Golem File Io Effect

File I/O in Effect Golem Agents

Effect agents run in wasm-rquickjs, which implements Node's filesystem APIs on top of WASI. Import node:fs or node:fs/promises exactly as in an ordinary TypeScript Golem component. The component build keeps wasm-rquickjs-provided Node modules external, while installed third-party npm packages remain bundleable.

Read a Provisioned File

Load golem-add-initial-files to provision a local file through the agent's files: manifest section. Agent code reads the configured targetPath; it must not use the local sourcePath.

Use Effect.promise when failure is impossible by contract, or Effect.tryPromise when the method must model filesystem errors. Method handlers must return an Effect rather than being declared async:

import { readFile } from "node:fs/promises";
import { Effect, Schema } from "effect";
import { defineAgent, method } from "@golemcloud/effect-golem";

export const ProvisionedFileAgent = defineAgent({
  name: "ProvisionedFileAgent",
  mode: "durable",
  id: {
    name: Schema.String,
  },
  methods: {
    readConfig: method({
      input: {},
      success: Schema.String,
    }),
    fileStats: method({
      input: {},
      success: Schema.String,
    }),
  },
}).implement({
  init: () => Effect.void,
  methods: () => ({
    readConfig: () =>
      Effect.promise(() => readFile("/data/config.json", "utf8")),

    fileStats: () =>
      Effect.promise(() => readFile("/data/large.bin")).pipe(
        Effect.map((bytes) => {
          let sum = 0n;
          for (const byte of bytes) sum += BigInt(byte);
          return `size=${bytes.byteLength},sum=${sum}`;
        }),
      ),
  }),
});

Register the implementation from the component entry point:

// src/main.ts
import "./provisioned-file-agent.js";

readFile returns a Buffer, which is a Uint8Array, and wasm-rquickjs performs the low-level chunked reads needed for files larger than the WASI per-read allocation limit. Do not truncate the result or replace a provisioned-file read with generated fixture bytes.

Other Node Filesystem Operations

Use the usual node:fs/promises operations (writeFile, mkdir, readdir, stat, unlink, and others) and wrap their Promises in Effect. The synchronous node:fs APIs are also available; wrap them in Effect.sync or Effect.try. Paths are inside the agent's sandboxed WASI filesystem, not the developer machine.

Use Blobstore instead when the data is logically object storage rather than filesystem data. A Blobstore key is not a mounted path and cannot read a file provisioned through golem.yaml.

Key Constraints

• Import filesystem APIs from node:fs or node:fs/promises.
• Return Effects from method handlers; do not use plain async functions.
• Use the provisioned targetPath, such as /data/config.json, in application code.
• Let Golem handle durable host-operation retries; do not add manual retry loops.
• Use TypeScript camelCase for Effect method names and TypeScript syntax for CLI agent values.
• Installed third-party npm packages are bundled normally, but native Node addons cannot run in the WebAssembly component; choose pure JavaScript dependencies or wasm-rquickjs Node APIs.