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Interview question · Prompt Injection & Jailbreaks

What is a canary token in an LLM application, and what can it detect?

Interpret a canary hit using placement, known copies, audience and delivery evidence. Separate a local marker check from an active callback and from complete leak prevention.

mediumNewUpdated Nov 2026

Interpret a canary hit using placement, known copies, audience and delivery evidence. Separate a local marker check from an active callback and from complete leak prevention.

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What would you say before reading the answer?

  1. 01 State the assumptions your answer depends on.
  2. 02 Explain the mechanism and choose a control.
  3. 03 Name a condition that would change your decision.

The worked answer includes an approach, scoring criteria and follow-up questions. Compare the reasoning with your own.

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What if · the follow-ups in this answer

The answer works through 4 follow-ups like an interviewer would

A fictional legal-summary assessment uses inert markers in synthetic restricted documents. Reviewers must distinguish expected processing, requester echoes, blocked candidates and unauthorized delivery.

  1. The marker appears in a permitted reviewer export. Is that automatically a breach?
  2. A requester posts a previously public marker and receives it back. What does that establish?
  3. A scanner blocks every matching reply, but a private fact leaves without the marker. Did the scanner fail its exact-match rule?
  4. What should the canary detection plan specify?

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UP NEXT ON YOUR JOURNEY
Next in this trackHow do prompt injections hidden in images, audio and documents work, and how would you test for them?Next in this trackA safety bypass works in one language but not in English. What does that tell you, and what do you do?Next in this trackA colleague says the new input filter catches jailbreaks, so the agent is safe to launch. How do you push back?
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