Fuse – an open source MCP/CLI tool to speed up Claude Code on C# codebases

Fuse, an open-source .NET tool, speeds up Claude Code on C# codebases by creating a persistent semantic index via MSBuild and Roslyn, reducing token usage and enabling compiler verification before writing code. It provides tools for service resolution, context reduction, and impact analysis, running locally and offline.

Fuse is a local .NET tool with a persistent semantic index, typed-graph wiring resolution, reduced task-scoped source, and pre-write compiler verification for coding agents. It indexes a solution through MSBuild and Roslyn, stores the result in .fuse/fuse.db , and reuses it across agent turns instead of rediscovering the same structure through repeated file reads and text searches. From a .NET project directory: dotnet tool install -g Fuse fuse mcp install --rules Reload your MCP client, then ask: Resolve IOrderService to its implementation, then check the proposed OrderService.cs edit with fuse check before writing it. When the MCP server starts, its shared local daemon begins warming .fuse/fuse.db in the background. A cold read waits for a bounded syntax-first pass and reports when the semantic graph is still upgrading. Run fuse index when you want a synchronous full index before connecting the agent. fuse mcp install --rules also adds .fuse/ to .gitignore at project scope. Analysis runs locally and can work offline. Fuse walks a typed graph of DI registrations, handlers, routes, and callers, emits reduced source for a selected scope, and lets a coding agent typecheck proposed single-file content before writing it. No model is required. The optional update check can contact NuGet, and build-grade operations can use the package feeds configured for the repository. Coding agents can inspect a repository through file reads, grep, and regex. On a large solution, those operations can rediscover the same symbols, references, registrations, and project structure across several turns. Fuse performs that discovery through MSBuild and Roslyn, persists the result, and incrementally re-indexes files as they change. When a project loads semantically, the graph records DI registrations, request handlers, routes, options bindings, and call edges. When it does not, Fuse falls back to syntax-level indexing for that project and reports the mode. Resolve wiring. fuse find traces a service, request, route, or configuration section to the code that handles it. Text search finds IOrderService ; Fuse follows the registration to the implementation that runs. Pack branch context. fuse review seeds on the git diff and returns related callers, handlers, and tests with provenance. On 69 recorded pull requests the median response was 1,026 tokens at 93.4 percent precision review.json . Return less source. fuse context reduces the selected files under a token budget and records why each file was included. Across four recorded repositories, skeleton reduction removed 38 to 44 percent of tokens while retaining every measured public and protected type name reduce.json . Read warm. On the recorded NodaTime run semantic tier, 14,760 symbols , exact symbol lookup took 1.8 ms at the median, task localization 15.7 ms, and review planning 106.3 ms performance.json ; timings are environment-dependent . fuse check checks the proposed content of one file and returns compiler diagnostics without changing the working tree. Oracle grade reuses compiler state captured from the real build. Build grade runs a scoped dotnet build for the owning project when captured state is unavailable. Supported API-shape errors can include a repair packet; in the recorded run, the top suggestion repaired 20 of 20 near-miss member and type errors diagbench.json .- Before changing a public method, fuse impact finds callers, implementations, and referencing types. Given a package id and two versions, it returns the break set for that NuGet upgrade. - When a signature must change, fuse refactor stages the refactor as a diff and returns it only when the compiler reports no new diagnostic. - After an edit, fuse test selects and runs the test types that reach the changed symbol instead of starting with the whole suite. Every answer names how it was produced. Fuse calls this the verification grade : oracle grade checks against the compilation captured from the real build, build grade runs a scoped dotnet build , and when neither compiler path can answer, Fuse abstains and names the missing prerequisite instead of guessing. Every result below comes from tests/benchmarks/results and has a reproduction command on the benchmarks page https://fuse.codes/docs/project/benchmarks . - Across 1,000 compiler-labeled edits in the recorded OrderingApp test app 500 breaking, 500 neutral , fuse check reported zero broken edits as clean and rejected zero valid edits checkgate.json . - In the same app, Fuse matched all 24 expected .NET wiring links with no extra matches semantics.json . - Across 69 real pull requests, branch review retained every git-changed file by construction at 93.4 percent precision with a median size of 1,026 tokens review.json . - In the reduced-scope agent-loop run, the Fuse arm's edits passed the project's own tests on the first attempt in 89 percent of scored rollouts versus 82 percent for native tools, with overlapping confidence intervals. The Fuse arm declared success on a failing edit 8 times versus 9 for native. Build and test calls were essentially equal at 3.1 versus 3.2 loop.json . - Across four recorded repositories, skeleton reduction removed 38 to 44 percent of tokens while keeping every public and protected type name and 96.3 to 99.4 percent of method names reduce.json . The opt-in resident workspace answered repeated fuse check calls in 31.2 ms at the median on the recorded NodaTime run resident-latency.json . That path is faster than a full build for speculative checks; it does not replace normal builds or tests before merge. These measurements have defined limits. Read the full methods and results https://fuse.codes/docs/project/benchmarks before comparing tools or applying the figures to another repository. Fuse reads source, compiler state, git metadata, and its local .fuse/fuse.db index. Read, check, impact, refactor, and review operations do not write the working tree. fuse workspace with action=apply is the one explicit tree-write path, and it is a dry run unless write=true . Compiler-backed wiring analysis is .NET-only. Other languages receive syntax-level search and reduction. Repository indexes, code graphs, and language-server tools already exist. CodeGraphContext https://github.com/CodeGraphContext/CodeGraphContext provides a local multi-language graph, Serena https://github.com/oraios/serena exposes language-server-backed symbol tools, and Sourcegraph https://sourcegraph.com/code-search covers multi-repository search and code intelligence. Fuse concentrates on local .NET work through MSBuild and Roslyn: framework wiring, reduced scoped context, compiler-backed proposed-file checks, change impact, and covering-test selection. It can run alongside the index or search built into a coding client. The peer comparison https://fuse.codes/docs/project/benchmarks peer-comparison-fuse-versus-codegraph-coa-codesearch-and-serena records a bounded, dated experiment and its sampling limits. It is not a general ranking of code-intelligence tools. Apache 2.0. 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