Docs / Effect System / Mocking with using

Mocking with using

The using keyword overrides a function or tool implementation inside a test block. This is Cleat's dependency injection mechanism — no framework, no interfaces, just using.

Basic mocking

cleat

test "mock HTTP calls" {
    using fetch = fn(url: string) -> Result[string, string] {
        return Ok("{\"status\": \"ok\"}")
    }
    let result = fetch("https://api.example.com")
    assert(result.IsOk())
}

Mocking tools

cleat

tool search(query: string) -> Result[string, string]
    needs { net }
    timeout: 10s
{ return Ok("real results") }

test "mock the search tool" {
    using search = fn(query: string) -> Result[string, string] {
        return Ok("mock results for: ${query}")
    }
    let r = search("test")
    assert(r.IsOk())
}

The mock signature must match the original tool's parameter and return types.

Why this matters

Test agents without API keys — mock the tools, verify the agent's behavior
Test server handlers without network — mock HTTP calls
Test file operations without disk — mock fs.read/fs.write
Deterministic, fast, repeatable tests — no external dependencies

Effect bypass in tests

Test blocks can call tools without declaring their effects. This lets you test effectful code without effect boilerplate:

cleat

tool send(text: string) -> Result[string, string]
    timeout: 5s
{ return Ok("sent") }

test "send works" {
    let r = send("hello")  // no needs { } required in test blocks
    assert(r.IsOk())
}

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