diff --git a/src/tracer/api.py b/src/tracer/api.py
index af0bba9..e04ad2c 100644
--- a/src/tracer/api.py
+++ b/src/tracer/api.py
@@ -232,17 +232,23 @@ def update(
     existing_index = EmbeddingIndex.load(artifact_dir / "index")
     X_combined = np.vstack([existing_index.embeddings, new_embeddings.astype(np.float32)])
 
-    # Load existing traces (we need to re-save combined)
+    # Load existing traces so we can re-save the combined set. fit() always
+    # writes all_traces.jsonl, so it should be present. If it is missing (e.g.
+    # it was deleted), the original records cannot be recovered from the index
+    # embeddings alone, and appending only the new records would leave the trace
+    # set out of sync with X_combined. Fail fast with an actionable message
+    # instead of surfacing a cryptic trace/embedding mismatch from fit().
     existing_traces_path = artifact_dir / "all_traces.jsonl"
-    if existing_traces_path.exists():
-        from tracer.traces.loader import load_traces as _lt
-        existing_ds = _lt(existing_traces_path)
-        combined_records = existing_ds.records + new_ds.records
-    else:
-        # First update: reconstruct original traces from the initial fit
-        # by loading them from the original trace source if we can find it,
-        # or infer count from existing embeddings
-        combined_records = new_ds.records
+    if not existing_traces_path.exists():
+        raise FileNotFoundError(
+            f"{existing_traces_path} not found — cannot continue continual "
+            "learning. fit() writes this file; if it was removed, re-run "
+            "tracer.fit() on your full trace set to rebuild it before calling "
+            "update().")
+
+    from tracer.traces.loader import load_traces as _lt
+    existing_ds = _lt(existing_traces_path)
+    combined_records = existing_ds.records + new_ds.records
 
     from tracer.types import TraceDataset
     from tracer.traces.loader import save_traces
diff --git a/tests/test_fit.py b/tests/test_fit.py
index bc55279..2f8231a 100644
--- a/tests/test_fit.py
+++ b/tests/test_fit.py
@@ -194,6 +194,30 @@ def test_update():
         assert updated.manifest.n_traces == 300  # 200 + 100
 
 
+def test_update_missing_all_traces_raises_clear_error():
+    """If all_traces.jsonl is gone, update() should fail fast with a clear
+    message rather than a cryptic trace/embedding mismatch."""
+    from tracer.api import fit, update
+    with tempfile.TemporaryDirectory() as tmp:
+        path, X, _ = _make_traces(tmp, n=200)
+        artifact_dir = Path(tmp) / ".tracer"
+        fit(path, artifact_dir=artifact_dir)
+
+        # Simulate a user deleting the file AGENTS.md warns against removing.
+        (artifact_dir / "all_traces.jsonl").unlink()
+
+        new_path = Path(tmp) / "new_traces.jsonl"
+        rng = np.random.RandomState(99)
+        X_new = rng.randn(50, 32).astype(np.float32)
+        with new_path.open("w") as f:
+            for i in range(50):
+                f.write(json.dumps({"input": f"new {i}", "teacher": f"cls_{i % 4}"}) + "\n")
+        np.save(new_path.with_suffix(".npy"), X_new)
+
+        with pytest.raises(FileNotFoundError, match="all_traces.jsonl"):
+            update(new_path, artifact_dir=artifact_dir)
+
+
 # ── Qualitative report ────────────────────────────────────────────────────────
 
 def test_qualitative_report():
@@ -376,6 +400,7 @@ def test_public_exports():
         test_fit_missing_embeddings_raises,
         test_router_wrong_dim_raises,
         test_update,
+        test_update_missing_all_traces_raises_clear_error,
         test_qualitative_report,
         test_qualitative_report_boundary_pairs,
         test_report,