AI-Assisted Scientific Bug Triage & Root Cause Investigation

You are a Principal Systems Debugger and Engineering Investigator. When developers troubleshoot bugs with AI, AI models frequently suggest speculative code changes that mask the real issue or introduce secondary regressions. We will follow a rigorous scientific debugging method instead of guessing. Problem Description & Symptoms: Intermittent HTTP 500 error when users update their profile avatar. It succeeds 80% of the time, but fails consistently for users uploading PNG files larger than 2MB. Observed Behavior vs Expected Behavior: Expected: Image is resized to 400x400, uploaded to S3, and profile updated. Observed: Server crashes with `PayloadTooLargeError` or hangs until Lambda 30-second timeout. Environment, Telemetry & Logs: Next.js 15 on AWS Amplify / Lambda. Memory allocation: 512MB. CloudWatch logs: 'Task timed out after 30.03 seconds'. Memory used: 498MB out of 512MB. Relevant Code Snippets / Call Sites: export async function POST(req: Request) { const formData = await req.formData(); const file = formData.get('file') as File; const buffer = Buffer.from(await file.arrayBuffer()); const resized = await sharp(buffer).resize(400, 400).png({ compressionLevel: 9 }).toBuffer(); await s3.putObject({ Bucket: 'avatars', Key: `${userId}.png`, Body: resized }); return NextResponse.json({ ok: true }); } Execute the Scientific Debugging Protocol: 1. Competing Hypotheses: Propose 3 distinct, testable hypotheses for what is causing this bug, ranked from most probable to least probable. For each hypothesis, explain the underlying mechanism. 2. Disproving Experiments & Probes: For each hypothesis, specify the exact single log statement, breakpoint, or curl request that will prove or disprove it decisively. 3. Minimal Reproduction Test Case: Write a minimal, standalone unit test or script that fails under the buggy condition and passes once fixed. 4. Differential Diagnosis Analysis: Based on the provided logs, eliminate at least one common false lead and explain why it is NOT the root cause. 5. Root Cause Remediation Plan: Once the bug is proven, provide the surgical patch and explain how to prevent regressions with a permanent automated test.