diff --git a/builders/server/core/runtime/validator.py b/builders/server/core/runtime/validator.py
index b59ce5d..cb1fc8f 100644
--- a/builders/server/core/runtime/validator.py
+++ b/builders/server/core/runtime/validator.py
@@ -8,9 +8,13 @@ class ValidationError(Exception):
 def validate(data: dict, schema: dict[str, SchemaType]) -> None:
     """Validate that data matches the declared schema.
 
-    Checks that all declared keys are present and values match declared types.
-    Raises ValidationError on failure.
+    Checks that all keys in data are in the schema, all schema keys are present in data,
+    and values match declared types. Raises ValidationError on failure.
     """
+    for key in data:
+        if key not in schema:
+            raise ValidationError(f"Unexpected key '{key}' in builder output")
+
     for key, schema_type in schema.items():
         if key not in data:
             raise ValidationError(f"Missing key '{key}' in builder output")
diff --git a/builders/server/tests/core/runtime/test_validator.py b/builders/server/tests/core/runtime/test_validator.py
index d8ee66c..6b772f3 100644
--- a/builders/server/tests/core/runtime/test_validator.py
+++ b/builders/server/tests/core/runtime/test_validator.py
@@ -11,14 +11,10 @@ def test_valid_data_passes() -> None:
     )
 
 
-def test_empty_schema_passes_anything() -> None:
-    """Empty schema means no constraints."""
-    validate({"anything": 123, "goes": "here"}, {})
-
-
-def test_extra_keys_allowed() -> None:
-    """Data with extra keys beyond schema passes."""
-    validate({"ticker": "AAPL", "extra": 999}, {"ticker": SchemaType.STR})
+def test_extra_keys_raises() -> None:
+    """Data with extra keys beyond schema fails."""
+    with pytest.raises(ValidationError, match="Unexpected key 'extra'"):
+        validate({"ticker": "AAPL", "extra": 999}, {"ticker": SchemaType.STR})
 
 
 def test_missing_key_raises() -> None:
diff --git a/builders/server/tests/core/service/test_worker.py b/builders/server/tests/core/service/test_worker.py
index 5b986a7..122e2b2 100644
--- a/builders/server/tests/core/service/test_worker.py
+++ b/builders/server/tests/core/service/test_worker.py
@@ -82,7 +82,9 @@ def test_missing_timestamps_built_and_inserted(
 @patch("core.service.worker.registry")
 def test_builder_failure_no_partial_insert(mock_registry, mock_db, mock_runner) -> None:
     """If builder fails on timestamp 3 of 5, no rows are inserted."""
-    mock_registry.get_config.return_value = _cfg(name="ds")
+    mock_registry.get_config.return_value = _cfg(
+        name="ds", schema={"val": SchemaType.INT}
+    )
     mock_db.get_existing_timestamps.return_value = []  # all missing
 
     call_count = 0
@@ -112,7 +114,9 @@ def fail_on_third(*args, **kwargs):
 @patch("core.service.worker.registry")
 def test_cancelled_event_stops_early(mock_registry, mock_db, mock_runner) -> None:
     """When cancelled is set, worker stops before building remaining timestamps."""
-    mock_registry.get_config.return_value = _cfg(name="ds")
+    mock_registry.get_config.return_value = _cfg(
+        name="ds", schema={"val": SchemaType.INT}
+    )
     mock_db.get_existing_timestamps.return_value = []
     mock_runner.run_builder.return_value = [{"val": 1}]
 
@@ -146,6 +150,7 @@ def test_lookback_dep_uses_get_rows_range(mock_registry, mock_db, mock_runner) -
     """Dependency with lookback fetches data via get_rows_range."""
     mock_registry.get_config.return_value = _cfg(
         name="ds",
+        schema={"val": SchemaType.INT},
         dependencies={
             "dep": DependencyInfo(version=V010, lookback_subtract=timedelta(days=4)),
         },
@@ -174,6 +179,7 @@ def test_no_lookback_dep_uses_get_rows_timestamps(
     """Dependency without lookback fetches data via get_rows_timestamps."""
     mock_registry.get_config.return_value = _cfg(
         name="ds",
+        schema={"val": SchemaType.INT},
         dependencies={
             "dep": DependencyInfo(version=V010),
         },
diff --git a/builders/server/tests/integration/test_errors.py b/builders/server/tests/integration/test_errors.py
index 83d38fd..b0d15d6 100644
--- a/builders/server/tests/integration/test_errors.py
+++ b/builders/server/tests/integration/test_errors.py
@@ -140,3 +140,34 @@ def build(dependencies, timestamp: datetime) -> list[dict]:
     )
     assert resp.status_code == 500
     assert _row_count(db_conn, name) == 0
+
+
+def test_schema_unexpected_key(client, db_conn, write_temp_builder):
+    """builder returns key not in the schema -> 500, 0 rows."""
+    name, version = write_temp_builder(
+        "unexpected-key",
+        "0.1.0",
+        """\
+name = "unexpected-key"
+version = "0.1.0"
+builder = "builder.py"
+calendar = "everyday"
+granularity = "1d"
+start-date = "2020-01-01"
+
+[schema]
+value = "int"
+""",
+        """\
+from datetime import datetime
+
+def build(dependencies, timestamp: datetime) -> list[dict]:
+    return [{"value": 1, "unexpected_key": "value"}]
+""",
+    )
+    resp = client.post(
+        f"/api/v1/build/{name}/{version}",
+        params={"start": "2024-01-02", "end": "2024-01-02"},
+    )
+    assert resp.status_code == 500
+    assert _row_count(db_conn, name) == 0