diff --git a/.github/scripts/install-exotic-toolchains.sh b/.github/scripts/install-exotic-toolchains.sh
new file mode 100755
index 0000000..fe0eade
--- /dev/null
+++ b/.github/scripts/install-exotic-toolchains.sh
@@ -0,0 +1,124 @@
+#!/usr/bin/env bash
+set -euo pipefail
+
+: "${RUNNER_TEMP:?RUNNER_TEMP must be set}"
+: "${GITHUB_PATH:?GITHUB_PATH must be set}"
+: "${GITHUB_ENV:?GITHUB_ENV must be set}"
+
+readonly ARK_VERSION="4.7.1"
+readonly ARK_SHA256="3d9e46ac5a8ecb19c9e4e87391fc5a8bf7c582ad6cea9599c1976f843d662861"
+readonly FAT_IMAGE="fatscript/fry@sha256:fa8bfe0d759ce4bd7146ce9c97521b2cc72f1905d496468a1537fd9e36addc80"
+readonly BRUIJN_COMMIT="9f1ac8856d09666a7444874f80b91b876929eecf"
+readonly BRUIJN_SHA256="5186ffd7898f081d2961b1e2a4f9b207bc6104f1cff0372761c513d46815b7ca"
+readonly LANGUAGE84_VERSION="0.8"
+readonly LANGUAGE84_SHA256="b51807d1b7d06f1fdba762fdbf99c8c7fe762ec1a0c96c0be054657c9551cec0"
+
+dotenv_excludes=(
+  "--exclude=.env"
+  "--exclude=*/.env"
+  "--exclude=*.env"
+  "--exclude=.env.*"
+  "--exclude=*/.env.*"
+  "--exclude=*.env.*"
+)
+
+download_and_verify() {
+  local url=$1
+  local output=$2
+  local sha256=$3
+
+  curl --fail --location --retry 3 --show-error --silent \
+    --output "$output" "$url"
+  printf '%s  %s\n' "$sha256" "$output" | sha256sum --check -
+}
+
+install_arkscript() {
+  local archive="$RUNNER_TEMP/arkscript-${ARK_VERSION}.zip"
+  local root="$RUNNER_TEMP/arkscript-${ARK_VERSION}"
+
+  download_and_verify \
+    "https://github.com/ArkScript-lang/Ark/releases/download/v${ARK_VERSION}/linux-clang-16.zip" \
+    "$archive" \
+    "$ARK_SHA256"
+
+  mkdir -p "$root"
+  unzip -q "$archive" -d "$root" -x \
+    '.env' '*/.env' '*.env' '.env.*' '*/.env.*' '*.env.*'
+  chmod +x "$root/arkscript"
+  "$root/arkscript" --version
+  printf '%s\n' "$root" >> "$GITHUB_PATH"
+}
+
+install_fatscript() {
+  docker pull "$FAT_IMAGE"
+}
+
+install_bruijn() {
+  local cache="$RUNNER_TEMP/bruijn-tool-v1.tar.gz"
+  local tool="$RUNNER_TEMP/bruijn-tool"
+
+  mkdir -p "$tool"
+  if [ -f "$cache" ]; then
+    tar "${dotenv_excludes[@]}" -xzf "$cache" -C "$tool"
+  else
+    local archive="$RUNNER_TEMP/bruijn-${BRUIJN_COMMIT}.tar.gz"
+    local source="$RUNNER_TEMP/bruijn-source"
+    local stack_root="$RUNNER_TEMP/bruijn-stack-root"
+
+    download_and_verify \
+      "https://github.com/marvinborner/bruijn/archive/${BRUIJN_COMMIT}.tar.gz" \
+      "$archive" \
+      "$BRUIJN_SHA256"
+
+    mkdir -p "$source" "$tool/bin" "$tool/share"
+    tar "${dotenv_excludes[@]}" -xzf "$archive" -C "$source" \
+      --strip-components=1
+
+    (
+      cd "$source"
+      stack --stack-root "$stack_root" \
+        --local-bin-path "$tool/bin" \
+        --no-terminal \
+        install
+    )
+
+    local install_root
+    local data_dir
+    install_root=$(
+      cd "$source"
+      stack --stack-root "$stack_root" --no-terminal path --local-install-root
+    )
+    data_dir=$(printf '%s\n' "$install_root"/share/*/bruijn-0.1.0.0)
+    [ -f "$data_dir/config" ]
+
+    tar "${dotenv_excludes[@]}" -cf - -C "$data_dir" . |
+      tar "${dotenv_excludes[@]}" -xf - -C "$tool/share"
+    tar "${dotenv_excludes[@]}" -czf "$cache" -C "$tool" .
+  fi
+
+  [ -x "$tool/bin/bruijn" ]
+  [ -f "$tool/share/config" ]
+  printf '%s\n' "$tool/bin" >> "$GITHUB_PATH"
+  printf 'bruijn_datadir=%s\n' "$tool/share" >> "$GITHUB_ENV"
+}
+
+install_language84() {
+  local archive="$RUNNER_TEMP/language84-${LANGUAGE84_VERSION}.tar.xz"
+  local root="$RUNNER_TEMP/language84-${LANGUAGE84_VERSION}"
+
+  download_and_verify \
+    "https://norstrulde.org/language84/language84-${LANGUAGE84_VERSION}.tar.xz" \
+    "$archive" \
+    "$LANGUAGE84_SHA256"
+
+  mkdir -p "$root"
+  tar "${dotenv_excludes[@]}" -xJf "$archive" -C "$root" \
+    --strip-components=1
+  [ -f "$root/84_stable.c" ]
+  printf 'LANGUAGE84_HOME=%s\n' "$root" >> "$GITHUB_ENV"
+}
+
+install_arkscript
+install_fatscript
+install_bruijn
+install_language84
diff --git a/.github/workflows/test.yml b/.github/workflows/test.yml
index f727812..b0c7e4f 100644
--- a/.github/workflows/test.yml
+++ b/.github/workflows/test.yml
@@ -11,8 +11,8 @@ jobs:
     steps:
       - uses: actions/checkout@v4
 
-      # Python, Node.js, Perl, GCC/G++ 14, and the .NET SDK (F#) come
-      # preinstalled on the runner.
+      # Python, Node.js, Perl, GCC/G++ 14, Clang, Stack, Docker, and the
+      # .NET SDK (F#) come preinstalled on the runner.
       - name: Install toolchains not preinstalled on the runner
         run: |
           sudo apt-get update
@@ -28,5 +28,16 @@ jobs:
 
       - uses: oven-sh/setup-bun@v2
 
+      # Bruijn has no release binary. Cache only the finished, relocatable
+      # interpreter bundle; the source and all build dependencies remain pinned.
+      - name: Cache built Bruijn interpreter
+        uses: actions/cache@v4
+        with:
+          path: ${{ runner.temp }}/bruijn-tool-v1.tar.gz
+          key: bruijn-${{ runner.os }}-${{ runner.arch }}-9f1ac885-lts-22.12-v1
+
+      - name: Install exotic language toolchains
+        run: .github/scripts/install-exotic-toolchains.sh
+
       - name: Run all language tests
         run: ./run-tests.sh
diff --git a/ROADMAP.md b/ROADMAP.md
index d5715ea..8dda1db 100644
--- a/ROADMAP.md
+++ b/ROADMAP.md
@@ -35,10 +35,13 @@ Folders without a `test.sh` are skipped until backfilled.
       numeral typing so PRED works; add test.sh + expected output to that
       branch, let CI validate it, then merge #29. Kotlin: runner has a JDK;
       install kotlinc in the workflow.
-- [ ] **Step 6 — exotic languages: ArkScript, FatScript, bruijn, Language 84.**
-      Each needs its own toolchain acquisition (GitHub releases, cargo/stack
-      installs, or building from source). If one is genuinely unobtainable in
-      CI, document that in its folder instead of leaving it silently untested.
+- [x] **Step 6 — exotic languages: ArkScript, FatScript, bruijn, Language 84.**
+      ArkScript uses the checksum-verified official 4.7.1 archive; FatScript
+      uses its official 4.5.0 image pinned by digest in a read-only,
+      network-disabled container; bruijn is built from a pinned upstream commit
+      and locked Stack snapshot, then cached as a relocatable bundle; Language
+      84 uses the checksum-verified official 0.8 source and its documented Clang
+      build. All four expected-output baselines came from observed runs.
 - [ ] **Step 7 — flip the default and update the front door.** Once all
       folders have tests: make `run-tests.sh` fail on folders *missing* a
       test.sh, update README contribution instructions to require
diff --git a/languages/a/arkscript/README.md b/languages/a/arkscript/README.md
index 394ea16..12e5cfe 100644
--- a/languages/a/arkscript/README.md
+++ b/languages/a/arkscript/README.md
@@ -1,26 +1,26 @@
 # ArkScript
 
-Running this solution requires either ArkScript 4.0.0 or a pre-release of ArkScript 4.0.0.
+This implementation runs on ArkScript 4.
 
-## Running the solution (docker)
+GitHub Actions downloads the official ArkScript 4.7.1 Linux archive and
+verifies its SHA-256 digest before running the program. For another platform,
+use the matching archive from the
+[official ArkScript releases](https://github.com/ArkScript-lang/Ark/releases).
+Keep the interpreter and its supplied libraries together when extracting it.
 
-```
-docker pull arkscript/nightly:latest
-docker run -it --rm -v $(pwd):/tmp:ro arkscript/nightly /tmp/lambda-core.ark
-```
+## Run
 
-## Running the solution (binary)
+Place the ArkScript directory on your PATH, then run:
 
-Download the binary for your platform in [the latest 4.0.0 pre-release](https://github.com/ArkScript-lang/Ark/releases/tag/v4.0.0-10).
+    arkscript -fno-cache lambda-core.ark
 
-Extract the zip, you will need both `arkscript` and `libArkReactor.(so|dll)` in the same directory.
+The no-cache flag prevents ArkScript from creating a generated
+`__arkscript__` directory beside this source file.
 
-```
-chmod u+x arkscript
-./arkscript lambda-core.ark
-```
+## Test
 
-## Other way of installing ArkScript
-
-See the documentation: [arkscript-lang.dev/tutorials/building.html](https://arkscript-lang.dev/tutorials/building.html).
+    sh test.sh
 
+The repository test harness compares that command's output byte-for-byte with
+`expected-output.txt`. If ArkScript is not installed, the test is reported as
+a local toolchain skip; a missing interpreter is a failure in CI.
diff --git a/languages/a/arkscript/expected-output.txt b/languages/a/arkscript/expected-output.txt
new file mode 100644
index 0000000..60e94ea
--- /dev/null
+++ b/languages/a/arkscript/expected-output.txt
@@ -0,0 +1,30 @@
+Logic
+-----
+True / False
+true
+false
+Not
+false
+true
+And
+true
+false
+false
+false
+Or
+true
+true
+true
+false
+
+Church Numerals
+---------------
+Zero / Succ
+0
+1
+2
+3
+Pred
+0
+1
+2
diff --git a/languages/a/arkscript/test.sh b/languages/a/arkscript/test.sh
new file mode 100755
index 0000000..1e52fce
--- /dev/null
+++ b/languages/a/arkscript/test.sh
@@ -0,0 +1,7 @@
+#!/bin/sh
+set -eu
+
+command -v arkscript >/dev/null 2>&1 || exit 42
+
+dir=$(CDPATH= cd -- "$(dirname -- "$0")" && pwd)
+arkscript -fno-cache "$dir/lambda-core.ark"
diff --git a/languages/b/bruijn/README.md b/languages/b/bruijn/README.md
index 7b781a8..05f4990 100644
--- a/languages/b/bruijn/README.md
+++ b/languages/b/bruijn/README.md
@@ -1,14 +1,34 @@
 # bruijn
 
-To run, follow the installation instructions on [bruijn's
-wiki](https://bruijn.marvinborner.de/wiki/introduction/installation/)
-and run
-
-``` bash
-# to run main and check the tests!
-bruijn -v lambda-core.bruijn
-```
-
-Further examples can be found in
-[examples](https://bruijn.marvinborner.de/samples/) or in the [standard
-library](https://bruijn.marvinborner.de/std/).
+This implementation requires the
+[bruijn interpreter](https://github.com/marvinborner/bruijn). Bruijn does not
+publish release binaries, so GitHub Actions builds upstream commit
+`9f1ac8856d09666a7444874f80b91b876929eecf` using the project's locked
+Stack snapshot (LTS 22.12).
+
+## Install
+
+    git clone https://github.com/marvinborner/bruijn.git
+    cd bruijn
+    git checkout 9f1ac8856d09666a7444874f80b91b876929eecf
+    stack install
+
+## Run
+
+    bruijn -v lambda-core.bruijn
+
+Verbose mode prints one success line for every `:test` expression in the
+source. The final `main` term is printed after those checks.
+
+## Test
+
+    sh test.sh
+
+The interpreter always emits terminal color codes, even when output is not a
+terminal. The test removes only those ANSI color codes before comparing the
+actual text with `expected-output.txt`; all test results and the final term
+remain in the comparison.
+
+Further examples are available in bruijn's
+[samples](https://bruijn.marvinborner.de/samples/) and
+[standard library](https://bruijn.marvinborner.de/std/).
diff --git a/languages/b/bruijn/expected-output.txt b/languages/b/bruijn/expected-output.txt
new file mode 100644
index 0000000..4fe2fd6
--- /dev/null
+++ b/languages/b/bruijn/expected-output.txt
@@ -0,0 +1,15 @@
+ OK  test passed: (¬‣ true) = false
+ OK  test passed: (¬‣ false) = true
+ OK  test passed: (true ⋀ true) = true
+ OK  test passed: (true ⋀ false) = false
+ OK  test passed: (false ⋀ true) = false
+ OK  test passed: (false ⋀ false) = false
+ OK  test passed: (true ⋁ true) = true
+ OK  test passed: (true ⋁ false) = true
+ OK  test passed: (false ⋁ true) = true
+ OK  test passed: (false ⋁ false) = false
+ OK  test passed: (++‣ (++‣ zero)) = [[(1 (1 0))]]
+ OK  test passed: (--‣ (++‣ zero)) = zero
+ OK  test passed: (--‣ [[(1 (1 0))]]) = [[(1 0)]]
+ OK  test passed: (--‣ [[(1 (1 (1 0)))]]) = [[(1 (1 0))]]
+*> [0]
diff --git a/languages/b/bruijn/test.sh b/languages/b/bruijn/test.sh
new file mode 100755
index 0000000..559c538
--- /dev/null
+++ b/languages/b/bruijn/test.sh
@@ -0,0 +1,12 @@
+#!/bin/sh
+set -eu
+
+command -v bruijn >/dev/null 2>&1 || exit 42
+command -v sed >/dev/null 2>&1 || exit 42
+
+dir=$(CDPATH= cd -- "$(dirname -- "$0")" && pwd)
+raw=$(mktemp)
+trap 'rm -f "$raw"' 0 1 2 15
+
+bruijn -v "$dir/lambda-core.bruijn" >"$raw"
+LC_ALL=C sed 's/\x1b\[[0-9;]*m//g' "$raw"
diff --git a/languages/f/fatscript/README.md b/languages/f/fatscript/README.md
index 24172ae..d7d4310 100644
--- a/languages/f/fatscript/README.md
+++ b/languages/f/fatscript/README.md
@@ -1,19 +1,23 @@
-# FatScript interpreter setup
+# FatScript
 
-https://fatscript.org/en/general/setup.html
+This implementation requires the
+[fry interpreter](https://fatscript.org/en/general/setup.html).
 
-# Run code
+## Run with a native fry installation
 
-```sh
-chmod +x ./lambda-core.fat
+    fry lambda-core.fat
 
-./lambda-core.fat
-```
+## Run and test with Docker
 
-# Run on web playground (alternative)
+The automated test uses the official FatScript 4.5.0 image pinned by its
+multi-platform digest:
 
-> no installation required
+    docker pull fatscript/fry@sha256:fa8bfe0d759ce4bd7146ce9c97521b2cc72f1905d496468a1537fd9e36addc80
+    sh test.sh
 
-https://fatscript.org/playground/
+The container receives this folder read-only and runs without network access.
+The test does not silently switch to a newer image. Its output is compared
+byte-for-byte with `expected-output.txt`.
 
-Select `lambda-core.fat` file on "run fat code" input.
+The [FatScript playground](https://fatscript.org/playground/) is also useful
+for interactive exploration, but it is not used for automated verification.
diff --git a/languages/f/fatscript/expected-output.txt b/languages/f/fatscript/expected-output.txt
new file mode 100644
index 0000000..1b055f1
--- /dev/null
+++ b/languages/f/fatscript/expected-output.txt
@@ -0,0 +1,30 @@
+LOGIC
+---------------
+TRUE/FALSE
+t
+f
+NOT
+f
+t
+AND
+t
+f
+f
+f
+OR
+t
+t
+t
+f
+
+CHURCH NUMERALS
+---------------
+ZERO/SUCC
+0
+1
+2
+3
+PRED
+0
+1
+2
diff --git a/languages/f/fatscript/test.sh b/languages/f/fatscript/test.sh
new file mode 100755
index 0000000..c7a35f6
--- /dev/null
+++ b/languages/f/fatscript/test.sh
@@ -0,0 +1,13 @@
+#!/bin/sh
+set -eu
+
+command -v docker >/dev/null 2>&1 || exit 42
+
+image='fatscript/fry@sha256:fa8bfe0d759ce4bd7146ce9c97521b2cc72f1905d496468a1537fd9e36addc80'
+docker image inspect "$image" >/dev/null 2>&1 || exit 42
+
+dir=$(CDPATH= cd -- "$(dirname -- "$0")" && pwd)
+docker run --rm --network none --read-only \
+  -v "$dir:/app:ro" \
+  "$image" \
+  lambda-core.fat
diff --git a/languages/l/language84/README.md b/languages/l/language84/README.md
index bd9bd76..8f71c78 100644
--- a/languages/l/language84/README.md
+++ b/languages/l/language84/README.md
@@ -1,19 +1,35 @@
+# Language 84
 
-(Note: You need x86 Linux to run this code.)
+Language 84 version 0.8 runs on x86-64 Linux and uses a self-hosting compiler.
+Build it with Clang. During verification, the 0.8 bootstrap compiler
+segfaulted when built with GCC 13.3, while the documented Clang build completed
+and ran successfully.
 
-Get Language 84 version 0.8 from https://norstrulde.org/language84/ and unpack.
+## Build and run
 
-    $ wget https://norstrulde.org/language84/language84-0.8.tar.xz
-    $ tar xfJ language84-0.8.tar.xz
+Download the
+[official Language 84 0.8 archive](https://norstrulde.org/language84/language84-0.8.tar.xz)
+and unpack it. Its SHA-256 digest is:
 
-Copy `lambda-core.84` and `local.make` from this repo into the `language84-0.8` directory created above.
+    b51807d1b7d06f1fdba762fdbf99c8c7fe762ec1a0c96c0be054657c9551cec0
 
-Build
+Copy `lambda-core.84` and `local.make` from this folder into the extracted
+`language84-0.8` directory, then run:
 
-    $ make CC=clang
+    make CC=clang
+    ./lambda-core
 
-Run
+## Test
 
-    $ ./lambda-core
+Point the test at the extracted, otherwise unmodified Language 84 source tree:
 
-Note that there are some functions defined with and without syntactic sugar to show both forms and how they relate. Look for `BITTER`, `SWEET`, `SWEETEST`.
+    LANGUAGE84_HOME=/path/to/language84-0.8 sh test.sh
+
+The test copies only the compiler's required `.84`, `.c`, and `.h`
+sources into a temporary directory, builds with Clang, runs the resulting
+program, and removes the temporary build. The output is compared byte-for-byte
+with `expected-output.txt`.
+
+Several functions are shown both with and without Language 84's syntactic
+sugar. Search the source for `BITTER`, `SWEET`, and `SWEETEST` to compare
+the forms.
diff --git a/languages/l/language84/expected-output.txt b/languages/l/language84/expected-output.txt
new file mode 100644
index 0000000..36e6e85
--- /dev/null
+++ b/languages/l/language84/expected-output.txt
@@ -0,0 +1,62 @@
+
+LOGIC
+---
+
+TRUE/FALSE
+t
+f
+
+NOT
+f
+t
+
+AND
+t
+f
+f
+f
+
+OR
+t
+t
+t
+f
+
+CHURCH NUMERALS
+---
+
+ZERO/SUCC
+0
+1
+2
+3
+
+PRED
+0
+0
+1
+2
+
+ADD
+5
+
+MUL
+0
+0
+6
+
+FACT
+1
+1
+2
+6
+24
+120
+
+FIB
+1
+1
+2
+3
+5
+8
diff --git a/languages/l/language84/test.sh b/languages/l/language84/test.sh
new file mode 100755
index 0000000..7779c8a
--- /dev/null
+++ b/languages/l/language84/test.sh
@@ -0,0 +1,24 @@
+#!/bin/sh
+set -eu
+
+command -v clang >/dev/null 2>&1 || exit 42
+command -v make >/dev/null 2>&1 || exit 42
+
+language84_home=${LANGUAGE84_HOME:-}
+[ -n "$language84_home" ] || exit 42
+[ -f "$language84_home/84_stable.c" ] || exit 42
+
+dir=$(CDPATH= cd -- "$(dirname -- "$0")" && pwd)
+build_dir=$(mktemp -d)
+trap 'rm -r "$build_dir"' 0 1 2 15
+
+cp "$language84_home"/*.84 "$build_dir"/
+cp "$language84_home/84_stable.c" "$language84_home/support.c" \
+  "$language84_home/support.h" "$language84_home/Makefile" "$build_dir"/
+cp "$dir/lambda-core.84" "$dir/local.make" "$build_dir"/
+
+(
+  cd "$build_dir"
+  make CC=clang 1>&2
+)
+"$build_dir/lambda-core"