Getting it into your agent
One page per mod, every tool's command on it. A separate URL per tool would split the same page into five that compete with each other.
npx skills add SummerEngine/summer-engine-agent --skill design-npcgit clone --depth 1 https://github.com/SummerEngine/summer-engine-agentWrote this? Show the measurements
A badge with what this costs and how it scanned, read live from this page, so it follows the numbers instead of freezing them. Markdown for a README, HTML for a documentation site or a project page.
[](https://agentmods.dev/skills/summerengine/summer-engine-agent/design-npc)<a href="https://agentmods.dev/skills/summerengine/summer-engine-agent/design-npc"><img src="https://agentmods.dev/badge/skills/summerengine/summer-engine-agent/design-npc/github.svg" alt="Measured on agentmods" height="20"></a>Or the 80×15 button, for a site that already has a row of RSS and ATOM ones. Only the verdict fits; the numbers stay here.
<a href="https://agentmods.dev/skills/summerengine/summer-engine-agent/design-npc"><img src="https://agentmods.dev/badge/skills/summerengine/summer-engine-agent/design-npc.svg" alt="Reviewed on agentmods" width="80" height="20"></a>- NVIDIA SkillSpector pass
What it costs to keep this loaded
Counted locally with the o200k_base tokenizer, which is exact for GPT models; Claude uses its own tokenizer and its counts differ. Treat this as one consistent yardstick across the catalogue rather than a bill. Prices are per million input tokens.
| Model | Per session | Once invoked |
|---|---|---|
| Fable 5.1 | $0.00101 | $0.07069 |
| Opus 5 | $0.00051 | $0.03535 |
| Sonnet 5 | $0.00020 | $0.01414 |
| Haiku 4.5 | $0.00010 | $0.00707 |
Grade A, and why
design-npc scanned grade A with 0 findings against 26 rules in 11 categories — prompt injection, anti-refusal, data exfiltration, privilege escalation, supply chain, agent snooping, system-prompt leakage, SSRF and excessive agency — measured 9d ago.
A static scan of the body, not an audit. Every finding is printed with the line that produced it so you can judge whether it matters here. A mod is markdown that instructs an agent; that is exactly why what it instructs is worth reading.
Nothing flagged
None of the 26 patterns this scan looks for appear in this file: no shell pipes, no recursive deletes, no credential paths, no hidden text, no instruction-override or anti-refusal phrasing, no agent-config snooping. That is not a guarantee, it is the absence of the things that are checkable.
Copies of this mod
1 near-identical copy found in the catalogue:
- design-npc — 89% identical, 72 lines differ
How it starts
The opening of the file, as written. The whole thing — 506 lines — stays where its author put it; the contents beside it link to each section on GitHub.
/design-npc — Design an NPC's Behavior End-to-End
Overview
A good NPC is legible and alive. The player can read its mood, predict its next move, and feel the consequence of theirs — and the four enemies in a patrol squad don't all behave like clones of one designer-authored script. This skill walks the canonical NPC AI pattern from the inside out: what the NPC perceives, what it wants (intent), what it does (action), what tells signal each state, and how it dies.
Core principle: behavior is what the player sees. Two layers separate doing from wanting: an action state machine the body runs, and an intent layer above it that reads perception + personality and tells the action SM what to aim at.
Second principle: identical NPCs are predictable, and predictable is boring. Every spawned instance jitters its personality knobs so a squad of four grunts produces emergent variety with zero designer authoring.
Steps
1. Pick the NPC archetype
Open with the question. Wait.
What's the NPC for? Pick the closest:
- Basic enemy (melee/ranged grunt, dies in 1–3 hits)
- Boss (named, scripted phases, 30–120 sec fight)
- Quest-giver (talks, gives objectives, doesn't fight)
- Friendly companion (follows player, helps in combat)
- Civilian / ambient (reacts to events, no combat AI)
- Wave-spawned mob (cheap, many at once, simple behavior)
Each archetype maps to a different complexity budget:
| Archetype | Action states | Intents | Perception | Personality range |
|---|---|---|---|---|
| Basic enemy | 4 (calm / alert / aggressive / defeated) | 5 (idle / investigate / hunt / kill / retreat) | Sight cone + LOS raycast | Wide |
| Boss | 6–10 (per phase) | 5–7 incl. RESCUE_ALLY |
Sight + hearing + scripted | Narrow, biased high |
| Quest-giver | 2 (idle / talking) | 1 (idle) | Trigger area | None — fixed |
| Companion | 5 | 4 (follow / support / engage / retreat) | Sight + group | Narrow, biased low aggression |
| Civilian | 3 (wander / panic / dead) | 2 (idle / retreat) | Hearing | Caution biased high |
| Wave mob | 2 (charge / dead) | 1 (kill) | Direct line to player | None |
What this file has done since we first saw it
Hashed on every crawl. A supply-chain change to an agent config is a question of when, not whether, so the history is kept rather than the latest state alone.
- 9d ago First seen · 506 lines · 101 tokens per session scan A 34aa846f15c5
design-npc is a skill published in the GitHub repository SummerEngine/summer-engine-agent (59 stars, last pushed yesterday), licensed MIT. It adds 101 tokens to every session and 7,069 once invoked, about $0.0005 per session on Opus 5. A static security scan graded it A with 0 findings. No closer match exists in the catalogue, so it is treated as the original; first seen 2026-08-30.
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