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.

agent/anthropic_adapter.py

@@ -1692,6 +1692,29 @@ def _convert_assistant_message(m: Dict[str, Any]) -> Dict[str, Any]:
     reasoning_content injection for Kimi/DeepSeek endpoints.
     """
     content = m.get("content", "")
+    # Anthropic interleaved-thinking fast path: when this turn carries a
+    # verbatim, order-preserving block list (set by normalize_response only
+    # for turns that interleave SIGNED thinking with tool_use), replay it
+    # unchanged. Reconstructing from the parallel reasoning_details +
+    # tool_calls fields front-loads thinking and reorders signed blocks,
+    # which Anthropic rejects with HTTP 400 ("thinking ... blocks in the
+    # latest assistant message cannot be modified"). Block order — and thus
+    # each thinking block's signature — must survive verbatim. tool_use IDs
+    # are sanitized to match the tool_result IDs produced elsewhere; the
+    # downstream mcp_ prefixing pass handles tool names on these blocks.
+    ordered_blocks = m.get("anthropic_content_blocks")
+    if isinstance(ordered_blocks, list) and ordered_blocks:
+        replayed: List[Dict[str, Any]] = []
+        for b in ordered_blocks:
+            if not isinstance(b, dict):
+                continue
+            blk = copy.deepcopy(b)
+            if blk.get("type") == "tool_use" and "id" in blk:
+                blk["id"] = _sanitize_tool_id(blk.get("id", ""))
+            replayed.append(blk)
+        if replayed:
+            return {"role": "assistant", "content": replayed}
+
     blocks = _extract_preserved_thinking_blocks(m)
     if content:
         if isinstance(content, list):

agent/chat_completion_helpers.py

@@ -952,6 +952,18 @@ def build_assistant_message(agent, assistant_message, finish_reason: str) -> dic
         if preserved:
             msg["reasoning_details"] = preserved
 
+    # Anthropic interleaved-thinking replay: when a turn interleaves signed
+    # thinking blocks with tool_use, the parallel reasoning_details +
+    # tool_calls fields lose the cross-type ordering, and reconstruction
+    # front-loads thinking — reordering signed blocks and triggering HTTP 400
+    # ("thinking ... blocks in the latest assistant message cannot be
+    # modified"). Carry the verbatim ordered block list so the adapter can
+    # replay the latest assistant message unchanged. See
+    # agent/transports/anthropic.py and agent/anthropic_adapter.py.
+    ordered_blocks = getattr(assistant_message, "anthropic_content_blocks", None)
+    if ordered_blocks:
+        msg["anthropic_content_blocks"] = ordered_blocks
+
     # Codex Responses API: preserve encrypted reasoning items for
     # multi-turn continuity. These get replayed as input on the next turn.
     codex_items = getattr(assistant_message, "codex_reasoning_items", None)

agent/transports/anthropic.py

@@ -94,13 +94,27 @@ class AnthropicTransport(ProviderTransport):
         reasoning_parts = []
         reasoning_details = []
         tool_calls = []
+        # Verbatim, order-preserving copy of every content block in the turn.
+        # Anthropic signs each thinking block against the turn content that
+        # PRECEDES it at its position; when a turn interleaves thinking and
+        # tool_use (adaptive/interleaved thinking, Claude 4.6+), the parallel
+        # reasoning_details + tool_calls lists below lose that cross-type
+        # ordering. Replaying the latest assistant message in the wrong order
+        # invalidates the signatures -> HTTP 400 "thinking ... blocks in the
+        # latest assistant message cannot be modified". Preserve the exact
+        # block sequence here so the adapter can replay it unchanged. See
+        # tests/agent/test_anthropic_thinking_block_order.py.
+        ordered_blocks = []
 
         for block in response.content:
+            block_dict = _to_plain_data(block)
+            if isinstance(block_dict, dict):
+                ordered_blocks.append(block_dict)
             if block.type == "text":
                 text_parts.append(block.text)
-            elif block.type == "thinking":
-                reasoning_parts.append(block.thinking)
-                block_dict = _to_plain_data(block)
+            elif block.type in ("thinking", "redacted_thinking"):
+                if block.type == "thinking":
+                    reasoning_parts.append(block.thinking)
                 if isinstance(block_dict, dict):
                     reasoning_details.append(block_dict)
             elif block.type == "tool_use":
@@ -130,6 +144,23 @@ class AnthropicTransport(ProviderTransport):
         provider_data = {}
         if reasoning_details:
             provider_data["reasoning_details"] = reasoning_details
+        # Only worth carrying the ordered-blocks channel when the turn
+        # actually interleaves signed thinking with tool_use — that's the
+        # only shape the parallel lists reconstruct incorrectly. A turn that
+        # is purely text, or thinking-then-tools with a single leading
+        # thinking block, replays correctly without it.
+        _has_signed_thinking = any(
+            isinstance(b, dict)
+            and b.get("type") in ("thinking", "redacted_thinking")
+            and (b.get("signature") or b.get("data"))
+            for b in ordered_blocks
+        )
+        _has_tool_use = any(
+            isinstance(b, dict) and b.get("type") == "tool_use"
+            for b in ordered_blocks
+        )
+        if _has_signed_thinking and _has_tool_use:
+            provider_data["anthropic_content_blocks"] = ordered_blocks
 
         return NormalizedResponse(
             content="\n".join(text_parts) if text_parts else None,

agent/transports/types.py

@@ -121,6 +121,18 @@ class NormalizedResponse:
         pd = self.provider_data or {}
         return pd.get("reasoning_details")
 
+    @property
+    def anthropic_content_blocks(self):
+        """Verbatim, order-preserving Anthropic content blocks for a turn.
+
+        Present only when an Anthropic turn interleaves signed thinking with
+        tool_use — the one shape the parallel reasoning_details + tool_calls
+        lists reconstruct in the wrong order, invalidating thinking-block
+        signatures on replay. See agent/transports/anthropic.py.
+        """
+        pd = self.provider_data or {}
+        return pd.get("anthropic_content_blocks")
+
     @property
     def codex_reasoning_items(self):
         pd = self.provider_data or {}