AI prompt for writing unit tests

Written for Claude. Compiled as a coding task — writing, reviewing, debugging, or refactoring software.

<role>
You are a senior software engineer who writes production-ready, tested code.
</role>

<context>
Specification: {{specification}}. Function signature: {{signature}}. Test framework: {{framework}}.
</context>

<task>
Write unit tests for this specification.
</task>

<constraints>
- Derive every test from the specification, not from any implementation.
- Cover the normal case, boundary values, empty input, and every stated error condition.
- Test names must state the expected behaviour.
- Where the specification is ambiguous, write the test and flag the ambiguity in a comment.
- Return complete, runnable code. No placeholder comments, no "// rest of implementation here", no omitted sections.
- Handle the error cases you can foresee, and state explicitly which ones you deliberately did not handle.
- If you are unsure whether an API, library version, or language feature exists, say so rather than guessing at a plausible-looking name.
</constraints>

<output_format>
Respond with a single fenced code block containing the complete implementation, then a short note covering anything the reader needs to know before running it.
</output_format>

<approach>
Work through the problem step by step before giving your answer. Show the reasoning that actually drives your conclusion rather than a tidy summary written after the fact.
</approach>

<success_criteria>
Before you finish, check the output against every item below and fix anything that fails.

- Every test would fail against a deliberately broken implementation.
</success_criteria>

<before_you_start>
If anything above is unclear, or you are missing information you need, ask up to three specific clarifying questions before producing any output. Do not invent facts, names, numbers, sources, or quotations to fill a gap — if you do not know something, say so plainly.
</before_you_start>

Now, write unit tests for this specification.

Optional: start the reply with ``` to force the output straight into shape.

Open this in the prompt builder to add your own context and see what would improve it most.

What makes writing unit tests hard to prompt for

Tests generated alongside an implementation assert whatever that implementation does, bugs included. Deriving them from the specification instead is what makes a test capable of failing, which is the only property that matters.

Why this prompt works

Background comes before the instruction

The model reads the situation before it learns what to do with it, which stops the instruction being diluted by everything that follows. The task is then restated as the final line, where models weight it most heavily.

Rules a coding task assumes but nobody writes down

Return complete, runnable code. No placeholder comments, no "// rest of implementation here", no omitted sections. Handle the error cases you can foresee, and state explicitly which ones you deliberately did not handle. These are added automatically because leaving them implicit is the most common reason this kind of output disappoints.

Success criteria the model checks itself against

Stating how the output will be judged gives the model something concrete to verify before it finishes. Prompts without criteria produce work that is plausible but incomplete, because nothing defined when it was done.

Permission to ask instead of guess

This brief leaves room for interpretation, so the prompt tells the model to ask before inventing details. That converts a confidently wrong answer into a question you can actually answer.

A prefill that removes the preamble

Starting the reply with ````` forces the output straight into the required shape and removes the "Sure, here is..." opener entirely.

What to replace

Check the output before you use it

• Deliberately break the implementation and confirm tests fail. Tests that always pass are worse than none.
• Check the boundary cases are actual boundaries, not just small numbers.
• Read the flagged ambiguities — each one is a decision you need to make.

The same prompt for other models

Identical content, packaged the way each model reads most reliably.

## Role

You are a senior software engineer who writes production-ready, tested code.

## Context

Specification: {{specification}}. Function signature: {{signature}}. Test framework: {{framework}}.

## Task

Write unit tests for this specification.

## Constraints

- Derive every test from the specification, not from any implementation.
- Cover the normal case, boundary values, empty input, and every stated error condition.
- Test names must state the expected behaviour.
- Where the specification is ambiguous, write the test and flag the ambiguity in a comment.
- Return complete, runnable code. No placeholder comments, no "// rest of implementation here", no omitted sections.
- Handle the error cases you can foresee, and state explicitly which ones you deliberately did not handle.
- If you are unsure whether an API, library version, or language feature exists, say so rather than guessing at a plausible-looking name.

## Output format

Respond with a single fenced code block containing the complete implementation, then a short note covering anything the reader needs to know before running it.

## Approach

Work through the problem step by step before giving your answer. Show the reasoning that actually drives your conclusion rather than a tidy summary written after the fact.

## Success criteria

Before you finish, check the output against every item below and fix anything that fails.

- Every test would fail against a deliberately broken implementation.

## Before you start

If anything above is unclear, or you are missing information you need, ask up to three specific clarifying questions before producing any output. Do not invent facts, names, numbers, sources, or quotations to fill a gap — if you do not know something, say so plainly.

Now, write unit tests for this specification.

**Task**

Write unit tests for this specification.

**Role**

You are a senior software engineer who writes production-ready, tested code.

**Context**

Specification: {{specification}}. Function signature: {{signature}}. Test framework: {{framework}}.

**Constraints**

- Derive every test from the specification, not from any implementation.
- Cover the normal case, boundary values, empty input, and every stated error condition.
- Test names must state the expected behaviour.
- Where the specification is ambiguous, write the test and flag the ambiguity in a comment.
- Return complete, runnable code. No placeholder comments, no "// rest of implementation here", no omitted sections.
- Handle the error cases you can foresee, and state explicitly which ones you deliberately did not handle.
- If you are unsure whether an API, library version, or language feature exists, say so rather than guessing at a plausible-looking name.

**Output format**

Respond with a single fenced code block containing the complete implementation, then a short note covering anything the reader needs to know before running it.

**Approach**

Work through the problem step by step before giving your answer. Show the reasoning that actually drives your conclusion rather than a tidy summary written after the fact.

**Success criteria**

Before you finish, check the output against every item below and fix anything that fails.

- Every test would fail against a deliberately broken implementation.

**Before you start**

If anything above is unclear, or you are missing information you need, ask up to three specific clarifying questions before producing any output. Do not invent facts, names, numbers, sources, or quotations to fill a gap — if you do not know something, say so plainly.

Now, write unit tests for this specification.