Critical route
A critical route is the minimum chain of cause-and-effect steps needed to get a user from a specific starting situation to a specific finished state. Each step produces something a later step needs; if removing a step wouldn't break the chain, it doesn't belong on the critical route. Picking up a whisk while making pancakes doesn't belong, since a fork would do just as well; but "the batter is mixed" does, because nothing downstream produces pancakes without it.
Definition
The causal-dependency map between committed starting conditions and committed completion criteria — what any route bridging those endpoints must satisfy.
Two commitments produce the critical route
The critical route is the causal sequence that connects a given starting state (the user's present state the path will accept) to completion criteria (the conditions of the resulting state that must hold for the transition to count as complete). Given a specific pair of starting conditions and completion criteria, the critical route is what the causal terrain between them entails.
Underspecifying starting conditions widens the served cohort but lengthens the critical route — more entry territory means more causal ground the path must cover. Narrowing starting conditions shortens the critical route by serving a tighter cohort. The trade-off is inclusivity against coverage. The common failure mode is widening entry without extending the path, producing a bridge to nowhere: users enter at conditions the path's first steps don't accept.
Construction works backward from completion criteria
For each completion criterion, ask what causes it. For each cause, trace back through prior causes until reaching the starting conditions. Each cause is a sub-transition that produces its target; some causes require prior causes; those form a dependency chain. The critical route is the union of all such dependency chains required to satisfy all completion criteria from the committed starting conditions.
The causal-minimum standard
The critical route includes what's causally required given the committed endpoints. Sub-activity that's descriptively valid but not causally needed sits outside it. Picking up a whisk is part of making pancakes descriptively, but "pancakes exist" doesn't require "whisk was used" — a fork would work, given starting conditions where some combining instrument is needed at all.
What sits outside the critical route falls into two categories the framework treats differently. Auxiliary routes are commonly-present user activity that's in-scope for the parent transition but not causally required — reading reviews while buying a car, using a meat thermometer while grilling. Provider design additions — scaffolding, examples, reassurance — sit on top of the structural minimum plus auxiliary-route coverage. Evaluating either category requires the critical route to be settled first.
Singular per endpoint pair
Given a specific pair of starting conditions and completion criteria, there is one critical route. Apparent route plurality between the same endpoints reduces to specification: either the endpoint conditions weren't actually identical, or differing route steps' impacts on sub-conditions weren't accounted for. Route plurality lives at coarser grain — when starting conditions differ, the critical routes differ.
Example
Take a self-serve invoicing tool and the transition "first invoice sent to a real client," with starting conditions of a signed-up user who has not yet billed anyone. Completion criteria might include: an invoice exists, it has line items with amounts, it's addressed to a client, and it was delivered to that client.
Working backward: "delivered to client" requires a send action, which requires a deliverable invoice and a destination. The destination requires the client to be in the system. The deliverable invoice requires line items, which require an invoice to exist to attach them to. Invoice existence requires invoice creation.
The critical route is the union of those chains: client added → invoice created → line items added → invoice sent. Configuring a logo, exploring report views, or reading the help docs are descriptively common but causally absent. A team treating logo configuration as part of the critical route imports a false dependency. A team treating client-addition as optional ships a path that misses a structural requirement.