From d9f07c9c8fb299de62278b5192fb380008481605 Mon Sep 17 00:00:00 2001
From: Aleksandr Gladkov <algladkov@microsoft.com>
Date: Thu, 23 Jul 2026 17:53:23 +0200
Subject: [PATCH] Bug 643397: Fix Miapp base branch resolution

---
 build/scripts/Miapp/MicroAppGitHelper.psm1 | 8 +++++++-
 build/scripts/Miapp/README.md              | 2 +-
 2 files changed, 8 insertions(+), 2 deletions(-)

diff --git a/build/scripts/Miapp/MicroAppGitHelper.psm1 b/build/scripts/Miapp/MicroAppGitHelper.psm1
index 87d2c656ef..f183a672fa 100644
--- a/build/scripts/Miapp/MicroAppGitHelper.psm1
+++ b/build/scripts/Miapp/MicroAppGitHelper.psm1
@@ -440,7 +440,13 @@ function Initialize-MiappRepoBranchName {
     param()
 
     if ($env:RepoBranchName) {
-        return $env:RepoBranchName
+        git show-ref --verify --quiet "refs/remotes/origin/$env:RepoBranchName"
+        if ($LASTEXITCODE -eq 0) {
+            return $env:RepoBranchName
+        }
+
+        Write-Warning "RepoBranchName '$env:RepoBranchName' does not exist on origin. Falling back to origin/HEAD."
+        $env:RepoBranchName = $null
     }
 
     [string] $originHeadRef = (git symbolic-ref --quiet --short refs/remotes/origin/HEAD 2>$null)
diff --git a/build/scripts/Miapp/README.md b/build/scripts/Miapp/README.md
index e87189d930..cd1e99327f 100644
--- a/build/scripts/Miapp/README.md
+++ b/build/scripts/Miapp/README.md
@@ -266,7 +266,7 @@ $MiappConfig = @{
 
 ### Environment Variables
 
-- `$env:RepoBranchName` - Base branch name used for comparisons (defaults to `origin/HEAD`, typically `main`, when not set)
+- `$env:RepoBranchName` - Base branch name used for comparisons. A valid explicit value is preserved. An unset value, or a stale value that does not exist on the current repository's `origin`, falls back to `origin/HEAD` (typically `main`).
 - `$env:MIAPP_DIR` - Directory for miapp temporary files (default: $env:USERPROFILE)
 
 ### Output