diff --git a/.crow/build-all-versions-install-deps.yaml b/.crow/build-all-versions-install-deps.yaml
index 1c2df64..97610df 100644
--- a/.crow/build-all-versions-install-deps.yaml
+++ b/.crow/build-all-versions-install-deps.yaml
@@ -10,21 +10,10 @@ variables:
       - arm64
     default: amd64
   OS:
-    description: "Base OS image name."
-    options:
-      - alpine
-      - redhat
-      - ubuntu
+    description: "Base OS image name (e.g. alpine, redhat, ubuntu)."
     default: alpine
   OS_VERSION:
-    description: "OS image tag. Must match OS (alpine: 3.24; redhat: 8/9/10; ubuntu: jammy/noble)."
-    options:
-      - "3.24"
-      - "8"
-      - "9"
-      - "10"
-      - "jammy"
-      - "noble"
+    description: "OS version / image tag (e.g. 3.24, 9, jammy, noble)."
     default: "3.24"
   R_VERSION:
     description: "Primary R version under /opt/R."
diff --git a/.crow/build-all-versions.yaml b/.crow/build-all-versions.yaml
index ed392fd..b2ede89 100644
--- a/.crow/build-all-versions.yaml
+++ b/.crow/build-all-versions.yaml
@@ -11,21 +11,10 @@ variables:
       - arm64
     default: amd64
   OS:
-    description: "Base OS image name."
-    options:
-      - alpine
-      - redhat
-      - ubuntu
+    description: "Base OS image name (e.g. alpine, redhat, ubuntu)."
     default: alpine
   OS_VERSION:
-    description: "OS image tag. Must match OS (alpine: 3.24; redhat: 8/9/10; ubuntu: jammy/noble)."
-    options:
-      - "3.24"
-      - "8"
-      - "9"
-      - "10"
-      - "jammy"
-      - "noble"
+    description: "OS version / image tag (e.g. 3.24, 9, jammy, noble)."
     default: "3.24"
   R_VERSION:
     description: "Primary R version under /opt/R."
@@ -101,6 +90,13 @@ 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: ${R_PKG_CACHE_DIR}
       R_LIBS_USER: /mnt/cache/R-pkgs
diff --git a/local/build-all.R b/local/build-all.R
index 6c3725c..f8e8efd 100644
--- a/local/build-all.R
+++ b/local/build-all.R
@@ -68,31 +68,8 @@ chunk <- chunk[!chunk$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.
-# 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"
+platform <- paste(Sys.getenv("OS"), gsub("[.]", "", Sys.getenv("OS_VERSION")), sep = "-")
+arch <- Sys.getenv("ARCH")
 con <- DBI::dbConnect(
   RPostgres::Postgres(),
   dbname = "build_metadata",