The most important architecture decision in Fusion is about where things run. The default answer: not on our servers.

The execution ladder

Fusion runs climb a ladder of execution environments, and each rung is a deliberate trust decision:

Tier 0, local. The default today. The harness runs on your machine, with your existing CLI logins and your keys in your OS keychain. Candidates, gates, judging: all of it executes inside your walls. The hosted side receives run metadata, verdicts, and receipts, plus diffs only if you opt in to sharing them.

Tier 1, your CI. For runs triggered by a pull request when no laptop is open. The run executes inside your own CI runner: your infrastructure checks out the code, your secrets store holds the model keys, and the finished result posts back a status check and a receipt. Fusion coordinates the run; your compute performs it.

Tier 2, managed sandboxes. For teams that explicitly do not want to bring compute. Ephemeral, isolated sandboxes that are created for one run, restricted in what they can reach, and destroyed afterward. This is a paid, opt-in tier, and receipts from it are attested differently than customer-run tiers, on purpose.

Saying it precisely

You will notice we do not say "your code never leaves your machine, period." That absolute would be false the moment you choose Tier 2, and a trust company cannot afford wording that its own product menu contradicts.

The precise claim is this: in default local and customer-CI modes, source code and model keys stay inside your chosen trust boundary. The hosted control plane coordinates runs, stores metadata and receipts, and only receives code artifacts when you explicitly enable a managed or sharing workflow. Every mode's boundary is stated, and the receipt records which mode produced the result.

Why local-first is also a quality decision

Privacy is the obvious reason, but not the only one:

  • Real environments. Code verified in the environment where it will actually run, with your toolchain and your data shapes, produces evidence that transfers. A sandbox that only resembles production produces evidence that mostly transfers.
  • Your keys, your rates. Bring-your-own-keys means model spend stays in accounts you already audit, at rates you already negotiated. We are not a token reseller marking up the middle.
  • No custody, less ceremony. The fastest security review is the one where the answer to "what do you store?" is a short list: metadata, verdicts, receipts.

What the hosted side is for

Coordination and record-keeping: the run graph, the receipts archive, status checks on pull requests, the dashboard your team reads. Think of it as the conductor, keeping time while the orchestra plays in your building.

The ladder exists so that the trust decision is always yours, always explicit, and always written on the receipt.

Source ledger

  • Tech stack decision record, sections 7 and 8.
  • Merged strategy, section 7.
  • Sourcery consensus, privacy honesty guardrails.