diff --git a/backend/internal/service/openai_gateway_grok.go b/backend/internal/service/openai_gateway_grok.go
index a99f37ac2a..efcbaa8d44 100644
--- a/backend/internal/service/openai_gateway_grok.go
+++ b/backend/internal/service/openai_gateway_grok.go
@@ -74,8 +74,14 @@ func (s *OpenAIGatewayService) forwardGrokResponses(
 		}
 	}
 	// Derive the identity from the request xAI will actually see. This makes
-	// Codex Responses Lite additional_tools part of the stable tool prefix.
-	cacheIdentity := resolveGrokCacheIdentity(c, patchedBody, "", upstreamModel)
+	// Codex Responses Lite additional_tools part of the stable tool prefix. If
+	// the client supplied a Claude Code session only through metadata.user_id,
+	// keep using that identity even though metadata is stripped before xAI.
+	cacheIdentityBody := patchedBody
+	if extractClaudeCodeSessionIDFromPayload(body) != "" {
+		cacheIdentityBody = body
+	}
+	cacheIdentity := resolveGrokCacheIdentity(c, cacheIdentityBody, "", upstreamModel)
 	mixedCacheIntentBody := append([]byte(nil), patchedBody...)
 	patchedBody, err = applyGrokResponsesCacheIdentity(patchedBody, body, cacheIdentity, account.IsGrokOAuth())
 	if err != nil {
@@ -547,7 +553,7 @@ func patchGrokResponsesBodyBase(body []byte, upstreamModel string) ([]byte, erro
 	if err != nil {
 		return nil, err
 	}
-	for _, unsupportedField := range []string{"prompt_cache_retention", "safety_identifier"} {
+	for _, unsupportedField := range []string{"prompt_cache_retention", "safety_identifier", "metadata"} {
 		if gjson.GetBytes(out, unsupportedField).Exists() {
 			out, err = sjson.DeleteBytes(out, unsupportedField)
 			if err != nil {
@@ -1053,6 +1059,8 @@ var grokResponsesSupportedToolTypes = map[string]struct{}{
 	"x_search":           {},
 }
 
+const grokSafeFunctionParameters = `{"type":"object","properties":{},"additionalProperties":true}`
+
 func sanitizeGrokResponsesTools(body []byte) ([]byte, error) {
 	tools := gjson.GetBytes(body, "tools")
 	if !tools.Exists() {
@@ -1088,6 +1096,19 @@ func sanitizeGrokResponsesTools(body []byte) ([]byte, error) {
 				}
 				raw = encoded
 				toolsChanged = true
+			} else if toolType == "function" && grokFunctionParametersHaveInvalidUnionRoot(tool.Get("parameters")) {
+				var err error
+				raw, err = sjson.SetRawBytes(raw, "parameters", []byte(grokSafeFunctionParameters))
+				if err != nil {
+					return nil, err
+				}
+				if strict := tool.Get("strict"); strict.Exists() && strict.Bool() {
+					raw, err = sjson.SetBytes(raw, "strict", false)
+					if err != nil {
+						return nil, err
+					}
+				}
+				toolsChanged = true
 			}
 			filteredTools = append(filteredTools, raw)
 		}
@@ -1139,6 +1160,28 @@ func sanitizeGrokResponsesTools(body []byte) ([]byte, error) {
 	return body, nil
 }
 
+func grokFunctionParametersHaveInvalidUnionRoot(parameters gjson.Result) bool {
+	if !parameters.Exists() || !parameters.IsObject() {
+		return false
+	}
+	for _, keyword := range []string{"anyOf", "oneOf"} {
+		branches := parameters.Get(keyword)
+		if !branches.IsArray() {
+			continue
+		}
+		values := branches.Array()
+		if len(values) == 0 {
+			continue
+		}
+		for _, branch := range values {
+			if !strings.EqualFold(strings.TrimSpace(branch.Get("type").String()), "object") {
+				return true
+			}
+		}
+	}
+	return false
+}
+
 func grokRawToolsContainType(tools []json.RawMessage, want string) bool {
 	for _, tool := range tools {
 		if strings.TrimSpace(gjson.GetBytes(tool, "type").String()) == want {
diff --git a/backend/internal/service/openai_gateway_grok_test.go b/backend/internal/service/openai_gateway_grok_test.go
index d1c966ac07..d3256eac93 100644
--- a/backend/internal/service/openai_gateway_grok_test.go
+++ b/backend/internal/service/openai_gateway_grok_test.go
@@ -325,7 +325,7 @@ func TestPatchGrokResponsesBodyDropsNestedUnsupportedFields(t *testing.T) {
 	require.True(t, json.Valid(patched))
 	require.False(t, strings.Contains(string(patched), "external_web_access"))
 	require.Equal(t, "kept_fn", gjson.GetBytes(patched, "tools.0.name").String())
-	require.Equal(t, "9007199254740993", gjson.GetBytes(patched, "metadata.large_id").Raw)
+	require.False(t, gjson.GetBytes(patched, "metadata").Exists())
 }
 
 func TestStripAnthropicThinkingSignaturesPreservesLargeIntegers(t *testing.T) {
@@ -395,6 +395,62 @@ func TestSanitizeGrokResponsesToolsRemovesDeferredFlagsWithToolSearch(t *testing
 	require.True(t, gjson.GetBytes(patched, `tools.#(name=="apply_patch")`).Exists())
 }
 
+func TestSanitizeGrokResponsesToolsSimplifiesInvalidRootUnion(t *testing.T) {
+	body := []byte(`{"tools":[
+		{"type":"function","name":"mcp__codex_app__automation_update","strict":true,"parameters":{"oneOf":[{"type":"object","properties":{"id":{"type":"string"}}},{"type":"null"}]}},
+		{"type":"function","name":"object_only","strict":true,"parameters":{"type":"object","anyOf":[{"type":"object","properties":{"a":{"type":"string"}}},{"type":"object","properties":{"b":{"type":"integer"}}}]}}
+	]}`)
+
+	patched, err := sanitizeGrokResponsesTools(body)
+	require.NoError(t, err)
+	require.True(t, json.Valid(patched))
+
+	mixed := gjson.GetBytes(patched, `tools.#(name=="mcp__codex_app__automation_update")`)
+	require.Equal(t, "object", mixed.Get("parameters.type").String())
+	require.True(t, mixed.Get("parameters.properties").IsObject())
+	require.True(t, mixed.Get("parameters.additionalProperties").Bool())
+	require.False(t, mixed.Get("parameters.oneOf").Exists())
+	require.Equal(t, gjson.False, mixed.Get("strict").Type)
+
+	objectOnly := gjson.GetBytes(patched, `tools.#(name=="object_only")`)
+	require.True(t, objectOnly.Get("parameters.anyOf").Exists())
+	require.Equal(t, gjson.True, objectOnly.Get("strict").Type)
+}
+
+func TestPatchGrokResponsesBodySimplifiesTypedInvalidRootUnion(t *testing.T) {
+	body := []byte(`{
+		"model":"grok-4.6",
+		"metadata":{"session_id":"abc"},
+		"tools":[{
+			"type":"namespace",
+			"name":"mcp__codex_app",
+			"tools":[{
+				"type":"function",
+				"name":"automation_update",
+				"strict":true,
+				"parameters":{
+					"type":"object",
+					"oneOf":[{"$ref":"#/$defs/update"},{"type":"null"}],
+					"$defs":{"update":{"type":"object","properties":{"id":{"type":"string"}}}}
+				}
+			}]
+		}]
+	}`)
+
+	patched, _, err := patchGrokResponsesBodyWithClientTools(body, "grok-4.6")
+	require.NoError(t, err)
+	require.True(t, json.Valid(patched))
+	require.False(t, gjson.GetBytes(patched, "metadata").Exists())
+
+	tool := gjson.GetBytes(patched, `tools.#(name=="mcp__codex_app__automation_update")`)
+	require.Equal(t, "object", tool.Get("parameters.type").String())
+	require.True(t, tool.Get("parameters.properties").IsObject())
+	require.True(t, tool.Get("parameters.additionalProperties").Bool())
+	require.False(t, tool.Get("parameters.oneOf").Exists())
+	require.False(t, tool.Get("parameters.$defs").Exists())
+	require.Equal(t, gjson.False, tool.Get("strict").Type)
+}
+
 func TestSanitizeGrokResponsesToolsKeepsToolChoiceOnlyWithSupportedTools(t *testing.T) {
 	t.Parallel()
 
@@ -1949,7 +2005,7 @@ func TestForwardGrokResponsesAPIKeyUsesXAIResponses(t *testing.T) {
 
 	recorder := httptest.NewRecorder()
 	c, _ := gin.CreateTestContext(recorder)
-	body := []byte(`{"model":"grok","input":"hi","stream":true}`)
+	body := []byte(`{"model":"grok","input":"hi","metadata":{"session_id":"abc"},"stream":true}`)
 	c.Request = httptest.NewRequest(http.MethodPost, "/v1/responses", bytes.NewReader(body))
 	c.Request.Header.Set("Content-Type", "application/json")
 
@@ -1984,11 +2040,63 @@ func TestForwardGrokResponsesAPIKeyUsesXAIResponses(t *testing.T) {
 	require.Empty(t, upstream.lastReq.Header.Get("X-Grok-Client-Version"))
 	require.NotEqual(t, defaultGrokUpstreamUserAgent(), upstream.lastReq.Header.Get("User-Agent"))
 	require.Equal(t, "grok-4.6", gjson.GetBytes(upstream.lastBody, "model").String())
+	require.False(t, gjson.GetBytes(upstream.lastBody, "metadata").Exists())
 	require.Equal(t, "resp_grok_api_key", result.ResponseID)
 	require.Equal(t, 2, result.Usage.InputTokens)
 	require.Equal(t, 1, result.Usage.OutputTokens)
 }
 
+func TestForwardGrokResponsesUsesMetadataSessionForCacheIdentityWithoutForwardingMetadata(t *testing.T) {
+	gin.SetMode(gin.TestMode)
+
+	firstBody := []byte(`{"model":"grok","input":"first turn","metadata":{"user_id":"{\"session_id\":\"metadata-session\"}"},"stream":false}`)
+	secondBody := []byte(`{"model":"grok","input":"different second turn","metadata":{"user_id":"{\"session_id\":\"metadata-session\"}"},"stream":false}`)
+	account := &Account{
+		ID:          5401,
+		Name:        "grok-api-key",
+		Platform:    PlatformGrok,
+		Type:        AccountTypeAPIKey,
+		Concurrency: 2,
+		Credentials: map[string]any{
+			"api_key":  "xai-test-key",
+			"base_url": "https://api.x.ai/v1",
+		},
+	}
+	upstream := &httpUpstreamRecorder{responses: []*http.Response{
+		{
+			StatusCode: http.StatusOK,
+			Header:     http.Header{"Content-Type": []string{"application/json"}},
+			Body:       io.NopCloser(strings.NewReader(`{"id":"resp_first","object":"response","model":"grok-4.6","status":"completed","output":[],"usage":{"input_tokens":2,"output_tokens":1}}`)),
+		},
+		{
+			StatusCode: http.StatusOK,
+			Header:     http.Header{"Content-Type": []string{"application/json"}},
+			Body:       io.NopCloser(strings.NewReader(`{"id":"resp_second","object":"response","model":"grok-4.6","status":"completed","output":[],"usage":{"input_tokens":3,"output_tokens":1}}`)),
+		},
+	}}
+	svc := &OpenAIGatewayService{httpUpstream: upstream}
+	newContext := func(body []byte) *gin.Context {
+		recorder := httptest.NewRecorder()
+		c, _ := gin.CreateTestContext(recorder)
+		c.Request = httptest.NewRequest(http.MethodPost, "/v1/responses", bytes.NewReader(body))
+		c.Set("api_key", &APIKey{ID: 5401})
+		return c
+	}
+
+	_, err := svc.forwardGrokResponses(context.Background(), newContext(firstBody), account, firstBody, "grok", false, time.Now())
+	require.NoError(t, err)
+	_, err = svc.forwardGrokResponses(context.Background(), newContext(secondBody), account, secondBody, "grok", false, time.Now())
+	require.NoError(t, err)
+	require.Len(t, upstream.bodies, 2)
+
+	firstIdentity := gjson.GetBytes(upstream.bodies[0], "prompt_cache_key").String()
+	secondIdentity := gjson.GetBytes(upstream.bodies[1], "prompt_cache_key").String()
+	require.NotEmpty(t, firstIdentity)
+	require.Equal(t, firstIdentity, secondIdentity)
+	require.False(t, gjson.GetBytes(upstream.bodies[0], "metadata").Exists())
+	require.False(t, gjson.GetBytes(upstream.bodies[1], "metadata").Exists())
+}
+
 func TestForwardGrokResponsesRetriesInvalidEncryptedContentOnce(t *testing.T) {
 	gin.SetMode(gin.TestMode)
 
@@ -2056,8 +2164,8 @@ func TestForwardGrokResponsesRetriesInvalidEncryptedContentOnce(t *testing.T) {
 	require.False(t, gjson.GetBytes(upstream.bodies[1], "input.0.encrypted_content").Exists())
 	require.Equal(t, "keep this summary", gjson.GetBytes(upstream.bodies[1], "input.0.summary.0.text").String())
 	require.Equal(t, "message", gjson.GetBytes(upstream.bodies[1], "input.1.type").String())
-	require.Equal(t, "9007199254740993", gjson.GetBytes(upstream.bodies[0], "metadata.large_id").Raw)
-	require.Equal(t, "9007199254740993", gjson.GetBytes(upstream.bodies[1], "metadata.large_id").Raw)
+	require.False(t, gjson.GetBytes(upstream.bodies[0], "metadata").Exists())
+	require.False(t, gjson.GetBytes(upstream.bodies[1], "metadata").Exists())
 
 	firstIdentity := gjson.GetBytes(upstream.bodies[0], "prompt_cache_key").String()
 	secondIdentity := gjson.GetBytes(upstream.bodies[1], "prompt_cache_key").String()
diff --git a/backend/internal/service/openai_responses_tool_schema.go b/backend/internal/service/openai_responses_tool_schema.go
index c77640d0e3..a53eefb982 100644
--- a/backend/internal/service/openai_responses_tool_schema.go
+++ b/backend/internal/service/openai_responses_tool_schema.go
@@ -28,9 +28,9 @@ var errOpenAIResponsesToolSchemaLimit = errors.New("OpenAI Responses tool schema
 
 // shouldRepairOpenAIResponsesNullToolSchemaType reports whether the upstream
 // path requires a concrete object type at a function tool's parameter root.
-// This defect is shared by the OpenAI, Anthropic, and CN-compatible paths.
+// This defect is shared by the OpenAI, Anthropic, Grok, and CN-compatible paths.
 func shouldRepairOpenAIResponsesNullToolSchemaType(platform string) bool {
-	return platform == PlatformOpenAI || platform == PlatformAnthropic || IsCNProvider(platform)
+	return platform == PlatformOpenAI || platform == PlatformAnthropic || platform == PlatformGrok || IsCNProvider(platform)
 }
 
 // shouldSanitizeOpenAIResponsesToolSchemaPatterns is intentionally narrower:
diff --git a/backend/internal/service/openai_responses_tool_schema_test.go b/backend/internal/service/openai_responses_tool_schema_test.go
index e899e17098..b2dfba46bc 100644
--- a/backend/internal/service/openai_responses_tool_schema_test.go
+++ b/backend/internal/service/openai_responses_tool_schema_test.go
@@ -250,7 +250,7 @@ func TestOpenAIResponsesToolSchemaCapabilities_PlatformBoundary(t *testing.T) {
 		{PlatformKimi, true, false},
 		{PlatformZhipu, true, false},
 		{PlatformDeepseek, true, false},
-		{PlatformGrok, false, false},
+		{PlatformGrok, true, false},
 		{PlatformGemini, false, false},
 		{PlatformAntigravity, false, false},
 		{PlatformComposite, false, false},
@@ -270,7 +270,7 @@ func TestSanitizeOpenAIResponsesToolSchemasForPlatform_ReplayBoundary(t *testing
 	// A malformed tool definition may be replayed after account failover. Every
 	// compatible account must repair it, while non-OpenAI providers retain their
 	// supported regex semantics.
-	for _, platform := range []string{PlatformAnthropic, PlatformKimi, PlatformZhipu, PlatformDeepseek} {
+	for _, platform := range []string{PlatformAnthropic, PlatformGrok, PlatformKimi, PlatformZhipu, PlatformDeepseek} {
 		t.Run(platform, func(t *testing.T) {
 			for attempt := 0; attempt < 2; attempt++ {
 				normalized, changed, err := sanitizeOpenAIResponsesToolSchemasForPlatform(body, platform)
@@ -288,12 +288,23 @@ func TestSanitizeOpenAIResponsesToolSchemasForPlatform_ReplayBoundary(t *testing
 	require.Equal(t, "object", gjson.GetBytes(openAI, "tools.0.parameters.type").String())
 	require.False(t, gjson.GetBytes(openAI, "tools.0.parameters.properties.query.pattern").Exists())
 
-	unsupported, changed, err := sanitizeOpenAIResponsesToolSchemasForPlatform(body, PlatformGrok)
+	unsupported, changed, err := sanitizeOpenAIResponsesToolSchemasForPlatform(body, PlatformGemini)
 	require.NoError(t, err)
 	require.False(t, changed)
 	require.Equal(t, string(body), string(unsupported))
 }
 
+func TestSanitizeOpenAIResponsesToolSchemasForPlatform_GrokObjectOnlyRootUnion(t *testing.T) {
+	body := []byte(`{"tools":[{"type":"function","name":"codex_app__automation_update","parameters":{"oneOf":[{"type":"object","properties":{"id":{"type":"string"}}},{"type":"object","properties":{}}]}}]}`)
+
+	sanitized, changed, err := sanitizeOpenAIResponsesToolSchemasForPlatform(body, PlatformGrok)
+
+	require.NoError(t, err)
+	require.True(t, changed)
+	require.Equal(t, "object", gjson.GetBytes(sanitized, "tools.0.parameters.type").String())
+	require.True(t, gjson.GetBytes(sanitized, "tools.0.parameters.oneOf").Exists())
+}
+
 // 索引映射：只有坏条目被改，前后兄弟条目按原下标保持不变。
 func TestSanitizeOpenAIResponsesToolParameterTypes_OnlyOffendingIndexRewritten(t *testing.T) {
 	body := []byte(`{"tools":[