A time in no particular zone, and a preview missing what it previews

The first audit pass: everything from 0.8.1 to 0.9.8 read as a whole rather
than one feature at a time, starting with what a model is actually told.

Four of these had shipped as correct. The date line carried a timezone
variable that resolves to nothing until somebody chooses one -- so every
default account was told times were "in  unless they say otherwise", while
two comments asserted the line disappeared instead. The prompt preview
built its variables without a chat, which is what eleven fragments are
gated on, so the whole agent surface was absent from it whatever was
ticked. Plan mode was instructed to keep its plan current with a tool that
mode withdraws. And knowledge_get returned a document whole where every
sibling reader caps and says so, its description promising exactly that.

The subagent guidance was wrong in both directions at once: it denied a
documented parameter and named seven of twenty-three allowed commands.
Both halves are pinned by tests against the real list and the real schema
now, because prose and a constant drift the moment one is edited alone.

docs/notes/audit-0.9.md carries the findings that are not fixed here, with
why -- the ones whose fix would change what a feature does are the user's
call, not this pass's.

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

tests/test_agent_tools.py

@@ -986,6 +986,67 @@ async def test_an_ordinary_chat_is_not_asked_to_narrate(db, user_id, machine):
     assert "Settle what you are setting out to achieve" not in text
 
 
+def _give_a_plan(db, chat):
+    """A plan on the chat, the way `_plan_of` finds one: a message carrying it
+    and the chat pointing at that message."""
+    from lembas.services import plans
+
+    message = Message(
+        chat_id=chat.id,
+        role=ROLE_ASSISTANT,
+        content="",
+        plan_json=plans.build(title="Fix the parser", steps="Read it\nChange it"),
+    )
+    db.add(message)
+    db.commit()
+    chat.plan_message_id = message.id
+    db.commit()
+
+
+async def test_plan_mode_is_not_told_to_use_a_tool_it_does_not_have(db, user_id, machine):
+    """`agent/tools.py` withdraws `plan_update` in Plan mode -- that mode ends
+    with `plan_submit` instead. Its guidance was gated on {{plan}}, which is set
+    whenever a plan exists in any mode, so a model in Plan mode was told to
+    "keep it current with plan_update as you go" about a tool that was not in
+    its list, directly under `core.tool_list` saying anything unnamed does not
+    exist. The fragment's own hint asserted the two coincided.
+    """
+    from lembas.services import harness
+
+    chat, _profile = _setup(db, user_id, machine, mode=policy.MODE_PLAN)
+    _give_a_plan(db, chat)
+    user = db.get(User, user_id)
+
+    offered = tools_service.resolve_tools(db, chat, user)
+    assert "plan_update" not in offered.by_name
+
+    values = harness.context_variables(db, user, offered.schemas, chat)
+    # The plan is still shown -- a model that cannot see it cannot submit a
+    # better one. Only the instruction to *edit* it goes.
+    assert values["plan"]
+    assert values["plan_editable"] == ""
+
+    text = harness.compose(db, user, offered.schemas, chat)
+    assert "Fix the parser" in text
+    assert "plan_update" not in text
+
+
+async def test_a_mode_that_has_plan_update_is_still_told_about_it(db, user_id, machine):
+    """The other half, or the gate is just a deletion."""
+    from lembas.services import harness
+
+    chat, _profile = _setup(db, user_id, machine, mode=policy.MODE_EDIT)
+    _give_a_plan(db, chat)
+    user = db.get(User, user_id)
+
+    offered = tools_service.resolve_tools(db, chat, user)
+    assert "plan_update" in offered.by_name
+
+    values = harness.context_variables(db, user, offered.schemas, chat)
+    assert values["plan_editable"] == values["plan"] != ""
+    assert "plan_update" in harness.compose(db, user, offered.schemas, chat)
+
+
 async def test_an_agent_chat_is_told_its_real_round_budget(db, user_id, machine):
     """MAX_ROUNDS is one. An agent chat gets hundreds, and telling it one would
     be a false fact about its own budget on every turn."""