What causes "Invalid JSON in request body" in coleam00/Archon?

JSON.stringify the payload object and send it as the body with Content-Type: application/json.

Invalid JSON in request body

Error message

Invalid JSON in request body

What it means

The conversation-message route expected a JSON body ({ message: string }) but c.req.json() threw because the body is not valid JSON. The server logs message.json_parse_failed with the parse error and conversationId, then returns 400 with this message, indicating a malformed client request rather than a server fault.

Source

Thrown at packages/server/src/routes/api.ts:2928

      }

      const fileEntries = fileList.filter((e): e is File => e instanceof File);
      if (fileEntries.length > 0) {
        const result = await persistUploadedFiles(conversationId, fileEntries);
        if (!result.ok) {
          return c.json({ error: result.error }, result.status);
        }
        savedFiles = result.savedFiles;
        uploadDir = result.uploadDir;
        getLog().info({ conversationId, fileCount: savedFiles.length }, 'message.files_uploaded');
      }
    } else {
      let body: { message?: unknown };
      try {
        body = await c.req.json();
      } catch (parseErr: unknown) {
        getLog().warn({ err: parseErr, conversationId }, 'message.json_parse_failed');
        return c.json({ error: 'Invalid JSON in request body' }, 400);
      }

      if (typeof body.message !== 'string' || !body.message) {
        return c.json({ error: 'message must be a non-empty string' }, 400);
      }
      message = body.message;
    }

    // Look up conversation for message persistence
    let conv: Awaited<ReturnType<typeof conversationDb.findConversationByPlatformId>> = null;
    try {
      conv = await conversationDb.findConversationByPlatformId(conversationId);
    } catch (e: unknown) {
      getLog().error({ err: e, conversationId }, 'conversation_lookup_failed');
    }

    // Persist user message and pass DB ID to adapter for assistant message persistence
    if (conv) {

View on GitHub (pinned to 0773b97458)

Solutions

JSON.stringify the payload object and send it as the body with Content-Type: application/json.
Validate the body parses locally (JSON.parse in a test) before sending.
Check for body truncation through proxies/timeouts on large messages.
If the payload includes files, switch to the multipart variant of the route instead of embedding binary data in JSON.

Example fix

// before
await fetch(url, { method: 'POST', headers: { 'Content-Type': 'application/json' }, body: `{ message: "${msg}" }` });
// after
await fetch(url, { method: 'POST', headers: { 'Content-Type': 'application/json' }, body: JSON.stringify({ message: msg }) });

Defensive patterns

Strategy: validation

Validate before calling

function buildMessageBody(message: string): string {
  if (typeof message !== 'string' || !message) throw new Error('message must be a non-empty string');
  return JSON.stringify({ message }); // validate locally what the route expects
}
const body = buildMessageBody(msg); // JSON.parse(body) round-trips safely

Type guard

function isValidMessageBody(raw: string): boolean {
  try {
    const parsed: unknown = JSON.parse(raw);
    return typeof (parsed as { message?: unknown })?.message === 'string' && (parsed as { message: string }).message.length > 0;
  } catch { return false; }
}

Try / catch

try {
  const res = await sendMessage(body);
  if (res.status === 400) {
    const { error } = await res.json();
    if (error === 'Invalid JSON in request body') throw new Error('Client sent non-JSON body; check JSON.stringify/content-type');
  }
} catch (err) { /* surface client-side serialization bug */ }

Prevention

Always JSON.stringify request objects; never interpolate strings into JSON templates.
Send Content-Type: application/json together with a stringified body.
Validate the payload with JSON.parse in tests before shipping the client call.
Use the multipart route for file payloads instead of embedding binary in JSON.

When it happens

Trigger: POSTing to the conversation message endpoint with Content-Type application/json but a body that is empty, truncated, or invalid JSON (unquoted keys, trailing commas, plain text, binary data).

Common situations: Client sending form-encoded or raw-text body while claiming JSON content type; a proxy/CDN truncating the body; string-template code building JSON without escaping; forgetting JSON.stringify on the payload.

Understand the failure class

Parsing and encoding errors: unexpected token, malformed input — why parsers reject input and how to find the real culprit.

Related errors

AI-assisted analysis of coleam00/Archon@0773b97458 (2026-09-01). Data as JSON: /api/errors/434554c877397da6. Report an issue: GitHub.