diff --git a/py/src/braintrust/framework.py b/py/src/braintrust/framework.py
index 3b4c2e52..4099a552 100644
--- a/py/src/braintrust/framework.py
+++ b/py/src/braintrust/framework.py
@@ -1236,10 +1236,14 @@ def __init__(
             self.update({"expected": expected})
         self.update({"trial_index": trial_index})
         self._span = None
-        if tags is not None:
-            self.update({"tags": tags})
-        else:
+        if tags is None:
             self.update({"tags": []})
+        elif isinstance(tags, (list, tuple, set)):
+            # Copy so hook mutations don't leak into the caller's tags.
+            self.update({"tags": list(tags)})
+        else:
+            # Leave other values as-is instead of splitting a string or dict into tags.
+            self.update({"tags": tags})
 
         self._report_progress = report_progress
         self._parameters = parameters
@@ -1756,6 +1760,7 @@ async def run_evaluator_task(datum, trial_index=0):
             # In most cases this will be a no-op span, but if the parent is set, it will use that ctx.
             root_span = start_span(state=state, **base_event)
 
+        hooks = None
         with root_span:
             try:
 
@@ -1950,7 +1955,10 @@ async def ensure_spans_flushed():
                     root_span.log(metadata=metadata, tags=tags)
             except Exception as e:
                 exc_type, exc_value, tb = sys.exc_info()
-                root_span.log(error=stringify_exception(exc_type, exc_value, tb))
+                if hooks is not None:
+                    # Keep tags the task added before it failed.
+                    tags = hooks.tags if hooks.tags else None
+                root_span.log(error=stringify_exception(exc_type, exc_value, tb), tags=tags)
 
                 error = e
                 # Python3.10 has a different set of arguments to format_exception than earlier versions,
diff --git a/py/src/braintrust/test_framework.py b/py/src/braintrust/test_framework.py
index c3284e53..e01076cb 100644
--- a/py/src/braintrust/test_framework.py
+++ b/py/src/braintrust/test_framework.py
@@ -1,4 +1,5 @@
 import asyncio
+import copy
 import importlib.util
 import re
 import sys
@@ -1444,6 +1445,91 @@ def task_with_hooks(input, hooks):
     assert root_span[0].get("tags") == expected_tags
 
 
+@pytest.mark.parametrize("use_trials", [False, True], ids=["shared-row-tags", "trials"])
+@pytest.mark.parametrize("initial_tags", [None, ["seed"], ("seed",), {"seed"}], ids=["none", "list", "tuple", "set"])
+def test_hooks_tags_mutation_is_isolated_per_evaluation(with_memory_logger, use_trials, initial_tags):
+    original_tags = copy.copy(initial_tags)
+    base_tags = list(initial_tags or [])
+    inputs = ["row"] if use_trials else ["first", "second"]
+
+    def task(input, hooks):
+        own_tag = f"{input}:{hooks.trial_index}"
+        hooks.tags.append(own_tag)
+        return own_tag
+
+    def tag_isolation(output, tags):
+        return float(tags == [*base_tags, output])
+
+    result = Eval(
+        "isolated-hook-tags",
+        data=[EvalCase(input=value, tags=initial_tags) for value in inputs],
+        task=task,
+        scores=[tag_isolation],
+        trial_count=2 if use_trials else 1,
+        max_concurrency=1,
+        no_send_logs=True,
+        enable_cache=False,
+    )
+
+    assert [row.error for row in result.results] == [None, None]
+    assert [row.tags for row in result.results] == [[*base_tags, row.output] for row in result.results]
+    assert [row.scores["tag_isolation"] for row in result.results] == [1.0, 1.0]
+    assert initial_tags == original_tags
+
+
+@pytest.mark.asyncio
+async def test_hooks_tags_are_kept_when_task_raises(with_memory_logger, with_simulate_login):
+    def task(input, hooks):
+        hooks.tags.append("failed-step")
+        raise ValueError("task failed")
+
+    evaluator = Evaluator(
+        project_name="test-project",
+        eval_name="test-evaluator",
+        data=[EvalCase(input="row", tags=["seed"])],
+        task=task,
+        scores=[],
+        experiment_name=None,
+        metadata=None,
+        summarize_scores=False,
+    )
+
+    result = await run_evaluator(
+        experiment=init_test_exp("test-evaluator", "test-project"),
+        evaluator=evaluator,
+        position=None,
+        filters=[],
+    )
+
+    assert isinstance(result.results[0].error, ValueError)
+    assert result.results[0].tags == ["seed", "failed-step"]
+    root_spans = [log for log in with_memory_logger.pop() if not log["span_parents"]]
+    assert [span.get("tags") for span in root_spans] == [["seed", "failed-step"]]
+
+
+@pytest.mark.asyncio
+@pytest.mark.parametrize("tags", ["prod", {"prod": True}], ids=["str", "dict"])
+async def test_non_sequence_eval_case_tags_are_rejected(with_memory_logger, with_simulate_login, tags):
+    evaluator = Evaluator(
+        project_name="test-project",
+        eval_name="test-evaluator",
+        data=[EvalCase(input="row", tags=tags)],
+        task=lambda input: input,
+        scores=[],
+        experiment_name=None,
+        metadata=None,
+        summarize_scores=False,
+    )
+
+    with pytest.raises(ValueError, match="tags must be a list, set, or tuple of strings"):
+        await run_evaluator(
+            experiment=init_test_exp("test-evaluator", "test-project"),
+            evaluator=evaluator,
+            position=None,
+            filters=[],
+        )
+
+
 @pytest.mark.asyncio
 @pytest.mark.parametrize(
     ("tags", "expected_tags"),