Handlers and aliases are lambdas; the calling convention is fixed by surface form

A handler or alias body is applied, never merely forced, so its arity is part of its contract. Every per-name handler (within [handlers: [name: …]]) and every alias must be a unary lambda { |args| … }, and every catch-all (within [handler: …]) must be a binary lambda { |name args| … }. The calling convention is chosen by the surface position — per-name/alias versus catch-all — not inferred from the value’s runtime shape, and arity is validated at install time: a bare block { … }, a non-lambda value, or a lambda of the wrong arity is rejected with a clear error.

Why arity cannot be inferred from the value

A per-name handler is invoked with the command’s argument list; a catch-all is invoked with the name and the argument list. Currying (cbpv, docs/SPEC.md §4.5) makes the two indistinguishable by shape after the fact: a binary lambda applied to a single argument does not error — it returns the inner unary closure. So a binary lambda installed where a unary handler is expected would not fault; it would hand the body a partial-application closure as the intercepted command’s result — a silent wrong answer, exactly the structural-bug-prevention failure mode where a category error survives as a plausible-looking value. Pinning the convention to the surface form and checking arity at install time makes that bad state unconstructable rather than a runtime surprise.

What changes

• Nullary is removed. A bare block { … } previously installed as a zero-argument handler, treated as nullary at dispatch. That form is gone: a bare block in handler, catch-all, or alias position is now an install-time error. The only valid handler/alias values are unary and binary lambdas.
• The convention is positional. The per-name slot demands unary; the catch-all slot demands binary; the alias NAME { |args| … } builtin and the rc/plugin aliases: maps (docs/SPEC.md §9) demand unary. Dispatch (handler-dispatch) invokes the matched entry under the convention its install site fixed, never by inspecting the stored value.
• Arity is checked once, at install. within-frame installation, install_alias, and the rc/plugin alias loaders all reject a malformed value before it can reach lookup, so no wrong-arity entry is ever dispatched.

The hard rule

A per-name handler and every alias is { |args| … }; a catch-all is { |name args| … }. Anything else — a bare block, a non-lambda, a unary catch-all, a binary per-name handler — is rejected where it is installed, with a diagnostic naming the expected arity. The convention is read off the surface form, never off the value.

See also effects-handlers, handler-dispatch, structural-bug-prevention, alias-head-defines-its-modes; the surface forms are docs/SPEC.md §3.2 (handlers) and §9 (aliases).