Claude Agent SDK Hooks, Prerequisite Gates and Human Handoffs

Prompt instructions are probabilistic: even a well-written rule fails a small percentage of the time. When a business rule must always hold — verify identity before a refund, never refund over a limit — the exam expects programmatic enforcement (Task Statements 1.4 and 1.5).

✍️ Prompt guidance vs programmatic enforcement

🚧 Prerequisite gates

A gate blocks a downstream tool until an upstream step has succeeded — for example, process_refund is rejected unless get_customer returned a verified customer ID in this session.

def guard(tool_name, tool_input, session):
    if tool_name in {"lookup_order", "process_refund"} and not session.verified_customer_id:
        return deny("Verify the customer with get_customer first.")
    if tool_name == "process_refund" and tool_input["amount"] > 500:
        return redirect("escalate_to_human", reason="Refund above $500 needs approval")
    return allow()

🪝 The two hook patterns the exam tests

• Tool-call interception (pre-tool) — inspects outgoing calls and blocks or redirects policy-violating ones (refunds over a threshold, rm -rf, writes to production paths).
• PostToolUse — transforms tool results before the model sees them, e.g. normalising Unix epochs, ISO-8601 strings and numeric status codes from different MCP servers into one format, or trimming verbose results to the relevant fields.

📘 Multi-concern requests

“My item arrived broken, I was double-charged, and my address is wrong” is three concerns. Decompose them, investigate each (in parallel where possible) using shared context, then send one combined resolution. Having several issues is not, by itself, a reason to escalate.

🤝 Structured human handoffs

Human agents usually can’t see the bot transcript. When escalating, pass a structured summary: customer ID, the issue, the root cause found, actions already taken, amounts, and the recommended next step.

🧪 Exam-Style Practice Questions

Try these before opening the answers. They use the same scenario style as the real exam.

Question 1. Identity must be verified before any refund. What's the strongest implementation?

• A. Few-shot examples
• B. A post-hoc audit log
• C. A system-prompt rule
• D. A programmatic gate that rejects `process_refund` unless `get_customer` returned a verified ID in this session

Question 2. What does a PostToolUse hook typically do in this exam's context?

• A. Blocks dangerous outgoing tool calls
• B. Compacts the conversation
• C. Transforms or normalises tool results before the model processes them
• D. Selects which tool to call

Question 3. When should you choose a hook over a prompt instruction?

• A. Whenever you want shorter prompts
• B. When a business rule needs a guaranteed outcome
• C. Never; hooks add latency
• D. Only for logging

📝 Assignment

Do these tasks in order. Each one has a Done when check, so you know when it is complete. The stretch task is optional but close to what the exam expects from an architect. Keep your answers in one notes file or Git repository, and revisit them in your final week of revision together with the cheat sheet in the last lesson of this course.

1. Task 1: Write pseudo-code for a gate that blocks process_refund until get_customer has verified the customer.
   ✅ Done when: Your gate denies the refund call when verification is missing and allows it once get_customer has returned verified.
2. Task 2: Add a threshold rule that redirects refunds over $500 to escalate_to_human.
   ✅ Done when: A refund of $501 is redirected to escalation, while a refund of $499 goes through unchanged.
3. Stretch: write a PostToolUse function that converts Unix epochs and ISO-8601 strings into one date format.
   ✅ Done when: Both 1731000000 and '2024-11-07T17:20:00Z' come out of your function in the same format.

❓ Frequently Asked Questions

🤔 When should I use a hook instead of a prompt instruction?

When a rule must hold every time, such as identity checks, refund limits or blocking destructive commands. Prompts are probabilistic.

🤔 What does a PostToolUse hook do?

It transforms a tool's result before Claude sees it, for example normalising formats or trimming irrelevant fields.

🤔 Should a multi-issue request be escalated?

Not automatically. Split it into separate concerns, investigate each, and send one combined resolution.

🤔 What should a human handoff include?

Customer ID, the issue, the root cause found, actions taken, amounts involved and the recommended next step.

🏁 Summary

• “Must always” → hooks and gates. “Should usually” → prompts.
• Pre-tool interception blocks or redirects calls; PostToolUse normalises results.
• Escalations carry a structured handoff summary.

Next, continue with Task Decomposition and Session Management in Claude Code.

Independent study material by SQA Tools. Not affiliated with, endorsed by or sponsored by Anthropic. Claude is a trademark of Anthropic. Mock questions are original practice items, not real exam content. Exam facts checked against Anthropic's official exam guides and certification FAQ on 11 November 2026.