The v1.3 agent-tooling surface

Five CLI subcommands, one unified diagnostic envelope, and an LSP. Each piece is structured to be machine-consumable first and human-consumable second — the reverse of how most language toolchains evolved.

fastc explain

Emits a JSON document of every fn's public surface: signature, required capabilities, purity / panics annotations, contracts. The v1.3 release adds a modules array listing the project's module graph. This is the stable agent-facing artifact — agent loops should read fastc explain output, not parse compiler stderr.

{
  "module": "myapp",
  "functions": [
    {
      "name": "main",
      "signature": "fn main(caps: Caps) -> i32",
      "caps_required": ["CapFsRead"],
      "purity": "impure",
      "panics": ["E_OOM"],
      "requires": [],
      "ensures": []
    }
  ],
  "modules": ["cli", "log", "json"]
}

Full reference: docs.skelfresearch.com/fastc/cli/explain/

fastc context

Dumps the project's pub type surface — every exported struct, enum, function signature, and constant — as markdown or JSON, optimized for AI context windows. Where explain is exhaustive, context is budget-aware: it deduplicates, abbreviates, and ranks by likely-relevance to a query. Pair with fastc explain when you need the agent to edit the project, not just read it.

Full reference: docs.skelfresearch.com/fastc/cli/context/

fastc diff

Semantic AST-level diff between two snapshots. Reports added, removed, and signature-changed pub items. CI gates consume the JSON output to enforce semver: a removed pub item forces a major bump; an added one forces a minor. fastc diff is the structural answer to "did this commit break the public API".

Full reference: docs.skelfresearch.com/fastc/cli/diff/

fastc fix

Applies the Fixit registry's structured fixes — currently the missing-; insertion — plus the universal mechanical fix (re-run fastc fmt). The Fixit registry is the same mechanism the LSP surfaces as Quick Fix actions; running fastc fix on a project in CI gives you the same edits an editor would.

Full reference: docs.skelfresearch.com/fastc/cli/fix/

fastc mcp

A stdio JSON-RPC 2.0 server exposing the five agent-relevant tools as MCP endpoints: explain, check, caps_summary, context, diff. Wire it into Claude Code or Cursor with a one-line config:

{
  "mcpServers": {
    "fastc": {
      "command": "fastc",
      "args": ["mcp"]
    }
  }
}

Full reference: docs.skelfresearch.com/fastc/cli/mcp/

Unified diagnostic envelope

Every fastC diagnostic — compile error, P10 violation, capability violation, contract violation, discharge failure — serializes through one JSON shape. Editors, CI gates, and agent loops parse one envelope instead of five.

{
  "kind": "compile_error",
  "rule_id": "E_PARSE",
  "severity": "error",
  "span": { "file": "foo.fc", "start": 10, "end": 12 },
  "message": "expected ;",
  "hint": "add a semicolon"
}

Available via --output-format=json on compile, fmt, and check. The rule_id is the stable handle agents use to map diagnostics back to documented rule codes.

LSP capabilities

fastc-lsp ships as part of the v1.3 release. The server advertises:

• Code-actions. Every Fixit in the registry surfaces as a Quick Fix. Saving with auto-fix-on-save applies the same edits fastc fix would.
• Semantic tokens. fastC-specific highlighting that knows about @purity, @panics, and capability parameters. Capability params highlight in the same color across signatures and call sites.
• Workspace rename. Driven by the resolver's symbol table, so renames are correct across modules. The resolver's two-pass design (declare, then resolve) means rename never partially-applies on a file with a syntax error elsewhere.

Editor configuration: docs.skelfresearch.com/fastc/getting-started/editor-setup/