websockets/ws · error · Error

None of the extension offers can be accepted

Error message

None of the extension offers can be accepted

What it means

Thrown by `PerMessageDeflate.acceptAsServer()` when none of the client's extension offers satisfy the server's configured constraints. The server walks each offer and rejects it if (a) the server disabled `server_no_context_takeover` but the offer requests it, (b) the offer's `server_max_window_bits` is smaller than what the server requires, or (c) the server pinned `client_max_window_bits` to a number and the offer can't meet it. If every offer is rejected, negotiation fails with this Error.

Solutions

1. Loosen the server's `perMessageDeflate` options (drop `clientMaxWindowBits` lower bound, allow `serverNoContextTakeover`).
2. If compression is not required, disable it entirely with `perMessageDeflate: false` on the server.
3. Catch the error during the handshake (`verifyClient`/`'connection'` setup) and fall back to a no-compression connection or reject the client.

Example fix

// before
const pmd = new PerMessageDeflate({
  isServer: true,
  clientMaxWindowBits: 15,
  serverNoContextTakeover: false
});
pmd.accept([{ server_no_context_takeover: true }]); // throws

// after — align constraints with the offers you expect
const pmd = new PerMessageDeflate({ isServer: true });
pmd.accept([{ client_max_window_bits: true }]);

const perMessageDeflate = new PerMessageDeflate({ isServer: true });
try {
  perMessageDeflate.accept(parsedOffers);
} catch (err) {
  // No offer is compatible with the server's compression policy.
  // Fall back to a connection without compression, or reject the client.
  negotiateWithoutCompression(socket);
}

When it happens

Trigger: Server created with `new WebSocketServer({ perMessageDeflate: { serverNoContextTakeover: false, clientMaxWindowBits: 15 } })` connecting to a client whose offer has `server_no_context_takeover` set or a smaller `client_max_window_bits`; or any combination where the configured constraints are stricter than every offer received.

Common situations: An over-strict server `perMessageDeflate` config that no real client can satisfy; a custom client that sends only incompatible offers; version skew where a peer advertises window bits the server forbids.

Related errors

• Unexpected or invalid parameter "client_max_window_bits"
• Invalid value for parameter
• Parameter " " must have only a single value
• Unexpected parameter "client_no_context_takeover"
• Unknown parameter

AI-assisted analysis of websockets/ws@c791e707ea (2026-08-06). Data as JSON: /api/errors/439c93bee3957e46. Report an issue: GitHub.

Appendix: source

Thrown at lib/permessage-deflate.js:181

        (opts.serverNoContextTakeover === false &&
          params.server_no_context_takeover) ||
        (params.server_max_window_bits &&
          (opts.serverMaxWindowBits === false ||
            (typeof opts.serverMaxWindowBits === 'number' &&
              opts.serverMaxWindowBits > params.server_max_window_bits))) ||
        (typeof opts.clientMaxWindowBits === 'number' &&
          (typeof params.client_max_window_bits === 'number'
            ? opts.clientMaxWindowBits > params.client_max_window_bits
            : !params.client_max_window_bits))
      ) {
        return false;
      }

      return true;
    });

    if (!accepted) {
      throw new Error('None of the extension offers can be accepted');
    }

    if (opts.serverNoContextTakeover) {
      accepted.server_no_context_takeover = true;
    }
    if (opts.clientNoContextTakeover) {
      accepted.client_no_context_takeover = true;
    }
    if (typeof opts.serverMaxWindowBits === 'number') {
      accepted.server_max_window_bits = opts.serverMaxWindowBits;
    }
    if (typeof opts.clientMaxWindowBits === 'number') {
      accepted.client_max_window_bits = opts.clientMaxWindowBits;
    } else if (
      accepted.client_max_window_bits === true ||
      opts.clientMaxWindowBits === false
    ) {
      delete accepted.client_max_window_bits;

View on GitHub (pinned to c791e707ea)