From 32d87b70d7613d0f33527f6cef2842df63871de1 Mon Sep 17 00:00:00 2001
From: Kian Jones <11655409+kianjones9@users.noreply.github.com>
Date: Fri, 6 Feb 2026 17:11:51 -0800
Subject: [PATCH] fix(core): return 400 for ContextWindowExceededError instead
 of 500 (#9348)
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SystemPromptTokenExceededError (a subclass of ContextWindowExceededError)
was hitting the generic 500 handler in Datadog. This is a client-side
configuration issue (system prompt too large for context window), so it
should return 400 Bad Request.

Datadog: https://us5.datadoghq.com/error-tracking/issue/3360e252-d5f1-11f0-8467-da7ad0900000
Also covers: https://us5.datadoghq.com/error-tracking/issue/6e50c57c

🤖 Generated with [Letta Code](https://letta.com)

Co-authored-by: Letta <noreply@letta.com>
---
 letta/server/rest_api/app.py | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/letta/server/rest_api/app.py b/letta/server/rest_api/app.py
index 91d8905f..e984a37e 100644
--- a/letta/server/rest_api/app.py
+++ b/letta/server/rest_api/app.py
@@ -33,6 +33,7 @@ from letta.errors import (
     AgentNotFoundForExportError,
     BedrockPermissionError,
     ConcurrentUpdateError,
+    ContextWindowExceededError,
     ConversationBusyError,
     EmbeddingConfigRequiredError,
     HandleNotFoundError,
@@ -513,6 +514,7 @@ def create_application() -> "FastAPI":
     app.add_exception_handler(AgentFileImportError, _error_handler_400)
     app.add_exception_handler(EmbeddingConfigRequiredError, _error_handler_400)
     app.add_exception_handler(LettaImageFetchError, _error_handler_400)
+    app.add_exception_handler(ContextWindowExceededError, _error_handler_400)
     app.add_exception_handler(ValueError, _error_handler_400)
 
     # 404 Not Found errors