ADR 0003: Model tool calls are Scheme applications

• Status: accepted
• Date: 2026-09-20

Context

SPEC invariant 4: the Scheme interpreter is the only Scheme→Go surface and the guard sits on it. If the loop dispatched model tool calls straight to Go handlers there would be a second surface.

Decision

The loop turns a model tool call {name, input} into the Scheme application (name arg …) — argument order comes from the primitive's declared parameter list — and evaluates it through apply. The guard call site inside apply is therefore the one Scheme→Go exit, crossed identically by the model's calls, the prelude, and the user's .scm files.

Consequences

• A CI lint asserts that Prim.Handler is invoked from exactly one place.
• Tool argument JSON maps to Scheme values through the same json-parse rules the language documents.
• A Scheme-registered tool (register-tool) is indistinguishable from a builtin at the dispatch site.