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[WIP] MCP#8001

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zth wants to merge 5 commits intomasterfrom
experimental-mcp
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[WIP] MCP#8001
zth wants to merge 5 commits intomasterfrom
experimental-mcp

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@zth zth commented Nov 2, 2025

This bootstrap a simple MCP for ReScript, through Rewatch. Initially it adds a single command diagnose, which will run quick diagnostics for a single file. Diagnostics here means that we'll run a compilation only on the single file targeted, no dependents, so it's really quick. Warnings and errors are returned to the LLM.

Supersedes #7566.

@zth zth mentioned this pull request Nov 2, 2025
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nojaf
nojaf reviewed Nov 3, 2025
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rewatch/src/build/compile.rs
.concat())
}

pub fn compiler_args_for_diagnostics(
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@nojaf nojaf Nov 3, 2025

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Please add a comment why this exists.

rewatch/src/mcp.rs Outdated
fn list_tools(&self) -> Vec<mcp_spec::tool::Tool> {
vec![Tool::new(
"diagnose",
"Quick per-file diagnostics after edits; prefer before full build; no writes; only this file.",
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@nojaf nojaf Nov 3, 2025

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You might want to clarify what path is here.
I know it is a .res or .resi path, preferably the full path, but if left unspecified the LLM might call this with some other path.

rewatch/src/mcp.rs
let name = tool_name.to_string();
Box::pin(async move {
match name.as_str() {
"diagnose" => {
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This seems rather horrible that we need to parse the tool name ourselves. Is there no better abstraction for this?

rewatch/src/mcp.rs
fn instructions(&self) -> String {
"ReScript MCP server.\n\n\
Tools:\n\
- diagnose(path): Quick per-file diagnostics after edits. Prefer this before a full build.\n\
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@nojaf nojaf Nov 3, 2025

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This tool call does feel a bit like duplication of one of the LSP endpoints.
It makes me wonder if we should explore having an LSP client inside the MCP tool that can ask this information from the running LSP Server.

@zth zth force-pushed the experimental-mcp branch from 4e63942 to 60935ad Compare November 6, 2025 17:54
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pkg-pr-new bot commented Nov 6, 2025 •
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rescript

npm i https://pkg.pr.new/rescript-lang/rescript@8001

@rescript/darwin-arm64

npm i https://pkg.pr.new/rescript-lang/rescript/@rescript/darwin-arm64@8001

@rescript/darwin-x64

npm i https://pkg.pr.new/rescript-lang/rescript/@rescript/darwin-x64@8001

@rescript/linux-arm64

npm i https://pkg.pr.new/rescript-lang/rescript/@rescript/linux-arm64@8001

@rescript/linux-x64

npm i https://pkg.pr.new/rescript-lang/rescript/@rescript/linux-x64@8001

@rescript/runtime

npm i https://pkg.pr.new/rescript-lang/rescript/@rescript/runtime@8001

@rescript/win32-x64

npm i https://pkg.pr.new/rescript-lang/rescript/@rescript/win32-x64@8001

commit: ffdd6de

@zth zth force-pushed the experimental-mcp branch from 488eeb2 to 5902225 Compare November 20, 2025 21:38
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tsnobip commented Jan 16, 2026

@zth what's the status of this? It'd be cool to have it.

Can this MCP server respond while rewatch is in watch mode ? This is a frustrating limit today that you can't both have rescript in watch mode and have your agent run a build to get feedback from the compiler.

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@zth @tsnobip @nojaf