fix(compression): replace alias task headings during snapshot grounding

Align the iterative update instruction with HISTORICAL_TASK_HEADING and
treat leftover ## Active Task sections as the same snapshot so grounding
replaces them instead of prepending a second live-looking task heading.

Fixes #114479

agent/context_compressor.py

@@ -1016,7 +1016,12 @@ _PATH_MENTION_RE = re.compile(r"(?:/|~/?|[A-Za-z]:\\)[^\s`'\")\]}<>]+")
 # MEDIA delivery directives must not reach the summarizer — if one leaks into the summary, the downstream
 # model may re-emit it as an active directive on the next turn, triggering bogus attachment sends (#14665).
 _MEDIA_DIRECTIVE_RE = re.compile(r"MEDIA:\S+")
-_HISTORICAL_TASK_SECTION_RE = re.compile(rf"(?ms)^{re.escape(HISTORICAL_TASK_HEADING)}\s*\n.*?(?=^## |\Z)")
+# Pre-#44454 alias. A summarizer that still emits it must be replaced, not prepended.
+_LEGACY_ACTIVE_TASK_HEADING = "## Active Task"
+_TASK_SNAPSHOT_HEADINGS = (HISTORICAL_TASK_HEADING, _LEGACY_ACTIVE_TASK_HEADING)
+_HISTORICAL_TASK_SECTION_RE = re.compile(
+    rf"(?ms)^(?:{'|'.join(re.escape(heading) for heading in _TASK_SNAPSHOT_HEADINGS)})\s*\n.*?(?=^## |\Z)"
+)
 
 
 def _redact_compaction_text(text: Any) -> str:
@@ -3784,8 +3789,8 @@ Write only the summary body. Do not include any preamble or prefix."""
         """Reject user attribution when the source transcript has no user."""
         if has_user_turn:
             return
-        match = re.search(rf"(?ms)^{re.escape(HISTORICAL_TASK_HEADING)}\s*\n(.*?)(?=\n##\s|\Z)", summary)
-        task_snapshot = match.group(1).strip() if match else ""
+        match = _HISTORICAL_TASK_SECTION_RE.search(summary)
+        task_snapshot = match.group(0).split("\n", 1)[-1].strip() if match else ""
         # The "User asked:" scan can false-positive on quoted tool output; acceptable, since
         # the RuntimeError only costs one retry on the existing fallback path.
         if task_snapshot != _NO_USER_TASK_SENTINEL or re.search(r"\bUser\s+asked\s*:", summary, re.IGNORECASE):

tests/agent/test_task_snapshot_heading_contract.py

@@ -0,0 +1,73 @@
+"""Task-snapshot heading identity across prompt, template, and grounding.
+
+The iterative summarizer prompt, the emitted template, and deterministic
+grounding must share one heading. A leftover ``## Active Task`` section is
+not disclaimed by SUMMARY_PREFIX and reads as live work (#114479 / #44454).
+"""
+
+from types import SimpleNamespace
+
+from agent.context_compressor import (
+    ContextCompressor,
+    HISTORICAL_TASK_HEADING,
+)
+
+_LEGACY_ACTIVE_TASK_HEADING = "## Active Task"
+
+
+def _iterative_update_prompt() -> str:
+    stub = SimpleNamespace(
+        tail_mode="lean",
+        _previous_summary="PREVIOUS SUMMARY BODY",
+        _bound_summary_input=lambda text: text,
+    )
+    stub._summary_template_sections = ContextCompressor._summary_template_sections
+    stub._build_summary_prompt = ContextCompressor._build_summary_prompt.__get__(stub)
+    return stub._build_summary_prompt("NEW TURNS", 2000, None, "", True)
+
+
+def _headings(text: str) -> list[str]:
+    return [line for line in text.splitlines() if line.startswith("## ")]
+
+
+def test_iterative_update_instruction_names_canonical_heading():
+    prompt = _iterative_update_prompt()
+    assert f'Update "{HISTORICAL_TASK_HEADING}"' in prompt
+    assert f'Update "{_LEGACY_ACTIVE_TASK_HEADING}"' not in prompt
+    assert HISTORICAL_TASK_HEADING in prompt
+
+
+def test_grounding_replaces_legacy_active_task_heading():
+    body = (
+        f"{_LEGACY_ACTIVE_TASK_HEADING}\n"
+        "User asked: 'do X'\n\n"
+        "## Goal\n"
+        "thing\n"
+    )
+    grounded = ContextCompressor._ground_historical_task_snapshot.__func__(
+        ContextCompressor,
+        body,
+        [{"role": "user", "content": "do X"}],
+    )
+    headings = _headings(grounded)
+    assert headings.count(HISTORICAL_TASK_HEADING) == 1
+    assert _LEGACY_ACTIVE_TASK_HEADING not in headings
+    assert "## Goal" in headings
+
+
+def test_grounding_keeps_single_canonical_heading():
+    body = (
+        f"{HISTORICAL_TASK_HEADING}\n"
+        "User asked: 'stale'\n\n"
+        "## Goal\n"
+        "thing\n"
+    )
+    grounded = ContextCompressor._ground_historical_task_snapshot.__func__(
+        ContextCompressor,
+        body,
+        [{"role": "user", "content": "fresh ask"}],
+    )
+    headings = _headings(grounded)
+    assert headings.count(HISTORICAL_TASK_HEADING) == 1
+    assert _LEGACY_ACTIVE_TASK_HEADING not in headings
+    assert "fresh ask" in grounded