ADR 0001: Implement wisp in Swift, linking FoundationModels directly

Date: 2026-09-17. Status: accepted.

Context

The project brief allows any language. The requirement is a tool-using harness over the on-device Apple model. Two integration routes exist: wrap the fm CLI or its fm serve HTTP API from any language, or link FoundationModels.framework from Swift.

Verified on macOS 27.0 with Xcode 27.0 (see fm-cli.md):

• fm respond/fm chat accept --tool only for built-ins (barcode, ocr).
• fm serve ignores the tools parameter and never returns tool_calls.
• A Swift probe using LanguageModelSession(tools:) with a Tool whose Arguments are @Generable had the model call the tool and use its result.

Decision

Swift, linking FoundationModels directly. The framework’s Tool protocol and @Generable macro are the only supported path to user-defined tool calling.

Consequences

• The project is macOS-only, which the objective already required.
• Xcode (not just the Command Line Tools) is required to build, because the FoundationModelsMacros plugin ships only with Xcode.
• A hand-rolled tool loop over fm respond --schema was rejected: it would re-implement what the framework already does, with worse reliability.