LLM-BabyBench: Can Language Models Plan in Worlds They Can Simulate?

When an interactive benchmark reports a single success rate for a language-model agent, it is rarely clear what that number measures. A failure can come from perception, ambiguous instructions, retrieval, missing commonsense about what actions do, an incorrect model of the dynamics, or planning, and an aggregate score does not separate them. LLM-BabyBench recasts the procedurally generated BabyAI gridworld as a fully observable, purely textual environment in which every source of failure but planning is removed by construction. The whole grid is serialised into the prompt, instructions come from a small formal grammar, every object's coordinate is stated, the six actions and their effects are specified, and a deterministic expert validates each answer by executing it rather than judging it. On this substrate we define the PPD suite: Predict asks for the state that follows an action sequence, Plan for an action sequence that reaches a goal, and Decompose for a subgoal sequence that achieves a mission, scored by three assistance-aware metrics that separate understanding a mission from sequencing it. Across seven frontier and open models, simulation is far ahead of planning for every model, near saturation for the strongest and well short of it for the weakest, and the length of the required solution, not grid size or obstacle count, governs planning failure. Each model has a characteristic horizon beyond which single-attempt success collapses. Where enough instances are solved to support the ratio, returned plans stay near-optimal. Models that write out their working show why: they commit to one family of corridor-shaped route and verify it with no means of backing out, so what they return is either near-optimal or invalid. The same pattern holds one level up: decomposition precision falls to zero on long missions even where comprehension persists.