feat(langfuse): add tool-level spans with input/output to traces

Wrap each MCP tool handler in withSpan()/withSpanContext() to create
individual named spans per tool call:
- ask262_search_spec_sections
- ask262_get_section_content
- ask262_evaluate_in_engine262

Each span includes:
- langfuse.observation.input: JSON of tool arguments (query, sectionIds, code)
- tool name and relevant metadata as span attributes

This makes the Langfuse trace hierarchy show which MCP tool was called
and what input it received, nested under the root HTTP/stdio request span.

src/mcp-server-http.ts

@@ -36,7 +36,7 @@ import {
 import { DEFAULT_PORT, STORAGE_DIR as STORAGE_DIR_REL } from "./constants.js";
 import { createEmbeddings } from "./lib/embeddings-factory.js";
 import { LogOperation, logger } from "./lib/logger.js";
-import { withSpan } from "./lib/tracing.js";
+import { getSessionMetadata, setupTracing, withSpan } from "./lib/tracing.js";
 
 // Resolve storage path relative to this script's directory
 const __filename = fileURLToPath(import.meta.url);
@@ -90,6 +90,14 @@ async function createMcpServer() {
       },
     },
     async ({ query }) => {
+      return await withSpan(
+        "ask262_search_spec_sections",
+        {
+          "langfuse.observation.input": JSON.stringify({ query }),
+          tool: searchSpecToolName,
+          query,
+        },
+        async () => {
           const result = await searchSpecTool({ query });
           return {
             content: [{ type: "text", text: JSON.stringify(result, null, 2) }],
@@ -98,6 +106,8 @@ async function createMcpServer() {
           };
         },
       );
+    },
+  );
 
   // Register get section content tool
   server.registerTool(
@@ -113,6 +123,17 @@ async function createMcpServer() {
       },
     },
     async ({ sectionIds, recursive }) => {
+      return await withSpan(
+        "ask262_get_section_content",
+        {
+          "langfuse.observation.input": JSON.stringify({
+            sectionIds,
+            recursive,
+          }),
+          tool: sectionContentToolName,
+          section_count: sectionIds.length,
+        },
+        async () => {
           const result = await getSectionContentTool({ sectionIds, recursive });
           return {
             content: [{ type: "text", text: JSON.stringify(result, null, 2) }],
@@ -121,6 +142,8 @@ async function createMcpServer() {
           };
         },
       );
+    },
+  );
 
   // Register evaluate in engine262 tool
   server.registerTool(
@@ -136,9 +159,21 @@ async function createMcpServer() {
       },
     },
     async ({ code }) => {
+      return await withSpan(
+        "ask262_evaluate_in_engine262",
+        {
+          "langfuse.observation.input": JSON.stringify({
+            code: code.slice(0, 200),
+          }),
+          tool: evaluateToolName,
+          code_length: code.length,
+        },
+        async () => {
           const result = await evaluateTool({ code });
           const isError = result.error !== undefined;
-      const text = isError ? result.error : JSON.stringify(result, null, 2);
+          const text = isError
+            ? result.error
+            : JSON.stringify(result, null, 2);
           return {
             content: [{ type: "text", text }],
             structuredContent: result,
@@ -146,6 +181,8 @@ async function createMcpServer() {
           };
         },
       );
+    },
+  );
 
   // Register prompt for tool orchestration guidance
   server.registerPrompt("ask", askPromptMetadata, async ({ question }) =>
@@ -156,6 +193,9 @@ async function createMcpServer() {
 }
 
 export async function main() {
+  // Initialize tracing (registers Langfuse processor when enabled)
+  setupTracing();
+
   // Initialize HTTP server logger
   const log = await logger.forComponent("http-server");
 
@@ -217,6 +257,7 @@ export async function main() {
     }
 
     // Handle request within trace context (passing trace ID from header if available)
+    const sessionMetadata = getSessionMetadata("http");
     return await withSpan(
       LogOperation.HANDLING_MCP_HTTP_REQUEST,
       { method: c.req.method, client_ip: clientIp },
@@ -257,6 +298,7 @@ export async function main() {
         }
       },
       traceId,
+      sessionMetadata,
     );
   });
 

src/mcp-server-stdio.ts

@@ -37,7 +37,12 @@ import {
 import { STORAGE_DIR as STORAGE_DIR_REL } from "./constants.js";
 import { createEmbeddings } from "./lib/embeddings-factory.js";
 import { LogOperation, logger } from "./lib/logger.js";
-import { createProcessScopedTrace, withSpanContext } from "./lib/tracing.js";
+import {
+  createProcessScopedTrace,
+  getSessionMetadata,
+  setupTracing,
+  withSpanContext,
+} from "./lib/tracing.js";
 
 // Resolve storage path relative to this script's directory
 const __filename = fileURLToPath(import.meta.url);
@@ -90,6 +95,9 @@ export interface SearchSpecMCPOutput extends McpToolOutputBase {
 const embeddings = createEmbeddings();
 
 export async function main() {
+  // Initialize tracing (registers Langfuse processor when enabled)
+  setupTracing();
+
   // Initialize stdio server logger
   const log = await logger.forComponent("stdio-server");
 
@@ -139,8 +147,12 @@ export async function main() {
     async ({ query }: SearchSpecMCPInput): Promise<SearchSpecMCPOutput> => {
       return await withSpanContext(
         sessionTraceId,
-        "vector_search",
+        "ask262_search_spec_sections",
-        { tool: searchSpecToolName, query },
+        {
+          "langfuse.observation.input": JSON.stringify({ query }),
+          tool: searchSpecToolName,
+          query,
+        },
         async () => {
           const result = await searchSpecTool({ query });
           return {
@@ -149,6 +161,7 @@ export async function main() {
             isError: false,
           };
         },
+        getSessionMetadata("stdio"),
       );
     },
   );
@@ -172,8 +185,15 @@ export async function main() {
     }: GetSectionContentMCPInput): Promise<GetSectionContentMCPOutput> => {
       return await withSpanContext(
         sessionTraceId,
-        "section_fetch",
+        "ask262_get_section_content",
-        { tool: sectionContentToolName, section_count: sectionIds.length },
+        {
+          "langfuse.observation.input": JSON.stringify({
+            sectionIds,
+            recursive,
+          }),
+          tool: sectionContentToolName,
+          section_count: sectionIds.length,
+        },
         async () => {
           const result = await getSectionContentTool({ sectionIds, recursive });
           return {
@@ -182,6 +202,7 @@ export async function main() {
             isError: false,
           };
         },
+        getSessionMetadata("stdio"),
       );
     },
   );
@@ -202,8 +223,14 @@ export async function main() {
     async ({ code }: EvaluateToolMCPInput): Promise<EvaluateToolMCPOutput> => {
       return await withSpanContext(
         sessionTraceId,
-        "code_execution",
+        "ask262_evaluate_in_engine262",
-        { tool: evaluateToolName, code_length: code.length },
+        {
+          "langfuse.observation.input": JSON.stringify({
+            code: code.slice(0, 200),
+          }),
+          tool: evaluateToolName,
+          code_length: code.length,
+        },
         async () => {
           const result = await evaluateTool({ code });
           const isError = result.error !== undefined;
@@ -214,6 +241,7 @@ export async function main() {
             isError,
           };
         },
+        getSessionMetadata("stdio"),
       );
     },
   );