diff --git a/.changes/baseball-comfort-dock-appliance.json b/.changes/baseball-comfort-dock-appliance.json
new file mode 100644
index 00000000..697a0e89
--- /dev/null
+++ b/.changes/baseball-comfort-dock-appliance.json
@@ -0,0 +1 @@
+{"type":"MAJOR","changes":["Make totalTokens in CountTokensResponse nullable"]}
diff --git a/common/src/main/kotlin/com/google/ai/client/generativeai/common/Response.kt b/common/src/main/kotlin/com/google/ai/client/generativeai/common/Response.kt
index 009f4743..ac7cec57 100644
--- a/common/src/main/kotlin/com/google/ai/client/generativeai/common/Response.kt
+++ b/common/src/main/kotlin/com/google/ai/client/generativeai/common/Response.kt
@@ -30,7 +30,9 @@ data class GenerateContentResponse(
   val usageMetadata: UsageMetadata? = null
 ) : Response
 
-@Serializable data class CountTokensResponse(val totalTokens: Int) : Response
+@Serializable
+data class CountTokensResponse(val totalTokens: Int, val totalBillableCharacters: Int? = null) :
+  Response
 
 @Serializable data class GRpcErrorResponse(val error: GRpcError) : Response
 
diff --git a/common/src/test/java/com/google/ai/client/generativeai/common/APIControllerTests.kt b/common/src/test/java/com/google/ai/client/generativeai/common/APIControllerTests.kt
index b30d01f2..959f10dd 100644
--- a/common/src/test/java/com/google/ai/client/generativeai/common/APIControllerTests.kt
+++ b/common/src/test/java/com/google/ai/client/generativeai/common/APIControllerTests.kt
@@ -160,7 +160,8 @@ internal class RequestFormatTests {
 
   @Test
   fun `countTokenRequest doesn't include the model name`() = doBlocking {
-    val response = JSON.encodeToString(CountTokensResponse(totalTokens = 10))
+    val response =
+      JSON.encodeToString(CountTokensResponse(totalTokens = 10, totalBillableCharacters = 10))
     val mockEngine = MockEngine {
       respond(response, HttpStatusCode.OK, headersOf(HttpHeaders.ContentType, "application/json"))
     }