Arbiter

A detection engine that reports what it did not check.

Most checkers answer one question: what is wrong? When they return nothing, you cannot tell whether they looked and found nothing, or never looked at all. Those two results are printed identically, and only one of them is good news.

Arbiter separates them. Every evaluation that declines to run is recorded — with a machine-readable reason — alongside the findings, and every pass reports how many evaluations it attempted. A clean result means these invariants were tested and held, and it is distinguishable from nothing was testable.

The envelope

A detection pass returns findings, declines, and a denominator:

problems              what was found
not_evaluated         what was NOT evaluated, and why
evaluations_attempted how many (axiom, entity, indicator) evaluations were tried

not_evaluated entries carry a reason from a closed vocabulary of eight — not_applicable, insufficient_samples, missing_property, missing_config, missing_entity_type, no_threshold, wrong_indicator_type, checker_error — so a decline is data, not a log line.

Two of the eight will account for most of what you see. insufficient_samples reports both the count it had and the count it needed, so it tells you how much longer to collect. not_applicable means the checker decided the axiom does not apply to that indicator at all — worth reading closely, because some checkers make that decision from the indicator's NAME, which is a rough edge rather than a rule to infer.

Why the denominator matters. Findings and declines do not sum to the total: an evaluation that ran and found nothing appears in neither. Without evaluations_attempted, the statement checked N invariants has no honest value of N — and an envelope reporting a fabricated denominator is the exact failure the envelope exists to prevent.

All eight axiom checkers emit declines (15 call sites). This is not a property of one checker that the others aspire to.

The eight axioms

Declared per-indicator in a domain model, not in code:

Axiom	Asks
BOUNDEDNESS	does this stay within its bounds?
STABILITY	does it settle, or oscillate?
HOMEOSTASIS	does it return to baseline after disturbance?
MONOTONICITY	does it move only in the permitted direction?
CONSERVATION	does what goes in come out?
CONNECTIVITY	is the topology intact?
CONSISTENCY	do related values agree?
RESPONSIVENESS	does it react within its deadline?

- name: cpuUsageNanoCores
  type: NUMERIC
  axioms: [STABILITY, BOUNDEDNESS, HOMEOSTASIS]
  warning: 3
  critical: 10
  window: 1h

An empty axioms: [] is meaningful — the values flow into observation history without a per-cycle check. Silence is a declaration here, not an omission.

role: — two axioms need to know what kind of quantity they are reading

RESPONSIVENESS and CONSISTENCY carry rules about a quantity, not about an entity: a deadline applies to a latency, and 0 <= x <= 100 applies to a percentage. Declare which:

- name: setpoint_error_pct
  role: latency            # latency | count | percentage | ratio
  axioms: [RESPONSIVENESS]
  warning: 5
  critical: 12

Leave it out and the engine infers a role from the indicator's name — response/latency for the first, count/percent/pct/ratio for the second — so models written before this field existed behave exactly as they did. That inference is a guess about English, and when it misses, the axiom declines not_applicable and the cycle stays green: an indicator called pulldown_error_c could declare RESPONSIVENESS, be accepted, be listed by model_describe, and never once evaluate.

You do not have to run a cycle to find that out. model_describe reports unreachable_declarations — every declared (indicator, axiom) pair that cannot fire under any input, each with the remedy — and the loader logs the same list. An empty list is the target.

Quickstart

pip install arbiter-engine          # requires numpy and pyyaml, and nothing else

python3 -c "
from importlib.resources import files
from arbiter_engine.api import EngineSession, model_describe

s = EngineSession()
s.load_model(files('arbiter_engine').joinpath('examples/water_tank.yaml').read_text())
print(model_describe(s).to_dict()['checked'])"

The example is read out of the INSTALLED PACKAGE rather than off a relative path, and that detail is load-bearing rather than stylistic. A wheel ships only what lives under the package directory, so the copy at examples/ in this repository reaches the source distribution and not the wheel. This block used to open examples/water_tank.yaml directly: correct from a clone, and FileNotFoundError for anyone who installed the package instead — a failure that could not appear until the install line above stopped saying git clone. Both copies are here, written from one source: examples/ for reading, the packaged one for running.

That prints {'invariants': 0, 'entities': 3, 'declared_invariants': 10} — three entities, ten declared invariants, and zero evaluated, because no observations have been supplied yet. The zero is the point: it is reported rather than left for you to infer from an empty finding list.

Everything above is on the supported surface. Until 2026-08-11 this example imported load_domain from a deep module path — which works, and which this same README calls importable and unsupported three sections down. The first thing a reader runs should not be the one thing the document tells them not to depend on.

examples/water_tank.yaml is a deliberately synthetic two-tank water system that declares all eight axioms in one file, so it doubles as the schema reference. It is not one of the curated domain models — those are not published — and reading it is the fastest way to learn the shape.

Dependencies are two, and that was measured rather than assumed. numpy and pyyaml are required. scipy and rdflib are extras ([confidence], [rdf]) because they are reached only through two deep modules that the public API never touches — so the naive reading of the import list says four, and the measurement says two.

The public API

11 names. Everything else in the package is importable and unsupported — reaching for a deeper path is legitimate and unpromised, and those paths may move without a major version.

from arbiter_engine import (
    TopologyTraverser,          # the kernel: problem-solving as graph traversal
    UnifiedAxiomReasoner,       # evaluates axioms, produces the envelope
    DomainModel,                # your YAML, loaded
    InMemoryObservationHistory, # a concrete history, so it runs without a store
    Entity, Problem, RelationshipGraph, Observation, Axiom, Severity,
    api,                        # the tool surface — see below
)

Ten of those are types and the kernel; the eleventh is a module, and the split is deliberate. arbiter_engine.api is the tool surface: five verbs over a session, each returning the envelope above.

from arbiter_engine.api import EngineSession, model_describe, check, traverse, gaps, attest

session = EngineSession()
session.load_model("examples/water_tank.yaml")
model_describe(session)   # what is declared: entity types, indicators, axioms
check(session)            # evaluate the declared invariants over supplied observations
gaps(session)             # what the model says should exist and nothing has been observed

Three kinds of input, one feeder each. A session takes the current value of a property, the series behind it, and the edges between entities — and every axiom reads one or both of the first two, except CONNECTIVITY, which reads only the third.

session.add_entity("pump1", "Pump", properties={"speed_rpm": 2900})
session.add_observations("pump1", "speed_rpm", [2900, 2905, 2890, ...])
session.add_relationship("pump1", "feeds", "header")   # source, relation, target

The first two are easy to conflate and worth separating deliberately: threshold checks read the entity's current properties, and the temporal axioms read observation history. Supplying one and not the other is the commonest way to get a clean result over a value that is plainly out of range — the threshold checker never saw it, because the current value lives on the entity.

Omit the third and CONNECTIVITY will report a missing relationship, which is correct: a model that declares a pump must feed a tank is asserting something, and an absent edge falsifies it. That finding is not a complaint that you forgot to load edges — the engine cannot tell those apart, so it reports what the model asserted and lets you decide which it was.

They are a supported contract, and they are listed here as one name rather than six because they serve a different caller: an agent invoking tools, not a library user composing objects. check is not a peer of Entity, and flattening them into one namespace would say it was. The module is the promise; its membership is documented here and does not change inside a minor version.

The same five are exposed over MCP by arbiter_engine.mcp.server, which is a thin transport over exactly these functions and needs the optional mcp extra. That module is a deep path — importable, and not part of the eleven.

What is not here, and why

The engine is open. The knowledge and the operations are not.

• Domain models. The engine reads them; the curated packs are not published. The mechanism is the contribution — the models are the accumulated work. examples/water_tank.yaml is a synthetic teaching model, deliberately not one of them.
• The operator half. Clinic, planning, the Kubernetes executor, the introspection layer. These are welded to a running deployment and are not v0.1.
• Two lazy imports reach outside the cut, and they behave differently. One root-cause wiring module and an LLM client are imported lazily and are not shipped, so the package still imports cleanly. The root-cause wiring degrades to a no-op — its callsite is guarded and the feature it reports is optional telemetry. The LLM path raises, with a message saying so; it is reachable only through NLTraversalTranslator, which is not part of the supported surface, and the deterministic translate() needs no client. Both measured by running them, not read off the imports.

Status

v0.1. 57 Python files, 55 modules importing on the declared dependencies alone, 11 supported names — counted in this repository, which is the package you are holding.

That basis is stated because it is easy to get wrong in a way nobody notices. The build adds one __init__.py per package level, so a count taken before the build is smaller than the package you are holding — and this line published the smaller figure until 2026-08-12, where any reader could falsify it with find . -name '*.py' | wc -l. A checkable false claim, in the Status section of a project whose subject is checkable claims. Count the artifact, never an earlier stage of it.

The import figure carries the same hazard one layer down, and it depends on what you have installed. Sweeping the package where scipy happens to be present imports 56; on the declared dependencies alone it is the 55 above, because propagation.lp_confidence is the one module that needs scipy and it is a deep path outside the supported surface. Count the artifact in the state the reader will have it, not in the state the person measuring happens to be standing in — this line quoted the with-scipy figure until 2026-08-12, which no reader installing normally could reproduce.

Honest boundaries, stated because you would otherwise find them yourself:

• PREDICT is plumbed but unfed. The traversal mode exists and nothing produces projected values outside a test. It is not a working forecast.
• Not published to any index yet, so installation is from a clone.
• One worked example ships, not a library of them. Modelling a real system is your work.
• Stage I and Stage II of this project are archived, not running. Anything describing them as production is out of date.

Documentation

Evidence and technical write-ups live in evidence/ — architecture, deployment runbook, fault-scenario catalogue, and the observation logs from the closed-loop alpha, including the findings that went against us.

Licence

Apache License 2.0. See LICENSE and NOTICE.

TRADEMARK.md is separate and narrower: Apache Section 6 withholds any trademark grant, and that file says what use of the name is permitted. Arbiter is a project name, not a licence grant.

Contributing

See CONTRIBUTING.md. Adversarial findings are the most useful thing you can send: if the engine reports a clean pass over something it did not actually evaluate, that is the bug this project most wants to hear about.