denoland/deno · error · TypeError

ERR_HTTP2_ALTSVC_LENGTH

ERR_HTTP2_ALTSVC_LENGTH

Error message

HTTP/2 ALTSVC frames are limited to 16382 bytes

What it means

An ALTSVC frame's payload is capped at kMaxALTSVC = 2**14 - 2 = 16382 bytes. altsvc() throws ERR_HTTP2_ALTSVC_LENGTH when alt.length plus origin.length (when an origin string is present) exceeds that budget, because the frame cannot carry more without violating the protocol's 16-bit length field.

Source

Thrown at ext/node/polyfills/http2.ts:4628

          "number",
          "URL",
          "object",
        ], originOrStream);
      } else if (origin === "null" || origin.length === 0) {
        throw new ERR_HTTP2_ALTSVC_INVALID_ORIGIN();
      }
    }

    validateString(alt, "alt");
    if (!kQuotedString.test(alt)) {
      throw new ERR_INVALID_CHAR("alt");
    }

    // Max length permitted for ALTSVC
    if (
      (alt.length + (origin !== undefined ? origin.length : 0)) > kMaxALTSVC
    ) {
      throw new ERR_HTTP2_ALTSVC_LENGTH();
    }

    this[kHandle].altsvc(stream, origin || "", alt);
  }

  // Submits an origin frame to be sent.
  origin(...origins) {
    if (this.destroyed) {
      throw new ERR_HTTP2_INVALID_SESSION();
    }

    if (origins.length === 0) {
      return;
    }

    let arr = "";
    let len = 0;
    const count = origins.length;

View on GitHub (pinned to 9ad36f7a2c)

Solutions

1. Split into multiple altsvc() calls — each call is its own frame, so advertise one (or few) alternates per call
2. Trim the origin and alt to what the peer can actually use (typically 'h2=":port"', ~12 bytes)
3. Cap config-driven alt values at a sane length (e.g. 256 bytes) before submission

Example fix

// before
const alt = endpoints.map((e) => `h2="${e.host}:${e.port}"`).join(',');
session.altsvc(alt, origin); // can exceed 16382

// after
for (const e of endpoints) session.altsvc(`h2="${e.host}:${e.port}"`, origin);

const originLen = typeof origin === 'string' ? origin.length : 0;
if (alt.length + originLen > 16382) {
  throw new RangeError('alt + origin exceeds the ALTSVC frame limit of 16382 bytes');
}
session.altsvc(alt, origin);

When it happens

Trigger: A single altsvc call whose alt value and origin together exceed 16382 characters — e.g. advertising many protocols/hosts in one alt string built by concatenation.

Common situations: Generating the alt value from a large service catalog or long list of alternative endpoints; unbounded user-supplied alt-svc config; a loop that appends entries to one alt string instead of issuing one altsvc() per entry.

Related errors

• ERR_HTTP2_ALTSVC_INVALID_ORIGIN
• ERR_INVALID_CHAR
• ERR_HTTP2_ORIGIN_LENGTH
• ERR_INVALID_URL
• ERR_HTTP2_NO_SOCKET_MANIPULATION

AI-assisted analysis of denoland/deno@9ad36f7a2c (2026-08-20). Data as JSON: /api/errors/842059557efb5bb7. Report an issue: GitHub.