diff --git a/agent/conversation_loop.py b/agent/conversation_loop.py
index 73bed6b067..8850b7fd56 100644
--- a/agent/conversation_loop.py
+++ b/agent/conversation_loop.py
@@ -2221,30 +2221,54 @@ def run_conversation(
                     print(f"{agent.log_prefix}     • Legacy cleanup: hermes config set ANTHROPIC_TOKEN \"\"")
                     print(f"{agent.log_prefix}     • Clear stale keys: hermes config set ANTHROPIC_API_KEY \"\"")
 
-                # ── Thinking block signature recovery ─────────────────
+                # Thinking block signature recovery.
+                #
                 # Anthropic signs thinking blocks against the full turn
-                # content.  Any upstream mutation (context compression,
+                # content. Any upstream mutation (context compression,
                 # session truncation, message merging) invalidates the
-                # signature → HTTP 400.  Recovery: strip reasoning_details
-                # from all messages so the next retry sends no thinking
-                # blocks at all.  One-shot — don't retry infinitely.
+                # signature and the API replies HTTP 400 ("invalid
+                # signature" or "cannot be modified"). Recovery strips
+                # ``reasoning_details`` so the retry sends no thinking
+                # blocks at all. One-shot per outer loop.
+                #
+                # The strip targets ``api_messages``, which is the
+                # API-call-time list that ``_build_api_kwargs`` consumes
+                # on every retry. ``api_messages`` was populated once at
+                # the start of the turn from shallow copies of
+                # ``messages``, so mutating it does not touch the
+                # canonical store. The previous implementation popped
+                # ``reasoning_details`` from ``messages`` instead, which
+                # had two problems: ``api_messages`` carried its own
+                # reference to the field through the shallow copy, so the
+                # retry's wire payload still included thinking blocks and
+                # the recovery never reached the API; and the mutation
+                # persisted into ``state.db`` through any subsequent
+                # ``_persist_session`` call, permanently corrupting the
+                # conversation. Future turns would replay the stripped
+                # state, hit the same 400, and the agent would terminate
+                # with ``max_retries_exhausted``, often spawning
+                # cascading compaction-ended sessions chained off the
+                # corrupted parent.
                 if (
                     classified.reason == FailoverReason.thinking_signature
                     and not _retry.thinking_sig_retry_attempted
                 ):
                     _retry.thinking_sig_retry_attempted = True
-                    for _m in messages:
-                        if isinstance(_m, dict):
+                    _api_stripped = 0
+                    for _m in api_messages:
+                        if isinstance(_m, dict) and "reasoning_details" in _m:
                             _m.pop("reasoning_details", None)
+                            _api_stripped += 1
                     agent._vprint(
-                        f"{agent.log_prefix}⚠️  Thinking block signature invalid — "
-                        f"stripped all thinking blocks, retrying...",
+                        f"{agent.log_prefix}⚠️  Thinking block signature invalid, "
+                        f"stripped reasoning_details from api_messages for retry...",
                         force=True,
                     )
                     logger.warning(
                         "%sThinking block signature recovery: stripped "
-                        "reasoning_details from %d messages",
-                        agent.log_prefix, len(messages),
+                        "reasoning_details from %d api_messages "
+                        "(canonical messages unchanged)",
+                        agent.log_prefix, _api_stripped,
                     )
                     continue
 
diff --git a/tests/run_agent/test_thinking_sig_recovery_persistence.py b/tests/run_agent/test_thinking_sig_recovery_persistence.py
new file mode 100644
index 0000000000..e518af5145
--- /dev/null
+++ b/tests/run_agent/test_thinking_sig_recovery_persistence.py
@@ -0,0 +1,93 @@
+"""Regression tests for the thinking-block signature recovery.
+
+The recovery in ``agent/conversation_loop.py`` strips ``reasoning_details``
+from ``api_messages`` (the API-call-time list rebuilt on every retry) and
+leaves ``messages`` (the canonical store) untouched. The previous
+implementation popped from ``messages`` directly, which never reached
+``api_messages`` because each entry in ``api_messages`` was a shallow
+copy of the corresponding entry in ``messages``, and the mutation also
+landed in ``state.db`` on the next ``_persist_session`` call, corrupting
+the conversation.
+
+These tests cover the surface that the recovery touches in isolation:
+shallow copies share inner field references; popping a key from one dict
+does not remove it from the other; and a list of shallow copies behaves
+the same way.
+"""
+
+
+def _shallow_copies(messages):
+    return [m.copy() for m in messages]
+
+
+def test_pop_on_shallow_copy_does_not_affect_source():
+    rd = [{"type": "thinking", "thinking": "r", "signature": "s"}]
+    src = {"role": "assistant", "content": "x", "reasoning_details": rd}
+    cp = src.copy()
+
+    cp.pop("reasoning_details", None)
+
+    assert "reasoning_details" not in cp
+    assert "reasoning_details" in src
+    assert src["reasoning_details"] is rd
+
+
+def test_strip_api_messages_leaves_canonical_messages_intact():
+    """Mirrors the recovery: pop reasoning_details from api_messages only.
+
+    The canonical ``messages`` list keeps its reasoning_details so future
+    persists carry the original signed blocks.
+    """
+    rd_one = [{"type": "thinking", "thinking": "one", "signature": "sig_one"}]
+    rd_two = [{"type": "thinking", "thinking": "two", "signature": "sig_two"}]
+    messages = [
+        {"role": "user", "content": "q1"},
+        {"role": "assistant", "content": "a1", "reasoning_details": rd_one},
+        {"role": "user", "content": "q2"},
+        {"role": "assistant", "content": "a2", "reasoning_details": rd_two},
+    ]
+    api_messages = _shallow_copies(messages)
+
+    stripped = 0
+    for m in api_messages:
+        if isinstance(m, dict) and "reasoning_details" in m:
+            m.pop("reasoning_details", None)
+            stripped += 1
+
+    assert stripped == 2
+    assert all("reasoning_details" not in m for m in api_messages)
+    canonical_rd = [
+        m.get("reasoning_details") for m in messages if m["role"] == "assistant"
+    ]
+    assert canonical_rd == [rd_one, rd_two]
+
+
+def test_strip_is_idempotent_when_run_twice():
+    """A second strip is a no-op when reasoning_details has already been
+    removed from api_messages. Guards against a duplicate firing path.
+    """
+    api_messages = [
+        {"role": "assistant", "content": "a", "reasoning_details": [{"x": 1}]},
+        {"role": "user", "content": "q"},
+    ]
+    for _ in range(2):
+        for m in api_messages:
+            if isinstance(m, dict) and "reasoning_details" in m:
+                m.pop("reasoning_details", None)
+
+    assert all("reasoning_details" not in m for m in api_messages)
+
+
+def test_strip_skips_messages_without_reasoning_details():
+    api_messages = [
+        {"role": "user", "content": "q"},
+        {"role": "assistant", "content": "a"},
+        {"role": "tool", "tool_call_id": "1", "content": "ok"},
+    ]
+    snapshot = [dict(m) for m in api_messages]
+
+    for m in api_messages:
+        if isinstance(m, dict) and "reasoning_details" in m:
+            m.pop("reasoning_details", None)
+
+    assert api_messages == snapshot