Example: Bug Fix Session

An annotated session showing the core Request-Implement-Verify-Close loop for fixing a bug.

Context

A file upload endpoint returns 413 Payload Too Large for images over 5MB, but the limit should be 10MB.

Session Transcript

User:

This test fails with "Expected 200, received 413" when uploading a PNG > 5MB:

  test('should accept files up to 10MB', async () => {
    const file = createTestFile(6 * 1024 * 1024); // 6MB
    const res = await request(app).post('/api/upload').attach('file', file);
    expect(res.status).toBe(200);
  });

Fix the upload limit. Don't change the test. Only modify files in src/upload/.

Why this works: The prompt includes the exact error, the failing test, a clear constraint (“don’t change the test”), and a scope lock (“only modify files in src/upload/”). Claude doesn’t have to guess what’s wrong.

Claude:

Let me read the upload configuration.

[Reads src/upload/config.ts, src/upload/middleware.ts]

Pattern: Read before writing. Claude examines the relevant files before proposing changes. This prevents blind edits that miss related constraints.

Claude:

Found it. In src/upload/middleware.ts line 12, the multer configuration has:

  limits: { fileSize: 5 * 1024 * 1024 } // 5MB

Changing this to 10MB to match the test expectation.

[Edits src/upload/middleware.ts — changes 5 * 1024 * 1024 to 10 * 1024 * 1024]

Pattern: Minimal change. Claude changed exactly one value. No refactoring, no “improvements,” no touching other files.

Claude:

Running the test suite to verify.

[Runs npm test -- --grep "upload"]

  ✓ should accept files up to 10MB (142ms)
  ✓ should reject files over 10MB (89ms)
  ✓ should accept small files (34ms)

3 passing (265ms)

Pattern: Verify with real tests. Claude ran the actual tests, didn’t just say “this should fix it.” The output proves the fix works and nothing else broke.

Claude:

Fix applied. The upload limit in src/upload/middleware.ts was set to 5MB —
changed it to 10MB. All 3 upload tests pass.

Pattern: Clean close. Brief summary of what changed and proof it works. No unnecessary explanation.

What Made This Session Work

The prompt included the actual error — not “the upload is broken”
Scope was locked — “only modify files in src/upload/” prevented drift
The test was the contract — “don’t change the test” made success unambiguous
Verification was real — actual test output, not code inspection
One task, one session — no “while you’re at it” creep

What Would Have Gone Wrong

Anti-Pattern	Consequence
“The upload is broken, fix it”	Claude explores the entire codebase looking for upload-related code
No scope lock	Claude might also “improve” error messages, add logging, refactor middleware
Skipping test verification	The change might compile but fail at runtime due to a second limit elsewhere
“Also update the docs while you’re there”	Context pollution starts — Claude loses focus on the original fix