# Agent system-prompt best practices (2026)

Synthesised from current (2026) prompt-engineering guidance. These are the rules
the **agent-creator** meta-prompt enforces when it drafts a new agent, and the
checklist to apply when writing any agent here.

## Core philosophy
- **It's information architecture, not magic words.** Better context beats
  longer prompts. Engineer *what the model knows and when*, not verbosity.
- **A system prompt is operational policy, not a script.** Define the agent's
  identity, job, scope boundaries, success criteria, and stop/escalate rules —
  not rigid step-by-step instructions (the model runs many turns/tools).

## Structure (in this order)
1. **Identity & job** — what the agent *is*, its single job, and explicitly
   *where the job begins and ends*.
2. **Operating rules** — tone, language, output format constraints, length.
3. **Tools** — which MCP tools it has, **when and why** to use each, and when
   *not* to. Models under-use tools unless told explicitly.
4. **Procedure** — the ordered steps/sections to produce (use a numbered list).
5. **Boundaries & failure** — permission limits, what to never do, and graceful
   degradation ("if a tool fails, write '(nicht verfügbar)' and continue —
   never invent values").
6. **Closing actions** — side effects (notify, log) stated explicitly.

## Mechanics
- **Explicit delimiters.** Separate *rules* from *content the model processes*
  with stable markers (XML-ish tags or clear headers). Most failures come from
  the model conflating instructions with data.
- **No assumed state.** Dynamic facts (date/time, sensor values) must come from
  a tool call, never the model's prior. State this in the prompt.
- **Determinism aids.** Give concrete IDs/keys; constrain output shape; show one
  short example only when tone/format matters.
- **Escalation/stop conditions.** Say when to stop, when to ask, when to bail.
- **Idempotency/self-logging** where relevant (e.g. an agent that logs its own
  output so a later run can find it).

## Anti-patterns
- Walls of prose; mixing rules and data; "be helpful/creative" with no scope;
  assuming the current date; vague tool guidance; no failure path.

The Morning Briefing agent (`morning-briefing-agent.md`) is the worked example
that follows all of the above.