diff --git a/Dockerfile b/Dockerfile
index b5c26f9..b087878 100644
--- a/Dockerfile
+++ b/Dockerfile
@@ -10,7 +10,7 @@ COPY . .
 
 # Go half of fmt-check only: this image has no node, so no prettier. The
 # markdown half runs in the mdfmt stage below. The image has no gofumpt
-# either; script/gofumpt builds the version it pins with `go run`.
+# either; script/gofumpt builds the version tools/go.mod requires.
 RUN script/gofumpt --check
 # The linter directly, not `make lint`: script/lint builds this stage, and
 # there is no docker inside this build.
@@ -31,7 +31,7 @@ COPY . .
 RUN script/prettier --check
 
 # Build stage — tests and compilation
-# golang:1.23 (2026-03-14)
+# golang:1.23.12, 2026-03-14
 FROM golang@sha256:60deed95d3888cc5e4d9ff8a10c54e5edc008c6ae3fba6187be6fb592e19e8c0 AS builder
 
 # Force BuildKit to run the lint and mdfmt stages by creating stage dependencies
diff --git a/README.md b/README.md
index 65f89ed..2a352eb 100644
--- a/README.md
+++ b/README.md
@@ -99,9 +99,10 @@ provide:
 - `script/fmt` — format all code and docs (writes): `script/gofumpt --write` and
   `script/prettier --write`
 - `script/gofumpt` — run `gofumpt` over every Go file in the repository in the
-  given mode, `--write` or `--check`, at the one version it pins (built on
-  demand by `go run`, so nothing installs it); `script/fmt`, `script/fmt-check`
-  and the Docker lint stage all go through it, so they cannot disagree about Go
+  given mode, `--write` or `--check`, at the version `tools/go.mod` requires
+  (built on demand by `go tool` from source checked against the hashes in
+  `tools/go.sum`, so nothing installs it); `script/fmt`, `script/fmt-check` and
+  the Docker lint stage all go through it, so they cannot disagree about Go
   formatting
 - `script/prettier` — run prettier over the repository's canonical file set
   (Markdown and JSON, minus `.prettierignore`) in the given mode, `--write` or
diff --git a/package.json b/package.json
index cca61f6..1a23c56 100644
--- a/package.json
+++ b/package.json
@@ -1,6 +1,5 @@
 {
     "name": "mfer",
-    "version": "0.1.0",
     "private": true,
     "description": "Development tooling for the mfer repository: prettier, used by script/fmt and script/fmt-check to format and verify Markdown and JSON.",
     "license": "WTFPL",
diff --git a/script/bootstrap b/script/bootstrap
index 1ee831f..5a5f9c9 100755
--- a/script/bootstrap
+++ b/script/bootstrap
@@ -132,15 +132,18 @@ main() {
     # ---- JS / docs repos ----
     # This is a Go repo, but node and yarn are required anyway: prettier
     # formats the Markdown and JSON, and script/fmt-check verifies it.
-    # The version is pinned by package.json/yarn.lock, whose integrity
-    # hashes --frozen-lockfile enforces.
+    # The version is pinned by package.json/yarn.lock: yarn checks every
+    # package it fetches against its yarn.lock integrity hash, and
+    # --frozen-lockfile fails instead of rewriting a yarn.lock that no
+    # longer matches package.json.
     ensure_node
     ensure_yarn
     install_js_deps
 
     # ---- Go repos ----
     if missing go; then pkg_install go golang go go; fi
-    # No golangci-lint: script/lint runs it in Docker only.
+    # No golangci-lint: script/lint runs it in Docker only. No gofumpt:
+    # script/gofumpt builds the version tools/go.mod requires.
     go mod download
 
     # ---- Python repos ----
diff --git a/script/gofumpt b/script/gofumpt
index 1622d46..a9fc585 100755
--- a/script/gofumpt
+++ b/script/gofumpt
@@ -10,10 +10,9 @@ set -eu
 
 ROOT="$(cd "$(dirname "$0")/.." && pwd -P)"
 
-# gofumpt v0.12.0, 2026-10-04. `go run` fetches and builds exactly this
-# version, so neither a developer machine nor the lint image needs
-# gofumpt installed.
-GOFUMPT="mvdan.cc/gofumpt@v0.12.0"
+# gofumpt v0.12.0, 2026-10-04, required by tools/go.mod. `go tool` run in
+# tools/ builds it from source checked against the hashes in tools/go.sum,
+# so neither a developer machine nor the lint image needs it installed.
 
 usage() {
     echo "usage: script/gofumpt --write|--check" >&2
@@ -22,15 +21,15 @@ usage() {
 
 main() {
     [ "$#" -eq 1 ] || usage
-    cd "$ROOT"
-    # Every Go file in the repo. gofumpt holds generated files, such as
-    # mfer/mf.pb.go, to gofmt's rules only.
+    cd "$ROOT/tools"
+    # Every Go file in the repo, from $ROOT down. gofumpt holds generated
+    # files, such as mfer/mf.pb.go, to gofmt's rules only.
     case "$1" in
-        --write) go run "$GOFUMPT" -l -w . ;;
+        --write) go tool gofumpt -l -w "$ROOT" ;;
         --check)
             # Own line: a failing command inside `[ -n "$(...)" ]` does
             # not trip `set -e`, so a gofumpt that never ran would pass.
-            unformatted="$(go run "$GOFUMPT" -l .)"
+            unformatted="$(go tool gofumpt -l "$ROOT")"
             if [ -n "$unformatted" ]; then
                 echo "gofumpt: files need formatting (run make fmt):" >&2
                 echo "$unformatted" >&2
diff --git a/script/prettier b/script/prettier
index b2f66a3..3519278 100755
--- a/script/prettier
+++ b/script/prettier
@@ -18,24 +18,9 @@ usage() {
     exit 2
 }
 
-# Prefer the version pinned by package.json/yarn.lock so that CI and
-# developer machines format identically. Fall back to a prettier on PATH,
-# but say so, because a different version formats differently.
-find_prettier() {
-    if [ -x "$ROOT/node_modules/.bin/prettier" ]; then
-        printf '%s\n' "$ROOT/node_modules/.bin/prettier"
-        return 0
-    fi
-    if command -v prettier >/dev/null 2>&1; then
-        echo "prettier: node_modules/.bin/prettier is absent; using the" \
-            "prettier on PATH, which may be a different version than the" \
-            "one pinned in package.json. Run script/bootstrap to install" \
-            "the pinned version." >&2
-        command -v prettier
-        return 0
-    fi
-    return 1
-}
+# Only the prettier yarn installed from yarn.lock, never one on PATH: a
+# different version formats differently.
+PRETTIER="$ROOT/node_modules/.bin/prettier"
 
 main() {
     [ "$#" -eq 1 ] || usage
@@ -46,10 +31,17 @@ main() {
 
     cd "$ROOT"
 
-    if ! prettier_bin="$(find_prettier)"; then
-        echo "prettier: not found." >&2
-        echo "  Install it with: script/bootstrap" >&2
-        echo "  (installs the version pinned in package.json/yarn.lock)" >&2
+    # node_modules keeps the old prettier after package.json moves to a new
+    # one, until script/bootstrap runs again, so compare the two. A prettier
+    # that runs also means node is on PATH, which reading package.json needs.
+    if ! installed="$("$PRETTIER" --version 2>/dev/null)"; then
+        echo "prettier: not installed; run script/bootstrap" >&2
+        exit 1
+    fi
+    pinned="$(node -p 'require("./package.json").devDependencies.prettier')"
+    if [ "$installed" != "$pinned" ]; then
+        echo "prettier: package.json pins $pinned but $installed is" \
+            "installed; run script/bootstrap" >&2
         exit 1
     fi
 
@@ -62,8 +54,8 @@ main() {
     # patterns always match at least one tracked file (README.md,
     # package.json), so an empty match means the glob broke, and prettier
     # erroring out is exactly what we want rather than a vacuous pass.
-    "$prettier_bin" "$mode" "**/*.md"
-    "$prettier_bin" "$mode" "**/*.json"
+    "$PRETTIER" "$mode" "**/*.md"
+    "$PRETTIER" "$mode" "**/*.json"
 }
 
 main "$@"
diff --git a/tools/go.mod b/tools/go.mod
new file mode 100644
index 0000000..8d0bd50
--- /dev/null
+++ b/tools/go.mod
@@ -0,0 +1,15 @@
+// The developer tools this repo runs with `go tool`, kept out of the mfer
+// module so they add nothing to what mfer's users download. `go tool` builds
+// exactly the source whose hashes go.sum here records.
+module sneak.berlin/go/mfer/tools
+
+go 1.26.0
+
+tool mvdan.cc/gofumpt
+
+require (
+	golang.org/x/mod v0.40.0 // indirect
+	golang.org/x/sync v0.22.0 // indirect
+	golang.org/x/tools v0.49.0 // indirect
+	mvdan.cc/gofumpt v0.12.0 // indirect
+)
diff --git a/tools/go.sum b/tools/go.sum
new file mode 100644
index 0000000..34c5e53
--- /dev/null
+++ b/tools/go.sum
@@ -0,0 +1,20 @@
+github.com/go-quicktest/qt v1.102.0 h1:HSQxCeh5YZH3EL3W39ixjtyaEhcWSXQHtHnMBzSs474=
+github.com/go-quicktest/qt v1.102.0/go.mod h1:p4lGIVX+8Wa6ZPNDvqcxq36XpUDLh42FLetFU7odllI=
+github.com/google/go-cmp v0.7.0 h1:wk8382ETsv4JYUZwIsn6YpYiWiBsYLSJiTsyBybVuN8=
+github.com/google/go-cmp v0.7.0/go.mod h1:pXiqmnSA92OHEEa9HXL2W4E7lf9JzCmGVUdgjX3N/iU=
+github.com/kr/pretty v0.3.1 h1:flRD4NNwYAUpkphVc1HcthR4KEIFJ65n8Mw5qdRn3LE=
+github.com/kr/pretty v0.3.1/go.mod h1:hoEshYVHaxMs3cyo3Yncou5ZscifuDolrwPKZanG3xk=
+github.com/kr/text v0.2.0 h1:5Nx0Ya0ZqY2ygV366QzturHI13Jq95ApcVaJBhpS+AY=
+github.com/kr/text v0.2.0/go.mod h1:eLer722TekiGuMkidMxC/pM04lWEeraHUUmBw8l2grE=
+github.com/rogpeppe/go-internal v1.16.0 h1:O9DK+vNMDVGLr2BeZqmpLeMjiMNkuXfcqntWbZV6S5g=
+github.com/rogpeppe/go-internal v1.16.0/go.mod h1:DrUVZyrJU+txYW5/1kwtXQSMFio52ZOxX7yM1VHvnxs=
+golang.org/x/mod v0.40.0 h1:hUv+3cXcdRHz08UmSiOob7sadHig73uo5bkXxQ/tvUs=
+golang.org/x/mod v0.40.0/go.mod h1:0/weTWkPWGBikyTWAX3dkjVztMmBA5hM0DH6BElSupE=
+golang.org/x/sync v0.22.0 h1:SZjpbeLmrCk4xhRSZFNZW5gFUeCeFgjekvI/+gfScek=
+golang.org/x/sync v0.22.0/go.mod h1:9xrNwdLfx4jkKbNva9FpL6vEN7evnE43NNNJQ2LF3+0=
+golang.org/x/sys v0.47.0 h1:o7XGOvZQCADBQQ4Y7VNq2dRWQR7JmOUW8Kxx4ZsNgWs=
+golang.org/x/sys v0.47.0/go.mod h1:4GL1E5IUh+htKOUEOaiffhrAeqysfVGipDYzABqnCmw=
+golang.org/x/tools v0.49.0 h1:3NI7VXzL9+1WZD52Dx2ttoPwD5DWrFGpl9mFZDlmisI=
+golang.org/x/tools v0.49.0/go.mod h1:SJNXV9DBKT0UbdttsQjbfJlAE/q+y36++zo3uL3N0Oo=
+mvdan.cc/gofumpt v0.12.0 h1:1Lbudkz2kpM9Cjz2pL4M19u7q+GaEhCTNf7N9mfpcho=
+mvdan.cc/gofumpt v0.12.0/go.mod h1:SmBHHrljiZu/uoypeKup3rFzP6eoC9UwCp2iH5E3jZA=