# Policy Sim

## What this page is for

A standalone, deeper version of the Simulate tab in Policy Builder. Run a draft Rego rule against historical events without affecting production traffic. Output is a side-by-side "what would have flipped" report.

## Sidebar location & role gating

* **Sidebar group**: Settings → Developer.
* **Path**: `/policy-sim`.
* **Keyboard hint**: none.
* **Minimum role**: `ADMIN` or `SECURITY`. Simulate is computationally a read but is treated as a write in the platform's RBAC because the simulator runs OPA evaluation against the live policy bundle.

## What you see

* **Rule editor** — Rego text area with line numbers.
* **Agent picker** — UUID input plus the sidebar `useAgents` quick-select.
* **Time range picker** — 1 hour / 6 hours / 24 hours / 7 days.
* **Run button** — fires the simulation.
* **Result table** — decisions that would flip under the draft. Columns: tool, current decision, draft decision, finding.
* **Summary banner** — "No decision changes" if the draft is a no-op, or a per-tool count of flips.
* **Read-only banner** — "No OPA calls, no writes — read-only dry run" reinforces the safety guarantee.

## Backend calls

| Action           | HTTP | API path           | Service      |
| ---------------- | ---- | ------------------ | ------------ |
| Run a simulation | POST | `/policy/simulate` | policy → OPA |

## Auto-refresh & realtime

* **No auto-refresh.** Operator-driven.

## Per-agent scoping

Yes. The simulation always operates against one agent. Without a selection, the input is required.

## Empty states

| Condition               | Copy shown                                   | What to do                                                                           |
| ----------------------- | -------------------------------------------- | ------------------------------------------------------------------------------------ |
| Draft produces no flips | `No decision changes in the sampled events.` | Either the draft is a no-op or the window has no triggering events; widen the range. |

## Edge cases & known gotchas

* **Simulation timeout**: very wide windows (over 6 hours) can take 30+ seconds; capped at 1000 sampled events. If you need broader coverage, run multiple narrower windows.
* **403 on Run**: caller is `VIEWER` or `AUDITOR`. Re-login as `ADMIN` or `SECURITY`.
* **Rego syntax error**: surfaces in the response body with line/column. Fix and re-run.
* **Per-EC2 flap**: `/policy/simulate` is stable.

## Related docs

* [Policy service](/services/hot-path/policy.md)
* [Policy Builder UI](/ui/primary-nav/policies.md)
* [Policy Analytics UI](/ui/settings-sub-pages/policy-analytics.md)

## Screenshot


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