diff --git a/src/auth-cli.ts b/src/auth-cli.ts
index d072a88..e20a343 100644
--- a/src/auth-cli.ts
+++ b/src/auth-cli.ts
@@ -20,6 +20,18 @@ import * as path from 'path';
 import * as os from 'os';
 
 const home = process.env.HOME || os.homedir();
+
+// ── session ID (Langfuse tracing) ──
+// Priority: SKILL_SESSION_ID env > auto-generate
+const SESSION_ID = process.env.SKILL_SESSION_ID || (() => {
+  const ts = new Date();
+  const pad = (n: number) => String(n).padStart(2, '0');
+  const tsPart = `${ts.getFullYear()}${pad(ts.getMonth()+1)}${pad(ts.getDate())}-${pad(ts.getHours())}${pad(ts.getMinutes())}${pad(ts.getSeconds())}`;
+  const rand = Math.random().toString(36).slice(2, 6);
+  return `skill-${tsPart}-${rand}`;
+})();
+process.env.SKILL_SESSION_ID = SESSION_ID;
+
 const AUTH_RT_BIN = process.env.AUTH_RT_BIN
   || (() => {
     // Check if auth-rt is in PATH