Bun v1.4 Lands After an 11-Day AI Rewrite from Zig to Rust

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2026 is a watershed year for Bun.

Previously, Bun was written in Zig. Although it ran astonishingly fast, the Zig version always had potential memory safety hazards (such as use-after-free issues). After Bun was acquired by Anthropic at the end of 2025, the project officially entered a "full-speed evolution" mode.

The craziest move: in just 11 days, the development team, assisted by Claude Fable 5, directly rewrote Bun from Zig to Rust. This "brutal" upgrade directly led to the release of Bun v1.4 in August 2026—the first production version running entirely on the Rust engine.

1. The Radical Rewrite: The 11-Day Leap from `Zig` to `Rust`

The process of this rewrite was almost like science fiction: 535,496 lines of Zig code became over 1 million lines of Rust code, generating a total of 6,502 commits. To handle this mountain of code, the team even deployed up to 64 Claude agents working simultaneously.

Although this "AI-driven rewrite" sparked considerable controversy in the community (some complained that the generated Rust code contained over 13,000 unsafe blocks, and this epic PR was almost entirely completed by AI without direct human review), the results ultimately silenced the critics.

The benefits of the rewrite are very tangible:

Safer Memory: Rust can avoid the memory overflow and leak risks present in the Zig version at compile time.
Smaller Size: The binary file size shrank by about 20%.
Performance Surge: The memory leak in Bun.build() plummeted from 6745 MB to 609 MB.

In Anthropic's own production environment (specifically the Claude Code project), after switching to the Rust version, the p99 CPU usage dropped directly from 24% to 10%.

2. Feature Explosion: Eliminating Your `node_modules`

Bun's core logic has always been clear: Don't let your package.json become bloated.

Previously, writing a full-stack project might require installing sharp for image processing, puppeteer for automation, pg for database connections, jest for testing, webpack for bundling... Now, Bun v1.4 attempts to make all of these "native".

1. Natively Integrated High-Efficiency APIs

Look at how much is now built into Bun:

Bun.Image: Image processing, 7 times faster than sharp.
Bun.WebView: No need for Playwright or Puppeteer; directly calls the system kernel's browser for headless testing.
Bun.SQL: Built-in SQL client, perfectly supporting PostgreSQL, MySQL, MariaDB, and SQLite.
Bun.cron: Write scheduled tasks directly in the runtime, eliminating the need for node-cron.
Bun.Archive: Built-in compression/decompression, goodbye to tar or archiver.

2. TypeScript Support: `Bun` vs `Node.js`

In 2026's Node.js v24, type stripping has become the default behavior. But if you pursue the ultimate development experience, Bun still has its moat.

Capability	`Bun v1.4`	`Node.js v24+`
Run `.ts` files	✅ `bun app.ts`	✅ `node app.ts`
`JSX` Support	✅ Built-in	❌ Not supported
`Path aliases` (`@app/*`)	✅ Supported (`tsconfig.json`)	❌ Ignored
`Enums`, `namespaces`	✅ Supported	⚠️ Requires experimental flag
Startup Speed	~5.1ms (`Linux`)	~19ms
Built-in Bundler	✅ `bun build`	❌ None

A reminder: Bun does not perform static type checking. Although it can run .ts files directly, if you need to ensure types are completely correct in a CI environment, you still have to run tsc --noEmit as the final step.

3. Benchmarked Performance: How Big Is the Leap?

If the upgrade from v1.3 to v1.4 was a "minor patch," the migration from Zig to Rust is a "qualitative change."

By comparing Bun v1.4 with Node.js, we can see Bun's dominance in startup speed and throughput.

Metric	`Bun v1.4`	`Node.js v26`
Cold Start Speed (`Linux`)	~5.1ms	~19ms
HTTP Throughput	290k req/s	71k req/s

A "dimensional strike" on installation speed:

When testing a T3 Stack (Next.js) project, Bun's installation speed makes npm look very "clunky":

# Dependency installation comparison
# Scenario: Fresh install
npm install     # ~38s
pnpm install    # ~15s
bun install     # 7s

# Scenario: Cached install
npm install     # ~18s
pnpm install    # ~5.5s
bun install     # 2.5s

4. Conclusion: Should You Migrate Your Project to `Bun` in 2026?

After seeing so many updates, my personal advice is: Embrace it as much as possible, but choose the right scenario.

You can use Bun without hesitation in these scenarios:

Local Development and CLI Tools: The startup speed and native TypeScript support are something you can't go back from once you've tried them.
CI/CD Pipelines: Bun install can save you a lot of waiting time; just run bun install --frozen-lockfile to fly through the process.
Full-stack TS Projects: If you don't want to clutter your package.json with fragmented packages like dotenv, jsonc, tar, etc., Bun is currently the best choice.

But also pay attention to the following points:

Experimental Features: For example, HTTP/3 support in Bun.serve() is still experimental; it's advisable to wait and see for production environments.
Ecosystem Compatibility: Although Node.js compatibility has greatly improved, some complex modules that depend on native N-API might still require manual verification.

In short, Bun's goal is not to become "yet another" runtime for Node.js, but to shrink the entire node_modules to a minimum. From this perspective, it wins decisively.

References:

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Bun v1.4 Lands After an 11-Day AI Rewrite from Zig to Rust

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1. The Radical Rewrite: The 11-Day Leap from Zig to Rust

2. Feature Explosion: Eliminating Your node_modules