# EXEC Schemes and Builtins ## The schemes | Scheme | Runs | Reference | |---|---|---| | `http(s)://…` | Your service, over HTTP | [ops](../ops.md) | | `op://NAME` | Sandboxed wasm nano-op on the chassis | [ops](../ops.md), `sdk/op` | | `txco://…` | A chassis builtin (table below) | this page | | `ai://chat` | A chat model via the chassis's AI registry | [ai](../ai.md) | | `mcp+http(s)://…` | A tool on an external MCP server | [mcp](./protocols/mcp.md) | | `/` | Unschemed stage jump (synthesized into `@goto`) | [resonators](../resonators.md) | ## The builtin registry | Builtin | What it does | |---|---| | `txco://noop` | Returns `{}`. Placeholder / structural. | | `txco://static` | Serve static files with layered lookup: the stack's `FILES/` → workspace `FILES/` → embedded defaults. Caps: 1 MiB/file, 2048 files, 64 MiB total. See `examples/quickstart-hello-world` for the rule pattern. | | `txco://read-file` | Read a stack's `FILES/` asset(s) into the document as data (templates, fixtures, config) — the read-into-the-tree counterpart to `static`. See [read-file](./read-file.md). | | `txco://web-render` | Read a source path, optionally render Markdown→HTML, set `@web.res.*`, halt. Pages without a backend. | | `txco://sendmail` | Render + submit outbound email from the `_sendmail` contract — see [sendmail](./protocols/sendmail.md). | | `txco://relay` | Forward an inbound message VERBATIM (the `.forward` primitive) — see [relay](./protocols/relay.md). Only fires from the inbound-mail path (LMTP). | | `txco://hmac-sign` | Compute an HMAC signature (key via `WITH secrets.*`). | | `txco://hmac-verify` | Verify an HMAC, constant-time; result lands under `@computed.*`. | | `txco://basic-auth-encode` | Encode `user:pass` to a basic-auth header value. | | `txco://copy` | Path-to-path copy inside the envelope (what `SET` can't do with computed paths). | | `txco://kv/get` · `kv/set` · `kv/delete` · `kv/incr` · `kv/cas` | Read + write durable state across requests — counters, flags, locks, caches (`boltdb` local / `redis` shared). See [kv](./kv.md). | | `txco://detect-tenant` | Boot-pipeline: hostname/listener → tenant resolution. Used by the scaffolded `_sys/boot` rules; you rarely call it directly. | | `txco://route` | Boot-pipeline: promote a routing proposal (`@route.*`) into `@goto` + `@tenant`. Companion to `detect-tenant`. | | `txco://continuation-result` | Poll handler behind `?_txc.continuation=` ([continuations](../continuations.md)). Wired by the chassis; not called from rules. | Builtins pay normal [fuel](./fuel.md) and appear in [traces](./trace.md) like any other op.