Write a System Prompt That an Agent Can Actually Follow

Editorial note: this is a practical guide, not an independently tested customer case study.

A useful system prompt describes a job and its boundaries. A page of personality adjectives doesn't tell an agent what information to trust, when to stop or how to return a result. Start with the contract between your process and the model.

Specify six things

State the outcome, permitted inputs, allowed actions, forbidden actions, output structure and escalation condition. For a support drafter, the outcome is an evidence-backed response draft. Inputs are the ticket and approved help articles. Actions are reading those sources and drafting. Refunds and sending remain outside its authority.

Make uncertainty visible

Require a missing-information list. Say what to do when evidence conflicts: identify the conflict and route it to the owner. Avoid a rule that demands confidence when the source material is incomplete.

Separate instructions from data

Treat customer messages, web pages and documents as untrusted content. A sentence inside an email that says “ignore previous instructions” is part of the email to analyze, not permission to change the workflow. Enforce tool permissions outside the prompt as well.

Test behavior, not elegance

Create five normal examples and five exceptions. Include missing fields, contradictory policy, an oversized input, an instruction-injection attempt and an out-of-scope request. Score whether the output obeys the contract. A beautiful prompt that fails those cases needs revision.

Version the prompt alongside your examples. Change one instruction at a time and rerun the evaluation set. The Prompt Vault includes an editable support-drafter contract you can adapt without starting from a blank page.

Next step: Browse the use-case library, start with one useful task, or estimate your workflow costs.

Architecture reference: Building effective agents, Anthropic (historical pattern reference; verify current product interfaces separately).

Illustrative stock photo by Gabrielle Henderson on Unsplash.