diff --git a/README.md b/README.md
index 8960b0c..a68587f 100644
--- a/README.md
+++ b/README.md
@@ -49,13 +49,15 @@ By default that installs the **latest release** — the installer resolves the
 release tag off GitHub's `releases/latest` redirect (no API, no token) and
 downloads exactly that tree, so two operators running it get the same box.
 If the resolution fails it says so and stops — it never silently hands out
-`main`. `BOX_REF` picks another channel (a set ref is tried as a tag first,
-then as a branch — [#83](https://github.com/heavy-duty/box/issues/83)):
+`main`. `BOX_REF` picks another channel: a named ref is tried as a tag first,
+then as a branch ([#83](https://github.com/heavy-duty/box/issues/83)), while a
+full 40-hex commit SHA selects GitHub's immutable commit archive:
 
 ```sh
 curl -fsSL .../install.sh | bash                  # the latest release (default)
 curl -fsSL .../install.sh | BOX_REF=0.6.0 bash    # pin a release
 curl -fsSL .../install.sh | BOX_REF=main bash     # the development tip
+curl -fsSL .../install.sh | BOX_REF=<full-commit-sha> bash  # exact tree
 ```
 
 (A dev tree's `VERSION` carries a `-dev` suffix, so it lands beside your
diff --git a/bin/box b/bin/box
index d50d8de..cc603cb 100755
--- a/bin/box
+++ b/bin/box
@@ -1096,36 +1096,48 @@ load_template() {
   T_DISK="${disk:-${BOX_DISK:-${T_DISK:-60GiB}}}"
 }
 
-# The ONE substitution a template gets — user-data.yaml is otherwise passed to
-# Incus verbatim. The tenant seeds preinstall rig, which inverts the rig→box
-# install edge (rig#28: rig installs box on hosts; now box guests install rig),
-# and that edge needs a pin point (#81): the seed carries @RIG_REPO@ /
-# @RIG_REF@ tokens, resolved here from the mint environment — RIG_REPO
-# (default heavy-duty/rig) and RIG_REF (default main). Both directions track
-# main unpinned today, said honestly (the same treatment rig#29 gave box's own
-# unpinned install) until a release flow exists (rig#32 / #83). The values are
+# The ONE substitution a template gets — @RIG_INSTALL@ — while user-data.yaml
+# is otherwise passed to Incus verbatim. The tenant seeds preinstall rig, which
+# inverts the rig→box install edge (rig#28: rig installs box on hosts; now box
+# guests install rig), and that edge needs a pin point (#81). The renderer
+# resolves the mint environment's RIG_REPO (default heavy-duty/rig) and RIG_REF
+# (default main) into the whole installer command. Named refs keep the normal
+# tag/branch channel; a drill-pinned commit carries its immutable archive as
+# RIG_INSTALL_SOURCE (#152). Both values are
 # allowlist-validated BEFORE touching the YAML: they land inside a runcmd
 # shell line, so a quote, a space or a newline smuggled through the
 # environment must die on the host, never execute in the guest. bash's =~
 # anchors to the whole string — a multi-line value cannot sneak one clean
 # line past it the way a line-oriented grep would.
 # The rig pin, resolved from the mint environment, in ONE place: render_userdata
-# substitutes it into the seed, and the mint stamp (#103) records it onto the
+# builds it into the seed, and the mint stamp (#103) records it onto the
 # instance. Two spellings of the same default would eventually disagree, and a
 # stamp that disagrees with the seed is worse than no stamp at all.
 rig_repo() { printf '%s\n' "${RIG_REPO:-heavy-duty/rig}"; }
 rig_ref()  { printf '%s\n' "${RIG_REF:-main}"; }
 
 render_userdata() {
-  local f="$1" repo data ref
+  local f="$1" repo data ref install
   repo="$(rig_repo)"; ref="$(rig_ref)"
   [[ "$repo" =~ ^[A-Za-z0-9._-]+/[A-Za-z0-9._-]+$ ]] \
     || die "RIG_REPO must look like owner/repo: $repo"
   [[ "$ref" =~ ^[A-Za-z0-9._/-]+$ ]] \
     || die "RIG_REF must be a plain ref name (letters, digits, . _ / -): $ref"
+  if [[ "$ref" =~ ^[0-9a-f]{40}$ ]]; then
+    # rig's installer currently resolves named tags/branches. A release drill
+    # pins the moving ref before mint, so hand that installer the immutable
+    # commit archive as a local source instead of asking it to resolve again
+    # inside the guest (#152).
+    printf -v install '%s' \
+      "rig_source=/tmp/rig-$ref.tar.gz; rig_installer=/tmp/rig-install-$ref.sh; curl -fsSL https://github.com/$repo/archive/$ref.tar.gz -o \"\$rig_source\" && curl -fsSL https://raw.githubusercontent.com/$repo/$ref/install.sh -o \"\$rig_installer\" && HOME=/root RIG_REPO=\"$repo\" RIG_REF=\"$ref\" RIG_INSTALL_SOURCE=\"\$rig_source\" bash \"\$rig_installer\"; rig_rc=\$?; rm -f \"\$rig_source\" \"\$rig_installer\"; exit \"\$rig_rc\""
+  else
+    printf -v install 'curl -fsSL https://raw.githubusercontent.com/%s/%s/install.sh | HOME=/root RIG_REPO="%s" RIG_REF="%s" bash' \
+      "$repo" "$ref" "$repo" "$ref"
+  fi
   data="$(cat "$f")"
   data="${data//@RIG_REPO@/$repo}"
   data="${data//@RIG_REF@/$ref}"
+  data="${data//@RIG_INSTALL@/$install}"
   printf '%s\n' "$data"
 }
 
diff --git a/install.sh b/install.sh
index 3522d45..d673ebb 100755
--- a/install.sh
+++ b/install.sh
@@ -24,10 +24,11 @@ set -euo pipefail
 # downloading — for CI and the drill, so what lands is the code under review.
 
 REPO="${BOX_REPO:-heavy-duty/box}"
-# Three install channels, one knob (#83): BOX_REF unset installs the LATEST
+# Four install channels, one knob (#83, #152): BOX_REF unset installs the LATEST
 # RELEASE (the tag resolved from GitHub's releases/latest redirect, below);
 # BOX_REF=<tag> pins a release; BOX_REF=<branch> (say, main) is the dev
-# channel. A set ref is tried as a tag first, then as a branch.
+# channel; and a full 40-hex commit SHA is fetched immutably. A named ref is
+# tried as a tag first, then as a branch.
 REF="${BOX_REF:-}"
 # Root installs GLOBALLY, non-root installs per-user. box's install tree is
 # EXECUTED by other users (the multi-user host path: rig installs box once, every
@@ -232,17 +233,27 @@ else
   fi
   INSTALLED_FROM="$REPO@$REF"
   log "installing box from $REPO@$REF"
-  # A ref is a TAG first (the pinned-release channel), a branch second (the
-  # dev channel, BOX_REF=main) — and the fallback only exists for a ref the
-  # OPERATOR named: a resolved latest tag has no branch to fall through to.
-  URL="https://github.com/$REPO/archive/refs/tags/$REF.tar.gz"
-  log "downloading $URL"
-  if ! curl -fsSL "$URL" -o "$TMPDIR/box.tar.gz"; then
-    [ -n "${BOX_REF:-}" ] || die "failed to download $URL"
-    URL="https://github.com/$REPO/archive/refs/heads/$REF.tar.gz"
-    log "no tag '$REF' — trying it as a branch: $URL"
+  if [[ "$REF" =~ ^[0-9a-f]{40}$ ]]; then
+    # The drill resolves moving candidate refs before it mutates a host, then
+    # installs the exact object it measured (#152). GitHub's commit archive is
+    # outside refs/{tags,heads}; guessing either namespace makes every SHA 404.
+    URL="https://github.com/$REPO/archive/$REF.tar.gz"
+    log "downloading immutable commit $URL"
     curl -fsSL "$URL" -o "$TMPDIR/box.tar.gz" \
-      || die "failed to download it as either — '$REF' is neither a tag nor a branch of $REPO"
+      || die "failed to download commit $REF from $REPO"
+  else
+    # A named ref is a TAG first (the pinned-release channel), a branch second
+    # (the dev channel, BOX_REF=main). The fallback only exists for a ref the
+    # OPERATOR named: a resolved latest tag has no branch to fall through to.
+    URL="https://github.com/$REPO/archive/refs/tags/$REF.tar.gz"
+    log "downloading $URL"
+    if ! curl -fsSL "$URL" -o "$TMPDIR/box.tar.gz"; then
+      [ -n "${BOX_REF:-}" ] || die "failed to download $URL"
+      URL="https://github.com/$REPO/archive/refs/heads/$REF.tar.gz"
+      log "no tag '$REF' — trying it as a branch: $URL"
+      curl -fsSL "$URL" -o "$TMPDIR/box.tar.gz" \
+        || die "failed to download it as either — '$REF' is neither a tag nor a branch of $REPO"
+    fi
   fi
 
   log "extracting archive"
diff --git a/templates/claude-box/user-data.yaml b/templates/claude-box/user-data.yaml
index 9a7baf8..9a8bcee 100644
--- a/templates/claude-box/user-data.yaml
+++ b/templates/claude-box/user-data.yaml
@@ -30,4 +30,4 @@ runcmd:
   # HOME=/root: cloud-init runs runcmd as root but with NO $HOME in the
   # environment, and the rig installer (set -u) reads $HOME for its DEST —
   # measured live: the mint died with "HOME: unbound variable" without it.
-  - curl -fsSL https://raw.githubusercontent.com/@RIG_REPO@/@RIG_REF@/install.sh | HOME=/root RIG_REPO="@RIG_REPO@" RIG_REF="@RIG_REF@" bash
+  - '@RIG_INSTALL@'
diff --git a/templates/codex-box/user-data.yaml b/templates/codex-box/user-data.yaml
index 32fa730..1599fb0 100644
--- a/templates/codex-box/user-data.yaml
+++ b/templates/codex-box/user-data.yaml
@@ -30,4 +30,4 @@ runcmd:
   # HOME=/root: cloud-init runs runcmd as root but with NO $HOME in the
   # environment, and the rig installer (set -u) reads $HOME for its DEST —
   # measured live: the mint died with "HOME: unbound variable" without it.
-  - curl -fsSL https://raw.githubusercontent.com/@RIG_REPO@/@RIG_REF@/install.sh | HOME=/root RIG_REPO="@RIG_REPO@" RIG_REF="@RIG_REF@" bash
+  - '@RIG_INSTALL@'
diff --git a/templates/grok-box/user-data.yaml b/templates/grok-box/user-data.yaml
index 28c2ab3..fd2d840 100644
--- a/templates/grok-box/user-data.yaml
+++ b/templates/grok-box/user-data.yaml
@@ -30,4 +30,4 @@ runcmd:
   # HOME=/root: cloud-init runs runcmd as root but with NO $HOME in the
   # environment, and the rig installer (set -u) reads $HOME for its DEST —
   # measured live: the mint died with "HOME: unbound variable" without it.
-  - curl -fsSL https://raw.githubusercontent.com/@RIG_REPO@/@RIG_REF@/install.sh | HOME=/root RIG_REPO="@RIG_REPO@" RIG_REF="@RIG_REF@" bash
+  - '@RIG_INSTALL@'
diff --git a/templates/kimi-box/user-data.yaml b/templates/kimi-box/user-data.yaml
index e86ba31..a4f1519 100644
--- a/templates/kimi-box/user-data.yaml
+++ b/templates/kimi-box/user-data.yaml
@@ -30,4 +30,4 @@ runcmd:
   # HOME=/root: cloud-init runs runcmd as root but with NO $HOME in the
   # environment, and the rig installer (set -u) reads $HOME for its DEST —
   # measured live: the mint died with "HOME: unbound variable" without it.
-  - curl -fsSL https://raw.githubusercontent.com/@RIG_REPO@/@RIG_REF@/install.sh | HOME=/root RIG_REPO="@RIG_REPO@" RIG_REF="@RIG_REF@" bash
+  - '@RIG_INSTALL@'
diff --git a/templates/staging-box/user-data.yaml b/templates/staging-box/user-data.yaml
index 3a230d3..76cd330 100644
--- a/templates/staging-box/user-data.yaml
+++ b/templates/staging-box/user-data.yaml
@@ -31,4 +31,4 @@ runcmd:
   # HOME=/root: cloud-init runs runcmd as root but with NO $HOME in the
   # environment, and the rig installer (set -u) reads $HOME for its DEST —
   # measured live: the mint died with "HOME: unbound variable" without it.
-  - curl -fsSL https://raw.githubusercontent.com/@RIG_REPO@/@RIG_REF@/install.sh | HOME=/root RIG_REPO="@RIG_REPO@" RIG_REF="@RIG_REF@" bash
+  - '@RIG_INSTALL@'
diff --git a/test/cli.sh b/test/cli.sh
index 6dc71e0..6804fca 100644
--- a/test/cli.sh
+++ b/test/cli.sh
@@ -266,15 +266,28 @@ check "render_userdata: the pin's defaults came with it (guards the grep)" 0 "he
 check "render_userdata: the extracted function is valid bash"    0 "" bash -n "$RUFN"
 
 SEED="$(mktemp)"
-printf '#cloud-config\nruncmd:\n  - curl -fsSL https://raw.githubusercontent.com/@RIG_REPO@/@RIG_REF@/install.sh | RIG_REPO="@RIG_REPO@" RIG_REF="@RIG_REF@" bash\n' > "$SEED"
+printf "#cloud-config\nruncmd:\n  - '@RIG_INSTALL@'\n" > "$SEED"
 # shellcheck disable=SC2016  # $0/$1 expand in the child shell, by design
 rud() { # rud [VAR=val ...] — render the fixture seed through the real function
   env "$@" bash -c 'die() { echo "box: $*" >&2; exit 1; }; . "$0"; render_userdata "$1"' "$RUFN" "$SEED"
 }
 check "render_userdata: defaults pin heavy-duty/rig" 0 "githubusercontent.com/heavy-duty/rig/main/install.sh" rud
 check "render_userdata: defaults feed the installer's own env too" 0 'RIG_REPO="heavy-duty/rig" RIG_REF="main"' rud
+check "render_userdata: the rig install pins HOME=/root (runcmd has no \$HOME)" 0 "HOME=/root " rud
 check "render_userdata: RIG_REPO/RIG_REF override at mint" 0 "dan-claude-bot/rig/feat/bootstrap-roles/install.sh" \
   rud RIG_REPO=dan-claude-bot/rig RIG_REF=feat/bootstrap-roles
+RIG_COMMIT=89abcdef0123456789abcdef0123456789abcdef
+check "render_userdata: a rig commit downloads its immutable archive" 0 "archive/$RIG_COMMIT.tar.gz" \
+  rud RIG_REPO=heavy-duty/rig RIG_REF="$RIG_COMMIT"
+# $rig_source belongs to the rendered guest shell.
+# shellcheck disable=SC2016
+check "render_userdata: a rig commit feeds the archive to the installer" 0 'RIG_INSTALL_SOURCE="$rig_source"' \
+  rud RIG_REPO=heavy-duty/rig RIG_REF="$RIG_COMMIT"
+# Expansion belongs to the bash -c fixture.
+# shellcheck disable=SC2016
+check "render_userdata: a named rig ref keeps the normal channel" 1 "" \
+  bash -c 'die() { exit 1; }; RIG_REPO=heavy-duty/rig RIG_REF=main; . "$1"; render_userdata "$2" | grep -q RIG_INSTALL_SOURCE' \
+  _ "$RUFN" "$SEED"
 # shellcheck disable=SC2016  # $0/$1 expand in the child shells, by design
 check "render_userdata: no token survives the render" 1 "" \
   bash -c 'env bash -c "die() { echo box: \$*; exit 1; }; . \"\$0\"; render_userdata \"\$1\"" "$1" "$2" | grep -q @RIG_' _ "$RUFN" "$SEED"
@@ -326,26 +339,13 @@ for d in "$ROOT"/templates/*/; do
   # The thin-template contract (#81), both halves per template:
   #
   # THE SEED — a template that names a tenant role (BOX_BOOTSTRAP_ROLE) must
-  # preinstall rig carrying BOTH pin tokens, on the installer URL and on the
-  # installer's own env, or the pin is a half-truth: a mint would fetch one
-  # ref's installer and install another ref's tree.
+  # delegate its rig install to render_userdata. That one renderer owns both
+  # the named-ref channel and the immutable-commit archive channel.
   # ------------------------------------------------------------------------
   trole="$(tpl "$ROOT" "$t" | sed -n 's/.*ROLE=\([^ ]*\).*/\1/p')"
   if [ -n "$trole" ]; then
-    check "template '$t': the seed installs rig (role '$trole')" 0 "" \
-      grep -q 'install.sh' "$d/user-data.yaml"
-    # shellcheck disable=SC2016  # $1 expands in the child shell, by design
-    check "template '$t': the rig install carries the @RIG_REPO@ pin token" 0 "" \
-      bash -c 'grep "install.sh" "$1" | grep -q "@RIG_REPO@/@RIG_REF@"' _ "$d/user-data.yaml"
-    # shellcheck disable=SC2016
-    check "template '$t': the pin reaches the installer's env too" 0 "" \
-      bash -c 'grep "install.sh" "$1" | grep -q "RIG_REPO=\"@RIG_REPO@\" RIG_REF=\"@RIG_REF@\""' _ "$d/user-data.yaml"
-    # HOME=/root: a scar found live — cloud-init's runcmd has no $HOME and
-    # rig's installer (set -u) dies on it (rig#39). The pin must survive
-    # every seed rewrite.
-    # shellcheck disable=SC2016
-    check "template '$t': the rig install pins HOME=/root (runcmd has no \$HOME)" 0 "" \
-      bash -c 'grep "install.sh" "$1" | grep -q "HOME=/root "' _ "$d/user-data.yaml"
+    check "template '$t': the seed installs rig through the renderer (role '$trole')" 0 "" \
+      grep -q "^[[:space:]]*- '@RIG_INSTALL@'\$" "$d/user-data.yaml"
   fi
   # ------------------------------------------------------------------------
   # THE ABSENCE — no tenant content in ANY template, ever again. Everything a
diff --git a/test/release.sh b/test/release.sh
index d47372d..62461ad 100644
--- a/test/release.sh
+++ b/test/release.sh
@@ -109,6 +109,16 @@ check "dev channel: tries the tag first" 0 "refs/tags/main.tar.gz" head -1 "$L3"
 check "dev channel: then downloads the branch" 0 "" \
   grep -qF "archive/refs/heads/main.tar.gz" "$L3"
 
+COMMIT_REF=0123456789abcdef0123456789abcdef01234567
+H5="$WORK/h5"; B5="$WORK/b5"; L5="$WORK/c5.log"
+check "commit channel: installs an immutable full SHA" 0 "installing box from heavy-duty/box@$COMMIT_REF" \
+  ninst "$H5" "$B5" BOX_REF="$COMMIT_REF" FAKE_CURL_LOG="$L5" \
+    FAKE_SERVE_URL="https://github.com/heavy-duty/box/archive/$COMMIT_REF.tar.gz"
+check "commit channel: downloads the commit archive directly" 0 "archive/$COMMIT_REF.tar.gz" cat "$L5"
+check "commit channel: never guesses tag or branch namespaces" 1 "" grep -Eq 'refs/(tags|heads)' "$L5"
+check "commit channel: records the immutable source" 0 "heavy-duty/box@$COMMIT_REF" \
+  cat "$H5/versions/9.9.9/INSTALLED_FROM"
+
 H4="$WORK/h4"; B4="$WORK/b4"; L4="$WORK/c4.log"
 check "resolution failure: names the latest-release probe" 1 "could not resolve the latest release" \
   ninst "$H4" "$B4" FAKE_CURL_RC=6 FAKE_CURL_LOG="$L4"
@@ -122,6 +132,7 @@ check "unknown ref names both attempted channels" 1 "neither a tag nor a branch"
 check "README documents the latest-release channel" 0 "" grep -qF 'latest release' "$ROOT/README.md"
 check "README documents the pinned channel" 0 "" grep -qF 'BOX_REF=0.6.0' "$ROOT/README.md"
 check "README documents the dev channel" 0 "" grep -qF 'BOX_REF=main' "$ROOT/README.md"
+check "README documents the immutable commit channel" 0 "" grep -qF 'BOX_REF=<full-commit-sha>' "$ROOT/README.md"
 
 echo "---"
 echo "$PASS passed, $FAIL failed"