traceparent,
+			tracestate,
+			progressToken: 'tok-1',
+		});
+
+		transport.simulateReceiveMessage({
+			jsonrpc: MCP.JSONRPC_VERSION,
+			id: callRequest.id,
+			result: { content: [] },
+		});
+
+		await callPromise;
+	});
 });
 
 suite.skip('Workbench - MCP - McpTask', () => { // TODO@connor4312 https://github.com/microsoft/vscode/issues/280126

src/vscode-dts/vscode.proposed.chatParticipantPrivate.d.ts

@@ -303,6 +303,17 @@ declare module 'vscode' {
 		 * Unique ID for the subagent invocation, used to group tool calls from the same subagent run together.
 		 */
 		subAgentInvocationId?: string;
+		/**
+		 * W3C trace context `traceparent` header value identifying the active distributed
+		 * tracing span. When provided to a tool implementation backed by an MCP server, this
+		 * value is forwarded as `_meta.traceparent` on the JSON-RPC `tools/call` request so
+		 * downstream servers can correlate their spans (MCP SEP-414).
+		 */
+		traceparent?: string;
+		/**
+		 * Optional W3C trace context `tracestate` header value paired with `traceparent`.
+		 */
+		tracestate?: string;
 		/**
 		 * Pre-tool-use hook result, if the hook was already executed by the caller.
 		 * When provided, the tools service will skip executing its own preToolUse hook