diff --git a/crates/language_models/src/provider/copilot_chat.rs b/crates/language_models/src/provider/copilot_chat.rs
index 489837f159..fe11458282 100644
--- a/crates/language_models/src/provider/copilot_chat.rs
+++ b/crates/language_models/src/provider/copilot_chat.rs
@@ -219,32 +219,24 @@ impl LanguageModel for CopilotChatLanguageModel {
         cx: &App,
     ) -> BoxFuture<'static, Result<usize>> {
         match self.model {
-            CopilotChatModel::Claude3_5Sonnet => count_anthropic_tokens(request, cx),
-            CopilotChatModel::Claude3_7Sonnet => count_anthropic_tokens(request, cx),
-            CopilotChatModel::Claude3_7SonnetThinking => count_anthropic_tokens(request, cx),
+            CopilotChatModel::Claude3_5Sonnet
+            | CopilotChatModel::Claude3_7Sonnet
+            | CopilotChatModel::Claude3_7SonnetThinking => count_anthropic_tokens(request, cx),
             CopilotChatModel::Gemini20Flash | CopilotChatModel::Gemini25Pro => {
                 count_google_tokens(request, cx)
             }
-            _ => {
-                let model = match self.model {
-                    CopilotChatModel::Gpt4o => open_ai::Model::FourOmni,
-                    CopilotChatModel::Gpt4 => open_ai::Model::Four,
-                    CopilotChatModel::Gpt4_1 => open_ai::Model::FourPointOne,
-                    CopilotChatModel::Gpt3_5Turbo => open_ai::Model::ThreePointFiveTurbo,
-                    CopilotChatModel::O1 => open_ai::Model::O1,
-                    CopilotChatModel::O3Mini => open_ai::Model::O3Mini,
-                    CopilotChatModel::O3 => open_ai::Model::O3,
-                    CopilotChatModel::O4Mini => open_ai::Model::O4Mini,
-                    CopilotChatModel::Claude3_5Sonnet
-                    | CopilotChatModel::Claude3_7Sonnet
-                    | CopilotChatModel::Claude3_7SonnetThinking
-                    | CopilotChatModel::Gemini20Flash
-                    | CopilotChatModel::Gemini25Pro => {
-                        unreachable!()
-                    }
-                };
-                count_open_ai_tokens(request, model, cx)
+            CopilotChatModel::Gpt4o => count_open_ai_tokens(request, open_ai::Model::FourOmni, cx),
+            CopilotChatModel::Gpt4 => count_open_ai_tokens(request, open_ai::Model::Four, cx),
+            CopilotChatModel::Gpt4_1 => {
+                count_open_ai_tokens(request, open_ai::Model::FourPointOne, cx)
             }
+            CopilotChatModel::Gpt3_5Turbo => {
+                count_open_ai_tokens(request, open_ai::Model::ThreePointFiveTurbo, cx)
+            }
+            CopilotChatModel::O1 => count_open_ai_tokens(request, open_ai::Model::O1, cx),
+            CopilotChatModel::O3Mini => count_open_ai_tokens(request, open_ai::Model::O3Mini, cx),
+            CopilotChatModel::O3 => count_open_ai_tokens(request, open_ai::Model::O3, cx),
+            CopilotChatModel::O4Mini => count_open_ai_tokens(request, open_ai::Model::O4Mini, cx),
         }
     }
 
diff --git a/crates/language_models/src/provider/open_ai.rs b/crates/language_models/src/provider/open_ai.rs
index e186a5151e..4f2a750c5a 100644
--- a/crates/language_models/src/provider/open_ai.rs
+++ b/crates/language_models/src/provider/open_ai.rs
@@ -539,7 +539,7 @@ struct RawToolCall {
 
 pub fn count_open_ai_tokens(
     request: LanguageModelRequest,
-    model: open_ai::Model,
+    model: Model,
     cx: &App,
 ) -> BoxFuture<'static, Result<usize>> {
     cx.background_spawn(async move {
@@ -559,11 +559,33 @@ pub fn count_open_ai_tokens(
             .collect::<Vec<_>>();
 
         match model {
-            open_ai::Model::Custom { .. }
-            | open_ai::Model::O1Mini
-            | open_ai::Model::O1
-            | open_ai::Model::O3Mini => tiktoken_rs::num_tokens_from_messages("gpt-4", &messages),
-            _ => tiktoken_rs::num_tokens_from_messages(model.id(), &messages),
+            Model::Custom { max_tokens, .. } => {
+                let model = if max_tokens >= 100_000 {
+                    // If the max tokens is 100k or more, it is likely the o200k_base tokenizer from gpt4o
+                    "gpt-4o"
+                } else {
+                    // Otherwise fallback to gpt-4, since only cl100k_base and o200k_base are
+                    // supported with this tiktoken method
+                    "gpt-4"
+                };
+                tiktoken_rs::num_tokens_from_messages(model, &messages)
+            }
+            // Not currently supported by tiktoken_rs. All use the same tokenizer as gpt-4o (o200k_base)
+            Model::O1
+            | Model::FourPointOne
+            | Model::FourPointOneMini
+            | Model::FourPointOneNano
+            | Model::O3Mini
+            | Model::O3
+            | Model::O4Mini => tiktoken_rs::num_tokens_from_messages("gpt-4o", &messages),
+            // Currently supported by tiktoken_rs
+            Model::ThreePointFiveTurbo
+            | Model::Four
+            | Model::FourTurbo
+            | Model::FourOmni
+            | Model::FourOmniMini
+            | Model::O1Preview
+            | Model::O1Mini => tiktoken_rs::num_tokens_from_messages(model.id(), &messages),
         }
     })
     .boxed()