diff --git a/.github/workflows/private-names.yml b/.github/workflows/private-names.yml
new file mode 100644
index 0000000..aa7345b
--- /dev/null
+++ b/.github/workflows/private-names.yml
@@ -0,0 +1,17 @@
+name: Private names
+
+on:
+  pull_request:
+  push:
+    branches: [main]
+
+jobs:
+  check:
+    runs-on: ubuntu-latest
+    timeout-minutes: 5
+    steps:
+      - uses: actions/checkout@v4
+      - name: The check catches what it should
+        run: python3 scripts/check-private-names.py --self-test
+      - name: No private machine, person, or agent names
+        run: python3 scripts/check-private-names.py
diff --git a/scripts/check-private-names.py b/scripts/check-private-names.py
new file mode 100755
index 0000000..f1bc39f
--- /dev/null
+++ b/scripts/check-private-names.py
@@ -0,0 +1,148 @@
+#!/usr/bin/env python3
+"""Keep private machine, person, and agent names out of this public repository.
+
+Fails when a tracked file names one of the fleet's machines, a home directory
+under /home or /Users, a person/ ID, or an internal agent or mission ID
+(agent/fleet/, mission/fleet/, mission-run/fleet/). Write invented
+examples instead: web1.example.com, /home/example, /Users/example,
+person/alex, agent/example/worker.
+
+Machine names are listed as SHA-256 digests of the lowercase name, so this
+file does not publish the names it keeps out. To add a machine, append the
+output of `printf %s NAME | shasum -a 256`.
+
+A line that must stay as it is goes in .private-names-allow: one regular
+expression per line, searched in `path:text of the line`. Blank lines and
+lines starting with # are ignored.
+
+Run it from anywhere inside the repository: python3 scripts/check-private-names.py
+With --self-test it checks its own patterns instead of the repository.
+"""
+
+import hashlib
+import re
+import subprocess
+import sys
+from pathlib import Path
+
+MACHINE_DIGESTS = {
+    "a994ffcab684f9e2136b0681c20d2a5ef5f23a5cb7b64ab8934d15a888cc4ea3",
+    "2c619e6895b513481a07dae6c4b5781282ef1eee7a7f16744fafe7dbbd3e5ca2",
+    "46b6312339466d3b206325f6f402e1fae56cd65f117c779e3b9259833ffbcdf0",
+    "9fe430e5274621be518924ed90d2355f39eeb1d4d98fb00ea0022f3a26ba5595",
+    "5881d66b8738b483578c8479c59f09856a5447b47bbd2660cff7dc92e1607a2d",
+    "294aa8d75483b8331e3ba6a7f24aea15202747f36de65197e7bc6194880b2558",
+    "151ec47d45b057450da03010df462bf604183571812f9ba66598b9fb8f8b692c",
+}
+
+ALLOW_FILE = ".private-names-allow"
+
+WORD = re.compile(r"[a-z0-9]+(?:-[a-z0-9]+)*")
+HOME = re.compile(r"/(?:home|Users)/([A-Za-z0-9._-]+)")
+PERSON = re.compile(r"(?<![A-Za-z0-9_-])person/([A-Za-z0-9._-]+)")
+FLEET_ID = re.compile(r"(?<![A-Za-z0-9_-])(?:agent|mission|mission-run)/fleet/")
+INVENTED_PEOPLE = {"alex"}
+
+
+def invented(name):
+    return name.lower().startswith("example")
+
+
+def digest(word):
+    return hashlib.sha256(word.encode()).hexdigest()
+
+
+def names_a_machine(line):
+    for word in WORD.findall(line.lower()):
+        for part in {word, *word.split("-")}:
+            if digest(part) in MACHINE_DIGESTS:
+                return True
+    return False
+
+
+def problems(line):
+    if names_a_machine(line):
+        yield "names a private machine"
+    for match in HOME.finditer(line):
+        if not invented(match.group(1)):
+            yield f"home path {match.group(0)!r}: use /home/example or /Users/example"
+    for match in PERSON.finditer(line):
+        name = match.group(1)
+        if not invented(name) and name.lower() not in INVENTED_PEOPLE:
+            yield f"person ID {match.group(0)!r}: use person/alex"
+    if FLEET_ID.search(line):
+        yield "internal agent or mission ID: use agent/example/..."
+
+
+def self_test():
+    """Prove each kind of name is caught and each invented example is not."""
+    MACHINE_DIGESTS.add(digest("plantedhost"))
+    caught = [
+        "ssh plantedhost", "PlantedHost.local (cli)", "plantedhost-pty",
+        "cd /home/someone/src", "/Users/someone", "person/someone",
+        "agent/fleet/x/standing/y", "mission/fleet/x", "mission-run/fleet/x/1",
+    ]
+    allowed = [
+        "ssh example-host", "plantedhostx", "/home/example/src", "/Users/Example99/Proj",
+        "person/alex", "person/example-2", "agent/example/worker",
+    ]
+    wrong = [line for line in caught if not list(problems(line))]
+    wrong += [line for line in allowed if list(problems(line))]
+    for line in wrong:
+        print(f"self test: wrong answer for {line!r}")
+    return 1 if wrong else 0
+
+
+def main():
+    if sys.argv[1:] == ["--self-test"]:
+        return self_test()
+    root = Path(
+        subprocess.run(
+            ["git", "rev-parse", "--show-toplevel"],
+            check=True, capture_output=True, text=True,
+        ).stdout.strip()
+    )
+    script = Path(__file__).resolve()
+    this = script.relative_to(root).as_posix() if script.is_relative_to(root) else None
+    allow_path = root / ALLOW_FILE
+    allowed = []
+    if allow_path.exists():
+        for raw in allow_path.read_text().splitlines():
+            if raw.strip() and not raw.startswith("#"):
+                allowed.append(re.compile(raw))
+
+    files = subprocess.run(
+        ["git", "ls-files", "-z"],
+        cwd=root, check=True, capture_output=True,
+    ).stdout.decode().split("\0")
+
+    found = 0
+    for path in files:
+        if not path or path in (this, ALLOW_FILE):
+            continue
+        full = root / path
+        if not full.is_file():
+            continue
+        data = full.read_bytes()
+        if b"\0" in data:
+            continue
+        for number, line in enumerate(data.decode("utf-8", "replace").splitlines(), 1):
+            located = f"{path}:{line}"
+            if any(pattern.search(located) for pattern in allowed):
+                continue
+            for problem in problems(line):
+                print(f"{path}:{number}: {problem}")
+                found += 1
+
+    if found:
+        print(
+            f"\n{found} private name(s) found. Replace each with an invented "
+            f"example, or add a pattern to {ALLOW_FILE} if the line must stay.",
+            file=sys.stderr,
+        )
+        return 1
+    return 0
+
+
+if __name__ == "__main__":
+    sys.exit(main())