diff --git a/backend/ai.py b/backend/ai.py
index d16befa..5bfff01 100644
--- a/backend/ai.py
+++ b/backend/ai.py
@@ -8,7 +8,7 @@
 from anthropic import Anthropic
 from dotenv import load_dotenv
 
-from database import db_get
+from database import db_get, escape_filter_value
 
 load_dotenv()
 
@@ -28,14 +28,16 @@ def _get_client() -> Anthropic:
 
 
 def _fetch_ticket(ticket_id: str) -> dict[str, Any]:
-    result = db_get("tickets", f"id=eq.{ticket_id}")
+    result = db_get("tickets", f"id=eq.{escape_filter_value(ticket_id)}")
     if not isinstance(result, list) or not result:
         raise ValueError(f"Ticket {ticket_id} not found")
     return result[0]
 
 
 def _fetch_notes(ticket_id: str) -> list[dict[str, Any]]:
-    notes = db_get("ticket_notes", f"ticket_id=eq.{ticket_id}&order=created_at.asc")
+    notes = db_get(
+        "ticket_notes", f"ticket_id=eq.{escape_filter_value(ticket_id)}&order=created_at.asc"
+    )
     return notes if isinstance(notes, list) else []
 
 
diff --git a/backend/test_ai.py b/backend/test_ai.py
new file mode 100644
index 0000000..4b77744
--- /dev/null
+++ b/backend/test_ai.py
@@ -0,0 +1,27 @@
+"""Unit tests for ai.py -- PostgREST filter injection guard on ticket_id."""
+
+import pytest
+
+import ai
+
+
+@pytest.mark.parametrize(
+    "fetch",
+    [ai._fetch_ticket, ai._fetch_notes],
+    ids=["_fetch_ticket", "_fetch_notes"],
+)
+def test_fetch_helpers_escape_injected_ticket_id(monkeypatch, fetch):
+    captured = {}
+
+    def fake_db_get(table, params=""):
+        captured["params"] = params
+        return [{"id": "t1"}]
+
+    monkeypatch.setattr(ai, "db_get", fake_db_get)
+
+    malicious_id = "t1&status=eq.Closed"
+    fetch(malicious_id)
+
+    params = captured["params"]
+    assert "&status=eq.Closed" not in params
+    assert "status=eq.Closed" not in params