fix(grok): simplify invalid tool union roots

backend/internal/service/openai_gateway_grok.go

@@ -1057,6 +1057,8 @@ 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() {
@@ -1092,6 +1094,19 @@ 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)
 		}
@@ -1143,6 +1158,29 @@ func sanitizeGrokResponsesTools(body []byte) ([]byte, error) {
 	return body, nil
 }
 
+func grokFunctionParametersHaveInvalidUnionRoot(parameters gjson.Result) bool {
+	if !parameters.Exists() || !parameters.IsObject() ||
+		strings.EqualFold(strings.TrimSpace(parameters.Get("type").String()), "object") {
+		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 {

backend/internal/service/openai_gateway_grok_test.go

@@ -395,6 +395,28 @@ 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":{"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 TestSanitizeGrokResponsesToolsKeepsToolChoiceOnlyWithSupportedTools(t *testing.T) {
 	t.Parallel()