diff --git a/scripts/build.ps1 b/scripts/build.ps1
index 0a608b7..727102f 100644
--- a/scripts/build.ps1
+++ b/scripts/build.ps1
@@ -209,8 +209,8 @@ $leftoverPlaceholders = @(Get-ChildItem $output -File -Recurse | ForEach-Object
     # __AI_CONFIG_ROOT__ is resolved at install time, once the destination is known; it is
     # expected to remain in generated output. -cmatch keeps this case-sensitive so Python
     # dunder names (__main__, __name__) in flashbang.sh are not mistaken for placeholders.
-    $content = (Get-Content -LiteralPath $_.FullName -Raw) -replace '__AI_CONFIG_ROOT__|__POWERSHELL_COMMAND__', ''
-    if ($content -cmatch '__[A-Z0-9_]+__' -and $content -notmatch '__CLAUDE_CODE_MAX_TOOL_USE_CONCURRENCY__') { $_.FullName }
+    $content = (Get-Content -LiteralPath $_.FullName -Raw) -replace '__AI_CONFIG_ROOT__|__POWERSHELL_COMMAND__|__CLAUDE_CODE_MAX_TOOL_USE_CONCURRENCY__', ''
+    if ($content -cmatch '__[A-Z0-9_]+__') { $_.FullName }
 })
 if ($leftoverPlaceholders.Count) { throw "Unresolved template placeholders in: $($leftoverPlaceholders -join ', ')" }
 
diff --git a/scripts/test-build-validation.ps1 b/scripts/test-build-validation.ps1
index de821ff..16d02ff 100644
--- a/scripts/test-build-validation.ps1
+++ b/scripts/test-build-validation.ps1
@@ -24,6 +24,9 @@ function Test-BuildFails([string]$Label, [scriptblock]$Mutate) {
     if (-not $failed) { throw "Expected build to fail for case '$Label' but it succeeded." }
 }
 
+$baseline = New-RepoCopy
+& (Join-Path $baseline 'scripts/build.ps1') -Summary | Out-Null
+
 Test-BuildFails 'invalid Claude settings.json' {
     param($copy)
     Add-Content (Join-Path $copy 'adapters/claude/config/settings.json') '} this is not json {'
@@ -47,6 +50,12 @@ Test-BuildFails 'leftover template placeholder' {
     Add-Content (Join-Path $copy 'shared/global-instructions.md') "`n__NOT_A_REAL_PLACEHOLDER__"
 }
 
+Test-BuildFails 'unknown placeholder beside an allowed installer placeholder' {
+    param($copy)
+    $path = Join-Path $copy 'adapters/claude/config/settings.json'
+    (Get-Content $path -Raw).Replace('__CLAUDE_CODE_MAX_TOOL_USE_CONCURRENCY__', '__CLAUDE_CODE_MAX_TOOL_USE_CONCURRENCY____UNKNOWN_PLACEHOLDER__') | Set-Content $path
+}
+
 Remove-Item $work -Recurse -Force -ErrorAction SilentlyContinue
 if ($Summary) { Write-Output 'Tests: PASS | build validation' } else { Write-Output 'PASS build validation: invalid JSON, duplicate agent, duplicate plugin, and leftover placeholders are all rejected' }
 exit 0
diff --git a/scripts/test-build-validation.sh b/scripts/test-build-validation.sh
index d80bda5..32534bc 100755
--- a/scripts/test-build-validation.sh
+++ b/scripts/test-build-validation.sh
@@ -31,6 +31,9 @@ test_build_fails() {
   rm -rf -- "$copy"
 }
 
+baseline=$(new_repo_copy)
+bash "$baseline/scripts/build.sh" --summary >/dev/null
+
 mutate_invalid_json() {
   printf '} this is not json {' >> "$1/adapters/claude/config/settings.json"
 }
@@ -52,4 +55,9 @@ mutate_leftover_placeholder() {
 }
 test_build_fails 'leftover template placeholder' mutate_leftover_placeholder
 
+mutate_mixed_placeholders() {
+  sed -i 's/__CLAUDE_CODE_MAX_TOOL_USE_CONCURRENCY__/__CLAUDE_CODE_MAX_TOOL_USE_CONCURRENCY__ __UNKNOWN_PLACEHOLDER__/' "$1/adapters/claude/config/settings.json"
+}
+test_build_fails 'unknown placeholder beside an allowed installer placeholder' mutate_mixed_placeholders
+
 if [ "$summary" = true ]; then printf 'Tests: PASS | build validation\n'; else printf 'PASS build validation: invalid JSON, duplicate agent, duplicate plugin, and leftover placeholders are all rejected\n'; fi