fix: also prefix next system block with Claude Code banner

backend/internal/service/gateway_prompt_test.go

@@ -2,6 +2,7 @@ package service
 
 import (
 	"encoding/json"
+	"strings"
 	"testing"
 
 	"github.com/stretchr/testify/require"
@@ -134,6 +135,8 @@ func TestSystemIncludesClaudeCodePrompt(t *testing.T) {
 }
 
 func TestInjectClaudeCodePrompt(t *testing.T) {
+	claudePrefix := strings.TrimSpace(claudeCodeSystemPrompt)
+
 	tests := []struct {
 		name           string
 		body           string
@@ -162,7 +165,7 @@ func TestInjectClaudeCodePrompt(t *testing.T) {
 			system:         "Custom prompt",
 			wantSystemLen:  2,
 			wantFirstText:  claudeCodeSystemPrompt,
-			wantSecondText: "Custom prompt",
+			wantSecondText: claudePrefix + "\n\nCustom prompt",
 		},
 		{
 			name:          "string system equals Claude Code prompt",
@@ -178,7 +181,7 @@ func TestInjectClaudeCodePrompt(t *testing.T) {
 			// Claude Code + Custom = 2
 			wantSystemLen:  2,
 			wantFirstText:  claudeCodeSystemPrompt,
-			wantSecondText: "Custom",
+			wantSecondText: claudePrefix + "\n\nCustom",
 		},
 		{
 			name: "array system with existing Claude Code prompt (should dedupe)",
@@ -190,7 +193,7 @@ func TestInjectClaudeCodePrompt(t *testing.T) {
 			// Claude Code at start + Other = 2 (deduped)
 			wantSystemLen:  2,
 			wantFirstText:  claudeCodeSystemPrompt,
-			wantSecondText: "Other",
+			wantSecondText: claudePrefix + "\n\nOther",
 		},
 		{
 			name:          "empty array",

backend/internal/service/gateway_service.go

@@ -2479,6 +2479,10 @@ func injectClaudeCodePrompt(body []byte, system any) []byte {
 		"text":          claudeCodeSystemPrompt,
 		"cache_control": map[string]string{"type": "ephemeral"},
 	}
+	// Opencode plugin applies an extra safeguard: it not only prepends the Claude Code
+	// banner, it also prefixes the next system instruction with the same banner plus
+	// a blank line. This helps when upstream concatenates system instructions.
+	claudeCodePrefix := strings.TrimSpace(claudeCodeSystemPrompt)
 
 	var newSystem []any
 
@@ -2490,16 +2494,32 @@ func injectClaudeCodePrompt(body []byte, system any) []byte {
 		if strings.TrimSpace(v) == "" || strings.TrimSpace(v) == strings.TrimSpace(claudeCodeSystemPrompt) {
 			newSystem = []any{claudeCodeBlock}
 		} else {
-			newSystem = []any{claudeCodeBlock, map[string]any{"type": "text", "text": v}}
+			// Mirror opencode behavior: keep the banner as a separate system entry,
+			// but also prefix the next system text with the banner.
+			merged := v
+			if !strings.HasPrefix(v, claudeCodePrefix) {
+				merged = claudeCodePrefix + "\n\n" + v
+			}
+			newSystem = []any{claudeCodeBlock, map[string]any{"type": "text", "text": merged}}
 		}
 	case []any:
 		newSystem = make([]any, 0, len(v)+1)
 		newSystem = append(newSystem, claudeCodeBlock)
+		prefixedNext := false
 		for _, item := range v {
 			if m, ok := item.(map[string]any); ok {
 				if text, ok := m["text"].(string); ok && strings.TrimSpace(text) == strings.TrimSpace(claudeCodeSystemPrompt) {
 					continue
 				}
+				// Prefix the first subsequent text system block once.
+				if !prefixedNext {
+					if blockType, _ := m["type"].(string); blockType == "text" {
+						if text, ok := m["text"].(string); ok && strings.TrimSpace(text) != "" && !strings.HasPrefix(text, claudeCodePrefix) {
+							m["text"] = claudeCodePrefix + "\n\n" + text
+							prefixedNext = true
+						}
+					}
+				}
 			}
 			newSystem = append(newSystem, item)
 		}