Spline V2 turns the 3D editor into an endpoint, with the desktop app as the only door

The undo history is doing more work here than the MCP server is. Spline says every change an agent makes is treated as a normal editor operation, landing in the undo stack and syncing to everyone else in the file ^[1]. That puts the rollback path in the same place a designer already looks, which is a cheaper form of review than inspecting whatever an agent wrote to disk.

Getting there requires the app to insert itself into your tooling. When the desktop application starts, it scans for supported clients and writes the server into their configuration files; the developer then restarts the client and prompts normally ^[8]. Five clients are covered ^[17], and a single connection spans two surfaces, with Spline deciding whether a request is scene work for the 3D editor or interface work for Hana ^[14]. Convenient, and also a reminder that discovery is the vendor's call rather than the client's.

The asymmetry is on the way out. Spline's runtime API can control a published scene, but the company documents no path for changes made in exported code to flow back into the original 3D file; an agent that needs to revise the source has to return through MCP and work in the live editor ^[2]. Set that against the packaging and you are left with one writable entrance to the source of truth, which is an MCP call executed against an open tab on somebody's desktop ^[16]. Anything an agent edits after export is a branch with no merge.

Hana's half of the round trip is narrower than it sounds. The agent can turn an existing component into an editable frame and carry visual changes back into application code, using stable references on exported elements to find the right place in the source ^[12]. Hana supports only a subset of HTML and CSS built around flexbox, though, so complex markup and framework-specific components have to be reconciled with the existing code rather than replaced outright ^[13]. Reconciliation is human work, and it is where the closed loop between design file and repository actually leaks.

Spline already ships its own agent that will build and edit a scene from a prompt ^[3], so the pitch for MCP is not that a model can produce 3D. It is that the coding agent already holds the codebase and the work in progress, and nobody wants to rebuild that context in a separate Spline conversation [c2b]. That is an argument about where context should live, and it has a consequence the marketing does not mention: the context stays in the client, while the artefact stays in Spline, and only Spline's application can carry a write between them.