create_summarization_middleware

Create a Deep Agents SummarizationMiddleware with model-aware defaults.

Why this exists in deepagents

The Deep Agents SummarizationMiddleware wraps langchain.agents.middleware.SummarizationMiddleware to add behavior long-running, file-aware agents need. Prefer LangChain's middleware directly if none of the below apply:

• Backend offload of evicted history. Evicted messages are appended to /conversation_history/{session_id}.md (default path) on the configured backend before the summary replaces them, and the summary embeds that path so the agent can re-open it via read_file when FilesystemMiddleware is registered. LangChain drops evicted messages with no recovery path.
• Pre-summarization tool-arg truncation. Large write_file / edit_file arguments in older messages are clipped at a lower threshold than full compaction, often reclaiming enough context to skip summarizing. Configured via truncate_args_settings.
• ContextOverflowError fallback. On a provider over-budget rejection the middleware summarizes and retries instead of bubbling the error up.
• Non-mutating message state. Summarization is tracked in a private _summarization_event field via wrap_model_call, leaving state["messages"] intact. LangChain rewrites it with RemoveMessage(id=REMOVE_ALL_MESSAGES) from before_model. Preserving the raw log enables replay, evals, and shared state with SummarizationToolMiddleware's compact_conversation tool.
• Auto-selected trigger/keep thresholds. LangChain accepts fraction-based thresholds but defaults to trigger=None and keep=("messages", 20). This factory picks fraction-based defaults from the model's profile when max_input_tokens is exposed, falling back to fixed counts otherwise — see compute_summarization_defaults.