Append-only event log
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A data structure in which records are only ever added, never modified or deleted, used in long-horizon agent architectures to represent the full history of an agent session as an immutable sequence of events.
Role in long-horizon agent architectures
Lance Martin of Anthropic describes the session as "an append-only event log" as a core design principle in Claude Managed Agents and the broader approach to Claude for Long-Horizon Tasks. →
The append-only event log is one half of a two-part architectural pattern: the agent harness (execution environment) is decoupled from the session state, with the harness implemented as a stateless process and all session history preserved in the immutable log. Martin advocates that this separation makes long-horizon agents both more reliable and more secure. →
The key properties this design provides:
- Statelessness of the harness: because all state lives in the log rather than in the process, the execution environment can be restarted, replaced, or scaled without losing session context.
- Auditability and reproducibility: an immutable, append-only record means the full sequence of agent actions and observations can be replayed or inspected after the fact.
- Security: preventing in-place mutation of the session record limits the ability of a compromised step in the pipeline to retroactively alter the agent's history.
Points of disagreement
No dissenting positions on the append-only event log pattern are recorded in the available material.