Deterministic workflows and AI reasoning: when to use each

One of the most consequential design decisions in AI operations is also one of the least discussed: when to use a deterministic workflow, and when to use an AI worker.

The difference is not just technical. It reflects a fundamental choice about how a step in your operation should behave under uncertainty — and what it should do when the input does not match expectations.

A deterministic workflow executes a fixed sequence of steps. Given the same inputs, it produces the same outputs — every time, without exception. It does not reason. It does not improvise. It does not hallucinate. If a step fails or an input is missing, it follows a defined error path.

This is exactly what you want for steps that need to be fast, cheap, reliable, and auditable. Credit score pulls. DTI calculations. Compliance rule checks against a known policy set. Data transformations. System record updates. These steps have known inputs, known logic, and known outputs. They do not benefit from AI reasoning — they benefit from reliable execution. Applying AI to them introduces unnecessary latency, cost, and variability.

An AI worker reasons. It reads context, weighs options, applies judgment, uses tools, and produces an output that reflects its understanding of the situation. This is what you want for steps where the right action depends on context that cannot be fully captured in a rule.

Reviewing an unusual applicant profile that does not fit standard underwriting criteria. Drafting a recommendation letter that needs to synthesize information from multiple sources. Triaging an inbound document that does not match a known template. Interpreting a compliance clause that is ambiguous in a specific context. These steps require judgment. A rigid workflow will either fail or produce a wrong answer. An AI worker, with appropriate tools and constraints, can handle them.

Most teams make one of two mistakes. They apply AI reasoning to steps that should be deterministic, adding cost and variability where reliability was needed. Or they apply rigid workflows to steps that require judgment, producing brittle systems that fail on edge cases and require constant human intervention.

Both are expensive. AI reasoning applied to a rule-based calculation costs more per run, takes longer, and introduces the possibility of wrong answers. A deterministic workflow applied to a judgment-heavy step requires human escalation every time the input deviates from the template — which, in practice, is often.

The practical heuristic is simple: if you could write the step as a function — if the logic is fully specifiable in advance, with defined inputs and outputs — use a workflow. If the right answer depends on context that varies in ways you cannot enumerate ahead of time, use a worker.

Most real operations are neither fully deterministic nor fully judgment-dependent. A mortgage prequalification operation might pull a credit report deterministically, calculate DTI with a fixed formula, run compliance rules against a known policy set — and then use an AI worker to synthesize those results into a recommendation for a human reviewer. The first steps are workflows. The last step is a worker. Both are necessary.

Miji is built to run both in the same operation. The workflow carries the deterministic steps — conditions, loops, parallel branches, exact decimal arithmetic rather than a model doing sums — and an AI step sits inside it wherever judgment helps, with the model you choose and its output on the same record. Versioned rules and approval gates decide what happens next. Choosing which kind of step goes where is one of the first things we work through with a team designing an operation, and it is one of the most important decisions for getting both cost and quality right.