diff --git a/pkg/backend/anthropic_stream_test.go b/pkg/backend/anthropic_stream_test.go
index 6b7c6be..b61e5cb 100644
--- a/pkg/backend/anthropic_stream_test.go
+++ b/pkg/backend/anthropic_stream_test.go
@@ -441,3 +441,73 @@ func TestToolResultCaching_LastLargeResult(t *testing.T) {
 		t.Errorf("last large result missing cache_control:ephemeral; got %+v", last.Content[1].CacheControl)
 	}
 }
+
+// --- image ContentBlocks ---
+
+func TestBuildAnthropicMessages_UserImageBlocks(t *testing.T) {
+	_, out := buildAnthropicMessages([]Message{{
+		Role: "user",
+		ContentBlocks: []ContentBlock{
+			{Type: "text", Text: "what is this?"},
+			{Type: "image", ImageSource: &ImageSource{Type: "base64", MediaType: "image/png", Data: "iVBOR"}},
+		},
+	}})
+	if len(out) != 1 {
+		t.Fatalf("len = %d; want 1", len(out))
+	}
+	if len(out[0].Content) != 2 {
+		t.Fatalf("content blocks = %d; want 2", len(out[0].Content))
+	}
+	if out[0].Content[0].Type != "text" || out[0].Content[0].Text != "what is this?" {
+		t.Errorf("block[0] = %+v", out[0].Content[0])
+	}
+	if out[0].Content[1].Type != "image" || out[0].Content[1].Source == nil {
+		t.Fatalf("block[1] = %+v", out[0].Content[1])
+	}
+	if out[0].Content[1].Source.Type != "base64" || out[0].Content[1].Source.MediaType != "image/png" || out[0].Content[1].Source.Data != "iVBOR" {
+		t.Errorf("source = %+v", out[0].Content[1].Source)
+	}
+}
+
+func TestBuildAnthropicMessages_ToolImageBlocks(t *testing.T) {
+	_, out := buildAnthropicMessages([]Message{
+		{Role: "user", Content: "read image"},
+		{Role: "assistant", ToolCalls: []ToolCall{{ID: "c1", Name: "file_read", Arguments: json.RawMessage(`{}`)}}},
+		{Role: "tool", ToolCallID: "c1", Content: "[image]", ContentBlocks: []ContentBlock{
+			{Type: "image", ImageSource: &ImageSource{Type: "base64", MediaType: "image/jpeg", Data: "AAAA"}},
+			{Type: "text", Text: "[image: image/jpeg]"},
+		}},
+	})
+	// Last message is the batched tool result (user role).
+	last := out[len(out)-1]
+	if last.Role != "user" {
+		t.Fatalf("role = %q; want user", last.Role)
+	}
+	// Should have image block + tool_result block.
+	var hasImage, hasToolResult bool
+	for _, b := range last.Content {
+		if b.Type == "image" && b.Source != nil && b.Source.MediaType == "image/jpeg" {
+			hasImage = true
+		}
+		if b.Type == "tool_result" && b.ToolUseID == "c1" {
+			hasToolResult = true
+		}
+	}
+	if !hasImage {
+		t.Error("missing image block in tool result")
+	}
+	if !hasToolResult {
+		t.Error("missing tool_result block")
+	}
+}
+
+func TestBuildAnthropicMessages_UserPlainTextFallback(t *testing.T) {
+	// When ContentBlocks is nil, should fall back to Content string.
+	_, out := buildAnthropicMessages([]Message{{Role: "user", Content: "hello"}})
+	if len(out) != 1 || len(out[0].Content) != 1 {
+		t.Fatalf("unexpected: %+v", out)
+	}
+	if out[0].Content[0].Type != "text" || out[0].Content[0].Text != "hello" {
+		t.Errorf("block = %+v", out[0].Content[0])
+	}
+}
diff --git a/pkg/backend/openai_encode_test.go b/pkg/backend/openai_encode_test.go
index 5db196a..79fc5a8 100644
--- a/pkg/backend/openai_encode_test.go
+++ b/pkg/backend/openai_encode_test.go
@@ -362,6 +362,57 @@ func TestOpenAI_NoCacheControl(t *testing.T) {
 	}
 }
 
+func TestEncodeMessages_UserWithImageContentBlocks(t *testing.T) {
+	wire := encodeOpenAIMessages([]Message{{
+		Role: "user",
+		ContentBlocks: []ContentBlock{
+			{Type: "text", Text: "what is this?"},
+			{Type: "image", ImageSource: &ImageSource{Type: "base64", MediaType: "image/png", Data: "iVBOR"}},
+		},
+	}})
+	if len(wire) != 1 {
+		t.Fatalf("len = %d; want 1", len(wire))
+	}
+	if wire[0].Content != nil {
+		t.Error("Content should be nil when ContentBlocks is used")
+	}
+	if len(wire[0].ContentBlocks) != 2 {
+		t.Fatalf("ContentBlocks len = %d; want 2", len(wire[0].ContentBlocks))
+	}
+	if wire[0].ContentBlocks[0].Type != "text" || wire[0].ContentBlocks[0].Text != "what is this?" {
+		t.Errorf("block[0] = %+v", wire[0].ContentBlocks[0])
+	}
+	if wire[0].ContentBlocks[1].Type != "image_url" || wire[0].ContentBlocks[1].ImageURL == nil {
+		t.Fatalf("block[1] = %+v", wire[0].ContentBlocks[1])
+	}
+	if wire[0].ContentBlocks[1].ImageURL.URL != "data:image/png;base64,iVBOR" {
+		t.Errorf("image url = %q", wire[0].ContentBlocks[1].ImageURL.URL)
+	}
+}
+
+func TestEncodeMessages_UserWithImageContentBlocks_JSON(t *testing.T) {
+	wire := encodeOpenAIMessages([]Message{{
+		Role: "user",
+		ContentBlocks: []ContentBlock{
+			{Type: "text", Text: "look"},
+			{Type: "image", ImageSource: &ImageSource{Type: "base64", MediaType: "image/jpeg", Data: "AAAA"}},
+		},
+	}})
+	data, _ := json.Marshal(wire[0])
+	var raw map[string]json.RawMessage
+	json.Unmarshal(data, &raw)
+	// content must be an array
+	if raw["content"][0] != '[' {
+		t.Errorf("content should be array; got %s", raw["content"])
+	}
+	if !strings.Contains(string(raw["content"]), "image_url") {
+		t.Errorf("content missing image_url: %s", raw["content"])
+	}
+	if !strings.Contains(string(raw["content"]), "data:image/jpeg;base64,AAAA") {
+		t.Errorf("content missing data URI: %s", raw["content"])
+	}
+}
+
 func TestEncodeRequest_ToolsOmittedWhenEmpty(t *testing.T) {
 	req := openAIChatRequest{
 		Model:    "m",