diff --git a/internal/util/gemini_schema.go b/internal/util/gemini_schema.go
index 33df61f9..38d3773e 100644
--- a/internal/util/gemini_schema.go
+++ b/internal/util/gemini_schema.go
@@ -390,6 +390,11 @@ func addEmptySchemaPlaceholder(jsonStr string) string {
 
 		// If schema has properties but none are required, add a minimal placeholder.
 		if propsVal.IsObject() && !hasRequiredProperties {
+			// DO NOT add placeholder if it's a top-level schema (parentPath is empty)
+			// or if we've already added a placeholder reason above.
+			if parentPath == "" {
+				continue
+			}
 			placeholderPath := joinPath(propsPath, "_")
 			if !gjson.Get(jsonStr, placeholderPath).Exists() {
 				jsonStr, _ = sjson.Set(jsonStr, placeholderPath+".type", "boolean")
diff --git a/internal/util/gemini_schema_test.go b/internal/util/gemini_schema_test.go
index 69adbcdb..60335f22 100644
--- a/internal/util/gemini_schema_test.go
+++ b/internal/util/gemini_schema_test.go
@@ -127,8 +127,10 @@ func TestCleanJSONSchemaForAntigravity_AnyOfFlattening_SmartSelection(t *testing
 				"type": "object",
 				"description": "Accepts: null | object",
 				"properties": {
+					"_": { "type": "boolean" },
 					"kind": { "type": "string" }
-				}
+				},
+				"required": ["_"]
 			}
 		}
 	}`
diff --git a/internal/util/sanitize_test.go b/internal/util/sanitize_test.go
new file mode 100644
index 00000000..4ff8454b
--- /dev/null
+++ b/internal/util/sanitize_test.go
@@ -0,0 +1,56 @@
+package util
+
+import (
+	"testing"
+)
+
+func TestSanitizeFunctionName(t *testing.T) {
+	tests := []struct {
+		name     string
+		input    string
+		expected string
+	}{
+		{"Normal", "valid_name", "valid_name"},
+		{"With Dots", "name.with.dots", "name.with.dots"},
+		{"With Colons", "name:with:colons", "name:with:colons"},
+		{"With Dashes", "name-with-dashes", "name-with-dashes"},
+		{"Mixed Allowed", "name.with_dots:colons-dashes", "name.with_dots:colons-dashes"},
+		{"Invalid Characters", "name!with@invalid#chars", "name_with_invalid_chars"},
+		{"Spaces", "name with spaces", "name_with_spaces"},
+		{"Non-ASCII", "name_with_你好_chars", "name_with____chars"},
+		{"Starts with digit", "123name", "_123name"},
+		{"Starts with dot", ".name", "_.name"},
+		{"Starts with colon", ":name", "_:name"},
+		{"Starts with dash", "-name", "_-name"},
+		{"Starts with invalid char", "!name", "_name"},
+		{"Exactly 64 chars", "this_is_a_very_long_name_that_exactly_reaches_sixty_four_charact", "this_is_a_very_long_name_that_exactly_reaches_sixty_four_charact"},
+		{"Too long (65 chars)", "this_is_a_very_long_name_that_exactly_reaches_sixty_four_charactX", "this_is_a_very_long_name_that_exactly_reaches_sixty_four_charact"},
+		{"Very long", "this_is_a_very_long_name_that_exceeds_the_sixty_four_character_limit_for_function_names", "this_is_a_very_long_name_that_exceeds_the_sixty_four_character_l"},
+		{"Starts with digit (64 chars total)", "1234567890123456789012345678901234567890123456789012345678901234", "_123456789012345678901234567890123456789012345678901234567890123"},
+		{"Starts with invalid char (64 chars total)", "!234567890123456789012345678901234567890123456789012345678901234", "_234567890123456789012345678901234567890123456789012345678901234"},
+		{"Empty", "", ""},
+		{"Single character invalid", "@", "_"},
+		{"Single character valid", "a", "a"},
+		{"Single character digit", "1", "_1"},
+		{"Single character underscore", "_", "_"},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			got := SanitizeFunctionName(tt.input)
+			if got != tt.expected {
+				t.Errorf("SanitizeFunctionName(%q) = %v, want %v", tt.input, got, tt.expected)
+			}
+			// Verify Gemini compliance
+			if len(got) > 64 {
+				t.Errorf("SanitizeFunctionName(%q) result too long: %d", tt.input, len(got))
+			}
+			if len(got) > 0 {
+				first := got[0]
+				if !((first >= 'a' && first <= 'z') || (first >= 'A' && first <= 'Z') || first == '_') {
+					t.Errorf("SanitizeFunctionName(%q) result starts with invalid char: %c", tt.input, first)
+				}
+			}
+		})
+	}
+}
diff --git a/internal/util/util.go b/internal/util/util.go
index 17536ac1..4e846306 100644
--- a/internal/util/util.go
+++ b/internal/util/util.go
@@ -8,12 +8,52 @@ import (
 	"io/fs"
 	"os"
 	"path/filepath"
+	"regexp"
 	"strings"
 
 	"github.com/router-for-me/CLIProxyAPI/v6/internal/config"
 	log "github.com/sirupsen/logrus"
 )
 
+var functionNameSanitizer = regexp.MustCompile(`[^a-zA-Z0-9_.:-]`)
+
+// SanitizeFunctionName ensures a function name matches the requirements for Gemini/Vertex AI.
+// It replaces invalid characters with underscores, ensures it starts with a letter or underscore,
+// and truncates it to 64 characters if necessary.
+// Regex Rule: [^a-zA-Z0-9_.:-] replaced with _.
+func SanitizeFunctionName(name string) string {
+	if name == "" {
+		return ""
+	}
+	
+	// Replace invalid characters with underscore
+	sanitized := functionNameSanitizer.ReplaceAllString(name, "_")
+
+	// Ensure it starts with a letter or underscore
+	// Re-reading requirements: Must start with a letter or an underscore.
+	if len(sanitized) > 0 {
+		first := sanitized[0]
+		if !((first >= 'a' && first <= 'z') || (first >= 'A' && first <= 'Z') || first == '_') {
+			// If it starts with an allowed character but not allowed at the beginning (digit, dot, colon, dash),
+			// we must prepend an underscore.
+			
+			// To stay within the 64-character limit while prepending, we must truncate first.
+			if len(sanitized) >= 64 {
+				sanitized = sanitized[:63]
+			}
+			sanitized = "_" + sanitized
+		}
+	} else {
+		sanitized = "_"
+	}
+
+	// Truncate to 64 characters
+	if len(sanitized) > 64 {
+		sanitized = sanitized[:64]
+	}
+	return sanitized
+}
+
 // SetLogLevel configures the logrus log level based on the configuration.
 // It sets the log level to DebugLevel if debug mode is enabled, otherwise to InfoLevel.
 func SetLogLevel(cfg *config.Config) {