# Tenant guardrail policy (TEN-WP-0006)

Consumer contract for tenant guardrails: the **ceilings** a tenant is subject
to. Primary consumer: `flex-auth`, which joins a tenant's limits with observed
consumption to reach a decision.

`tenant-engine` owns *policy* — what a tenant is permitted to spend and how
many entities or actions it may hold. It does **not**:

- **meter.** Consumption counters are produced by whoever meters the resource.
  This service never observes spend and never counts actions.
- **bill.** Money movement belongs to no repo yet identified; plan *terms*
  belong to `adaptive-pricing`.
- **decide.** `flex-auth` is the PDP. A guardrail read is an input to a
  decision, never the decision itself. Never invert this.

**A guardrail is a safety ceiling, not an entitlement.** It answers "how far
may this tenant go before something must stop it", not "what has this tenant
bought". Entitlement is `adaptive-pricing`'s question.

---

## Limit kinds

| Kind | Meaning | Unit |
|---|---|---|
| `spend` | money the tenant may commit over a period | integer minor units + ISO-4217 currency |
| `entity_count` | how many of a thing the tenant may hold at once | integer, no period |
| `action_count` | how many times the tenant may do a thing per period | integer + period |

Amounts are **integer minor units** (cents), never floats — a budget compared
with a float is a budget that rounds the wrong way at the boundary.

**Rate limiting is out of scope.** A per-second request ceiling is a gateway
concern with entirely different latency and storage characteristics; it does
not fall out of this model for free. `action_count` is a business-period
quota (per month, per day), not a traffic shaper.

### Limit keys and the registry

A limit is addressed by a dotted **limit key**. Keys are declared in an
explicit **registry**; a key that is not registered does not resolve.

| Prefix | Kind | Example |
|---|---|---|
| `spend.` | `spend` | `spend.monthly` |
| `entity.` | `entity_count` | `entity.workspace` |
| `action.` | `action_count` | `action.export.monthly` |

`spend.monthly` is the only key seeded by this workplan. Entity and action
keys are registered as consuming services arrive, each naming its owner in
the registry entry — `tenant-engine` stores the ceiling and never learns what
a `workspace` is.

Reading an **unregistered** key is an error (`unknown_limit_key`), not a zero
and not an unlimited. Both of those would be lies: zero would break every
caller whose key is merely misspelled, and unlimited would fail open.

The registry is validated at startup: every registered key must supply a
default for **every** grouping in `GROUPINGS`. A registry that does not is a
startup failure, so an unmapped grouping cannot reach production.

---

## Resolution

Every tenant resolves to exactly **one** effective value per registered limit
key. Resolution is a total, side-effect-free function of
`(grouping, plan_id, overrides, lifecycle)`.

Precedence is evaluated **per key**, highest first — not per set. A plan that
supplies `spend.monthly` does not thereby wipe out a grouping-derived
`entity.workspace`.

| # | Layer | Provenance | Source |
|---|---|---|---|
| 1 | per-tenant override | `override` | explicit, audited, authorized write |
| 2 | plan-derived | `plan` | derived from the assigned `adaptive-pricing` plan id |
| 3 | grouping default | `grouping` | the table below (ADR-0013) |
| 4 | fail-closed floor | `fail_closed` | the registry's declared floor |

Layer 4 exists so resolution is total. It is reached only when the registry
is internally inconsistent — a grouping added to `GROUPINGS` with no default
declared. It resolves to the most restrictive value the key admits, and its
provenance is `fail_closed` so the condition is visible in a read rather than
mistaken for policy.

**There is no "unset means unlimited".** Absence at every layer yields the
floor, never an open ceiling.

### Unlimited

`unlimited` exists as an **explicit sentinel**, subject to four rules:

1. it is never a default;
2. it is never the result of absence, silence, or a parse failure;
3. it can only arrive by explicit declaration — an override, a plan-derived
   limit, or the reserved profile below;
4. setting it is audited like any other limit change.

The prohibition is on inferring an open ceiling, not on choosing one.

### Lifecycle clamp

The clamp is applied **after** precedence resolution and may only **reduce**:

| Lifecycle | Effect |
|---|---|
| `active` | resolved value stands |
| `retired` | every limit clamps to the floor; provenance becomes `lifecycle` |

This follows the TEN-WP-0005 precedent — operations that only reduce privilege
stay available while retired, loosening ones do not. A retired tenant's
guardrails remain **readable**; it is the values that clamp, not the endpoint.

---

## Grouping defaults

Monthly spend budget per grouping (ADR-0013), in minor units of the
deployment's canonical currency (`GUARDRAIL_CURRENCY`, default `EUR`).

| Grouping | `spend.monthly` | Rationale |
|---|---|---|
| `trial` | **0** | ADR-0013 mandate — a trial tenant commits nothing |
| `friendly` | 0 | comped relationship; spend is the platform's, not the tenant's |
| `consumer` | 2 000 (€20) | single natural person |
| `single` | 5 000 (€50) | one-person business |
| `family` | 5 000 (€50) | household, consumer-shaped |
| `community` | 5 000 (€50) | volunteer-run, low commercial exposure |
| `agentic` | 10 000 (€100) | autonomous spender — deliberately tight |
| `small` | 25 000 (€250) | |
| `association` | 25 000 (€250) | small-business-shaped, member-funded |
| `medium` | 100 000 (€1 000) | |
| `large` | 500 000 (€5 000) | |
| `enterprise` | 2 000 000 (€20 000) | |

> **Assumption flagged for product sign-off.** Only the `trial` = 0 row is
> canon (ADR-0013). The rest are conservative opening ceilings chosen so that
> no grouping starts unbounded. They are guardrails, not prices — raising one
> is an override or a plan-derived limit, both audited. Adjusting the table is
> a config change, not a code change.

`agentic` sits below `small` on purpose: an autonomous agent can exhaust a
budget far faster than a human operator can notice, so its default ceiling is
set for the blast radius, not the buying power.

### Grouping can change (TEN-WP-0010)

A tenant's grouping is **mutable**, through
`POST /tenants/{id}/grouping` (action `tenant.grouping.set`). Reclassifying a
tenant moves every grouping-derived ceiling with it — a `trial` tenant
reclassified to `medium` goes from a zero spend budget to €1 000.

Two consequences for anyone reading guardrails:

- **A ceiling can change with no guardrail write against it.** The audit record
  is the `tenant_grouping_changed` event on the tenant, not an entry in the
  guardrail change log. That is deliberate: a guardrail trail containing
  entries no guardrail write produced would be worse than one that is complete
  but needs two event types read together.
- **Precedence is unchanged.** A per-tenant override still wins over the new
  grouping default. Reclassification moves the grouping layer only.

Grouping is read from the tenant record, never parsed from the identifier's
middle segment — that segment is historical, and the two are expected to
diverge after a reclassification.

### Currency

Spend limits carry their currency explicitly. Because precedence resolves
per key and the highest layer wins outright, two currencies never combine
within one resolution — so `tenant-engine` never converts currencies, and has
no exchange rate anywhere in it. A plan-derived limit in a currency other
than the resolved read's is simply the value that won.

---

## Reserved identifiers

`tenant:platform` and `tenant:coulomb` are ungrouped (`grouping is None`), so
layer 3 cannot apply to them. They do not fall through to the floor either —
falling through would clamp the platform's own identity to zero and take the
platform down with it.

They resolve to an explicit **`reserved` profile**, provenance `reserved`,
slotted at layer 3 in place of the grouping default:

| Key kind | Reserved value | Why |
|---|---|---|
| `spend.*` | **0** | infrastructure identities are not billable spenders; platform cost is not metered per tenant |
| `entity.*`, `action.*` | `unlimited` (explicit) | the platform tenant operates the platform |

This is the sentinel's rule 3 — an explicit declaration, not an inference.

---

## Consumption

**Consumption counters do not live in this repo.** Decided under T01.

`tenant-engine` is a low-write policy authority with strong audit and
compare-and-swap semantics. Consumption is high-frequency telemetry with
opposite durability, latency, and retention needs. Storing it here would
drag metering into a service whose `SCOPE.md` explicitly disowns it, and the
audit trail that makes a limit *change* reconstructible is pure overhead on a
counter that ticks continuously.

So the guardrail read returns **limits and provenance only**. A PDP joins
them with consumption obtained from the meter.

> **Open boundary question.** No repo currently owns metering — the same gap
> `SCOPE.md` records for payment processing. Until one is named in
> `net-kingdom` canon, `flex-auth` can enforce presence-and-ceiling semantics
> (is there a limit? is it zero?) but not consumption-relative ones (has the
> tenant used it up?). A zero budget is fully enforceable today without any
> meter, which is what makes the ADR-0013 `trial` default land immediately.

---

## Audit

A limit change is a privilege change, and is audited exactly like a
`RoleGrant`: append-only, carrying actor, reason, and correlation id. A
guardrail's history must be as reconstructible as a role's — "who raised this
tenant's ceiling, when, and why" is an answerable question.

---

## What a read returns

Effective limits **with provenance** — which layer each value came from —
so a consumer can tell a deliberate enterprise ceiling from a floor reached
by accident.

```json
{
  "tenant_id": "t-1",
  "identifier": "tenant:trial:binky",
  "lifecycle": "active",
  "limits": {
    "spend.monthly": {
      "kind": "spend",
      "value": 0,
      "currency": "EUR",
      "period": "P1M",
      "provenance": "grouping"
    }
  }
}
```

---

## Endpoints

### `GET /tenants/{tenant_id}/guardrails?actor=<actor>`

Every registered key, resolved, with provenance — the shape above. `actor` is
required: the read is authorized, so there is no anonymous caller.

### `PUT /tenants/{tenant_id}/guardrails/{limit_key}`

```http
PUT /tenants/t-1/guardrails/spend.monthly
Idempotency-Key: 4f1c…
If-Match: "1"

{"limit": {"kind": "spend", "amount": "9000", "currency": "EUR", "period": "P1M"},
 "actor": "ops", "reason": "raised for pilot", "correlation_id": "corr-1"}
```

`amount` is a **string** so the `unlimited` sentinel and an integer share one
field, and so a spend amount in minor units never round-trips through a float.

### `DELETE /tenants/{tenant_id}/guardrails/{limit_key}`

Same headers; body is `{"actor", "reason", "correlation_id"}`. Clears the
override so resolution falls back to the plan, then the grouping default.

Both mutations return the new effective limit plus `ETag`, `change_id`, and
`Idempotent-Replay: true|false`:

```json
{"tenant_id": "t-1", "limit_key": "spend.monthly", "version": 2,
 "change_id": "9f2c…", "cleared": false,
 "effective": {"kind": "spend", "amount": 9000, "currency": "EUR",
               "period": "P1M", "provenance": "override"}}
```

A guardrail change **bumps the tenant's version**, so it invalidates the
record ETag exactly as a metadata edit does. There is no separately versioned
guardrail resource to race against the tenant record.

`change_id` is derived from `(tenant_id, limit_key, Idempotency-Key)`, not
random, so a genuine retry replays the original audit record rather than
minting a second one for a mutation that happened once.

---

## Authorization

Two **distinct** actions, so policy can give `flex-auth` the read without
giving anything the write:

| Action | Resource type |
|---|---|
| `tenant.guardrail.read` | `guardrail` |
| `tenant.guardrail.set` | `guardrail` |

These extend package `tenant-engine.write-api.mutate` alongside the existing
seven actions. **Both are new and require a policy-package change in
`flex-auth` before this surface functions in production** — until then every
check correctly resolves to deny.

Authorization runs **before** the store is touched on every guardrail route,
so an unauthorized caller cannot use status codes or timing to probe which
tenants exist or which limit keys are registered. A denied read of a real
tenant and a denied read of a nonexistent one are byte-identical.

---

## Errors

| Status | `error_code` | Cause |
|---|---|---|
| 400 | `idempotency_key_required` | `Idempotency-Key` header missing |
| 400 | `invalid_if_match` | `If-Match` is `*` or not a version ETag |
| 400 | `invalid_limit` | malformed limit — negative amount, spend without currency, wrong shape |
| 403 | `write_denied` | flex-auth denied the action |
| 404 | `tenant_not_found` | unknown tenant |
| 404 | `unknown_limit_key` | key is not in the registry |
| 409 | `version_conflict` | stale `If-Match` — re-read and retry |
| 409 | `idempotency_key_conflict` | key reused for a different request |
| 409 | `guardrail_loosening_denied` | change would raise a retired tenant's ceiling |
| 422 | *(schema)* | unknown body field, or empty `reason`/`correlation_id` |
| 428 | `if_match_required` | `If-Match` header missing |
| 503 | `tenant_authority_unavailable` | store or authority unavailable |
| — | `guardrail_registry_invalid` | startup validation failed — a grouping has no default |

`guardrail_registry_invalid` is a **startup** failure, not a response: the
service refuses to come up rather than serving an incomplete registry.

Errors are redacted: never a policy internal, a store path, or a registry
dump in `detail`.

---

## What is not wired yet

The **plan-derived layer has no feed.** Precedence layer 2 exists, resolves,
and is tested, but nothing populates it: `adaptive-pricing` owns plan terms
and does not yet expose plan-derived ceilings. Until it does, a tenant's
limits come from its override or its grouping default. Inventing a plan→limit
mapping here would duplicate terms this repo does not own.