Should-BeString does not truncate identical prefix in diff message

[fflaten]

Discussed in #2946

^{Originally posted by Der-Bayer July 31, 2026}
We are migrating a Pester test suite from Pester 5.7.1 to Pester 6.

Our tests compare the complete content of large generated files. The files can contain 10,000 and more lines. In Pester 5, we currently use:

$expected = Get-Content -LiteralPath $expectedPath -Raw
Get-Content -LiteralPath $actualPath -Raw | Should -BeExactly $expected

When this fails, Should -BeExactly provides useful output: it identifies the first differing character and truncates the unchanged prefix and suffix. This remains readable even for very large strings.

For the Pester 6 assertion syntax we tried:

$actual = Get-Content -LiteralPath $actualPath -Raw
$expected = Get-Content -LiteralPath $expectedPath -Raw

$actual | Should-BeString -Expected $expected -CaseSensitive

What is the recommended Pester 6 assertion, switch, or configuration setting to get the same diagnostic behavior as classic Should -BeExactly for large strings—specifically, showing the first difference and a limited context before and after it instead of rendering the full string?

If this behavior is not currently available, would it be reasonable to add it to Should-BeString?

Sidenote: TestDrive is not working for me, it expands to C:

Complete Pester File:

Describe 'Vergleich klassisch vs. neu' {
    BeforeAll {
        $actualPath = "TestDrive:\actual.txt"
        $expectedPath = "TestDrive:\expected.txt"

        $actualLines = 1..10000 | ForEach-Object {
            "Line {0:D5}" -f $_
        }

        $expectedLines = [string[]]$actualLines.Clone()
        $expectedLines[5432] = "Line 99999" # Change one line to make the test fail

        Set-Content -LiteralPath $actualPath   -Value $actualLines
        Set-Content -LiteralPath $expectedPath -Value $expectedLines
    }

    It 'V5 Syntax: BeExactly' {
        $expected = Get-Content $expectedPath -Raw
        Get-Content $actualPath -Raw | Should -BeExactly $expected
    }

    It 'V6 Syntax: Should-BeString' {
        $actual = Get-Content $actualPath -Raw
        $expected = Get-Content $expectedPath -Raw
        $actual | Should-BeString $expected 
    }
}

Collapsed Output (Cuttet / too long for GitHub)

Pester v6.1.0-alpha2

Running tests from 1 files.

Running tests from 'C:\Code\Pester-BeExactlyV6.Tests.ps1'
Describing Vergleich klassisch vs. neu
  [-] V5 Syntax: BeExactly 38ms
   Expected strings to be the same, but they were different.
   String lengths are both 120000.
   Strings differ at index 65189.
   Expected: '...5420␍␊Line 05421␍␊Line 05422␍␊Line 05423␍␊Line 05424␍␊Line 05425␍␊Line 05426␍␊Line 05427␍␊Line 05428␍␊Line 05429␍␊Line 05430␍␊Line 05431␍␊Line 05432␍␊Line 99999␍␊Line 05434␍␊Line 05435␍␊...'
   But was:  '...5420␍␊Line 05421␍␊Line 05422␍␊Line 05423␍␊Line 05424␍␊Line 05425␍␊Line 05426␍␊Line 05427␍␊Line 05428␍␊Line 05429␍␊Line 05430␍␊Line 05431␍␊Line 05432␍␊Line 05433␍␊Line 05434␍␊Line 05435␍␊...'
              --------------------------------------------------------------------------------------------------------------------------------------------------------------^
   at Get-Content $actualPath -Raw | Should -BeExactly $expected, C:\Code\Pester-BeExactlyV6.Tests.ps1:19
  [-] V6 Syntax: Should-BeString 26ms
   Expected strings to be the same, but they were different.
   String lengths are both 120000.
   Strings differ at index 65189.
   Expected: 'Line 00001␍␊Line 00002␍␊Line 00003␍␊Line 00004␍␊Line 00005␍␊Line 00006␍␊Line 00007␍␊Line 00008␍␊Line 00009␍␊Line 00010␍␊Line 00011␍␊Line 00012␍␊Line 00013␍␊Line 00014␍␊Line 00015␍␊Line 00016␍␊Line 00017␍␊Line 00018␍␊Line 00019␍␊Line 00020␍␊Line 00021␍␊Line 00022␍␊Line 00023␍␊Line 00024␍␊Line 00025␍␊Line 00026␍␊Line 00027␍␊Line 00028␍␊Line 00029␍␊Line 00030␍␊Line 00031␍␊Line 00032␍␊Line 00033␍␊Line 00034␍␊Line 00035␍␊Line 00036␍␊Line 00037␍␊Line 00038␍␊Line 00039␍␊Line 00040␍␊Line 00041␍␊Line 00042␍␊Line 00043␍␊Line 00044␍␊Line 00045␍␊Line 00046␍␊Line 00047␍␊Line [Cuttet to long for GitHub]05432␍␊Line 05433␍␊Line 05434␍␊Line 05435␍␊Line 05436␍␊Line 05437␍␊Line 05438␍␊Line 05439␍␊Line 05440␍␊Line 05441␍␊Line 05442␍␊Line 05443␍␊Line 05444␍␊Line 05445␍␊Line 05446␍␊Line 05447␍␊Line 05448␍␊Line 05449␍␊Line 05450␍␊Line 05451␍␊Line 05452␍␊Line 05453␍␊Line 05454␍␊Line 05455␍␊Line 05456␍␊Line 05457␍␊Line 05458␍␊Line 05459␍␊Line 05460␍␊Line 05461␍␊Line 05462␍␊Line 05463␍␊Line 05464␍␊Line 05465␍␊Line 05466␍␊Line 05467␍␊Line 05468␍␊Line 05469␍␊Line 05470␍␊Line 05471␍␊Line 05472␍␊Line 05473␍␊Line 05474␍␊Line 05475␍␊Line 05476␍␊Line 05477␍␊Line 05478␍␊Line 05479␍␊Line 05480␍␊Line 05481␍␊Line 05482␍␊Line 05483␍␊Line 05484␍␊Line 05485␍␊Line 05486␍␊Line 05487␍␊Line 05488␍␊Line 05489␍␊Line 05490␍␊Line 05491␍␊Line 05492␍␊Line 05493␍␊Line 05494␍␊Line 05495␍␊Line 05496␍␊Line 05497␍␊Line 05498␍␊Line 05499␍␊Line 05500␍␊Line 05501␍␊Line 05502␍␊Line 05503␍␊Line 05504␍␊Line 05505␍␊Line 05506␍␊Line 05507␍␊Line 05508␍␊Line 05509␍␊Line 05510␍␊Line 05511␍␊Line 05512␍␊Line 05513␍␊Line 05514␍␊Line 05515␍␊Line 05516␍␊Line 05517␍␊Line 05518␍␊Line 05519␍␊Line 05520␍␊Line 05521␍␊Line 05522␍␊Line 05523␍␊Line 05524␍␊Line 05525␍␊Line 05526␍␊Line 05527␍␊Line 05528␍␊Line 05529␍␊Line 05530␍␊Line 05531␍␊Line 05532␍␊Line 05533␍␊Line 05534␍␊Line 05535␍␊Line 05536␍␊Line 05537␍␊Line 05538␍␊Line 05539␍␊Line 05540␍␊Line 05541␍␊Line 05542␍␊Line 05543␍␊Line 05544␍␊Line 05545␍␊Line 05546␍␊Line 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GitHub]--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------^
   at $actual | Should-BeString $expected -CaseSensitive, C:\Code\Pester-BeExactlyV6.Tests.ps1:25
Tests completed in 206ms
Tests Passed: 0, Failed: 2, Skipped: 0, Inconclusive: 0, NotRun: 0

[fflaten]

Missing equivalent to Format-AsExcerpt in new Get-StringDifferenceMessage helper.