Look around the machine before deciding to talk about it

The terminal and the canvas both needed a Chat, so they were missing from the
one screen where you are choosing which machine to work on. A draft is the
smallest thing that fixes it: an id, and the three facts behind it.

The trick is that a draft resolves to a *transient* Chat -- constructed, never
added to a session. `canvas.agent_ready`, `_executor`, `_load_agent`, `_save_agent`
and `agent_session.resolve` read exactly four attributes between them and none
of them queries or writes the row, so all of it works unchanged and nothing had
to learn what a draft is. Proven against a real sshd rather than a stub: a
transient chat opens and saves a project file over the same SFTP path a real one
uses, and the database stays empty throughout.

Chats are still created lazily. A draft is not a chat and never becomes one;
when the first prompt makes the real one, the shell is re-keyed into it and the
open tabs are copied across. `terminal.rekey` moves the registry key *and*
`session.chat_id`, because close_for_profile, close_for_owner and the reaper all
pop by the field -- a stale one would leave a dead session that `get` keeps
handing out. The shell is only adopted when its profile and directory match the
chat as finally resolved, since `_new_chat` settles an empty directory to the
connection's own; otherwise it is left alone rather than transplanted onto a
chat that says it runs elsewhere.

Two canvas sources are refused on a draft, by name, and one of them is a hole
rather than an inconvenience. `_load_file` authorises with
`attachment.chat_id != chat.id`, and an upload made on the new-chat screen is
stored with `chat_id=None` -- so a draft whose chat carried no id would make that
comparison `None != None`, which is False, and open every unclaimed attachment
its owner has. `as_chat` does set an id, so it already fails; the refusal is
stated anyway, because a guarantee that lives in an id-shaped coincidence is one
the next change breaks without noticing.

Adoption needed almost no JavaScript: start_chat already answers with
HX-Redirect, so the page reloads and the canvas adopts by construction while the
terminal reconnects to the re-keyed session and replays its scrollback -- the "a
reload is indistinguishable from a second tab" property working for us. What
re-points them mid-screen is a `lembas:agent-target` event, dispatched from
`setDir` and the connection select because assigning to a hidden field's value
fires nothing on its own. Driven under a DOM stub before committing.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

src/lembas/api/canvas.py

@@ -21,6 +21,7 @@ from lembas.api.deps import Db, RequiredUser
 from lembas.db.models import Chat, User
 from lembas.services import canvas as canvas_service
 from lembas.services import generation as generation_service
+from lembas.services.agent import draft as draft_service
 from lembas.services.agent.base import Conflict
 from lembas.services.markdown import highlight_code, render_markdown
 from lembas.web.templating import templates
@@ -32,13 +33,38 @@ router = APIRouter(prefix="/api/chats", tags=["canvas"])
 
 def _owned_chat(db: DBSession, chat_id: str, user_id: str) -> Chat:
     """404 rather than 403 for somebody else's chat: whether it exists at all is
-    not this account's business."""
+    not this account's business.
+
+    A draft id resolves to a transient `Chat` -- constructed, never saved --
+    which is what lets the canvas work on the new-chat screen without any of the
+    six sources learning that drafts exist. See services/agent/draft.py.
+    """
+    if draft_service.is_draft(chat_id):
+        draft = draft_service.get(chat_id, user_id)
+        if draft is None:
+            raise HTTPException(status.HTTP_404_NOT_FOUND, "That chat no longer exists.")
+        return draft_service.as_chat(draft)
+
     chat = db.get(Chat, chat_id)
     if chat is None or chat.user_id != user_id:
         raise HTTPException(status.HTTP_404_NOT_FOUND, "That chat no longer exists.")
     return chat
 
 
+def _remember_tabs(chat: Chat, state: dict) -> bool:
+    """Put the tab strip back where it came from. True when it was a draft.
+
+    A draft's tabs live in the registry rather than on a row, so the two write
+    paths below fork here rather than each remembering to check.
+    """
+    if not draft_service.is_draft(chat.id):
+        return False
+    draft = draft_service.get(chat.id, chat.user_id)
+    if draft is not None:
+        draft.canvas_json = dict(state or {})
+    return True
+
+
 async def _panel(
     request: Request,
     db: DBSession,
@@ -121,6 +147,16 @@ async def open_tab(
     """
     chat = _owned_chat(db, chat_id, user.id)
 
+    # Two of the six sources need a real row behind them, and one of those is a
+    # hole rather than an inconvenience -- see draft.SOURCES_NEEDING_A_CHAT.
+    # Refused by source name, here, rather than left to fall out of an id
+    # comparison somewhere further in.
+    if draft_service.is_draft(chat.id) and draft_service.refuses(key.split(":", 1)[0]):
+        return await _panel(
+            request, db, user, chat,
+            message="That can only be opened once this chat exists. Send a message first.",
+        )
+
     try:
         doc = await canvas_service.load(db, user, chat, key)
     except canvas_service.Refused as exc:
@@ -133,7 +169,8 @@ async def open_tab(
     # Reassigned rather than mutated: an in-place edit of a JSON column is not
     # reliably detected as a change.
     chat.canvas_json = state
-    db.commit()
+    if not _remember_tabs(chat, state):
+        db.commit()
 
     # A reply running right now holds its own snapshot, seeded when it started.
     # Without this the next frame it sends would contradict what was just
@@ -151,7 +188,8 @@ async def close_tab(
 ):
     chat = _owned_chat(db, chat_id, user.id)
     chat.canvas_json = canvas_service.close_tab(dict(chat.canvas_json or {}), key)
-    db.commit()
+    if not _remember_tabs(chat, chat.canvas_json):
+        db.commit()
 
     live = generation_service.running_for(chat.id)
     if live is not None: