gchq/CyberChef · error · Error

Schema Error: Schema not defined

Error message

Schema Error: Schema not defined

What it means

Thrown by Protobuf.encode when, after parsing the supplied .proto schema, no main message name could be determined (mainMessageName is null). encode() needs a concrete message type to serialise against; without one it cannot proceed.

Source

Thrown at src/core/lib/Protobuf.mjs:91

     * @param {byteArray} input
     * @returns {number}
     */
    static varIntDecode(input) {
        const pb = new Protobuf(input);
        return pb._varInt();
    }

    /**
     * Encode input JSON according to the given schema
     *
     * @param {Object} input
     * @param {Object []} args
     * @returns {Object}
     */
    static encode(input, args) {
        this.updateProtoRoot(args[0]);
        if (!this.mainMessageName) {
            throw new Error("Schema Error: Schema not defined");
        }
        const message = this.parsedProto.root.nested[this.mainMessageName];

        // Convert input into instance of message, and verify instance
        input = message.fromObject(input);
        const error = message.verify(input);
        if (error) {
            throw new Error("Input Error: " + error);
        }
        // Encode input
        const output = message.encode(input).finish();
        return new Uint8Array(output).buffer;
    }

    /**
     * Parse Protobuf data
     *
     * @param {byteArray} input

View on GitHub (pinned to 4290ea7539)

Solutions

1. Provide a .proto schema that declares at least one top-level `message`.
2. Verify the schema parses and yields a mainMessageName (call Protobuf.updateProtoRoot(protoText) and inspect mainMessageName) before encode.
3. Check that the schema text was not truncated or passed as the wrong arg index.

Example fix

// before
Protobuf.encode(input, ['']);
// after
Protobuf.encode(input, ['syntax = "proto3";\nmessage Person { string name = 1; }']);

Protobuf.updateProtoRoot(protoText);
if (!Protobuf.mainMessageName) {
  throw new Error('Schema has no top-level message; cannot encode.');
}
return Protobuf.encode(input, [protoText]);

function schemaHasMessage(protoText) {
  try {
    protobuf.parse(protoText);
    return /\bmessage\s+\w+/m.test(protoText);
  } catch { return false; }
}

try {
  return Protobuf.encode(input, [protoText]);
} catch (e) {
  if (/Schema not defined/.test(e.message)) {
    // prompt user for a .proto containing at least one top-level message
  }
  throw e;
}

When it happens

Trigger: Calling Protobuf.encode(input, [protoText]) where protoText is empty, contains only enums/services, defines only nested submessages that updateMainMessageName rejects, or where updateMainMessageName found zero top-level protobuf.Type entries.

Common situations: Empty schema string passed from the UI; a .proto that only declares `enum` or `package` with no `message`; schema whose only messages are filtered out; forgotten schema argument.

Related errors

• Input Error: ${error}
• Schema ${error}
• Input ${error}
• Exhausted Buffer
• Unknown type 0x${type.toString(16)}

AI-assisted analysis of gchq/CyberChef@4290ea7539 (2026-08-13). Data as JSON: /api/errors/45143ab7f0cb5c31. Report an issue: GitHub.