Strict Function-Call Coercer

Models emit arguments that are nearly right: "true" instead of a boolean, "5" instead of 5, an enum with different casing or a trailing space, a single object where an array of one was declared. Most code either passes these straight through -- so a handler runs on the string "false", which is truthy -- or uses a permissive coercer that accepts anything, so a typo’d enum silently becomes the first valid option. Both produce a handler that runs and returns something plausible.

ENUMS ARE STRICT BY DEFAULT. An unrecognised value is an error, never mapped to the nearest match or to enum[0]. Case-insensitive and whitespace-trimmed matching are explicit opt-ins, off by default, and even then never fuzzy. Silently mapping a typo to the first valid option is how a handler runs the wrong branch entirely.

COERCION HAPPENS ONLY WHERE THE INTENT IS UNAMBIGUOUS. "5" becomes 5 for a declared number; a lone value becomes [value] for a declared array. "false", "0" and "" never become truthy booleans -- that is the classic one. null, undefined and missing are three different things for an optional field and are treated distinctly.

UNIONS RESOLVE ONLY WHEN EXACTLY ONE MEMBER MATCHES. Ambiguity is an error, not a coin flip.

It returns a discriminated result with a path to each offending field, and never throws for ordinary validation failure -- an agent needs to read the error and retry, not catch an exception.

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Interface

What you call, and what comes back. Types and signatures only — the implementation ships with the source.

  export function coerceValue(schema: JSONSchema, value: unknown, path: string, options: CoerceOptions, errors: FieldError[]): unknown;
  export function coerce<T = unknown>(schema: JSONSchema, value: unknown, options: CoerceOptions = {}): CoerceResult<T>;

  export type CoerceResult<T = unknown> = { ok: true;