Hire F# developers

The honest answer is: it depends on the half-life of your F# surface area. If your roadmap leans heavily on domain models with discriminated unions and records and you expect to keep investing in .NET over the next 18-24 months, a specialist will out-deliver a generalist on day-30 throughput and incident response.

If your team is smaller than ten engineers, or F# is one of three or four core technologies, hire a strong generalist who has shipped F# in anger at least twice. The cross-stack pattern recognition will pay for itself the first time you need to integrate Giraffe with another part of the system.

On Haystack we surface both - filtered by whether the candidate self-identifies as a F# specialist and verified against their last two roles. Expect to pay around £70k–£95k for a mid-level UK hire, scaling toward £100k–£145k for senior.

A great F# engineer is not the one with the most stars on GitHub - it is the one who has paged at 3am for a F# service they wrote, and changed how they build because of it. Across the core engineering hires we have placed in 2025-2026, the same patterns keep showing up.

Every stack has its own pattern of plausible-sounding answers that fall apart in production. With F#, these are the patterns that most often correlate with a six-month regret hire on the employer side.

When teams ask us how to evaluate F# engineers beyond a CV, we recommend a 90-minute paid take-home that mirrors real work, not algorithm puzzles. The brief below is one we have refined with employers hiring core engineering teams.

Give the candidate a small, intentionally imperfect F# service that already does domain models with discriminated unions and records. Their task is to add a second capability - f# giraffe / saturn web services - while keeping existing behaviour green. Grade in three parts.

By week one, the new F# engineer should have shipped a small change to production - typically a docs fix, a .NET dependency bump or a minor refactor in domain models with discriminated unions and records. The goal is to validate the development loop, not to ship anything heroic.

By week two, expect them on the on-call rota in a shadow capacity, pair-programming on at least one feature, and asking pointed questions about why specific F# patterns were chosen. If they are not asking those questions, the hire is going to plateau.

By day 30, they should own one cleanly-scoped slice of the F# surface area, have a public ramp-up document, and be the named reviewer on PRs touching that area. Every Haystack employer gets a structured onboarding template - so you are not reinventing the playbook for each hire.