diff --git a/anthropic/anthropic.go b/anthropic/anthropic.go
index bd1465792..f4d9a6e23 100644
--- a/anthropic/anthropic.go
+++ b/anthropic/anthropic.go
@@ -78,12 +78,14 @@ type MessageParam struct {
 	Content any    `json:"content"` // string or []ContentBlock
 }
 
-// ContentBlock represents a content block in a message
+// ContentBlock represents a content block in a message.
+// Text and Thinking use pointers so they serialize as the field being present (even if empty)
+// only when set, which is required for SDK streaming accumulation.
 type ContentBlock struct {
 	Type string `json:"type"` // text, image, tool_use, tool_result, thinking
 
-	// For text blocks (no omitempty - SDK requires field to be present for accumulation)
-	Text string `json:"text"`
+	// For text blocks - pointer so field only appears when set (SDK requires it for accumulation)
+	Text *string `json:"text,omitempty"`
 
 	// For image blocks
 	Source *ImageSource `json:"source,omitempty"`
@@ -98,9 +100,9 @@ type ContentBlock struct {
 	Content   any    `json:"content,omitempty"` // string or []ContentBlock
 	IsError   bool   `json:"is_error,omitempty"`
 
-	// For thinking blocks (no omitempty - SDK requires field to be present for accumulation)
-	Thinking  string `json:"thinking"`
-	Signature string `json:"signature,omitempty"`
+	// For thinking blocks - pointer so field only appears when set (SDK requires it for accumulation)
+	Thinking  *string `json:"thinking,omitempty"`
+	Signature string  `json:"signature,omitempty"`
 }
 
 // ImageSource represents the source of an image
@@ -458,14 +460,14 @@ func ToMessagesResponse(id string, r api.ChatResponse) MessagesResponse {
 	if r.Message.Thinking != "" {
 		content = append(content, ContentBlock{
 			Type:     "thinking",
-			Thinking: r.Message.Thinking,
+			Thinking: ptr(r.Message.Thinking),
 		})
 	}
 
 	if r.Message.Content != "" {
 		content = append(content, ContentBlock{
 			Type: "text",
-			Text: r.Message.Content,
+			Text: ptr(r.Message.Content),
 		})
 	}
 
@@ -579,7 +581,7 @@ func (c *StreamConverter) Process(r api.ChatResponse) []StreamEvent {
 					Index: c.contentIndex,
 					ContentBlock: ContentBlock{
 						Type:     "thinking",
-						Thinking: "",
+						Thinking: ptr(""),
 					},
 				},
 			})
@@ -620,7 +622,7 @@ func (c *StreamConverter) Process(r api.ChatResponse) []StreamEvent {
 					Index: c.contentIndex,
 					ContentBlock: ContentBlock{
 						Type: "text",
-						Text: "",
+						Text: ptr(""),
 					},
 				},
 			})
@@ -760,3 +762,8 @@ func generateID(prefix string) string {
 func GenerateMessageID() string {
 	return generateID("msg")
 }
+
+// ptr returns a pointer to the given string value
+func ptr(s string) *string {
+	return &s
+}
diff --git a/anthropic/anthropic_test.go b/anthropic/anthropic_test.go
index 3cebe6710..8228bd37b 100644
--- a/anthropic/anthropic_test.go
+++ b/anthropic/anthropic_test.go
@@ -447,7 +447,7 @@ func TestToMessagesResponse_Basic(t *testing.T) {
 	if len(result.Content) != 1 {
 		t.Fatalf("expected 1 content block, got %d", len(result.Content))
 	}
-	if result.Content[0].Type != "text" || result.Content[0].Text != "Hello there!" {
+	if result.Content[0].Type != "text" || result.Content[0].Text == nil || *result.Content[0].Text != "Hello there!" {
 		t.Errorf("unexpected content: %+v", result.Content[0])
 	}
 	if result.StopReason != "end_turn" {
@@ -516,8 +516,8 @@ func TestToMessagesResponse_WithThinking(t *testing.T) {
 	if result.Content[0].Type != "thinking" {
 		t.Errorf("expected first block type 'thinking', got %q", result.Content[0].Type)
 	}
-	if result.Content[0].Thinking != "Let me think about this..." {
-		t.Errorf("unexpected thinking content: %q", result.Content[0].Thinking)
+	if result.Content[0].Thinking == nil || *result.Content[0].Thinking != "Let me think about this..." {
+		t.Errorf("unexpected thinking content: %v", result.Content[0].Thinking)
 	}
 	if result.Content[1].Type != "text" {
 		t.Errorf("expected second block type 'text', got %q", result.Content[1].Type)
@@ -825,7 +825,7 @@ func TestContentBlockJSON_EmptyFieldsPresent(t *testing.T) {
 			name: "text block includes empty text field",
 			block: ContentBlock{
 				Type: "text",
-				Text: "",
+				Text: ptr(""),
 			},
 			wantKeys: []string{"type", "text"},
 		},
@@ -833,7 +833,7 @@ func TestContentBlockJSON_EmptyFieldsPresent(t *testing.T) {
 			name: "thinking block includes empty thinking field",
 			block: ContentBlock{
 				Type:     "thinking",
-				Thinking: "",
+				Thinking: ptr(""),
 			},
 			wantKeys: []string{"type", "thinking"},
 		},
@@ -841,7 +841,7 @@ func TestContentBlockJSON_EmptyFieldsPresent(t *testing.T) {
 			name: "text block with content",
 			block: ContentBlock{
 				Type: "text",
-				Text: "hello",
+				Text: ptr("hello"),
 			},
 			wantKeys: []string{"type", "text"},
 		},