> [!IMPORTANT]
> **Breaking change to API to fix NPC AI issues**
## Symptom
`FamiliarAITests.HiddenCaster_FamiliarRefusesRetaliation` fails on main at `Assert.False(wolf.Warmode)`: the familiar's `Combatant` stays null as intended, but it is left in war stance. In-game the familiar of a hidden caster flips into warmode for a tick when struck, until `FamiliarAI.StandDown()` clears it on the next think.
CI never saw it because the familiar tests are `SkippableFact` (they need client tiledata).
## Root cause
#2644 and #2645 landed a day apart and are individually correct but conflict at one seam.
Since #2645, a familiar (`Commandable == false`) is no longer "told to stand down", so `BaseCreature.AggressiveAction` continues past `AIObject.OnAggressiveAction(aggressor)` to its generic fallback:
```cs
else if (Combatant == null && !BardPacified)
{
Warmode = true;
Combatant = aggressor;
}
```
`FamiliarAI.OnAggressiveAction` had already declined (caster hidden), but the hook was `void`, so the creature could not tell. The fallback raised `Warmode` before `BaseFamiliar.Combatant` vetoed the target.
## Fix
This is the follow-up agreed in #2645: the retaliation policy moves into a virtual `BaseAI.OnAggressiveAction(aggressor) -> bool`.
- **`BaseAI.OnAggressiveAction`** now holds the stand-down verdict, the closer-aggressor swap (`PreferCloserAggressor`), and the `IssueOrder(Attack)` vs `Combatant =` decision. Returns `false` when the creature stands down.
- **`BaseCreature.AggressiveAction`** keeps only bookkeeping — `ControlMaster.Aggressors` entry, `StopFlee`, `ForceReacquire`, ethics `FinishShield` — keyed off the return, so an AI that declines is never second-guessed.
- **`FamiliarAI`** overrides the whole policy: no orders to stand down on, never fights for a hidden caster or a hidden aggressor.
Behaviour for every other AI is unchanged. Moving the fallback ahead of `StopFlee`/`ForceReacquire` is order-safe: both are pure field resets, `IssueAttack` does not `Think()`, and `OnCombatantChange` only derives `Warmode`.
Two commits: the first is the minimal `bool` hook that alone fixes the test, the second the policy move.
## Not changed (noted for later)
- `Warmode = true; Combatant = x;` (also in `FamiliarAI.Act`, `IssueAttack`) is redundant when the setter accepts — `BaseCreature.OnCombatantChange` derives `Warmode` — and is the dangling-warmode hazard whenever a `Combatant` setter vetoes.
- Since #2614 a stood-down pet skips the ethics `FinishShield`; RunUO ran it unconditionally. Preserved as-is.
## Tests
`HiddenCaster_FamiliarRefusesRetaliation` fails on main and passes here. Full `UOContent.Tests`: 1126 passed, 0 failed, 2 skipped (spawner fixture capture + benchmark).
## Problem
`BaseFamiliar.OnThink` drove its own movement (`WalkMobileRange` toward the master) while the familiar was also a controlled pet running `Obey()`. `Summon → SetControlMaster` issues `Come`; `DoOrderCome` converts it to `Stay` within two tiles and anchors `Home`; from then on `OnThink` walked toward the caster while `DoOrderStay` greedy-stepped back toward the stale post — the backtracking. Combat never approached anything: main only copied `Combatant` while already adjacent to the caster. `CurrentSpeed = 0.01` was a 10 ms think / 50 ms step sprint hack, and `RangeCheck` teleported the familiar to a spot eight tiles *from* the caster.
## Change
A dedicated `FamiliarAI : BaseAI` (registered through `ForcedAI`, like `CloneAI`) owns every familiar decision, for both the controlled (`Obey`) and uncontrolled (`Think`) dispatch:
1. **Lifecycle** — caster gone → drop pack, delete. Caster on another map → stand down and wait for `TeleportPets`.
2. **Herding** (Dark Tides) — stands down, then `CheckHerding()`. Outranks combat.
3. **Assist** — combat-capable familiars (dark wolf, vampire bat, horde minion) engage the caster's target; otherwise anything in a fight with the caster's side — attacked the caster or the familiar, or attacked by the caster (a pet's attack is credited to the caster) — that is still fighting the caster, the familiar, or one of the caster's pets. Leashed to `RangePerception` of the caster; dropped when the caster hides. Shadow wisp and death adder never fight (`AssistsMaster => false`, enforced at the `Combatant` setter so no path can hand them a target).
4. **Follow** — `MoveTo(master, 1)` through the centralized `ApproachTarget` (greedy step / persistent `PathFollower` / stall detection).
5. **Keep-up** — snap to a validated tile beside the caster (on the caster's floor) when outpaced on open ground beyond 10 tiles, or when `ApproachTarget` gave up; never while a detour is working.
**Command immunity** is expressed inside the order machinery rather than around it: `FamiliarAI.IssueOrder` does nothing and rests the order on `Come`, so `TeleportPets` keeps working and no system-issued `Attack` (retaliation on ML's stand-down rule) can strand the familiar. `StandsDownOnCommand => false` so the ML rule never mutes it.
**Visibility** mirrors the caster from the familiar's own state (a step reveals a hidden NPC in `Mobile.OnMove`; the old cache compared the caster's previous state), `RevealingAction` is suppressed while the caster is hidden, and becoming hidden drops Warmode so no swing gives the caster away.
**Speed** is a flat 0.1 (`ReduceSpeedWithDamage => false`).
### Engine-side (all `Projects/UOContent`)
- `ApproachTarget` records which exit it took in `BaseAI.LastApproach` (`ApproachOutcome`: Arrived / Waiting / DirectProgress / Routing / Blocked / GaveUp / InvalidGoal). Callers' booleans are unchanged; keep-up reads this instead of running a second scheduler. `MoveTo`'s arrival return now also clears the move intent, as `ApproachTarget`'s own arrival does.
- `MoveToPoint(goal, range = 1)`; `CheckHerding` passes 0. **Fixes a main regression from #2591:** herding stopped one tile short, never cleared `TargetLocation`, and left the creature pinned to the herding pace — affects the shepherd's crook and the Dark Tides scroll fetch for every herded creature, not just familiars.
- `ChangeAIType` reads `ForcedAI` once. It read it twice, and each `BaseAI` ctor activates its timer for a non-sector-gated creature, so a `ForcedAI` creature with `PlayerRangeSensitive => false` got an orphan AI ticking it.
## Tests
`FamiliarAITests` are timer-wheel driven (the real `AITimer` thinks and moves; `PetPacingTests` style) against live Trammel statics, gated on client map data: follow without backtracking, the five-way assist theory, leash, retaliation, aggressor fallback (caster's own `Combatant` expired; caster's pet in the fight), target dropped when it stops fighting, keep-up on open ground / not while routing / after give-up, hidden mirror across steps, herding priority with a visible fighting caster, stand-down when left behind, no stale move intent. `ApproachOutcomeTests`, `HerdingTests` (fails on main), `ForcedAITests` (fails on main) cover the engine-side pieces.
Against `origin/main` with the familiar tests dropped in: 16/16 fail, including the reported backtracking. On this branch: `UOContent.Tests` 1082 passed / 2 skipped, `Server.Tests` 891/891, solution builds with 0 warnings.
Kamron Batman