fix(anthropic): avoid Fable OAuth system prompt refusal

修复 Fable OAuth 请求因系统提示词形态导致空响应的问题。

backend/internal/service/gateway_claude_oauth_body.go

@@ -391,6 +391,7 @@ func (s *GatewayService) applyClaudeCodeOAuthMimicryToBody(
 	systemPromptInjectionEnabled, systemPrompt, systemPromptBlocks := s.claudeOAuthSystemPromptInjectionSettings(ctx)
 	systemRewritten := false
 	if systemPromptInjectionEnabled {
+		systemPromptBlocks = claudeOAuthSystemPromptBlocksForModel(model, systemPromptBlocks)
 		body = rewriteSystemForNonClaudeCodeWithPromptBlocks(body, normalizeSystemParam(systemRaw), systemPrompt, systemPromptBlocks)
 		systemRewritten = true
 	}
@@ -698,6 +699,24 @@ type claudeOAuthSystemPromptBlocksEnvelope struct {
 	Blocks []claudeOAuthSystemPromptBlockConfig `json:"blocks"`
 }
 
+// claudeFableOAuthSystemPromptBlocks keeps the Claude Code identity required by
+// OAuth credentials without the generic CLI expansion block. Fable 5 rejects
+// that expansion upstream with stop_reason=refusal and zero output tokens,
+// while the native billing + identity shape is accepted. Original client
+// system instructions are still migrated into the message history by
+// rewriteSystemForNonClaudeCodeWithPromptBlocks.
+const claudeFableOAuthSystemPromptBlocks = `[
+	{"type":"text","text":"{billing_header}"},
+	{"type":"text","text":"{claude_code_system_prompt}"}
+]`
+
+func claudeOAuthSystemPromptBlocksForModel(model, configured string) string {
+	if isAnthropicFableModel(model) {
+		return claudeFableOAuthSystemPromptBlocks
+	}
+	return configured
+}
+
 func defaultClaudeOAuthExpansionPrompt(expansionPrompt string) string {
 	expansionPrompt = strings.TrimSpace(expansionPrompt)
 	if expansionPrompt == "" {

backend/internal/service/gateway_context_management_test.go

@@ -358,6 +358,25 @@ func TestApplyClaudeCodeOAuthMimicryToBody_HaikuRewritesSystem(t *testing.T) {
 	require.Equal(t, "claude-haiku-4-5-20251001", gjson.GetBytes(out, "model").String())
 }
 
+func TestApplyClaudeCodeOAuthMimicryToBody_FableOmitsRefusedExpansion(t *testing.T) {
+	account := &Account{ID: 406, Platform: PlatformAnthropic, Type: AccountTypeOAuth}
+	body := []byte(`{"model":"claude-fable-5","system":"Project instructions","messages":[{"role":"user","content":"hello"}]}`)
+	svc := &GatewayService{cfg: &config.Config{}}
+
+	out := svc.applyClaudeCodeOAuthMimicryToBody(
+		context.Background(), nil, account, body, "Project instructions", "claude-fable-5",
+	)
+
+	system := gjson.GetBytes(out, "system").Array()
+	require.Len(t, system, 2)
+	require.Contains(t, system[0].Get("text").String(), "x-anthropic-billing-header:")
+	require.Equal(t, claudeCodeSystemPrompt, system[1].Get("text").String())
+	require.NotContains(t, string(out), claudeCodeSystemPromptExpansion)
+	require.Contains(t, gjson.GetBytes(out, "messages.0.content.0.text").String(), "Project instructions")
+	require.Equal(t, "Understood. I will follow these instructions.", gjson.GetBytes(out, "messages.1.content.0.text").String())
+	require.Equal(t, "hello", gjson.GetBytes(out, "messages.2.content").String())
+}
+
 // ============================================================================
 // passthrough 集成测试：buildUpstreamRequest-
 // AnthropicAPIKeyPassthrough 与 buildCountTokensRequestAnthropicAPIKeyPassthrough