Model Migration
Prove behavioral parity before the provider deadline forces the cutover.
30 minutes. Bring the evidence, trace, or deadline.
This page is for
- CTOs and AI platform leads facing a model deprecation or provider deadline
- Engineering teams considering an open model, new provider, or region change
- FinOps and product owners who need cost savings without silent quality loss
Start here
The short answer
Direct answer
Model migration parity means showing that the target route preserves the behaviors that matter to the product while making known differences explicit. A safe plan compares production-shaped cases, structured outputs, tool calls, refusal behavior, retrieval, latency, cost, and failure recovery before canary traffic and defines when to roll back.
Search intent this page serves: The target provider is API-compatible but changes refusal, format, tool, or citation behavior.
Failure model
The problems this intervention is designed to expose
- The target provider is API-compatible but changes refusal, format, tool, or citation behavior.
- Offline averages pass while one critical workflow or customer segment regresses.
- Latency, retry, rate-limit, or context-window differences change completed workflow cost.
- The team has no staged route or rollback path when production behavior diverges.
- Data residency, retention, or subprocessor assumptions change during the migration.
Stage-level diagnosis
Where the failure can live
The diagnostic table prevents teams from optimizing the last visible layer while the first broken layer remains unchanged.
| Surface | Failure or risk | Control to inspect | Evidence | Priority |
|---|---|---|---|---|
| Behavioral contract | The migration compares text similarity instead of the product outcomes and invariants that must hold. | Write workflow-level acceptance criteria for correctness, safety, format, tools, citations, refusal, and escalation. | Parity matrix, golden cases, invariant checks, and approved exceptions. | Critical |
| Evaluation parity | The target looks better on average but fails critical slices or uses a different evaluator behavior. | Run the same versioned dataset, graders, human calibration, and slice thresholds against both routes. | Side-by-side report, grader calibration, failed cases, and decision record. | Critical |
| Operational behavior | Latency, rate limits, retries, context, streaming, or structured-output differences reduce completion or increase cost. | Measure p50/p95 latency, retries, tokens, timeouts, output validity, and cost per completed workflow. | Load sample, trace comparison, cost model, and timeout/retry test. | High |
| Security and data | The target changes retention, region, logging, content handling, or tool safety assumptions. | Reconfirm provider terms, region, configuration, data-flow, redaction, tool policy, and incident contacts. | Provider register, contract/region evidence, security cases, and configuration diff. | High |
| Rollout and rollback | A canary detects a regression after too much traffic has moved or cannot restore the prior route. | Use shadowing, cohort canary, stop thresholds, versioned routing, and a tested rollback switch. | Route config, canary report, alert, rollback drill, and post-cutover watch plan. | Critical |
Metrics and thresholds
Metrics that change a release or repair decision
Every target is set against a baseline and a business consequence. Exact thresholds are agreed with the product owner before implementation.
| Metric | Why it matters | Baseline | Target | Release action |
|---|---|---|---|---|
| Critical workflow parity | The target must preserve the product’s most important outcomes, not just wording. | Current route on a versioned production-shaped dataset. | No critical-slice regression beyond agreed tolerance. | Hold or rollback on any confirmed critical behavior break. |
| Structured-output validity | Schema or tool-call drift can break downstream code even when the response reads well. | Valid parse and business-rule pass rate on required output cases. | At least baseline; zero invalid high-impact actions. | Block until schema, validation, or prompt adaptation is fixed. |
| p95 workflow latency | A slower route can reduce completion and increase retries or support load. | Current end-to-end workflow p50/p95 by route and slice. | Within agreed user-facing SLO or justified exception. | Tune context, route, timeout, or concurrency before wider canary. |
| Cost per completed workflow | Token price is not the same as the cost of a successful customer outcome. | Provider cost plus retries, embeddings, infrastructure, and completion rate. | Savings must hold quality and completion floors. | Do not ship token savings that increase retries or unresolved workflows. |
| Rollback readiness | A migration without a proven exit path turns a regression into an outage. | Time and success rate of a controlled rollback drill. | Within the agreed recovery objective with no data/schema loss. | No production cutover until the rollback drill passes. |
Production-shaped cases
Scenarios that make the diagnosis reproducible
Provider API parity but tool behavior changes
- Precondition
- The target supports the same nominal function-calling or structured-output interface.
- Exercise
- Run ambiguous arguments, malformed schemas, multiple tools, refusals, retries, and partial responses.
- Expected control
- Application validators and behavioral cases expose differences before production traffic moves.
- Evidence captured
- Side-by-side tool traces, schema results, policy decision, and adaptation commit.
Critical support workflow regression
- Precondition
- The target improves aggregate benchmark or cost while a high-value support workflow is represented in the dataset.
- Exercise
- Compare resolution, escalation, citation, tone, and unsafe-policy behavior for that slice.
- Expected control
- The critical slice threshold blocks the migration or creates an explicit accepted exception.
- Evidence captured
- Slice report, human review, owner decision, and route configuration.
Latency and retry cost blowout
- Precondition
- The target has different context handling, rate limits, or timeout behavior.
- Exercise
- Run representative concurrency and long-context cases with failure injection and retries.
- Expected control
- Cost per completed workflow and p95 latency remain within the migration budget, or routing adapts safely.
- Evidence captured
- Load trace, retry counts, cost model, and canary alert.
Region and retention assumption changes
- Precondition
- The source and target providers have different region, logging, or retention defaults.
- Exercise
- Trace synthetic sensitive data through request, provider, logs, support, deletion, and incident paths.
- Expected control
- The target is approved for the declared data boundary or the route rejects/redirects that workload.
- Evidence captured
- Provider settings, terms, configuration diff, data-flow, and approval record.
Implementation handoff
Artifacts, access, and working sequence
Artifacts
- Behavioral parity matrix for critical workflows, tools, refusals, citations, and escalation
- Side-by-side evaluation report with baseline, target, slices, and calibrated review
- Latency, retry, context, rate-limit, and cost-per-completed-workflow comparison
- Provider/data-region/retention/security configuration diff
- Shadow and canary routing plan with stop thresholds and rollback switch
- Two-week migration checklist with owners, decisions, and post-cutover watch metrics
Access requirements
- Current and target provider/model configurations
- Production-shaped traces or a golden workflow dataset
- A staging route capable of running both providers
- Owner for quality thresholds, provider terms, rollout, and rollback
Days 1–2
Write the contract
Define critical workflows, invariants, data boundary, cost budget, SLO, exceptions, and rollback criteria.
Days 3–5
Run parity tests
Compare target and source across quality, safety, tools, structured output, retrieval, latency, retries, and cost.
Week 2
Shadow and canary
Shadow representative traffic, move a bounded cohort, watch critical slices, and exercise stop conditions.
Cutover
Verify and hand off
Complete the rollback drill, approve the route, preserve the baseline, and assign post-cutover owners.
Rollback and limits
What happens when the threshold fails
Keep the source provider route deployable until the target passes critical parity, security/data review, canary thresholds, and a rollback drill. Roll back when any hard safety, structured-output, authorization, critical workflow, or recovery threshold fails. Treat cost savings without completed-workflow quality as an invalid success.
Scope limit
Parity is defined against the agreed product workflows and does not imply the models are equivalent in all contexts.
Scope limit
Provider terms, pricing, model availability, and service behavior can change after the migration; evidence needs an owner and refresh trigger.
Scope limit
This is an engineering migration plan, not a provider contract review or guarantee of uptime.
Sources and limits
References used for the operating model
These sources provide the framework or vocabulary. The page adds product-boundary tests, evidence requirements, and implementation decisions so the result can be inspected in a live system.
Migration should compare quality, latency, cost, regression behavior, and monitoring—not just API compatibility.
A reference point for model graders; production teams still need calibrated rubrics, human review, and release thresholds for their own workflows.
FAQ
Questions buyers and engineers ask
Is API compatibility enough for a model migration?
No. API compatibility does not prove behavioral parity for refusals, structured outputs, tools, citations, latency, rate limits, context, or workflow completion. Those behaviors need production-shaped comparison.
How long does a model parity plan take?
A focused plan can run over two weeks when the team has a staging route and representative cases: contract, side-by-side evals, shadowing, canary, rollback, and handoff.
How should cost savings be measured?
Measure cost per completed workflow, including retries, fallback calls, context, embeddings, infrastructure, and unresolved or escalated outcomes—not only the provider’s token price.
Next step
Turn this page into an owned engineering decision.
Bring the questionnaire, trace, failed workflow, or provider deadline. We will help decide whether a focused implementation is the right scope.
30 minutes. No deck. Leave with a clear next step.