agent: Handle context window exceeded errors from Anthropic (#28688)

![CleanShot 2025-04-14 at 11 15
38@2x](https://github.com/user-attachments/assets/9e803ffb-74fd-486b-bebc-2155a407a9fa)

Release Notes:

- agent: Handle context window exceeded errors from Anthropic

crates/anthropic/src/anthropic.rs

@@ -724,4 +724,54 @@ impl ApiError {
     pub fn is_rate_limit_error(&self) -> bool {
         matches!(self.error_type.as_str(), "rate_limit_error")
     }
+
+    pub fn match_window_exceeded(&self) -> Option<usize> {
+        let Some(ApiErrorCode::InvalidRequestError) = self.code() else {
+            return None;
+        };
+
+        parse_prompt_too_long(&self.message)
+    }
+}
+
+pub fn parse_prompt_too_long(message: &str) -> Option<usize> {
+    message
+        .strip_prefix("prompt is too long: ")?
+        .split_once(" tokens")?
+        .0
+        .parse::<usize>()
+        .ok()
+}
+
+#[test]
+fn test_match_window_exceeded() {
+    let error = ApiError {
+        error_type: "invalid_request_error".to_string(),
+        message: "prompt is too long: 220000 tokens > 200000".to_string(),
+    };
+    assert_eq!(error.match_window_exceeded(), Some(220_000));
+
+    let error = ApiError {
+        error_type: "invalid_request_error".to_string(),
+        message: "prompt is too long: 1234953 tokens".to_string(),
+    };
+    assert_eq!(error.match_window_exceeded(), Some(1234953));
+
+    let error = ApiError {
+        error_type: "invalid_request_error".to_string(),
+        message: "not a prompt length error".to_string(),
+    };
+    assert_eq!(error.match_window_exceeded(), None);
+
+    let error = ApiError {
+        error_type: "rate_limit_error".to_string(),
+        message: "prompt is too long: 12345 tokens".to_string(),
+    };
+    assert_eq!(error.match_window_exceeded(), None);
+
+    let error = ApiError {
+        error_type: "invalid_request_error".to_string(),
+        message: "prompt is too long: invalid tokens".to_string(),
+    };
+    assert_eq!(error.match_window_exceeded(), None);
 }