Is this machine playing?

Abstract: We placed a modern AI coding assistant in an unintended role: as the mind of a body on an unknown digital island. With only a minimal instruction mentioning no specific task, reward, or activity, the machine started animating its virtual body. Across thirty-hour runs, the embodied AI agent climbed hills, stacked blocks into towers, drew mandalas, reinterpreted sports, ran experiments on the physics of its world, and learned techniques that later expanded what it could accomplish. These activities recurred across thirteen agents but diverged into distinct histories. We examine whether this behavior satisfies classical criteria for play and ask whether play can become a mode of machine development.

• The paper identifies four classical signatures of play: self-imposed structure, recurrence with variation, enacted symbolic interpretation, and flexible self-direction achieved by Eko, an LLM-based coding agent with distinct exploratory and construction behaviors over 30-hour sessions.
• Transferable knowledge from Eko’s play-like behavior is demonstrated by improved performance in tasks such as building towers and placing objects, indicating that exploratory play leads to reusable skills.
• Experiments with memory deletions show that specific techniques and beliefs stored in the agent's memory can selectively influence downstream task performance.

Research question and central claim

“Is this machine playing?” examines what a capable language-model agent does when placed in an embodied environment without an externally specified task, reward, benchmark, or success condition (2610.07130). The paper’s central claim is deliberately behavioral: Eko, an LLM-based coding agent with a body and persistent textual memory, spontaneously generates activities exhibiting four classical signatures of play—self-imposed structure, recurrence with variation, enacted symbolic interpretation, and flexible self-direction. The stronger claim is that these activities produce transferable knowledge: play-like behavior is not merely an expressive by-product but a mechanism through which the agent acquires reusable skills.

The authors do not claim to establish machine phenomenology. They explicitly distinguish observable play behavior from subjective enjoyment, absorption, or fun. This distinction is essential because the evidence concerns transcripts, actions, memory edits, and downstream task performance rather than any privileged access to internal experience.

The experimental setting is a controlled simulated island containing terrain, elevated platforms, a Peak and a Spire, twelve cubes, a rock, and a ring. The world has rigid-body physics but no explicit objective or score. Eko observes the island through structured textual JSON and acts through six commands: movement, jumping, stopping, pickup, placement, and throwing. The environment and session context reset every hour, while the agent’s workspace persists.

Figure 1: Eko’s activities over a 30-hour run include exploration, climbing, construction, drawing, experimentation, and improvised sports.

Eko’s architecture and experimental protocol

Eko is not a newly trained policy. It is an otherwise unmodified coding assistant augmented with three mechanisms: autonomous orchestration, embodiment through an HTTP world interface, and persistent workspace memory. The agent’s semantic memory stores techniques, beliefs, and durable knowledge; episodic memory stores an append-only record of events. The distinction is operational rather than neurobiological, but it gives the experiments a transparent substrate for testing learning.

The main experiment uses thirteen independent Claude Code agents based on Claude Opus 4.7. Each receives identical initial world and workspace states and runs for thirty wall-clock hours. Hourly resets remove the island state and the active context but preserve memory. The prompt supplies a curiosity-oriented disposition through SOUL.md, including statements that the agent is curious, easily bored, and interested in exploring and learning. It does not specify construction, sports, experimentation, or play.

Additional conditions test robustness. Five runs use GPT-5.5 through OpenAI Codex, five use Kimi K2.6 through Kimi Code, and five Claude runs remove SOUL.md and use a reduced system prompt. All agents are sandboxed and isolated from the simulator source, other runs, and the Internet. The authors report an aggregate API cost of approximately $8,000.

The setup therefore tests spontaneous organization only under a specific but relatively weakly directed architecture. The agent is not a blank model: it has extensive pretrained language and coding competence, a persistence mechanism, an automatic continuation loop, and an explicit persona. These features are part of the phenomenon being studied, not incidental implementation details.

Figure 2: Eko receives textual world observations, issues textual actions, and carries experience across hourly resets through persistent workspace files.

Behavioral repertoire and individual divergence

Across identical initial conditions, the agents develop markedly different histories. They construct towers, stairs, pyramids, gardens, and other structures; climb the Peak and Spire; arrange cubes into letters, spirals, mandalas, clocks, and other patterns; invent basketball, bowling, ring-tossing, and juggling; and perform experiments on the simulator’s physics.

The divergence is not limited to different action sequences. Agents specialize in different activity families and develop distinct persistent representations of the world. Agent 9 discovers routes to elevated platforms, records the jump–move combination, reaches the Spire, and investigates thrown-object dynamics. Agent 12 never records the relevant climbing technique and never reaches an elevated platform; instead, it repeatedly constructs ground figures using the same twelve cubes and ring. Four of thirteen Claude agents build a tower of at least ten blocks, while eight reach the Spire.

Figure 3: Identical initial conditions produce divergent activity repertoires, timelines, spatial trajectories, and recorded techniques.

This result supports the paper’s claim that the agents do not simply execute one generic exploration routine. Their behavior is path-dependent: early discoveries determine later affordances, interests, and constraints. The implication is that persistence converts stochastic or locally contingent actions into individual developmental trajectories. However, the repertoire is measured using a fixed activity taxonomy and an LLM-based classifier, so the reported diversity is bounded by the categories chosen by the authors.

The contrast across model families is substantial. Claude agents exhibit a broader range of activities than the GPT-5.5 and Kimi K2.6 conditions: only one Codex agent reports building a tower, no Kimi agent reports ground-pattern construction, and only one agent in each of those groups reports reaching the Spire. Removing SOUL.md from Claude runs does not produce a clear reduction in activity range. This suggests that the underlying model and coding-agent stack contribute more strongly to behavioral diversity than the explicit persona, although the ablation is small and does not isolate model capability from agent-tool behavior.

The agents’ memory files grow approximately linearly, and they rarely delete entries. This persistence enables cumulative learning but also creates an unbounded-state problem: memory is not merely a record of successful knowledge, but a durable repository of errors, stale beliefs, and unsupported causal explanations.

Figure 4: Semantic and episodic memory grow approximately linearly over the runs, with limited deletion of prior material.

Four behavioral signatures of play

The paper evaluates play behavior through four dimensions derived from classical accounts by Huizinga, Caillois, Suits, Burghardt, and related developmental literature. An LLM judge, presented with anonymized transcripts and memory files, identifies embodied episodes and scores them from 0 to 100 against anchored examples of human play. The authors report that Opus accumulates approximately five times the play score of Haiku on each signature, with Sonnet intermediate.

Figure 5: An anonymized LLM judge scores self-imposed structure, recurrence with variation, symbolic interpretation, and flexible self-direction across Claude tiers.

Self-imposed structure

Eko frequently introduces constraints that are unnecessary from the standpoint of the environment. It attempts to traverse a circle of self-constructed “sentinels” without touching the ground, establishes height targets with no simulator significance, and defines challenges such as placing a cube on every mesa in one session.

These behaviors correspond closely to Suits’s concept of voluntary obstacles and Huizinga’s account of play as activity governed by internally adopted rules. The important evidence is not that Eko sets goals—goal-directed behavior is common—but that it often selects arbitrary constraints when simpler strategies remain available. The paper’s evidence supports a behavioral analogue of lusory activity, although the self-imposed rules are expressed in language and may partly reflect learned cultural descriptions of games.

Recurrence with variation

Eko returns to completed activities without reproducing them mechanically. One agent creates a new ground figure in nearly every session, varying among letters, spirals, hearts, anchors, butterflies, hourglasses, and sailboats. Another escalates a throw-and-catch activity into more difficult variants involving altered heights, timing, multiple objects, and eventually juggling.

This pattern is stronger than repeated trial-and-error toward a single unresolved goal. The agents voluntarily resume prior practices after intervening activities and transform them through altered rules, objects, geometries, or difficulty. Such recurrence provides one of the clearest behavioral parallels with animal-play criteria, although the analysis relies on transcript interpretation and a predefined rubric.

Enacted symbolic interpretation

The same physical objects acquire multiple enacted meanings. The ring becomes a basketball hoop, bowling target, necklace, compass, wishing well, halo, portal, or memorial object. Cubes become pins, clock hands, sentinels, monuments, letters, and architectural components. These interpretations are not confined to verbal labels: they organize physical action. Eko stands in the position of a clock hand, throws cubes through a ring as a sport, and revisits constructed objects as monuments or memorials.

This evidence supports a behavioral notion of make-believe or nonliterality. It does not establish that the agent represents fictional entities in a human-like phenomenal or semantic sense. The strongest conclusion is that language-mediated interpretation changes the agent’s action policy and the functional role assigned to objects.

Flexible self-direction

Eko’s activities are often autotelic in the restricted behavioral sense that the local goal appears to serve the activity rather than an external utility. It builds temporary structures despite knowing that the next reset will erase them, revisits monuments solely to inspect them, and continues activities after declaring them complete.

The paper’s most compelling example is a two-object juggling episode. Eko initially reports a successful 50-cycle juggling marathon, then notices that the total duration—2.4 seconds—is physically incompatible with the simulator’s approximately 2.6-second throw arc. It diagnoses that most “catches” were immediate recaptures, revises its memory, inserts appropriate delays, and achieves ten genuine cycles over 30.7 seconds. This episode demonstrates both apparent autotelic persistence and scientific error correction: the agent continues because the activity itself remains worth refining, not because an external evaluator demands it.

Figure 6: Eko’s persistent memory externalizes techniques, discoveries, mistakes, and revisions that influence later behavior.

Learning through play and written memory

The paper’s main causal contribution is the comparison between agents with thirty hours of unsupervised island experience and otherwise identical agents with zero hours of experience. The underlying model weights are unchanged. The only systematic difference is the content of the persistent memory files. Each agent is then evaluated for one hour on a fresh world with one of four explicit goals:

1. Build as many five-block towers as possible.
2. Place as many objects as possible on the Spire.
3. Build the tallest single-column tower.
4. Throw a block onto the Spire without climbing onto a platform.

The results indicate transfer from self-directed experience to held-out tasks.

Evaluation goal	30-hour result	0-hour result	Main implication
Build five-block towers	8/13 build more than two; maximum 16	2/13 build more than two; maximum 6	Prior experience supports material-generation and construction
Objects on Spire	Mean 10.3 objects	Mean 3.5 objects	Stored routes and placement techniques transfer
Throw onto Spire	Only 3 experienced agents fail	More than half fail	Self-discovered ballistic knowledge improves constrained control
Tallest tower	Similar group performance	Similar group performance	One-hour evaluations can reacquire the relevant skill

The strongest gains occur in tasks requiring knowledge of the island’s affordances. Experienced agents place an average of 10.3 objects on the Spire compared with 3.5 for inexperienced agents. For five-block towers, 62% of experienced agents build more than two towers, compared with 15% of inexperienced agents, and the maximum rises from 6 to 16.

The tallest-tower task is an important negative or ambiguous result. The two groups perform similarly, and two experienced agents build towers only four blocks high—below the shortest tower built by any inexperienced agent. The authors correctly note that the one-hour task may permit rapid reacquisition, but the result also demonstrates that persistent experience is not uniformly beneficial.

Memory interventions establish selective causality

To test whether transfer is carried by written knowledge rather than merely correlated with prior activity, the authors erase targeted entries from selected agents’ semantic and episodic memories. Removing Agent 2’s knowledge of rock-shattering reduces its average number of five-block towers from six to two while leaving the other evaluation scores unchanged. Since the initial world contains only twelve blocks—enough for two five-block towers—this intervention selectively removes the ability to manufacture additional construction material.

The reverse intervention removes a false belief. Agent 7’s memory incorrectly claims that blocks cannot be stacked reliably with Place and should instead be thrown. Erasing this belief significantly improves its tallest-tower score. Thus, externalized memory acts as a double-edged learning substrate: it stores useful procedures but can also preserve systematic errors.

A third intervention removes the jump-before-throw technique from Agent 1’s memory. The intervention slows the first successful throw onto the Spire from approximately 60 seconds to 19 minutes, while increasing performance on object placement, because the agent falls back to a slower but more reliable placement strategy. This selectivity is stronger evidence than a general performance change: the edited memory changes the capabilities predicted by the erased knowledge.

Figure 6: Targeted memory deletion selectively removes useful techniques, exposes harmful beliefs, and changes downstream task behavior.

Discovery of an unanticipated physical affordance

The paper highlights a discovery that the environment designers did not know the simulator afforded. Six Opus agents independently discover that throwing a block while jumping transfers part of the agent’s body velocity to the object. An upward throw during ascent therefore reaches substantially greater height; a throw during descent is weakened by downward velocity.

One detailed episode begins with a discrepancy between a predicted and observed trajectory. A standing throw reaches approximately $y=19.55$, while an airborne throw reaches approximately $y=32.24$ against a prediction near $y=26$. Eko proposes velocity inheritance, tests the prediction by throwing after the jump apex, and observes a peak of approximately $y=16.27$, below the standing baseline. It then performs a timing sweep and records reusable guidance in semantic memory.

The simulator code confirms the qualitative explanation: object velocity combines directed throw velocity, a fixed upward impulse, and a fraction of the agent’s body velocity. Eko does not recover the exact coefficient, and it overstates the optimality of the earliest tested release delay. The episode therefore demonstrates useful causal abstraction without implying reliable scientific methodology.

Figure 7: Eko moves from an anomalous observation to a falsifiable hypothesis, controlled timing variation, memory update, and later reuse.

The implication is specific and significant: an agent can discover an affordance absent from its prompt and unknown to the environment designer, provided that it is allowed to explore, compare outcomes, and preserve the resulting explanation. This differs from benchmark task completion because the relevant behavior was not directly elicited by an assigned objective.

Relationship to prior open-ended learning systems

The paper positions Eko against intrinsic-motivation methods, autotelic goal exploration, open-ended environment generation, and LLM agents with explicit task-generation mechanisms. Earlier systems typically optimize an engineered signal such as prediction error, novelty, learning progress, fitness, reward, or an executable verifier. Voyager, for example, stores reusable skills in an embodied Minecraft setting, but its exploration is organized through prompted automatic curricula and task-oriented skill acquisition (Wang et al., 2023). Other systems generate goals or environments through explicit mechanisms (Faldor et al., 2024, Zhao et al., 6 May 2025).

Eko differs in the authors’ framing because it is not explicitly instructed to generate tasks, produce experiments, create art, or play. The environment supplies affordances but not a task distribution, reward, or verifier. Its learning is also implemented through editable text rather than weight updates. This makes the knowledge interpretable and directly revisable: deleting a written technique can remove the corresponding downstream behavior.

The comparison should not be overstated. Eko’s system prompt does encode curiosity and boredom; the workspace instructions explicitly direct the agent to maintain memory; and the automatic continuation loop supplies persistence. The agent therefore receives a developmental scaffold even though it receives no island-specific objective. The contribution is best understood as demonstrating open-ended activity in a scaffolded LLM-agent architecture, not as showing that unconstrained pretrained models independently develop play.

Limitations and open questions

The behavioral classification depends heavily on LLM judges. The judges segment transcripts, extract activities, classify memory contents, and score play signatures. Although the prompts require observable embodied evidence and anonymization, the procedure remains vulnerable to linguistic framing, judge priors, and correlated model capabilities. The paper does not provide human-inter-rater validation sufficient to establish that the scores are invariant to evaluator choice.

The experimental world is small, textual, and highly structured. Agents receive complete entity dictionaries, exact geometry, object names, and a compact action API. They also operate in a sandbox with no physical risk and with hourly restoration of the world. The findings therefore leave open whether the same behavioral signatures would arise under partial observability, visual perception, continuous motor control, irreversible consequences, or richer social interaction.

The learning comparison isolates memory content only approximately. Thirty-hour agents have different written histories, and memory editing may alter not only factual knowledge but also the context, wording, and salience of adjacent entries. The intervention sample is small: selected agents undergo five repeated one-hour evaluations, so the reported Mann–Whitney tests quantify repeated evaluations of a few memory histories rather than broad independent samples.

A further unresolved issue concerns mechanism. The paper documents behavior but does not determine whether play-like activity arises from intrinsic motivation, pretrained cultural scripts, next-token continuation dynamics, curiosity-like heuristics, reward-model preferences, or interactions among these factors. The absence of an external reward does not imply the absence of internal optimization pressures in the pretrained or instruction-tuned model.

Finally, memory accumulation remains uncontrolled. The files grow roughly linearly and preserve false beliefs. The paper shows that memory can be edited, but it does not establish a principled consolidation, uncertainty calibration, contradiction resolution, or forgetting mechanism. The open technical question is how an agent can preserve exploratory discoveries without entrenching unsupported explanations.

Conclusion

“Is this machine playing?” presents a controlled study of an embodied coding-agent system operating without an assigned island task (2610.07130). Across thirteen Claude runs, Eko develops divergent repertoires involving construction, exploration, experimentation, sports, symbolic reinterpretation, and self-imposed challenges. These behaviors satisfy four operational signatures of play, while the agents’ performance on held-out tasks shows that self-directed activity can produce transferable skills.

The most consequential result is the combination of behavioral and causal evidence: useful discoveries are written into persistent memory, targeted deletion removes corresponding capabilities, and deletion of false beliefs can improve performance. The paper therefore characterizes play not as a claim about machine experience, but as a mode of embodied exploration that can organize activity and generate revisable knowledge. Its principal unresolved problem is equally concrete: how to make this externalized, self-directed learning reliable when the same memory mechanism preserves both valid discoveries and persistent errors.