langflow-ai/langflow · error · Error

Invalid JSON format.

Error message

Invalid JSON format.

What it means

Thrown by extractMcpServersFromJson in mcpUtils when the input string cannot be parsed as JSON even after a fallback that wraps it in braces (to support fragment input like '"mcpServers": {...}'). Both JSON.parse attempts failed, so the text is not recoverable JSON and MCP server extraction stops.

Source

Thrown at src/frontend/src/utils/mcpUtils.ts:36

 * Extracts all MCP servers from a JSON string or object.
 * Supports:
 * 1. { mcpServers: { ... } }
 * 2. { ... } (object with server keys)
 * 3. a single server object
 * Returns: Array<MCPServerType> or throws an error.
 */
export function extractMcpServersFromJson(
  json: string | object,
): MCPServerType[] {
  let parsed: any = json;
  if (typeof json === "string") {
    try {
      parsed = JSON.parse(json);
    } catch (_e) {
      try {
        parsed = JSON.parse(`{${json}}`);
      } catch (_e) {
        throw new Error("Invalid JSON format.");
      }
    }
  }

  let serverEntries: [string, any][] = [];

  // Case 1: { mcpServers: { ... } }
  if (
    parsed &&
    typeof parsed === "object" &&
    parsed.mcpServers &&
    typeof parsed.mcpServers === "object"
  ) {
    serverEntries = Object.entries(parsed.mcpServers);
  }
  // Case 2: { ... } (object with server keys)
  else if (
    parsed &&

View on GitHub (pinned to 976ec789d2)

Solutions

1. Paste the text into a JSON linter (editor, jsonlint) and fix syntax errors: trailing commas, single→double quotes, smart→straight quotes
2. Wrap the whole thing in { } once — the helper only tries adding outer braces, not fixing inner structure
3. Prefer exporting real JSON from the MCP client config file instead of re-typing it
4. If passing programmatically, pass an object instead of a string — objects skip parsing entirely

Example fix

// before (trailing comma + single quotes)
const input = "{'mcpServers': {'fs': {'command': 'npx',}},}";

// after
const input = JSON.stringify({
  mcpServers: { fs: { command: "npx" } },
});

function tryParseMcpJson(text: string): object | null {
  try { return JSON.parse(text); } catch { /* fall through */ }
  try { return JSON.parse(`{${text}}`); } catch { return null; }
}
const parsed = tryParseMcpJson(clipboardText);
if (!parsed) warnUserAboutJsonSyntax();

const isJsonObject = (v: unknown): v is Record<string, unknown> =>
  typeof v === "object" && v !== null && !Array.isArray(v);

try {
  const servers = extractMcpServersFromJson(text);
} catch (e) {
  if (e instanceof Error && e.message === "Invalid JSON format.") {
    highlightJsonEditorErrors(); // keep user in the editor
  } else throw e;
}

When it happens

Trigger: Pasting MCP config text with trailing commas, single quotes, comments, smart quotes from a chat/docs page, or unquoted keys — e.g. copying a Claude/mcpServers snippet from prose where curly quotes replaced straight quotes.

Common situations: Copy-paste from websites or chat apps that 'beautify' quotes; hand-typing JSON; trailing commas which strict JSON.parse rejects; missing closing brace that the brace-wrapper cannot fix.

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

• No valid MCP server found in the input.
• Invalid flow data
• Failed to install MCP
• Deployment name is required
• error: invalid JSON in {source}: {exc}

AI-assisted analysis of langflow-ai/langflow@976ec789d2 (2026-08-14). Data as JSON: /api/errors/8d11773ae38bd6b0. Report an issue: GitHub.