diff --git a/src/lembas/api/chats.py b/src/lembas/api/chats.py
index 0554681..b879a60 100644
--- a/src/lembas/api/chats.py
+++ b/src/lembas/api/chats.py
@@ -34,6 +34,7 @@ router = APIRouter(prefix="/api/chats", tags=["chats"])
 # Well under nginx's 60s default; see services/sse.py:KEEPALIVE.
 KEEPALIVE_AFTER = 15.0
 
+
 def _owned_chat(db: DBSession, chat_id: str, user_id: str) -> Chat:
     chat = db.get(Chat, chat_id)
     # 404 rather than 403 for someone else's chat: whether a given id exists is
@@ -43,7 +44,14 @@ def _owned_chat(db: DBSession, chat_id: str, user_id: str) -> Chat:
     return chat
 
 
-def _new_chat(db: DBSession, user: User, *, folder_id: str = "", model_id: str = "") -> Chat:
+def _new_chat(
+    db: DBSession,
+    user: User,
+    *,
+    folder_id: str = "",
+    model_id: str = "",
+    temporary: bool = False,
+) -> Chat:
     """Create a chat row, resolving which model it should use."""
     chosen = None
     if model_id:
@@ -60,6 +68,7 @@ def _new_chat(db: DBSession, user: User, *, folder_id: str = "", model_id: str =
         folder_id=folder_id or None,
         model_id=chosen[0] if chosen else "",
         connection_id=chosen[1] if chosen else None,
+        temporary=temporary,
     )
     db.add(chat)
     db.commit()
@@ -74,6 +83,7 @@ async def start_chat(
     file_ids: list[str] = Form(default=[]),
     folder_id: str = Form(""),
     model_id: str = Form(""),
+    temporary: bool = Form(False),
 ) -> Response:
     """Create a chat from its first message.
 
@@ -86,7 +96,9 @@ async def start_chat(
     if not content and not file_ids:
         return Response(status_code=status.HTTP_204_NO_CONTENT)
 
-    chat = _new_chat(db, user, folder_id=folder_id, model_id=model_id)
+    chat = _new_chat(
+        db, user, folder_id=folder_id, model_id=model_id, temporary=temporary
+    )
 
     user_message = chat_service.create_message(db, chat, ROLE_USER, content)
     if file_ids:
@@ -106,6 +118,25 @@ async def start_chat(
 # /start when the first message is actually sent.
 
 
+@router.post("/{chat_id}/keep")
+async def keep_chat(db: Db, user: RequiredUser, chat_id: str) -> Response:
+    """Stop a temporary chat being temporary.
+
+    A conversation that turns out to matter has to have a way out; without one,
+    the sweep destroys it a day later with no recourse, and people discover that
+    exactly once.
+    """
+    chat = _owned_chat(db, chat_id, user.id)
+    chat.temporary = False
+    db.commit()
+
+    response = Response(status_code=status.HTTP_204_NO_CONTENT)
+    # The sidebar has to gain a row and the topbar has to lose a badge; a full
+    # refresh is one line against a handful of out-of-band fragments.
+    response.headers["HX-Refresh"] = "true"
+    return response
+
+
 @router.get("/unread")
 async def unread_poll(db: Db, user: RequiredUser) -> Response:
     """Dots for the sidebar, and a toast for anything newly arrived.
@@ -119,7 +150,13 @@ async def unread_poll(db: Db, user: RequiredUser) -> Response:
     """
     chats = list(
         db.scalars(
-            select(Chat).where(Chat.user_id == user.id, Chat.archived.is_(False))
+            select(Chat).where(
+                Chat.user_id == user.id,
+                Chat.archived.is_(False),
+                # A temporary chat has no sidebar row, so a dot has nowhere to
+                # land and the toast would name a chat nobody can navigate to.
+                Chat.temporary.is_(False),
+            )
         )
     )
 
diff --git a/src/lembas/api/pages.py b/src/lembas/api/pages.py
index 79a79d2..961b31b 100644
--- a/src/lembas/api/pages.py
+++ b/src/lembas/api/pages.py
@@ -83,6 +83,7 @@ def sidebar_context(db: DBSession, user: User) -> dict:
                 Chat.user_id == user.id,
                 Chat.folder_id.is_(None),
                 Chat.archived.is_(False),
+                Chat.temporary.is_(False),
             )
             .order_by(Chat.pinned.desc(), Chat.updated_at.desc())
         )
@@ -164,11 +165,15 @@ async def offline(request: Request) -> Response:
 
 
 @router.get("/chat")
-async def chat_index(request: Request, db: Db, user: RequiredUser, model: str = ""):
+async def chat_index(
+    request: Request, db: Db, user: RequiredUser, model: str = "", temporary: bool = False
+):
     """A composer with no chat behind it yet.
 
     `?model=` preselects one, which is how the pinned shortcuts work without
-    creating a row for a chat that may never be sent.
+    creating a row for a chat that may never be sent. `?temporary=1` is the
+    same idea for the temporary flag: it lives in the URL rather than in
+    JavaScript, so it survives a reload and can be bookmarked.
     """
     context = _chat_context(db, user, None)
 
@@ -195,6 +200,7 @@ async def chat_index(request: Request, db: Db, user: RequiredUser, model: str =
             "bodies": {},
             **context,
             "current_model": preselected,
+            "starting_temporary": temporary,
             **sidebar_context(db, user),
         },
     )
diff --git a/src/lembas/db/models/chat.py b/src/lembas/db/models/chat.py
index e4dca77..0eedd64 100644
--- a/src/lembas/db/models/chat.py
+++ b/src/lembas/db/models/chat.py
@@ -58,7 +58,7 @@ class Folder(UUIDPrimaryKey, Timestamps, Base):
 
         Ordered like the unfiled list: pinned first, then most recently touched.
         """
-        kept = [chat for chat in self.chats if not chat.archived]
+        kept = [chat for chat in self.chats if not chat.archived and not chat.temporary]
         kept.sort(key=lambda chat: chat.updated_at, reverse=True)
         kept.sort(key=lambda chat: not chat.pinned)
         return kept
@@ -95,6 +95,13 @@ class Chat(UUIDPrimaryKey, Timestamps, Base):
     pinned: Mapped[bool] = mapped_column(Boolean, default=False, nullable=False)
     archived: Mapped[bool] = mapped_column(Boolean, default=False, nullable=False)
 
+    # Never listed in the sidebar, and swept a day after the last thing said in
+    # it. A real row rather than something held in the browser, so a reload or a
+    # dropped connection does not lose the conversation -- and `Keep` clears the
+    # flag, because a temporary chat that turns out to matter must have a way
+    # out. See services/chat.py:sweep_temporary.
+    temporary: Mapped[bool] = mapped_column(Boolean, default=False, nullable=False)
+
     # A reply landed while nobody was watching this chat. Cleared when the chat
     # is next opened. `unread_notified` stops the same arrival being announced
     # on every poll.
diff --git a/src/lembas/main.py b/src/lembas/main.py
index c1c6287..dcf0bed 100644
--- a/src/lembas/main.py
+++ b/src/lembas/main.py
@@ -62,6 +62,7 @@ async def lifespan(app: FastAPI) -> AsyncIterator[None]:
     # disk forever. Cheap, and startup is the natural moment for it.
     try:
         from lembas.db.session import session_scope
+        from lembas.services.chat import sweep_temporary
         from lembas.services.files import sweep_orphans
         from lembas.services.library.documents import sweep_unfiled
 
@@ -70,6 +71,9 @@ async def lifespan(app: FastAPI) -> AsyncIterator[None]:
             # Documents that predate knowledge bases have nowhere to live until
             # this runs; see services/library/documents.py.
             sweep_unfiled(db)
+            # Temporary chats older than a day. Startup only, like the sweeps
+            # above it -- see services/chat.py:sweep_temporary.
+            sweep_temporary(db)
     except Exception:  # noqa: BLE001 - housekeeping must never block startup
         log.exception("orphaned upload sweep failed")
 
diff --git a/src/lembas/services/chat.py b/src/lembas/services/chat.py
index 54c642c..956eded 100644
--- a/src/lembas/services/chat.py
+++ b/src/lembas/services/chat.py
@@ -3,9 +3,10 @@
 from __future__ import annotations
 
 import logging
+from datetime import UTC, datetime, timedelta
 from typing import Any
 
-from sqlalchemy import select
+from sqlalchemy import func, select
 from sqlalchemy.orm import Session as DBSession
 
 from lembas.db.models import (
@@ -32,6 +33,9 @@ FORWARDED_PARAMS = frozenset(
 
 MAX_TITLE_LENGTH = 60
 
+# How long a temporary chat survives after the last thing said in it.
+TEMPORARY_LIFETIME = timedelta(hours=24)
+
 
 def resolve_endpoint(db: DBSession, chat: Chat) -> tuple[Endpoint, str]:
     """Find the connection and model a chat should use.
@@ -387,8 +391,49 @@ def create_message(
     return message
 
 
+def sweep_temporary(db: DBSession, older_than: timedelta = TEMPORARY_LIFETIME) -> int:
+    """Delete temporary chats nobody has touched for a day.
+
+    Age is measured from the newest message rather than from the chat row's own
+    timestamps. `created_at` would destroy a conversation still in use at hour
+    23, and `updated_at` does not move when a message is inserted -- `onupdate`
+    fires on an UPDATE of the chat, and adding a message is not one.
+
+    Startup only, like files.sweep_orphans beside it. A server that runs for a
+    month sweeps once; that is the trade the existing sweep already makes, and a
+    scheduler is a whole new concern for a single-worker application.
+    """
+    cutoff = datetime.now(UTC) - older_than
+    newest = (
+        select(Message.chat_id, func.max(Message.created_at).label("last"))
+        .group_by(Message.chat_id)
+        .subquery()
+    )
+    stale = list(
+        db.scalars(
+            select(Chat)
+            .outerjoin(newest, newest.c.chat_id == Chat.id)
+            .where(
+                Chat.temporary.is_(True),
+                func.coalesce(newest.c.last, Chat.created_at) < cutoff,
+            )
+        )
+    )
+    if not stale:
+        return 0
+
+    files_service.remove_files_for_chats(db, [chat.id for chat in stale])
+    for chat in stale:
+        db.delete(chat)
+    db.commit()
+    log.info("swept %d temporary chat(s)", len(stale))
+    return len(stale)
+
+
 def user_chats(db: DBSession, user_id: str, *, folder_id: str | None = None) -> list[Chat]:
-    query = select(Chat).where(Chat.user_id == user_id, Chat.archived.is_(False))
+    query = select(Chat).where(
+        Chat.user_id == user_id, Chat.archived.is_(False), Chat.temporary.is_(False)
+    )
     if folder_id is not None:
         query = query.where(Chat.folder_id == folder_id)
     return list(db.scalars(query.order_by(Chat.pinned.desc(), Chat.updated_at.desc())))
diff --git a/src/lembas/services/files.py b/src/lembas/services/files.py
index 8e2787e..26f500b 100644
--- a/src/lembas/services/files.py
+++ b/src/lembas/services/files.py
@@ -435,6 +435,26 @@ def claim(db: DBSession, *, ids: list[str], user_id: str, message_id: str) -> li
     return pending
 
 
+def remove_files_for_chats(db: DBSession, chat_ids: list[str]) -> int:
+    """Unlink the files belonging to these chats' attachments.
+
+    Deleting a Chat cascades to its Message and Attachment *rows* but leaves the
+    files on disk -- only `sweep_orphans` unlinks anything, and it only looks at
+    uploads that were never attached. Anything that deletes chats has to call
+    this first, while the rows still say which files to remove.
+    """
+    if not chat_ids:
+        return 0
+
+    removed = 0
+    for attachment in db.scalars(select(Attachment).where(Attachment.chat_id.in_(chat_ids))):
+        path = stored_path(attachment.stored_name)
+        if path is not None and path.exists():
+            path.unlink(missing_ok=True)
+            removed += 1
+    return removed
+
+
 def sweep_orphans(db: DBSession, older_than: timedelta = ORPHAN_AGE) -> int:
     """Delete uploads that were never attached to a message.
 
diff --git a/src/lembas/services/generation.py b/src/lembas/services/generation.py
index b2913ec..2e4d9dc 100644
--- a/src/lembas/services/generation.py
+++ b/src/lembas/services/generation.py
@@ -443,8 +443,10 @@ def _persist(generation: Generation, title: str, elapsed: float) -> None:
                 chat.title_generated = True
 
             # Nobody watching when it landed, so it is news. The chat page
-            # clears this when it is next opened.
-            if generation.followers == 0:
+            # clears this when it is next opened. Not for a temporary chat:
+            # there is no sidebar row for the dot, and the toast would name a
+            # chat nobody can navigate to.
+            if generation.followers == 0 and not chat.temporary:
                 chat.unread = True
                 chat.unread_notified = False
 
diff --git a/src/lembas/web/static/css/app.css b/src/lembas/web/static/css/app.css
index ca59b6b..ae33297 100644
--- a/src/lembas/web/static/css/app.css
+++ b/src/lembas/web/static/css/app.css
@@ -345,6 +345,7 @@ button, input, textarea, select {
 .badge--leaf { background: var(--leaf-soft); color: var(--leaf); }
 .badge--success { background: var(--success-soft); color: var(--success); }
 .badge--danger { background: var(--danger-soft); color: var(--danger); }
+.badge--warning { background: var(--warning-soft); color: var(--warning); }
 
 /* --- Application shell ----------------------------------------------------- */
 .shell { display: flex; height: 100dvh; overflow: hidden; }
diff --git a/src/lembas/web/templates/chat/_composer.html b/src/lembas/web/templates/chat/_composer.html
index bc67d42..02ae468 100644
--- a/src/lembas/web/templates/chat/_composer.html
+++ b/src/lembas/web/templates/chat/_composer.html
@@ -50,6 +50,9 @@
       {% if not chat and current_model %}
       <input type="hidden" name="model_id" value="{{ current_model.model_id }}">
       {% endif %}
+      {% if not chat and starting_temporary %}
+      <input type="hidden" name="temporary" value="true">
+      {% endif %}
 
       <div class="composer__row">
         {% if can.get("files.upload") %}
diff --git a/src/lembas/web/templates/chat/index.html b/src/lembas/web/templates/chat/index.html
index 0b8809c..5e6ccfa 100644
--- a/src/lembas/web/templates/chat/index.html
+++ b/src/lembas/web/templates/chat/index.html
@@ -25,6 +25,32 @@
       </h1>
 
       <div class="topbar__actions">
+        {#
+          A link, not a script: the flag lives in the URL, so it survives a
+          reload and can be bookmarked. On an existing temporary chat the same
+          corner explains what temporary means and offers the way out -- without
+          one, a conversation that turns out to matter is destroyed a day later
+          with no recourse.
+        #}
+        {% if chat and chat.temporary %}
+        <span class="badge badge--warning"
+              title="Not listed in the sidebar, and removed 24 hours after the last message.">
+          Temporary
+        </span>
+        <button class="btn btn--sm" type="button"
+                hx-post="/api/chats/{{ chat.id }}/keep" hx-swap="none"
+                title="Keep this chat and list it in the sidebar">
+          {{ icon("pin", "icon--sm") }} Keep
+        </button>
+        {% elif not chat and can.get("chat.create") %}
+        <a class="btn btn--icon {{ 'is-active' if starting_temporary }}"
+           href="{{ '/chat' if starting_temporary else '/chat?temporary=1' }}"
+           aria-label="Temporary chat"
+           title="{% if starting_temporary %}Starting a temporary chat. Click to go back to a normal one.{% else %}Start a temporary chat: not listed in the sidebar, and removed after a day.{% endif %}">
+          {{ icon("clock") }}
+        </a>
+        {% endif %}
+
         {% if models %}
           {% if can.get("chat.model_select") or not chat %}
             {% include "chat/_model_picker.html" %}
diff --git a/src/lembas/web/templates/partials/icons.html b/src/lembas/web/templates/partials/icons.html
index 30c13bb..a8c15da 100644
--- a/src/lembas/web/templates/partials/icons.html
+++ b/src/lembas/web/templates/partials/icons.html
@@ -114,6 +114,10 @@
     <rect x="3.5" y="4.5" width="17" height="4" rx="1.2"/>
     <path d="M5 8.5v9a2 2 0 0 0 2 2h10a2 2 0 0 0 2-2v-9M10 12.5h4"/>
   </symbol>
+  <symbol id="i-clock" viewBox="0 0 24 24">
+    <circle cx="12" cy="12" r="8.5"/>
+    <path d="M12 7.2V12l3.2 2"/>
+  </symbol>
   <symbol id="i-warning" viewBox="0 0 24 24">
     <path d="M12 4.2 21 19.5H3Z"/>
     <path d="M12 10v4M12 16.8h.01"/>
diff --git a/tests/test_temporary_chat.py b/tests/test_temporary_chat.py
new file mode 100644
index 0000000..08270f5
--- /dev/null
+++ b/tests/test_temporary_chat.py
@@ -0,0 +1,222 @@
+"""Temporary chats: hidden from the sidebar, and swept a day later."""
+
+from __future__ import annotations
+
+from datetime import UTC, datetime, timedelta
+
+from fastapi.testclient import TestClient
+from sqlalchemy import select
+
+from lembas.db.models import Chat, Connection, Folder, Message, Model
+from lembas.services import chat as chat_service
+from lembas.services.crypto import encrypt
+
+
+def _connection(db) -> Connection:
+    connection = Connection(
+        name="Test", base_url="http://127.0.0.1:1", api_key_encrypted=encrypt("")
+    )
+    db.add(connection)
+    db.commit()
+    db.add(Model(connection_id=connection.id, model_id="test-model"))
+    db.commit()
+    return connection
+
+
+def _temporary(db, chat_id: str, *, title: str = "Passing thought") -> Chat:
+    chat = db.get(Chat, chat_id)
+    chat.temporary = True
+    chat.title = title
+    db.commit()
+    return chat
+
+
+# --- Hidden ------------------------------------------------------------------
+def test_a_temporary_chat_is_not_in_the_sidebar(client: TestClient, db, registered, make_chat):
+    _connection(db)
+    _temporary(db, make_chat())
+    assert "Passing thought" not in client.get("/chat").text
+
+
+def test_a_temporary_chat_inside_a_folder_is_not_listed(
+    client: TestClient, db, registered, make_chat
+):
+    """The folder branch renders through the relationship, which is why this
+    needs asserting separately from the unfiled list."""
+    _connection(db)
+    client.post("/api/folders", data={"name": "Quests"})
+    folder = db.scalar(select(Folder))
+
+    chat_id = make_chat()
+    client.patch(f"/api/chats/{chat_id}", data={"folder_id": folder.id})
+    _temporary(db, chat_id)
+
+    page = client.get("/chat").text
+    assert "Passing thought" not in page
+    assert "Empty" in page
+
+
+def test_a_temporary_chat_gets_no_unread_dot(client: TestClient, db, registered, make_chat):
+    """There is no sidebar row for the dot, and the toast would name a chat
+    nobody can navigate to."""
+    _connection(db)
+    chat = _temporary(db, make_chat())
+    chat.unread = True
+    db.commit()
+
+    response = client.get("/api/chats/unread")
+    assert chat.id not in response.text
+    assert "HX-Trigger" not in response.headers
+
+
+def test_user_chats_excludes_temporary_ones(db, registered, make_chat, user_id):
+    _connection(db)
+    _temporary(db, make_chat())
+    assert chat_service.user_chats(db, user_id) == []
+
+
+def test_a_finished_temporary_reply_is_not_unread(db, registered, make_chat):
+    from lembas.services import generation as generation_service
+
+    _connection(db)
+    chat_id = make_chat()
+    _temporary(db, chat_id)
+    reply = Message(chat_id=chat_id, role="assistant", content="", complete=False)
+    db.add(reply)
+    db.commit()
+
+    generation = generation_service.Generation(chat_id=chat_id, message_id=reply.id)
+    generation.content.append("Waybread.")
+    generation_service._persist(generation, "", 0.0)
+
+    db.expire_all()
+    assert db.get(Chat, chat_id).unread is False
+
+
+# --- Starting one -------------------------------------------------------------
+def test_the_new_chat_screen_carries_the_flag(client: TestClient, db, registered):
+    _connection(db)
+    assert 'name="temporary"' not in client.get("/chat").text
+    assert 'name="temporary"' in client.get("/chat?temporary=1").text
+
+
+def test_starting_a_temporary_chat_sets_the_flag(client: TestClient, db, registered):
+    _connection(db)
+    client.post("/api/chats/start", data={"content": "hello", "temporary": "true"})
+    assert db.scalar(select(Chat)).temporary is True
+
+
+def test_starting_an_ordinary_chat_does_not(client: TestClient, db, registered):
+    _connection(db)
+    client.post("/api/chats/start", data={"content": "hello"})
+    assert db.scalar(select(Chat)).temporary is False
+
+
+# --- Keeping one --------------------------------------------------------------
+def test_keeping_a_chat_clears_the_flag(client: TestClient, db, registered, make_chat):
+    _connection(db)
+    chat = _temporary(db, make_chat())
+
+    response = client.post(f"/api/chats/{chat.id}/keep")
+    assert response.status_code == 204
+    assert response.headers["HX-Refresh"] == "true"
+
+    db.refresh(chat)
+    assert chat.temporary is False
+    assert "Passing thought" in client.get("/chat").text
+
+
+def test_keeping_someone_elses_chat_is_not_found(client: TestClient, db, registered, make_chat):
+    from lembas.db.models import User
+    from lembas.security.passwords import hash_password
+
+    _connection(db)
+    chat = _temporary(db, make_chat())
+    other = User(name="Sam", email="s@shire.test", password_hash=hash_password("x"))
+    db.add(other)
+    db.commit()
+    chat.user_id = other.id
+    db.commit()
+
+    assert client.post(f"/api/chats/{chat.id}/keep").status_code == 404
+
+
+# --- The sweep ----------------------------------------------------------------
+def _aged(db, chat_id: str, hours: float) -> None:
+    """Backdate the chat's newest message, which is what the sweep measures."""
+    when = datetime.now(UTC) - timedelta(hours=hours)
+    message = Message(chat_id=chat_id, role="user", content="hello", created_at=when)
+    db.add(message)
+    db.commit()
+
+
+def test_the_sweep_removes_a_stale_temporary_chat(db, registered, make_chat):
+    _connection(db)
+    chat_id = make_chat()
+    _temporary(db, chat_id)
+    _aged(db, chat_id, 25)
+
+    assert chat_service.sweep_temporary(db) == 1
+    assert db.get(Chat, chat_id) is None
+
+
+def test_the_sweep_keeps_one_still_in_use(db, registered, make_chat):
+    """Age is measured from the newest message. A conversation still going at
+    hour 23 must not vanish mid-sentence."""
+    _connection(db)
+    chat_id = make_chat()
+    _temporary(db, chat_id)
+    _aged(db, chat_id, 40)
+    _aged(db, chat_id, 0.5)
+
+    assert chat_service.sweep_temporary(db) == 0
+    assert db.get(Chat, chat_id) is not None
+
+
+def test_the_sweep_leaves_ordinary_chats_alone(db, registered, make_chat):
+    _connection(db)
+    chat_id = make_chat()
+    _aged(db, chat_id, 500)
+
+    assert chat_service.sweep_temporary(db) == 0
+    assert db.get(Chat, chat_id) is not None
+
+
+def test_a_temporary_chat_with_no_messages_ages_from_its_own_row(db, registered, make_chat):
+    _connection(db)
+    chat_id = make_chat()
+    chat = _temporary(db, chat_id)
+    chat.created_at = datetime.now(UTC) - timedelta(hours=30)
+    db.commit()
+
+    assert chat_service.sweep_temporary(db) == 1
+
+
+def test_the_sweep_unlinks_the_files_too(client: TestClient, db, registered, make_chat):
+    """Deleting a chat cascades the rows but leaves the files on disk. Anything
+    that deletes chats has to remove them while the rows still say which."""
+    from lembas.db.models import Attachment
+    from lembas.services.files import stored_path
+
+    _connection(db)
+    chat_id = make_chat()
+    client.post("/api/files", files={"file": ("notes.txt", b"some words", "text/plain")})
+    attachment = db.scalar(select(Attachment))
+    attachment.chat_id = chat_id
+    message = Message(chat_id=chat_id, role="user", content="look")
+    db.add(message)
+    db.commit()
+    attachment.message_id = message.id
+    db.commit()
+
+    path = stored_path(attachment.stored_name)
+    assert path is not None and path.exists()
+
+    _temporary(db, chat_id)
+    db.query(Message).filter(Message.id == message.id).update(
+        {"created_at": datetime.now(UTC) - timedelta(hours=30)}
+    )
+    db.commit()
+
+    chat_service.sweep_temporary(db)
+    assert not path.exists()