We Gave 5 AI Coding Tools a Real Goroutine Leak to Fix — Only One Found the Actual Bug

Disclosure: We link out to every tool we test. None of the five tools below currently has a confirmed, active affiliate relationship with us, so nothing here is commission-motivated.

Go doesn't leave much room for "almost right." gofmt means there's one accepted formatting. Explicit error returns mean there's no ambiguity about whether a function can fail. So instead of asking which AI coding tool "feels" best for Go, we gave five of them the same production bug and the same four project types — an HTTP service, a CLI, a gRPC microservice, and a concurrent worker pool — and watched what they actually did with it.

The bug

A worker pool was occasionally leaking goroutines under load. The real cause: a select statement that didn't handle context cancellation, so workers blocked forever on a channel send after the parent context was cancelled. The fix was three lines.

Claude Code traced it in minutes — correctly identified the missing case <-ctx.Done(), explained why the goroutines were piling up, and proposed the three-line fix.

Cursor suggested a plausible-looking fix: add a timeout. It compiled, it "worked," and it masked the leak instead of fixing it — the workers still blocked, just for a bounded time instead of forever.

That's the gap that doesn't show up in a feature checklist: one tool diagnosed the actual failure mode, the other patched the symptom.

Where the rest fell down

We also asked each tool to add a database method to an existing service, and to refactor a *sql.DB-based service into one that takes an interface, for testability.

• Cursor wrote the method with correct errors.Is handling and defer-closed the *sql.Rows with error handling on the close. Copilot missed the defer-close error check. Windsurf used %v instead of %w for error wrapping, which silently drops the error chain — the kind of bug that only bites you three services downstream when errors.Is stops matching.
• On the interface refactor, Cursor produced a tight Querier interface with only the methods the service actually called. Copilot's interface was too wide, pulling in methods nothing used. Claude Code's interface was the cleanest of all three, but it's terminal-based, so you're applying the diff by hand.

The numbers

Compile-on-first-try rate across all four project types, tracked over three weeks on the same Go 1.22 codebase:

• Cursor: ~85%
• Copilot: ~75%
• Windsurf: ~65%
• Tabnine: ~55%

Claude Code doesn't fit this metric — it's a task-based terminal agent, not inline autocomplete — which is exactly why it's the one that caught the goroutine leak nobody asked it to look for twice: once in the original bug, once in a context-propagated tracing task where it ended every span correctly on error paths and Cursor missed one.

The honest tradeoff

None of this makes one tool strictly better. Cursor is the best daily driver for Go — highest compile rate, best module/go.mod awareness, cleanest table-driven tests. Claude Code is what you reach for when the bug is subtle and the stakes are a 3am page, not a missing semicolon. Copilot at $10/month is the value pick if you're already in the GitHub ecosystem. Windsurf is a budget option only if someone senior is reviewing its concurrency code before it ships.

The full writeup has the complete testing methodology, the gRPC and CLI results, and current pricing for all five tools.