One round for a chat, as many as it takes for an agent

Two different jobs were sharing one number. A plain conversation asking a
question is one round of looking things up and then an answer; the rounds after
that were a small model that had decided searching was the answer searching
until the context ran out, at a full request each. MAX_ROUNDS is 1 now. Several
tools can still be called within that round, which is the thing worth telling
the model.

The trade is real and worth naming: a plain chat can no longer search and then
read one of the results, because reading is a second round. That is what an
agent chat is for.

An agent chat is sized by Limits instead, where steps is now a runaway backstop
and not a working budget. It was 40 and it was reached -- a step count low
enough to be the thing that ends a reply is a count that ends it halfway. What
bounds one now is the wall clock and a new completion-token ceiling, with zero
meaning no ceiling, the same convention index_chars already uses.

That ceiling would have been decorative. generation.completion_tokens is only
populated when the endpoint sends a usage block, and llama.cpp, Ollama and
friends never do; the fallback estimate is computed once, in _run's finally,
long after the loop that needs it. So _written takes the larger of reported and
estimated, and there is a test that runs the whole thing against a stream
reporting no usage at all. A limit that works on OpenAI and silently does
nothing everywhere else is the worst kind: one that looks configured.

core.rounds could not stay one fragment. "You get at most N rounds" is not the
same sentence with a different number in it -- a model told it has a budget
rations it and stops early to report progress, which is exactly the behaviour
that strands a long piece of work. So it splits: core.rounds keeps the
one-round case and gates on a new round_budget variable that _agent_values
blanks, and core.keep_working says the other thing to an agent chat.

A queued message during a one-round reply is now never taken mid-reply -- there
is no work under way to steer -- and falls through to _drain, which gives it a
reply of its own. No code change went with that; it falls out of the guard, and
there is a test so that "it happens to work" and "it is meant to work" stop
looking the same.

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

src/lembas/services/prompts.py

@@ -128,6 +128,13 @@ VARIABLES: tuple[Variable, ...] = (
     Variable("user_name", "User's name", "The name of the person in the conversation."),
     Variable("model_name", "Model", "The display name of the model answering."),
     Variable("max_rounds", "Tool rounds", "How many rounds of tool calls one reply may take."),
+    Variable(
+        "round_budget",
+        "Round budget applies",
+        "Set in an ordinary chat and blank in an agent chat. Nothing renders it; "
+        "it exists so a fragment can say `requires=('round_budget',)` and appear "
+        "for one and not the other.",
+    ),
     Variable(
         "memory_limit",
         "Memory length",
@@ -572,19 +579,64 @@ BUILTIN: tuple[Fragment, ...] = (
             "permission first."
         ),
     ),
+    Fragment(
+        key="core.tool_list",
+        label="What you have",
+        group=GROUP_CORE,
+        order=105,
+        when_tools=True,
+        variables=("tool_names",),
+        requires=("tool_names",),
+        hint="The names of the tools offered on THIS request, which is not the "
+        "same as the tools that exist -- a chat can narrow them, a model's "
+        "capabilities can, a permission can. A model that has to discover its "
+        "own list by calling something and being told it does not exist spends "
+        "a round finding out, and in an ordinary chat that round is the whole "
+        "reply. It is also what stops a model hunting for a skill when there "
+        "are none.",
+        default=(
+            "The tools you have on this request are: {{tool_names}}. That is the whole "
+            "list. Anything not named there does not exist here — calling it costs a "
+            "round and returns nothing."
+        ),
+    ),
     Fragment(
         key="core.rounds",
         label="The round budget",
         group=GROUP_CORE,
         order=110,
         when_tools=True,
-        variables=("max_rounds",),
-        hint="A model that plans six searches gets cut off after three. Better it "
-        "knows the budget than discovers it.",
+        requires=("round_budget",),
+        hint="An ordinary chat only. It gets ONE round of tool calls, and the "
+        "thing worth saying about one round is 'ask for everything at once' — "
+        "which is different in kind from what is true of an agent chat's two "
+        "hundred, not a different number in the same sentence. So this is "
+        "gated on `round_budget`, which `_agent_values` blanks, and the agent "
+        "case is its own fragment below.",
         default=(
-            "You get at most {{max_rounds}} rounds of tool calls before you have to "
-            "answer with what you have. Several tools can be called in one round. Plan "
-            "within that budget: two careful searches beat six that run out halfway."
+            "You get one round of tool calls, and then you have to answer with what "
+            "came back. Ask for everything you need at once — several tools can be "
+            "called in the same round. If what comes back is not enough, say what you "
+            "would look up next rather than answering as though it were."
+        ),
+    ),
+    Fragment(
+        key="core.keep_working",
+        label="Working until it is done",
+        group=GROUP_CORE,
+        order=111,
+        families=("agent",),
+        hint="An agent chat only, and the counterpart to the round budget above. "
+        "A model told it has a budget rations it and stops early to report "
+        "progress; the step count here is a runaway backstop, not an "
+        "allowance, and saying so is what makes a long piece of work run.",
+        default=(
+            "Keep working until the task is actually done. You are not rationing a "
+            "round budget: call tools as many times as the work needs, one step "
+            "informing the next. What ends a reply is finishing it, being stopped, or "
+            "running past the time and output an administrator allowed — and if that "
+            "happens you are told so and can be asked to carry on. Do not stop halfway "
+            "to report progress and wait to be told to continue."
         ),
     ),
     Fragment(