diff --git a/backend/internal/pkg/antigravity/claude_types.go b/backend/internal/pkg/antigravity/claude_types.go
index 23601c0457..61e32b615d 100644
--- a/backend/internal/pkg/antigravity/claude_types.go
+++ b/backend/internal/pkg/antigravity/claude_types.go
@@ -122,6 +122,20 @@ type ClaudeContentItem struct {
 	Input any    `json:"input,omitempty"`
 }
 
+// MarshalJSON 保证 thinking 块始终带 thinking/signature 字段（即使为空），
+// 与 Anthropic 官方一致；Grok Build 等严格客户端缺少 signature 会解析失败。
+func (c ClaudeContentItem) MarshalJSON() ([]byte, error) {
+	type claudeContentItem ClaudeContentItem
+	if c.Type != "thinking" {
+		return json.Marshal(claudeContentItem(c))
+	}
+	return json.Marshal(struct {
+		Thinking  string `json:"thinking"`
+		Signature string `json:"signature"`
+		claudeContentItem
+	}{Thinking: c.Thinking, Signature: c.Signature, claudeContentItem: claudeContentItem(c)})
+}
+
 // ClaudeUsage Claude 用量统计
 type ClaudeUsage struct {
 	InputTokens              int `json:"input_tokens"`
diff --git a/backend/internal/pkg/antigravity/claude_types_test.go b/backend/internal/pkg/antigravity/claude_types_test.go
index 4b49fdb9b1..1aa5bca312 100644
--- a/backend/internal/pkg/antigravity/claude_types_test.go
+++ b/backend/internal/pkg/antigravity/claude_types_test.go
@@ -1,6 +1,9 @@
 package antigravity
 
-import "testing"
+import (
+	"encoding/json"
+	"testing"
+)
 
 func TestDefaultModels_ContainsNewAndLegacyImageModels(t *testing.T) {
 	t.Parallel()
@@ -44,3 +47,23 @@ func TestDefaultModels_ContainsNewAndLegacyImageModels(t *testing.T) {
 		}
 	}
 }
+
+func TestClaudeContentItem_ThinkingAlwaysHasSignature(t *testing.T) {
+	t.Parallel()
+
+	data, err := json.Marshal(ClaudeContentItem{Type: "thinking", Thinking: "plan"})
+	if err != nil {
+		t.Fatal(err)
+	}
+	if got, want := string(data), `{"thinking":"plan","signature":"","type":"thinking"}`; got != want {
+		t.Fatalf("thinking block = %s, want %s", got, want)
+	}
+
+	data, err = json.Marshal(ClaudeContentItem{Type: "text", Text: "hi"})
+	if err != nil {
+		t.Fatal(err)
+	}
+	if got, want := string(data), `{"type":"text","text":"hi"}`; got != want {
+		t.Fatalf("text block = %s, want %s", got, want)
+	}
+}
diff --git a/backend/internal/pkg/antigravity/stream_transformer.go b/backend/internal/pkg/antigravity/stream_transformer.go
index 13471ddddc..faecf233ec 100644
--- a/backend/internal/pkg/antigravity/stream_transformer.go
+++ b/backend/internal/pkg/antigravity/stream_transformer.go
@@ -296,8 +296,9 @@ func (p *StreamingProcessor) processThinking(text, signature string) []byte {
 	// 开始或继续 thinking 块
 	if p.blockType != BlockTypeThinking {
 		_, _ = result.Write(p.startBlock(BlockTypeThinking, map[string]any{
-			"type":     "thinking",
-			"thinking": "",
+			"type":      "thinking",
+			"thinking":  "",
+			"signature": "",
 		}))
 	}
 
@@ -475,8 +476,9 @@ func (p *StreamingProcessor) emitEmptyThinkingWithSignature(signature string) []
 	var result bytes.Buffer
 
 	_, _ = result.Write(p.startBlock(BlockTypeThinking, map[string]any{
-		"type":     "thinking",
-		"thinking": "",
+		"type":      "thinking",
+		"thinking":  "",
+		"signature": "",
 	}))
 	_, _ = result.Write(p.emitDelta("thinking_delta", map[string]any{
 		"thinking": "",
diff --git a/backend/internal/pkg/apicompat/anthropic_responses_test.go b/backend/internal/pkg/apicompat/anthropic_responses_test.go
index 63d355eb78..5902128b97 100644
--- a/backend/internal/pkg/apicompat/anthropic_responses_test.go
+++ b/backend/internal/pkg/apicompat/anthropic_responses_test.go
@@ -844,7 +844,7 @@ func TestStreamingReasoning(t *testing.T) {
 
 	sse, err := ResponsesAnthropicEventToSSE(events[0])
 	require.NoError(t, err)
-	assert.Contains(t, sse, `"content_block":{"thinking":"","type":"thinking"}`)
+	assert.Contains(t, sse, `"content_block":{"thinking":"","signature":"","type":"thinking"}`)
 
 	// reasoning text delta
 	events = ResponsesEventToAnthropicEvents(&ResponsesStreamEvent{
diff --git a/backend/internal/pkg/apicompat/thinking_signature_wire_test.go b/backend/internal/pkg/apicompat/thinking_signature_wire_test.go
new file mode 100644
index 0000000000..33924fd498
--- /dev/null
+++ b/backend/internal/pkg/apicompat/thinking_signature_wire_test.go
@@ -0,0 +1,24 @@
+package apicompat
+
+import (
+	"encoding/json"
+	"testing"
+
+	"github.com/stretchr/testify/require"
+)
+
+// Thinking blocks must always carry the `signature` key (Anthropic wire shape);
+// strict clients such as Grok Build fail with "missing field `signature`".
+func TestAnthropicContentBlock_ThinkingAlwaysHasSignature(t *testing.T) {
+	data, err := json.Marshal(AnthropicContentBlock{Type: "thinking", Thinking: "plan"})
+	require.NoError(t, err)
+	require.JSONEq(t, `{"type":"thinking","thinking":"plan","signature":""}`, string(data))
+
+	data, err = json.Marshal(AnthropicContentBlock{Type: "thinking", Signature: "sig"})
+	require.NoError(t, err)
+	require.JSONEq(t, `{"type":"thinking","thinking":"","signature":"sig"}`, string(data))
+
+	data, err = json.Marshal(AnthropicContentBlock{Type: "text", Text: "hi"})
+	require.NoError(t, err)
+	require.JSONEq(t, `{"type":"text","text":"hi"}`, string(data))
+}
diff --git a/backend/internal/pkg/apicompat/types.go b/backend/internal/pkg/apicompat/types.go
index c0a833f107..b5864f2f2f 100644
--- a/backend/internal/pkg/apicompat/types.go
+++ b/backend/internal/pkg/apicompat/types.go
@@ -95,10 +95,14 @@ func (b AnthropicContentBlock) MarshalJSON() ([]byte, error) {
 			anthropicContentBlock
 		}{Text: b.Text, anthropicContentBlock: anthropicContentBlock(b)})
 	case "thinking":
+		// Anthropic always sends `signature` on thinking blocks (empty on
+		// content_block_start); strict clients such as Grok Build reject the
+		// block with "missing field `signature`" when the key is absent.
 		return json.Marshal(struct {
-			Thinking string `json:"thinking"`
+			Thinking  string `json:"thinking"`
+			Signature string `json:"signature"`
 			anthropicContentBlock
-		}{Thinking: b.Thinking, anthropicContentBlock: anthropicContentBlock(b)})
+		}{Thinking: b.Thinking, Signature: b.Signature, anthropicContentBlock: anthropicContentBlock(b)})
 	default:
 		return json.Marshal(base)
 	}