backend: 100% coverage for buildAnthropicMessages and streamAnthropicSSE

- Test all buildAnthropicMessages branches: system concat, assistant
  tool-calls-only, nil arguments default, tool batching, unknown role,
  full multi-turn conversation
- Test streamAnthropicSSE done re-entry: events after message_stop and
  error are ignored
- Remove unreachable done guard in process closure

pkg/backend/anthropic.go

@@ -193,9 +193,6 @@ func streamAnthropicSSE(body io.Reader, ch chan<- StreamEvent) {
 	done := false
 
 	process := func(typ, data string) {
-		if done {
-			return
-		}
 		raw := []byte(data)
 		switch typ {
 		case "message_start":

pkg/backend/anthropic_stream_test.go

@@ -1,6 +1,7 @@
 package backend
 
 import (
+	"encoding/json"
 	"fmt"
 	"strings"
 	"testing"
@@ -184,3 +185,133 @@ func TestMapAnthropicStopReason(t *testing.T) {
 		}
 	}
 }
+
+// --- buildAnthropicMessages ---
+
+func TestBuildAnthropicMessages_SystemConcat(t *testing.T) {
+	sys, _ := buildAnthropicMessages([]Message{
+		{Role: "system", Content: "a"},
+		{Role: "system", Content: "b"},
+	})
+	if sys != "a\n\nb" {
+		t.Errorf("system = %q", sys)
+	}
+}
+
+func TestBuildAnthropicMessages_AssistantToolCallsOnly(t *testing.T) {
+	_, out := buildAnthropicMessages([]Message{
+		{Role: "assistant", ToolCalls: []ToolCall{{ID: "c1", Name: "f", Arguments: json.RawMessage(`{"k":"v"}`)}}},
+	})
+	if len(out) != 1 {
+		t.Fatalf("len = %d", len(out))
+	}
+	// No text block, only tool_use
+	for _, b := range out[0].Content {
+		if b.Type == "text" {
+			t.Error("should not have text block when Content is empty")
+		}
+	}
+	if out[0].Content[0].Type != "tool_use" {
+		t.Errorf("type = %q", out[0].Content[0].Type)
+	}
+}
+
+func TestBuildAnthropicMessages_NilArguments(t *testing.T) {
+	_, out := buildAnthropicMessages([]Message{
+		{Role: "assistant", ToolCalls: []ToolCall{{ID: "c1", Name: "f", Arguments: nil}}},
+	})
+	block := out[0].Content[0]
+	if string(block.Input) != "{}" {
+		t.Errorf("input = %s; want {}", block.Input)
+	}
+}
+
+func TestBuildAnthropicMessages_ToolBatching(t *testing.T) {
+	_, out := buildAnthropicMessages([]Message{
+		{Role: "tool", Content: "r1", ToolCallID: "c1"},
+		{Role: "tool", Content: "r2", ToolCallID: "c2"},
+	})
+	// Two consecutive tool messages should be batched into one user message.
+	if len(out) != 1 {
+		t.Fatalf("len = %d; want 1", len(out))
+	}
+	if out[0].Role != "user" {
+		t.Errorf("role = %q", out[0].Role)
+	}
+	if len(out[0].Content) != 2 {
+		t.Fatalf("content len = %d; want 2", len(out[0].Content))
+	}
+	if out[0].Content[0].Type != "tool_result" || out[0].Content[0].ToolUseID != "c1" {
+		t.Errorf("block[0] = %+v", out[0].Content[0])
+	}
+	if out[0].Content[1].ToolUseID != "c2" {
+		t.Errorf("block[1] = %+v", out[0].Content[1])
+	}
+}
+
+func TestBuildAnthropicMessages_UnknownRole(t *testing.T) {
+	_, out := buildAnthropicMessages([]Message{
+		{Role: "unknown", Content: "x"},
+		{Role: "user", Content: "hi"},
+	})
+	// Unknown role should be skipped, user message should still appear.
+	if len(out) != 1 || out[0].Role != "user" {
+		t.Errorf("out = %+v", out)
+	}
+}
+
+func TestBuildAnthropicMessages_FullConversation(t *testing.T) {
+	sys, out := buildAnthropicMessages([]Message{
+		{Role: "system", Content: "sys"},
+		{Role: "user", Content: "hi"},
+		{Role: "assistant", Content: "let me check", ToolCalls: []ToolCall{{ID: "c1", Name: "f", Arguments: json.RawMessage(`{}`)}}},
+		{Role: "tool", Content: "result", ToolCallID: "c1"},
+		{Role: "assistant", Content: "done"},
+	})
+	if sys != "sys" {
+		t.Errorf("system = %q", sys)
+	}
+	if len(out) != 4 {
+		t.Fatalf("len = %d; want 4", len(out))
+	}
+	// assistant[1] should have both text and tool_use blocks
+	if len(out[1].Content) != 2 {
+		t.Errorf("assistant content len = %d; want 2", len(out[1].Content))
+	}
+}
+
+// --- streamAnthropicSSE: done re-entry and scanner break ---
+
+func TestAnthropicSSE_EventsAfterStop(t *testing.T) {
+	// Events after message_stop should be ignored (done flag).
+	evs := collectAnthropic(
+		"event: message_delta\ndata: {\"delta\":{\"stop_reason\":\"end_turn\"},\"usage\":{\"output_tokens\":1}}\n\n" +
+			"event: message_stop\ndata: {\"type\":\"message_stop\"}\n\n" +
+			"event: content_block_delta\ndata: {\"index\":0,\"delta\":{\"type\":\"text_delta\",\"text\":\"ghost\"}}\n\n",
+	)
+	// Should get exactly one Done event, no "ghost" text.
+	var text string
+	var doneCount int
+	for _, ev := range evs {
+		text += ev.Content
+		if ev.Done {
+			doneCount++
+		}
+	}
+	if text != "" {
+		t.Errorf("text = %q; want empty", text)
+	}
+	if doneCount != 1 {
+		t.Errorf("done count = %d; want 1", doneCount)
+	}
+}
+
+func TestAnthropicSSE_EventsAfterError(t *testing.T) {
+	evs := collectAnthropic(
+		"event: error\ndata: {\"error\":{\"message\":\"bad\"}}\n\n" +
+			"event: content_block_delta\ndata: {\"index\":0,\"delta\":{\"type\":\"text_delta\",\"text\":\"ghost\"}}\n\n",
+	)
+	if len(evs) != 1 || !evs[0].Done {
+		t.Fatalf("expected single Done event; got %d", len(evs))
+	}
+}