fix: truncation agent token calculations (#915)

crates/goose/src/agents/truncate.rs

@@ -43,6 +43,8 @@ impl TruncateAgent {
         &self,
         messages: &mut Vec<Message>,
         estimate_factor: f32,
+        system_prompt: &str,
+        tools: &mut Vec<Tool>,
     ) -> anyhow::Result<()> {
         // Model's actual context limit
         let context_limit = self
@@ -57,20 +59,37 @@ impl TruncateAgent {
         // Our token count is an estimate since model providers often don't provide the tokenizer (eg. Claude)
         let context_limit = (context_limit as f32 * estimate_factor) as usize;
 
-        // Calculate current token count
+        // Take into account the system prompt, and our tools input and subtract that from the
+        // remaining context limit
+        let system_prompt_token_count = self.token_counter.count_tokens(system_prompt);
+        let tools_token_count = self.token_counter.count_tokens_for_tools(tools.as_slice());
+
+        // Check if system prompt + tools exceed our context limit
+        let remaining_tokens = context_limit
+            .checked_sub(system_prompt_token_count)
+            .and_then(|remaining| remaining.checked_sub(tools_token_count))
+            .ok_or_else(|| {
+                anyhow::anyhow!("System prompt and tools exceed estimated context limit")
+            })?;
+
+        let context_limit = remaining_tokens;
+
+        // Calculate current token count of each message, use count_chat_tokens to ensure we
+        // capture the full content of the message, include ToolRequests and ToolResponses
         let mut token_counts: Vec<usize> = messages
             .iter()
-            .map(|msg| self.token_counter.count_tokens(&msg.as_concat_text()))
+            .map(|msg| {
+                self.token_counter
+                    .count_chat_tokens("", std::slice::from_ref(msg), &[])
+            })
             .collect();
 
-        let _ = truncate_messages(
+        truncate_messages(
             messages,
             &mut token_counts,
             context_limit,
             &OldestFirstTruncation,
-        );
-
-        Ok(())
+        )
     }
 }
 
@@ -229,7 +248,7 @@ impl Agent for TruncateAgent {
                             // Create an error message & terminate the stream
                             // the previous message would have been a user message (e.g. before any tool calls, this is just after the input message.
                             // at the start of a loop after a tool call, it would be after a tool_use assistant followed by a tool_result user)
-                            yield Message::assistant().with_text("Error: Context length exceeds limits even after multiple attempts to truncate.");
+                            yield Message::assistant().with_text("Error: Context length exceeds limits even after multiple attempts to truncate. Please start a new session with fresh context and try again.");
                             break;
                         }
 
@@ -243,7 +262,11 @@ impl Agent for TruncateAgent {
                         // release the lock before truncation to prevent deadlock
                         drop(capabilities);
 
-                        self.truncate_messages(&mut messages, estimate_factor).await?;
+                        if let Err(err) = self.truncate_messages(&mut messages, estimate_factor, &system_prompt, &mut tools).await {
+                            yield Message::assistant().with_text(format!("Error: Unable to truncate messages to stay within context limit. \n\nRan into this error: {}.\n\nPlease start a new session with fresh context and try again.", err));
+                            break;
+                        }
+
 
                         // Re-acquire the lock
                         capabilities = self.capabilities.lock().await;