From bc7cce592057d785c5dfc7467b8bdb84ac43660f Mon Sep 17 00:00:00 2001
From: James <james@keppylab.com>
Date: Fri, 25 Sep 2026 11:57:33 -0700
Subject: [PATCH] docs(rust): describe the null arguments mapping and decline
 coverage

Fix the tool call doc comment that still claimed a non-string arguments
declines, and spell out the null-to-None mapping it now shares with the
deserializer. Note the deliberate absence of deny_unknown_fields, since
python reads only function.arguments and the extra id, type and name keys
are ignored rather than declined. Restore decline coverage for malformed
tool call shapes: tool_calls that is not a list, a tool call with no
function key, and a non-string non-null arguments such as a number or an
object, all surfaced as the crate parse error so the host falls back to
the python counter
---
 litellm-rust/crates/token-counter/src/types.rs       | 12 +++++++++---
 .../crates/token-counter/tests/token_counter.rs      | 12 ++++++++++++
 2 files changed, 21 insertions(+), 3 deletions(-)

diff --git a/litellm-rust/crates/token-counter/src/types.rs b/litellm-rust/crates/token-counter/src/types.rs
index 7f01219d4c0..73c21039fcf 100644
--- a/litellm-rust/crates/token-counter/src/types.rs
+++ b/litellm-rust/crates/token-counter/src/types.rs
@@ -139,9 +139,15 @@ impl<'de> Visitor<'de> for TextValueVisitor {
 
 /// The parts of an assistant tool call Python counts: only the `arguments`
 /// string contributes (`_count_function_call_tokens` in
-/// `litellm_core_utils/token_counter.py`). Absent arguments count as the empty
-/// string; a non-string `arguments` declines so Python handles the fallback
-/// instead of miscounting.
+/// `litellm_core_utils/token_counter.py`). An absent arguments key counts as the
+/// empty string, while an explicit null counts as the string "None" the way
+/// Python's `str(None)` does; any other non-string `arguments` declines so
+/// Python handles the fallback instead of miscounting.
+///
+/// `deny_unknown_fields` is deliberately absent from this shape, and from
+/// `ToolCallFunction` and `LegacyFunctionCall`: Python reads only
+/// `function.arguments` off a tool call, so the extra `id`, `type` and `name`
+/// keys the OpenAI shape carries are ignored rather than declined.
 #[derive(Clone, Debug, Deserialize, PartialEq)]
 pub(crate) struct ToolCall {
     pub(crate) function: ToolCallFunction,
diff --git a/litellm-rust/crates/token-counter/tests/token_counter.rs b/litellm-rust/crates/token-counter/tests/token_counter.rs
index 10dd5abbbb0..77dc618db38 100644
--- a/litellm-rust/crates/token-counter/tests/token_counter.rs
+++ b/litellm-rust/crates/token-counter/tests/token_counter.rs
@@ -262,6 +262,18 @@ mod json {
 #[rstest]
 #[case::not_json(b"not json" as &[u8])]
 #[case::messages_not_a_list(br#"{"model":"m","messages":"hi"}"#)]
+#[case::tool_calls_not_a_list(
+    br#"{"model":"m","messages":[{"role":"assistant","tool_calls":"nope"}]}"#
+)]
+#[case::tool_call_without_function(
+    br#"{"model":"m","messages":[{"role":"assistant","tool_calls":[{"id":"1","type":"function"}]}]}"#
+)]
+#[case::tool_call_arguments_is_a_number(
+    br#"{"model":"m","messages":[{"role":"assistant","tool_calls":[{"id":"1","type":"function","function":{"name":"f","arguments":42}}]}]}"#
+)]
+#[case::tool_call_arguments_is_an_object(
+    br#"{"model":"m","messages":[{"role":"assistant","tool_calls":[{"id":"1","type":"function","function":{"name":"f","arguments":{"city":"Tokyo"}}}]}]}"#
+)]
 #[case::dict_content(
     br#"{"model":"m","messages":[{"role":"user","content":{"type":"text","text":"x"}}]}"#
 )]