diff --git a/src/openultrasast/cpg/backend.py b/src/openultrasast/cpg/backend.py
index b1cd311..89244b4 100644
--- a/src/openultrasast/cpg/backend.py
+++ b/src/openultrasast/cpg/backend.py
@@ -1129,7 +1129,7 @@ class JoernBackend:
                 command += ["--param", f"{name}={value}"]
             completed = self._run(command, timeout=self.query_timeout, cwd=cpg_path.parent)
             if completed is None or completed.returncode != 0:
-                detail = (completed.stderr or "")[-400:] if completed is not None else "timeout"
+                detail = _engine_failure(completed)
                 logger.warning("cpg batch %s failed: %s", query, detail)
                 return None
             body = completed.stdout or ""
@@ -1162,7 +1162,7 @@ class JoernBackend:
             # it can never land in the repository being analysed.
             completed = self._run(command, timeout=self.query_timeout, cwd=cpg_path.parent)
             if completed is None or completed.returncode != 0:
-                detail = (completed.stderr or "")[-400:] if completed is not None else "timeout"
+                detail = _engine_failure(completed)
                 logger.warning("cpg query %s failed: %s", query, detail)
                 return None
             body = completed.stdout or ""
@@ -1541,6 +1541,23 @@ def _configured_timeout() -> int:
     return QUERY_TIMEOUT_SECONDS
 
 
+def _engine_failure(completed: subprocess.CompletedProcess[str] | None) -> str:
+    """What the engine said, keeping the line that names the cause.
+
+    A stack trace's exception is on its FIRST line and its frames follow, so logging only the tail
+    reports frames and discards the cause. Four PHP investigations were spent on `cpg batch taint
+    failed: dure1.java:10)`, which is the middle of a frame and says nothing at all.
+    """
+    if completed is None:
+        return "timeout"
+    text = (completed.stderr or completed.stdout or "").strip()
+    if not text:
+        return f"exit {completed.returncode} with no output"
+    lines = [line for line in text.splitlines() if line.strip() and not line.strip().startswith("at ")]
+    head = " | ".join(lines[:4])[:600]
+    return head or text[:300]
+
+
 def _render(value: object) -> str:
     """CPGQL parameters are strings; a sequence becomes a comma-separated list the script splits."""
     if isinstance(value, str):
diff --git a/tests/test_cpg_backend.py b/tests/test_cpg_backend.py
index 6a45803..386ca32 100644
--- a/tests/test_cpg_backend.py
+++ b/tests/test_cpg_backend.py
@@ -1217,3 +1217,16 @@ def test_a_batch_no_part_can_answer_is_still_unavailable(tmp_path, monkeypatch):
     graph.write_bytes(b"graph")
     # None means "could not ask", which must never become an empty answer.
     assert backend.query_batch(graph, "taint", {str(i): {"file": f"f{i}.php"} for i in range(8)}) is None
+
+
+def test_an_engine_failure_reports_the_line_that_names_the_cause(tmp_path, monkeypatch):
+    """A stack trace's exception is on its first line; logging the tail reports frames only."""
+    from openultrasast.cpg.backend import JoernBackend, _engine_failure
+
+    trace = "java.lang.OutOfMemoryError: Java heap space\n\tat foo.Bar.baz(Bar.java:10)\n\tat x.Procedure1.java:10)"
+    named = _engine_failure(subprocess.CompletedProcess(["joern"], 1, "", trace))
+    assert "OutOfMemoryError" in named and "Java heap space" in named
+    assert "Bar.java:10" not in named, "frames are noise; the cause is not"
+    assert _engine_failure(None) == "timeout"
+    assert "no output" in _engine_failure(subprocess.CompletedProcess(["joern"], 3, "", ""))
+    assert isinstance(JoernBackend(), JoernBackend)