The surface similarity
Both hand you engineers you don't employ, working against your roadmap, billed by the engagement. From a procurement spreadsheet, forward-deployed engineering (FDE) and a dev agency look like the same line item.
Where they actually differ
An agency typically works from a spec: you write requirements, they build to them, and the relationship is transactional — ship the ticket, invoice, repeat. A forward-deployed pod embeds inside your stack and your team, sits in your standups, and is accountable for the thing working in production, not just the code compiling.
That changes what gets asked. An agency asks "what do you want built?" A forward-deployed pod asks "what's actually stuck, and what does 'done' mean for the business?" — then owns the deployment, the integration, and the rough edges that only show up once real traffic hits it.
The tell: what happens at handoff
Agencies optimize for shipping the deliverable. Forward-deployed engineering optimizes for the client owning a clean system afterward — documented, version-controlled, no lock-in, no dependency on the vendor to keep it running. If a vendor's incentive is more tickets rather than less need for them, that's the agency model wearing FDE language.
When each one fits
A well-specified, bounded piece of work (a marketing site, a known integration) is a legitimate agency job — you know what you want, you just need hands. FDE fits when the work is ambiguous, high-stakes, or genuinely stuck: a launch that keeps slipping, a migration nobody wants to own, a codebase you're about to invest in and need read honestly. The signal isn't project size — it's whether you need someone accountable for the outcome, not just the output.