fix(anthropic): smart thinking block signature management (#6112)

Anthropic signs thinking blocks against the full turn content. Any
upstream mutation (context compression, session truncation, orphan
stripping, message merging) invalidates the signature, causing HTTP 400
'Invalid signature in thinking block' — especially in long-lived
gateway sessions.

Strategy (following clawdbot/OpenClaw pattern):

1. Strip thinking/redacted_thinking from all assistant messages EXCEPT
   the last one — preserves reasoning continuity on the current
   tool-use chain while avoiding stale signature errors on older turns.

2. Downgrade unsigned thinking blocks to plain text — Anthropic can't
   validate them, but the reasoning content is preserved.

3. Strip cache_control from thinking/redacted_thinking blocks to
   prevent cache markers from interfering with signature validation.

4. Drop thinking blocks from the second message when merging
   consecutive assistant messages (role alternation enforcement).

5. Error recovery: on HTTP 400 mentioning 'signature' and 'thinking',
   strip all reasoning_details from the conversation and retry once.
   This is the safety net for edge cases the proactive stripping
   misses.

Addresses the issue reported in PR #6086 by @mingginwan while
preserving reasoning continuity (their PR stripped ALL thinking
blocks unconditionally).

Files changed:
- agent/anthropic_adapter.py: thinking block management in
  convert_messages_to_anthropic (strip old turns, downgrade unsigned,
  strip cache_control, merge-time strip)
- run_agent.py: one-shot signature error recovery in retry loop
- tests/test_anthropic_adapter.py: 10 new tests covering all cases

agent/anthropic_adapter.py

@@ -1102,7 +1102,15 @@ def convert_messages_to_anthropic(
                         curr_content = [{"type": "text", "text": curr_content}]
                     fixed[-1]["content"] = prev_content + curr_content
             else:
-                # Consecutive assistant messages — merge text content
+                # Consecutive assistant messages — merge text content.
+                # Drop thinking blocks from the *second* message: their
+                # signature was computed against a different turn boundary
+                # and becomes invalid once merged.
+                if isinstance(m["content"], list):
+                    m["content"] = [
+                        b for b in m["content"]
+                        if not (isinstance(b, dict) and b.get("type") in ("thinking", "redacted_thinking"))
+                    ]
                 prev_blocks = fixed[-1]["content"]
                 curr_blocks = m["content"]
                 if isinstance(prev_blocks, list) and isinstance(curr_blocks, list):
@@ -1120,6 +1128,68 @@ def convert_messages_to_anthropic(
             fixed.append(m)
     result = fixed
 
+    # ── Thinking block signature management ──────────────────────────
+    # Anthropic signs thinking blocks against the full turn content.
+    # Any upstream mutation (context compression, session truncation,
+    # orphan stripping, message merging) invalidates the signature,
+    # causing HTTP 400 "Invalid signature in thinking block".
+    #
+    # Strategy (following clawdbot/OpenClaw pattern):
+    # 1. Strip thinking/redacted_thinking from all assistant messages
+    #    EXCEPT the last one — preserves reasoning continuity on the
+    #    current tool-use chain while avoiding stale signature errors.
+    # 2. Downgrade unsigned thinking blocks (no signature) to text —
+    #    Anthropic can't validate them and will reject them.
+    # 3. Strip cache_control from thinking/redacted_thinking blocks —
+    #    cache markers can interfere with signature validation.
+    _THINKING_TYPES = frozenset(("thinking", "redacted_thinking"))
+
+    last_assistant_idx = None
+    for i in range(len(result) - 1, -1, -1):
+        if result[i].get("role") == "assistant":
+            last_assistant_idx = i
+            break
+
+    for idx, m in enumerate(result):
+        if m.get("role") != "assistant" or not isinstance(m.get("content"), list):
+            continue
+
+        if idx != last_assistant_idx:
+            # Strip ALL thinking blocks from non-latest assistant messages
+            stripped = [
+                b for b in m["content"]
+                if not (isinstance(b, dict) and b.get("type") in _THINKING_TYPES)
+            ]
+            m["content"] = stripped or [{"type": "text", "text": "(thinking elided)"}]
+        else:
+            # Latest assistant: keep signed thinking blocks for reasoning
+            # continuity; downgrade unsigned ones to plain text.
+            new_content = []
+            for b in m["content"]:
+                if not isinstance(b, dict) or b.get("type") not in _THINKING_TYPES:
+                    new_content.append(b)
+                    continue
+                if b.get("type") == "redacted_thinking":
+                    # Redacted blocks use 'data' for the signature payload
+                    if b.get("data"):
+                        new_content.append(b)
+                    # else: drop — no data means it can't be validated
+                elif b.get("signature"):
+                    # Signed thinking block — keep it
+                    new_content.append(b)
+                else:
+                    # Unsigned thinking — downgrade to text so it's not lost
+                    thinking_text = b.get("thinking", "")
+                    if thinking_text:
+                        new_content.append({"type": "text", "text": thinking_text})
+            m["content"] = new_content or [{"type": "text", "text": "(empty)"}]
+
+        # Strip cache_control from any remaining thinking/redacted_thinking
+        # blocks — cache markers interfere with signature validation.
+        for b in m["content"]:
+            if isinstance(b, dict) and b.get("type") in _THINKING_TYPES:
+                b.pop("cache_control", None)
+
     return system, result