ContextOverflowError: input exceeds the model context limit

ContextOverflowError

langchain_core.exceptions.ContextOverflowError means your messages plus the expected output do not fit the model's context limit. Not retryable.

Fixes

1. Summarize the conversation when it gets long. In create_agent, add SummarizationMiddleware with a token trigger so history condenses automatically.
2. Trim messages explicitly with trim_messages from langchain.messages / langchain-core before the call.
3. Check for accidental duplication: the same big document appended every turn is the usual culprit in agent loops. Log message token counts per turn until you find the growth.
4. Switch to a model with a larger window if the input is legitimately large.

Rule

Never "fix" this by catching and retrying. The fix is always smaller input. If the error appears mid-agent-run, the conversation history is growing unboundedly; find the growth before the next run.

Error catalog entry grounded in the official model exceptions docs and v1 middleware (summarization), verified 2026-09-26. Models: https://docs.langchain.com/oss/python/langchain/models ; middleware: https://docs.langchain.com/oss/python/langchain/middleware/built-in