diff --git a/skills/x-twitter/SKILL.md b/skills/x-twitter/SKILL.md
index ebf7f3f65..c3d373265 100644
--- a/skills/x-twitter/SKILL.md
+++ b/skills/x-twitter/SKILL.md
@@ -32,9 +32,13 @@ python3 skills/x-twitter/x-post.py engagement 2040817066199195818
 
 ## Setup
 
-1. Go to https://developer.x.com and sign in
-2. Create a Project + App
-3. Generate keys and add to `.env`:
+1. Install Python dependencies (one-time):
+   ```
+   pip3 install requests requests-oauthlib
+   ```
+2. Go to https://developer.x.com and sign in
+3. Create a Project + App
+4. Generate keys and add to `.env`:
    ```
    X_API_KEY=...
    X_API_SECRET=...
diff --git a/skills/x-twitter/x-post.py b/skills/x-twitter/x-post.py
index d002a0d61..2891d278d 100644
--- a/skills/x-twitter/x-post.py
+++ b/skills/x-twitter/x-post.py
@@ -37,10 +37,11 @@ def _require_requests():
         import requests as _requests
         from requests_oauthlib import OAuth1 as _OAuth1
     except ImportError:
-        print("Installing required packages...")
-        os.system("pip3 install --break-system-packages requests requests-oauthlib")
-        import requests as _requests
-        from requests_oauthlib import OAuth1 as _OAuth1
+        sys.exit(
+            "x-post: missing dependencies. Install them with:\n"
+            "  pip3 install requests requests-oauthlib\n"
+            "or add them to your project's requirements."
+        )
     requests = _requests
     OAuth1 = _OAuth1
 
diff --git a/src/agent-api.py b/src/agent-api.py
index f12b5f3c3..e0c5bb4e2 100644
--- a/src/agent-api.py
+++ b/src/agent-api.py
@@ -51,11 +51,13 @@ def _safe_id(raw: str) -> str:
 
 
 def validate_twilio_signature(handler, body: str) -> bool:
-    """Validate X-Twilio-Signature if TWILIO_AUTH_TOKEN is configured.
-    Returns True if valid or if token not configured (local dev)."""
+    """Validate X-Twilio-Signature against TWILIO_AUTH_TOKEN.
+    Fails closed — returns False when the token is not configured so that
+    unauthenticated requests cannot create tasks via the /twilio/* endpoints.
+    TWILIO_AUTH_TOKEN must be set in .env for these endpoints to accept webhooks."""
     auth_token = os.environ.get("TWILIO_AUTH_TOKEN", "")
     if not auth_token:
-        return True
+        return False
     import hmac, hashlib, base64
     from urllib.parse import parse_qs
 
diff --git a/tests/agent-api-twilio-auth.test.py b/tests/agent-api-twilio-auth.test.py
new file mode 100644
index 000000000..f90c76229
--- /dev/null
+++ b/tests/agent-api-twilio-auth.test.py
@@ -0,0 +1,94 @@
+#!/usr/bin/env python3
+"""Security regression: validate_twilio_signature must fail CLOSED when
+TWILIO_AUTH_TOKEN is not set.
+
+Finding #2 from the 2026-07-06 source-code security audit: the original
+implementation returned True (accept all) when auth_token was empty,
+allowing unauthenticated Twilio webhook requests to create tasks in the
+agent.
+
+The fix: return False when TWILIO_AUTH_TOKEN is unset so that /twilio/*
+endpoints always reject unauthenticated requests. Operators must set the
+token in .env for Twilio webhooks to work.
+
+Run: python3 tests/agent-api-twilio-auth.test.py
+Exit: 0 on pass, 1 on fail.
+"""
+from __future__ import annotations
+
+import importlib.util
+import os
+import sys
+import unittest
+from pathlib import Path
+from unittest.mock import MagicMock
+
+REPO = Path(__file__).resolve().parent.parent
+
+
+def _load_agent_api() -> object:
+    spec = importlib.util.spec_from_file_location("agent_api", REPO / "src" / "agent-api.py")
+    mod = importlib.util.module_from_spec(spec)
+    assert spec.loader is not None
+    spec.loader.exec_module(mod)
+    return mod
+
+
+class TwilioAuthTests(unittest.TestCase):
+    def setUp(self):
+        # Ensure no token in env for the fail-closed tests.
+        self._orig = os.environ.pop("TWILIO_AUTH_TOKEN", None)
+        self.mod = _load_agent_api()
+
+    def tearDown(self):
+        if self._orig is not None:
+            os.environ["TWILIO_AUTH_TOKEN"] = self._orig
+        else:
+            os.environ.pop("TWILIO_AUTH_TOKEN", None)
+
+    def _make_handler(self, headers: dict | None = None) -> MagicMock:
+        h = MagicMock()
+        h.headers = {**(headers or {})}
+        h.path = "/twilio/voice"
+        return h
+
+    def test_no_token_rejects_request(self):
+        """TWILIO_AUTH_TOKEN unset → validate_twilio_signature returns False (fail closed)."""
+        os.environ.pop("TWILIO_AUTH_TOKEN", None)
+        handler = self._make_handler()
+        result = self.mod.validate_twilio_signature(handler, "CallSid=CA123&To=%2B1555")
+        self.assertFalse(result, "Must reject when TWILIO_AUTH_TOKEN is not configured")
+
+    def test_empty_token_rejects_request(self):
+        """TWILIO_AUTH_TOKEN='' (explicitly empty) → rejects."""
+        os.environ["TWILIO_AUTH_TOKEN"] = ""
+        handler = self._make_handler()
+        result = self.mod.validate_twilio_signature(handler, "CallSid=CA123")
+        self.assertFalse(result)
+
+    def test_valid_token_with_missing_signature_rejects(self):
+        """Token configured but X-Twilio-Signature header absent → rejects."""
+        os.environ["TWILIO_AUTH_TOKEN"] = "test_token_abc"
+        handler = self._make_handler({"Host": "example.com"})
+        result = self.mod.validate_twilio_signature(handler, "CallSid=CA123")
+        self.assertFalse(result)
+
+    def test_valid_token_with_wrong_signature_rejects(self):
+        """Token configured but signature doesn't match → rejects."""
+        os.environ["TWILIO_AUTH_TOKEN"] = "test_token_abc"
+        handler = self._make_handler({
+            "Host": "example.com",
+            "X-Forwarded-Proto": "https",
+            "X-Twilio-Signature": "badsig",
+        })
+        os.environ["TWILIO_WEBHOOK_URL"] = "https://example.com"
+        try:
+            result = self.mod.validate_twilio_signature(handler, "CallSid=CA123")
+        finally:
+            os.environ.pop("TWILIO_WEBHOOK_URL", None)
+        self.assertFalse(result)
+
+
+if __name__ == "__main__":
+    result = unittest.main(exit=False).result
+    sys.exit(0 if result.wasSuccessful() else 1)