Depth pruning removes whole Transformer blocks to make a language model smaller and faster, but the damage is not spread evenly across what the model can do, and a short fine-tuning run, called healing, decides what comes back. We measure this for small models used as agents.
Four instruction-tuned models of 1.2-1.7B parameters (Llama-3.2-1B, Qwen2.5-1.5B, SmolLM2-1.7B and OLMo-2-1B) are pruned and healed, and six capabilities are tracked: structured output, function calling, instruction following, mathematical reasoning, safety refusal and knowledge.
Three results hold across the families.
- First, function calling and math collapse in every model (retention 0.00-0.07 of the unpruned score), and the order in which capabilities break agrees closely across the four (Kendall's W = 0.92; 0.62 after correcting for chance floors).
- Second, which parameters are trained matters as much as which data: fully tuning the last three blocks instead of adding low-rank adapters to all blocks costs up to 0.18 in capped mean retention on three families, and one 300k-token pilot per scope detects this.
- Third, we test a planner that allocates a 1M-token healing budget from 21 short pilot heals. At a fixed budget it raise