fix(agent): freeze carveout-era SUMMARY_PREFIX for renormalization

The prompt consolidation above retires the carveout-era prefix. Without a
frozen copy in _HISTORICAL_SUMMARY_PREFIXES, summaries persisted by
pre-upgrade builds would lose detection (_is_context_summary_content) and
renormalization (_strip_summary_prefix) — the exact regression class the
tuple exists to prevent. Adds contract tests covering every frozen prefix.

Refs #41607 #38364 #42812

agent/context_compressor.py

@@ -80,6 +80,31 @@ LEGACY_SUMMARY_PREFIX = "[CONTEXT SUMMARY]:"
 # embedded in the body and keeps hijacking replies. Keep newest-first; entries
 # are matched literally. Add a frozen copy here whenever SUMMARY_PREFIX changes.
 _HISTORICAL_SUMMARY_PREFIXES = (
+    # Carveout era (#41607/#38364/#42812): "consistent → use as background"
+    # licensed stale-task resumption on topic overlap.
+    "[CONTEXT COMPACTION — REFERENCE ONLY] Earlier turns were compacted "
+    "into the summary below. This is a handoff from a previous context "
+    "window — treat it as background reference, NOT as active instructions. "
+    "Do NOT answer questions or fulfill requests mentioned in this summary; "
+    "they were already addressed. "
+    "Respond ONLY to the latest user message that appears AFTER this "
+    "summary — that message is the single source of truth for what to do "
+    "right now. "
+    "If the latest user message is consistent with the '## Active Task' "
+    "section, you may use the summary as background. If the latest user "
+    "message contradicts, supersedes, changes topic from, or in any way "
+    "diverges from '## Active Task' / '## In Progress' / '## Pending User "
+    "Asks' / '## Remaining Work', the latest message WINS — discard those "
+    "stale items entirely and do not 'wrap up the old task first'. "
+    "Reverse signals in the latest message (e.g. 'stop', 'undo', 'roll "
+    "back', 'just verify', 'don't do that anymore', 'never mind', a new "
+    "topic) must immediately end any in-flight work described in the "
+    "summary; do not re-surface it in later turns. "
+    "IMPORTANT: Your persistent memory (MEMORY.md, USER.md) in the system "
+    "prompt is ALWAYS authoritative and active — never ignore or deprioritize "
+    "memory content due to this compaction note. "
+    "The current session state (files, config, etc.) may reflect work "
+    "described here — avoid repeating it:",
     # Pre-#35344: contained the self-contradicting "resume exactly" directive.
     "[CONTEXT COMPACTION — REFERENCE ONLY] Earlier turns were compacted "
     "into the summary below. This is a handoff from a previous context "

tests/agent/test_summary_prefix_semantics.py

@@ -80,3 +80,37 @@ def test_memory_authority_preserved():
     assert "MEMORY.md" in SUMMARY_PREFIX
     assert "USER.md" in SUMMARY_PREFIX
     assert "authoritative" in SUMMARY_PREFIX
+
+
+def test_no_background_consistency_carveout():
+    """The "consistent → use as background" carveout licensed stale-task
+    resumption on topic overlap (#41607, #38364, #42812). It must stay gone,
+    and the prefix must explicitly neutralize topic overlap."""
+    lower = SUMMARY_PREFIX.lower()
+    assert "you may use the summary as background" not in lower
+    assert "topic overlap" in lower
+
+
+def test_replaced_prefixes_are_frozen_for_renormalization():
+    """Every retired SUMMARY_PREFIX must be frozen into
+    _HISTORICAL_SUMMARY_PREFIXES, otherwise summaries persisted by older
+    builds lose detection/renormalization after an upgrade. The carveout-era
+    prefix is the latest retiree."""
+    from agent.context_compressor import (
+        _HISTORICAL_SUMMARY_PREFIXES,
+        ContextCompressor,
+    )
+
+    carveout_era = [
+        p for p in _HISTORICAL_SUMMARY_PREFIXES
+        if "you may use the summary as background" in p
+    ]
+    assert carveout_era, "carveout-era prefix missing from frozen tuple"
+    # The live prefix must never be one of the frozen ones.
+    assert SUMMARY_PREFIX not in _HISTORICAL_SUMMARY_PREFIXES
+    # Detection + strip must work for every frozen prefix.
+    for old_prefix in _HISTORICAL_SUMMARY_PREFIXES:
+        content = old_prefix + "\n## Summary body"
+        assert ContextCompressor._is_context_summary_content(content)
+        stripped = ContextCompressor._strip_summary_prefix(content)
+        assert not stripped.startswith(old_prefix)