Capability retention under fine-tuning

Every specialist we train risks eroding the base model’s general intelligence. We have direct, measured evidence (tool-calling-frontier-parity.md §8.4–8.5): a file-ops tool-calling distill that hit 100% on its domain dropped out-of-domain agentic from 60→42% and single-turn from 87→83%; the multi-backend gold-distill dropped breadth to 31%. As we add more specialists (tool-callers, founder-voice / style LoRAs), “how much intelligence did we keep?” must be a first-class metric, not an afterthought.

Two halves

1. Measure retention — a regression suite for intelligence

A fixed “general capability” battery run before and after every fine-tune, reporting a retention delta per axis:

• general reasoning (GSM8K slice + a few MMLU-style items),
• instruction-following,
• other agentic backends (the breadth gate — already built),
• single-turn tool-calling (BFCL — already built),
• coherence/fluency (perplexity on held-out generic text).

Output: a retention scorecard, e.g. “tool-caller: +42 file-ops / −18 breadth / −4 single-turn / −X reasoning.” Make it a gate — a specialist that craters general capability is flagged, not shipped blind. Half the battery already exists (breadth gate + single-turn BFCL).

2. Preserve retention — techniques to try

• Data mixing / replay — blend a slice of general / other-domain data into the specialist SFT (the direct fix for the negative transfer we measured).
• LoRA hygiene — lower rank, fewer epochs, lower LR, fewer target layers (we saw val loss → 0.01 = overfit; less is more).
• KL-to-reference (the GRPO lever) — bound drift from the base.
• Model merging — TIES-merge (shipped) a specialist back toward the base, or merge multiple specialists, to recover breadth.
• Routing instead of merging — keep the specialist narrow and route to it only on its domain (the “stock 4B is the best generalist” finding makes this attractive).

Why now / connection

This is the dual of the self-improving loop (self-improving-agents.md): self-improvement adds capability; retention measures what specialization subtracts. The auto-curriculum (training across the whole distribution) is itself a retention strategy.

Acceptance (when picked up)

• A retention-battery that scores a model on N general axes + emits a before/after scorecard.
• Run it on the existing distilled specialists (quantify what we already lost).
• Demonstrate data-mixing + LoRA-hygiene recovering breadth without losing depth.

References: catastrophic forgetting (McCloskey & Cohen 1989; the modern LLM version); replay / rehearsal; TIES-merge (Yadav et al. 2023); journey §8.4–8.5; the breadth gate; the self-improving PRD.