diff --git a/gateway/hosted_rooms_common.py b/gateway/hosted_rooms_common.py
new file mode 100644
index 0000000000..a08de5b43a
--- /dev/null
+++ b/gateway/hosted_rooms_common.py
@@ -0,0 +1,139 @@
+"""Shared leaf helpers for the gateway hosted-room modules.
+
+Every hosted-room module validates identifiers, bounded integers, exact field
+sets and canonical JSON with its own error class and its own error strings
+(tests pin those strings). These helpers take the error class and message
+templates as parameters so each caller keeps byte-identical failures while the
+logic lives once. This module must stay a leaf: never import a hosted_room*
+origin module from here (import cycle).
+"""
+
+from __future__ import annotations
+
+import json
+import re
+import sqlite3
+from contextlib import contextmanager
+from pathlib import Path
+from typing import Any, Callable, Iterator, Mapping
+
+IDENTIFIER_RE = re.compile(r"^[A-Za-z0-9][A-Za-z0-9._:-]*$")
+
+
+def identifier(
+    value: Any,
+    *,
+    label: str,
+    error: type[Exception],
+    max_chars: int = 128,
+    pattern: re.Pattern[str] = IDENTIFIER_RE,
+    invalid: str | None = None,
+) -> str:
+    """Strip and validate a bounded identifier; ``invalid`` overrides the fail message."""
+    if not isinstance(value, str):
+        raise error(f"{label} must be a string")
+    value = value.strip()
+    if not value or len(value) > max_chars or not pattern.fullmatch(value):
+        raise error(invalid or f"invalid {label}")
+    return value
+
+
+def positive_int(value: Any, *, error: type[Exception], message: str) -> int:
+    """Reject bools, non-ints and values below 1 (``message`` is the exact error text)."""
+    if isinstance(value, bool) or not isinstance(value, int) or value < 1:
+        raise error(message)
+    return value
+
+
+def non_negative_int(value: Any, *, error: type[Exception], message: str) -> int:
+    if isinstance(value, bool) or not isinstance(value, int) or value < 0:
+        raise error(message)
+    return value
+
+
+def exact_fields(
+    value: Any,
+    *,
+    label: str,
+    required: frozenset[str] | set[str],
+    optional: frozenset[str] | set[str] = frozenset(),
+    error: type[Exception],
+    not_object: str | None = None,
+    missing_fmt: str = "{label} is missing fields: {fields}",
+    unknown_fmt: str = "{label} has unknown fields: {fields}",
+) -> Mapping[str, Any]:
+    """Require exactly ``required`` (+ any ``optional``) keys; formats name the offenders sorted."""
+    if not isinstance(value, Mapping):
+        raise error(not_object or f"{label} must be an object")
+    keys = frozenset(value)
+    missing = required - keys
+    unknown = keys - required - optional
+    if missing:
+        raise error(missing_fmt.format(label=label, fields=", ".join(sorted(missing))))
+    if unknown:
+        raise error(unknown_fmt.format(label=label, fields=", ".join(sorted(unknown))))
+    return value
+
+
+def canonical_json(
+    value: Any,
+    *,
+    error: type[Exception],
+    label: str,
+    max_bytes: int,
+    ensure_ascii: bool,
+) -> str:
+    """Sorted, compact JSON bounded by ``max_bytes`` of UTF-8."""
+    try:
+        encoded = json.dumps(
+            value, ensure_ascii=ensure_ascii, sort_keys=True, separators=(",", ":")
+        )
+    except (TypeError, ValueError, RecursionError) as exc:
+        raise error(f"{label} must be JSON-serializable") from exc
+    if len(encoded.encode("utf-8")) > max_bytes:
+        raise error(f"{label} is too large")
+    return encoded
+
+
+def utf8_len(*parts: str) -> int:
+    return len("".join(parts).encode("utf-8"))
+
+
+def open_sqlite(path: Path | str, *, timeout: float = 10) -> sqlite3.Connection:
+    """Row-factory connection with foreign keys on; no journal or schema work."""
+    conn = sqlite3.connect(path, timeout=timeout)
+    conn.row_factory = sqlite3.Row
+    conn.execute("PRAGMA foreign_keys=ON")
+    return conn
+
+
+def table_exists(conn: sqlite3.Connection, table: str) -> bool:
+    row = conn.execute(
+        "SELECT 1 FROM sqlite_master WHERE type='table' AND name=?", (table,)
+    ).fetchone()
+    return row is not None
+
+
+def table_columns(conn: sqlite3.Connection, table: str) -> frozenset[str]:
+    return frozenset(row[1] for row in conn.execute(f"PRAGMA table_info({table})"))
+
+
+@contextmanager
+def transaction(
+    connect: Callable[[Path | str], sqlite3.Connection],
+    db_path: Path | str,
+    *,
+    immediate: bool,
+) -> Iterator[sqlite3.Connection]:
+    """Open via ``connect``, optionally ``BEGIN IMMEDIATE``, commit on success, always close."""
+    conn = connect(db_path)
+    try:
+        if immediate:
+            conn.execute("BEGIN IMMEDIATE")
+        yield conn
+        conn.commit()
+    except Exception:
+        conn.rollback()
+        raise
+    finally:
+        conn.close()