Pester Migration

---
name: pester-migration
description: 'Experimental (preview) Pester migration skill for upgrading PowerShell Pester test suites across major versions — v3→v4, v4→v5, and v5→v6. The v5→v6 path tracks Pester 6, which is still a release candidate, so that guidance may change. Covers the Discovery/Run two-phase model, moving setup into BeforeAll, $PSScriptRoot vs $MyInvocation, mock changes (Assert-MockCalled → Should -Invoke, removed fall-through), Invoke-Pester parameters → PesterConfiguration, data-driven -ForEach/-TestCases, and the v6 breaking changes. Use when the user asks to upgrade, migrate, or modernize Pester tests, fix *.Tests.ps1 files that broke after bumping the Pester version, or convert legacy Should / Invoke-Pester syntax.'
---

# Pester Migration

> **Experimental / preview.** The **v5→v6** guidance tracks Pester 6 while it is a release
> candidate and may change; verify against the current
> [release notes](https://github.com/pester/Pester/releases). v3→v4 and v4→v5 cover stable releases.

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

| Reference | When to load |
|---|---|
| [v3-to-v4.md](references/v3-to-v4.md) | `Should Be` → `Should -Be`, `Contain` → `FileContentMatch`, `Assert-VerifiableMocks` → `Assert-VerifiableMock`, array-assertion edge cases. |
| [v4-to-v5.md](references/v4-to-v5.md) | The big one. Discovery/Run phases, `BeforeAll` setup, `$PSScriptRoot`, `BeforeDiscovery`, `-ForEach`, mock scoping, `Should -Throw` wildcards, `Invoke-Pester` → `New-PesterConfiguration`. |
| [v5-to-v6.md](references/v5-to-v6.md) | PowerShell 5.1/7.4+ only, per-file discovery+run, empty `-ForEach` throws, duplicate setup blocks throw, name `<...>` templates evaluate, `Assert-MockCalled` removed, mocks no longer fall through, code-coverage tracer, legacy `Invoke-Pester` params removed. |

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.

```powershell
# 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:

| You see in `*.Tests.ps1` / build scripts | Suite was written for |
|---|---|
| `Should Be` / `Should Contain` (no dash) | v3 or earlier → start at [v3-to-v4](references/v3-to-v4.md) |
| `$MyInvocation.MyCommand.Path` + dot-source at the **top** of the file; arbitrary code directly under `Describe` | v4 → [v4-to-v5](references/v4-to-v5.md) |
| `Assert-MockCalled`, `Assert-VerifiableMock`, `Set-ItResult -Pending` | v4 / early-v5 (these are **removed in v6**) |
| `Invoke-Pester -Script … -OutputFile … -CodeCoverage …` (legacy params) | v4 invocation → map to config |
| `BeforeAll { . $PSScriptRoot/… }`, `New-PesterConfiguration`, `Should -Invoke` | already v5-style → [v5-to-v6](references/v5-to-v6.md) |

Install the target version when ready:

```powershell
# Latest stable v5 — pin the major so this keeps installing v5 even after v6 goes GA
Install-Module Pester -MaximumVersion 5.99.99 -Force

# Pester 6 (currently a release candidate — needs -AllowPrerelease)
Install-Module Pester -AllowPrerelease -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.
   ```powershell
   # 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.
6. **Green, diff, commit.** Re-run until the result matches the baseline (or better). Review the
   diff, then commit. Migrating in small commits makes regressions trivial to bisect.

## What actually changes (scope per jump)

| Jump | Difficulty | Nature |
|---|---|---|
| v3 → v4 | Low | Assertion-syntax rename (`Should -Be`). Largely script-automatable. |
| v4 → v5 | **High** | New two-phase runtime. Test **structure** changes: setup must move into `BeforeAll`, discovery-time code into `BeforeDiscovery`, file location via `$PSScriptRoot`. Not a pure find-replace. |
| v5 → v6 | Low–Medium | Backwards-compatible runtime; deprecated features now throw. Mostly small, targeted fixes. Your `Should -Be` assertions keep working unchanged. |

## Quick cheat sheets

### v4 → v5 (most common fixes)
```powershell
# 1. Move file import into BeforeAll, use $PSScriptRoot (NOT $MyInvocation.MyCommand.Path)
# BEFORE
$here = Split-Path -Parent $MyInvocation.MyCommand.Path
. "$here\Get-Thing.ps1"
# AFTER
BeforeAll { . $PSScriptRoot/Get-Thing.ps1 }

# 2. Any code that DISCOVERS/generates tests must be in BeforeDiscovery
BeforeDiscovery { $cases = Get-Content $PSScriptRoot/cases.json | ConvertFrom-Json }

# 3. Should -Throw matches with -like wildcards, not .Contains
{ throw 'a long message' } | Should -Throw '*long*'

# 4. Invoke-Pester legacy params → New-PesterConfiguration (see reference for full map)
```
Full details, scoping rules, and the parameter→config table: [references/v4-to-v5.md](references/v4-to-v5.md).

### v5 → v6 (most common fixes)
```powershell
# 1. Mock assertions: removed verbs — rename (old -> new):
#    Assert-MockCalled     -> Should -Invoke
#    Assert-VerifiableMock -> Should -InvokeVerifiable
Should -Invoke Get-Thing -Times 1 -Exactly
Should -InvokeVerifiable

# 2. Add a default mock — unmatched calls no longer run the real command
Mock Get-Thing { 'default' }
Mock Get-Thing -ParameterFilter { $Name -eq 'a' } -MockWith { 'a' }

# 3. Empty/$null -ForEach now throws; allow it only where empty is expected
Describe 'Optional' -ForEach $cases -AllowNullOrEmptyForEach { }

# 4. Combine duplicate BeforeAll/BeforeEach/AfterAll/AfterEach in the same block into one
```
Full breaking-change list with symptoms and fixes: [references/v5-to-v6.md](references/v5-to-v6.md).

## Safety rules

- **Tests are the spec.** Migration must not change what a test asserts — only how the suite is
  structured and invoked. If a test starts passing/failing differently for any reason other than a
  documented breaking change, investigate before accepting it.
- **Automated migration scripts produce false positives.** The community scripts (linked in the
  references) help with `Should` syntax and dot-sourcing, but always review the diff and re-run the
  suite afterward. Never bulk-edit and commit unchecked.
- **Mind file encoding** when scripting replacements over `*.Tests.ps1` — preserve the original
  encoding (UTF-8 vs ASCII) so you don't mangle non-ASCII test names.
- **Work on a branch, commit per file/concern.** Small commits keep `git bisect` useful if a
  migrated test goes red later.

# Migrating Pester v3 → v4

This is the smallest jump — mostly an assertion-syntax rename. Many suites need only minor changes,
some need none. It is largely script-automatable, but always review the diff and re-run the suite.

Official guide: https://pester.dev/docs/migrations/v3-to-v4

> Heads-up: if your goal is a modern Pester (5 or 6), v3→v4 is only the first step. Do it, get
> green, then continue with [v4-to-v5.md](v4-to-v5.md) and [v5-to-v6.md](v5-to-v6.md).

---

## Change 1 — Dashed `Should` assertion syntax

v4 introduced the parameter-style `Should` syntax. The bareword form still ran in v4 but is
**removed in v5**, so converting now saves a later migration.

```powershell
# v3 (bareword)
It 'checks something' { 10 | Should Be 10 }

# v4+ (dashed)
It 'checks something' { 10 | Should -Be 10 }
```

The rename applies to every operator: `Be`, `BeExactly`, `Match`, `Throw`, `BeNullOrEmpty`,
`Contain`, etc. → `-Be`, `-BeExactly`, `-Match`, `-Throw`, `-BeNullOrEmpty`, …

There is a well-known AST-based converter, `Update-PesterTest` (Chris Dent / Wojciech Sciesinski),
that inserts the dashes safely by parsing the file rather than regexing it:
https://gist.github.com/indented-automation/aeb14825e39dd8849beee44f681fbab3 — it's also reproduced
in the official v3→v4 guide. Review its output, especially for non-UTF-8/ASCII files, where it can
change encoding.

---

## Change 2 — `Contain` → `FileContentMatch`

The `Contain` assertion was renamed to `FileContentMatch` (it tests file **contents**, which the
old name made ambiguous against collection containment).

```powershell
# Should Contain      -> Should -FileContentMatch
# Should Not Contain  -> Should -Not -FileContentMatch
'app.config' | Should -FileContentMatch 'setting'
'app.config' | Should -Not -FileContentMatch 'secret'
```

A simple regex-based migration script from the official guide (verify results — it can produce
false positives):

```powershell
$content = Get-Content -Path $file -Encoding $encoding
$content = $content -replace 'Should\s+\-?Contain',        'Should -FileContentMatch'
$content = $content -replace 'Should\s+\-?Not\s*-?Contain', 'Should -Not -FileContentMatch'
$content = $content -replace 'Assert-VerifiableMocks',      'Assert-VerifiableMock'
$content | Set-Content -Path $file -Encoding $encoding
```

---

## Change 3 — `Assert-VerifiableMocks` → `Assert-VerifiableMock`

The cmdlet was renamed (dropped the trailing `s`). Rename all occurrences.

> In Pester 5 this is *deprecated* and in Pester 6 it is *removed* — when you continue past v4,
> switch to `Should -InvokeVerifiable`. See [v5-to-v6.md](v5-to-v6.md).

---

## Change 4 — Array assertions (watch for edge cases)

`Should` gained array assertions in v4. This is transparent for most tests, but there are edge
cases where an array test that passed under v3 fails under v4. If an array-related test changes
result after the rename, inspect it manually rather than forcing it to pass. Background:
https://github.com/pester/Pester/issues/873.

Mocking also shifted subtly when Pester moved from functions to aliases; there are no required
changes, but if mocked-command behavior looks off, see
https://github.com/pester/Pester/issues/810 and https://github.com/pester/Pester/issues/812.

---

## v3 → v4 checklist

- [ ] Suite runs on v3 first (baseline).
- [ ] All `Should <Operator>` converted to `Should -<Operator>` (prefer the AST converter).
- [ ] `Should Contain` → `Should -FileContentMatch` (and the `-Not` form).
- [ ] `Assert-VerifiableMocks` → `Assert-VerifiableMock`.
- [ ] Array-assertion behavior changes reviewed manually.
- [ ] File encoding preserved by any scripted replacement.
- [ ] Suite green on v4; diff reviewed; committed.

# Migrating Pester v4 → v5

This is the hard jump. v5 introduced a new runtime that splits a test run into two phases —
**Discovery** and **Run** — and that changes how you must *structure* tests. It is not a pure
find-and-replace. Read this whole file before editing a suite.

Official guide: https://pester.dev/docs/migrations/v4-to-v5 ·
Breaking changes: https://pester.dev/docs/migrations/breaking-changes-in-v5

---

## The one concept that explains everything: Discovery and Run

A v5+ run happens in two passes:

- **Discovery** — Pester executes each `*.Tests.ps1` file top to bottom but only to *find* tests.
  It invokes the `Describe`/`Context` script blocks to collect the tree of `It`s, evaluates `It`
  `-Name` strings and `-TestCases`/`-ForEach` data, and records `BeforeAll`/`It`/etc. script blocks
  **without running them**.
- **Run** — Pester then executes the recorded setups, tests, and teardowns with correct scoping.

**The two rules that make a suite v5-correct:**

1. Put **all** test code inside `It`, `BeforeAll`, `BeforeEach`, `AfterAll`, or `AfterEach`.
2. Put **no** test code directly in `Describe`/`Context` bodies or at the top of the file —
   unless it is meant to build tests, in which case it goes in `BeforeDiscovery`.

Code that sits loose in a `Describe` body or at file top-level runs during **Discovery**, and its
results are usually **not** available during **Run**. This is the root cause of most "it worked in
v4, it's `$null` in v5" bugs.

---

## Fix 1 — Move file setup into `BeforeAll` and use `$PSScriptRoot`

The classic v4 header dot-sources the system-under-test at file scope using
`$MyInvocation.MyCommand.Path`. Both the placement and that variable break in v5.

```powershell
# BEFORE (v4)
$here = Split-Path -Parent $MyInvocation.MyCommand.Path
$sut  = (Split-Path -Leaf $MyInvocation.MyCommand.Path).Replace('.Tests.', '.')
. "$here\$sut"

Describe 'Get-Cactus' {
    It 'Returns 🌵' { Get-Cactus | Should -Be '🌵' }
}
```

```powershell
# AFTER (v5+)
BeforeAll {
    # Do NOT use $MyInvocation.MyCommand.Path here.
    . $PSScriptRoot/Get-Cactus.ps1
    # or, by convention from the test file name:
    # . $PSCommandPath.Replace('.Tests.ps1', '.ps1')
}

Describe 'Get-Cactus' {
    It 'Returns 🌵' { Get-Cactus | Should -Be '🌵' }
}
```

Why `$MyInvocation.MyCommand.Path` fails: it returns the path only when evaluated directly in the
script body. Inside *any* function or script block (and `BeforeAll` is a script block) `Path` is
empty. Use `$PSScriptRoot` (the test file's directory) or `$PSCommandPath` (the test file's full
path) instead. `string.Replace('.Tests.ps1','.ps1')` is case-sensitive — keep the `.Tests.ps1`
casing exact.

> `$MyInvocation.MyCommand.Path` is fine *inside your module/product code* — the change only
> affects the test-file header pattern. See
> https://pester.dev/docs/usage/importing-tested-functions#migrating-from-pester-v4.

There is a community migration script that does the BeforeAll wrap for you (review its output):
https://gist.github.com/nohwnd/d488bd14ab4572f92ae77e208f476ada

---

## Fix 2 — Generate tests with `BeforeDiscovery` + `-ForEach`, not loose `foreach`

A very common v4 pattern builds tests from data with a top-level `foreach`. In v5 the data often
isn't defined at Discovery time, so no tests get generated, or the per-item variable is missing
inside `It`.

```powershell
# BROKEN in v5: $files is set in BeforeAll (Run phase), but the foreach runs in Discovery
BeforeAll { $files = Get-ChildItem *.ps1 }
foreach ($file in $files) {
    Describe "$file is correct" {
        It 'has empty line at end' { }
    }
}
```

Two things must change: build the data in `BeforeDiscovery` (so it exists during Discovery), and
pass per-item data into the test with `-ForEach`/`-TestCases` (so `It` can see it during Run):

```powershell
BeforeDiscovery {
    $files = Get-ChildItem *.ps1            # runs during Discovery
}

Describe 'script <_> is correct' -ForEach $files {
    It 'has an empty line at the end' {
        # $_ is the current file here
    }
}
```

Prefer `-ForEach` on the block/`It` over a hand-written `foreach`; it both creates the copies and
makes the current item available. Use `<_>` (or `<Name>` for hashtable items) in the name to
template per-item titles. Reference: https://pester.dev/docs/usage/data-driven-tests.

---

## Fix 3 — `-Skip` and `-TestCases` are evaluated during Discovery

Because filters and data are resolved during Discovery, conditions computed in `BeforeAll` are not
available yet.

```powershell
# DOES NOT skip: $isSkipped is set in BeforeAll (Run), but -Skip is read in Discovery
Describe 'd' {
    BeforeAll { $isSkipped = Get-IsSkipped }
    It 'i' -Skip:$isSkipped { }
}
```

Move cheap skip logic to file scope (it runs on every Discovery) or, better, base it on a static
global like `$IsWindows`:

```powershell
$isSkipped = -not $IsWindows
Describe 'd' {
    It 'i' -Skip:$isSkipped { }
}
```

Keep Discovery-time code cheap — it runs every time the file is discovered, which can be often.

---

## Fix 4 — Variables don't leak from Discovery into the test

Variables defined during Discovery are **not** visible in `BeforeAll/-Each`, `AfterAll/-Each`, or
`It`. If you compute something while generating tests and need it at run time, attach it to the
test via `-ForEach`/`-TestCases`. (`TestDrive` is Run-only and likewise can't be used in
`-ForEach`.)

---

## Fix 5 — `Should -Throw` matches with `-like` wildcards

In v5, `Should -Throw <message>` matches the exception message with `-like`, not `.Contains()`. A
substring that used to match now needs wildcards.

```powershell
# v4: matched a substring
{ throw 'connection failed: timeout' } | Should -Throw 'timeout'
# v5+: use wildcards to match part of the message
{ throw 'connection failed: timeout' } | Should -Throw '*timeout*'
```

---

## Fix 6 — Mocks: scoping, debugging, and `InModuleScope`

- **Scope follows placement.** In v5, mocks (and their call counts) are scoped to where you put
  them — the current block/test — not the entire `Describe`/`Context`. Define the `Mock` in the
  same `BeforeAll`/`It` scope where it applies, and assert counts in that scope.
- **`Assert-VerifiableMocks` was removed.** Use `Should -InvokeVerifiable`. (`Assert-MockCalled`
  and `Assert-VerifiableMock` still exist but are *deprecated* in v5 and **removed in v6** — prefer
  `Should -Invoke` / `Should -InvokeVerifiable` now to save a second migration. See
  [v5-to-v6.md](v5-to-v6.md).)
- **Mocks are debuggable.** v5 no longer rewrites your mock script block, so you can set
  breakpoints inside `-MockWith` and inside `-ParameterFilter`.
- **Avoid `InModuleScope` around `Describe`/`It`.** It loads the module during Discovery (slowing
  it down) and lets you test internals instead of the published surface. Prefer `-ModuleName` on
  `Mock` and on `Should -Invoke`; if you must use `InModuleScope`, keep it inside `It`. See
  https://pester.dev/docs/usage/mocking.

```powershell
# Prefer this over wrapping the whole Describe in InModuleScope
Mock Get-Internal -ModuleName MyModule { 'mocked' }
Should -Invoke Get-Internal -ModuleName MyModule -Times 1 -Exactly
```

---

## Fix 7 — `Invoke-Pester` parameters → `New-PesterConfiguration`

`Invoke-Pester`'s interface was overhauled. v5 kept a **deprecated** compatibility set so v4 calls
mostly still run (with a warning), but you should move to either the **Simple** parameters or the
**Advanced** `-Configuration` object. (v6 removes the legacy set entirely — migrate now.)

**Simple interface** (parameter → config property):

| Simple parameter | Configuration property |
|---|---|
| `-Path` | `Run.Path` |
| `-ExcludePath` | `Run.ExcludePath` |
| `-Tag` | `Filter.Tag` |
| `-ExcludeTag` | `Filter.ExcludeTag` |
| `-FullNameFilter` | `Filter.FullName` |
| `-Output` | `Output.Verbosity` |
| `-CI` | `TestResult.Enabled` + `Run.Exit` (both `$true`) |
| `-PassThru` | `Run.PassThru` |

**Legacy (v4) parameters → config:**

| v4 parameter | Configuration property |
|---|---|
| `-Script` | `Run.Path` (paths only — no hashtables) |
| `-EnableExit` | `Run.Exit` |
| `-TestName` | replaced by `-FullNameFilter` / `Filter.FullName` |
| `-CodeCoverage` | `CodeCoverage.Path` (+ `CodeCoverage.Enabled = $true`) |
| `-CodeCoverageOutputFile` | `CodeCoverage.OutputPath` |
| `-CodeCoverageOutputFileEncoding` | `CodeCoverage.OutputEncoding` |
| `-CodeCoverageOutputFileFormat` | `CodeCoverage.OutputFormat` |
| `-OutputFile` | `TestResult.OutputPath` (+ `TestResult.Enabled = $true`) |
| `-OutputFormat` | `TestResult.OutputFormat` |
| `-Show` / `-Output` | `Output.Verbosity` (see mapping below) |
| `-PesterOption`, `-Strict` | ignored / not available |

`-Show` value → `Output.Verbosity`: `All`/`Default`/`Detailed` → `Detailed`; `Fails`/`Normal` →
`Normal`; `Diagnostic` → `Diagnostic`; `Minimal` → `Minimal`; `None` → `None`.

```powershell
# BEFORE (v4 legacy)
Invoke-Pester -Script ./tests -CodeCoverage ./src/*.ps1 `
    -OutputFile result.xml -OutputFormat NUnitXml -EnableExit

# AFTER (v5 Advanced)
$config = New-PesterConfiguration
$config.Run.Path = './tests'
$config.Run.Exit = $true
$config.CodeCoverage.Enabled = $true
$config.CodeCoverage.Path = './src'
$config.TestResult.Enabled = $true
$config.TestResult.OutputPath = 'result.xml'
$config.TestResult.OutputFormat = 'NUnitXml'
Invoke-Pester -Configuration $config
```

`-Output Diagnostic` is your best friend while migrating — it shows Discovery/Skip/Mock decisions.

---

## The new result object

The v5 result object is much richer and is what Pester uses internally. To keep a v4-era CI
pipeline working, convert it with `ConvertTo-Pester4Result`. For NUnit output use
`ConvertTo-NUnitReport`, or pass `-CI` to enable NUnit output, code coverage, and a failing exit
code in one switch. Each test's `-TestCases`/`-ForEach` item is available on the test object's
`Data` property.

---

## Other removed / changed things in v5

- **PowerShell 2** is no longer supported.
- **Legacy `Should Be`** (no dash) is removed — convert to `Should -Be` ([v3-to-v4.md](v3-to-v4.md)).
- **Gherkin** was removed — stay on Pester v4 if you need it.
- `-Output`/`-Show` reduced to `None`, `Normal`, `Detailed`, `Diagnostic`.
- `-TestName` → `-FullNameFilter`; `-Script` → `-Path` (paths only); `-PesterOption` removed.

---

## v4 → v5 checklist

- [ ] Suite runs green on v4 first (baseline).
- [ ] File import moved into `BeforeAll`; `$MyInvocation.MyCommand.Path` replaced with
      `$PSScriptRoot`/`$PSCommandPath`.
- [ ] No loose code in `Describe`/`Context` bodies or at file top-level; test-generating code moved
      to `BeforeDiscovery`.
- [ ] `foreach`-generated tests converted to `-ForEach`; per-item data passed via `-ForEach`/`-TestCases`.
- [ ] `-Skip:` conditions don't depend on `BeforeAll` variables.
- [ ] `Should -Throw` messages use `-like` wildcards (`*...*`).
- [ ] Mocks defined in the right scope; `Assert-VerifiableMocks` → `Should -InvokeVerifiable`;
      `InModuleScope` removed from around `Describe`/`It` in favor of `-ModuleName`.
- [ ] `Invoke-Pester` call converted to Simple params or `New-PesterConfiguration`.
- [ ] Suite green on v5 with `-Output Detailed`; diff reviewed; committed.

# Migrating Pester v5 → v6

Pester 6 builds on v5 and is **largely backwards-compatible** — most v5 suites run on v6 with no
changes, and your existing `Should -Be` assertions keep working. The work is fixing a handful of
previously-deprecated behaviors that now throw. This is a low-to-medium effort jump.

> Pester 6 is currently a **release candidate**. Install with
> `Install-Module Pester -AllowPrerelease -Force` (add `-SkipPublisherCheck` on Windows
> PowerShell 5.1). Some details may still change before final — check the release notes:
> https://github.com/pester/Pester/releases.

Official guide: https://pester.dev/docs/migrations/v5-to-v6

---

## Quick upgrade checklist

For most suites this is low risk. Run through these, then read the details for anything that bites:

1. Run on **Windows PowerShell 5.1** or **PowerShell 7.4+** (older PS is dropped).
2. Check any `-ForEach`/`-TestCases` that can be empty — it now **throws** unless you add
   `-AllowNullOrEmptyForEach`.
3. Remove duplicate `BeforeAll`/`BeforeEach`/`AfterAll`/`AfterEach` in the same block.
4. Replace `Assert-MockCalled` / `Assert-VerifiableMock` with `Should -Invoke` /
   `Should -InvokeVerifiable`.
5. Add a default `Mock` where some calls don't match a `-ParameterFilter` — mocks no longer fall
   through to the real command.
6. If you call `Invoke-Pester` with v4-style params (`-Script`, `-OutputFile`, …), switch to
   `New-PesterConfiguration`.

---

## Breaking changes (symptom → fix)

### PowerShell 5.1 and 7.4+ only
PowerShell 3, 4, 6, and early/unsupported 7 are dropped (all out of support from Microsoft), which
let Pester move its C# to .NET 8 (net462 for Windows PowerShell 5.1).

- **Symptom:** Pester won't import on an older PowerShell.
- **Fix:** Update your machines and CI agents to Windows PowerShell 5.1 or PowerShell 7.4+.

### Discovery and Run now happen per file
In v5 the run had two global phases: discover **every** file, then run every file. In v6 the unit
of work is a single file — Pester discovers a file and runs it before moving to the next. This is
what enables the experimental parallel runner, and serial runs follow the same model.

- **Symptom:** a file that relied on *another* file's discovery-time side effect fails — e.g. a
  module imported at the top of one file, a global variable, a changed working directory, or
  `-ForEach` data defined in a different file.
- **Fix:** make each test file self-contained. Do discovery-time setup in `BeforeDiscovery`, and
  import the modules the file needs in its own `BeforeAll`. For setup every file needs, use
  `Run.BeforeContainer` (config) or a `Pester.BeforeContainer.ps1` at the repo root.

```powershell
BeforeDiscovery {
    $cases = Get-Content "$PSScriptRoot/cases.json" | ConvertFrom-Json
}
Describe 'MyModule' {
    BeforeAll { Import-Module "$PSScriptRoot/MyModule.psm1" }
    It 'handles <Name>' -ForEach $cases { Invoke-Thing $Name | Should -Be 'ok' }
}
```

On-screen output also changes: one `Running tests from N files.` banner, per-file results, then a
single grand total. The old `Starting discovery in N files.` / `Discovery found X tests` framing is
gone for a normal run. The parallel runner (`Run.Parallel`) keeps the same result object with a few
edge cases — see https://pester.dev/docs/usage/result-object#parallel-runner-edge-cases.

### Empty or `$null` `-ForEach` throws
`-ForEach` (or `-TestCases`) given `$null` or `@()` now throws instead of silently skipping. This
catches the common bug of pointing `-ForEach` at a variable that wasn't defined in
`BeforeDiscovery`, or external data that failed to load.

- **Symptom:**
  ```
  Value can not be null or empty array. If this is expected, use -AllowNullOrEmptyForEach
  on this Describe, or set the Run.FailOnNullOrEmptyForEach configuration option to $false ...
  ```
- **Fix:** when the data can legitimately be empty, allow it on that specific block/test:
  ```powershell
  Describe 'Optional cases' -ForEach $cases -AllowNullOrEmptyForEach {
      It 'runs only when there is data' { }
  }
  ```
  You *can* disable the check for the whole run with `Run.FailOnNullOrEmptyForEach = $false`, but
  that brings back the silent skipping it's meant to catch — prefer fixing the data or using
  `-AllowNullOrEmptyForEach` where empty is genuinely expected.

### Duplicate setup/teardown blocks throw
A block may have only one of each `BeforeAll`/`BeforeEach`/`AfterAll`/`AfterEach`. Two of the same
(a common copy-paste bug) was silently allowed in v5; v6 throws.

- **Symptom:** `BeforeAll is already defined in this block. Each block can only have one BeforeAll.`
- **Fix:** combine them into one block:
  ```powershell
  Describe 'd' {
      BeforeAll {
          $a = 1
          $b = 2   # was a second BeforeAll
      }
  }
  ```

### Test names evaluate `<...>` templates as expressions
In v6 the content of every `<...>` token in a `Describe`/`Context`/`It` name is evaluated as a
PowerShell expression in the test's run scope (current `-ForEach` item and its properties, in-scope
variables, arithmetic, method calls). Everything outside `<...>` is kept literal. In v5 only simple
data/variable/property references were substituted.

- **Symptom:** a name containing expression-like content inside `<...>` that used to render
  literally now evaluates.
  ```powershell
  # v5 renders literally: "adds up to <($a + $b)>"; v6 evaluates: "adds up to 3"
  It 'adds up to <($a + $b)>' -ForEach @(@{ a = 1; b = 2 }) { }
  ```
- **Fix (to keep the literal text):** escape the leading `<` with a backtick:
  ```powershell
  It 'adds up to `<($a + $b)`>' -ForEach @(@{ a = 1; b = 2 }) { }
  ```

### `Assert-MockCalled` and `Assert-VerifiableMock` removed
Deprecated in v5, removed in v6.

- **Symptom:** `The term 'Assert-MockCalled' is not recognized ...`
- **Fix:**
  ```powershell
  # Assert-MockCalled     -> Should -Invoke
  # Assert-VerifiableMock -> Should -InvokeVerifiable
  Should -Invoke Get-Thing -Times 1 -Exactly
  Should -InvokeVerifiable
  ```

### Mocks no longer fall through to the real command
In v5, a call to a mocked command that matched none of your `-ParameterFilter` mocks quietly ran
the **real** command. v6 removes that implicit fall-through.

- **Symptom:**
  ```
  No mock for command 'Get-Thing' matched the call: none of the parameter filters matched,
  and there is no default mock to fall back to. Add a default mock ...
  ```
- **Fix:** add a default mock (no `-ParameterFilter`) for the unfiltered calls, or widen a filter:
  ```powershell
  Mock Get-Thing -MockWith { 'default' }                                # everything else
  Mock Get-Thing -ParameterFilter { $Name -eq 'a' } -MockWith { 'a' }   # special case
  ```

### `Set-ItResult -Pending` removed
`Pending` was never fully implemented in v5 and is gone.

- **Symptom:** `Parameter set cannot be resolved using the specified named parameters.`
- **Fix:** use `-Inconclusive` or `-Skipped`, or mark the test with `It … -Skip`:
  ```powershell
  Set-ItResult -Inconclusive -Because 'not implemented yet'
  ```

### Code coverage uses the Profiler tracer by default
Coverage no longer sets a breakpoint on every command; it uses the Profiler's tracer, which is much
faster on large code bases. `CodeCoverage.UseBreakpoints` is no longer experimental and defaults to
`$false`.

- **Symptom:** coverage numbers differ from v5 and you want the old behavior.
- **Fix:** `$config.CodeCoverage.UseBreakpoints = $true`.

### `CodeCoverage.OutputFormat = 'CoverageGutters'` removed
All coverage output is now relative to the repo root (`Run.RepoRoot`, found from the `.git`
directory), so plain `JaCoCo` already works with the Coverage Gutters extension.

- **Symptom:** setting `OutputFormat` to `'CoverageGutters'` throws an invalid-value error.
- **Fix:** use `JaCoCo` (default) or `Cobertura`.

### `Invoke-Pester` legacy (v4) parameters removed
Only the **Simple** set (`-Path`, `-Output`, `-Container`, `-Tag`, …) and the **Advanced** set
(`-Configuration`) remain. The v4-style parameter set is gone.

- **Symptom:** calls like `Invoke-Pester -Script … -OutputFile … -OutputFormat … -EnableExit
  -CodeCoverage …` fail with a parameter-binding error.
- **Fix:** use a configuration object (the full parameter→config map is in the
  "`Invoke-Pester` parameters → `New-PesterConfiguration`" section of [v4-to-v5.md](v4-to-v5.md)):
  ```powershell
  $config = New-PesterConfiguration
  $config.Run.Path = './tests'
  $config.Run.Exit = $true
  $config.TestResult.Enabled  = $true
  $config.TestResult.OutputPath = 'result.xml'
  $config.TestResult.OutputFormat = 'NUnitXml'
  $config.CodeCoverage.Enabled = $true
  $config.CodeCoverage.Path = './src'
  Invoke-Pester -Configuration $config
  ```

> Config convenience in v6: setting **any** non-default option in the `TestResult` or
> `CodeCoverage` section auto-enables that section, so you no longer have to set `.Enabled = $true`
> separately for a report to be written.

---

## New `Should-*` assertions — optional, NOT part of upgrading

v6 adds a new family of assertions — `Should-Be`, `Should-Throw`, `Should-Invoke`, and ~40 more
(note the hyphen: `Should-Be`, a distinct command, vs. the classic `Should -Be`) — with clearer
failure messages. **You do not need to touch your existing `Should -Be` assertions to upgrade**;
they keep working. Treat any `Should -Be` → `Should-Be` rewrite as a separate, later effort, not
part of this migration. Reference: https://pester.dev/docs/commands/Should-Be.

If you do try them: the new assertions take the actual value from the pipeline or `-Actual`. The
pipeline unwraps input (so `@(1)` becomes `1`, `@()` becomes `$null`, and a collection is
re-collected as `[object[]]`, losing types like `[int[]]`). Use `-Actual` when you need the exact
value or concrete collection type.

---

## v5 → v6 checklist

- [ ] Suite runs green on v5 first (baseline).
- [ ] Running on Windows PowerShell 5.1 or PowerShell 7.4+.
- [ ] `-ForEach`/`-TestCases` that can be empty marked `-AllowNullOrEmptyForEach` (or data fixed).
- [ ] Duplicate `Before*`/`After*` blocks combined.
- [ ] `Assert-MockCalled` → `Should -Invoke`; `Assert-VerifiableMock` → `Should -InvokeVerifiable`.
- [ ] Default `Mock` added wherever calls can miss every `-ParameterFilter`.
- [ ] `Set-ItResult -Pending` → `-Inconclusive`/`-Skipped`.
- [ ] `<...>` test-name templates that should stay literal are backtick-escaped.
- [ ] Each test file is self-contained (no reliance on another file's discovery-time state).
- [ ] `Invoke-Pester` legacy params → `New-PesterConfiguration`.
- [ ] Coverage `OutputFormat` is `JaCoCo`/`Cobertura`; `UseBreakpoints` set only if old numbers needed.
- [ ] Suite green on v6; diff reviewed; committed.