diff --git a/bin/cast b/bin/cast
index b0922b2..bb69c71 100755
--- a/bin/cast
+++ b/bin/cast
@@ -48,6 +48,20 @@ pkg_version() {
   CAST_PKG_PATH="$1/package.json" node -p 'require(process.env.CAST_PKG_PATH).version' 2>/dev/null || true
 }
 
+# Flip <root>/current to versions/<v> atomically: build the new link beside
+# it, rename(2) over. Plain ln -sfn is unlink+create — a window where current
+# names nothing and a concurrent 'cast' invocation dies mid-chain. The rename
+# rides node's fs.renameSync because the coreutils spelling is not portable —
+# GNU mv says "replace, don't descend" with -T, BSD/macOS says -h — while
+# rename(2) itself is POSIX and node is a cast prerequisite on every
+# platform. install.sh carries a byte-identical copy; test/install-sh.test.ts
+# diffs the two so they cannot drift.
+flip_current() {   # $1 = install root, $2 = version
+  ln -sfn "versions/$2" "$1/current.new.$$"
+  CAST_FLIP_NEW="$1/current.new.$$" CAST_FLIP_CUR="$1/current" \
+    node -e 'const fs = require("node:fs"); fs.renameSync(process.env.CAST_FLIP_NEW, process.env.CAST_FLIP_CUR);'
+}
+
 # The PATH symlinks that could ride this install: the one this invocation came
 # in on, CAST_BIN's, and the tier default's. Candidates only — every consumer
 # checks where a link actually points before touching it, so a symlink that is
@@ -99,9 +113,7 @@ cmd_use() {
   ir="$(install_root)" || die "this cast runs from a working tree ($ROOT), not a versioned install — nothing to switch"
   valid_version "$v" || die "not a sane version name: '$v' (a version is a directory name under versions/ — see 'cast versions')"
   [ -d "$ir/versions/$v" ] || die "no such version: $v (see 'cast versions')"
-  # An atomic flip, not unlink+create: ln -sfn leaves a window where current
-  # is missing; a rename over it does not.
-  ln -sfn "versions/$v" "$ir/current.new.$$" && mv -Tf "$ir/current.new.$$" "$ir/current"
+  flip_current "$ir" "$v"
   converge_bin_links "$ir"
   # Assert the EFFECTIVE result, not the intent: current must resolve to the
   # version asked for, and the chain's own binary must answer that version —
@@ -139,7 +151,7 @@ uninstall_confirm() {   # $1 = question
 # 'cast uninstall' — trees and symlinks, and it ENDS by PROVING the absence —
 # the last word is a re-check, not a hope.
 cmd_uninstall() {
-  local ir a ver="" all=0 force=0 cur p t leftover=""
+  local ir a ver="" all=0 force=0 cur p t leftover="" deduped=""
   local targets=()
   for a in "$@"; do
     case "$a" in
@@ -202,7 +214,10 @@ cmd_uninstall() {
     [ -n "$t" ] || t="$(readlink "$p" 2>/dev/null || true)"
     case "$t" in "$ir"/*) targets+=("$p") ;; esac
   done < <(bin_links)
-  mapfile -t targets < <(printf '%s\n' "${targets[@]}" | awk '!seen[$0]++')
+  # De-dup, portably: macOS ships bash 3.2, which has no mapfile.
+  deduped="$(printf '%s\n' "${targets[@]}" | awk '!seen[$0]++')"
+  targets=()
+  while IFS= read -r p; do targets+=("$p"); done <<<"$deduped"
 
   # rm's exit code is not the verdict — the absence assert below is (a
   # half-removed tree must be reported as INCOMPLETE by name, not as a crash).
diff --git a/install.sh b/install.sh
index b710490..d4b5c6f 100644
--- a/install.sh
+++ b/install.sh
@@ -79,6 +79,13 @@ command -v npm  >/dev/null 2>&1 || die "npm is required but was not found."
 NODE_MAJOR="$(node -p 'process.versions.node.split(".")[0]')"
 [ "$NODE_MAJOR" -ge 22 ] || die "node >=22.12 is required (found $(node -v))."
 
+# readlink -f is load-bearing across the layout (the launcher and every verb
+# resolve the symlink chain with it). GNU always has it; Apple's readlink
+# grew -f in macOS 12.3 (March 2022). Probe once and refuse loudly on the
+# museum pieces, instead of failing weirdly mid-flip later.
+readlink -f / >/dev/null 2>&1 \
+  || die "this system's readlink does not support -f (macOS older than 12.3?) — upgrade, or 'brew install coreutils'."
+
 # age is what decrypts the state repo's secrets — apply/diff shell out to it.
 if ! command -v age >/dev/null 2>&1; then
   warn "age not found — 'cast apply' and 'cast diff' will fail until it is installed."
@@ -92,13 +99,18 @@ else
   SRCDESC="$REPO@$REF"
 fi
 
-# Flip $DEST/current to versions/<v> atomically: build the new link beside it,
-# rename over. Plain ln -sfn is unlink+create — a window where current names
-# nothing and a concurrent 'cast' invocation dies mid-chain. bin/cast's
-# cmd_use flips with the same pattern.
-flip_current() {
-  ln -sfn "versions/$1" "$DEST/current.new.$$"
-  mv -Tf "$DEST/current.new.$$" "$DEST/current"
+# Flip <root>/current to versions/<v> atomically: build the new link beside
+# it, rename(2) over. Plain ln -sfn is unlink+create — a window where current
+# names nothing and a concurrent 'cast' invocation dies mid-chain. The rename
+# rides node's fs.renameSync because the coreutils spelling is not portable —
+# GNU mv says "replace, don't descend" with -T, BSD/macOS says -h — while
+# rename(2) itself is POSIX and node is a cast prerequisite on every
+# platform. bin/cast carries a byte-identical copy; test/install-sh.test.ts
+# diffs the two so they cannot drift.
+flip_current() {   # $1 = install root, $2 = version
+  ln -sfn "versions/$2" "$1/current.new.$$"
+  CAST_FLIP_NEW="$1/current.new.$$" CAST_FLIP_CUR="$1/current" \
+    node -e 'const fs = require("node:fs"); fs.renameSync(process.env.CAST_FLIP_NEW, process.env.CAST_FLIP_CUR);'
 }
 
 # --- migrate a pre-versioning flat install -----------------------------------
@@ -119,7 +131,7 @@ if [ -e "$DEST/bin/cast" ] && [ ! -d "$DEST/versions" ]; then
   mv "$DEST" "$staging"
   mkdir -p "$DEST/versions"
   mv "$staging" "$DEST/versions/$flat_ver"
-  flip_current "$flat_ver"
+  flip_current "$DEST" "$flat_ver"
   mkdir -p "$BINDIR"
   ln -sfn "$DEST/current/bin/cast" "$BINDIR/cast"
   log "migrated: it now lives at $DEST/versions/$flat_ver (still current)"
@@ -244,7 +256,7 @@ fi
 cur="$(readlink -f "$DEST/current" 2>/dev/null || true)"
 want="$(readlink -f "$VDIR")"
 if [ -z "$cur" ] || [ ! -d "$cur" ]; then
-  flip_current "$new_ver"
+  flip_current "$DEST" "$new_ver"
   log "default version: $new_ver"
 elif [ "$cur" = "$want" ]; then
   : # already the default — nothing to flip
@@ -255,7 +267,7 @@ elif [ "$newly_installed" -eq 0 ]; then
   log "the default stays $(basename "$cur") — 'cast use $new_ver' switches."
 else
   old_ver="$(basename "$cur")"
-  flip_current "$new_ver"
+  flip_current "$DEST" "$new_ver"
   log "default version switched: $old_ver -> $new_ver ('cast use $old_ver' switches back)"
 fi
 
diff --git a/test/install-sh.test.ts b/test/install-sh.test.ts
index c6fb83e..4397795 100644
--- a/test/install-sh.test.ts
+++ b/test/install-sh.test.ts
@@ -330,7 +330,7 @@ describe("the shared gates cannot drift", () => {
     return text.slice(start, end + 2);
   }
 
-  for (const fn of ["valid_version", "pkg_version"]) {
+  for (const fn of ["valid_version", "pkg_version", "flip_current"]) {
     it(`${fn}() is byte-identical between install.sh and bin/cast`, () => {
       expect(extractFunction(INSTALL_SH, fn)).toBe(
         extractFunction(REAL_BIN_CAST, fn),
@@ -338,3 +338,21 @@ describe("the shared gates cannot drift", () => {
     });
   }
 });
+
+describe("portability — cast runs on the operator's own machine, macOS included", () => {
+  // box#79/rig#36 could assume GNU userland and bash 4+ because boxes run
+  // Linux; cast cannot. These greps pin the two spellings that bit (or
+  // nearly bit) for real: `mv -T` (GNU-only — BSD/macOS mv has no -T, the
+  // flip now rides rename(2) via node) and `mapfile` (bash 4 — macOS ships
+  // bash 3.2). A reintroduction fails HERE, not on the first operator Mac.
+  for (const file of [INSTALL_SH, REAL_BIN_CAST]) {
+    const name = file.split("/").pop();
+    it(`${name} carries no GNU mv -T and no bash-4 mapfile`, () => {
+      const text = readFileSync(file, "utf8");
+      // "mv -T" as an invocation — the comments explaining WHY it is absent
+      // spell it "mv says … with -T", which this must not match.
+      expect(text).not.toMatch(/\bmv\s+-[A-Za-z]*T/);
+      expect(text).not.toMatch(/^\s*mapfile\b/m);
+    });
+  }
+});