diff --git a/src/core/event-log.ts b/src/core/event-log.ts
index a2833fe..3254f97 100644
--- a/src/core/event-log.ts
+++ b/src/core/event-log.ts
@@ -64,6 +64,13 @@ export interface HookEvent {
    * Optional — absent when no workflow is active.
    */
   workflow_id?: string;
+  /**
+   * Loom spine link — id of the Loom task this event belongs to, when the
+   * session was launched by Loom (LOOM_TASK_ID set). Lets Loom attribute token
+   * savings to a task exactly. Optional — absent outside Loom, so standalone
+   * token-pilot events stay byte-identical to before.
+   */
+  task_id?: string;
   event:
     | "denied"
     | "allowed"
@@ -235,8 +242,14 @@ export async function appendEvent(
       event.workflow_id ??
       process.env.TOKEN_PILOT_WORKFLOW_ID ??
       process.env.CLAUDE_CODE_WORKFLOW_ID ??
+      process.env.LOOM_WORKFLOW_ID ??
       undefined;
-    const tagged = wf ? { ...event, workflow_id: wf } : event;
+    // Loom spine: tag the task id when the session was launched by Loom.
+    // Env-driven so call sites stay unchanged; absent outside Loom → no field.
+    const taskId = event.task_id ?? process.env.LOOM_TASK_ID ?? undefined;
+    let tagged = event;
+    if (wf) tagged = { ...tagged, workflow_id: wf };
+    if (taskId) tagged = { ...tagged, task_id: taskId };
     await ensureLogDir(projectRoot);
     await rotateIfNeeded(projectRoot);
     const line = JSON.stringify(tagged) + "\n";
diff --git a/tests/core/event-log.test.ts b/tests/core/event-log.test.ts
index 3c5969a..fae0aa1 100644
--- a/tests/core/event-log.test.ts
+++ b/tests/core/event-log.test.ts
@@ -222,6 +222,59 @@ describe("appendEvent / loadEvents", () => {
   });
 });
 
+// ─── appendEvent — Loom spine tagging (task_id / workflow_id from env) ─────────
+
+describe("appendEvent — Loom spine tagging", () => {
+  const base: HookEvent = {
+    ts: 1_700_000_000_000,
+    session_id: "s1",
+    agent_type: null,
+    agent_id: null,
+    event: "denied",
+    file: "src/big.ts",
+    lines: 500,
+    estTokens: 2000,
+    summaryTokens: 400,
+    savedTokens: 1600,
+  };
+
+  it("tags task_id from LOOM_TASK_ID when set", async () => {
+    const project = await makeTmp();
+    tmpDirs.push(project);
+    process.env.LOOM_TASK_ID = "tj-xyz";
+    try {
+      await appendEvent(project, { ...base });
+    } finally {
+      delete process.env.LOOM_TASK_ID;
+    }
+    const events = await loadEvents(project);
+    expect(events[0].task_id).toBe("tj-xyz");
+  });
+
+  it("tags workflow_id from LOOM_WORKFLOW_ID when set", async () => {
+    const project = await makeTmp();
+    tmpDirs.push(project);
+    process.env.LOOM_WORKFLOW_ID = "wf-1";
+    try {
+      await appendEvent(project, { ...base });
+    } finally {
+      delete process.env.LOOM_WORKFLOW_ID;
+    }
+    const events = await loadEvents(project);
+    expect(events[0].workflow_id).toBe("wf-1");
+  });
+
+  it("adds no task_id field when LOOM_TASK_ID is unset (back-compat)", async () => {
+    const project = await makeTmp();
+    tmpDirs.push(project);
+    delete process.env.LOOM_TASK_ID;
+    await appendEvent(project, { ...base });
+    const events = await loadEvents(project);
+    expect(events[0].task_id).toBeUndefined();
+    expect("task_id" in events[0]).toBe(false);
+  });
+});
+
 // ─── applyRetention ──────────────────────────────────────────────────────────
 
 describe("applyRetention", () => {