diff --git a/.chronus/changes/fix-python-prerelease-version-stamping-2026-5-27-13-30-0.md b/.chronus/changes/fix-python-prerelease-version-stamping-2026-5-27-13-30-0.md
new file mode 100644
index 00000000000..5cb580941de
--- /dev/null
+++ b/.chronus/changes/fix-python-prerelease-version-stamping-2026-5-27-13-30-0.md
@@ -0,0 +1,7 @@
+---
+changeKind: fix
+packages:
+  - "@typespec/http-client-python"
+---
+
+Fix CI publish failures by stamping prerelease version in Build-Packages.ps1. The `-Prerelease` flag was accepted but unused, causing every CI build to produce the same version and fail with a 409 conflict on the DevOps feed.
diff --git a/eng/emitters/pipelines/templates/steps/build-step.yml b/eng/emitters/pipelines/templates/steps/build-step.yml
index 2352b172f38..4dc72eb5cfa 100644
--- a/eng/emitters/pipelines/templates/steps/build-step.yml
+++ b/eng/emitters/pipelines/templates/steps/build-step.yml
@@ -85,6 +85,9 @@ steps:
       arguments: -UseTypeSpecNext:$${{ parameters.UseTypeSpecNext }}
       workingDirectory: $(selfRepositoryPath)
 
+  - script: npm install -g pnpm
+    displayName: Install pnpm
+
   - task: PowerShell@2
     displayName: "Run build script"
     name: ci_build
diff --git a/packages/http-client-python/eng/scripts/Build-Packages.ps1 b/packages/http-client-python/eng/scripts/Build-Packages.ps1
index 16fd147f5db..fe8e120cea8 100644
--- a/packages/http-client-python/eng/scripts/Build-Packages.ps1
+++ b/packages/http-client-python/eng/scripts/Build-Packages.ps1
@@ -1,30 +1,4 @@
 #Requires -Version 7.0
-<#
-.SYNOPSIS
-    Builds and packages the TypeSpec Python emitter for publishing.
-
-.DESCRIPTION
-    This script is called by the CI pipeline to create publishable packages.
-    It runs:
-      1. npm run build     - Compile TypeScript emitter and build Python wheel
-      2. npm run lint      - Run linting (Linux only)
-      3. npm pack          - Create npm tarball for publishing
-
-.PARAMETER BuildNumber
-    The build number for versioning.
-
-.PARAMETER Output
-    Output directory for built packages. Defaults to ./ci-build.
-
-.PARAMETER Prerelease
-    Flag indicating if this is a prerelease build.
-
-.PARAMETER PublishType
-    Type of publish: "internal" for dev feed, otherwise public.
-
-.EXAMPLE
-    ./Build-Packages.ps1 -Output ./dist
-#>
 
 param(
     [string] $BuildNumber,
@@ -35,13 +9,14 @@ param(
 
 $ErrorActionPreference = 'Stop'
 Set-StrictMode -Version 3.0
-
-# Setup paths and helpers
 $packageRoot = (Resolve-Path "$PSScriptRoot/../..").Path.Replace('\', '/')
 . "$packageRoot/../../eng/emitters/scripts/CommandInvocation-Helpers.ps1"
 Set-ConsoleEncoding
 
-# Helper function to write package info for downstream publishing
+Write-Host "Building packages for BuildNumber: '$BuildNumber', Output: '$Output', Prerelease: '$Prerelease', PublishType: '$PublishType'"
+
+$outputPath = $Output ? $Output : "$packageRoot/ci-build"
+
 function Write-PackageInfo {
     param(
         [string] $packageName,
@@ -55,66 +30,75 @@ function Write-PackageInfo {
     }
 
     @{
-        Name          = $packageName
-        Version       = $version
+        Name = $packageName
+        Version = $version
         DirectoryPath = $directoryPath
-        SdkType       = "client"
-        IsNewSdk      = $true
+        SdkType = "client"
+        IsNewSdk = $true
         ReleaseStatus = "Unreleased"
     } | ConvertTo-Json | Set-Content -Path "$packageInfoPath/$packageName.json"
 }
 
-Write-Host "Building packages for BuildNumber: '$BuildNumber', Output: '$Output', Prerelease: '$Prerelease', PublishType: '$PublishType'"
+function Set-VersionVariable {
+    param(
+        [string] $variableName,
+        [string] $version
+    )
 
-# Setup output directory
-$outputPath = $Output ? $Output : "$packageRoot/ci-build"
+    Write-Host "Setting output variable '$variableName' to $version"
+    Write-Host "##vso[task.setvariable variable=$variableName;isOutput=true]$version"
+}
+
+# create the output folders
 $outputPath = New-Item -ItemType Directory -Force -Path $outputPath | Select-Object -ExpandProperty FullName
 New-Item -ItemType Directory -Force -Path "$outputPath/packages" | Out-Null
 
-# Get package version
+Write-Host "Getting existing versions"
 $emitterVersion = node -p -e "require('$packageRoot/package.json').version"
-Write-Host "Package version: $emitterVersion"
 
+if ($BuildNumber) {
+    # set package versions
+    $versionTag = $Prerelease ? "-alpha" : "-beta"
+
+    $emitterVersion = "$emitterVersion$versionTag.$BuildNumber"
+    Set-VersionVariable -variableName "emitterVersion" -version $emitterVersion
+}
+
+# build and pack the emitter
 Push-Location "$packageRoot"
 try {
-    # Step 1: Build the emitter and generator
-    Write-Host "`n=== Building emitter and generator ===" -ForegroundColor Cyan
+    Write-Host "Working in $PWD"
+
     Invoke-LoggedCommand "npm run build" -GroupOutput
 
-    # Step 2: Run linting (Linux only, as CI runs on Linux)
-    if ($IsLinux) {
-        Write-Host "`n=== Running lint checks ===" -ForegroundColor Cyan
-        Invoke-LoggedCommand "npm run lint" -GroupOutput
-    }
+    if ($BuildNumber) {
+        Write-Host "Updating package.json to version: $emitterVersion`n"
 
-    # Step 3: Create npm package
-    Write-Host "`n=== Creating npm package ===" -ForegroundColor Cyan
-    Invoke-LoggedCommand "npm pack"
-    Copy-Item "typespec-http-client-python-$emitterVersion.tgz" -Destination "$outputPath/packages"
+        $packageJson = Get-Content -Raw "package.json" | ConvertFrom-Json -AsHashtable
+        $packageJson.version = $emitterVersion
+        $packageJson | ConvertTo-Json -Depth 100 | Out-File -Path "package.json" -Encoding utf8 -NoNewline -Force
+    }
 
-    # Step 4: Verify package can be installed
-    Write-Host "`n=== Verifying package installation ===" -ForegroundColor Cyan
-    Invoke-LoggedCommand "npm install typespec-http-client-python-$emitterVersion.tgz" -GroupOutput
+    # pack the emitter
+    $file = Invoke-LoggedCommand "npm pack -q"
+    Copy-Item $file -Destination "$outputPath/packages"
 
-    # Write package info for publishing pipeline
-    Write-PackageInfo -packageName "typespec-http-client-python" `
-                      -directoryPath "packages/http-client-python/emitter/src" `
-                      -version $emitterVersion
+    Write-PackageInfo -packageName "typespec-http-client-python" -directoryPath "packages/http-client-python/emitter/src" -version $emitterVersion
 }
 finally {
     Pop-Location
 }
 
-# Generate override URLs for internal publishing
-$overrides = @{}
 if ($PublishType -eq "internal") {
     $feedUrl = "https://pkgs.dev.azure.com/azure-sdk/public/_packaging/azure-sdk-for-js/npm/registry"
-    $overrides["@typespec/http-client-python"] = "$feedUrl/@typespec/http-client-python/-/http-client-python-$emitterVersion.tgz"
+
+    $overrides = @{
+        "@typespec/http-client-python" = "$feedUrl/@typespec/http-client-python/-/http-client-python-$emitterVersion.tgz"
+    }
+} else {
+    $overrides = @{}
 }
+
 $overrides | ConvertTo-Json | Set-Content "$outputPath/overrides.json"
 
-# Write package version matrix
 @{ "emitter" = $emitterVersion } | ConvertTo-Json | Set-Content "$outputPath/package-versions.json"
-
-Write-Host "`n=== Build complete ===" -ForegroundColor Green
-Write-Host "Output: $outputPath"