diff --git a/docs/openapi.json b/docs/openapi.json
index 9de76e47..0a40e8bc 100644
--- a/docs/openapi.json
+++ b/docs/openapi.json
@@ -1508,6 +1508,25 @@
           }
         }
       },
+      "FunctionCall": {
+        "type": "object",
+        "required": [
+          "name",
+          "arguments"
+        ],
+        "properties": {
+          "arguments": {
+            "type": "string"
+          },
+          "description": {
+            "type": "string",
+            "nullable": true
+          },
+          "name": {
+            "type": "string"
+          }
+        }
+      },
       "FunctionDefinition": {
         "type": "object",
         "required": [
@@ -2280,7 +2299,7 @@
         ],
         "properties": {
           "function": {
-            "$ref": "#/components/schemas/FunctionDefinition"
+            "$ref": "#/components/schemas/FunctionCall"
           },
           "id": {
             "type": "string"
@@ -2406,4 +2425,4 @@
       "description": "Hugging Face Text Generation Inference API"
     }
   ]
-}
+}
\ No newline at end of file
diff --git a/docs/source/basic_tutorials/using_guidance.md b/docs/source/basic_tutorials/using_guidance.md
index e389fbbc..6540cb6d 100644
--- a/docs/source/basic_tutorials/using_guidance.md
+++ b/docs/source/basic_tutorials/using_guidance.md
@@ -305,7 +305,7 @@ chat = client.chat_completion(
 )
 
 print(chat.choices[0].message.tool_calls)
-# [ChatCompletionOutputToolCall(function=ChatCompletionOutputFunctionDefinition(arguments={'format': 'fahrenheit', 'location': 'Brooklyn, New York', 'num_days': 7}, name='get_n_day_weather_forecast', description=None), id=0, type='function')]
+# [ChatCompletionOutputToolCall(function=ChatCompletionOutputFunctionCall(arguments="{\"format\": \"fahrenheit\", \"location\": \"Brooklyn, New York\", \"num_days\": 7}", name='get_n_day_weather_forecast', description=None), id=0, type='function')]
 
 ```
 
diff --git a/router/src/lib.rs b/router/src/lib.rs
index e8c875a8..089e30df 100644
--- a/router/src/lib.rs
+++ b/router/src/lib.rs
@@ -1142,6 +1142,15 @@ pub struct FunctionDefinition {
     pub arguments: serde_json::Value,
 }
 
+#[derive(Clone, Debug, Deserialize, Serialize, ToSchema, Default, PartialEq)]
+pub(crate) struct FunctionCall {
+    #[serde(default)]
+    pub description: Option<String>,
+    pub name: String,
+    #[serde(alias = "parameters")]
+    pub arguments: String,
+}
+
 #[derive(Clone, Debug, Deserialize, Serialize, ToSchema)]
 #[cfg_attr(test, derive(PartialEq))]
 pub(crate) struct Tool {
@@ -1165,7 +1174,7 @@ pub(crate) struct ChatTemplateInputs<'a> {
 pub struct ToolCall {
     pub id: String,
     pub r#type: String,
-    pub function: FunctionDefinition,
+    pub function: FunctionCall,
 }
 
 #[derive(Clone, Deserialize, ToSchema, Serialize, Debug, PartialEq)]
@@ -1718,19 +1727,20 @@ mod tests {
             tool_calls: vec![ToolCall {
                 id: "0".to_string(),
                 r#type: "function".to_string(),
-                function: FunctionDefinition {
+                function: FunctionCall {
                     description: None,
                     name: "myfn".to_string(),
                     arguments: json!({
                         "format": "csv"
-                    }),
+                    })
+                    .to_string(),
                 },
             }],
         });
         let serialized = serde_json::to_string(&message).unwrap();
         assert_eq!(
             serialized,
-            r#"{"role":"assistant","tool_calls":[{"id":"0","type":"function","function":{"description":null,"name":"myfn","arguments":{"format":"csv"}}}]}"#
+            r#"{"role":"assistant","tool_calls":[{"id":"0","type":"function","function":{"description":null,"name":"myfn","arguments":"{\"format\":\"csv\"}"}}]}"#
         );
     }
 
diff --git a/router/src/server.rs b/router/src/server.rs
index e9aa4612..71e8c663 100644
--- a/router/src/server.rs
+++ b/router/src/server.rs
@@ -12,7 +12,6 @@ use crate::sagemaker::{
 };
 use crate::validation::ValidationError;
 use crate::vertex::vertex_compatibility;
-use crate::ChatTokenizeResponse;
 use crate::{
     usage_stats, BestOfSequence, Details, ErrorResponse, FinishReason, FunctionName,
     GenerateParameters, GenerateRequest, GenerateResponse, GrammarType, HubModelInfo,
@@ -27,6 +26,7 @@ use crate::{
     ChatRequest, Chunk, CompatGenerateRequest, Completion, CompletionComplete, CompletionFinal,
     CompletionRequest, CompletionType, DeltaToolCall, Function, Prompt, Tool,
 };
+use crate::{ChatTokenizeResponse, FunctionCall};
 use crate::{FunctionDefinition, HubPreprocessorConfig, ToolCall, ToolChoice};
 use crate::{MessageBody, ModelInfo, ModelsInfo};
 use async_stream::__private::AsyncStream;
@@ -1438,10 +1438,10 @@ pub(crate) async fn chat_completions(
                     let tool_calls = vec![ToolCall {
                         id: "0".to_string(),
                         r#type: "function".to_string(),
-                        function: FunctionDefinition {
+                        function: FunctionCall {
                             description: None,
                             name,
-                            arguments,
+                            arguments: arguments.to_string(),
                         },
                     }];
                     (Some(tool_calls), None)
@@ -1574,6 +1574,7 @@ Tool,
 ToolCall,
 Function,
 FunctionDefinition,
+FunctionCall,
 ToolChoice,
 ModelInfo,
 ChatTokenizeResponse,