Bun.serve: reject a WebSocket idleTimeout outside 0..960 instead of truncating it to 16 bits

[robobun]

Problem

• Bun.serve({ websocket: { idleTimeout } }) cast the value to u16 before the range check, so idleTimeout: 65537 (18 hours) was accepted and the server closed idle clients after 8 seconds with WebSocket timed out from inactivity. idleTimeout: 2 ** 31 became 0 and disabled the idle timeout. -1 was accepted as 0.
• The cause is src/runtime/server/WebSocketServerContext.rs:334: value.to_int64().max(0) as u16 ran before if idle_timeout > 960. The HTTP idleTimeout on the same constructor already rejects out-of-range values.

Fix

• Keep the value as an i64 and reject anything outside 0..=960 with websocket expects idleTimeout to be between 0 and 960. The narrowing to u16 happens after the check. In-range values behave as before, including the round-up of 1..7 to 8 seconds.
• The maximum is now documented on the type and in the WebSocket docs.
• Verified: test/js/bun/http/bun-serve-args.test.ts (two new tests; stock bun 1.4.0 fails the first). Also test/js/bun/websocket/websocket-server.test.ts and test/integration/bun-types/bun-types.test.ts.

Background

• idleTimeout is the number of seconds a WebSocket connection may stay silent before uWebSockets closes it. uWS stores it as unsigned short and Bun caps it at 960 seconds (16 minutes), the most its 4-second sweep timer can track per socket.
• Bun's validation for this option runs once at Bun.serve() time in WebSocketServerContext::on_create, which builds the uws::WebSocketBehavior passed to uWS.

Notes

Reproduced on bun 1.4.0 with a raw loopback client that finishes the upgrade and stays silent:

const s = Bun.serve({ port: 0, fetch(r, srv) { srv.upgrade(r); }, websocket: { idleTimeout: 65537, message() {},
  close(ws, code, why) { console.log(((performance.now() - t0) / 1e3).toFixed(1) + "s", code, why); process.exit(0); } } });
const t0 = performance.now();
const c = require("net").connect(s.port, "127.0.0.1", () => c.write("GET / HTTP/1.1\r\nHost: x\r\nUpgrade: websocket\r\nConnection: Upgrade\r\nSec-WebSocket-Key: dGhlIHNhbXBsZSBub25jZQ==\r\nSec-WebSocket-Version: 13\r\n\r\n"));
// 1.4.0: "8.0s 1006 WebSocket timed out from inactivity" (configured: 65537 s)

With this change the same script throws at Bun.serve(). -1, 65537, 2 ** 31 and 961 all throw; 0 and 960 are accepted.

Negative values used to map to 0 (timeout disabled). They now throw. The docs only ever described 0 as the way to disable the timeout.

maxPayloadLength and backpressureLimit in the same function narrow with as u32 without a range check. They are not timeouts and are left alone here.

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no test proof · iteration 0 · platform-specific test(s) that do not run on this machine, deferring to CI, which covers all platforms: test/js/bun/http/bun-serve-args.test.ts

[robobun]

Status: reproduced on bun 1.4.0. A server with websocket: { idleTimeout: 65537 } closed a silent upgraded client after 8.0 s with 1006 WebSocket timed out from inactivity, and idleTimeout: 2 ** 31 was accepted as 0. With this change both throw at Bun.serve(). The new tests in test/js/bun/http/bun-serve-args.test.ts fail on stock 1.4.0 and pass with the debug build.

b909f5f addresses the review threads: the uws bound comment is one line, and the docs and type comment state the 0..960 range, that 0 disables the timeout, and the 1..7 round-up to 8. Waiting for CI.

[robobun]

^{Updated 6:07 AM PT - Aug 22nd, 2026}

✅ @robobun, your commit b909f5f07373ebead87f263e1ad7b0a75aef49b5 passed in Build #103578! 🎉

---

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Walkthrough

The WebSocket idleTimeout range is now limited to 0..=960 seconds. Positive values below 8 seconds remain rounded up to 8 seconds. Tests and documentation describe the updated behavior.

Changes

WebSocket timeout handling

Layer / File(s)	Summary
Timeout validation src/runtime/server/WebSocketServerContext.rs	Rejects idleTimeout values below 0 or above 960. Positive values below 8 remain rounded up to 8 seconds.
Timeout contract tests and documentation test/js/bun/http/bun-serve-args.test.ts, docs/runtime/http/websockets.mdx, packages/bun-types/serve.d.ts	Tests cover invalid values and the 0 and 960 boundaries. Documentation states the maximum, disabled state, and rounding behavior.

Suggested reviewers: alii

🚥 Pre-merge checks | ✅ 4

✅ Passed checks (4 passed)

Check name	Status	Explanation
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
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Title check	✅ Passed	The title clearly summarizes the primary validation change for WebSocket idleTimeout values.
Description check	✅ Passed	The description explains the problem, fix, behavior, documentation updates, and verification details.

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Actionable comments posted: 1

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Inline comments:
In `@docs/runtime/http/websockets.mdx`:
- Line 278: Update the idleTimeout documentation in
docs/runtime/http/websockets.mdx at lines 278-278 and
packages/bun-types/serve.d.ts at lines 497-498 to state that accepted values are
integers from 0 through 960, values 1 through 7 are rounded up to 8 seconds, and
negative or fractional values are rejected.

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• src/runtime/server/WebSocketServerContext.rs
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[claude]

LGTM — small, well-scoped validation fix.

What was reviewed:

• The i64 range check in WebSocketServerContext.rs: 0..=960 is checked before the as u16 narrow, so 65537, 2³¹, 961, and -1 all correctly reject; the 1..7 → 8 round-up and 0 → disabled paths are preserved.
• Confirmed is_any_int() gates to_int64() so no exception path is skipped.
• New tests cover both the throwing inputs and the 0/960 boundary accepts; docs and .d.ts match the runtime contract.

Extended reasoning...

Overview

This PR fixes silent 16-bit truncation of the WebSocket idleTimeout option in Bun.serve(). The core change in src/runtime/server/WebSocketServerContext.rs keeps the parsed value as i64, range-checks it against 0..=960, and only then narrows to u16. Previously .max(0) as u16 ran first, so 65537 wrapped to 1 (rounded to 8s) and 2**31 wrapped to 0 (timeout disabled). The remaining changes are one docs sentence, one JSDoc paragraph in serve.d.ts, and two new tests in bun-serve-args.test.ts.

Security risks

None. This is input validation on a numeric server config option, tightening it from silent-wrap to explicit-throw. No auth, crypto, filesystem, or network parsing paths touched.

Level of scrutiny

Low-to-medium. The Rust change is ~10 effective lines in a config-parsing block whose neighbors follow the same pattern. The logic is easy to verify by inspection: after !(0..=960).contains(&seconds) rejects, seconds is provably in [0, 960], so seconds as u16 cannot truncate, and the > 0 branch preserves the existing 1..7 → 8 round-up while 0 stays 0. The preceding is_any_int() guard means to_int64() cannot throw here, so no exception check is missing.

Other factors

• The one behavior change beyond the bug fix — negative values now throw instead of silently mapping to 0 — is called out in the PR description and is consistent with the docs, which only ever named 0 as the disable value.
• Tests cover the exact wrap cases from the bug (65537, 2³¹), the boundary just past valid (961), a negative (-1), and both accepted endpoints (0, 960). They follow the existing file's conventions (using server, port: 0).
• Both prior review threads (comment-cop on the long comment, CodeRabbit on docs completeness) are resolved in b909f5f and confirmed by their respective bots. No outstanding reviewer feedback.

[robobun]

I reached the same fix from the fuzz ledger entry on websocket.idleTimeout and stopped before opening a second PR. My branch is here for comparison: main...farm/46fa9507/ws-idle-timeout-range (commit a7a0490).

Differences from this PR, in case any are useful to pick up:

• The out-of-range path throws a RangeError through throw_range_error, so the message carries the received value: The value of "options.websocket.idleTimeout" is out of range. It must be >= 0 and <= 960. Received 65536. This matches the port check in ServerConfig.rs and the S3 option checks.
• The integer check uses is_integer() (Number.isInteger semantics) instead of is_any_int(). is_any_int() is isInt52, so idleTimeout: 2 ** 53 - 1 reports expects idleTimeout to be an integer instead of an out-of-range error.
• The HTTP idleTimeout gets the same treatment (0..=255). Without it Bun.serve({ idleTimeout: -1 }) is still clamped to 0 while websocket: { idleTimeout: -1 } throws.
• 16 cases in test/js/bun/http/bun-serve-args.test.ts fail on bun 1.4.0 and pass with the change, including 65536, 65544, 66497, 2 ** 32, 2 ** 53, 1e21, -1 and -65536.

#36998 is a third open PR for the same wrap, and it also saturates maxPayloadLength and backpressureLimit (u32) instead of wrapping them.