The problem
A large regulated enterprise set out to run agentic AI at portfolio scale: dozens of use cases across credit, financial crime, servicing and operations, not a handful of pilots. Each new team was about to build its own agents, its own prompts, its own retrieval and its own controls. That path is familiar and it fails predictably. Cost multiplies, quality varies by team, and nobody can answer the question a regulator eventually asks: how many agents do you run, what may each one touch, and who certified them?
I designed the architecture for a reusable agent estate, and teams I worked with took it through build and into live use. The premise was simple: agents should be assembled from certified parts, not written from scratch every time.
Why it was hard
Agents are not services. A service does the same thing tomorrow that it did today. An agent's behaviour depends on a model, a prompt, retrieved context and accumulated memory, and any of those can drift without a code change. The controls an enterprise already trusts, change management, access control and code review, all assume determinism, so none of them catch the failure modes that matter here.
Reuse made this harder, not easier. A summarisation skill written for credit gets picked up by financial crime, and now a defect travels. Shared memory is the same story: it is the most valuable thing to share and the most dangerous, because a poisoned write in one flow can surface as a wrong decision in another weeks later.
Underneath everything sat model risk requirements. Financial regulators expect an enterprise to know what a system does, prove it keeps doing it, and show that a human stays accountable for consequential outcomes. An estate that could not produce that evidence per agent would not survive assurance.
Key decisions
The estate is layered by how general the work is. Foundation agents carry common skills: extraction, entity resolution, summarisation, validation and audit. Domain agents carry enterprise logic: eligibility, obligations, restricted parties, conduct. Process agents run end-to-end flows such as decision memo preparation or periodic customer review. Reuse is highest at the foundation layer and falls as you move up, which is exactly the shape you want.
Every agent ships with a versioned contract: inputs, outputs, tool permissions, memory rights, an accuracy baseline and a cost profile. Every agent holds a signed identity in a central registry with scoped capability tokens, so what an agent may touch is a property of its credential, not its prompt.
Each skill runs on the cheapest capable model that clears a numeric acceptance bar. Small models carry the high-volume skills; reasoning models are reserved for the calls that genuinely need judgment.
Authority is one-way. A generative step can request escalation, but it can never overwrite a verdict issued by a deterministic control below it. No amount of fluent reasoning talks past a rule.
Architecture
Three agent layers sit between a shared control plane on one side and a governance plane on the other. The registry and the tiered memory feed all three layers; guardrails and evaluation apply to all three; anything consequential exits through a human oversight gate.
Agents never talk freely. Every agent-to-agent message crosses a schema-validated, injection-screened channel on an allow-listed topology, and the topology itself is a governed artefact. Adding an edge between two agents is an architecture decision with a review, not a configuration change. The design goal is a computable blast radius: when one agent is compromised or simply wrong, the set of agents it can influence is a number written into the design, not a discovery made during an incident.
Trade-offs
The estate taxes the first use case to subsidise every later one. Writing a contract, an accuracy baseline and an evaluation pack for a single agent feels like overhead when a one-off bot would ship faster. The bet is that the second and tenth use cases assemble rather than build, and that bet paid off.
Small models are cheaper but less forgiving: they need calibration, confidence thresholds and an escalation path to a stronger model, all of which is machinery a frontier-model-everywhere design avoids. The strict topology slows integration, deliberately. Central controls can become a bottleneck, which is why certification is designed as a lifecycle teams move through rather than a committee they queue for.
Governance
Agents move through a certification lifecycle the way an enterprise already manages models: certified, monitored, re-certified, retired. Autonomy is metered, rather than only permitted; each step carries a budget of unsupervised actions before a mandatory checkpoint. A human stays in the loop on any decision that cannot be undone, every agent action lands in an audit trail, and a kill switch exists at the estate level, not per bot. Drift sentinels watch each agent's tool-selection distribution over time, because the earliest symptom of contaminated memory is not a wrong answer but a changed habit.
Outcome
Teams I worked with took the estate through build and assurance at a large regulated enterprise, and it carries a broad portfolio of agentic use cases built by many teams. Most of what a new use case needs now arrives prebuilt: the foundation layer is reused almost everywhere, domain agents travel across related flows, and high-volume work runs on small models at a fraction of frontier-model cost. The programme scaled from proof of concept to production because the governance evidence was a by-product of the design rather than a document written afterwards.
What I learned
Reuse is a governance property before it is an engineering one: nobody reuses an agent they cannot trust, and trust needs a certificate, not a demo. The registry turned out to be the real product; once identity, permissions and certification state lived in one place, everything else followed. And the discipline of the cheapest capable model, enforced by acceptance bars rather than taste, ended more arguments than any benchmark ever did.