diff --git a/libs/hackbot-runtime/hackbot_runtime/actions/phabricator.py b/libs/hackbot-runtime/hackbot_runtime/actions/phabricator.py
index 098822398c..88773e51c0 100644
--- a/libs/hackbot-runtime/hackbot_runtime/actions/phabricator.py
+++ b/libs/hackbot-runtime/hackbot_runtime/actions/phabricator.py
@@ -13,9 +13,10 @@
 
 from __future__ import annotations
 
+import re
 from typing import Annotated
 
-from agent_tools.registry import tool, tools_in
+from agent_tools.registry import ToolError, tool, tools_in
 from pydantic import Field
 
 from hackbot_runtime.actions.recorder import ActionsRecorder
@@ -25,11 +26,29 @@
 # in ``context.publish_changes`` — has to cover both types.
 PATCH_ACTION_TYPES = frozenset({"phabricator.submit_patch", "phabricator.update_patch"})
 
+_PHABRICATOR_TEST_PLAN_HEADER_RE = re.compile(
+    r"^(?:Test Plan|Testplan|Tested|Tests):",
+    re.IGNORECASE | re.MULTILINE,
+)
+
 
 def _confirm(recorder: ActionsRecorder, action_type: str) -> str:
     return f"Recorded {action_type} (#{len(recorder.actions) - 1})."
 
 
+def _validate_summary(summary: str | None) -> None:
+    if not summary:
+        return
+
+    match = _PHABRICATOR_TEST_PLAN_HEADER_RE.search(summary)
+    if match:
+        raise ToolError(
+            f'Invalid Phabricator summary: "{match.group()}" at the beginning of '
+            "a line is interpreted as a Test Plan field. Call submit_patch again "
+            "with that fixed."
+        )
+
+
 @tool
 async def submit_patch(
     recorder: ActionsRecorder,
@@ -80,6 +99,7 @@ async def submit_patch(
     in the same run, written as `{{actions.<ref>.url}}` (for example, inside a
     bug comment).
     """
+    _validate_summary(summary)
     recorder.record(
         "phabricator.submit_patch",
         {"bug_id": bug_id, "title": title, "summary": summary},
diff --git a/libs/hackbot-runtime/tests/test_phabricator_actions.py b/libs/hackbot-runtime/tests/test_phabricator_actions.py
index b8a947d24d..8364f340eb 100644
--- a/libs/hackbot-runtime/tests/test_phabricator_actions.py
+++ b/libs/hackbot-runtime/tests/test_phabricator_actions.py
@@ -1,6 +1,7 @@
 """Tests for the phabricator recording tools (submit/update patch, comment)."""
 
 import pytest
+from agent_tools.registry import ToolError
 from hackbot_runtime.actions import ActionsRecorder, phabricator
 
 
@@ -19,6 +20,47 @@ async def test_submit_records_create_params_only():
     assert "ref" not in action
 
 
+@pytest.mark.parametrize(
+    "header",
+    ["Tests", "tests", "Test Plan", "Testplan", "Tested"],
+)
+async def test_submit_rejects_test_plan_headers(header):
+    rec = ActionsRecorder()
+
+    with pytest.raises(ToolError) as exc:
+        await phabricator.submit_patch(
+            rec,
+            bug_id=1,
+            title="Fix",
+            reasoning="r",
+            summary=f"Explanation\n\n{header}: details",
+        )
+
+    assert header in str(exc.value)
+    assert "Call submit_patch again with that fixed" in str(exc.value)
+    assert rec.actions == []
+
+
+@pytest.mark.parametrize(
+    "summary",
+    [
+        None,
+        "",
+        "Testing: details",
+        "Some Tests: details",
+        " Tests: already indented",
+    ],
+)
+async def test_submit_accepts_safe_summary(summary):
+    rec = ActionsRecorder()
+
+    await phabricator.submit_patch(
+        rec, bug_id=1, title="Fix", reasoning="r", summary=summary
+    )
+
+    assert rec.actions[0]["params"]["summary"] == summary
+
+
 async def test_submit_requires_title():
     rec = ActionsRecorder()
     with pytest.raises(TypeError):
diff --git a/libs/hackbot-runtime/tests/test_phabricator_handler.py b/libs/hackbot-runtime/tests/test_phabricator_handler.py
index 1585dd2149..3d595bec75 100644
--- a/libs/hackbot-runtime/tests/test_phabricator_handler.py
+++ b/libs/hackbot-runtime/tests/test_phabricator_handler.py
@@ -123,6 +123,7 @@ async def test_submit_patch_creates_planned_changes_revision(monkeypatch):
     assert transactions["plan-changes"] is True
     assert "reviewers.add" not in transactions
     assert transactions["bugzilla.bug-id"] == "1"
+    assert transactions["summary"] == "s"
 
 
 async def test_submit_patch_sets_local_commits_property(monkeypatch):