fix: rewrite OpenCode identity sentence to Claude Code

backend/internal/service/gateway_sanitize_test.go

+package service
+
+import (
+	"strings"
+	"testing"
+
+	"github.com/stretchr/testify/require"
+)
+
+func TestSanitizeOpenCodeText_RewritesCanonicalSentence(t *testing.T) {
+	in := "You are OpenCode, the best coding agent on the planet."
+	got := sanitizeOpenCodeText(in)
+	require.Equal(t, strings.TrimSpace(claudeCodeSystemPrompt), got)
+}
+
+func TestSanitizeOpenCodeText_RewritesOpenCodeKeywords(t *testing.T) {
+	in := "OpenCode and opencode are mentioned."
+	got := sanitizeOpenCodeText(in)
+	require.Equal(t, "Claude Code and Claude are mentioned.", got)
+}

backend/internal/service/gateway_service.go

@@ -559,6 +559,14 @@ func sanitizeOpenCodeText(text string) string {
 	if text == "" {
 		return text
 	}
+	// Some clients include a fixed OpenCode identity sentence. Anthropic may treat
+	// this as a non-Claude-Code fingerprint, so rewrite it to the canonical
+	// Claude Code banner before generic "OpenCode"/"opencode" replacements.
+	text = strings.ReplaceAll(
+		text,
+		"You are OpenCode, the best coding agent on the planet.",
+		strings.TrimSpace(claudeCodeSystemPrompt),
+	)
 	text = strings.ReplaceAll(text, "OpenCode", "Claude Code")
 	text = opencodeTextRe.ReplaceAllString(text, "Claude")
 	return text