diff --git a/litellm-rust/crates/token-counter/src/types.rs b/litellm-rust/crates/token-counter/src/types.rs
index 16ab1d86f90..7f01219d4c0 100644
--- a/litellm-rust/crates/token-counter/src/types.rs
+++ b/litellm-rust/crates/token-counter/src/types.rs
@@ -147,15 +147,28 @@ pub(crate) struct ToolCall {
     pub(crate) function: ToolCallFunction,
 }
 
+/// Python runs `str(function.get("arguments", ""))`: a missing key counts as
+/// empty, but an explicit null counts as the string "None". `serde(default)`
+/// keeps a missing key at `None`, and the deserializer turns an explicit null
+/// into "None" so the counter sees what Python counts.
+fn explicit_null_arguments_as_none<'de, D: Deserializer<'de>>(
+    deserializer: D,
+) -> Result<Option<String>, D::Error> {
+    match Option::<String>::deserialize(deserializer)? {
+        Some(arguments) => Ok(Some(arguments)),
+        None => Ok(Some(String::from("None"))),
+    }
+}
+
 #[derive(Clone, Debug, Deserialize, PartialEq)]
 pub(crate) struct ToolCallFunction {
-    #[serde(default)]
+    #[serde(default, deserialize_with = "explicit_null_arguments_as_none")]
     pub(crate) arguments: Option<String>,
 }
 
 #[derive(Clone, Debug, Deserialize, PartialEq)]
 pub(crate) struct LegacyFunctionCall {
-    #[serde(default)]
+    #[serde(default, deserialize_with = "explicit_null_arguments_as_none")]
     pub(crate) arguments: Option<String>,
 }
 
diff --git a/litellm-rust/crates/token-counter/tests/token_counter.rs b/litellm-rust/crates/token-counter/tests/token_counter.rs
index 2161652f0b1..10dd5abbbb0 100644
--- a/litellm-rust/crates/token-counter/tests/token_counter.rs
+++ b/litellm-rust/crates/token-counter/tests/token_counter.rs
@@ -80,6 +80,17 @@ mod json {
     const ASSISTANT_CONTENT_WITH_TOOL_CALL: &str = r#"{"model":"claude-sonnet-4-5","messages":[
   {"role":"assistant","content":"calling","tool_calls":[{"id":"1","type":"function","function":{"name":"get_weather","arguments":"{\"city\": \"Paris, France\"}"}}]}]}"#;
 
+    const NULL_TOOL_CALL_ARGUMENTS: &str = r#"{"model":"claude-sonnet-4-5","messages":[
+  {"role":"assistant","tool_calls":[{"id":"1","type":"function","function":{"name":"f","arguments":null}}]},
+  {"role":"tool","tool_call_id":"1","content":"x"}]}"#;
+
+    const MISSING_TOOL_CALL_ARGUMENTS: &str = r#"{"model":"claude-sonnet-4-5","messages":[
+  {"role":"assistant","tool_calls":[{"id":"1","type":"function","function":{"name":"f"}}]},
+  {"role":"tool","tool_call_id":"1","content":"x"}]}"#;
+
+    const LEGACY_NULL_FUNCTION_CALL_ARGUMENTS: &str = r#"{"model":"claude-sonnet-4-5","messages":[
+  {"role":"assistant","function_call":{"name":"f","arguments":null}}]}"#;
+
     fn assert_count_request_matches_python_token_counter(
         load: JsonLoader,
         body: &str,
@@ -167,6 +178,9 @@ mod json {
             #[case::tool_calls_and_tool_result(super::TOOL_CALLS_AND_TOOL_RESULT, 24)]
             #[case::legacy_function_call(super::LEGACY_FUNCTION_CALL, 14)]
             #[case::assistant_content_with_tool_call(super::ASSISTANT_CONTENT_WITH_TOOL_CALL, 16)]
+            #[case::null_tool_call_arguments_count_as_none(super::NULL_TOOL_CALL_ARGUMENTS, 14)]
+            #[case::missing_tool_call_arguments_count_as_empty(super::MISSING_TOOL_CALL_ARGUMENTS, 13)]
+            #[case::legacy_null_function_call_arguments_count_as_none(super::LEGACY_NULL_FUNCTION_CALL_ARGUMENTS, 8)]
             fn count_request_matches_python_token_counter(
                 #[case] body: &str,
                 #[case] expected: usize,