Langgraph Decision Models

Converting an existing agent to route on a decision model

A six-step audit for finding where typed classification replaces a generative call, and for proving it was worth doing. The framing of steps 1–2 is adapted from jevify, a prompt for investigating what a decision model makes possible in an existing project.

Keep exploration separate from production changes. Do not edit the agent until step 6.

1. Understand the project as it is

Read the real code, not the README. For each model call, record: what it costs, what it returns, and what the caller actually uses from the return.

Tie every finding to a specific file and line. Look for:

• Generate-then-parse. A completion immediately fed to json.loads, a regex, or .startswith(...). The prose was never the product.

• Prompted classifiers. Prompts containing "respond with one of", "answer yes or no", "rate 1–5", "reply ONLY with".

• Budget-driven sampling. Code that checks the first N items, or runs a check only on weekends, because doing it always is too expensive.

• Brittle proxies. Keyword lists, regexes, or heuristics standing in for semantic judgment because a model call was too slow or costly.

• Re-reading. The same document sent to a model repeatedly, once per question.

2. Reconsider from first principles

The pivot question: if many semantic judgments were affordable inside your latency budget, what would you design differently?

Sort ideas into three buckets, and give the third real weight:

1. Direct savings — same behavior, lower cost or latency.

2. Better outcomes — same feature, better decisions (judging every item instead of sampling).

3. New capabilities — things currently impossible: reacting mid-interaction, continuously reassessing state, filtering a firehose.

Bucket 3 is where the leverage is. Bucket 1 is the easiest to justify and the least interesting.

3. Make each opportunity concrete

For every candidate, write down:

• The integration point — which node, which file.

• The input state — exactly what gets sent.

• Which primitives apply — Noul, Choice, or Score, and why.

• Which questions share one request versus genuinely depend on a prior answer. Questions in one request are answered independently; if B depends on A's answer, they are two requests and two nodes.

• How plain code consumes the result — the routing function, in full.

• What still needs generation or retrieval. Most workflows keep an LLM for the part that produces text; the decision model only picks which items get there.

Do not bury hard reasoning in a fuzzy question. "Is this code correct?" is not a classification. If a question needs multi-step reasoning to answer, it needs a reasoning model.

4. Test the economics honestly

Estimate the whole workflow, not one call: retries, fallbacks, the escalation path, and the generative work that remains.

• Separate per-request latency from end-to-end. Fan-out changes the second and not the first.

• Compare against the cheaper alternatives you skipped: deterministic code, caching, embeddings, a smaller model. Sometimes a regex really is the right answer.

• If you have no measurements yet, state the assumption and the break-even point rather than a number you cannot defend.

When benchmarking against an LLM baseline, make the baseline fair — native structured output (method="json_schema"), no tool-schema injection, caching enabled on any stable prompt prefix. An unfair baseline makes the result useless for deciding anything.

5. Design an evaluation that can falsify the idea

A decision model returns probabilities. Probabilities are signals whose calibration must be tested on your workload; vendor benchmarks do not transfer.

• Baselines — current behavior, and the cheapest non-model alternative.

• Asymmetric costs — a false negative on a privilege check is not a false positive on a spam check. Weight them.

• Latency distribution — p50 and p99, not the mean.

• Adversarial and ambiguous inputs — items that should land mid-rubric. These are where thresholds get decided.

• Threshold and fallback validation — sweep thresholds against labeled data. Do not hand-pick them from a handful of examples.

• Go/no-go criteria, written before you run it.

langsmith-skills covers dataset construction and evaluators for this step.

6. Deliver a recommendation

• A ranked table of opportunities with expected impact and confidence.

• Detailed designs for the top three.

• A sketch of how the system would look if designed this way from scratch — often different from the incremental patch.

• The smallest decisive experiment: the one change that would settle it.

• Rejected ideas, with reasons. These are as valuable as the accepted ones.

Migration shapes

Prompted classifier → one classifier node. The direct swap. The prompt's enumerated options become Choice.criteria; its rating scale becomes Score.criteria; its yes/no becomes a Noul. Delete the parsing code — that is the point.

Chain of LLM guards → one request, many questions. Sequential guardrail calls that each ask one yes/no collapse into a single request with several Nouls, then a routing function. Only do this where the guards are genuinely independent.

Sampling → full coverage. Where cost forced you to check 1 in 100, check all of them and route the uncertain ones to the expensive path. This usually improves the product, not just the bill.

Keyword rules → semantic questions. Replace regex allowlists with a Noul plus a threshold. Keep the regex as a fast path if it is precise; use the model for the tail it cannot cover.

Rollout

1. Run the decision model in shadow beside existing logic. Log both decisions; change nothing.

2. Compare on real traffic. Disagreements are your evaluation set — label them.

3. Tune thresholds against those labels, not against intuition.

4. Cut over the decisive bands first; keep humans or the LLM on the ambiguous middle.

5. Pin the model id and re-validate thresholds on every model change — calibration shifts between versions.