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7. Fallbacks and evaluation

The goal

Fail safely when a decision provider cannot answer, and verify the behavior without presenting a mock as live-provider evidence.

Each decision boundary owns a fallback

Each decision node overrides onDecisionError(context). EZGraph calls it after a provider error or timeout that outlasts the graph’s maxRetries: 2, or immediately after an answer that fails validation. Invalid answers are never retried.

  • RouterDecisionNode shows a pending notice if there is one. Otherwise it prompts for the next missing criterion, or says the saved criteria are ready and invites “search”.
  • CriteriaReadinessDecisionNode uses deterministic validation: it sends the first issue to its owner, or proceeds to SearchHotelsNode when there are none.
  • PresentationDecisionNode returns the deterministic rendering of the saved hotel list.

These are different fallbacks because the three decision boundaries have different responsibilities. Missing credentials, a missing @langchain/typesafe package, and authentication failures are configuration errors, not outages. They fail the turn instead of falling back.

Test the full journey

test/decision-hotel-graph/decision-hotel-graph.spec.ts drives 23 real GraphEngine turns through createTurnHarness(). Model replies come from a scripted gateway. Decisions come from an inline DecisionProviderAdapter that derives answers from the request it receives. The suite covers:

  • an unrelated request, impossible and past dates, an inverted budget, and a negative distance
  • a cross-step date correction and criteria review
  • a readiness judge that sends the customer back to confirm the budget
  • a rejected, invented presentation, empty results, and repeated revisions
  • validated booking

It also simulates a decision-provider outage at each of the three decision nodes. hotel-search.spec.ts checks the catalog search on its own.

npm run test:decision-hotel-graph
npm run test2:decision-hotel-graph

The live script sets USE_ENV=1 KEEP_SESSION=1, requires TYPESAFE_API_KEY and OPENAI_API_KEY, and semantically judges the sixteen-turn scenario turn by turn. It is separate from deterministic evidence; a skipped or failed live run must not be described as a provider success.

Next

Return to the DecisionHotelGraph tutorial overview or learn how a quote uses the same boundaries in QuoteGraph.