diff --git a/hooks/bus-send-guard.py b/hooks/bus-send-guard.py
new file mode 100644
index 0000000..4ec38e3
--- /dev/null
+++ b/hooks/bus-send-guard.py
@@ -0,0 +1,238 @@
+#!/usr/bin/env python3
+"""PreToolUse guard: stop a claude-bus message body from being executed by the shell.
+
+WHY THIS EXISTS
+---------------
+On 2026-08-09 10:23:25 PT the orchestrator ran (abridged):
+
+    node ~/.claude/claude-bus/bus.mjs send --to all \
+      --subject "... worktree - `git worktree remove --force` followed the link ..." \
+      --msg "... everything was committed and `git restore .` brought it all back ..."
+
+The Bash tool runs Git Bash (POSIX sh). Inside DOUBLE quotes a POSIX shell still
+expands backticks, $(...), ${...} and $VAR. So the shell ran `git restore .` in
+~/.claude BEFORE node ever started -- a repo-wide revert of every modified tracked
+file -- and delivered the message with the backticked text silently deleted.
+
+Nothing inside bus.mjs can prevent this: by the time node runs, the shell has
+already executed the payload. A PreToolUse hook is the ONLY layer that sees the
+command before the shell does. That is what this file is.
+
+POLICY
+------
+For a `bus.mjs send` command, the value of --msg / --body / --subject must be
+inert. Allowed:
+  * a single-quoted literal:            --msg 'text'
+  * a quoted-heredoc capture:           --msg "$(cat <<'EOF' ... EOF)"
+  * a plain file read:                  --msg "$(cat body.txt)"
+  * the safe routes:                    --file body.txt | --stdin | --json-stdin
+Denied: any backtick, $(...), ${...} or $VAR that the shell would expand inside a
+double-quoted or unquoted value.
+
+Fails OPEN on an internal error (exit 0, no opinion): a bug in this guard must
+never be able to block Bash across the fleet. It fails CLOSED only on a positive
+detection.
+"""
+import json
+import os
+import re
+import sys
+from datetime import datetime
+
+LOG = os.path.join(os.path.dirname(os.path.abspath(__file__)), "bus-send-guard.log")
+
+PAYLOAD_FLAGS = ("--msg", "--body", "--subject")
+# Commands whose output is literal text, so "$(cat ...)" is an inert capture.
+LITERAL_READERS = ("cat", "printf", "type")
+
+REMEDY = (
+    "Rewrite the send so the text never reaches the shell. Either:\n"
+    "  node ~/.claude/claude-bus/bus.mjs send --to <id> --subject 'plain text' --stdin <<'EOF'\n"
+    "  ...body...\n"
+    "  EOF\n"
+    "or write the body to a file with a quoted heredoc and pass --file <path>.\n"
+    "Single quotes around a short subject are also fine. See "
+    "~/.claude/claude-bus/SHELL-INJECTION-EVIDENCE-2026-08-09.md"
+)
+
+
+def log(line: str) -> None:
+    try:
+        with open(LOG, "a", encoding="utf-8") as f:
+            f.write(f"{datetime.now().isoformat(timespec='seconds')} {line}\n")
+    except OSError:
+        pass
+
+
+def decide(decision: str, reason: str) -> None:
+    print(json.dumps({"hookSpecificOutput": {
+        "hookEventName": "PreToolUse",
+        "permissionDecision": decision,
+        "permissionDecisionReason": reason,
+    }}))
+    sys.exit(0)
+
+
+def split_words(cmd: str):
+    """Split a shell command into words, keeping each word's RAW source text.
+
+    Tracks POSIX quoting so we can later ask "would the shell expand this?".
+    A $( ) or ` ` region is consumed whole -- whitespace inside it does not split a
+    word, which is what keeps `--msg "$(cat <<'EOF' ... EOF)"` a single value.
+    """
+    words, buf = [], []
+    i, n = 0, len(cmd)
+    while i < n:
+        c = cmd[i]
+        if c in " \t\n":
+            if buf:
+                words.append("".join(buf))
+                buf = []
+            i += 1
+            continue
+        if c == "\\" and i + 1 < n:
+            buf.append(cmd[i:i + 2])
+            i += 2
+            continue
+        if c == "'":
+            j = cmd.find("'", i + 1)
+            j = n if j == -1 else j + 1
+            buf.append(cmd[i:j])
+            i = j
+            continue
+        if c == '"':
+            j, i2 = i + 1, i
+            while j < n:
+                if cmd[j] == "\\":
+                    j += 2
+                    continue
+                if cmd[j] == '"':
+                    j += 1
+                    break
+                j += 1
+            buf.append(cmd[i2:j])
+            i = j
+            continue
+        if c == "$" and i + 1 < n and cmd[i + 1] == "(":
+            j, depth = i + 2, 1
+            while j < n and depth:
+                if cmd[j] == "(":
+                    depth += 1
+                elif cmd[j] == ")":
+                    depth -= 1
+                j += 1
+            buf.append(cmd[i:j])
+            i = j
+            continue
+        if c == "`":
+            j = cmd.find("`", i + 1)
+            j = n if j == -1 else j + 1
+            buf.append(cmd[i:j])
+            i = j
+            continue
+        buf.append(c)
+        i += 1
+    if buf:
+        words.append("".join(buf))
+    return words
+
+
+def _expansions(text: str):
+    """Shell expansions the shell WOULD perform in `text` (single-quoted spans skipped)."""
+    found, i, n = [], 0, len(text)
+    while i < n:
+        c = text[i]
+        if c == "'":  # literal span - nothing expands inside
+            j = text.find("'", i + 1)
+            i = n if j == -1 else j + 1
+            continue
+        if c == "\\" and i + 1 < n:  # escaped -> inert
+            i += 2
+            continue
+        if c == "`":
+            found.append("backtick command substitution (`...`)")
+            i += 1
+            continue
+        if c == "$" and i + 1 < n:
+            nxt = text[i + 1]
+            if nxt == "(":
+                found.append("command substitution $(...)")
+            elif nxt == "{":
+                found.append("parameter expansion ${...}")
+            elif nxt.isalpha() or nxt == "_":
+                m = re.match(r"\$[A-Za-z_][A-Za-z0-9_]*", text[i:])
+                found.append(f"variable expansion {m.group(0) if m else '$VAR'}")
+            elif nxt in "@*#?!0123456789":
+                found.append(f"variable expansion ${nxt}")
+            i += 1
+            continue
+        i += 1
+    return found
+
+
+def classify(raw: str):
+    """Return None if the value is inert, else a human reason it is dangerous."""
+    v = raw.strip()
+    if not v:
+        return None
+
+    # Fully single-quoted literal -> the shell expands nothing.
+    if len(v) >= 2 and v[0] == "'" and v[-1] == "'" and "'" not in v[1:-1]:
+        return None
+
+    # Peel one layer of surrounding double quotes for the checks below.
+    inner = v[1:-1] if len(v) >= 2 and v[0] == '"' and v[-1] == '"' else v
+
+    # Inert capture: "$(cat <<'EOF' ... EOF)" / "$(cat file)" / "$(printf ...)".
+    m = re.match(r"^\$\(\s*(\w+)", inner)
+    if m and inner.endswith(")") and m.group(1) in LITERAL_READERS:
+        # Only inert if EVERY heredoc delimiter is quoted; <<EOF expands inside.
+        for hd in re.finditer(r"<<-?\s*(\S)", inner):
+            if hd.group(1) not in ("'", '"'):
+                return ("heredoc delimiter is not quoted (<<EOF), so the shell expands "
+                        "backticks and $(...) inside the body - use <<'EOF'")
+        return None
+
+    hits = _expansions(inner)
+    if hits:
+        uniq = list(dict.fromkeys(hits))
+        return "the shell would execute/expand " + "; ".join(uniq[:4])
+    return None
+
+
+def main() -> None:
+    try:
+        data = json.load(sys.stdin)
+    except (ValueError, TypeError):
+        sys.exit(0)
+
+    if data.get("tool_name", "") != "Bash":
+        sys.exit(0)
+
+    cmd = (data.get("tool_input", {}) or {}).get("command", "") or ""
+    if "bus.mjs" not in cmd or not re.search(r"(?<![-\w])send(?![-\w])", cmd):
+        sys.exit(0)
+
+    try:
+        words = split_words(cmd)
+        for idx, w in enumerate(words):
+            if w not in PAYLOAD_FLAGS:
+                continue
+            value = words[idx + 1] if idx + 1 < len(words) else ""
+            reason = classify(value)
+            if reason:
+                log(f"DENY {w}: {reason} :: {cmd[:300]!r}")
+                decide("deny",
+                       f"BLOCKED: unsafe claude-bus send. The value of {w} is not inert - "
+                       f"{reason}. This is the exact bug that ran a repo-wide 'git restore' "
+                       f"in ~/.claude on 2026-08-09.\n\n{REMEDY}")
+        log(f"ALLOW :: {cmd[:200]!r}")
+    except Exception as e:  # never brick Bash on a guard bug
+        log(f"ERROR (failing open) {e!r}")
+        sys.exit(0)
+
+    sys.exit(0)
+
+
+if __name__ == "__main__":
+    main()
diff --git a/hooks/tests/test_bus_send_guard.py b/hooks/tests/test_bus_send_guard.py
new file mode 100644
index 0000000..6901f08
--- /dev/null
+++ b/hooks/tests/test_bus_send_guard.py
@@ -0,0 +1,105 @@
+#!/usr/bin/env python3
+"""Adversarial tests for hooks/bus-send-guard.py.
+
+Two properties matter equally:
+  1. Every shape of the 2026-08-09 injection is DENIED.
+  2. The idioms ~21 live sessions already use are still ALLOWED (a guard that
+     blocks legitimate sends would break the fleet, which is its own outage).
+
+Run:  python hooks/tests/test_bus_send_guard.py
+"""
+import json
+import os
+import subprocess
+import sys
+
+HOOK = os.path.join(os.path.dirname(os.path.abspath(__file__)), "..", "bus-send-guard.py")
+BUS = "node ~/.claude/claude-bus/bus.mjs send --to all "
+
+
+def run(cmd: str):
+    """Invoke the hook exactly as Claude Code does; return (decision, reason)."""
+    payload = json.dumps({"tool_name": "Bash", "tool_input": {"command": cmd}})
+    p = subprocess.run([sys.executable, HOOK], input=payload,
+                       capture_output=True, text=True, timeout=30)
+    if not p.stdout.strip():
+        return ("allow", "")  # silence == no opinion == allowed
+    out = json.loads(p.stdout)["hookSpecificOutput"]
+    return (out["permissionDecision"], out["permissionDecisionReason"])
+
+
+# (label, command, expected decision)
+CASES = [
+    # ---- the real incident, both halves -------------------------------------
+    ("incident: backtick in --msg",
+     BUS + '--msg "everything was committed and `git restore .` brought it back"', "deny"),
+    ("incident: backtick in --subject",
+     BUS + '--subject "worktree - `git worktree remove --force` followed the link" --msg \'x\'',
+     "deny"),
+
+    # ---- the full metacharacter set ----------------------------------------
+    ("$( ) command substitution",
+     BUS + '--msg "run $(touch /tmp/pwned) now"', "deny"),
+    ("${ } parameter expansion",
+     BUS + '--msg "path is ${HOME}/x"', "deny"),
+    ("$VAR expansion",
+     BUS + '--msg "home is $HOME"', "deny"),
+    ("$? special variable",
+     BUS + '--msg "exit was $?"', "deny"),
+    ("unquoted value with backtick",
+     BUS + "--msg hello`id`world", "deny"),
+    ("--body alias is covered",
+     BUS + '--body "oops `id`"', "deny"),
+    ("newline + semicolon inside double quotes",
+     BUS + '--msg "line one\nrm -rf /tmp/x; echo $USER"', "deny"),
+    ("unquoted heredoc delimiter still expands",
+     BUS + '--msg "$(cat <<EOF\nvalue is `id`\nEOF\n)"', "deny"),
+
+    # ---- idioms that MUST keep working -------------------------------------
+    ("single-quoted literal with backticks",
+     BUS + "--msg 'this `git restore .` is inert'", "allow"),
+    ("quoted heredoc capture (the fleet's current safe idiom)",
+     BUS + '--msg "$(cat <<\'EOF\'\nbody with `backticks` and $(subst) and ${braces}\nEOF\n)"',
+     "allow"),
+    ("plain file read",
+     BUS + '--msg "$(cat body.txt)"', "allow"),
+    ("--file route",
+     BUS + "--subject 'plain subject' --file body.txt", "allow"),
+    ("--stdin route with quoted heredoc",
+     BUS + "--subject 'plain subject' --stdin <<'EOF'\nbody `id` $(id)\nEOF", "allow"),
+    ("ordinary prose, no metacharacters",
+     BUS + '--subject "Build green" --msg "All 178 tests passed on branch fix/x"', "allow"),
+    ("escaped dollar is inert",
+     BUS + '--msg "costs \\$5 today"', "allow"),
+
+    # ---- must not touch unrelated commands ----------------------------------
+    ("non-send bus command untouched",
+     "node ~/.claude/claude-bus/bus.mjs inbox --peek", "allow"),
+    ("unrelated command with backticks untouched",
+     'echo "`date`"', "allow"),
+    ("word 'send' inside another program is not a bus send",
+     'python resend.py --msg "$HOME"', "allow"),
+]
+
+
+def main() -> int:
+    failures = []
+    for label, cmd, expected in CASES:
+        try:
+            decision, reason = run(cmd)
+        except Exception as e:  # noqa: BLE001
+            failures.append((label, expected, f"EXCEPTION {e!r}"))
+            continue
+        ok = (decision == expected)
+        print(f"{'PASS' if ok else 'FAIL'}  [{expected:5}] {label}")
+        if not ok:
+            failures.append((label, expected, f"got {decision}: {reason[:120]}"))
+
+    print(f"\n{len(CASES) - len(failures)}/{len(CASES)} passed")
+    for label, expected, got in failures:
+        print(f"  FAILED {label}: expected {expected}, {got}")
+    return 1 if failures else 0
+
+
+if __name__ == "__main__":
+    sys.exit(main())