Model Migration Planner

Model Migration Planner

Every few months a better/cheaper/faster model ships and every AI feature you run becomes a quiet question mark. Migration is where teams pay for eval debt: if you can't measure the feature, you can't migrate it — you can only gamble on it. This skill turns "the new model is out!" into a decision with evidence.

The prior to state up front: a model upgrade is a full change to your system's core dependency, wearing a minor-version costume. Prompts are tuned to a model's quirks; downstream parsers are tuned to its output shapes; users are tuned to its behavior. "Better on benchmarks" tells you almost nothing about your feature.

Step 0: Should you even migrate?

Three legitimate triggers — name which one this is, because it sets the deadline and the bar:

• Deprecation (forced): the question is only how and by when. Get the sunset date in writing; work back from it.
• Capability/quality pull: the new model might fix a known weakness. The bar: measurable improvement on the cases you currently fail, no regression elsewhere.
• Cost/latency pull: same quality, cheaper or faster. The bar: statistical parity on quality, with the savings quantified against migration cost (engineer-weeks are real money too).

"It's newer" is not a trigger. If there's no deprecation date and no pull, log a decision-log entry saying why not yet and stop here — declining a migration deliberately is a valid output of this skill.

Step 1: Inventory the blast radius

Most teams underestimate what's coupled to the model version. Enumerate:

• Call sites & pinned versions — every place the model id appears (code, config, env). Unpinned "latest" references are a finding in themselves.
• Prompts — system prompts and templates tuned to the old model's behavior (formatting quirks, instruction-following patterns, tone). These are the #1 silent breakage source.
• Output contracts — parsers, JSON schemas, downstream regex, UI assumptions about length/format.
• Guardrails and thresholds — confidence cutoffs, refusal handling, token/cost budgets calibrated to old-model behavior.
• Past decisions — search ~/bettersense-work-reflections/decision-log entries (and any ADRs the user has) mentioning the old model. Decisions made because of old-model behavior ("we chose few-shot because zero-shot was weak") may now be reversible — each is both a risk and an opportunity. This is exactly the payoff decision-log was built for.

Step 2: Eval before belief

• Golden set exists? Run it on the new model, unchanged prompts first. That's the honest baseline of "what migration costs for free."
• No golden set? Stop and say so plainly: the migration is unmeasurable and step one is a minimal eval (hand off to the-eval-designer — even 30 curated cases beats vibes).
• Compare per-case, not just aggregate: a new model that fixes 10 cases and breaks 8 different ones looks like "+2" in aggregate and feels like a regression to the 8 affected users. Categorize the flips.
• Then, and only then, re-tune prompts for the new model on the failing cases — and re-run the full set after every prompt change (fixing case 12 by breaking case 40 is the classic).
• Re-baseline cost and latency on real traffic shapes: tokenizer changes and different verbosity mean per-request economics move even when the price sheet looks better.

Step 3: Stage the rollout

• Shadow (run both, compare, serve old) → canary (5–10% of traffic, watch quality signals and complaint channels) → ramp — with an explicit gate at each step.
• Rollback criteria written before the canary starts: which metric, what threshold, who flips it back. If rollback isn't cheap (old model already sunset?), say so — that changes how conservative the ramp must be.
• Decide the comms: whether users are told, and what support/CS should watch for. A model migration is a product change; the-explainer and the-gtm-planner own the messaging when the behavior shift is user-visible.

Step 4: Close the loop

• decision-log entry: which model, why, what the eval showed, the rollback trigger, and a revisit date (the next model release, at the latest).
• Fold every case that flipped during migration into the golden set — this migration's surprises are the next migration's regression tests.
• Update the inventory from Step 1 (it's the starter kit for next time; there is always a next time).

Anti-patterns

• Benchmark transference. "It's better on SWE-bench" ≠ better at summarizing your contracts. Only your eval counts.
• Prompt archaeology skipped. Migrating prompts verbatim and calling the diff "model regression" — half of it is usually prompt-model mismatch, which is fixable.
• The big-bang cutover. 100% traffic on day one because "the evals passed." Evals are a sample; canaries exist because samples lie.
• Migration as a side quest. "Just swap the string" assigned to whoever's free. The inventory in Step 1 is the argument for treating it as a real project.

Composition

• the-eval-designer — builds the golden set this skill depends on
• decision-log — both input (old decisions to revisit) and output (the migration decision)
• ai-pm-frameworks — model-selection criteria when the migration is also a provider question
• the-vendor-evaluator — when deprecation forces a provider comparison, not just a version bump
• the-incident-responder — if a migration ships and quality craters, that's an incident; this skill's rollback criteria are its first input
• the-translator — explaining the migration (and its risks) to executives