diff --git a/pester/winoneshot-compat.Tests.ps1 b/pester/winoneshot-compat.Tests.ps1
new file mode 100644
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+++ b/pester/winoneshot-compat.Tests.ps1
@@ -0,0 +1,353 @@
+#===========================================================================
+# Tests - WinOneShot Catalog Compatibility
+#===========================================================================
+# WinOneShot downloads these files from WinUtil's main/config raw GitHub URLs.
+# Keep the known field types aligned with WinOneShot Models.cs; extra fields are allowed.
+
+$repoRoot = (Resolve-Path (Join-Path $PSScriptRoot "..")).Path
+$catalogCases = @(
+    @{
+        Name = "applications.json"
+        Path = Join-Path $repoRoot "config\applications.json"
+        ScalarFields = @{
+            category = "String"
+            choco = "String"
+            content = "String"
+            description = "String"
+            foss = "Boolean"
+            link = "String"
+            winget = "String"
+        }
+        ArrayFields = @{}
+    }
+    @{
+        Name = "appx.json"
+        Path = Join-Path $repoRoot "config\appx.json"
+        ScalarFields = @{
+            Category = "String"
+            Content = "String"
+            Description = "String"
+            Panel = "String"
+            PackageId = "String"
+            StoreId = "String"
+        }
+        ArrayFields = @{}
+    }
+    @{
+        Name = "feature.json"
+        Path = Join-Path $repoRoot "config\feature.json"
+        ScalarFields = @{
+            Content = "String"
+            Description = "String"
+            category = "String"
+            panel = "String"
+            Order = "String"
+            link = "String"
+            Type = "String"
+            ButtonWidth = "String"
+            function = "String"
+        }
+        ArrayFields = @{
+            feature = "StringArray"
+            InvokeScript = "StringArray"
+        }
+    }
+    @{
+        Name = "tweaks.json"
+        Path = Join-Path $repoRoot "config\tweaks.json"
+        ScalarFields = @{
+            Content = "String"
+            Description = "String"
+            category = "String"
+            panel = "String"
+            Order = "String"
+            Type = "String"
+            Checked = "String"
+            ButtonWidth = "String"
+            ComboItems = "String"
+            link = "String"
+        }
+        ArrayFields = @{
+            registry = "RegistryArray"
+            service = "ServiceArray"
+            ScheduledTask = "ScheduledTaskArray"
+            appx = "StringArray"
+            InvokeScript = "StringArray"
+            UndoScript = "StringArray"
+        }
+    }
+)
+$invalidCatalogCases = @(
+    @{
+        Name = "applications.json"
+        Json = '[{"App":{}}]'
+        ScalarFields = $catalogCases[0].ScalarFields
+        ArrayFields = $catalogCases[0].ArrayFields
+        ExpectedError = "must contain a JSON object"
+    }
+    @{
+        Name = "applications.json"
+        Json = "{}"
+        ScalarFields = $catalogCases[0].ScalarFields
+        ArrayFields = $catalogCases[0].ArrayFields
+        ExpectedError = "does not contain any entries"
+    }
+    @{
+        Name = "applications.json"
+        Json = '{"Broken":null}'
+        ScalarFields = $catalogCases[0].ScalarFields
+        ArrayFields = $catalogCases[0].ArrayFields
+        ExpectedError = "applications.json.Broken is null"
+    }
+    @{
+        Name = "applications.json"
+        Json = '{"App":{"foss":"yes"}}'
+        ScalarFields = $catalogCases[0].ScalarFields
+        ArrayFields = $catalogCases[0].ArrayFields
+        ExpectedError = "foss must be a Boolean"
+    }
+    @{
+        Name = "appx.json"
+        Json = '{"App":{"PackageId":[]}}'
+        ScalarFields = $catalogCases[1].ScalarFields
+        ArrayFields = $catalogCases[1].ArrayFields
+        ExpectedError = "PackageId must be a string or null"
+    }
+    @{
+        Name = "feature.json"
+        Json = '{"Feature":{"feature":"Example-Feature"}}'
+        ScalarFields = $catalogCases[2].ScalarFields
+        ArrayFields = $catalogCases[2].ArrayFields
+        ExpectedError = "feature must be an array or null"
+    }
+    @{
+        Name = "tweaks.json"
+        Json = '{"Tweak":{"registry":{}}}'
+        ScalarFields = $catalogCases[3].ScalarFields
+        ArrayFields = $catalogCases[3].ArrayFields
+        ExpectedError = "registry must be an array or null"
+    }
+)
+
+BeforeAll {
+    function script:Get-WinOneShotJsonKind {
+        param($Value)
+
+        if ($null -eq $Value) {
+            return "Null"
+        }
+        if ($Value -is [string]) {
+            return "String"
+        }
+        if ($Value -is [bool]) {
+            return "Boolean"
+        }
+        if ($Value -is [array]) {
+            return "Array"
+        }
+        if ($Value -is [pscustomobject]) {
+            return "Object"
+        }
+
+        return $Value.GetType().Name
+    }
+
+    function script:Get-WinOneShotProperty {
+        param(
+            [Parameter(Mandatory)]
+            [pscustomobject]$Object,
+
+            [Parameter(Mandatory)]
+            [string]$Name
+        )
+
+        foreach ($property in $Object.PSObject.Properties) {
+            if ($property.Name -ieq $Name) {
+                $property
+            }
+        }
+    }
+
+    function script:Get-WinOneShotTypeError {
+        param(
+            [Parameter(Mandatory)]
+            [string]$Location,
+
+            $Value,
+
+            [Parameter(Mandatory)]
+            [string]$ExpectedType
+        )
+
+        $actualKind = Get-WinOneShotJsonKind -Value $Value
+
+        if ($ExpectedType -eq "String") {
+            if ($actualKind -notin @("String", "Null")) {
+                return "$Location must be a string or null; found $actualKind"
+            }
+            return
+        }
+
+        if ($ExpectedType -eq "Boolean") {
+            if ($actualKind -ne "Boolean") {
+                return "$Location must be a Boolean; found $actualKind"
+            }
+            return
+        }
+
+        if ($actualKind -eq "Null") {
+            return
+        }
+        if ($actualKind -ne "Array") {
+            return "$Location must be an array or null; found $actualKind"
+        }
+
+        if ($ExpectedType -eq "StringArray") {
+            for ($index = 0; $index -lt $Value.Count; $index++) {
+                $elementKind = Get-WinOneShotJsonKind -Value $Value[$index]
+                if ($elementKind -notin @("String", "Null")) {
+                    "$Location[$index] must be a string or null; found $elementKind"
+                }
+            }
+            return
+        }
+
+        $nestedFields = switch ($ExpectedType) {
+            "RegistryArray" {
+                @("Path", "Name", "Value", "OriginalValue", "DefaultState", "Type")
+            }
+            "ServiceArray" {
+                @("Name", "StartupType", "OriginalType")
+            }
+            "ScheduledTaskArray" {
+                @("Name")
+            }
+            default {
+                throw "Unknown WinOneShot contract type: $ExpectedType"
+            }
+        }
+
+        for ($index = 0; $index -lt $Value.Count; $index++) {
+            $element = $Value[$index]
+            $elementKind = Get-WinOneShotJsonKind -Value $element
+            if ($elementKind -eq "Null") {
+                continue
+            }
+            if ($elementKind -ne "Object") {
+                "$Location[$index] must be an object or null; found $elementKind"
+                continue
+            }
+
+            foreach ($field in $nestedFields) {
+                foreach ($property in (Get-WinOneShotProperty -Object $element -Name $field)) {
+                    $propertyKind = Get-WinOneShotJsonKind -Value $property.Value
+                    if ($propertyKind -notin @("String", "Null")) {
+                        "$Location[$index].$($property.Name) must be a string or null; found $propertyKind"
+                    }
+                }
+            }
+        }
+    }
+
+    function script:Get-WinOneShotCatalogError {
+        param(
+            [Parameter(Mandatory)]
+            [string]$Name,
+
+            [Parameter(Mandatory)]
+            [string]$Json,
+
+            [Parameter(Mandatory)]
+            [hashtable]$ScalarFields,
+
+            [Parameter(Mandatory)]
+            [hashtable]$ArrayFields
+        )
+
+        try {
+            $catalog = $Json | ConvertFrom-Json -NoEnumerate -ErrorAction Stop
+        } catch {
+            return "WinOneShot cannot parse ${Name}: $_"
+        }
+
+        if ((Get-WinOneShotJsonKind -Value $catalog) -ne "Object") {
+            return "$Name must contain a JSON object."
+        }
+
+        $entries = @($catalog.PSObject.Properties)
+        if ($entries.Count -eq 0) {
+            return "$Name does not contain any entries."
+        }
+
+        foreach ($entry in $entries) {
+            $entryKind = Get-WinOneShotJsonKind -Value $entry.Value
+            if ($entryKind -eq "Null") {
+                "$Name.$($entry.Name) is null"
+                continue
+            }
+            if ($entryKind -ne "Object") {
+                "$Name.$($entry.Name) must be an object; found $entryKind"
+                continue
+            }
+
+            foreach ($field in $ScalarFields.Keys) {
+                foreach ($property in (Get-WinOneShotProperty -Object $entry.Value -Name $field)) {
+                    Get-WinOneShotTypeError -Location "$Name.$($entry.Name).$($property.Name)" -Value $property.Value -ExpectedType $ScalarFields[$field]
+                }
+            }
+
+            foreach ($field in $ArrayFields.Keys) {
+                foreach ($property in (Get-WinOneShotProperty -Object $entry.Value -Name $field)) {
+                    Get-WinOneShotTypeError -Location "$Name.$($entry.Name).$($property.Name)" -Value $property.Value -ExpectedType $ArrayFields[$field]
+                }
+            }
+        }
+    }
+}
+
+Describe "WinOneShot catalog download contract" {
+    foreach ($catalogCase in $catalogCases) {
+        It "publishes <Name> at the exact raw-download path" -TestCases $catalogCase {
+            param([string]$Path)
+
+            Test-Path -LiteralPath $Path -PathType Leaf | Should -BeTrue
+            $exactNameMatch = @(
+                Get-ChildItem -LiteralPath (Split-Path -Path $Path -Parent) -File |
+                    Where-Object { $_.Name -ceq (Split-Path -Path $Path -Leaf) }
+            )
+            $exactNameMatch.Count | Should -Be 1
+        }
+
+        It "loads <Name> with WinOneShot-compatible field types" -TestCases $catalogCase {
+            param(
+                [string]$Name,
+                [string]$Path,
+                [hashtable]$ScalarFields,
+                [hashtable]$ArrayFields
+            )
+
+            $json = Get-Content -LiteralPath $Path -Raw
+            $errors = @(Get-WinOneShotCatalogError -Name $Name -Json $json -ScalarFields $ScalarFields -ArrayFields $ArrayFields)
+
+            if ($errors.Count -gt 0) {
+                throw ($errors -join "`n")
+            }
+        }
+    }
+}
+
+Describe "WinOneShot catalog contract guard" {
+    It "rejects consumer-breaking <Name> fixture: <ExpectedError>" -TestCases $invalidCatalogCases {
+        param(
+            [string]$Name,
+            [string]$Json,
+            [hashtable]$ScalarFields,
+            [hashtable]$ArrayFields,
+            [string]$ExpectedError
+        )
+
+        $errors = @(Get-WinOneShotCatalogError -Name $Name -Json $Json -ScalarFields $ScalarFields -ArrayFields $ArrayFields)
+
+        $errors -join "`n" | Should -BeLike "*$ExpectedError*"
+    }
+}