Resident workspace

The resident workspace holds a repository's Roslyn compilations between calls. With it enabled, fuse_check typechecks an overlay against the held compilation with no disk write or new build and stamps the answer oracle grade. Without it, build capture can still provide an oracle-grade compilation, and the build-grade fallback can run the local toolchain.

This page describes what the resident workspace holds, how it stays current, and how Fuse degrades when it is not available. It is opt-in for now (see Opting in); the default remains the persistent store path until the latency gate promotes it.

State

A resident workspace holds, per repository root, the rehydrated Roslyn compilations for the repository's C# projects. The compilations are rehydrated once from a build capture (a binary log produced by the repository's own build) and then kept in memory. Only the rehydration closure is loaded in the process that holds a resident workspace; the MSBuild-based loader used by fuse index and fuse doctor is a separate path.

Each held compilation supports three in-memory operations, none of which writes the tree:

• Overlay check: apply a proposed file's content to the held compilation and return the changed document's diagnostics. This is the speculative typecheck behind resident-grade fuse_check.
• Apply edit / add / remove: replace, add, or remove a document in the held compilation to reflect a change on disk.
• Diagnostics baseline: the current error and warning set across the held compilations, which a delta check compares against to attribute newly introduced or resolved diagnostics to an edit.

Invalidation

A file watcher observes the repository tree and coalesces raw filesystem events to the net change per path over a short debounce window (a create followed by a delete cancels; a delete followed by a create becomes a change; otherwise the latest event wins). When the batch settles, Fuse applies it to the held compilations: created and changed C# files are read from disk and their documents edited or added, deleted files have their documents removed. Non-C# files and files under no held project are skipped.

A bulk change above the storm threshold (300 files in one batch) is not applied incrementally; instead the root is evicted to the store-backed path, whose freshness reconcile handles the staleness, and a later warm re-establishes the resident workspace. This keeps a large branch switch or generated-file sweep from turning incremental update into a re-index in disguise.

The degraded ladder

Fuse answers at the best available substrate and names which one answered in the availability header:

• resident: a live resident workspace serves the root. The header reads workspace resident (N project(s), current as of ...), and fuse_check answers oracle-grade from the held compilation with no build.
• store-backed: no resident workspace serves the root. The header reads workspace store-backed. Reads answer from the persistent semantic index, kept fresh by the freshness reconcile that re-indexes edited files on open. fuse_check still never shrugs: it uses the build-capture worker when build capture is configured, or the build-grade fallback (a scoped dotnet build) otherwise. See verification grades.

The store remains the substrate for degraded modes (repositories that do not build), cold boot, and the portable artifact. Resident truth is preferred when live; the store answers otherwise.

Opting in

The resident workspace is opt-in. Set FUSE_RESIDENT to 1, true, yes, or on for fuse mcp serve or fuse host. The host then warms the served root in the background and keeps held compilations current through the file watcher. With the variable unset or false, reads remain store-backed.