diff --git a/backend/internal/service/gateway_sanitize_test.go b/backend/internal/service/gateway_sanitize_test.go
index 3b0a07c9..a70c1a00 100644
--- a/backend/internal/service/gateway_sanitize_test.go
+++ b/backend/internal/service/gateway_sanitize_test.go
@@ -9,12 +9,12 @@ import (
 
 func TestSanitizeOpenCodeText_RewritesCanonicalSentence(t *testing.T) {
 	in := "You are OpenCode, the best coding agent on the planet."
-	got := sanitizeOpenCodeText(in)
+	got := sanitizeSystemText(in)
 	require.Equal(t, strings.TrimSpace(claudeCodeSystemPrompt), got)
 }
 
-func TestSanitizeOpenCodeText_RewritesOpenCodeKeywords(t *testing.T) {
+func TestSanitizeToolText_RewritesOpenCodeKeywords(t *testing.T) {
 	in := "OpenCode and opencode are mentioned."
-	got := sanitizeOpenCodeText(in)
+	got := sanitizeToolText(in)
 	require.Equal(t, "Claude Code and Claude are mentioned.", got)
 }
diff --git a/backend/internal/service/gateway_service.go b/backend/internal/service/gateway_service.go
index 47ea8593..40354e48 100644
--- a/backend/internal/service/gateway_service.go
+++ b/backend/internal/service/gateway_service.go
@@ -697,7 +697,10 @@ func normalizeParamNameForOpenCode(name string, cache map[string]string) string
 	return name
 }
 
-func sanitizeOpenCodeText(text string) string {
+// sanitizeSystemText rewrites only the fixed OpenCode identity sentence (if present).
+// We intentionally avoid broad keyword replacement in system prompts to prevent
+// accidentally changing user-provided instructions.
+func sanitizeSystemText(text string) string {
 	if text == "" {
 		return text
 	}
@@ -709,6 +712,17 @@ func sanitizeOpenCodeText(text string) string {
 		"You are OpenCode, the best coding agent on the planet.",
 		strings.TrimSpace(claudeCodeSystemPrompt),
 	)
+	return text
+}
+
+// sanitizeToolText is intentionally more aggressive than sanitizeSystemText because
+// tool descriptions are not user chat content, and some upstreams may flag "opencode"
+// strings as non-Claude-Code fingerprints.
+func sanitizeToolText(text string) string {
+	if text == "" {
+		return text
+	}
+	text = sanitizeSystemText(text)
 	text = strings.ReplaceAll(text, "OpenCode", "Claude Code")
 	text = opencodeTextRe.ReplaceAllString(text, "Claude")
 	return text
@@ -720,7 +734,7 @@ func sanitizeToolDescription(description string) string {
 	}
 	description = toolDescAbsPathRe.ReplaceAllString(description, "[path]")
 	description = toolDescWinPathRe.ReplaceAllString(description, "[path]")
-	return sanitizeOpenCodeText(description)
+	return sanitizeToolText(description)
 }
 
 func normalizeToolInputSchema(inputSchema any, cache map[string]string) {
@@ -795,7 +809,7 @@ func normalizeClaudeOAuthRequestBody(body []byte, modelID string, opts claudeOAu
 	if system, ok := req["system"]; ok {
 		switch v := system.(type) {
 		case string:
-			sanitized := sanitizeOpenCodeText(v)
+			sanitized := sanitizeSystemText(v)
 			if sanitized != v {
 				req["system"] = sanitized
 			}
@@ -812,7 +826,7 @@ func normalizeClaudeOAuthRequestBody(body []byte, modelID string, opts claudeOAu
 				if !ok || text == "" {
 					continue
 				}
-				sanitized := sanitizeOpenCodeText(text)
+				sanitized := sanitizeSystemText(text)
 				if sanitized != text {
 					block["text"] = sanitized
 				}