diff --git a/firebase-vertexai/src/main/kotlin/com/google/firebase/vertexai/internal/util/conversions.kt b/firebase-vertexai/src/main/kotlin/com/google/firebase/vertexai/internal/util/conversions.kt
index 506570d96e1..4c30459a6c4 100644
--- a/firebase-vertexai/src/main/kotlin/com/google/firebase/vertexai/internal/util/conversions.kt
+++ b/firebase-vertexai/src/main/kotlin/com/google/firebase/vertexai/internal/util/conversions.kt
@@ -153,16 +153,7 @@ internal fun Tool.toInternal() =
   com.google.firebase.vertexai.common.client.Tool(functionDeclarations.map { it.toInternal() })
 
 internal fun FunctionDeclaration.toInternal() =
-  com.google.firebase.vertexai.common.client.FunctionDeclaration(
-    name,
-    description,
-    Schema(
-      properties = parameters.mapValues { it.value.toInternal() },
-      required = parameters.keys.minus(optionalParameters.toSet()).toList(),
-      type = "OBJECT",
-      nullable = false,
-    ),
-  )
+  com.google.firebase.vertexai.common.client.FunctionDeclaration(name, "", schema.toInternal())
 
 internal fun com.google.firebase.vertexai.type.Schema.toInternal(): Schema =
   Schema(
diff --git a/firebase-vertexai/src/main/kotlin/com/google/firebase/vertexai/type/FunctionDeclaration.kt b/firebase-vertexai/src/main/kotlin/com/google/firebase/vertexai/type/FunctionDeclaration.kt
index e8949a4a6ba..f1c0bbd0090 100644
--- a/firebase-vertexai/src/main/kotlin/com/google/firebase/vertexai/type/FunctionDeclaration.kt
+++ b/firebase-vertexai/src/main/kotlin/com/google/firebase/vertexai/type/FunctionDeclaration.kt
@@ -41,4 +41,7 @@ class FunctionDeclaration(
   val description: String,
   val parameters: Map<String, Schema>,
   val optionalParameters: List<String> = emptyList(),
-)
+) {
+  internal val schema: Schema =
+    Schema.obj(properties = parameters, optionalProperties = optionalParameters, nullable = false)
+}
diff --git a/firebase-vertexai/src/main/kotlin/com/google/firebase/vertexai/type/Schema.kt b/firebase-vertexai/src/main/kotlin/com/google/firebase/vertexai/type/Schema.kt
index 4a3383e5c79..4d041c917d2 100644
--- a/firebase-vertexai/src/main/kotlin/com/google/firebase/vertexai/type/Schema.kt
+++ b/firebase-vertexai/src/main/kotlin/com/google/firebase/vertexai/type/Schema.kt
@@ -130,14 +130,20 @@ internal constructor(
       optionalProperties: List<String> = emptyList(),
       description: String? = null,
       nullable: Boolean = false,
-    ) =
-      Schema(
+    ): Schema {
+      if (!properties.keys.containsAll(optionalProperties)) {
+        throw IllegalArgumentException(
+          "All optional properties must be present in properties. Missing: ${optionalProperties.minus(properties.keys)}"
+        )
+      }
+      return Schema(
         description = description,
         nullable = nullable,
         properties = properties,
         required = properties.keys.minus(optionalProperties.toSet()).toList(),
         type = "OBJECT",
       )
+    }
 
     /**
      * Returns a schema for an array.