diff --git a/src/daemon/__tests__/request-execution-scope.test.ts b/src/daemon/__tests__/request-execution-scope.test.ts
index f0298db1f..2dfe85f2a 100644
--- a/src/daemon/__tests__/request-execution-scope.test.ts
+++ b/src/daemon/__tests__/request-execution-scope.test.ts
@@ -91,6 +91,29 @@ test('createRequestExecutionScope resolves session-scoped request and runner log
   expect(scope.runnerLogPath).toMatch(/cwd_[a-f0-9]{16}_default\/runner\.log$/);
 });
 
+test('a relative session name is rejected before any session artifact path is written', async () => {
+  const sessionStore = makeSessionStore('agent-device-request-scope-');
+  const stateDir = sessionStore.resolveDaemonStateDir();
+  const requestId = 'relative-session-1';
+
+  await withDiagnosticsScope({ command: 'snapshot', requestId, logPath: LOG_PATH }, async () => {
+    await expect(
+      createRequestExecutionScope({
+        req: makeRequest({ session: '..', meta: { requestId } }),
+        sessionStore,
+        leaseRegistry: new LeaseRegistry(),
+      }),
+    ).rejects.toMatchObject({
+      code: 'INVALID_ARGS',
+      message: expect.stringMatching(/session name/i),
+    });
+    flushDiagnosticsToSessionFile({ force: true });
+  });
+
+  // Nothing landed in the state dir itself (where `sessions/..` resolves to).
+  expect(fs.readdirSync(stateDir).filter((entry) => entry !== 'sessions')).toEqual([]);
+});
+
 test('request diagnostics flush into the effective session request log', async () => {
   const sessionStore = makeSessionStore('agent-device-request-scope-');
   const cwd = fs.mkdtempSync(path.join(TEST_ROOT, 'diag-scope-'));
diff --git a/src/daemon/__tests__/session-store.test.ts b/src/daemon/__tests__/session-store.test.ts
index 447f43482..32b4b03d9 100644
--- a/src/daemon/__tests__/session-store.test.ts
+++ b/src/daemon/__tests__/session-store.test.ts
@@ -3,6 +3,7 @@ import assert from 'node:assert/strict';
 import fs from 'node:fs';
 import os from 'node:os';
 import path from 'node:path';
+import { AppError } from '@agent-device/kernel/errors';
 import { SessionStore } from '../session-store.ts';
 import type { SessionState } from '../types.ts';
 import { buildRequestFinishedEvent } from '../session-event-log.ts';
@@ -114,6 +115,26 @@ test('defaultTracePath sanitizes session name', () => {
   assert.match(tracePath, /\.trace\.log$/);
 });
 
+test('resolveSessionDir keeps every session dir beneath the sessions dir', () => {
+  const sessionsDir = path.join(os.tmpdir(), 'agent-device-tests', 'sessions');
+  const store = new SessionStore(sessionsDir);
+  assert.equal(store.resolveSessionDir('a/b:c d'), path.join(sessionsDir, 'a_b_c_d'));
+  // `.` and `..` survive `safeSessionName` unchanged, so without an explicit
+  // refusal `path.join` resolves them to the sessions dir itself and its parent
+  // (the daemon state dir): a remote caller's `--session ..` would then land
+  // app.log / runner.log / requests/*.ndjson outside the sessions tree.
+  for (const name of ['.', '..', '']) {
+    assert.throws(
+      () => store.resolveSessionDir(name),
+      (error: unknown) =>
+        error instanceof AppError &&
+        error.code === 'INVALID_ARGS' &&
+        /session name/i.test(error.message),
+      `expected resolveSessionDir(${JSON.stringify(name)}) to reject`,
+    );
+  }
+});
+
 test('session lease metadata round-trips through the store', () => {
   const { store, session } = makeFixture('agent-device-session-lease-');
   session.lease = {
diff --git a/src/daemon/session-paths.ts b/src/daemon/session-paths.ts
index dc54376bb..4ac0f3a76 100644
--- a/src/daemon/session-paths.ts
+++ b/src/daemon/session-paths.ts
@@ -4,6 +4,20 @@ export function safeSessionName(name: string): string {
   return name.replace(/[^a-zA-Z0-9._-]/g, '_');
 }
 
+/**
+ * Whether `name` addresses exactly one filesystem entry under a session dir.
+ * `safeSessionName` already strips separators, so what is left to reject is the
+ * empty name and the two relative-directory names — the shapes that would make
+ * `path.join` leave the directory it was given. `SessionStore.resolveSessionDir`
+ * refuses such a name (so no request can steer session artifacts outside the
+ * sessions tree), and callers that accept a session or request id from the
+ * network refuse it before addressing a record.
+ */
+export function isSafeSessionSegment(name: string): boolean {
+  const safe = safeSessionName(name);
+  return safe.length > 0 && safe !== '.' && safe !== '..';
+}
+
 export function expandSessionPath(filePath: string, cwd?: string): string {
   return resolveUserPath(filePath, { cwd });
 }
diff --git a/src/daemon/session-store.ts b/src/daemon/session-store.ts
index 63919fc97..ec04bbfb5 100644
--- a/src/daemon/session-store.ts
+++ b/src/daemon/session-store.ts
@@ -1,9 +1,10 @@
 import path from 'node:path';
 import fs from 'node:fs';
+import { AppError } from '@agent-device/kernel/errors';
 import { emitDiagnostic } from '../utils/diagnostics.ts';
 import type { SessionRuntimeHints, SessionState } from './types.ts';
 import { recordActionEntry, type RecordActionEntry } from './session-action-recorder.ts';
-import { expandSessionPath, safeSessionName } from './session-paths.ts';
+import { expandSessionPath, isSafeSessionSegment, safeSessionName } from './session-paths.ts';
 import { NO_SCRIPT_PUBLICATION, isRepairCommittable } from './session-script-publication-state.ts';
 import { effectiveWriteForce } from './session-script-publication-capability.ts';
 import {
@@ -276,7 +277,19 @@ export class SessionStore {
     return path.join(this.sessionsDir, `${safeName}-${timestamp}.trace.log`);
   }
 
+  /**
+   * The one place a session name becomes a directory, so the invariant that every
+   * session dir lies beneath `sessionsDir` is enforced here rather than by each
+   * caller: `.` and `..` survive `safeSessionName` and would resolve to the
+   * sessions dir itself or the daemon state dir above it.
+   */
   resolveSessionDir(sessionName: string): string {
+    if (!isSafeSessionSegment(sessionName)) {
+      throw new AppError(
+        'INVALID_ARGS',
+        `Invalid session name ${JSON.stringify(sessionName)}: a session name cannot be empty, ".", or "..".`,
+      );
+    }
     return path.join(this.sessionsDir, safeSessionName(sessionName));
   }