Branch main Algo fallback selection

Algorithm: Fallback Selection

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Field	Value
Branch	main
Source	specs/algorithms/fallback_selection.pseudo.md
Commit	5b2aac39fcf8
Synced (UTC)	2026-06-04T06:01:36Z

Visual Context

Algorithm pages define deterministic platform-neutral behavior that downstream repos implement behind native adapters.

flowchart LR
  Input["Input contract"] --> Preconditions["Preconditions"]
  Preconditions --> Decision["Deterministic decision"]
  Decision --> Output["Output contract"]
  Decision --> Diagnostic["Diagnostic envelope"]
  Diagnostic --> Audit["Audit chain"]

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Reading Checklist

Question	Where to look
What is deterministic?	Read the pseudocode branch table and preconditions.
What blocks unsafe behavior?	Look for blocked, ambiguous, policy, or precondition paths.
How is the decision reconstructable?	Trace diagnostic and audit requirements.

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Source Content

Fallback Selection

Fallback is registry-driven and policy-constrained.

FUNCTION select_fallback(registry, diagnostic, policy) -> FallbackDecision
  candidates = registry.candidates ordered by registry priority
  FOR candidate IN candidates:
    IF policy allows candidate and candidate validates against ASH state semantics:
      RETURN SELECTED with diagnosticReference
  RETURN UNAVAILABLE with escalation diagnostic
END FUNCTION