Executive briefing, October 2026
Based on companion v0.14
Before AI acts.
An AI agent can act, seek more evidence or involve a person. Hexis is a research programme to make that choice inspectable, and to test whether it can be made better.
A six-minute read for business leaders. The companion guide and the research paper sit behind it. Simon Herko and Chris Pidcock
What should the agent do next?
Act
when the evidence supports the decision
Seek evidence
when a material fact is missing
Escalate
when a person needs to decide
We are testing whether a visible, structured way of selecting an agent’s approach improves the decisions it makes, and the moment at which it makes them.
The business question
The action is visible. What led to it?
An agent handling a refund request has a choice to make before it sends a message or changes an account: whether it has enough evidence to act, whether it should seek more, or whether it should refer the case to a person.
For the organisation deploying it, the practical questions are what the agent did, when it did it, and whether that action was appropriate to the information available at the time. These are questions of judgement, with consequences for customers, staff and the business.
Hexis makes the agent’s disposition an explicit part of the design. A disposition is a configuration of three capacities, each of which can be implicit or explicit.
- Planning
- Make the course of action explicit when the situation needs it.
- Monitoring
- Check progress against supplied evidence or an external check.
- Recall
- Retrieve the relevant record when it is available, and abstain when it is not.
The selected configuration is recorded outside the model’s weights and can change with the situation. Whether these configurations produce meaningfully different behaviour is the first thing the programme tests.
The names in this briefing
- Hexis
- the research programme
- Yarrow
- the planned open contribution,
yarrow-ai - TAI
- the proposed evaluation instrument
A decision in practice
A missing fact changes the decision.
A customer reports a damaged item after the ordinary return window. Policy allows an exception if the damage was reported within a stated period after delivery, so the agent needs the dates before it can apply the policy.
The request is familiar. The facts are incomplete.
The selected configuration and a reading of the situation yield a decision state. The tool layer refuses a refund call from that state.
What the record can establish: Which state was recorded, and that a tool call was refused.
Refund call refused
Make the missing evidence explicit.
A permitted change of disposition precedes a decision artefact: a structured record that identifies the relevant dates and requests the order record.
What the record can establish: The recorded transition and the artefact produced.
Refund call refused
The record changes the situation.
The surrounding software supplies the order record between agent turns, and the situation is read again with the relevant history available.
What the record can establish: That the record was available at this point in the sequence, and the updated reading.
No tool call
Permission to act still requires a sound decision.
A further permitted change reaches an execution state, where a refund call is permitted. The right response still depends on applying the policy to the evidence: refund if the record qualifies, decline if it does not.
What the record can establish: Whether and when a tool call occurred.
Refund call permitted
Illustrative. These are hand-assigned readings adapted from the research paper’s worked example, not observed router outputs and not TAI assessment items.
A complete record could show that the evidence arrived before the action. Whether the agent used it correctly is a separate question, and the programme’s behavioural tests are what would answer it.
What Hexis proposes
A fixed structure, and a boundary on action.
Eight configurations of the agent’s three capacities combine with eight readings of what the situation supplies, giving 64 states. Named relations constrain which changes are permitted, and Yarrow’s arithmetic module, yarrow.hexmath, validates a relation before a change is made.
The space is divided in half. An open escalation blocks tool execution in any state.
32
Decision states
Produce a typed decision artefact. Tool execution is refused.
32
Execution states
Tool execution is permitted, unless an escalation is open.
- A router
- Hexis-S1, a small non-generative router, returns structured state and transition values, or a hold or an escalation, and no text.
- A replay record
- For every turn: the state readings, the transition, the relation cited, the action and the operational measurements. A reviewer can inspect the sequence without relying on the model’s account of itself.
These are design requirements to build and test. The programme’s first harness phase tests each check, including a stop that arrives mid-generation, mid-change or mid-escalation, before any agent-performance claim is assessed.
Existing frameworks already offer approval pauses and resumable state. The question for Hexis is what the explicit disposition configuration and its fixed transition structure add. That contribution has to earn its place in comparison.
Measuring judgement
Judgement has a timing dimension.
The same action can be premature on one turn and appropriate on the next. TAI, the proposed Timeliness and Appropriateness Instrument, assesses which response a system chooses in a situation, and when.
- Appropriateness
- Does the chosen response fit the role, the facts available, the constraints and the people affected?
- Timeliness
- At which turn in the scenario does the system choose that response, given the information and the deadline?
TAI is intended for systems built on other architectures, so organisations can take part without adopting Yarrow. It remains a development scale until its validation requirements are met.
Validation must show that scores mean more than fluency, general ability or habitual caution. An independent expert key, prepared without knowledge of the structural reading, tests whether system rankings depend on Hexis’s assumptions. Timeliness and appropriateness would be reported as separate dimensions only if the evidence supports that separation.
The evidence threshold
What would justify adoption work?
The structural results are available to examine. The behavioural case is still to be made. The research paper states each claim with the test that would refute it, fixed before the evaluations are run, and reports no agent-performance results.
| Area | Established so far | Still needed |
|---|---|---|
| Structure | Definitions and structural results, with a checking script. | Evidence of effects on agent behaviour. |
| Agent behaviour | The architecture and its comparisons are specified. | Implementation evidence and behavioural evaluations. |
| TAI | The construct, development process and validity tests are specified. | Successful validation before use as a validated instrument. |
| Yarrow release | Early components are identified, with an intention to license code under Apache 2.0 and documents under CC-BY. | Release approval and confirmation of the artefacts available. |
The research paper also reports one corpus measurement. That result does not establish an agent-performance benefit.
Three questions for a business evaluation
Does it change decisions for the better?
Compare with existing workflow controls and the specified baselines at matched compute, including premature action, unnecessary delay and appropriate escalation.
Can its controls be relied on?
Test the execution boundary, a stop during escalation and the completeness of the record, including failures and awkward timing.
Is the benefit worth the operational burden?
Measure the whole task: extra turns, routing, retrieval, latency, integration effort and human intervention. Matched compute alone does not establish lower operating cost.
An invitation
Start with one decision that matters.
Bring one bounded workflow where acting too early, waiting too long or escalating poorly has a consequence. Together we can define the evidence an adoption decision would need.
Architecture route
Evaluate Hexis and Yarrow
Help scope an evaluation in one workflow: its permissions, a credible comparison and the integration effort. For organisations adopting agents, solution integrators and platform providers.
Instrument route
Help validate TAI
Contribute domain or evaluation expertise: realistic decisions, independent judgement and tests for confounds. Taking part does not require adopting the Hexis architecture.
A useful first message is a non-confidential description of one workflow or validation question, the route that interests you, and a technical or domain contact. Arrangements for contributing data would be agreed before collection, under the programme’s approved protocols.
Write to Simon Herko: simon.herko@trigrammatic.com
Read further
Companion guide
The full reading copy: the worked example, what the architecture adds, the claims and their refuters, and the route to release.
Download the companion (PDF) 108 KB
Research paper
Herko, S. and Pidcock, C. (2026). Hexis: a fixed 64-state structure for governing cognitive disposition in language-model agents — definitions, structural results and a pre-registered programme.