diff --git a/crates/language_models/src/provider/open_ai.rs b/crates/language_models/src/provider/open_ai.rs
index 5c519b84a0..5b20f8f9da 100644
--- a/crates/language_models/src/provider/open_ai.rs
+++ b/crates/language_models/src/provider/open_ai.rs
@@ -673,10 +673,11 @@ pub fn count_open_ai_tokens(
             | Model::O1
             | Model::O3
             | Model::O3Mini
-            | Model::O4Mini
-            | Model::Five
-            | Model::FiveMini
-            | Model::FiveNano => tiktoken_rs::num_tokens_from_messages(model.id(), &messages),
+            | Model::O4Mini => tiktoken_rs::num_tokens_from_messages(model.id(), &messages),
+            // GPT-5 models don't have tiktoken support yet; fall back on gpt-4o tokenizer
+            Model::Five | Model::FiveMini | Model::FiveNano => {
+                tiktoken_rs::num_tokens_from_messages("gpt-4o", &messages)
+            }
         }
         .map(|tokens| tokens as u64)
     })