diff --git a/scripts/ci/ensure-windows-toolchain.ps1 b/scripts/ci/ensure-windows-toolchain.ps1
index 8146182d..7c20ecca 100644
--- a/scripts/ci/ensure-windows-toolchain.ps1
+++ b/scripts/ci/ensure-windows-toolchain.ps1
@@ -16,6 +16,34 @@ trap {
   exit 1
 }
 
+# --- reclaim disk before building -----------------------------------------------------------------
+# The windows-amd64 runner's system volume is intentionally small (100 GB) and a full Windows CI pass
+# writes ~50 GB of cargo target output into C:\t (x64) / C:\t-a64 (arm64). Left to accumulate across
+# runs that overflows the disk and the build dies with "no space on device" (os error 112) - exactly
+# what took the Windows host build down. The runner bakes in a reclaimer + a scheduled task that keeps
+# an idle box lean (unom/infra's setup-gitea-runner-base.ps1 ->
+# C:\Users\Public\act-runner\clean-runner-disk.ps1); call it here too so THIS job starts with headroom
+# regardless of when that task last ran. Threshold mode (no -Force): it only prunes when actually low,
+# so incremental-compile caches survive when there's room. Best-effort - a cleanup hiccup must never
+# fail the build.
+$reclaimer = 'C:\Users\Public\act-runner\clean-runner-disk.ps1'
+try {
+  if (Test-Path $reclaimer) {
+    & powershell.exe -NoProfile -ExecutionPolicy Bypass -File $reclaimer
+  }
+  else {
+    # Fallback for a runner not yet re-baked with the infra reclaimer: prune the big target dirs when low.
+    $freeGb = [math]::Round((Get-PSDrive C).Free / 1GB, 1)
+    Write-Host "[ensure-toolchain] clean-runner-disk.ps1 absent; C: free ${freeGb} GB"
+    if ($freeGb -lt 35) {
+      foreach ($d in 'C:\t', 'C:\t-a64') {
+        if (Test-Path $d) { Write-Host "  reclaiming $d"; Remove-Item $d -Recurse -Force -ErrorAction SilentlyContinue }
+      }
+    }
+  }
+}
+catch { Write-Warning "disk reclaim step failed (non-fatal): $_" }
+
 $ciDir = $PSScriptRoot
 & "$ciDir\provision-windows-wdk.ps1"
 & "$ciDir\provision-windows-punktfunk-extras.ps1"