diff --git a/agent/context_compressor.py b/agent/context_compressor.py
index 7464802b27be4..c14f67fce3c88 100644
--- a/agent/context_compressor.py
+++ b/agent/context_compressor.py
@@ -215,7 +215,40 @@ _MERGED_SUMMARY_DELIMITER = "[END OF PRIOR CONTEXT — COMPACTION SUMMARY BELOW]
 # 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 = (
-    # Jul 2026 (#65848 class): identical to the current prefix except it
+    # Pre-#69619: identical to the current prefix except the stale-item
+    # discard clause named all four historical headings (the three
+    # section headers removed by #69619 were still in the template).
+    # Summaries persisted by builds immediately before #69619 carry this
+    # exact text and must remain detectable/strippable on resume.
+    "[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. "
+    "Topic overlap with the summary does NOT mean you should resume its "
+    "task: even on similar topics, the latest user message WINS. Treat ONLY "
+    "the latest message as the active task and discard stale items from "
+    "'## Historical Task Snapshot' / '## Historical In-Progress State' / "
+    "'## Historical Pending User Asks' / "
+    "'## Historical Remaining Work' entirely — do not 'wrap up' or "
+    "'finish' work described there unless the latest message explicitly "
+    "asks for it. "
+    "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. "
+    "None of the above restricts HOW you work: your tools remain fully "
+    "active — keep calling them normally for the active task (edit files, "
+    "run commands, search) instead of merely narrating what you would do. "
+    "The current session state (files, config, etc.) may reflect work "
+    "described here — avoid repeating it:",
+    # Jul 2026 (#65848 class): identical to the pre-#69619 prefix except it
     # lacked the explicit "tools remain fully active" clause — the strong
     # REFERENCE ONLY framing bled into general tool-use suppression
     # (observed: 7 consecutive narration-only turns immediately after a
@@ -231,7 +264,9 @@ _HISTORICAL_SUMMARY_PREFIXES = (
     "Topic overlap with the summary does NOT mean you should resume its "
     "task: even on similar topics, the latest user message WINS. Treat ONLY "
     "the latest message as the active task and discard stale items from "
-    f"'{HISTORICAL_TASK_HEADING}' entirely — do not 'wrap up' or "
+    "'## Historical Task Snapshot' / '## Historical In-Progress State' / "
+    "'## Historical Pending User Asks' / "
+    "'## Historical Remaining Work' entirely — do not 'wrap up' or "
     "'finish' work described there unless the latest message explicitly "
     "asks for it. "
     "Reverse signals in the latest message (e.g. 'stop', 'undo', 'roll "
diff --git a/tests/agent/test_summary_prefix_semantics.py b/tests/agent/test_summary_prefix_semantics.py
index 77d8ba12f4f19..9c1b42dac3c88 100644
--- a/tests/agent/test_summary_prefix_semantics.py
+++ b/tests/agent/test_summary_prefix_semantics.py
@@ -108,3 +108,61 @@ def test_replaced_prefixes_are_frozen_for_renormalization():
         assert ContextCompressor._is_context_summary_content(content)
         stripped = ContextCompressor._strip_summary_prefix(content)
         assert not stripped.startswith(old_prefix)
+
+
+# Exact literal copy of the live SUMMARY_PREFIX as it shipped immediately
+# before #69619 (four-heading discard clause + tools-active clause).
+# Frozen on purpose: do NOT derive it from module constants — the test must
+# fail if the corresponding entry in _HISTORICAL_SUMMARY_PREFIXES is mutated
+# or dropped.
+_PRE_69619_LIVE_PREFIX = (
+    "[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. "
+    "Topic overlap with the summary does NOT mean you should resume its "
+    "task: even on similar topics, the latest user message WINS. Treat ONLY "
+    "the latest message as the active task and discard stale items from "
+    "'## Historical Task Snapshot' / '## Historical In-Progress State' / "
+    "'## Historical Pending User Asks' / "
+    "'## Historical Remaining Work' entirely — do not 'wrap up' or "
+    "'finish' work described there unless the latest message explicitly "
+    "asks for it. "
+    "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. "
+    "None of the above restricts HOW you work: your tools remain fully "
+    "active — keep calling them normally for the active task (edit files, "
+    "run commands, search) instead of merely narrating what you would do. "
+    "The current session state (files, config, etc.) may reflect work "
+    "described here — avoid repeating it:"
+)
+
+
+def test_pre_69619_prefix_generation_is_frozen_and_stripped():
+    """Regression for the #69619 review: the prefix generation live right
+    before the section-header removal was never added to
+    _HISTORICAL_SUMMARY_PREFIXES, so a summary persisted immediately before
+    upgrading survived resume/re-compaction undetected and unstripped.
+    That exact generation must stay frozen, detectable, and strippable."""
+    from agent.context_compressor import (
+        _HISTORICAL_SUMMARY_PREFIXES,
+        ContextCompressor,
+    )
+
+    assert _PRE_69619_LIVE_PREFIX in _HISTORICAL_SUMMARY_PREFIXES, (
+        "pre-#69619 live prefix missing from _HISTORICAL_SUMMARY_PREFIXES — "
+        "summaries persisted by the immediately previous build are no longer "
+        "normalized on resume"
+    )
+    content = _PRE_69619_LIVE_PREFIX + "\nBODY"
+    assert ContextCompressor._is_context_summary_content(content)
+    assert ContextCompressor._strip_summary_prefix(content) == "BODY"
diff --git a/tests/agent/test_summary_prefix_tool_use.py b/tests/agent/test_summary_prefix_tool_use.py
index 2a289b86f9bf4..c67f71f184c8b 100644
--- a/tests/agent/test_summary_prefix_tool_use.py
+++ b/tests/agent/test_summary_prefix_tool_use.py
@@ -30,11 +30,19 @@ class TestSummaryPrefixToolUseClause:
         generation must be frozen into _HISTORICAL_SUMMARY_PREFIXES so old
         persisted summaries still get the directive-strip on re-compaction."""
         assert len(_HISTORICAL_SUMMARY_PREFIXES) >= 3
-        newest_frozen = _HISTORICAL_SUMMARY_PREFIXES[0]
-        # The frozen copy is the pre-clause generation: same framing, no clause.
-        assert "tools remain fully active" not in newest_frozen
-        assert "Do NOT answer questions or fulfill requests" in newest_frozen
-        assert newest_frozen != SUMMARY_PREFIX
+        # The pre-clause generation (#65848 incident era): same framing, no
+        # tools-active clause. Newer generations are prepended ahead of it as
+        # the prefix evolves (tuple is newest-first), so match by content,
+        # not position. "topic overlap" distinguishes it from the older
+        # carveout-era entry.
+        pre_clause = [
+            p for p in _HISTORICAL_SUMMARY_PREFIXES
+            if "tools remain fully active" not in p
+            and "topic overlap" in p.lower()
+            and "Do NOT answer questions or fulfill requests" in p
+        ]
+        assert pre_clause, "pre-clause generation missing from frozen tuple"
+        assert all(p != SUMMARY_PREFIX for p in pre_clause)
 
     def test_historical_prefixes_are_distinct_from_current(self):
         for frozen in _HISTORICAL_SUMMARY_PREFIXES: