pester-testing

name	pester-testing
description	Write idiomatic Pester 5 tests for PowerShell modules. Use this skill whenever a user asks to add tests, fill coverage gaps, or validate AI-generated PowerShell code. Targets Pester 5+ syntax (BeforeAll/Describe/It, no Pester 4 patterns).
domain	powershell-testing
confidence	high
source	earned
tools	[]

Context

Apply this skill when the user wants to write or extend Pester tests for a PowerShell module. The user has already provided the module under test (or will point you at it) and expects tests that follow Pester 5 conventions and the project's existing style.

Patterns

• Pester 5 only. Use BeforeAll, Describe, Context, It, Should -Be, Should -Throw. Do not use Pester 4 patterns like In blocks or v4-style mocks unless the user explicitly requests them.
• Test the module surface, not internals. Import via the .psd1 manifest and assert on ExportedFunctions, parameter validation, and output shape. Prefer (Get-Module X).ExportedFunctions.Keys | Should -Be ... over reaching into private functions.
• Validate parameter attributes at bind time. { Get-Foo -BadParam X } | Should -Throw proves the validator works without needing the function body to run.
• Skip platform-specific tests on the wrong platform. Use -Skip:(-not $IsWindows) or -Skip:(-not $IsLinux) rather than wrapping in if. Skips show up in test output; ifs disappear.
• One BeforeAll to import the module at the file level, one AfterAll to remove it. Don't import inside each Describe.
• Discover real gaps first. Before writing tests, scan the public functions for: missing parameter-validation tests, missing shape tests, missing error-case tests. Report the gap list, then offer to fill them.

Examples

Minimal Pester 5 file for a module:

#requires -Modules @{ ModuleName = 'Pester'; ModuleVersion = '5.0' }

BeforeAll {
    $script:Manifest = Join-Path $PSScriptRoot '..\MyModule\MyModule.psd1' | Resolve-Path
    Remove-Module MyModule -ErrorAction SilentlyContinue
    Import-Module $script:Manifest -Force
}

AfterAll {
    Remove-Module MyModule -ErrorAction SilentlyContinue
}

Describe 'MyModule surface' {
    It 'exports the expected functions' {
        (Get-Module MyModule).ExportedFunctions.Keys | Should -Be @('Get-Foo','Set-Foo')
    }
}

Describe 'Get-Foo' {
    It 'rejects an invalid value at bind time' {
        { Get-Foo -Status 'Bogus' } | Should -Throw
    }
}

Anti-patterns

• Don't write tests that require the production target (real services, real hosts) unless explicitly asked. Parameter-validation and shape tests run anywhere.
• Don't use -Verifiable mocks with Assert-VerifiableMock (Pester 5 prefers Should -Invoke).
• Don't paste an entire generated test suite without first showing the user a one-line summary of what gaps you found. They need to confirm the scope before you fill it.