Bun 1.4.3 Adds bun check, a Built-In TypeScript Type Checker That Bun Says Runs 3x to 6.4x Faster Than tsc 7.0.2

Overview

Bun v1.4.3, published on October 10, 2026 according to the Bun release notes (the GitHub release tag is timestamped 2026-10-10T05:24:54Z), adds bun check, a TypeScript type checker built into the runtime. Bun describes it as a port of typescript-go, the native compiler behind TypeScript 7, which The Machine Herald previously reported reached general availability on July 8. All performance and conformance figures below come from Bun’s own post; this article found no independent replication.

What We Know

What it does. Per the release notes, bun check “reads your tsconfig.json, reports the same errors as tsc, and uses every CPU core,” and the typescript package does not need to be installed. The notes add a limit: “It only type checks. It doesn’t emit JavaScript or .d.ts files, and there’s no language server, so your editor keeps using TypeScript.”

Where it plugs in. According to Bun, a --check flag is available on three commands. bun run --check type checks and then runs, so a type error stops execution. bun test --check checks the test files and everything they import before running tests. bun build --check fails the build on a type error and writes nothing. Bun.build accepts check: true.

Speed and memory (vendor figures). Bun reports that bun check is “3x to 6.4x faster than tsc 7.0.2, on a 16-core Apple silicon Mac.” In its table, the VS Code src tree of 9,795 files took 1.24 seconds against 5.98 seconds for tsc (4.8x), and Next.js packages/next took 0.28 seconds against 1.82 seconds (6.4x). Bun also says it uses “2.2x to 4.9x less memory,” listing 2.14 GB against 7.97 GB for the VS Code tree.

Conformance claims (vendor figures). Bun states that the checker “passes 100% of TypeScript 7.0.2’s conformance test suite,” a total of 51,210 of 51,210 tests. It also reports deliberately misconfiguring 72 open-source repositories and comparing output with tsc: across 1,103 configurations, 1,102 were identical, with one mismatch in vuejs/core. Larger offline fuzzing runs covered 3,032,182 programs with 191 mismatches, of which Bun says 162 were “in code that refers to itself while it’s being declared.” Bun invites users to diff bun check --no-pretty output against tsc --noEmit --pretty false and says “that’s a bug in Bun” if they differ.

Other Changes in the Release

The release notes also list several runtime changes:

• Bun.FetchSession: gives a group of requests their own TLS, proxy and keep-alive settings and their own connection pool, per Bun.
• Proxy handling in fetch(): NO_PROXY now accepts wildcards, CIDR blocks, bare IPv6 addresses and host:port. When a proxy refuses CONNECT with a non-2xx status, fetch() now rejects with ERR_PROXY_TUNNEL; before, it “resolved with the proxy’s reply as if it came from the origin.”
• --disallow-code-generation-from-strings: Bun says it “used to accept the flag and ignore it”; eval() and new Function() now throw.
• Bun.ModuleGraph (experimental): runs many instances of one app in a single process with separate module state. Bun says it “is not a security sandbox.”
• node:http large responses: Bun reports up to 2.7x faster responses over 16 KB. In its four-write 16 KB benchmark, requests per second rose from 6,983 to 19,160, against 17,735 for Node.js 26.3. Bun says node:http is now faster than Node.js in all eight of its large-response benchmarks.
• CompressionStream levels: accepts a level option, for example zstd 1-22.

The post’s section heading also claims “67–89% less CPU while your server sleeps,” but that section contains only an image, so no methodology or baseline could be read.

What We Don’t Know

• No independent benchmark of bun check was located. Every speed, memory and conformance number above is Bun’s, measured on its own hardware and projects.
• Bun’s comparison is against tsc 7.0.2, the native TypeScript compiler. The notes do not compare it with the older JavaScript-based tsc.
• The notes do not say whether editor tooling or other TypeScript API consumers can use bun check; they say only that there is no language server.
• The idle-CPU claim has no readable detail in the text of the release notes.