write-assistant

name: write-assistant description: Use when creating, updating, refactoring, or reviewing an AI assistant or sub-agent system prompt - persona, role, capabilities, tools, output format, examples, and constraints. Covers Claude Code agents, OpenAI Codex / Responses-API agents, Pi assistants, and OpenCode agents. Use even if the user only says "agent prompt", "assistant", "subagent", "persona", or names the artefact by file path.

Write Assistant

Author and maintain agent system prompts: the always-loaded persona, capabilities, and constraints that define a sub-agent. One artefact, two flows: create from requirements or update in place.

Decide first

• Create vs update vs split. Split an agent when one prompt mixes different tools, guardrails, models, or output styles that diverge in practice.
• Examples vs no examples. Add 1-2 examples for subjective style or judgment work; omit for procedural agents.
• Triggers vs direct invocation. Sub-agents selected by routing need a trigger-rich description; user-invoked agents only need a name.

Required structure

---
name: <agent-name>
description: <one-sentence trigger summary>
---

# <Name> - <Role>

## Role & Approach

<2-4 sentences: persona, tone, core focus>

## Expertise

<bullet list of specific capabilities>

## Tool Usage

<only if tool guidance is non-default>

## Examples

<1-2 demonstrations for subjective or judgment tasks; omit for procedural agents>

## Output Format

<templates, JSON schema, or shape contract>

## Constraints

<what NOT to do>

The seven-element template (role, mission, capabilities, process, constraints, output format, examples) is the consensus across Anthropic, OpenAI, and Google guidance. See references/structure.md for filled examples per platform.

Voice

• Second person, imperative. "You are…", "Use X when Y."
• No first person ("I will…" reduces adherence).
• No "you should" - use the bare imperative.
• No hedging or filler. Skip "IMPORTANT" / "YOU MUST" caps; Opus 4.5+ and Sonnet 4.6 over-trigger on aggressive language.
• One default per decision. Mention alternatives only if behaviour diverges.

See references/voice.md for imperative-vs-descriptive rewrites.

Token budgets

• 400-700 words / 500-3,000 chars / ≤200 lines.
• Up to 1,200 words / ~10,000 chars hard cap for prompts with examples.
• Keep OpenCode direct startup near the measured floor: about 15K tokens with MCPs disabled.
• Keep /ready close to direct startup and do not eager-load delegate-task; delegated sessions around 17K-18K are acceptable today.
• Under-10K total context remains useful compliance evidence, not a current OpenCode startup target; reduce delegate-task and always-listed skill or agent metadata first.
• Terse instructions outperform verbose ones (~5x fewer tokens, ~8% better compliance).

Identifier rules

• 3-50 characters. Lowercase a-z, 0-9, hyphens.
• 2-4 words. No underscores. No reserved words (anthropic, claude).
• Avoid generic terms (helper, assistant, agent, bot).
• Match the name: field, the directory name, and the file basename exactly.

Description and triggers

• One sentence. Front-load the use case.
• For sub-agents on Claude Code, add 2-4 <example> blocks (Context / user / assistant / <commentary>) inside the description. See references/triggers.md.
• For portable agents that target Codex, OpenCode, or Pi as well, keep the description plain prose; extra <example> blocks degrade gracefully on those platforms but add noise.

Description craft mirrors write-skill's doctrine for SKILL.md descriptions; the difference is voice. Agent descriptions read in the third person from the routing layer's perspective but are triggered by user intent, exactly like skills.

Tool guidance

• List tools the agent owns and tools it must not touch (least privilege).
• Tool documentation deserves as much engineering as the prompt itself.
• Omit the section entirely if the agent uses the default tool set.

Examples policy

Required when:

• Output style is subjective (voice, tone, structure).
• Judgement decides what to include or exclude.
• The format is complex enough that prose description loses fidelity.

Optional when:

• The task is procedural with a single correct shape.
• Output is short, structured, or fully constrained by a schema.

Keep examples compact. Use <example_input> / <example_output> tags.

Update flow

1. Read the agent prompt and any companion command prompts.
2. Identify original intent before changing it.
3. Diagnose: voice, structure, redundancy, missing examples, contradictions.
4. Flag contradictions explicitly before editing (e.g. "be concise" alongside "be thorough"); pick a default in the rewrite.
5. Apply surgical edits. Preserve output format templates, few-shot examples, decision criteria, explicit constraints, tool-specific guidance, and numeric limits.
6. Record the change as a changelog.

Output

When invoked to create, produce the complete agent prompt at the requested path.

When invoked to update, produce the edited prompt plus this changelog:

| Removed     | Rationale           |
| ----------- | ------------------- |
| `<section>` | `<why ineffective>` |

| Preserved   | Rationale          |
| ----------- | ------------------ |
| `<element>` | `<why it matters>` |

| Added       | Rationale        |
| ----------- | ---------------- |
| `<element>` | `<gap it fills>` |

Append a word count and a flag for any contradictions surfaced before editing.

If invoked as a sub-agent for routing reasons, follow the response contract from delegate-task.

Anti-patterns

• Pre/post checklists or self-review instructions.
• Persona descriptions beyond 2-3 sentences.
• Vague terms with no criteria ("appropriate", "when needed", "if relevant").
• Repeated constraints across sections.
• Contradictions left unresolved.
• "Always use tools" / "never use tools" without a hierarchy.
• Output format described in prose rather than shown.
• Generic LLM behaviours ("be proactive", "ask clarifying questions").
• AGENTS.md content (project rules, build commands) inside the agent prompt - that belongs in AGENTS.md; see write-agents-md.
• Frontmatter fields beyond name and description on portable agents; see write-skill/references/portability.md for the field matrix.

References

• references/structure.md - filled examples of the seven-element template per platform (Claude Code, Codex, Pi/OpenCode).
• references/voice.md - imperative-vs-descriptive rewrites; do/don't pairs.
• references/triggers.md - sub-agent description craft and <example> block format.

Related skills: write-skill (for SKILL.md files; shares description-craft doctrine), write-agents-md (for project instruction files).