Precision tier (regex vs Roslyn)

Fuse analyzes C# two ways. By default it uses fast, regex-based extraction for skeletons, dependency edges, type location, and outlines. An opt-in precision tier swaps in Roslyn syntax analysis for those same jobs when accuracy matters more than the default's speed and Native AOT cleanliness.

The default: regex

The default path reads C# with regular expressions. It is fast, deterministic, uses no runtime reflection, and is Native AOT clean, which is why it ships in the AOT binary. For most codebases its skeletons and dependency graph are good enough.

Its limit shows on hard C#. Conditional compilation, partial classes, and braces inside strings can desynchronize the regex skeleton, so it drops signatures. On the benchmark corpus this is stark on Newtonsoft.Json: the regex skeleton kept only 4 percent of method signatures.

The precision tier: Roslyn

Enable it with the --semantic flag, or the FUSE_SEMANTIC environment variable for the MCP server:

fuse dotnet --directory ./src --skeleton --semantic

The tier replaces the regex extractors with Roslyn ones behind the same interfaces. It parses each file, so it does not lose signatures to conditional compilation or partial classes, and it captures references the regex misses, such as return types, generics, and object creations. It is syntax-level analysis, not full semantic binding, so it is more accurate than regex but not guaranteed complete.

On the same Newtonsoft.Json case, the Roslyn skeleton keeps 100 percent of method signatures where regex kept 4 percent:

The trade-off

Higher fidelity means larger output, because the regex skeleton's deep cut was partly a fidelity failure, not real saving. Newtonsoft.Json moves from 96,953 tokens (regex, 93.4 percent reduction but 4 percent method fidelity) to 694,862 tokens (Roslyn, 52.7 percent reduction at 100 percent fidelity). The precision tier does not weaken fidelity to inflate reduction; it gives you a faithful signatures-only cut.

The tier also unlocks symbol-level scoping: a Type.Member focus seed scopes the seed file to that one member, keeping its body in full and reducing the rest of the file to signatures.

Availability

The Roslyn tier is not Native AOT compatible, so it is isolated in a separate assembly that the AOT package does not reference. The AOT binary always uses the regex tier; the framework-dependent tool can use either. The mechanism is described in Scoping internals.

Next

See Scoping for the modes the tier sharpens, or the measured fidelity in Benchmarks.