Pester Migration

Pester is the test framework for PowerShell. Test files end in *.Tests.ps1 and use Describe / Context / It blocks with Should assertions. This skill upgrades an existing suite from one major Pester version to the next and gets it green again.

Mental model: each major jump has a different character. v3→v4 is mostly a syntax rename. v4→v5 is a fundamental runtime change (the Discovery/Run split) and is the hard one. v5→v6 is largely backwards-compatible — a handful of previously-deprecated things now throw. Migrate one major at a time; never skip a version.

Detailed, symptom-driven guides live in references/ — load the one(s) for the jump you are doing.

References

Canonical source: the official migration guides at https://pester.dev/docs/migrations/ — this skill mirrors them. When in doubt, prefer the website.

Step 0 — Detect where you are and where you're going

Find the installed version(s) and the version the tests were written for. These can differ.

# Installed Pester version(s) on this machine
Get-Module Pester -ListAvailable | Select-Object Name, Version, Path

# Version currently imported in the session
(Get-Module Pester).Version

Tell the source version from the test code with these heuristics:

Install the target version when ready:

# Latest stable v5 — pin the major to avoid installing Pester 6
Install-Module Pester -MaximumVersion 5.99.99 -Force

# Pester 6
Install-Module Pester -Force

On Windows PowerShell 5.1 the OS ships a Microsoft-signed built-in Pester 3 that PowerShellGet won't overwrite with the differently-signed newer Pester — add -SkipPublisherCheck there to install side-by-side. Not needed on PowerShell 7+. See https://pester.dev/docs/introduction/installation.

Migration workflow

Run this loop for each major jump. Do not jump two majors at once — go v4→v5, then v5→v6.

1. Baseline. Run the suite on the current version first and record pass/fail. You need a known-good (or known) starting point so you can tell migration regressions apart from pre-existing failures.

   # Bare Invoke-Pester works on every major; exact parameters differ
   # (v3/v4: -Script/-OutputFile; v5+/v6: -Path/-Output).
   Invoke-Pester
   

2. Read the reference for this jump (table above) so you know the full scope before editing.
3. Edit file by file. Apply the mechanical changes (see per-jump cheat sheets below and in the reference). Keep changes small and reviewable — one file or one concern at a time.
4. Switch versions with Install-Module (Step 0), then re-import: Remove-Module Pester; Import-Module Pester (or start a fresh session).
5. Run and fix. Re-run with -Output Detailed; use -Output Diagnostic (v4→v5) or read the explicit v6 error messages to locate problems. Match each failure to the symptom → fix tables in the reference.

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