When Does Memory Help? A Cost-Aware Evaluation of Long-Term Memory in Tool-Using LLM Agents

AuthorsShweta Mishra, Shashank Mishra

arXiv 20262026

TL;DR

MERIT benchmark + leak-checked dependent tasks + cost-aware metrics show memory lifts task success from 0.00 to up to 1.00 while some architectures collapse on updates.

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THE PROBLEM

Agents Hold Correct Facts But Ignore Them (Ignore Rate 0.45–0.53)

MERIT shows agents sometimes hold the correct fact in context yet fail to act on it, with Ignore Rate 0.45–0.53 across implementations.

This breaks tool-using agents on dependent tasks, where stale or unused memory leads to confidently wrong actions like incorrect refunds or misapplied configuration fixes.

HOW IT WORKS

MERIT: Memory Evaluation for Realistic Instrumented Tasks

MERIT’s core mechanism is a grid of memory conditions C0–C5, episodic tool-use tasks, controlled memory corruption, and cost-adjusted metrics like CAMU and MUR.

Think of MERIT as a lab where agent memory is swapped like hardware modules, while tasks and tools stay fixed, revealing how each memory behaves under stress.

This design lets MERIT expose when architectures like retrieval-only memory fail on updated facts, something a plain context window or conversational QA benchmark cannot reveal.

DIAGRAM

Cross-Episode Memory Dependency in MERIT Arcs

This diagram shows how MERIT structures arcs into plant, update, and probe episodes to enforce leak-free memory-dependent tasks.

DIAGRAM

MERIT Evaluation Grid and Cost-Aware Metrics

This diagram shows how MERIT’s evaluation grid spans domains, difficulty tiers, memory conditions, and computes CAMU and Ignore Rate.

PROCESS

How MERIT Handles an Episodic Tool-Use Task

  1. 01

    Environment and Arc Generation

    MERIT creates arcs of 4–6 episodes in domains like customer support, IT operations, and personal assistant with deterministic seeded worlds.

  2. 02

    Plant and Update Episodes

    MERIT runs plant episodes to establish gold facts and optional update episodes to revise them, while users forbid acting on these facts.

  3. 03

    Leak Check and Probe Episodes

    MERIT’s leak check ensures gold values are absent from probe inputs and tools, then runs probe episodes that require memory to succeed.

  4. 04

    Metric and Cost Computation

    MERIT computes TSR, MUR, Ignore Rate, SMH, and CAMU over 23,440 scored episodes with full token and dollar metering.

KEY CONTRIBUTIONS

Key Contributions

  • 01

    Task Suite with Controllable Memory Dependency

    MERIT provides episodic tool-use tasks in three domains with a difficulty ladder ending in updated-fact recall, verified by an automated leak check over 23,440 episodes.

  • 02

    Controlled Memory Corruption and Stale Harm

    MERIT injects stale, contradictory, and distractor records at known rates ρ∈{0.1,0.3} and measures Stale-Memory Harm, revealing up to +0.25 TSR drop for hybrid memory.

  • 03

    New Metrics: MUR, Ignore Rate, CAMU

    MERIT introduces Memory Utilization Rate, Ignore Rate up to 0.53, and Cost-Adjusted Marginal Utility, showing full replay is 2.7–3.9× less economical than the best memory.

RESULTS

By the Numbers

Dependent-task TSR (C0 floor)

0.000

-0.55 to -1.00 vs all memory conditions on easy tier

Dependent-task TSR (C2 hard)

0.30–0.95

spread across models and domains for embedding retrieval

Dependent-task TSR (C3 hard)

0.70–1.00

+0.28 to +0.44 over C2 across agent models

CAMU best condition

2.7–3.9× marginal utility per dollar

vs full replay which reaches only 1,222–1,741 pts/$

MERIT evaluates memory on episodic tool-use tasks in customer support, IT operations, and personal assistant domains. These results show MERIT’s overwrite-style memories maintain 0.70–1.00 TSR on updated facts while embedding retrieval varies between 0.30 and 0.95, and that cost-aware metrics reveal full replay is never the economical choice.

BENCHMARK

By the Numbers

MERIT evaluates memory on episodic tool-use tasks in customer support, IT operations, and personal assistant domains. These results show MERIT’s overwrite-style memories maintain 0.70–1.00 TSR on updated facts while embedding retrieval varies between 0.30 and 0.95, and that cost-aware metrics reveal full replay is never the economical choice.

BENCHMARK

Dependent-Task TSR on Hard Tier for D1 Commerce (Real Implementations)

Task Success Rate on D1 commerce hard tier, comparing MERIT memory architectures.

KEY INSIGHT

The Counterintuitive Finding

MERIT finds that hybrid memory combining a fact store with retrieval scores worse than the fact store alone on updated facts, dropping to 0.50–0.80 vs 0.75–1.00.

This breaks the common assumption that adding retrieval to structured memory is always helpful, showing retrieval can re-import staleness the overwrite path had eliminated.

WHY IT MATTERS

What this unlocks for the field

MERIT gives practitioners a way to measure when memory actually changes agent actions, including Ignore Rate and cost-adjusted marginal utility across architectures.

Builders can now choose between replay, retrieval, summarization, and fact stores based on task success and dollars, rather than conversational recall scores alone, especially for updated operational facts.

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