One Dispatch Mechanism
The YAML orchestration language had two separate dispatch systems. match/cases used MatchPattern — a sealed interface with ValuePattern, StructuralPattern, and DefaultPattern — for step-level branching. State machine transitions used direct enum identity comparison inside EventRouter. Same concept, different implementations.
The regularity principle says: learn one mechanism, apply it everywhere. If pattern matching is how you dispatch in step-level branching, it should be how you dispatch in state machine transitions too. The shapes are identical — value equality, structural subset matching, catch-all defaults. The only thing missing was AnyOfPattern for the from: [PENDING, ACTIVE] case, where a transition applies from multiple source states.
Adding AnyOfPattern to the sealed interface was trivial — four lines of matches() logic wrapping a list iteration. The interesting part was threading it through EventRouter. The old design keyed mappings by event string in a Map<String, List<EventMapping>> for O(1) lookup. The new design uses a flat list with pattern matching on both the event (on) and the source state (from). For state machines with a handful of transitions this is fine; the EventRouter matches current.name() against fromPattern so the YAML layer can work with strings throughout.
The Builder stays backward-compatible — the existing on(String event, S from, S to) methods create ValuePattern instances internally. The new overloads take MatchPattern directly, and registerTransitionsForPattern() expands patterns into the state machine’s internal transition map so the CAS validation still works. AnyOfPattern resolves each value to an enum constant; DefaultPattern registers from every non-terminal state.
The session also landed an ADR formalising the three-layer evaluation model — imperative (if, match), reactive (when), and a future rules engine with its own distinct keywords. The when → if rename from the previous session established this boundary; the ADR records it as a standing constraint so future language extensions can’t accidentally reuse keywords across layers.
Four hardening issues closed alongside: StepWalker now has a depth limit (32 levels, with nesting path in error messages), DefaultProcessExecutor enforces maxOutputBytes with a bounded read loop instead of readAllBytes(), five duplicate parseTimeout methods collapsed to DurationParser.parseOrNull(), and McpStepCatalogWiring dropped its reflection-based dispatch in favour of a ToolDispatcher interface in platform-api.
The queue still has four items — runtime evaluators for structural step types and decorators (#465-#466), then try/catch/finally and CSP select (#463-#464). Those are M-to-L scale and need their own design sessions. The groundwork laid today — MatchPattern shapes flowing through EventRouter, the depth-limited StepWalker, the evaluation model vocabulary contract — gives them a solid foundation to build on.