diff --git a/router/src/lib.rs b/router/src/lib.rs
index 76e70bb7..6f3a130b 100644
--- a/router/src/lib.rs
+++ b/router/src/lib.rs
@@ -107,8 +107,8 @@ pub(crate) struct GenerateParameters {
     #[schema(default = "false", example = true)]
     pub do_sample: bool,
     #[serde(default = "default_max_new_tokens")]
-    #[schema(exclusive_minimum = 0, exclusive_maximum = 512, default = "20")]
-    pub max_new_tokens: u32,
+    #[schema(exclusive_minimum = 0, exclusive_maximum = 512, default = "null")]
+    pub max_new_tokens: Option<u32>,
     #[serde(default)]
     #[schema(nullable = true, default = "null", example = false)]
     pub return_full_text: Option<bool>,
@@ -140,8 +140,8 @@ pub(crate) struct GenerateParameters {
     pub top_n_tokens: Option<u32>,
 }
 
-fn default_max_new_tokens() -> u32 {
-    20
+fn default_max_new_tokens() -> Option<u32> {
+    None
 }
 
 fn default_parameters() -> GenerateParameters {
diff --git a/router/src/validation.rs b/router/src/validation.rs
index 36cbfb9b..96d8b6d2 100644
--- a/router/src/validation.rs
+++ b/router/src/validation.rs
@@ -67,8 +67,8 @@ impl Validation {
         &self,
         inputs: String,
         truncate: Option<usize>,
-        max_new_tokens: u32,
-    ) -> Result<(String, usize), ValidationError> {
+        max_new_tokens: Option<u32>,
+    ) -> Result<(String, usize, u32), ValidationError> {
         // If we have a fast tokenizer
         if let Some(sender) = &self.sender {
             // Create response channel
@@ -84,6 +84,11 @@ impl Validation {
             let (inputs, input_length) = response_receiver.await.unwrap()?;
 
             // Get total tokens
+            let max_new_tokens: u32 = if let Some(max_new_tokens) = max_new_tokens{
+                max_new_tokens
+            }else{
+                self.max_total_tokens.saturating_sub(input_length) as u32
+            };
             let total_tokens = input_length + max_new_tokens as usize;
 
             // Validate MaxTotalTokens
@@ -104,7 +109,7 @@ impl Validation {
             }
 
             metrics::histogram!("tgi_request_input_length", input_length as f64);
-            Ok((inputs, input_length))
+            Ok((inputs, input_length, max_new_tokens))
         }
         // Return inputs without validation
         else {
@@ -112,6 +117,11 @@ impl Validation {
             // However, the inputs will be truncated by the python servers
             // We make sure that truncate + max_new_tokens <= self.max_total_tokens
             let input_length = truncate.unwrap_or(self.max_input_length);
+            let max_new_tokens: u32 = if let Some(max_new_tokens) = max_new_tokens{
+                max_new_tokens
+            }else{
+                self.max_total_tokens.saturating_sub(input_length) as u32
+            };
 
             // Validate MaxNewTokens
             if (input_length as u32 + max_new_tokens) > self.max_total_tokens as u32 {
@@ -121,7 +131,7 @@ impl Validation {
                 ));
             }
 
-            Ok((inputs, input_length))
+            Ok((inputs, input_length, max_new_tokens))
         }
     }
 
@@ -200,7 +210,7 @@ impl Validation {
             })
             .unwrap_or(Ok(0))?;
 
-        if max_new_tokens == 0 {
+        if max_new_tokens == Some(0) {
             return Err(ValidationError::NegativeMaxNewTokens);
         }
 
@@ -247,7 +257,7 @@ impl Validation {
             .unwrap_or(Ok(None))?;
 
         // Validate inputs
-        let (inputs, input_length) = self
+        let (inputs, input_length, max_new_tokens) = self
             .validate_input(request.inputs, truncate, max_new_tokens)
             .await?;
 
@@ -426,7 +436,7 @@ mod tests {
 
         let max_new_tokens = 10;
         match validation
-            .validate_input("Hello".to_string(), None, max_new_tokens)
+            .validate_input("Hello".to_string(), None, Some(max_new_tokens))
             .await
         {
             Err(ValidationError::MaxNewTokens(1, 10)) => (),
@@ -455,7 +465,7 @@ mod tests {
 
         let max_new_tokens = 10;
         match validation
-            .validate_input("Hello".to_string(), None, max_new_tokens)
+            .validate_input("Hello".to_string(), None, Some(max_new_tokens))
             .await
         {
             Err(ValidationError::MaxTotalTokens(6, 1, 10)) => (),
@@ -534,7 +544,6 @@ mod tests {
                 inputs: "Hello".to_string(),
                 parameters: GenerateParameters {
                     top_p: Some(0.99),
-                    max_new_tokens: 1,
                     ..default_parameters()
                 },
             })
@@ -549,7 +558,6 @@ mod tests {
                 inputs: "Hello".to_string(),
                 parameters: GenerateParameters {
                     top_p: None,
-                    max_new_tokens: 1,
                     ..default_parameters()
                 },
             })
@@ -596,7 +604,6 @@ mod tests {
                 inputs: "Hello".to_string(),
                 parameters: GenerateParameters {
                     top_n_tokens: Some(4),
-                    max_new_tokens: 1,
                     ..default_parameters()
                 },
             })
@@ -608,7 +615,6 @@ mod tests {
                 inputs: "Hello".to_string(),
                 parameters: GenerateParameters {
                     top_n_tokens: Some(0),
-                    max_new_tokens: 1,
                     ..default_parameters()
                 },
             })
@@ -620,7 +626,6 @@ mod tests {
                 inputs: "Hello".to_string(),
                 parameters: GenerateParameters {
                     top_n_tokens: None,
-                    max_new_tokens: 1,
                     ..default_parameters()
                 },
             })