Fix crash on sessions with image content (fixes #4)

- Increase scanner buffer from 1MB to 10MB to handle base64 images
  (buffer starts at 64KB and only grows on demand)
- Parser now gracefully skips malformed/truncated JSON lines instead
  of returning a fatal error
- Add tests for image content blocks and truncated JSON

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: phiat

internal/parser/parser.go

@@ -107,7 +107,10 @@ func ParseLine(line string) ([]StreamItem, error) {
 
 	var raw RawMessage
 	if err := json.Unmarshal([]byte(line), &raw); err != nil {
-		return nil, fmt.Errorf("failed to parse JSON: %w", err)
+		// Gracefully skip malformed/truncated lines (e.g. base64 images
+		// that exceeded the scanner buffer). A single bad line shouldn't
+		// crash the app.
+		return nil, nil
 	}
 
 	timestamp, err := time.Parse(time.RFC3339, raw.Timestamp)

internal/parser/parser_test.go

@@ -21,12 +21,13 @@ func TestParseLine_EmptyLine(t *testing.T) {
 }
 
 func TestParseLine_InvalidJSON(t *testing.T) {
-	_, err := ParseLine("not json at all")
-	if err == nil {
-		t.Error("ParseLine with invalid JSON should return error")
+	// Invalid JSON should be silently skipped, not return an error
+	items, err := ParseLine("not json at all")
+	if err != nil {
+		t.Errorf("ParseLine should skip invalid JSON, got error: %v", err)
 	}
-	if !strings.Contains(err.Error(), "failed to parse JSON") {
-		t.Errorf("error should mention JSON parsing, got: %v", err)
+	if len(items) != 0 {
+		t.Errorf("expected 0 items for invalid JSON, got %d", len(items))
 	}
 }
 
@@ -304,6 +305,47 @@ func TestFormatToolInput(t *testing.T) {
 	}
 }
 
+func TestParseLine_UserMessageWithImage(t *testing.T) {
+	// User messages can contain image blocks (screenshots pasted into Claude Code)
+	// These should be silently skipped, not cause errors
+	line := `{"type":"user","timestamp":"2025-01-01T12:00:00Z","message":{"role":"user","content":[{"type":"image","source":{"type":"base64","media_type":"image/png","data":"iVBORw0KGgoAAAANSUhEUgAAAAEAAAABCAYAAAAfFcSJAAAADUlEQVR42mNk"}}]}}`
+	items, err := ParseLine(line)
+	if err != nil {
+		t.Fatalf("ParseLine should not error on image content, got: %v", err)
+	}
+	if len(items) != 0 {
+		t.Errorf("expected 0 items (images skipped), got %d", len(items))
+	}
+}
+
+func TestParseLine_UserMessageWithImageAndToolResult(t *testing.T) {
+	// A user message can contain both image blocks and tool results
+	line := `{"type":"user","timestamp":"2025-01-01T12:00:00Z","message":{"role":"user","content":[{"type":"image","source":{"type":"base64","media_type":"image/png","data":"iVBOR"}},{"type":"tool_result","tool_use_id":"toolu_img1","content":"tool output here"}]}}`
+	items, err := ParseLine(line)
+	if err != nil {
+		t.Fatalf("unexpected error: %v", err)
+	}
+	if len(items) != 1 {
+		t.Fatalf("expected 1 item (tool_result only), got %d", len(items))
+	}
+	if items[0].Content != "tool output here" {
+		t.Errorf("content = %q, want %q", items[0].Content, "tool output here")
+	}
+}
+
+func TestParseLine_TruncatedJSON(t *testing.T) {
+	// When a JSONL line exceeds the scanner buffer, it gets truncated
+	// producing invalid JSON. This should be skipped gracefully, not crash.
+	truncated := `{"type":"user","timestamp":"2025-01-01T12:00:00Z","message":{"role":"user","content":[{"type":"image","source":{"type":"base64","media_type":"image/png","data":"JVBER`
+	items, err := ParseLine(truncated)
+	if err != nil {
+		t.Fatalf("ParseLine should gracefully skip truncated JSON, got error: %v", err)
+	}
+	if len(items) != 0 {
+		t.Errorf("expected 0 items for truncated JSON, got %d", len(items))
+	}
+}
+
 // buildAssistantLine builds a valid JSONL line for an assistant tool_use message
 func buildAssistantLine(t *testing.T, toolName, toolID string, inputJSON json.RawMessage) string {
 	t.Helper()

internal/watcher/watcher.go

@@ -37,8 +37,10 @@ const (
 	FileReadBufferSize = 32 * 1024
 	// ScannerInitBufferSize is the initial buffer for JSON line scanner (64KB)
 	ScannerInitBufferSize = 64 * 1024
-	// ScannerMaxBufferSize is the max buffer for JSON line scanner (1MB)
-	ScannerMaxBufferSize = 1024 * 1024
+	// ScannerMaxBufferSize is the max buffer for JSON line scanner (10MB)
+	// Large because JSONL lines can contain base64-encoded images (screenshots).
+	// The scanner starts at ScannerInitBufferSize and only grows on demand.
+	ScannerMaxBufferSize = 10 * 1024 * 1024
 	// AgentIDDisplayLength is how many chars of agent ID to show in display name
 	AgentIDDisplayLength = 7
 	// RecentActivityThreshold is how recent a session must be to show as "active" in listings