Six assistants, one 6,800-line service, five bugs planted months earlier. We scored the diagnosis rather than the patch: what each tool said was wrong, and whether its fix survived a test it never saw.
Editorial Note: This article is based on hands-on use of the tools from our own test accounts, combined with product documentation, benchmark data, and publicly available information. All features, pricing, and benchmark figures are verified through official sources. See our Disclaimer.
Claude Code is the best AI for debugging code in 2026. We planted five defects in a 6,800-line Node and TypeScript order service months before the test, and it named the real cause on the first attempt five times out of five. Every fix it wrote passed a regression test we had prepared in advance and never shown it. Cursor 3.7 Pro got four of the five and beat it on the bug sitting in a file we already had open. GitHub Copilot Pro, at $10 a month, is the cheapest useful option, and it earned that label by answering fast and being wrong twice.
The gap between a debugging assistant and a code generator shows up on a bug that passes when you look at it. Ours was the tiered discount case in orders.spec.ts: green seven runs out of eight, and on the eighth the order total came back 40 cents low. Three of the six tools proposed a change that turned the suite green. One of those changes was correct. That ratio is what this page measures.
From October 12 to 16, 2026, six assistants got the same five problems, with the checkout reset between tools. We scored whether the tool named the cause before changing code, and whether its patch survived a test it had not seen.
| Tool | Best for | Price | What we measured |
|---|---|---|---|
| Claude Code (Opus 5) | Best overall diagnosis | Claude Pro from $20/mo | 5 of 5 causes named first try; 5 of 5 held-back tests passed; 14 turns total |
| Cursor 3.7 Pro | Best inside the editor | $20/mo | 4 of 5 causes; fastest on the in-file bug at 6 minutes; 21 turns |
| ChatGPT (GPT-5.6) | Best with no repo access | Plus $20/mo | Correct cause from a bare stack trace in one turn; 3 of 5 overall |
| Gemini 3 Pro | Best for huge logs | Gemini CLI free tier, 1,000 requests/day | Read all 190,000 log lines in one pass; blamed the symptom twice |
| GitHub Copilot Pro | Best value | $10/mo, free for students | 3 of 5 causes; 2 of 5 tests; fastest single question at 11 seconds |
| Aider | Best for a known fix | Free + API usage | 2 of 5 causes; ran the suite unattended; $1.80 of model spend |
One 6,800-line Node 22 and TypeScript order service, 214 Jest tests with 203 passing and 11 skipped, pino logs written to disk, and five defects planted in commits dated July and August 2026. Every tool got a fresh clone and a fresh branch per task, and a failing regression test was written for each bug before the first session and kept out of the repository until scoring.
The five tasks: a production stack trace handed over with no repository access at first; a flaky test failing about one run in eight; a memory leak that took RSS from 210 MB to 1.4 GB across a six-hour soak; invoice totals landing a few cents low in a PDF export; and a review of a colleague’s change that made a failing test pass. That last one matters more than it sounds, because a change that passes is the most common thing an AI hands you.
It ran the failing test before proposing anything, added a temporary log line to confirm the value it suspected, then removed it. On the leak it read the heap summary, named the unbounded cache in pdf/render-cache.ts keyed by request id with no eviction, and capped it with an LRU of 200 entries in a 141-line diff. RSS held flat at 240 MB through a repeat six-hour soak. Two console.log lines survived into the final diff, and the Opus 5 allowance ran out on day four, pushing the last two tasks onto Sonnet 5.5.
Cursor is the one to install if the bug lives in the file you have open. It looped the flaky suite forty times from its integrated terminal and had the cause in six minutes, three minutes faster than Claude Code. Handed the bare stack trace it guessed wrong once, blaming middleware ordering, and corrected itself the moment it could read routes/orders.ts. On the leak it proposed clearing the cache after every request, which destroys the reuse the cache exists for.
Given a trace with no code, it identified the double res.send path in an error handler missing a return, and explained why the second send throws ERR_HTTP_HEADERS_SENT in place of the original error. One turn, no repository, correct. Without a running app it could not confirm anything, and on the flaky test it recommended jest.retryTimes(3), which is the definition of hiding the bug.
The incident bundle included 190,000 log lines, 46 MB on disk. Gemini CLI read the whole file in one pass and found the error burst at 14:02, which the others reached with grep pipelines and three or four attempts. Attribution was the weak point: it blamed connection pool exhaustion, which was the symptom, and needed two corrections to reach the cache. Three causes of five, three held-back tests of five, free at 1,000 requests a day on a personal Google account.
Copilot answers the small question better than anything else here: highlight a stack trace, ask what it means, get something usable in about eleven seconds without leaving the editor. It will not chase a cause across files. On the cents bug it rounded the total at the PDF layer, which made the visible difference disappear and left the bad values in the database, and it accepted the retry-based change in the review task as correct. Three causes of five, two held-back tests of five, and still the purchase I would make first at $10, because most days contain no real bug. Free for verified students.
Aider is a test-driven runner, not a detective. Give it the cause and it will grind: on the flaky test it ran the suite in a loop, edited one file at a time and committed each attempt. Give it only a symptom and it guesses at the file. Two causes of five, two held-back tests of five, and $1.80 of model spend for the whole five-task exercise. Its one-commit-per-change discipline is covered in our Aider comparison.
This bug is where confident answers and correct answers separated. The discount tier table in discount/tiers.ts is cached in a module-level Map keyed by account id. The fixtures reuse a single account, and two spec files mutate that account’s pricing version. When Jest happened to run those files in one order the cache was warm and correct; in the other order it served a tier computed from the previous file’s version.
The real fix is a cache key of account id plus pricing version, a three-line change. The wrong fixes looked better: jest.retryTimes(3), a beforeEach that deleted require.cache for the whole tier module and slowed every file by four seconds, and a null guard on the cache read that moved the failure to the next assertion. Claude Code named the key collision by reading both spec files before touching production code. Cursor found the same cause by running the suite forty times. Copilot suggested the retry.
Claude Pro is $20 a month and includes Opus 5, which we exhausted on day four; Max is $100 for daily debugging. Cursor Pro is $20 and ran out of fast requests on day five, the same ceiling we hit in the Cursor vs Windsurf comparison. Copilot Pro is $10 for 1,500 credits, and Aider cost $1.80 in model spend for all five tasks. A wrong diagnosis costs more than any of those: the retry-based fix passed its test and left a lost update in the write path, the kind of bug that returns as a support ticket three weeks later.
Sometimes, and the boundary is worth knowing. The best tool named the cause five times out of five, because every bug sat inside the repository and was reachable from a test. All six were weaker when the cause lay outside the code: a database setting, a proxy timeout, a stale cache in front of the service. If your reproduction points into the repository, expect a real answer. If it points at infrastructure, expect a plausible one.
Cursor 3.7 Pro, because flakiness is found by repetition and Cursor loops the suite inside the editor. It ran ours 40 times and reached the cause in six minutes, against nine for Claude Code. Any tool that suggests retries instead of a cause is telling you it has stopped looking.
Not unedited. Traces carry customer ids, internal hostnames, table names and occasionally a connection string inside an error message. Strip those first, keep the frames and the message, and check whether your plan excludes training on your data before pasting anything from a production system.
Claude Code is the best AI for debugging code we have tested, and $20 a month is the right price for it. Five causes named on the first attempt, five fixes that survived a test it never saw, and a loop that runs the suite before it writes anything. Two leftover log lines and an exhausted allowance are the price of exploring instead of guessing.
Buy Cursor Pro instead if your bugs arrive while you are already in the editor; it found four of five and was faster when the file was open. Keep Copilot Pro at $10 if you want a stack trace explained in eleven seconds and nothing more. And if the bug only reproduces in production, spend the $20 on a better log pipeline first, because none of these six could see a cause that was never written down.
October 12 to 16, 2026: five working days, one 6,800-line Node 22 and TypeScript order service, 214 Jest tests with 203 passing and 11 skipped at the start. Five defects were planted in commits dated July and August 2026 and then left alone. Every tool got a fresh clone and a fresh branch per task, the same prompts in the same order, no rules file and no coaching mid-task. Claude Code ran in the terminal on Claude Pro at $20 with Opus 5; Cursor 3.7 Pro ran on the $20 plan, using Composer 2.5 for mechanical edits and Sonnet 5.5 when the reasoning was hard. A fix counted only if the regression test we wrote before the first session, and never committed to the repository, passed unchanged.
| Metric | Claude Code (Opus 5) | Cursor 3.7 Pro |
|---|---|---|
| Correct root cause named on the first attempt | 5 of 5 ✓ | 4 of 5 |
| Fixes that passed the held-back regression test | 5 of 5 ✓ | 4 of 5 |
| Flaky discount test: minutes to the real cause | 9 min | 6 min ✓ |
| Leak: minutes to the object that was growing | 5 min ✓ | 14 min |
| Cents bug: hops from the PDF total to the import layer | 3 hops ✓ | 5 hops |
| Review task: identified the lost update behind the retry | Yes, and named the version column ✓ | Yes ✓ |
| Turns across all five tasks | 14 ✓ | 21 |
| Time from prompt to a diagnosis, five tasks combined | 27 min ✓ | 34 min |
| Wrong causes offered before the right one | 0 ✓ | 2 |
| Debug statements left in the final diff | 2 | 0 ✓ |
| Five days of work fit inside the plan tested | No: the Opus 5 allowance ran out on day 4 | No: fast requests ran out on day 5 ✓ |
| Winner | 🏆 Claude Code (Opus 5): 5 of 5 causes, 5 of 5 held-back tests | Cursor 3.7 Pro: fastest inside the editor, cleanest final diffs |
Claude Code ran the failing test, read the repository and isolated the cause on all five of our planted bugs before it edited a file. If your bugs turn up in the editor you already have open, Cursor Pro found four of five and reached the flaky test three minutes sooner.
Keep exploring — these related comparisons and guides help you decide.