fix(anthropic): preserve interleaved thinking/tool_use block order on replay

Interleaved-thinking turns (adaptive thinking, Claude 4.6+/Opus 4.8) emit
content blocks like:

    thinking_1(signed) tool_use_1 thinking_2(signed) tool_use_2

Anthropic signs each thinking block against the turn content preceding it
at its position. normalize_response split the turn into two parallel lists
(reasoning_details + tool_calls), discarding cross-type order, and
_convert_assistant_message rebuilt it as [all thinking][text][all tool_use].
That moved thinking_2 ahead of tool_use_1, invalidating its signature, so
Anthropic rejected the latest assistant message with HTTP 400:

    messages.N.content.M: `thinking` or `redacted_thinking` blocks in the
    latest assistant message cannot be modified.

Observed repeatedly in agent.conversation_loop against api.anthropic.com /
claude-opus-4-8, recurring across sessions on multi-thinking-block turns.

Fix: carry a verbatim, order-preserving copy of the turn's content blocks
(anthropic_content_blocks) end-to-end - capture in normalize_response,
persist/restore through state.db, and replay unchanged for the latest
assistant message. Gated to turns that actually interleave signed thinking
with tool_use, so normal turns are unaffected.

Adds 3 regression tests including a SQLite round-trip covering the
crash-recovery reload path.

tests/agent/test_anthropic_thinking_block_order.py

@@ -0,0 +1,235 @@
+"""Regression test for the Anthropic interleaved thinking-block 400.
+
+Reproduces: HTTP 400 ``messages.N.content.M: thinking or redacted_thinking
+blocks in the latest assistant message cannot be modified. These blocks must
+remain as they were in the original response.``
+
+Root cause under test
+----------------------
+With adaptive / interleaved thinking (Claude 4.6+, e.g. Opus 4.8), a single
+assistant turn can emit content blocks in an interleaved order::
+
+    thinking_1 (signed) · tool_use_1 · thinking_2 (signed) · tool_use_2
+
+Anthropic signs each thinking block against the turn content that precedes it
+at its position.  ``thinking_2`` is signed with ``tool_use_1`` before it.
+
+``AnthropicTransport.normalize_response`` (agent/transports/anthropic.py)
+splits the turn into two *parallel* lists — ``reasoning_details`` (thinking
+blocks) and ``tool_calls`` (tool_use blocks) — discarding the cross-type
+ordering.  ``run_agent`` stores those as separate fields on the assistant
+message.  On replay, ``_convert_assistant_message`` (agent/anthropic_adapter.py)
+rebuilds the content as ``[all thinking][text][all tool_use]``, which reorders
+``thinking_2`` ahead of ``tool_use_1``.  The signature no longer matches its
+original position, so Anthropic rejects the latest assistant message with the
+400 above.
+
+This test asserts that an interleaved turn round-trips through
+normalize_response -> stored message -> convert_messages_to_anthropic with its
+block order preserved.  It FAILS on the current code (documenting the bug) and
+should PASS once block ordering is preserved on replay.
+"""
+
+import json
+from types import SimpleNamespace
+
+import pytest
+
+from agent.transports import get_transport
+from agent.anthropic_adapter import convert_messages_to_anthropic
+
+
+def _thinking_block(text: str, signature: str) -> SimpleNamespace:
+    """A signed Anthropic thinking block, shaped like the SDK object."""
+    return SimpleNamespace(type="thinking", thinking=text, signature=signature)
+
+
+def _tool_use_block(block_id: str, name: str, payload: dict) -> SimpleNamespace:
+    return SimpleNamespace(type="tool_use", id=block_id, name=name, input=payload)
+
+
+def _interleaved_response() -> SimpleNamespace:
+    """An assistant turn with thinking interleaved between two tool_use blocks."""
+    return SimpleNamespace(
+        content=[
+            _thinking_block("Plan: inspect file A first.", "sig-AAA"),
+            _tool_use_block("toolu_1", "read_file", {"path": "a.py"}),
+            _thinking_block("A looked fine; now inspect B.", "sig-BBB"),
+            _tool_use_block("toolu_2", "read_file", {"path": "b.py"}),
+        ],
+        stop_reason="tool_use",
+        usage=None,
+    )
+
+
+def _stored_assistant_message(normalized) -> dict:
+    """Reconstruct the OpenAI-style assistant message the way run_agent stores it.
+
+    run_agent.py persists assistant turns as separate fields: content,
+    reasoning_details (from provider_data), and tool_calls.  See
+    run_agent.py L1513-1516 and hermes_state.py.
+    """
+    provider_data = normalized.provider_data or {}
+    tool_calls = []
+    for tc in (normalized.tool_calls or []):
+        tool_calls.append({
+            "id": tc.id,
+            "type": "function",
+            "function": {"name": tc.name, "arguments": tc.arguments},
+        })
+    msg = {
+        "role": "assistant",
+        "content": normalized.content or "",
+        "reasoning_details": provider_data.get("reasoning_details"),
+        "tool_calls": tool_calls,
+    }
+    # build_assistant_message lifts the verbatim ordered-block channel onto
+    # the stored message; mirror that here.
+    blocks = provider_data.get("anthropic_content_blocks")
+    if blocks:
+        msg["anthropic_content_blocks"] = blocks
+    return msg
+
+
+def _original_block_order(response) -> list:
+    """The (type, key) sequence of the original interleaved response."""
+    order = []
+    for b in response.content:
+        if b.type == "thinking":
+            order.append(("thinking", b.signature))
+        elif b.type == "tool_use":
+            order.append(("tool_use", b.id))
+    return order
+
+
+def _replayed_block_order(assistant_content) -> list:
+    order = []
+    for b in assistant_content:
+        if not isinstance(b, dict):
+            continue
+        if b.get("type") in ("thinking", "redacted_thinking"):
+            order.append(("thinking", b.get("signature")))
+        elif b.get("type") == "tool_use":
+            order.append(("tool_use", b.get("id")))
+    return order
+
+
+class TestInterleavedThinkingBlockOrder:
+    def test_normalize_response_loses_interleaving(self):
+        """Confirm the lossy split: normalize_response stores thinking and
+        tool_use in independent fields with no positional linkage."""
+        transport = get_transport("anthropic_messages")
+        normalized = transport.normalize_response(_interleaved_response())
+
+        # Both thinking blocks are captured...
+        details = (normalized.provider_data or {}).get("reasoning_details")
+        assert details is not None and len(details) == 2
+        # ...and both tool calls...
+        assert normalized.tool_calls is not None and len(normalized.tool_calls) == 2
+        # ...but they live in separate fields. There is no single ordered
+        # structure recording that thinking_2 sat between the two tool calls.
+        # (This is the structural precondition for the reorder bug.)
+
+    def test_interleaved_order_preserved_on_replay(self):
+        """The latest assistant message must replay blocks in their ORIGINAL
+        order, or Anthropic rejects the signed thinking blocks with a 400.
+
+        FAILS on current code: _convert_assistant_message front-loads all
+        thinking blocks, producing
+            thinking_1 · thinking_2 · tool_use_1 · tool_use_2
+        instead of the original
+            thinking_1 · tool_use_1 · thinking_2 · tool_use_2
+        """
+        response = _interleaved_response()
+        original_order = _original_block_order(response)
+
+        transport = get_transport("anthropic_messages")
+        normalized = transport.normalize_response(response)
+        assistant_msg = _stored_assistant_message(normalized)
+
+        # Build a minimal conversation where this assistant turn is the LATEST
+        # assistant message (the one whose signed blocks are sent verbatim).
+        messages = [
+            {"role": "user", "content": "Inspect a.py and b.py."},
+            assistant_msg,
+            {"role": "tool", "tool_call_id": "toolu_1", "content": "a.py: ok"},
+            {"role": "tool", "tool_call_id": "toolu_2", "content": "b.py: ok"},
+        ]
+
+        _system, anthropic_messages = convert_messages_to_anthropic(
+            messages,
+            base_url=None,             # direct Anthropic
+            model="claude-opus-4-8",   # adaptive thinking family
+        )
+
+        # Find the (latest) assistant message in the converted output.
+        assistant_out = [m for m in anthropic_messages if m.get("role") == "assistant"]
+        assert assistant_out, "no assistant message in converted output"
+        replayed_order = _replayed_block_order(assistant_out[-1]["content"])
+
+        assert replayed_order == original_order, (
+            "Interleaved thinking/tool_use order was not preserved on replay.\n"
+            f"  original: {original_order}\n"
+            f"  replayed: {replayed_order}\n"
+            "Anthropic signs thinking blocks against their original position; "
+            "reordering invalidates the signature -> HTTP 400 'thinking blocks "
+            "in the latest assistant message cannot be modified'."
+        )
+
+    def test_interleaved_order_survives_db_roundtrip(self, tmp_path):
+        """The ordered-block channel must survive SQLite persistence + reload.
+
+        This is the exact path that fails after a gateway crash: the session
+        is reloaded from state.db via get_messages_as_conversation, then
+        replayed. If the verbatim block list is dropped or not deserialized,
+        the reconstruction reorders signed thinking blocks -> HTTP 400.
+        """
+        import hermes_state
+
+        response = _interleaved_response()
+        original_order = _original_block_order(response)
+
+        transport = get_transport("anthropic_messages")
+        normalized = transport.normalize_response(response)
+        assistant_msg = _stored_assistant_message(normalized)
+
+        db = hermes_state.SessionDB(tmp_path / "state.db")
+        sid = "sess_roundtrip"
+        db.create_session(sid, source="test")
+        db.append_message(
+            session_id=sid,
+            role="assistant",
+            content=assistant_msg["content"],
+            tool_calls=assistant_msg["tool_calls"],
+            reasoning_details=assistant_msg.get("reasoning_details"),
+            anthropic_content_blocks=assistant_msg.get("anthropic_content_blocks"),
+        )
+        db.append_message(session_id=sid, role="tool", tool_call_id="toolu_1", content="a ok")
+        db.append_message(session_id=sid, role="tool", tool_call_id="toolu_2", content="b ok")
+
+        # Reload via the conversation-restore path used on resume / crash recovery.
+        loaded = db.get_messages_as_conversation(sid)
+        reloaded_assistant = [m for m in loaded if m.get("role") == "assistant"]
+        assert reloaded_assistant, "no assistant message after DB reload"
+        # The ordered-block channel must come back as a deserialized list.
+        blocks = reloaded_assistant[0].get("anthropic_content_blocks")
+        assert isinstance(blocks, list) and len(blocks) == 4, (
+            "anthropic_content_blocks was not persisted/deserialized correctly"
+        )
+
+        _system, anthropic_messages = convert_messages_to_anthropic(
+            loaded, base_url=None, model="claude-opus-4-8",
+        )
+        assistant_out = [m for m in anthropic_messages if m.get("role") == "assistant"]
+        assert assistant_out, "no assistant message in converted output"
+        replayed_order = _replayed_block_order(assistant_out[-1]["content"])
+
+        assert replayed_order == original_order, (
+            "Interleaved block order was lost across the SQLite round-trip.\n"
+            f"  original: {original_order}\n"
+            f"  replayed: {replayed_order}"
+        )
+
+
+if __name__ == "__main__":
+    raise SystemExit(pytest.main([__file__, "-v"]))