Add invalid-request fallback routing

backend/internal/service/antigravity_gateway_service_test.go

@@ -1,10 +1,16 @@
 package service
 
 import (
+	"bytes"
+	"context"
 	"encoding/json"
+	"io"
+	"net/http"
+	"net/http/httptest"
 	"testing"
 
 	"github.com/Wei-Shaw/sub2api/internal/pkg/antigravity"
+	"github.com/gin-gonic/gin"
 	"github.com/stretchr/testify/require"
 )
 
@@ -81,3 +87,77 @@ func TestStripThinkingFromClaudeRequest_DoesNotDowngradeTools(t *testing.T) {
 	require.Equal(t, "secret plan", blocks[0]["text"])
 	require.Equal(t, "tool_use", blocks[1]["type"])
 }
+
+func TestIsPromptTooLongError(t *testing.T) {
+	require.True(t, isPromptTooLongError([]byte(`{"error":{"message":"Prompt is too long"}}`)))
+	require.True(t, isPromptTooLongError([]byte(`{"message":"Prompt is too long"}`)))
+	require.False(t, isPromptTooLongError([]byte(`{"error":{"message":"other"}}`)))
+}
+
+type httpUpstreamStub struct {
+	resp *http.Response
+	err  error
+}
+
+func (s *httpUpstreamStub) Do(_ *http.Request, _ string, _ int64, _ int) (*http.Response, error) {
+	return s.resp, s.err
+}
+
+func TestAntigravityGatewayService_Forward_PromptTooLong(t *testing.T) {
+	gin.SetMode(gin.TestMode)
+	writer := httptest.NewRecorder()
+	c, _ := gin.CreateTestContext(writer)
+
+	body, err := json.Marshal(map[string]any{
+		"model": "claude-opus-4-5",
+		"messages": []map[string]any{
+			{"role": "user", "content": "hi"},
+		},
+		"max_tokens": 1,
+		"stream":     false,
+	})
+	require.NoError(t, err)
+
+	req := httptest.NewRequest(http.MethodPost, "/v1/messages", bytes.NewReader(body))
+	c.Request = req
+
+	respBody := []byte(`{"error":{"message":"Prompt is too long"}}`)
+	resp := &http.Response{
+		StatusCode: http.StatusBadRequest,
+		Header:     http.Header{"X-Request-Id": []string{"req-1"}},
+		Body:       io.NopCloser(bytes.NewReader(respBody)),
+	}
+
+	svc := &AntigravityGatewayService{
+		tokenProvider: &AntigravityTokenProvider{},
+		httpUpstream:  &httpUpstreamStub{resp: resp},
+	}
+
+	account := &Account{
+		ID:          1,
+		Name:        "acc-1",
+		Platform:    PlatformAntigravity,
+		Type:        AccountTypeOAuth,
+		Status:      StatusActive,
+		Concurrency: 1,
+		Credentials: map[string]any{
+			"access_token": "token",
+		},
+	}
+
+	result, err := svc.Forward(context.Background(), c, account, body)
+	require.Nil(t, result)
+
+	var promptErr *PromptTooLongError
+	require.ErrorAs(t, err, &promptErr)
+	require.Equal(t, http.StatusBadRequest, promptErr.StatusCode)
+	require.Equal(t, "req-1", promptErr.RequestID)
+	require.NotEmpty(t, promptErr.Body)
+
+	raw, ok := c.Get(OpsUpstreamErrorsKey)
+	require.True(t, ok)
+	events, ok := raw.([]*OpsUpstreamErrorEvent)
+	require.True(t, ok)
+	require.Len(t, events, 1)
+	require.Equal(t, "prompt_too_long", events[0].Kind)
+}