diff --git a/apps/server/src/orchestration-v2/Adapters/ClaudeTaskOutputTail.test.ts b/apps/server/src/orchestration-v2/Adapters/ClaudeTaskOutputTail.test.ts
index 51c9a7848f100..e110b53fc562c 100644
--- a/apps/server/src/orchestration-v2/Adapters/ClaudeTaskOutputTail.test.ts
+++ b/apps/server/src/orchestration-v2/Adapters/ClaudeTaskOutputTail.test.ts
@@ -127,6 +127,38 @@ describe("tailClaudeTaskOutput", () => {
       assert.deepEqual(chunks, ["one\n", "two\n"]);
     }).pipe(Effect.provide(NodeServices.layer)),
   );
+
+  it.effect("reads nothing when a session or task id leaves the Claude task directory", () =>
+    Effect.scoped(
+      Effect.gen(function* () {
+        const fileSystem = yield* FileSystem.FileSystem;
+        const path = yield* Path.Path;
+        const base = yield* fileSystem.makeTempDirectoryScoped({ prefix: "claude-task-output-" });
+        const root = path.join(base, "claude-root");
+        yield* fileSystem.makeDirectory(path.join(root, "-private-tmp-project"), {
+          recursive: true,
+        });
+        const outside = path.join(base, "outside", "tasks");
+        yield* fileSystem.makeDirectory(outside, { recursive: true });
+        yield* fileSystem.writeFileString(path.join(outside, `${TASK_ID}.output`), "secret\n");
+        const chunks: Array<string> = [];
+        const stop = yield* Deferred.make<void>();
+        yield* Deferred.succeed(stop, undefined);
+        for (const ids of [
+          { sessionId: "../../outside", taskId: TASK_ID },
+          { sessionId: SESSION_ID, taskId: `../../../../outside/tasks/${TASK_ID}` },
+        ]) {
+          yield* tailClaudeTaskOutput({
+            root,
+            ...ids,
+            stop,
+            onChunk: (chunk) => Effect.sync(() => chunks.push(chunk)),
+          });
+        }
+        assert.deepEqual(chunks, []);
+      }),
+    ).pipe(Effect.provide(NodeServices.layer)),
+  );
 });
 
 describe("claudeTaskOutputRoot", () => {
diff --git a/apps/server/src/orchestration-v2/Adapters/ClaudeTaskOutputTail.ts b/apps/server/src/orchestration-v2/Adapters/ClaudeTaskOutputTail.ts
index 7eac250d7afde..55db935792b88 100644
--- a/apps/server/src/orchestration-v2/Adapters/ClaudeTaskOutputTail.ts
+++ b/apps/server/src/orchestration-v2/Adapters/ClaudeTaskOutputTail.ts
@@ -12,6 +12,8 @@ const CLAUDE_TASK_OUTPUT_POLL_MS = 250;
 const MAX_LOCATE_ATTEMPTS = 40;
 /** Skip ahead of fast writers; only the tail is displayed. */
 const MAX_READ_BYTES = 256 * 1024;
+/** Ids come from the SDK stream and are joined into a path; one plain segment keeps them inside root. */
+const SAFE_PATH_SEGMENT = /^[\w-]+$/;
 
 /** Claude Code's per-user temp root, or none where it has no uid (Windows). */
 export function claudeTaskOutputRoot(
@@ -65,6 +67,7 @@ export const tailClaudeTaskOutput = Effect.fn("ClaudeTaskOutputTail.tail")(funct
   readonly stop: Deferred.Deferred<void>;
   readonly onChunk: (chunk: string) => Effect.Effect<void>;
 }) {
+  if (!SAFE_PATH_SEGMENT.test(input.sessionId) || !SAFE_PATH_SEGMENT.test(input.taskId)) return;
   const fileSystem = yield* FileSystem.FileSystem;
   const pause = Deferred.await(input.stop).pipe(
     Effect.as(true),