---
title: "What is memory decay in AI agents?"
description: "Memory decay down-ranks, compresses, or expires old records. Definition, decay strategies, appropriate uses, and the risk of removing required evidence."
canonical: https://past.dev/glossary/memory-decay
last-updated: 2026-10-15
---
# Memory decay

Source: https://past.dev/glossary/memory-decay

Memory decay is a policy that reduces the retrieval weight or retention period of old or unused records. Implementations may down-rank, compress, or expire records. Age alone does not indicate that a fact is false. Validity periods and supersession should determine whether a fact remains current.

## The three decay strategies

- **Rank decay.** Older or less-used memories score lower at retrieval. Cheap, reversible, and the default in several memory products; mem0's documentation describes recency working as a ranking signal ([mem0 docs](https://github.com/mem0ai/mem0)).
- **Consolidation.** Batches of episodic detail are summarized into fewer records. This reduces token use and removes some detail.
- **Expiry.** Memories past a TTL are deleted. Appropriate for data with genuine retention limits, dangerous elsewhere.

## Age is not falsity

Decay policies encode an assumption: old means less relevant. For conversational preferences that is often right. For operational facts it is often wrong. A contract signed years ago governs until it ends. Treating age as a proxy for truth produces confident answers built on whichever fact is newest, which fails in the exact cases where memory matters most: backfills, corrections, and long-lived facts.

Use [validity windows](/glossary/validity-window) and [supersession](/glossary/fact-supersession) to track whether a fact is current. Use ranking for relevance and retention rules for deletion. These are separate controls.

## How past.dev treats old memories

A fact remains current until it is superseded. Retrieval ranks current facts before superseded facts, while history remains available for [point-in-time recall](/glossary/point-in-time-recall). [Privacy](/privacy) documents explicit deletion for privacy and retention requirements.

## When decay is the right tool

- Session scratch and working state, which should die with the task.
- High-volume telemetry where individual events stop mattering after aggregation.
- Data subject to legal retention limits that require expiry.

See [what is agent memory](/what-is-agent-memory) for where decay sits among the memory types, and the [benchmarks page](/benchmarks) for how long-horizon recall is measured. Implementation details live in the [docs](/docs/memory-api/quickstart).

## Frequently asked questions

### Does past.dev ever forget on its own?

No. Facts change state only through supersession, explicit deletion, or configured retention. Age alone does not remove a fact. Retained facts remain linked to their evidence.

## Related

- [Fact supersession](https://past.dev/glossary/fact-supersession)
- [Validity window](https://past.dev/glossary/validity-window)
- [Context rot](https://past.dev/glossary/context-rot)
- [What is agent memory?](https://past.dev/what-is-agent-memory)
- [Quickstart](https://past.dev/docs/memory-api/quickstart)