🔥 deoptkit

A V8 deoptimization toolkit for JavaScript and TypeScript on Node.js — visibility into inline cache states, deoptimizations, hidden-class (map) churn, and CPU profile data, packaged for AI agents and everyday dev workflows.

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deoptkit began as a rebuild of Microsoft's Deopt Explorer VSCode extension, re-targeted at AI agents instead of humans and updated for modern V8 (tested against V8 14 / Node 26; the original supported V8 8–9). Where the extension offered tree views and editor decorations to browse, deoptkit answers the question agents actually ask: "what should I fix first?" — with ranked findings, explanations, suggested fixes, and a first-class before/after comparison to verify the fix worked.

Background reading: the TypeScript team's Introducing Deopt Explorer article, where this class of analysis produced an 8–10% compiler speedup.

What's in the kit

deoptkit ships several surfaces over one engine, all from the deoptkit package:

Surface	What it is
MCP server (deoptkit / deoptkit mcp)	Eleven tools + a prompt that let an AI agent profile a workload, get ranked findings, drill in, and verify fixes.
deoptkit ci	Snapshot-style regression gate: fails CI when a change introduces new structural findings (megamorphic ICs, deopt loops, map churn).
deoptkit lsp	A language server that publishes findings as inline editor diagnostics (squiggles + Problems panel).
deoptkit/harness	mark() / observed() — plant in-log window markers and warm up hot code in benchmark scripts.
deoptkit/vitest + deoptkit/bench	A vitest bench preset that records V8 logs per worker, and benchObserved() to window each case.
deoptkit/serve	driveServer() — boot an SSR app in-process and drive it with requests so its runtime is observable.
library API (deoptkit)	parseLog, computeFindings, compareSessions, runWorkload, … for building your own tooling.

Quick start (AI agent + MCP)

Register the stdio server with your MCP host. For Claude Code, add to .mcp.json in your project (or run claude mcp add):

{
  "mcpServers": {
    "deopt": {
      "command": "npx",
      "args": ["-y", "deoptkit"]
    }
  }
}

For VSCode (Copilot Chat / MCP-capable extensions), add to .vscode/mcp.json:

{
  "servers": {
    "deopt": { "command": "npx", "args": ["-y", "deoptkit"] }
  }
}

The server advertises instructions describing the workflow, so a connected agent knows what to do without reading this file. The loop:

1. profile_run { command: ["node", "bench.js"] } — runs the workload under V8 logging flags (no flag knowledge needed) and loads the result as a session.
2. get_findings { sessionId } — everything wrong, ranked worst-first with severity 1–100, source locations, V8-level explanations, and suggested-fix categories.
3. Fix the top finding — at its original position when present (that's the source you wrote), otherwise its file/line.
4. profile_run again, then compare_sessions { baseSessionId, headSessionId } — reports the findings your change resolved, any regressions it introduced, and per-function CPU deltas.

Drill-down tools when a finding needs more context: get_function (annotated source + its ICs/deopts/ticks), get_map (a hidden class's transition chain and which call sites it polluted), list_ics, list_deopts, list_functions, load_log (analyze an existing v8.log), load_manifest (load every log from a vitest bench run), list_sessions.

Warm-up matters. V8 only optimizes hot code, so profile a workload that runs the target thousands of times — the observed() helper below encodes this. For libraries, profile the built output, not source: bundling changes shapes and inlining.

Benchmarking your own code

Standalone scripts

// bench/parse.bench.mjs
import { observed } from "deoptkit/harness";
import { parseRecord } from "../dist/index.mjs";

const inputs = makeVariedInputs(); // realistic, varied shapes — not homogeneous
observed("parse record", (i) => parseRecord(inputs[i % inputs.length]), {
  iterations: 50_000
});

Then profile_run { command: ["node", "bench/parse.bench.mjs"] } and window the findings with fromMark: "parse_record_start", toMark: "parse_record_end".

Schema-driven inputs (Valibot + Valimock) are the easiest way to get realistically varied shapes — see docs/recipes.md.

Vitest bench mode

// vite.config.ts
import { defineConfig } from "vitest/config";
import { deoptKit } from "deoptkit/vitest";

export default defineConfig({
  test: { ...deoptKit({ outDir: ".deopt" }) }
});

// src/__benchmarks__/parse.bench.ts
import { benchObserved } from "deoptkit/bench";
import { parseRecord } from "../parse";

benchObserved("parse record", () => parseRecord(next()));

Run vitest bench, then load_manifest { path: ".deopt/manifest.json" } to load every worker's log as a session. (Coverage must be off — the preset refuses to run otherwise, since instrumentation changes optimization behavior.)

Full recipes, including SSR apps (Next/Astro via deoptkit/serve) and builds, live in docs/recipes.md.

Inline editor diagnostics (LSP)

deoptkit ci (and the vitest workflow) write .deopt/findings.json. Point the language server at it to get deopt squiggles in your editor:

deoptkit lsp --findings .deopt/findings.json

The server watches the file and republishes on every change, so vitest bench --watch → re-run ci → squiggles update live. It publishes diagnostics at source-mapped original positions with the explanation and suggested fix in the hover; severity maps from the finding score (error ≥ 60, warning ≥ 25, info below), and findings older than the file they annotate downgrade to info with a re-run hint.

Editor wiring. The server speaks standard LSP (initialize + push-model publishDiagnostics), so any editor with a generic LSP client works today:

• Neovim: vim.lsp.start({ name = "deoptkit", cmd = { "npx", "deoptkit", "lsp" }, root_dir = vim.fn.getcwd() })
• Helix / Zed: register a language server with command npx deoptkit lsp and attach it to javascript/typescript.

VSCode has no built-in generic LSP client, so a dedicated deoptkit extension is the tracked next step (BENCHMARKING.md §8, v1.4). Until then, VSCode + Claude Code users get the same findings through the MCP server — the agent reads them via get_findings and reports them in chat, which needs no editor setup at all.

CI regression gating

deoptkit ci bench/parse.bench.mjs             # first run writes .deopt/baselines/…, passes
deoptkit ci bench/parse.bench.mjs             # exit 1 only on NEW structural findings
deoptkit ci --update bench/parse.bench.mjs    # accept current findings as the baseline

Baselines contain identity-only structural findings (no line numbers, severities, or timing), so they survive unrelated edits and noisy CI runners — this is the payoff of deoptkit's structural-vs-timing split. On GitHub Actions, new findings also emit ::warning annotations at their source-mapped positions. Commit .deopt/baselines/ to make "no new megamorphic sites" a guarded invariant.

What the signals mean

Signal	What it tells you
Megamorphic IC	A property/call site saw 5+ object shapes; V8 fell back to generic hash lookups there
Deopt loop	TurboFan repeatedly optimized and discarded the same code — type instability after warm-up
Map churn	One constructor/site produces many hidden classes for conceptually one type (conditional or variably-ordered property init) — the root cause behind polymorphic reads elsewhere
Profile ticks	Where CPU time actually goes; severity ranking weights every other signal by this

Contributing

The project uses Vite+ as a unified toolchain (Oxlint + Oxfmt + tsdown + Vitest) and Bumpy for versioning and release.

vp install           # install dependencies
vp check --fix       # format + lint + typecheck (with autofixes)
vp test              # run Vitest (generates real V8 logs from fixtures/workloads)
node fixtures/generate.mjs   # regenerate fixture logs by hand for inspection

Tests generate real V8 logs at run time by executing the pathological workloads in fixtures/workloads/ under logging flags — nothing is mocked, so a Node upgrade that changes the log format fails loudly. Design and roadmap: docs/SPEC.md and docs/BENCHMARKING.md.

Acknowledgements

deoptkit derives its log-parsing approach from deoptexplorer-vscode (MIT), which in turn incorporates code from V8's tick processor (BSD-3-Clause) and thlorenz/deoptigate (MIT). Portions of src/parser/csv.ts are derived from V8's tools/csvparser.mjs. See THIRD_PARTY_NOTICES.md.

License

Released under the MIT license © Drake Costa.