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zio-golem-base-image

Agent Building
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Explains the zio-golem WIT folder structure and how to regenerate the agent_guest.wasm base image. Use when working with WIT definitions, upgrading Golem versions, or regenerating the guest runtime WASM.

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Source SKILL.md: https://github.com/zio/zio-blocks/blob/HEAD/.claude/skills/zio-golem-base-image/SKILL.md

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zio-golem Base Image (agent_guest.wasm)

The base image agent_guest.wasm is a QuickJS-based WASM component that serves as the guest runtime for Scala.js agents on Golem. It must be regenerated whenever WIT definitions change.

WIT Folder Structure

golem/wit/
├── main.wit      # Hand-maintained world definition (golem:agent-guest)
├── deps.toml     # wit-deps manifest — points to golemcloud/golem main branch
├── deps.lock     # Auto-generated lock file (gitignored)
└── deps/         # Auto-populated by wit-deps (gitignored)
    ├── golem-core/
    ├── golem-agent/
    ├── golem-1.x/
    ├── golem-rdbms/
    ├── golem-durability/
    ├── blobstore/
    ├── cli/
    ├── clocks/
    ├── config/
    ├── ...
    └── sockets/
  • main.wit defines the golem:agent-guest world — the set of imports/exports the agent component uses. This file is checked in and maintained manually.
  • deps.toml declares a single dependency source: the golem repo's main branch tarball. wit-deps downloads and extracts the WIT packages from it.
  • deps/ and deps.lock are gitignored — they are populated by running wit-deps from golem/.

When to Regenerate

The base image must be regenerated whenever:

  1. wit/main.wit changes — adding/removing imports or exports
  2. WIT dependencies update — e.g., upgrading from Golem v1.5.0 to a newer version (update deps.toml then regenerate)
  3. wasm-rquickjs updates — a new version of the wrapper generator may produce different output

The generated agent_guest.wasm is checked in at two locations (embedded in the sbt and mill plugins):

  • golem/sbt/src/main/resources/golem/wasm/agent_guest.wasm
  • golem/mill/resources/golem/wasm/agent_guest.wasm

Prerequisites

1. Rust toolchain

curl --proto '=https' --tlsv1.2 -sSf https://sh.rustup.rs | sh
rustup target add wasm32-wasip1

2. cargo-component

cargo install cargo-component

3. wit-deps

cargo install wit-deps-cli

4. wasm-rquickjs (pinned to 0.1.0)

The script enforces a specific version of wasm-rquickjs and will refuse to run if the installed version does not match. The required version is defined by REQUIRED_WASM_RQUICKJS_VERSION in generate-agent-guest-wasm.sh.

cargo install wasm-rquickjs-cli@0.1.0

How to Regenerate

From the repository root (zio-blocks/):

./golem/scripts/generate-agent-guest-wasm.sh

The script performs these steps:

  1. Runs wit-deps update in golem/ to fetch the latest WIT dependencies into golem/wit/deps/
  2. Stages a clean WIT package in .generated/agent-wit-root/ (copies main.wit + deps/)
  3. Runs wasm-rquickjs generate-wrapper-crate to produce a Rust crate from the WIT
  4. Builds with cargo component build --release targeting wasm32-wasip1
  5. Installs the resulting agent_guest.wasm into both plugin resource directories

Fetching Dependencies Only

To update the WIT dependencies without regenerating the WASM:

cd golem && wit-deps

The generation script always uses wit-deps update to ensure deps are fresh. To update deps without a full regeneration:

cd golem && wit-deps update

How It Fits Together

At build time, the sbt/mill GolemPlugin extracts the embedded agent_guest.wasm from plugin resources and writes it to the user project's .generated/agent_guest.wasm. Then golem-cli uses this base runtime to compose the final component: it injects the user's Scala.js bundle into the QuickJS runtime and wraps it as a proper Golem agent component.

The Scala SDK does not parse WIT to generate Scala bindings. Instead, Scala macros + ZIO Schema produce AgentMetadata at compile time, and WitTypeBuilder maps schema types to WIT-compatible JS representations at runtime. The WIT definitions only flow through the WASM guest runtime.