The Tool Desk
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For repeatable steps, replace “remember to do this” prompt rules with hooks that run at a defined event. Keep prompts, project instructions, and human review for decisions that require context. In Claude Code, command, HTTP, and MCP hooks execute deterministically when configured for a matching event; that makes the handler’s execution more predictable, not its logic or the resulting code automatically correct.
What hooks change—and what they do not
A prompt rule asks an agent to follow an instruction as it reasons through a task. A lifecycle hook attaches a configured action to an event, such as a tool call, file edit, session start, or completion. Anthropic describes Claude Code hooks as user-defined commands, HTTP endpoints, or LLM prompts that fire at specific points in Claude Code’s lifecycle. Anthropic’s hook configuration guide describes eight hook events spanning the session lifecycle; available events and behavior are product-specific and can change.
The practical difference is the trigger: a written rule depends on the agent applying guidance, while a hook is invoked by a configured event. That does not make hooks universally safer or make them proof of correctness. A handler may be misconfigured, implement the wrong policy, fail to detect a problem, or produce an output that is not shown to the agent. Anthropic’s Claude Code guidance on steering distinguishes deterministic command, HTTP, and MCP hooks from prompt and agent hooks, which use Claude’s judgment.
Choose hooks for observable, repeatable work
Hooks are a strong fit when you can name the trigger, the action, and an outcome you can check. Anthropic documents patterns such as these for Claude Code:
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- Before a tool runs: inspect a proposed command or file path and block a known dangerous command or invalid write path, using the event and handler behavior the product supports.
- After an edit or tool action: run a formatter or record an audit entry. Formatting establishes that a formatting step ran; it does not establish that the change is functionally correct.
- At session start or resume: load useful context such as the current branch or project status. This provides context; it does not validate later edits.
- At completion: run an explicit check or report a failed check. A stop-time check can flag an omitted procedure, but a passing result alone does not establish that code is ready to deploy.
These are Claude Code examples, not a promise that every coding agent offers the same lifecycle events, matchers, return behavior, or ability to block an action. Check the documentation for the specific product and version you use before designing enforcement around an event.
Keep judgment in instructions and review
Use project instructions, prompts, skills, and review for work that has no single mechanical answer: deciding between architectural trade-offs, interpreting an ambiguous requirement, or assessing whether a change fits the project’s conventions. Those tasks depend on context rather than a simple trigger and pass/fail condition.
Claude Code’s hook system also includes prompt and agent hook types, but these rely on model judgment; they should not be treated as deterministic rule checks. A sensible division is to automate the mechanical step and reserve interpretation for the agent or a person. For example, a hook can run a configured test command at completion, while instructions and review determine whether the tests cover the requirement and whether the change is appropriate.
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Running a handler and informing the agent are separate concerns. Anthropic notes that hook output may not automatically enter the main conversation unless the configuration returns or saves it there. Decide whether a hook should merely enforce a check, report its result to the agent, or do both; then verify that the chosen output path works in your setup.
- Make the trigger narrow enough to match the intended action rather than every vaguely related tool call.
- Define what success and failure look like, and make failures visible to the person responsible for the change.
- For blocking behavior, confirm the product’s documented event and handler semantics instead of assuming a hook can veto any action.
- Review automatic actions for unintended side effects; an overbroad formatter, blocker, or script can disrupt legitimate work.
How to migrate a prompt rule
- Identify the repeated instruction. Look for a step that is procedural and often forgotten, rather than a judgment call.
- Specify a trigger and check. State exactly when the action should run and what observable result counts as success or failure.
- Verify product support. In Claude Code, consult the hook configuration guide for event and handler behavior; do not assume another agent implements the same lifecycle.
- Configure and observe the result. Determine whether the handler runs, whether it can block or modify the workflow, and whether its output reaches the agent or is saved elsewhere.
- Keep the remaining instruction. Explain the purpose and any contextual judgment in project guidance, and review the outcome rather than treating hook execution as a correctness certificate.
For example, “remember to format changed files” can become a configured post-edit formatting action. “Choose a design that fits the existing architecture” remains guidance for reasoning and review. Claims that a hook can enforce file dependencies or review related files should be treated as workflow proposals unless the chosen product documents the exact implementation and guarantee.
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