AgenticSTS: A Bounded-Memory Testbed for Long-Horizon LLM Agents

AuthorsXiangchen Cheng, Yunwei Jiang, Jianwen Sun et al.

arXiv 20262026

TL;DR

AgenticSTS uses a bounded, typed L1–L5 memory contract to keep prompts fixed-size and directionally lifts A0 wins from 3/10 to 6/10 with triggered skills.

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THE PROBLEM

Long-horizon agents drown in growing transcripts (zero A0 wins, 16% human)

Public Slay the Spire 2 benchmarks report zero A0 wins across five frontier LLM configurations, while humans win 16% at the same difficulty.

In this regime, transcript-appending agents face unbounded context growth, making individual memory components un-attributable and long-horizon decisions hard to debug or improve.

HOW IT WORKS

AgenticSTS bounded, typed memory contract

AgenticSTS builds every decision from five typed layers: L1 protocol instructions, L2 state-typed prompts, L3 game knowledge, L4 episodic memory, and L5 skill library.

You can think of AgenticSTS like a computer separating OS rules, schemas, a read-only game database, a case log, and a playbook instead of dumping a giant terminal log into RAM.

This typed contract lets AgenticSTS cap prompt size, toggle each layer independently, and attribute behavior changes to specific memory slots in ways a plain context window cannot.

DIAGRAM

Per-decision typed retrieval pipeline in AgenticSTS

This diagram shows how AgenticSTS composes a fresh user message from L1–L5 for each decision instead of appending a transcript.

DIAGRAM

Fixed-A0 ablation design for AgenticSTS

This diagram shows how AgenticSTS evaluates five memory configurations at A0, isolating prompt strictness, L5 skills, and L4 episodes.

PROCESS

How AgenticSTS Handles a Slay the Spire 2 Run

  1. 01

    Per-decision compositional context

    AgenticSTS receives the current game state and uses L1 protocol instructions plus L2 state-typed prompts to define the decision schema and legal actions.

  2. 02

    Five typed knowledge layers

    AgenticSTS queries L3 game knowledge for rules, L4 episodic memory for past summaries, and L5 skill library for triggered guides relevant to the state.

  3. 03

    Routing and combat truncation

    AgenticSTS routes the decision to fast, strategic, analysis, or evolution tiers and truncates combat context to a small conversation object instead of a full transcript.

  4. 04

    Skill discovery and Mode B

    After runs, AgenticSTS can perform mistake-driven skill discovery and Mode B stub-template-filled authoring to update the L5 skill library under a strict write gate.

KEY CONTRIBUTIONS

Key Contributions

  • 01

    Per-decision composition interface

    AgenticSTS defines a per-decision composition interface that assembles prompts from L1 protocol instructions through L5 skill library instead of a raw transcript, making each layer individually ablatable.

  • 02

    Evidence for triggered L5 skills

    AgenticSTS shows that enabling L5 skill library directionally lifts A0 wins from 3/10 to 6/10 in the fixed-A0 matrix, with Wilson 95% CIs [10.8%, 60.3%] and [31.3%, 83.2%].

  • 03

    Reusable 298-trajectory archive

    AgenticSTS releases 298 completed trajectories with condition tags, frozen L4 episodic memory and L5 skill library snapshots, prompt records, and Wilson/bootstrap scripts for reproducible long-horizon memory studies.

RESULTS

By the Numbers

Win

6/10

+3/10 over baseline-strict

Win

3/10

baseline-strict no scaffold

Score

82.1

full-frozen skills+episodes at A0

Score

70.4

baseline-strict at A0

On fixed A0 Slay the Spire 2 runs, AgenticSTS evaluates five memory configurations and finds the largest observed difference when L5 skill library is enabled, directionally improving wins from 3/10 to 6/10 under the bounded contract.

BENCHMARK

By the Numbers

On fixed A0 Slay the Spire 2 runs, AgenticSTS evaluates five memory configurations and finds the largest observed difference when L5 skill library is enabled, directionally improving wins from 3/10 to 6/10 under the bounded contract.

BENCHMARK

Fixed-A0 ablation (Table 2)

Win counts across AgenticSTS memory configurations at A0.

KEY INSIGHT

The Counterintuitive Finding

AgenticSTS shows that template-filled Mode B skills reach the same 6/10 A0 win rate as hand-authored Mode A skills, despite simpler prose.

This challenges the assumption that sophisticated human-written strategy text is necessary; the key seems to be having a structured L5 skill library, not its exact wording.

WHY IT MATTERS

What this unlocks for the field

AgenticSTS makes memory a first-class, ablatable interface, letting researchers toggle L3 game knowledge, L4 episodic memory, and L5 skill library independently on a hard long-horizon game.

Builders can now run controlled experiments on memory contracts, compare bounded versus accumulating context in one harness, and ship agents whose long-horizon behavior is auditable and reproducible.

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