anthropic: use pointer types for Text and Thinking fields

Use *string instead of string for Text and Thinking fields in ContentBlock
so that omitempty works correctly:
- nil pointer: field omitted from JSON (for blocks that don't use it)
- ptr(""): field present as "" (for SDK streaming accumulation)
- ptr("content"): field present with content

This keeps the JSON output clean (text blocks don't have thinking field,
thinking blocks don't have text field) while still satisfying SDK
requirements for field presence during streaming.

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"`
@@ -98,8 +100,8 @@ 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"`
+	// 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"`
 }
 
@@ -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
+}

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"},
 		},