diff --git a/ai/internal/anthropic/client.go b/ai/internal/anthropic/client.go
index 26a656e..1353b85 100644
--- a/ai/internal/anthropic/client.go
+++ b/ai/internal/anthropic/client.go
@@ -135,11 +135,11 @@ type request struct {
 }
 
 type response struct {
-	ID         string  `json:"id"`
-	Type       string  `json:"type"`
-	Model      string  `json:"model"`
-	StopReason string  `json:"stop_reason"`
-	Content    []block `json:"content"`
+	ID         string          `json:"id"`
+	Type       string          `json:"type"`
+	Model      string          `json:"model"`
+	StopReason string          `json:"stop_reason"`
+	Content    json.RawMessage `json:"content"` // kept verbatim: thinking blocks (with their signature) must be replayed unchanged
 	Usage      struct {
 		InputTokens              int `json:"input_tokens"`
 		OutputTokens             int `json:"output_tokens"`
@@ -275,8 +275,10 @@ func (c *Client) CreateResponse(ctx context.Context, req openai.Request) (*opena
 		out.ID = fmt.Sprintf("msg_local_%d", c.seq)
 		c.mu.Unlock()
 	}
+	var blocks []block
+	_ = json.Unmarshal(r.Content, &blocks)
 	var text strings.Builder
-	for _, bl := range r.Content {
+	for _, bl := range blocks {
 		switch bl.Type {
 		case "text":
 			text.WriteString(bl.Text)
@@ -296,7 +298,9 @@ func (c *Client) CreateResponse(ctx context.Context, req openai.Request) (*opena
 	out.Usage.OutputTokens = r.Usage.OutputTokens
 	out.Usage.TotalTokens = out.Usage.InputTokens + out.Usage.OutputTokens
 
-	// remember the conversation for the next turn (assistant content kept verbatim so tool_use ids line up)
+	// remember the conversation for the next turn: the assistant content is replayed as the raw JSON the API returned,
+	// so tool_use ids and thinking blocks (adaptive thinking is always on for Claude 5.5; a thinking block replayed
+	// without its text/signature is rejected with "thinking.thinking: Field required") stay intact
 	full := append(append([]message{}, msgs...), message{Role: "assistant", Content: r.Content})
 	c.mu.Lock()
 	c.conv[out.ID] = conversation{system: body.System, msgs: full}
diff --git a/ai/internal/anthropic/client_test.go b/ai/internal/anthropic/client_test.go
index 039ce9e..2b0f8ec 100644
--- a/ai/internal/anthropic/client_test.go
+++ b/ai/internal/anthropic/client_test.go
@@ -24,7 +24,7 @@ func TestTranslateTurns(t *testing.T) {
 		bodies = append(bodies, b)
 		switch len(bodies) {
 		case 1:
-			w.Write([]byte(`{"id":"msg_1","type":"message","model":"claude-sonnet-5-5","stop_reason":"tool_use","content":[{"type":"text","text":"checking"},{"type":"tool_use","id":"toolu_1","name":"get_probe_status","input":{}}],"usage":{"input_tokens":100,"output_tokens":20,"cache_creation_input_tokens":5,"cache_read_input_tokens":0}}`))
+			w.Write([]byte(`{"id":"msg_1","type":"message","model":"claude-sonnet-5-5","stop_reason":"tool_use","content":[{"type":"thinking","thinking":"let me probe","signature":"sig-abc"},{"type":"text","text":"checking"},{"type":"tool_use","id":"toolu_1","name":"get_probe_status","input":{}}],"usage":{"input_tokens":100,"output_tokens":20,"cache_creation_input_tokens":5,"cache_read_input_tokens":0}}`))
 		default:
 			w.Write([]byte(`{"id":"msg_2","type":"message","model":"claude-sonnet-5-5","stop_reason":"tool_use","content":[{"type":"tool_use","id":"toolu_2","name":"submit_diagnosis","input":{"root_cause_label":"x","confidence":0.9}}],"usage":{"input_tokens":200,"output_tokens":30}}`))
 		}
@@ -75,6 +75,10 @@ func TestTranslateTurns(t *testing.T) {
 	if as["role"] != "assistant" || !strings.Contains(mustJSON(as["content"]), `"tool_use"`) {
 		t.Fatalf("assistant turn not replayed: %v", as)
 	}
+	// the thinking block must come back verbatim with its text and signature (Claude 5.5 rejects a stripped one)
+	if ac := mustJSON(as["content"]); !strings.Contains(ac, `"thinking":"let me probe"`) || !strings.Contains(ac, `"signature":"sig-abc"`) {
+		t.Fatalf("thinking block not replayed verbatim: %s", ac)
+	}
 	last := msgs[2].(map[string]any)["content"].([]any)
 	if tr := last[0].(map[string]any); tr["type"] != "tool_result" || tr["tool_use_id"] != "toolu_1" || tr["content"] != `{"alb":1}` {
 		t.Fatalf("tool_result: %v", tr)