Runtime and SDK

This page describes the runtime that executes cloud functions: the supported languages and toolchains, what your function reads and writes, how it is triggered, the limits, the SDK, and the execution history. For a step-by-step walkthrough, read the Cloud functions guide.

Runtime model

A cloud function is a WebAssembly module that targets WASI (WebAssembly System Interface). Each invocation runs your module once, in a sandbox:

Languages and toolchains

Any language that compiles to a WASI module works. These toolchains are known to work:

On macOS, py2wasm does not work with the system Python or the Homebrew Python. Install the official Python build, for example with pyenv.

The language field of a function is descriptive metadata. The runtime executes the WebAssembly module whatever the source language.

Input

Your function receives this JSON document on stdin:

{
  "input": "<base64-encoded bytes>",
  "context": {
    "function_id": "<function-id>",
    "project_id": 1
  },
  "metadata": {
    "<key>": "<base64-encoded bytes>"
  }
}

Output

Write the result to stdout. Exit normally on success. If your module exits with an error, the execution is recorded as failed, with your stderr output and the error.

What happens to the output depends on the trigger:

• HTTP: the output is returned in the invocation response.
• Topic: the output is published on the connector's output topic, if it has one.

Triggers

Every function can be invoked over HTTP. Connectors cannot form a cycle: the API rejects a definition whose connectors loop back on themselves. See Topics and Connectors.

The HTTP invocation returns:

{
  "status": "success",
  "data": {
    "started_at": "2026-01-01T12:00:00Z",
    "execution_time": 0.42,
    "output": {
      "format": "json",
      "value": { "processed": 10 }
    }
  }
}

output.format is json when the output is valid JSON, unicode when it is valid UTF-8 text, and base64 otherwise.

Function definition

Create and update functions with the v2 functions operations or fun functions new and fun functions edit.

Upload the WebAssembly binary separately, as multipart/form-data with the file in the function field, on PUT /v2/projects/{project_id}/functions/{function_id}/file, or with fun functions upload <name> <file.wasm>. has_file_set tells you whether a binary was uploaded.

Limits

The first execution after an upload, or after the keep-alive timeout expires, includes compiling the module, which takes noticeably longer than later executions. Set keep_alive_timeout to keep a frequently used function loaded.

SDK

These host functions let your module interact with Fundamentum. They are imported from the fundamentum namespace:

Each argument is passed as a 64-bit integer: the upper 32 bits are a pointer into the module's memory and the lower 32 bits a length in bytes.

Go SDK

The Go SDK wraps the input and the host functions:

package main

import (
	"fmt"
	"os"

	sdk "<go-sdk-module-path>"
)

// ID of the project topic that receives the result.
const outputTopicID = "42"

func main() {
	input, err := sdk.ReadInput(os.Stdin)
	if err != nil {
		fmt.Fprintln(os.Stderr, "failed reading input:", err)
		os.Exit(1)
	}

	fmt.Fprintln(os.Stderr, "project", input.Context.ProjectID)

	sdk.ProduceMessage(outputTopicID, string(input.Input))
	fmt.Print(`{"ok":true}`)
}

In other languages, read stdin and parse the JSON yourself. The host functions are available as WASI imports from the fundamentum namespace.

Execution history

Each execution is recorded with:

List executions with fun functions executions list --name <name> or GET /v2/projects/{project_id}/functions/{function_id}/executions. Executions are kept for 7 days.

What's next

• Cloud functions guide
• Actions and custom encoders
• fun CLI reference