fix(codex-models): 非 GPT 模型的 Codex 提示词模板去掉 GPT 身份声明，避免 Antigravity 429

模型目录对所有模型都用 openai.CodexBaseInstructionsForModel 填 instructions_template，
gemini / claude 等因此下发 "You are Codex, a coding agent based on GPT-5."。Codex 把它作为
instructions 发出，Antigravity 对「Codex 提示词 + GPT-5 身份」返回 429 RESOURCE_EXHAUSTED，
并让账号进入限流冷却。

OpenAI GPT 模型保持原模板；其他模型族（含 gpt-oss）去掉 "based on GPT-N" /
"You are GPT-N" 身份声明，提示词其余部分不变。

Refs #7307

backend/internal/service/openai_codex_models_service.go

@@ -11,6 +11,7 @@ import (
 	"net"
 	"net/http"
 	"net/url"
+	"regexp"
 	"sort"
 	"strings"
 	"sync"
@@ -443,7 +444,7 @@ func newConfiguredCodexModelDescriptor(modelID string) configuredCodexModelDescr
 		Priority:                          configuredCodexModelPriority,
 		AdditionalSpeedTiers:              []string{},
 		ServiceTiers:                      []configuredCodexServiceTier{},
-		ModelMessages:                     configuredCodexModelMessages{InstructionsTemplate: openai.CodexBaseInstructionsForModel(modelID)},
+		ModelMessages:                     configuredCodexModelMessages{InstructionsTemplate: codexInstructionsTemplateForModel(modelID)},
 		SupportsReasoningSummaryParameter: true,
 		DefaultReasoningSummary:           "auto",
 		WebSearchToolType:                 "text",
@@ -636,6 +637,47 @@ func configuredCodexGPTReasoningLevels(modelID string) []configuredCodexReasonin
 	return levels
 }
 
+// codexGPTIdentityPatterns match the GPT self-identification that the bundled
+// Codex prompts open with:
+//   - "You are Codex, a coding agent based on GPT-5." / "…an agent based on GPT-6." / "You are Codex, based on GPT-5."
+//   - "You are GPT-5.1 running in the Codex CLI, …"
+var codexGPTIdentityPatterns = []struct {
+	re   *regexp.Regexp
+	repl string
+}{
+	{regexp.MustCompile(`,?\s*based on GPT-\d+(?:\.\d+)?(?:-[A-Za-z0-9]+)*`), ""},
+	{regexp.MustCompile(`\bYou are GPT-\d+(?:\.\d+)?(?:-[A-Za-z0-9]+)*`), "You are Codex"},
+}
+
+// codexInstructionsTemplateForModel returns the Codex base prompt advertised for
+// modelID in the models manifest.
+//
+// OpenAI GPT models keep the bundled prompt verbatim. Every other family the
+// manifest can list (Gemini, Claude, DeepSeek, Grok, gpt-oss, custom aliases)
+// gets the same prompt without the GPT self-identification: the claim is false
+// for those models, and Google's Antigravity backend answers requests whose
+// system prompt is the Codex prompt carrying a "GPT-5" identity with
+// 429 RESOURCE_EXHAUSTED — which also puts the account into rate-limit
+// cooldown. Codex sends this template as `instructions` for the selected model.
+func codexInstructionsTemplateForModel(modelID string) string {
+	base := openai.CodexBaseInstructionsForModel(modelID)
+	if codexModelKeepsGPTIdentity(modelID) {
+		return base
+	}
+	for _, p := range codexGPTIdentityPatterns {
+		base = p.re.ReplaceAllString(base, p.repl)
+	}
+	return base
+}
+
+// codexModelKeepsGPTIdentity reports whether modelID is an OpenAI GPT model
+// whose Codex prompt may claim a GPT identity. gpt-oss models are open-weight
+// and typically served by non-OpenAI providers, so they do not.
+func codexModelKeepsGPTIdentity(modelID string) bool {
+	return isOpenAICodexGPTModel(modelID) &&
+		!strings.HasPrefix(canonicalizeOpenAIModelAliasSpelling(modelID), "gpt-oss")
+}
+
 func isOpenAICodexGPTModel(modelID string) bool {
 	normalized := canonicalizeOpenAIModelAliasSpelling(modelID)
 	if normalized == "" || strings.HasPrefix(normalized, "gpt-image") {

backend/internal/service/openai_codex_models_service_test.go

@@ -3703,3 +3703,59 @@ func TestFetchCodexModelsManifestOAuthSharedAcrossGroupsWithIndependentFiltering
 	require.Equal(t, []string{"model-b"}, got[92])
 	require.EqualValues(t, 1, calls.Load(), "同一账号两个分组同时请求时只发一次上游请求")
 }
+
+// Scenario: 非 OpenAI GPT 模型的 Codex 提示词不声称自己是 GPT。
+// Antigravity(Google) 对「Codex 提示词 + GPT-5 身份」直接回 429 RESOURCE_EXHAUSTED。
+func TestBuildCodexModelsManifestStripsGPTIdentityForNonGPTModels(t *testing.T) {
+	t.Parallel()
+
+	nonGPT := []string{
+		"gemini-3-flash", "claude-sonnet-4-6", "deepseek-v4-flash", "grok-4.3",
+		"gpt-oss-120b-medium", "company-coding-model", "company-codex-alias",
+	}
+	body, err := BuildCodexModelsManifest(nonGPT)
+	require.NoError(t, err)
+
+	var manifest struct {
+		Models []struct {
+			Slug          string `json:"slug"`
+			ModelMessages struct {
+				InstructionsTemplate string `json:"instructions_template"`
+			} `json:"model_messages"`
+		} `json:"models"`
+	}
+	require.NoError(t, json.Unmarshal(body, &manifest))
+	require.Len(t, manifest.Models, len(nonGPT))
+	for _, m := range manifest.Models {
+		tmpl := m.ModelMessages.InstructionsTemplate
+		require.NotContains(t, tmpl, "GPT-", m.Slug)
+		require.True(t, strings.HasPrefix(tmpl, "You are Codex"), "%s: %.60q", m.Slug, tmpl)
+		// Only the identity clause is removed; the rest of the bundled prompt is kept.
+		base := openai.CodexBaseInstructionsForModel(m.Slug)
+		require.Less(t, len(base)-len(tmpl), 32, m.Slug)
+	}
+}
+
+// Scenario: OpenAI GPT 模型的 Codex 提示词保持原样。
+func TestBuildCodexModelsManifestKeepsGPTIdentityForGPTModels(t *testing.T) {
+	t.Parallel()
+
+	for _, model := range []string{"gpt-5.5", "gpt-6-astra", "gpt-5.2", "gpt-5.3-codex-spark"} {
+		require.Equal(t, openai.CodexBaseInstructionsForModel(model), codexInstructionsTemplateForModel(model), model)
+	}
+}
+
+// Scenario: 每份内置 Codex 提示词的身份声明都能被识别并去掉。
+func TestCodexGPTIdentityPatternsCoverBundledPrompts(t *testing.T) {
+	t.Parallel()
+
+	for _, model := range []string{"gpt-5.1", "gpt-5.2", "gpt-5.5", "gpt-6-astra", "gpt-5-codex"} {
+		tmpl := openai.CodexBaseInstructionsForModel(model)
+		require.Contains(t, tmpl, "GPT-", "bundled prompt for %s changed; update this test", model)
+		for _, p := range codexGPTIdentityPatterns {
+			tmpl = p.re.ReplaceAllString(tmpl, p.repl)
+		}
+		require.NotContains(t, tmpl, "GPT-", model)
+		require.True(t, strings.HasPrefix(tmpl, "You are Codex"), "%s: %.60q", model, tmpl)
+	}
+}