Fit every model request to its context window

Every turn re-sent the whole transcript with nothing checking it against
the model's window. On a local model that fails silently: Ollama drops
the start of an over-long prompt, which is where the system prompt and
the plan are. Measured on the Jetson's llama3.2, cut at 4096 tokens with
no error.

- The model catalog reads max_input_tokens and max_output_tokens from
  LiteLLM's model_info, and startup refuses a role on a model with no
  declared window
- BudgetedClient wraps both routes. It fits the canonical events before
  rendering: older tool results cut to head and tail, then the oldest
  turns dropped with a note, keeping calls with their results; a request
  that still cannot fit fails with ContextOverflow and is never sent.
  The output ceiling is the smaller of config and the model's limit
- Tokens are estimated from characters, pessimistically, and the
  estimate is corrected from reported prompt sizes, only ever upward
- The executor takes its scope from the log: the latest plan, which is
  the approved milestone plan, goes in its system prompt, and it sees
  only events after it
- Tool output keeps its head and, mostly, its tail, so a test runner's
  failure summary survives truncation
- tuning.context.keep_recent_turns and safety_margin are new knobs
- scripts/probe_context.py checks a declared window against what the
  server does; it confirmed llama3.2 cuts at the declared 4096
- The tuning guide covers the budget, declaring a local model's limits,
  and the probe

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

nisse-dev/src/nisse_dev/agent.py

@@ -15,6 +15,7 @@ from nisse_dev.approvals import PlanBoard
 from nisse_dev.cancel import Cancel
 from nisse_dev.catalog import ModelCatalog, select_client
 from nisse_dev.config import AgentConfig
+from nisse_dev.context import BudgetedClient
 from nisse_dev.control import Orchestrator
 from nisse_dev.delegation import Caps, Delegation
 from nisse_dev.executor import LLMExecutor
@@ -50,6 +51,10 @@ def discover_models(config: AgentConfig, oplog: OpLog) -> ModelCatalog:
     """
     catalog = ModelCatalog.discover(config.llm_base_url, config.llm_token)
     catalog.validate(config.roles.pinned())
+    # Every role's context window must be known before a request is built
+    # against it (A10): a guess that is too large reaches a server that may
+    # silently cut the prompt.
+    catalog.require_limits(config.roles.pinned())
     for role in TOOL_DRIVING_ROLES:
         facts = catalog.facts(config.roles.resolve(role))
         if facts.supports_tools is False:
@@ -156,14 +161,33 @@ def build_agent(
         max_output_chars=tuning.tools.max_output_chars,
     )
 
-    normalized = LiteLLMClient(
-        config.llm_base_url, config.llm_token, max_tokens=config.max_output_tokens
+    def limits_for(model: str) -> tuple[int, int | None]:
+        facts = catalog.facts(model)
+        assert facts.max_input_tokens is not None  # require_limits checked
+        return facts.max_input_tokens, facts.max_output_tokens
+
+    def budgeted(client: object) -> BudgetedClient:
+        # Every request, on either route, is fitted to its model's window.
+        return BudgetedClient(
+            client,
+            limits_for,
+            max_output_tokens=config.max_output_tokens,
+            keep_recent_turns=tuning.context.keep_recent_turns,
+            safety_margin=tuning.context.safety_margin,
+        )
+
+    normalized = budgeted(
+        LiteLLMClient(
+            config.llm_base_url, config.llm_token, max_tokens=config.max_output_tokens
+        )
     )
     # Delegations prefer the vendor-native route; fidelity is cheap
     # elsewhere (A5). Which route a given model gets is decided by
     # select_client, not by where the caller happens to sit.
-    passthrough = AnthropicPassthroughClient(
-        config.llm_base_url, config.llm_token, max_tokens=config.max_output_tokens
+    passthrough = budgeted(
+        AnthropicPassthroughClient(
+            config.llm_base_url, config.llm_token, max_tokens=config.max_output_tokens
+        )
     )
 
     transport = MatrixTransport(config.matrix)

nisse-dev/src/nisse_dev/catalog.py

@@ -35,12 +35,25 @@ class UnknownModel(LLMError):
     stale alias, or a key without access. Always a misconfiguration."""
 
 
+class UnknownLimits(LLMError):
+    """A model in use has no declared context window. LiteLLM knows the
+    limits of commercial models; for a local one they are whatever its server
+    allocates, and have to be declared in LiteLLM's ``model_info``. Guessing
+    is not safe: a server that silently truncates a prompt (Ollama does)
+    drops its start, which is where the system prompt and the plan are."""
+
+
 @dataclass(frozen=True)
 class ModelFacts:
     alias: str  # what we ask for, e.g. "remote-architecture"
     backing: str  # what LiteLLM calls, e.g. "anthropic/claude-opus-5"
     provider: str  # "anthropic", "openai", "ollama", ...
     supports_tools: bool | None  # None when LiteLLM does not say
+    # The whole context window, prompt plus response, as LiteLLM declares it
+    # (model_info.max_input_tokens); None when it does not say.
+    max_input_tokens: int | None = None
+    # The most one response may use (model_info.max_output_tokens).
+    max_output_tokens: int | None = None
 
     @property
     def native_anthropic(self) -> bool:
@@ -72,6 +85,8 @@ class ModelCatalog:
                 backing=backing,
                 provider=str(provider),
                 supports_tools=info.get("supports_function_calling"),
+                max_input_tokens=_positive_int(info.get("max_input_tokens")),
+                max_output_tokens=_positive_int(info.get("max_output_tokens")),
             )
         return cls(facts)
 
@@ -136,6 +151,24 @@ class ModelCatalog:
                 f"available: {', '.join(self.aliases()) or '(none)'}"
             )
 
+    def require_limits(self, roles: Mapping[str, str]) -> None:
+        """Refuse a role mapping that uses a model with no declared context
+        window, naming every such model at once."""
+        missing = {
+            role: alias
+            for role, alias in roles.items()
+            if self.facts(alias).max_input_tokens is None
+        }
+        if missing:
+            named = ", ".join(
+                f"{role}={alias!r}" for role, alias in sorted(missing.items())
+            )
+            raise UnknownLimits(
+                f"no context window is declared for: {named}. Set "
+                "model_info.max_input_tokens (and max_output_tokens) for these "
+                "in LiteLLM; docs/Tuning-Guide.md says how to find the values"
+            )
+
     def pinned(self, roles: Mapping[str, str]) -> dict[str, dict[str, Any]]:
         """The resolved mapping to record at task start (A11, A16).
 
@@ -154,6 +187,12 @@ class ModelCatalog:
         }
 
 
+def _positive_int(value: Any) -> int | None:
+    if isinstance(value, bool) or not isinstance(value, int | float):
+        return None
+    return int(value) if value > 0 else None
+
+
 def select_client(
     model: str,
     normalized: LLMClient,

nisse-dev/src/nisse_dev/config.py

@@ -86,12 +86,19 @@ class ToolsTuning:
     max_output_chars: int = 4000
 
 
+@dataclass(frozen=True)
+class ContextTuning:
+    keep_recent_turns: int = 6
+    safety_margin: float = 0.1
+
+
 @dataclass(frozen=True)
 class Tuning:
     executor: ExecutorTuning = field(default_factory=ExecutorTuning)
     expert: ExpertTuning = field(default_factory=ExpertTuning)
     delegation: DelegationTuning = field(default_factory=DelegationTuning)
     tools: ToolsTuning = field(default_factory=ToolsTuning)
+    context: ContextTuning = field(default_factory=ContextTuning)
 
 
 _TUNING_SECTIONS: dict[str, type] = {
@@ -128,6 +135,9 @@ def _parse_tuning(raw: Any, errors: list[str]) -> Tuning:
                 continue
             values[key] = number
         sections[name] = cls(**values)
+    margin = sections.get("context", ContextTuning()).safety_margin
+    if margin >= 1:
+        errors.append("tuning.context.safety_margin: must be less than 1")
     return Tuning(**sections)
 
 

nisse-dev/src/nisse_dev/context.py

@@ -0,0 +1,262 @@
+"""The context budget (spec A10: token budgeting is a render-time concern).
+
+Every model call re-sends a transcript that only grows, and a model server
+does not always refuse one that no longer fits: Ollama silently drops the
+start of the prompt, which is where the system prompt and the plan are. So a
+request is made to fit the model's declared window *before* it is sent, and
+one that cannot be made to fit is never sent.
+
+The log itself is never changed. Fitting works on the events about to be
+rendered, in three steps, stopping at the first that fits:
+
+1. Tool results older than the most recent turns are cut to their first and
+   last lines, with a note saying how much was left out.
+2. The oldest events are dropped, keeping tool calls with their results, and
+   a note at the start says how many.
+3. If even the recent turns and the system prompt do not fit, the call
+   fails with ContextOverflow. The caller treats it like any failed call:
+   the executor stops and asks the operator.
+
+Token counts are estimated from characters, conservatively, and the estimate
+is corrected from the input-token counts the server reports back. The
+correction only ever makes the estimate larger: underestimating is the
+dangerous direction.
+"""
+
+import json
+from collections.abc import Callable, Sequence
+from dataclasses import dataclass, replace
+from typing import Any
+
+from nisse_dev.canonical import (
+    AssistantText,
+    CanonicalEvent,
+    DelegationStarted,
+    PlanSubmitted,
+    StoredEvent,
+    ToolCall,
+    ToolResult,
+    UserMessage,
+)
+from nisse_dev.llm import DEFAULT_MAX_OUTPUT_TOKENS, LLMError
+
+# English and code run at about four characters a token. Three is deliberately
+# pessimistic, before any correction from what the server reports.
+CHARS_PER_TOKEN = 3.0
+# Role markers and formatting each rendered message costs beyond its text.
+MESSAGE_OVERHEAD_TOKENS = 8
+# How much of an older tool result survives step 1.
+ELIDED_RESULT_CHARS = 600
+
+# Events the model produced. A turn starts where these start.
+_MODEL_SIDE = (AssistantText, ToolCall, PlanSubmitted, DelegationStarted)
+
+
+class ContextOverflow(LLMError):
+    """The request cannot be made to fit the model's context window."""
+
+
+def head_and_tail(text: str, limit: int) -> str:
+    """Keep the start and, mostly, the end of a long text.
+
+    The end matters more: test runners and compilers put their summary of
+    what failed last. The start keeps the exit code and the first lines.
+    """
+    if len(text) <= limit:
+        return text
+    head = limit // 4
+    tail = limit - head
+    dropped = len(text) - limit
+    return f"{text[:head]}\n[... {dropped} characters omitted ...]\n{text[-tail:]}"
+
+
+def _event_chars(event: CanonicalEvent) -> int:
+    match event:
+        case ToolCall(tool=tool, arguments=arguments):
+            return len(tool) + len(json.dumps(arguments))
+        case ToolResult(result=result):
+            return len(result)
+        case _:
+            text = getattr(event, "text", None) or getattr(event, "plan", None)
+            if text is None:
+                text = json.dumps(getattr(event, "__dict__", {}), default=str)
+            return len(str(text))
+
+
+def estimate_tokens(
+    events: Sequence[StoredEvent],
+    system: str | None,
+    tools: list[dict[str, Any]] | None,
+    factor: float = 1.0,
+) -> int:
+    chars = sum(_event_chars(e.event) for e in events)
+    chars += len(system or "") + len(json.dumps(tools or []))
+    raw = chars / CHARS_PER_TOKEN + MESSAGE_OVERHEAD_TOKENS * (len(events) + 1)
+    return int(raw * factor) + 1
+
+
+def _turn_starts(events: Sequence[StoredEvent]) -> list[int]:
+    starts = []
+    previous_model_side = False
+    for index, stored in enumerate(events):
+        model_side = isinstance(stored.event, _MODEL_SIDE)
+        if model_side and not previous_model_side:
+            starts.append(index)
+        previous_model_side = model_side
+    return starts
+
+
+def _consistent(events: list[StoredEvent]) -> list[StoredEvent]:
+    """Drop results whose call is gone: a provider rejects a tool result
+    with nothing to answer."""
+    calls = {e.event.call_id for e in events if isinstance(e.event, ToolCall)}
+    return [
+        e
+        for e in events
+        if not isinstance(e.event, ToolResult) or e.event.call_id in calls
+    ]
+
+
+@dataclass(frozen=True)
+class Budget:
+    window: int  # prompt plus response, in tokens
+    max_output: int  # the ceiling sent with the request
+    keep_recent_turns: int
+    safety_margin: float  # fraction of the window held back
+
+    @property
+    def for_prompt(self) -> int:
+        return int(self.window * (1 - self.safety_margin)) - self.max_output
+
+
+def fit(
+    events: Sequence[StoredEvent],
+    system: str | None,
+    tools: list[dict[str, Any]] | None,
+    budget: Budget,
+    factor: float = 1.0,
+) -> list[StoredEvent]:
+    """The events to render: all of them if they fit, otherwise the result of
+    the three steps in the module docstring."""
+    limit = budget.for_prompt
+
+    def fits(candidate: Sequence[StoredEvent]) -> bool:
+        return estimate_tokens(candidate, system, tools, factor) <= limit
+
+    kept = list(events)
+    if fits(kept):
+        return kept
+
+    starts = _turn_starts(kept)
+    recent_from = (
+        starts[-budget.keep_recent_turns]
+        if len(starts) >= budget.keep_recent_turns
+        else 0
+    )
+
+    # Step 1: cut older tool results down.
+    kept = [
+        replace(
+            stored,
+            event=replace(
+                stored.event,
+                result=head_and_tail(stored.event.result, ELIDED_RESULT_CHARS),
+            ),
+        )
+        if index < recent_from and isinstance(stored.event, ToolResult)
+        else stored
+        for index, stored in enumerate(kept)
+    ]
+    if fits(kept):
+        return kept
+
+    # Step 2: drop the oldest events, one turn at a time, never the recent
+    # ones. The note keeps the transcript starting with a user message, which
+    # some chat templates require.
+    cuts = sorted({s for s in starts if 0 < s < recent_from} | {recent_from} - {0})
+    for cut in cuts:
+        remaining = _consistent(kept[cut:])
+        note = StoredEvent(
+            id=0,
+            ts="",
+            event=UserMessage(
+                text=(
+                    f"[{cut} earlier events omitted to fit the context window; "
+                    "the log still has them]"
+                ),
+                source="context-budget",
+            ),
+        )
+        candidate = [note, *remaining]
+        if fits(candidate):
+            return candidate
+
+    # Step 3: nothing more can go.
+    needed = estimate_tokens(kept[recent_from:], system, tools, factor)
+    raise ContextOverflow(
+        f"the system prompt and the last {budget.keep_recent_turns} turns need "
+        f"about {needed} tokens, but only {limit} fit a {budget.window}-token "
+        "window after the output ceiling and safety margin"
+    )
+
+
+class BudgetedClient:
+    """An LLM client that fits every request to the model's window.
+
+    Wraps a real client (either route) and takes each model's limits from
+    ``limits_for``, which the agent fills from LiteLLM's model catalog.
+    """
+
+    def __init__(
+        self,
+        inner: Any,
+        limits_for: Callable[[str], tuple[int, int | None]],
+        *,
+        max_output_tokens: int = DEFAULT_MAX_OUTPUT_TOKENS,
+        keep_recent_turns: int = 6,
+        safety_margin: float = 0.1,
+    ) -> None:
+        self._inner = inner
+        self._limits_for = limits_for
+        self._max_output_tokens = max_output_tokens
+        self._keep_recent_turns = keep_recent_turns
+        self._safety_margin = safety_margin
+        # Per model: how much the character estimate undercounted, as learned
+        # from reported usage. Starts at 1 and only grows.
+        self._factors: dict[str, float] = {}
+
+    @property
+    def last_usage(self) -> dict[str, int]:
+        return getattr(self._inner, "last_usage", {}) or {}
+
+    def budget_for(self, model: str) -> Budget:
+        window, model_max_output = self._limits_for(model)
+        ceiling = self._max_output_tokens
+        if model_max_output is not None:
+            ceiling = min(ceiling, model_max_output)
+        return Budget(
+            window=window,
+            max_output=ceiling,
+            keep_recent_turns=self._keep_recent_turns,
+            safety_margin=self._safety_margin,
+        )
+
+    def complete(
+        self,
+        events: list[StoredEvent],
+        tools: list[dict[str, Any]] | None,
+        model: str,
+        system: str | None = None,
+    ) -> list[CanonicalEvent]:
+        budget = self.budget_for(model)
+        factor = self._factors.get(model, 1.0)
+        fitted = fit(events, system, tools, budget, factor)
+        produced = self._inner.complete(
+            fitted, tools, model, system=system, max_tokens=budget.max_output
+        )
+        reported = int(self.last_usage.get("input", 0))
+        if reported > 0:
+            estimated = estimate_tokens(fitted, system, tools)
+            observed = reported / max(estimated, 1)
+            self._factors[model] = max(factor, observed)
+        return produced

nisse-dev/src/nisse_dev/executor.py

@@ -15,7 +15,7 @@ from dataclasses import dataclass
 from typing import Any, Literal
 
 from nisse_dev.cancel import Cancel, skip_calls
-from nisse_dev.canonical import ToolCall, ToolResult
+from nisse_dev.canonical import PlanSubmitted, StoredEvent, ToolCall, ToolResult
 from nisse_dev.delegation import Delegation
 from nisse_dev.expert import ASK_EXPERT_TOOL, BudgetExhausted, ExpertDesk
 from nisse_dev.llm import LLMClient, LLMError, RoleMap
@@ -24,7 +24,7 @@ from nisse_dev.tools import EXECUTOR_TOOLS, MUTATING_TOOLS, ToolOutcome, Workspa
 
 EXECUTOR_SYSTEM = """You are nisse-dev's executor, doing mechanical work.
 
-Follow the approved plan above exactly. It was written to be executed
+Follow the approved plan below exactly. It was written to be executed
 verbatim; do not redesign it, expand its scope, or skip steps.
 
 Work by calling tools. Verify your work by running the project's tests.
@@ -145,12 +145,34 @@ class LLMExecutor:
         except LLMError as exc:
             return ToolOutcome(f"the consultant did not answer: {exc}", success=False)
 
+    def _scope(self) -> tuple[str | None, int]:
+        """The approved plan, and where this milestone's events begin.
+
+        The most recent plan in the log is the approved milestone plan: no
+        plan is submitted between approval and execution, or during review
+        responses. The executor sees that plan and what came after it, and
+        not the planning conversation or earlier milestones, which a small
+        context window cannot afford and the plan already distils.
+        """
+        latest: StoredEvent | None = None
+        for stored in self._store.read_events():
+            if isinstance(stored.event, PlanSubmitted):
+                latest = stored
+        if latest is None:
+            return None, 0
+        assert isinstance(latest.event, PlanSubmitted)
+        return latest.event.plan, latest.id
+
     def run(self, milestone: int) -> ExecutorOutcome:
         model = self._roles.resolve("executor")
+        plan, after = self._scope()
         system = (
             f"{EXECUTOR_SYSTEM}\n\n{self._workspace.describe()}"
             f"\n\nYou are executing milestone {milestone}."
         )
+        if plan is not None:
+            # In the system prompt, so the context budget never drops it.
+            system += f"\n\nThe approved plan:\n{plan}"
         consecutive_failures = 0
         did_work = False
         challenged = False
@@ -162,7 +184,7 @@ class LLMExecutor:
                 return ExecutorOutcome("stopped", f"before turn {turn + 1}")
             try:
                 produced = self._client.complete(
-                    self._store.read_events(),
+                    self._store.read_events(after=after),
                     tools=self._tools(),
                     model=model,
                     system=system,

nisse-dev/src/nisse_dev/llm.py

@@ -149,12 +149,15 @@ class LiteLLMClient:
         tools: list[dict[str, Any]] | None,
         model: str,
         system: str | None = None,
+        *,
+        max_tokens: int | None = None,
     ) -> list[CanonicalEvent]:
         body = self._renderer.render(events, tools, model, system=system)
         # A ceiling on this route too: without one a runaway turn is bounded
         # only by the provider's default, and the same truncation check has
-        # nothing to check against.
-        body["max_tokens"] = self._max_tokens
+        # nothing to check against. A caller that knows the model's limits
+        # (the context budget) passes a tighter one.
+        body["max_tokens"] = max_tokens or self._max_tokens
         response = _post_with_rate_limit_retry(
             self._http, "/chat/completions", body, self._sleep
         )
@@ -175,8 +178,8 @@ class LiteLLMClient:
             raise LLMError(f"malformed completion response: {exc}") from exc
         if choice.get("finish_reason") == "length":
             raise TruncatedResponse(
-                f"{requested_model} hit its output limit mid-answer; "
-                "raise llm.max_output_tokens"
+                f"{requested_model} hit its output limit mid-answer; raise "
+                "llm.max_output_tokens, or the model's max_output_tokens in LiteLLM"
             )
         produced_by = data.get("model") or requested_model
         usage = data.get("usage") or {}
@@ -258,9 +261,15 @@ class AnthropicPassthroughClient:
         tools: list[dict[str, Any]] | None,
         model: str,
         system: str | None = None,
+        *,
+        max_tokens: int | None = None,
     ) -> list[CanonicalEvent]:
         body = self._renderer.render(
-            events, tools, model, system=system, max_tokens=self._max_tokens
+            events,
+            tools,
+            model,
+            system=system,
+            max_tokens=max_tokens or self._max_tokens,
         )
         response = _post_with_rate_limit_retry(
             self._http, "/v1/messages", body, self._sleep
@@ -277,8 +286,8 @@ class AnthropicPassthroughClient:
     ) -> list[CanonicalEvent]:
         if data.get("stop_reason") == "max_tokens":
             raise TruncatedResponse(
-                f"{requested_model} hit the output limit of {self._max_tokens} "
-                f"tokens mid-answer; raise llm.max_output_tokens"
+                f"{requested_model} hit its output limit mid-answer; raise "
+                "llm.max_output_tokens, or the model's max_output_tokens in LiteLLM"
             )
         content = data.get("content")
         if not isinstance(content, list):

nisse-dev/src/nisse_dev/probe.py

@@ -0,0 +1,114 @@
+"""Check a model's declared context window against what its server does.
+
+LiteLLM's ``max_input_tokens`` for a local model is only a declaration; the
+real limit is whatever the model server allocates. This sends one prompt
+half again as long as the declared window and reads back what the server did
+with it:
+
+- **It refused.** Over-long prompts fail loudly, which is the safe behaviour.
+- **It cut the prompt.** The reported prompt size is where it cut, which is
+  the server's real window. Ollama does this, silently.
+- **It took the whole prompt.** The server allows more than is declared, so
+  the declaration is conservative.
+
+Run it with ``scripts/probe_context.py``. It spends one small and one large
+request on the model, so it refuses windows above a ceiling unless told
+otherwise: commercial models' limits are already known to LiteLLM.
+"""
+
+from dataclasses import dataclass
+
+import httpx
+
+# Of a probe's size, how much must come back for it to count as uncut. A
+# chat template adds a little, and a tokenizer counts a little differently.
+_UNCUT = 0.9
+# How close the server's cut must be to the declaration to count as a match.
+_MATCH = 0.02
+
+
+@dataclass(frozen=True)
+class ProbeResult:
+    verdict: str  # "matches", "declared_too_high", "declared_conservative", ...
+    message: str
+    server_window: int | None = None
+
+
+def interpret(
+    declared: int, sent: int, reported: int | None, error: str | None
+) -> ProbeResult:
+    """What one over-long probe says about the server's window.
+
+    ``sent`` is roughly how many tokens the probe held; ``reported`` is the
+    prompt size the server reported, if it answered; ``error`` its error
+    text, if it refused.
+    """
+    if error is not None:
+        return ProbeResult(
+            "refuses",
+            f"the server refused a ~{sent}-token prompt ({error[:200]}). Over-long "
+            "prompts fail loudly here, which is safe; the declared window of "
+            f"{declared} was not contradicted.",
+        )
+    assert reported is not None
+    if reported >= sent * _UNCUT:
+        return ProbeResult(
+            "declared_conservative",
+            f"the server took the whole ~{sent}-token prompt ({reported} reported), "
+            f"more than the declared {declared}. The declaration is safe; raise it "
+            "if you want the room.",
+            server_window=None,
+        )
+    if abs(reported - declared) <= declared * _MATCH:
+        return ProbeResult(
+            "matches",
+            f"the server cut the prompt at {reported} tokens, matching the declared "
+            f"{declared}.",
+            server_window=reported,
+        )
+    if reported < declared:
+        return ProbeResult(
+            "declared_too_high",
+            f"the server cut the prompt at {reported} tokens, but LiteLLM declares "
+            f"{declared}. Prompts between the two would be silently cut: set "
+            f"max_input_tokens to {reported}.",
+            server_window=reported,
+        )
+    return ProbeResult(
+        "declared_conservative",
+        f"the server cut the prompt at {reported} tokens, above the declared "
+        f"{declared}. The declaration is safe; it could be raised to {reported}.",
+        server_window=reported,
+    )
+
+
+def _prompt(words: int) -> str:
+    return " ".join(f"item{i}" for i in range(words)) + "\n\nReply with just: ok"
+
+
+def _send(http: httpx.Client, model: str, words: int) -> tuple[int | None, str | None]:
+    response = http.post(
+        "/chat/completions",
+        json={
+            "model": model,
+            "messages": [{"role": "user", "content": _prompt(words)}],
+            "max_tokens": 5,
+        },
+    )
+    if response.status_code != 200:
+        return None, f"{response.status_code}: {response.text}"
+    usage = response.json().get("usage") or {}
+    return int(usage.get("prompt_tokens") or 0), None
+
+
+def probe(http: httpx.Client, model: str, declared: int) -> ProbeResult:
+    """Measure how many tokens a word of the probe costs on this model, then
+    send a prompt half again as long as the declared window."""
+    calibration_words = 200
+    tokens, error = _send(http, model, calibration_words)
+    if error is not None or not tokens:
+        return ProbeResult("failed", f"a short calibration request failed: {error}")
+    per_word = tokens / calibration_words
+    sent = int(declared * 1.5)
+    reported, error = _send(http, model, int(sent / per_word))
+    return interpret(declared, sent, reported, error)

nisse-dev/src/nisse_dev/tools.py

@@ -20,6 +20,8 @@ from dataclasses import dataclass
 from pathlib import Path
 from typing import Any
 
+from nisse_dev.context import head_and_tail
+
 # Tools that change or verify something. Reading a file or listing a
 # directory is a model looking around, not doing the work, so neither counts
 # as evidence that work was carried out.
@@ -112,11 +114,7 @@ class Workspace:
         self._max_output_chars = max_output_chars
 
     def _truncate(self, text: str) -> str:
-        limit = self._max_output_chars
-        if len(text) <= limit:
-            return text
-        dropped = len(text) - limit
-        return f"{text[:limit]}\n[... {dropped} characters truncated]"
+        return head_and_tail(text, self._max_output_chars)
 
     def _resolve(self, path: str) -> Path:
         candidate = Path(path)

nisse-dev/tests/test_agent_build.py

@@ -26,6 +26,8 @@ TEST_CATALOG = ModelCatalog.from_payload(
                 "model_info": {
                     "litellm_provider": "anthropic",
                     "supports_function_calling": True,
+                    "max_input_tokens": 200000,
+                    "max_output_tokens": 64000,
                 },
             }
             for alias in ("claude-opus-5", "claude-haiku-4-5")
@@ -185,8 +187,8 @@ def test_delegation_uses_the_passthrough_route(
 
     agent = build_offline(tmp_path, origin, monkeypatch)
     delegation = agent.orchestrator._delegate
-    assert isinstance(delegation._client, AnthropicPassthroughClient)
-    assert isinstance(agent.orchestrator._executor._client, LiteLLMClient)
+    assert isinstance(delegation._client._inner, AnthropicPassthroughClient)
+    assert isinstance(agent.orchestrator._executor._client._inner, LiteLLMClient)
     agent.stop()
 
 
@@ -194,8 +196,8 @@ def test_httpx_clients_are_not_shared_between_routes(
     tmp_path: Path, origin: Path, monkeypatch
 ) -> None:
     agent = build_offline(tmp_path, origin, monkeypatch)
-    normalized = agent.orchestrator._executor._client._http
-    passthrough = agent.orchestrator._delegate._client._http
+    normalized = agent.orchestrator._executor._client._inner._http
+    passthrough = agent.orchestrator._delegate._client._inner._http
     assert isinstance(normalized, httpx.Client)
     assert normalized is not passthrough  # different base URLs and headers
     agent.stop()
@@ -244,6 +246,7 @@ def test_a_tool_less_model_warns_but_still_boots(tmp_path: Path, monkeypatch) ->
                     "model_info": {
                         "litellm_provider": "openai",
                         "supports_function_calling": False,
+                        "max_input_tokens": 32768,
                     },
                 }
                 for alias in ("claude-opus-5", "claude-haiku-4-5")
@@ -338,6 +341,7 @@ def test_tuning_reaches_every_component(
     assert delegation._caps.wall_clock_s == 60
     assert executor._workspace._timeout_s == 300
     assert executor._workspace._max_output_chars == 2000
-    assert executor._client._max_tokens == 8192
-    assert delegation._client._max_tokens == 8192
+    assert executor._client._inner._max_tokens == 8192
+    assert delegation._client._inner._max_tokens == 8192
+    assert executor._client._max_output_tokens == 8192
     agent.stop()

nisse-dev/tests/test_config.py

@@ -297,3 +297,12 @@ def test_sibling_paths_are_fine(tmp_path: Path) -> None:
         {**SECRETS, "NISSE_ENV": str(tmp_path / ".env")},
     )
     assert config.workspace.name == "project"
+
+
+def test_context_tuning_is_read_and_checked() -> None:
+    config = build(tuning={"context": {"keep_recent_turns": 4, "safety_margin": 0.2}})
+    assert config.tuning.context.keep_recent_turns == 4
+    assert config.tuning.context.safety_margin == 0.2
+    assert build().tuning.context.keep_recent_turns == 6
+    with pytest.raises(ConfigError, match="safety_margin: must be less than 1"):
+        build(tuning={"context": {"safety_margin": 1}})

nisse-dev/tests/test_context.py

@@ -0,0 +1,294 @@
+"""The context budget (spec A10)."""
+
+import io
+from pathlib import Path
+from typing import Any
+
+import pytest
+
+from nisse_dev.canonical import (
+    AssistantText,
+    CanonicalEvent,
+    GateOpened,
+    PlanSubmitted,
+    StoredEvent,
+    ToolCall,
+    ToolResult,
+    UserMessage,
+)
+from nisse_dev.catalog import ModelCatalog, UnknownLimits
+from nisse_dev.context import (
+    Budget,
+    BudgetedClient,
+    ContextOverflow,
+    estimate_tokens,
+    fit,
+    head_and_tail,
+)
+from nisse_dev.executor import LLMExecutor
+from nisse_dev.llm import RoleMap
+from nisse_dev.oplog import OpLog
+from nisse_dev.state import StateStore
+
+
+def stored(events: list[CanonicalEvent]) -> list[StoredEvent]:
+    return [StoredEvent(id=i + 1, ts="t", event=e) for i, e in enumerate(events)]
+
+
+def turn(n: int, output: str) -> list[CanonicalEvent]:
+    """One executor turn: a command and its (possibly long) output."""
+    return [
+        ToolCall(call_id=f"c{n}", tool="run_command", arguments={"command": "make"}),
+        ToolResult(call_id=f"c{n}", result=output, success=True),
+    ]
+
+
+def transcript(turns: int, output_chars: int) -> list[StoredEvent]:
+    events: list[CanonicalEvent] = [UserMessage(text="start milestone 1", source="t")]
+    for n in range(turns):
+        events += turn(n, f"output {n} " + "x" * output_chars)
+    return stored(events)
+
+
+def budget(window: int, keep: int = 2) -> Budget:
+    return Budget(
+        window=window, max_output=500, keep_recent_turns=keep, safety_margin=0.1
+    )
+
+
+def results(events: list[StoredEvent]) -> dict[str, str]:
+    return {
+        e.event.call_id: e.event.result
+        for e in events
+        if isinstance(e.event, ToolResult)
+    }
+
+
+# --- keeping the end of long output ---------------------------------------
+
+
+def test_head_and_tail_keeps_the_failure_summary_at_the_end() -> None:
+    output = (
+        "exit code: 1\nstdout:\n"
+        + "collected 400 items\n" * 500
+        + "FAILED tests/test_app.py::test_greet - AssertionError\n"
+        + "==== 1 failed, 399 passed ====\n"
+    )
+    kept = head_and_tail(output, 1000)
+    assert len(kept) < 1100
+    assert kept.startswith("exit code: 1")  # the head keeps the exit code
+    assert "FAILED tests/test_app.py::test_greet" in kept
+    assert "1 failed, 399 passed" in kept
+    assert "characters omitted" in kept
+
+
+def test_short_text_is_untouched() -> None:
+    assert head_and_tail("fine", 1000) == "fine"
+
+
+# --- fitting, step by step -------------------------------------------------
+
+
+def test_a_transcript_that_fits_is_sent_unchanged() -> None:
+    events = transcript(turns=3, output_chars=100)
+    assert fit(events, "system", None, budget(100_000)) == events
+
+
+def test_step_one_cuts_older_tool_results_and_keeps_recent_ones_whole() -> None:
+    events = transcript(turns=6, output_chars=3000)
+    fitted = fit(events, "system", None, budget(4000, keep=2))
+
+    kept = results(fitted)
+    assert len(kept) == 6  # nothing dropped yet
+    for older in ("c0", "c1", "c2", "c3"):
+        assert "characters omitted" in kept[older]
+    for recent in ("c4", "c5"):
+        assert "characters omitted" not in kept[recent]
+
+
+def test_step_two_drops_the_oldest_turns_and_says_so() -> None:
+    events = transcript(turns=20, output_chars=3000)
+    fitted = fit(events, "system", None, budget(4000, keep=2))
+
+    first = fitted[0].event
+    assert isinstance(first, UserMessage)
+    assert "omitted to fit the context window" in first.text
+    kept = results(fitted)
+    assert "c0" not in kept  # the oldest went
+    assert {"c18", "c19"} <= kept.keys()  # the recent turns stayed
+    assert "characters omitted" not in kept["c19"]
+    # Every surviving result still has its call: a provider rejects an
+    # answer to a call it never saw.
+    calls = {e.event.call_id for e in fitted if isinstance(e.event, ToolCall)}
+    assert kept.keys() <= calls
+    assert estimate_tokens(fitted, "system", None) <= budget(4000).for_prompt
+
+
+def test_step_three_refuses_what_cannot_fit() -> None:
+    # Even the two most recent turns are bigger than the whole window.
+    events = transcript(turns=3, output_chars=20_000)
+    with pytest.raises(ContextOverflow, match="last 2 turns"):
+        fit(events, "system", None, budget(2000, keep=2))
+
+
+def test_a_huge_system_prompt_alone_overflows() -> None:
+    events = transcript(turns=1, output_chars=10)
+    with pytest.raises(ContextOverflow):
+        fit(events, "s" * 50_000, None, budget(4096))
+
+
+def test_the_output_ceiling_and_margin_come_out_of_the_window() -> None:
+    b = Budget(window=4096, max_output=1024, keep_recent_turns=6, safety_margin=0.1)
+    assert b.for_prompt == int(4096 * 0.9) - 1024
+
+
+# --- the client wrapper ----------------------------------------------------
+
+
+class RecordingClient:
+    def __init__(self, reported_input: int = 0) -> None:
+        self.calls: list[dict[str, Any]] = []
+        self.last_usage = {"input": reported_input, "output": 5}
+
+    def complete(self, events, tools, model, system=None, *, max_tokens=None):
+        self.calls.append({"events": events, "max_tokens": max_tokens})
+        return [AssistantText(text="ok", model=model)]
+
+
+def test_the_ceiling_is_the_smaller_of_config_and_model() -> None:
+    inner = RecordingClient()
+    client = BudgetedClient(inner, lambda m: (4096, 1024), max_output_tokens=16384)
+    client.complete(transcript(1, 10), None, "llama3.2")
+    assert inner.calls[0]["max_tokens"] == 1024
+
+    client = BudgetedClient(inner, lambda m: (200_000, 64_000), max_output_tokens=16384)
+    client.complete(transcript(1, 10), None, "claude-haiku-4-5")
+    assert inner.calls[1]["max_tokens"] == 16384
+
+
+def test_the_wrapper_sends_the_fitted_events() -> None:
+    inner = RecordingClient()
+    client = BudgetedClient(
+        inner, lambda m: (4000, 500), keep_recent_turns=2, safety_margin=0.1
+    )
+    client.complete(transcript(20, 3000), None, "m", system="system")
+    sent = inner.calls[0]["events"]
+    assert "omitted to fit" in sent[0].event.text
+
+
+def test_reported_usage_only_ever_raises_the_estimate() -> None:
+    events = transcript(1, 10)
+    estimate = estimate_tokens(events, None, None)
+
+    under = RecordingClient(reported_input=estimate * 2)  # we undercounted
+    client = BudgetedClient(under, lambda m: (100_000, None))
+    client.complete(events, None, "m")
+    assert client._factors["m"] == pytest.approx(2.0, rel=0.05)
+
+    over = RecordingClient(reported_input=estimate // 2)  # we overcounted
+    under_then_over = BudgetedClient(over, lambda m: (100_000, None))
+    under_then_over._factors["m"] = 2.0
+    under_then_over.complete(events, None, "m")
+    assert under_then_over._factors["m"] == 2.0  # never loosened
+
+
+def test_last_usage_passes_through() -> None:
+    client = BudgetedClient(RecordingClient(reported_input=7), lambda m: (100, None))
+    assert client.last_usage["input"] == 7
+
+
+# --- the limits come from LiteLLM's catalog --------------------------------
+
+
+CATALOG = ModelCatalog.from_payload(
+    {
+        "data": [
+            {
+                "model_name": "claude-haiku-4-5",
+                "litellm_params": {"model": "anthropic/claude-haiku-4-5"},
+                "model_info": {"max_input_tokens": 200000, "max_output_tokens": 64000},
+            },
+            {
+                "model_name": "llama3.2",
+                "litellm_params": {"model": "ollama/llama3.2"},
+                "model_info": {"max_input_tokens": 4096, "max_output_tokens": 1024},
+            },
+            {
+                "model_name": "undeclared",
+                "litellm_params": {"model": "ollama/qwen3"},
+                "model_info": {"max_input_tokens": None},
+            },
+        ]
+    }
+)
+
+
+def test_the_catalog_reads_each_models_limits() -> None:
+    facts = CATALOG.facts("llama3.2")
+    assert (facts.max_input_tokens, facts.max_output_tokens) == (4096, 1024)
+    assert CATALOG.facts("undeclared").max_input_tokens is None
+
+
+def test_a_role_on_a_model_without_a_window_is_refused() -> None:
+    CATALOG.require_limits({"executor": "llama3.2", "planner": "claude-haiku-4-5"})
+    with pytest.raises(UnknownLimits, match="executor='undeclared'"):
+        CATALOG.require_limits({"executor": "undeclared", "planner": "llama3.2"})
+
+
+# --- the executor sees the approved plan and what came after ---------------
+
+
+def test_the_executor_sees_only_its_milestone(tmp_path: Path) -> None:
+    store = StateStore(tmp_path / "state.db", oplog=OpLog("t", io.StringIO()))
+    for event in (
+        UserMessage(text="use greet: a long planning conversation", source="t"),
+        PlanSubmitted(plan="arch plan", model="m"),
+        UserMessage(text="approve", source="t"),
+        PlanSubmitted(plan="milestone 1: write greet.py", model="m"),
+        GateOpened(gate="milestone-1", reason="plan submitted"),
+        UserMessage(text="start milestone 1", source="t"),
+    ):
+        store.append_event(event)
+
+    class Workspace:
+        def describe(self) -> str:
+            return "workspace"
+
+    seen: dict[str, Any] = {}
+
+    class Client:
+        def complete(self, events, tools, model, system=None):
+            seen["events"] = [e.event for e in events]
+            seen["system"] = system
+            return [AssistantText(text="done", model=model)]
+
+    roles = RoleMap(executor="m", planner="m", delegate="m", consultant="m")
+    LLMExecutor(Client(), store, roles, Workspace()).run(1)
+
+    texts = [getattr(e, "text", None) for e in seen["events"]]
+    assert "start milestone 1" in texts
+    assert "use greet: a long planning conversation" not in texts
+    assert not any(isinstance(e, PlanSubmitted) for e in seen["events"])
+    # The plan rides in the system prompt, which the budget never drops.
+    assert "The approved plan:\nmilestone 1: write greet.py" in seen["system"]
+    assert "arch plan" not in seen["system"]
+
+
+def test_a_request_that_cannot_fit_stops_the_milestone(tmp_path: Path) -> None:
+    # Never sent: the executor stops and asks the operator, like any failed
+    # call, rather than handing a server a prompt it would silently cut.
+    store = StateStore(tmp_path / "state.db", oplog=OpLog("t", io.StringIO()))
+    store.append_event(UserMessage(text="start milestone 1", source="t"))
+
+    class Workspace:
+        def describe(self) -> str:
+            return "workspace " + "w" * 20_000  # a system prompt too big to fit
+
+    inner = RecordingClient()
+    client = BudgetedClient(inner, lambda m: (4096, 1024))
+    roles = RoleMap(executor="m", planner="m", delegate="m", consultant="m")
+    outcome = LLMExecutor(client, store, roles, Workspace()).run(1)
+
+    assert outcome.status == "escalate"
+    assert "fit" in outcome.note
+    assert inner.calls == []

nisse-dev/tests/test_probe.py

@@ -0,0 +1,53 @@
+import json
+
+import httpx
+
+from nisse_dev.probe import interpret, probe
+
+
+def test_a_cut_at_the_declared_window_matches() -> None:
+    result = interpret(declared=4096, sent=6144, reported=4096, error=None)
+    assert result.verdict == "matches"
+    assert result.server_window == 4096
+
+
+def test_a_cut_below_the_declaration_says_what_to_set() -> None:
+    # The dangerous case: prompts between the two would be silently cut.
+    result = interpret(declared=8192, sent=12288, reported=4096, error=None)
+    assert result.verdict == "declared_too_high"
+    assert "set max_input_tokens to 4096" in result.message
+
+
+def test_a_cut_above_the_declaration_is_conservative() -> None:
+    result = interpret(declared=4096, sent=6144, reported=5000, error=None)
+    assert result.verdict == "declared_conservative"
+
+
+def test_an_uncut_prompt_means_the_server_allows_more() -> None:
+    result = interpret(declared=4096, sent=6144, reported=6100, error=None)
+    assert result.verdict == "declared_conservative"
+    assert result.server_window is None
+
+
+def test_a_refusal_is_the_safe_behaviour() -> None:
+    result = interpret(declared=4096, sent=6144, reported=None, error="400: too long")
+    assert result.verdict == "refuses"
+
+
+def test_probe_calibrates_then_sends_half_again_the_window() -> None:
+    """Against a fake Ollama: 2 tokens a word, silently cut at 4096."""
+    sizes: list[int] = []
+
+    def handler(request: httpx.Request) -> httpx.Response:
+        words = len(json.loads(request.content)["messages"][0]["content"].split())
+        sizes.append(words * 2)
+        reported = min(words * 2, 4096)
+        return httpx.Response(200, json={"usage": {"prompt_tokens": reported}})
+
+    with httpx.Client(
+        base_url="http://litellm", transport=httpx.MockTransport(handler)
+    ) as http:
+        result = probe(http, "llama3.2", declared=4096)
+
+    assert result.verdict == "matches"
+    assert sizes[1] >= 4096 * 1.4  # the probe really exceeded the window

nisse-dev/tests/test_tools.py

@@ -126,7 +126,7 @@ def test_stdin_is_closed_so_prompts_fail_fast(ws: Workspace) -> None:
 def test_long_output_is_truncated(ws: Workspace) -> None:
     outcome = ws.run_command("printf 'x%.0s' $(seq 1 20000)")
     assert len(outcome.result) < MAX_OUTPUT_CHARS + 200
-    assert "truncated" in outcome.result
+    assert "characters omitted" in outcome.result
 
 
 def test_dispatch_unknown_tool_and_missing_argument(ws: Workspace) -> None: