diff --git a/.crow/build-all-versions.yaml b/.crow/build-all-versions.yaml
index 77f5b77..41987ad 100644
--- a/.crow/build-all-versions.yaml
+++ b/.crow/build-all-versions.yaml
@@ -90,13 +90,6 @@ steps:
         from_secret: GITHUB_PAT
       # normal env vars
       GIT_USER: pat-s
-      # Export the platform selectors as runtime env vars. They are otherwise
-      # only available for ${...} interpolation (image/volume), so build-all.R's
-      # `Sys.getenv("OS")/("OS_VERSION")/("ARCH")` would be empty and its
-      # already-built dedup query would match platform "-" and skip nothing.
-      OS: ${OS}
-      OS_VERSION: ${OS_VERSION}
-      ARCH: ${ARCH}
       # set the location of the 'pkgcache' cache dir which persists the R package dependencies needed to install the packages themselves
       R_PKG_CACHE_DIR: ${R_PKG_CACHE_DIR}
       R_LIBS_USER: /mnt/cache/R-pkgs
diff --git a/local/build-all.R b/local/build-all.R
index f8e8efd..6c3725c 100644
--- a/local/build-all.R
+++ b/local/build-all.R
@@ -68,8 +68,31 @@ chunk <- chunk[!chunk$Package %in% exclude, ]
 # pkgs_to_build.rds is a static snapshot from the install-deps step, so on a
 # restart it still lists everything an interrupted run already produced. The
 # metadata DB reflects that progress, so we re-derive the remaining set here.
-platform <- paste(Sys.getenv("OS"), gsub("[.]", "", Sys.getenv("OS_VERSION")), sep = "-")
-arch <- Sys.getenv("ARCH")
+# Derive platform + arch from the running container, mirroring the codename ->
+# platform mapping bincraft uses internally. The OS/OS_VERSION selectors are
+# workflow-level CI variables that are not injected into the container
+# environment, so Sys.getenv() would return "" and this pre-filter would query
+# platform "-" and skip nothing.
+codename <- bincraft::set_codename(NULL)
+platform <- switch(
+  codename,
+  jammy = "ubuntu-2204",
+  noble = "ubuntu-2404",
+  resolute = "ubuntu-2604",
+  rhel10 = "redhat-10",
+  rhel9 = "redhat-9",
+  rhel8 = "redhat-8",
+  alpine320 = "alpine-320",
+  alpine321 = "alpine-321",
+  alpine322 = "alpine-322",
+  alpine323 = "alpine-323",
+  alpine324 = "alpine-324",
+  alpine325 = "alpine-325",
+  alpine326 = "alpine-326",
+  NA_character_
+)
+local_machine <- Sys.info()[["machine"]]
+arch <- if (grepl("arm64|aarch64", local_machine)) "arm64" else "amd64"
 con <- DBI::dbConnect(
   RPostgres::Postgres(),
   dbname = "build_metadata",