diff --git a/docs/CLIENT_INSTALLATION.md b/docs/CLIENT_INSTALLATION.md
index 9a65b111..a5f7cde0 100644
--- a/docs/CLIENT_INSTALLATION.md
+++ b/docs/CLIENT_INSTALLATION.md
@@ -37,3 +37,42 @@ Compatibility exports refer to the same functions/constants. Shared protocol hel
 `tests/test_client_boundary.py` checks implementation ownership, compatibility exports, proof context, atomic-write failure protection and missing-dependency errors. `scripts/check_client_install.py` starts a fresh interpreter with server imports blocked, loads client features, saves/reloads a private key, and sends the same signed request through four real client adapters. Its HTTP endpoints use MockTransport rather than production.
 
 The client-boundary workflow builds wheel/sdist, installs the wheel in a new environment outside the source tree, checks dependencies, both help commands and all four transport probes. `--minimal-install` rejects server Python dependencies. Full PostgreSQL/Valkey, CLI/MCP, credential journal/recovery and conformance gates remain required separately. These checks do not establish physical-device, external-network, performance or production acceptance.
+
+## One-command, user-level installation
+
+```bash
+curl -fsSL https://msg.lmm.best/install | bash
+msg lightjunction@msg.lmm.best ""
+```
+
+The `/install` endpoint serves a packaged Bash script. It installs the client
+without `sudo` on Linux glibc x86_64/ARM64 and macOS Intel/Apple Silicon.
+It needs Bash, curl, tar, and a SHA-256 utility. No existing Python is required:
+the script verifies a pinned uv 0.12.20 native archive and obtains managed
+CPython 3.15.0rc2. CPython is currently a release candidate because this project
+requires Python 3.15.
+
+The client source is the published GitHub commit
+`5554f93819478f4559aa226d1d94cfc6202a1cee`, verified against its embedded SHA-256;
+there is no dependency on an unpublished MSG release asset. Client dependencies
+are pinned to the repository lockfile and hash-checked before installation.
+Only `msg` is exposed in `$HOME/.local/bin`; server dependencies and daemon
+services are not installed. Install data stays under
+`$XDG_DATA_HOME/msg/client-{releases,python,cache}` (default `~/.local/share`).
+Identity, certificates, and service connection configuration are untouched.
+
+If that bin directory is absent from PATH, the installer prints the line to add
+to your shell configuration. It does not edit shell files. Repeating the
+installation reuses the verified installed revision. An existing `msg` launcher
+from a different installer is retained unless you explicitly replace it:
+
+```bash
+curl -fsSL https://msg.lmm.best/install | bash -s -- --force
+```
+
+To publish a newer client, update the source revision and archive SHA-256 in
+`src/msg/data/install.sh`, regenerate its embedded client requirements with
+`uv export --no-dev --no-emit-project --format requirements-txt`, and exercise a
+fresh installation before deploying the `/install` endpoint. The installer UX
+was inspired by [Termium](https://github.com/codr1/termium); its browser and SSH
+implementation are not dependencies of MSG.
diff --git a/src/msg/data/install.sh b/src/msg/data/install.sh
new file mode 100644
index 00000000..adb8dccc
--- /dev/null
+++ b/src/msg/data/install.sh
@@ -0,0 +1,240 @@
+#!/usr/bin/env bash
+# Parse the complete function before a piped download can execute installation.
+# UX inspired by https://github.com/codr1/termium; implementation is MSG-specific.
+main() {
+    set -euo pipefail
+    fail() { printf 'msg: %s\n' "$*" >&2; exit 1; }
+    force=0
+    case "${1:-}" in
+        '') ;;
+        --force) force=1; shift ;;
+        --help) printf '%s\n' 'Install the MSG client: curl -fsSL https://msg.lmm.best/install | bash' 'Use bash -s -- --force to replace an existing unmanaged msg command.'; return ;;
+        *) fail 'Unknown option; use --help.' ;;
+    esac
+    [ "$#" -eq 0 ] || fail 'Unexpected arguments.'
+    case "$(uname -s)-$(uname -m)" in
+        Linux-x86_64) platform=x86_64-unknown-linux-gnu; uv_hash=6590717592ace991ff83a63fef799e3ad9d33ecc8f96c5d6bdd732496e79337f ;;
+        Linux-aarch64|Linux-arm64) platform=aarch64-unknown-linux-gnu; uv_hash=8a7aad7bc76a2fae5151566ff3e43eacce0b2a113d5e4de3e4afe3e58fa2441e ;;
+        Darwin-x86_64) platform=x86_64-apple-darwin; uv_hash=ac54283d211fd77cdc152b67606dbaf6406ff4ab03f3af4ae99468fa8e887141 ;;
+        Darwin-arm64) platform=aarch64-apple-darwin; uv_hash=848fdeb602ff1a1baacd4f6c8b7bdc6cf1ad026a6d9cf59475fda17c179743ca ;;
+        *) fail 'Supported: Linux glibc x86_64/ARM64 and macOS Intel/Apple Silicon.' ;;
+    esac
+    if [ "$(uname -s)" = Linux ]; then
+        getconf GNU_LIBC_VERSION >/dev/null 2>&1 || fail 'Linux requires glibc; musl is not supported by this installer.'
+    fi
+    for tool in curl tar mktemp; do command -v "$tool" >/dev/null || fail "$tool is required."; done
+    if command -v sha256sum >/dev/null; then
+        sha256() { sha256sum "$1" | awk '{print $1}'; }
+    elif command -v shasum >/dev/null; then
+        sha256() { shasum -a 256 "$1" | awk '{print $1}'; }
+    else fail 'SHA-256 verification is required.'; fi
+    revision=5554f93819478f4559aa226d1d94cfc6202a1cee
+    source_hash=7214ffbd73aaa247a91732f4f7229ee75f6d0d90cd86e73dbc4bbcbd96dbdc09
+    data_home=${XDG_DATA_HOME:-"$HOME/.local/share"}
+    case "$data_home" in /*) ;; *) fail 'XDG_DATA_HOME must be absolute.' ;; esac
+    bin_dir="$HOME/.local/bin"
+    release_root="$data_home/msg/client-releases"
+    release="$release_root/$revision"
+    if [ -e "$bin_dir/msg" ] || [ -L "$bin_dir/msg" ]; then
+        current=$(readlink "$bin_dir/msg" 2>/dev/null || true)
+        case "$current" in "$release_root"/*/bin/msg) ;; *) [ "$force" -eq 1 ] || fail 'An existing msg command is not managed by this installer. Re-run with --force to replace its launcher.' ;; esac
+    fi
+    work=$(mktemp -d "${TMPDIR:-/tmp}/msg-install.XXXXXXXX")
+    created=0
+    locked=0
+    launcher=''
+    cleanup() {
+        rm -rf -- "$work"
+        [ -z "$launcher" ] || rm -f -- "$launcher"
+        if [ "$created" -eq 1 ]; then rm -rf -- "$release"; fi
+        if [ "$locked" -eq 1 ]; then rmdir "$release_root/.install-lock"; fi
+    }
+    trap cleanup EXIT
+    trap 'exit 130' INT
+    trap 'exit 143' TERM
+    download() { curl --fail --location --proto '=https' --proto-redir '=https' --tlsv1.2 --retry 3 --connect-timeout 15 "$1" -o "$2"; }
+    mkdir -p "$release_root"
+    mkdir "$release_root/.install-lock" 2>/dev/null || fail 'Another installation is running (or a stale .install-lock remains).'
+    locked=1
+    [ ! -d "$bin_dir/msg" ] || fail 'The launcher path is a directory; refusing to replace it.'
+    if [ ! -x "$release/bin/msg" ]; then
+        [ ! -e "$release" ] || fail 'An incomplete release directory exists; remove it before retrying.'
+        printf 'Installing MSG client %s (no sudo)\n' "${revision:0:7}" >&2
+        download "https://github.com/astral-sh/uv/releases/download/0.12.20/uv-$platform.tar.gz" "$work/uv.tar.gz"
+        [ "$(sha256 "$work/uv.tar.gz")" = "$uv_hash" ] || fail 'uv checksum mismatch.'
+        tar -xzf "$work/uv.tar.gz" -C "$work"
+        uv="$work/uv-$platform/uv"
+        [ -x "$uv" ] || fail 'uv archive does not contain the expected executable.'
+        download "https://codeload.github.com/TokenNotIncluded/msg/zip/$revision" "$work/msg.zip"
+        [ "$(sha256 "$work/msg.zip")" = "$source_hash" ] || fail 'MSG source checksum mismatch.'
+        cat > "$work/requirements.txt" <<'MSG_CLIENT_REQUIREMENTS'
+# This file was autogenerated by uv via the following command:
+#    uv export --no-dev --no-emit-project --format requirements-txt --output-file /tmp/msg-client-requirements.txt
+anyio==4.15.1 \
+    --hash=sha256:6152fdbbf9a77fdec97731721bebf7c4c44f7c29b424b0065826173efc7ed101 \
+    --hash=sha256:9f28306018cbd6d329e64a36d58256edff76dd996fe423bc957326e578b82a94
+    # via httpx
+attrs==26.1.0 \
+    --hash=sha256:c647aa4a12dfbad9333ca4e71fe62ddc36f4e63b2d260a37a8b83d2f043ac309 \
+    --hash=sha256:d03ceb89cb322a8fd706d4fb91940737b6642aa36998fe130a9bc96c985eff32
+    # via
+    #   jsonschema
+    #   referencing
+certifi==2026.7.22 \
+    --hash=sha256:62f22742b58a1a33014a2b6b706588a8d7e2a88ae7bd1a6ebe8c992928483775 \
+    --hash=sha256:741e2c3b351ddf169a738da9f2c048608ff7f2c5cc02f1ebc6b118bb090d5d55
+    # via
+    #   httpcore
+    #   httpx
+cffi==2.1.1 ; platform_python_implementation != 'PyPy' \
+    --hash=sha256:06c72bb76605a4b0cd0aad6930b69d4baf7dd5d806cfc409b824191099700e66 \
+    --hash=sha256:246fa40ce8645a614ff682e0b70f37134e460eaf93a775e0cbe3cca585a67a80 \
+    --hash=sha256:27350daa11d4f10c540e6e89dada4c54feb7256ad03e9a4dc075ebad7ba360d1 \
+    --hash=sha256:2ae64be792b8966f2c69538199728b290e34726562896df1e5dc8ffd8d8188e8 \
+    --hash=sha256:363e05fa78e15116c3c32c210ee36884fd6b9afa6d440e47112c3bd511d64cb6 \
+    --hash=sha256:42a494cee34437f05546455144f2b5d9ac09b1face62bcfce597d2e521066688 \
+    --hash=sha256:471cee653ae88de62096552e6d24ccb4a5adb8c8c9f10b5054d0122c15bf2779 \
+    --hash=sha256:4be96343e422f2dfcd12ab5c9f5aebe03f82f737c6bffeca6830b3875cb44aab \
+    --hash=sha256:4f42141fc14250de6dde5ee7ea4432be017252d91f19c5ad043c084cea629cac \
+    --hash=sha256:507a24c282e0f42f8ed737cf048572cbf580468da5555764a8331735e9c736b6 \
+    --hash=sha256:770de9db11e84213beec501cfcaa013b019820ca881e03344dea5844f7876d94 \
+    --hash=sha256:7da0c5eff80f0197f3b3d1232ec5a682a9325f4ae9016a78f5f5ca35f9ced1f5 \
+    --hash=sha256:937c0052c05a31ca1daf18de3158eed4dbfcb9cc107adbea227728d647be701e \
+    --hash=sha256:a730a083190634c65cca36ba5f489531576ebd79bcd5c8e172130f6453127231 \
+    --hash=sha256:aeae0e330c9f6acd681f647d46cefd30c29f93e3392882e792e82080c9691399 \
+    --hash=sha256:c26608d2222fb1e94487e4a387d85f13eb55d5ed725cb25a0c589ac4ee60e7bc \
+    --hash=sha256:cc572dace3f60ef98d7b12ff411d20f5362feb31a0439eab0085bbfd349982d7 \
+    --hash=sha256:d18e5ac0f2f03f4f518d3e23db0f0cad7faa1da8620e9c09461d443bbf6e6692 \
+    --hash=sha256:d9c275eaacd24aa73f94ffd6de08fc3f932424d8b6c376f4bed7cde376fe7bc3 \
+    --hash=sha256:da0e573f9f97159390c89d9f1a9e41908b66d408cc5b58d08cf3847d844c531b \
+    --hash=sha256:dd31f52ea1086513bb9df30f8fcee9b8918323ae067a3d5b78bc826a000712be \
+    --hash=sha256:dddad92b554513a31f272570678ba307fb9f618f05e3d4a5eacafff9eae03e1d \
+    --hash=sha256:df423d40ee8654634421812bc3b196da3f9bd7d32929da813f8394c4348a5358 \
+    --hash=sha256:e6e8cff14d6fb0be70a09c0bdc58096f501952d04624ebf867e0e56da2df8960 \
+    --hash=sha256:fb92203a88b3d3053034db775110081c49d28be6551923805e039924093761e4
+    # via cryptography
+cryptography==46.0.7 \
+    --hash=sha256:128c5edfe5e5938b86b03941e94fac9ee793a94452ad1365c9fc3f4f62216832 \
+    --hash=sha256:1d25aee46d0c6f1a501adcddb2d2fee4b979381346a78558ed13e50aa8a59067 \
+    --hash=sha256:24402210aa54baae71d99441d15bb5a1919c195398a87b563df84468160a65de \
+    --hash=sha256:35719dc79d4730d30f1c2b6474bd6acda36ae2dfae1e3c16f2051f215df33ce0 \
+    --hash=sha256:397655da831414d165029da9bc483bed2fe0e75dde6a1523ec2fe63f3c46046b \
+    --hash=sha256:420b1e4109cc95f0e5700eed79908cef9268265c773d3a66f7af1eef53d409ef \
+    --hash=sha256:42a1e5f98abb6391717978baf9f90dc28a743b7d9be7f0751a6f56a75d14065b \
+    --hash=sha256:462ad5cb1c148a22b2e3bcc5ad52504dff325d17daf5df8d88c17dda1f75f2a4 \
+    --hash=sha256:506c4ff91eff4f82bdac7633318a526b1d1309fc07ca76a3ad182cb5b686d6d3 \
+    --hash=sha256:5ad9ef796328c5e3c4ceed237a183f5d41d21150f972455a9d926593a1dcb308 \
+    --hash=sha256:5d1c02a14ceb9148cc7816249f64f623fbfee39e8c03b3650d842ad3f34d637e \
+    --hash=sha256:5e51be372b26ef4ba3de3c167cd3d1022934bc838ae9eaad7e644986d2a3d163 \
+    --hash=sha256:73510b83623e080a2c35c62c15298096e2a5dc8d51c3b4e1740211839d0dea77 \
+    --hash=sha256:7bbc6ccf49d05ac8f7d7b5e2e2c33830d4fe2061def88210a126d130d7f71a85 \
+    --hash=sha256:84d4cced91f0f159a7ddacad249cc077e63195c36aac40b4150e7a57e84fffe7 \
+    --hash=sha256:8a469028a86f12eb7d2fe97162d0634026d92a21f3ae0ac87ed1c4a447886c83 \
+    --hash=sha256:91bbcb08347344f810cbe49065914fe048949648f6bd5c2519f34619142bbe85 \
+    --hash=sha256:935ce7e3cfdb53e3536119a542b839bb94ec1ad081013e9ab9b7cfd478b05006 \
+    --hash=sha256:9694078c5d44c157ef3162e3bf3946510b857df5a3955458381d1c7cfc143ddb \
+    --hash=sha256:a1529d614f44b863a7b480c6d000fe93b59acee9c82ffa027cfadc77521a9f5e \
+    --hash=sha256:abad9dac36cbf55de6eb49badd4016806b3165d396f64925bf2999bcb67837ba \
+    --hash=sha256:b36a4695e29fe69215d75960b22577197aca3f7a25b9cf9d165dcfe9d80bc325 \
+    --hash=sha256:cbd5fb06b62bd0721e1170273d3f4d5a277044c47ca27ee257025146c34cbdd1 \
+    --hash=sha256:cdf1a610ef82abb396451862739e3fc93b071c844399e15b90726ef7470eeaf2 \
+    --hash=sha256:cdfbe22376065ffcf8be74dc9a909f032df19bc58a699456a21712d6e5eabfd0 \
+    --hash=sha256:d23c8ca48e44ee015cd0a54aeccdf9f09004eba9fc96f38c911011d9ff1bd457 \
+    --hash=sha256:e4cfd68c5f3e0bfdad0d38e023239b96a2fe84146481852dffbcca442c245aa5 \
+    --hash=sha256:ea42cbe97209df307fdc3b155f1b6fa2577c0defa8f1f7d3be7d31d189108ad4 \
+    --hash=sha256:f247c8c1a1fb45e12586afbb436ef21ff1e80670b2861a90353d9b025583d246
+    # via msgctl
+h11==0.16.0 \
+    --hash=sha256:4e35b956cf45792e4caa5885e69fba00bdbc6ffafbfa020300e549b208ee5ff1 \
+    --hash=sha256:63cf8bbe7522de3bf65932fda1d9c2772064ffb3dae62d55932da54b31cb6c86
+    # via httpcore
+httpcore==1.0.9 \
+    --hash=sha256:2d400746a40668fc9dec9810239072b40b4484b640a8c38fd654a024c7a1bf55 \
+    --hash=sha256:6e34463af53fd2ab5d807f399a9b45ea31c3dfa2276f15a2c3f00afff6e176e8
+    # via httpx
+httpx==0.28.1 \
+    --hash=sha256:75e98c5f16b0f35b567856f597f06ff2270a374470a5c2392242528e3e3e42fc \
+    --hash=sha256:d909fcccc110f8c7faf814ca82a9a4d816bc5a6dbfea25d6591d6985b8ba59ad
+    # via msgctl
+idna==3.20 \
+    --hash=sha256:a7db850025b95ded1eae8a46181a1a6c56c92c96f0e2b005d9ff8dc0210cab44 \
+    --hash=sha256:ab7ae7122974553370f0bdb919e1a960b2cd1bc1ef0276416d896db81c14582c
+    # via
+    #   anyio
+    #   httpx
+jsonschema==4.26.0 \
+    --hash=sha256:0c26707e2efad8aa1bfc5b7ce170f3fccc2e4918ff85989ba9ffa9facb2be326 \
+    --hash=sha256:d489f15263b8d200f8387e64b4c3a75f06629559fb73deb8fdfb525f2dab50ce
+    # via msgctl
+jsonschema-specifications==2025.9.1 \
+    --hash=sha256:98802fee3a11ee76ecaca44429fda8a41bff98b00a0f2838151b113f210cc6fe \
+    --hash=sha256:b540987f239e745613c7a9176f3edb72b832a4ac465cf02712288397832b5e8d
+    # via jsonschema
+pycparser==3.0 ; implementation_name != 'PyPy' and platform_python_implementation != 'PyPy' \
+    --hash=sha256:600f49d217304a5902ac3c37e1281c9fe94e4d0489de643a9504c5cdfdfc6b29 \
+    --hash=sha256:b727414169a36b7d524c1c3e31839a521725078d7b2ff038656844266160a992
+    # via cffi
+referencing==0.37.0 \
+    --hash=sha256:381329a9f99628c9069361716891d34ad94af76e461dcb0335825aecc7692231 \
+    --hash=sha256:44aefc3142c5b842538163acb373e24cce6632bd54bdb01b21ad5863489f50d8
+    # via
+    #   jsonschema
+    #   jsonschema-specifications
+    #   msgctl
+rpds-py==2026.6.3 \
+    --hash=sha256:1967debc37f64f2c4dc90a7f563aec558b471966e12adcac4e1c4240496b6ebf \
+    --hash=sha256:1cebd1337c242e4ec2293e541f712b2da849b29f48f0c293684b71c0632625d4 \
+    --hash=sha256:23a439f31ccbeff1574e24889128821d1f7917470e830cf6544dced1c662262a \
+    --hash=sha256:24e9c5386e16669b674a69c156c8eeefcb578f3b3397b713b08e6d60f3c7b187 \
+    --hash=sha256:2f7c26fbc5acd2522b95d4177fe4710ffd8e9b20529e703ffbf8db4d93903f05 \
+    --hash=sha256:38a2fea2787428f811719ceb9114cb78964a3138838320c29ac39526c79c16ba \
+    --hash=sha256:67e3a721ffc5d8d2210d3671872298c4a84e4b8035cfe42ffd7cde35d772b146 \
+    --hash=sha256:6e84adbcf4bf841aed8116a8264b9f50b4cb3e7bd89b516122e616ac56ca269e \
+    --hash=sha256:79486287de1730dbaff3dbd124d0ca4d2ef7f9d29bf2544f1f93c09b5bcbbd12 \
+    --hash=sha256:808345f53cb952433ca2816f1604ff3515608a81784954f38d4452acfe8e61d5 \
+    --hash=sha256:8b95977e7211527ab0ba576e286d023389fbeeb32a6b7b771665d333c60e5342 \
+    --hash=sha256:8c2642a7603ec0b16ed77da4555db3b4b472341904873788327c0b0d7b95f1bb \
+    --hash=sha256:8e4320744c1ffdd95a603def63344bfab2d33edeab301c5007e7de9f9f5b3885 \
+    --hash=sha256:8f2e5c5ee828d42cb11760761c0af6507927bec42d0ad5458f97c9203b054617 \
+    --hash=sha256:913ca42ccad3f8cc6e292b587ae8ae49c8c823e5dce51a736252fc7c7cdfa577 \
+    --hash=sha256:a136d453475ac0fcbda502ef1e6504bd28d6d904700915d278deeab0d00fe140 \
+    --hash=sha256:a3086b538543802f84c843911242db20447de00d8752dd0efc936dbcf02218ba \
+    --hash=sha256:a9f4645593036b81bbdb36b9c8e0ea0d1c3fee968c4d59db0344c14087ef143a \
+    --hash=sha256:ae3d4fe8c0b9213624fdce7279d70e3b148b682ca20719ebd193a23ebfa47324 \
+    --hash=sha256:ae6dd8f10bd17aad820876d24caec9efdafd80a318d16c0a48edb5e136902c6b \
+    --hash=sha256:bdbd97738551fca3917c1bd7188bec1920bb520104f28e7e1007f9ceb17b7690 \
+    --hash=sha256:c60924535c75f1566b6eb75b5c31a48a43fef04fa2d0d201acbad8a9969c6107 \
+    --hash=sha256:d15fde0e6fb0d88a60d221204873743e5d9f0b7d29165e62cd86d0413ad74ba6 \
+    --hash=sha256:d483fe17f01ad64b7bf7cc38fcefff1ca9fb83f8c2b2542b68f97ffe0611b369 \
+    --hash=sha256:e059c5dde6452b44424bd1834557556c226b57781dee1227af23518459722b13 \
+    --hash=sha256:e55d236be29255554da47abe5c577637db7c24a02b8b46f0ca9524c855801868 \
+    --hash=sha256:ed0c1e5d10cdc7135537988c74a0188da68e2f3c30813ba3744ab1e42e0480f9 \
+    --hash=sha256:f0840b5b17057f7fd918b76183a4b5a0635f43e14eb2ce60dce1d4ee4707ea00 \
+    --hash=sha256:f826877d462181e5eb1c26a0026b8d0cab05d99844ecb6d8bf3627a2ca0c0442 \
+    --hash=sha256:faa679d19a6696fd54259ad321251ad77a13e70e03dd834daa762a44fb6196ef
+    # via
+    #   jsonschema
+    #   referencing
+MSG_CLIENT_REQUIREMENTS
+        mkdir -p "$release_root"
+        created=1
+        # Keep the runtime/cache in MSG's installation area, away from identities.
+        export UV_PYTHON_INSTALL_DIR="$data_home/msg/client-python"
+        export UV_CACHE_DIR="$data_home/msg/client-cache"
+        "$uv" venv --managed-python --python 3.15.0rc2 "$release"
+        "$uv" pip install --python "$release/bin/python" --require-hashes --only-binary :all: -r "$work/requirements.txt"
+        "$uv" pip install --python "$release/bin/python" --no-deps "$work/msg.zip"
+        "$release/bin/msg" --help >/dev/null
+    fi
+    mkdir -p "$bin_dir"
+    # mv on the same filesystem atomically replaces only the launcher, after validation.
+    launcher=$(mktemp "$bin_dir/.msg-install.XXXXXXXX")
+    rm -f -- "$launcher"
+    ln -s "$release/bin/msg" "$launcher"
+    mv -f -- "$launcher" "$bin_dir/msg"
+    created=0
+    printf '\nMSG installed: %s/msg\nTry: msg lightjunction@msg.lmm.best ""\n' "$bin_dir"
+    # shellcheck disable=SC2016 # Print literal shell variables for the user.
+    case ":$PATH:" in *":$bin_dir:"*) ;; *) printf '\nAdd to your shell configuration:\n  export PATH="$HOME/.local/bin:$PATH"\n' ;; esac
+}
+main "$@"
diff --git a/src/msg/transports/http_routes.py b/src/msg/transports/http_routes.py
index 21c2707b..30907963 100644
--- a/src/msg/transports/http_routes.py
+++ b/src/msg/transports/http_routes.py
@@ -1768,6 +1768,18 @@ async def dispatch(request: Request):
             if path == '/-' or path.startswith('/-/'):
                 raise Failure('not_found')
             require(request.method in {'GET', 'HEAD'}, 'method_not_allowed')
+            if path == '/install':
+                require(not request.url.query, 'unknown_query_parameter')
+                payload = files('msg.data').joinpath('install.sh').read_bytes()
+                return Response(
+                    b'' if request.method == 'HEAD' else payload,
+                    media_type='text/plain',
+                    headers={
+                        **BASE_HEADERS,
+                        'Content-Length': str(len(payload)),
+                        'Cache-Control': 'no-cache',
+                    },
+                )
             if path in {'/robots.txt', '/sitemap.xml'}:
                 require(not request.url.query, 'unknown_query_parameter')
                 origin = service.settings.service_url.rstrip('/')
diff --git a/tests/test_client_installer.py b/tests/test_client_installer.py
new file mode 100644
index 00000000..9bd4f35c
--- /dev/null
+++ b/tests/test_client_installer.py
@@ -0,0 +1,68 @@
+"""The installer is a read-only, packaged bootstrap with GET/HEAD parity."""
+
+import os
+import subprocess
+from importlib.resources import files
+
+import httpx
+import pytest
+
+from msg.transports.http import create_app
+
+
+@pytest.mark.asyncio
+async def test_install_endpoint_is_packaged_read_only_and_head_matches(installed):
+    app, _ = installed
+    script = files('msg.data').joinpath('install.sh').read_bytes()
+    async with httpx.AsyncClient(
+        transport=httpx.ASGITransport(app=create_app(app)), base_url=app.settings.service_url
+    ) as http:
+        response = await http.get('/install')
+        assert response.status_code == 200
+        assert response.content == script
+        assert response.headers['content-type'].startswith('text/plain')
+        assert response.headers['x-content-type-options'] == 'nosniff'
+        assert response.headers['cache-control'] == 'no-cache'
+        head = await http.head('/install')
+        assert head.status_code == 200 and not head.content
+        assert head.headers['content-length'] == str(len(script))
+        assert (await http.post('/install', content='overwrite')).status_code == 405
+        assert (await http.get('/install?source=elsewhere')).status_code == 400
+
+
+def test_checksum_failure_keeps_existing_launcher_and_identity(tmp_path):
+    home = tmp_path / 'home'
+    bin_dir = home / '.local/bin'
+    release_root = home / '.local/share/msg/client-releases'
+    old = release_root / 'previous/bin/msg'
+    old.parent.mkdir(parents=True)
+    old.write_text('#!/bin/sh\nexit 0\n')
+    old.chmod(0o755)
+    bin_dir.mkdir(parents=True)
+    launcher = bin_dir / 'msg'
+    launcher.symlink_to(old)
+    identity = home / '.local/share/msg/services/example.test/identity.key'
+    identity.parent.mkdir(parents=True)
+    identity.write_text('identity marker')
+    fake = tmp_path / 'fake-bin'
+    fake.mkdir()
+    curl = fake / 'curl'
+    curl.write_text('#!/bin/bash\nprintf "invalid download" > "${@: -1}"\n')
+    curl.chmod(0o755)
+    result = subprocess.run(
+        ['bash', str(files('msg.data').joinpath('install.sh'))],
+        env={
+            **os.environ,
+            'HOME': str(home),
+            'XDG_DATA_HOME': str(home / '.local/share'),
+            'PATH': str(fake) + os.pathsep + os.environ['PATH'],
+        },
+        capture_output=True,
+        text=True,
+        check=False,
+    )
+    assert result.returncode != 0 and 'uv checksum mismatch' in result.stderr
+    assert launcher.readlink() == old
+    assert identity.read_text() == 'identity marker'
+    assert not (release_root / '.install-lock').exists()
+    assert not (release_root / '5554f93819478f4559aa226d1d94cfc6202a1cee').exists()