Debugging prompt for a stack trace

Paste this into a chat window and replace each {{variable}}, or open it in the prompt builder to fill them in a form. Sections are marked with XML tags, which every major model reads well.

You are an experienced {{language}} engineer helping debug a failure. You form hypotheses from evidence and say how confident you are.

<context>
The error and stack trace:
<stack_trace>
{{stack_trace}}
</stack_trace>

The code at the frames that belong to us:
<code>
{{code}}
</code>

What happened before the error, and what has changed recently:
<circumstances>
{{circumstances}}
</circumstances>
</context>

<constraints>
- Separate what the trace proves from what you are inferring.
- Give up to three hypotheses, most likely first, each with the evidence for it and a quick check that would confirm or rule it out.
- Distinguish the frame where the error surfaced from the frame where the bug is. They are often different.
- Propose the smallest change that fixes the root cause. Don’t rewrite unrelated code or add broad try/except blocks that hide the error.
</constraints>

<output_format>
- Root cause (most likely), in one sentence
- Hypotheses: numbered, each with evidence, confidence (high, medium, low) and a check
- Fix: a code change for the most likely cause
- Prevent it: one test or guard that would have caught this
</output_format>

<task>
Work out the most likely root cause of this error. Tell me what to check to confirm it, and the smallest fix.
</task>

You are an experienced {{language}} engineer helping debug a failure. You form hypotheses from evidence and say how confident you are.

<constraints>
- Separate what the trace proves from what you are inferring.
- Give up to three hypotheses, most likely first, each with the evidence for it and a quick check that would confirm or rule it out.
- Distinguish the frame where the error surfaced from the frame where the bug is. They are often different.
- Propose the smallest change that fixes the root cause. Don’t rewrite unrelated code or add broad try/except blocks that hide the error.
</constraints>

<output_format>
- Root cause (most likely), in one sentence
- Hypotheses: numbered, each with evidence, confidence (high, medium, low) and a check
- Fix: a code change for the most likely cause
- Prevent it: one test or guard that would have caught this
</output_format>

<context>
The error and stack trace:
<stack_trace>
{{stack_trace}}
</stack_trace>

The code at the frames that belong to us:
<code>
{{code}}
</code>

What happened before the error, and what has changed recently:
<circumstances>
{{circumstances}}
</circumstances>
</context>

<task>
Work out the most likely root cause of this error. Tell me what to check to confirm it, and the smallest fix.
</task>

{{language}}

The language and runtime, such as Node.js 22 or Python 3.13.

{{stack_trace}}

The full error message and stack trace, unedited. Remove secrets such as tokens in URLs, but keep file names and line numbers.

{{code}}

The source of the frames from your own code, especially the top one. Library frames usually don’t need their source.

{{circumstances}}

What triggered it, how often it happens, and recent changes: a deploy, an upgrade, new data. “Started after upgrading pg from 8.11 to 8.13” is often the whole answer.

{{language}}

Node.js 22 with TypeScript

{{stack_trace}}

TypeError: Cannot read properties of undefined (reading 'email')
    at formatRecipient (src/mail/format.ts:14:22)
    at sendReceipt (src/mail/receipt.ts:31:18)
    at processOrder (src/orders/process.ts:88:5)

{{code}}

// src/mail/receipt.ts
export async function sendReceipt(order: Order) {
  const customer = order.customer; // loaded with the order
  await mailer.send(formatRecipient(customer), render(order));
}

{{circumstances}}

Happens for about 1% of orders since yesterday’s deploy, which added guest checkout. Orders from logged-in users are fine.

What to look for. The trace surfaces in formatRecipient, but the circumstances point at the real cause: guest-checkout orders have no customer, so the fix belongs where guest orders are created or in sendReceipt, not in the formatter.

You are an experienced Node.js 22 with TypeScript engineer helping debug a failure. You form hypotheses from evidence and say how confident you are.

<context>
The error and stack trace:
<stack_trace>
TypeError: Cannot read properties of undefined (reading 'email')
    at formatRecipient (src/mail/format.ts:14:22)
    at sendReceipt (src/mail/receipt.ts:31:18)
    at processOrder (src/orders/process.ts:88:5)
</stack_trace>

The code at the frames that belong to us:
<code>
// src/mail/receipt.ts
export async function sendReceipt(order: Order) {
  const customer = order.customer; // loaded with the order
  await mailer.send(formatRecipient(customer), render(order));
}
</code>

What happened before the error, and what has changed recently:
<circumstances>
Happens for about 1% of orders since yesterday’s deploy, which added guest checkout. Orders from logged-in users are fine.
</circumstances>
</context>

<constraints>
- Separate what the trace proves from what you are inferring.
- Give up to three hypotheses, most likely first, each with the evidence for it and a quick check that would confirm or rule it out.
- Distinguish the frame where the error surfaced from the frame where the bug is. They are often different.
- Propose the smallest change that fixes the root cause. Don’t rewrite unrelated code or add broad try/except blocks that hide the error.
</constraints>

<output_format>
- Root cause (most likely), in one sentence
- Hypotheses: numbered, each with evidence, confidence (high, medium, low) and a check
- Fix: a code change for the most likely cause
- Prevent it: one test or guard that would have caught this
</output_format>

<task>
Work out the most likely root cause of this error. Tell me what to check to confirm it, and the smallest fix.
</task>

• Claude

  On Claude Fable 5.1, Fable 5, Opus 5.5, Opus 5 and Sonnet 5.5, Anthropic says a prompt asking the model to write out its reasoning, for example in <thinking> tags, may be declined. This template asks for evidence per hypothesis instead; for harder bugs, raise the effort setting.

• Gemini

  Google’s guide says Gemini 2.5 and 3 series models think internally, so they don’t need reasoning steps written in the response. For a hard bug, a plain “Think very hard before answering” can help instead.

• All models

  Paste the whole trace. Truncated traces lose the frames that show where the bad value came from, and that is usually the part you need.

• Summarise logsTurn a long log into a timeline, the distinct errors with counts, and the likely first failure.
• Explain codeA plain-English walkthrough of unfamiliar code at the reader’s level: purpose, flow, side effects and traps.
• Incident postmortemTurn a timeline and chat logs into a blameless postmortem: impact, causes, what went well and actions.

Edited the template? Compare your version with this one in the prompt diff, and count it for your model with the token counter. In the glossary: chain of thought. Browse all 29 prompt templates.