upstash/context7 · error · Error

Unsupported TOML escape \\${escape}

Error message

Unsupported TOML escape \\${escape}

What it means

The parser hit a backslash escape inside a double-quoted TOML string that it does not support. Only the standard TOML escapes (b, t, n, f, r, quote, backslash) plus \u and \U unicode escapes are handled; anything else (e.g. \x, \0, \d) is rejected.

Source

Thrown at packages/cli/src/setup/toml-editor.ts:54

  let index = start + 1;
  while (index < source.length) {
    const char = source[index++];
    if (char === '"') return { value, start, end: index };
    if (char === "\n" || char === "\r") {
      throw new Error("Multiline strings are not supported in MCP args");
    }
    if (char !== "\\") {
      value += char;
      continue;
    }

    const escape = source[index++];
    if (Object.hasOwn(TOML_BASIC_ESCAPES, escape)) {
      value += TOML_BASIC_ESCAPES[escape];
      continue;
    }
    if (escape !== "u" && escape !== "U") {
      throw new Error(`Unsupported TOML escape \\${escape}`);
    }

    const width = escape === "u" ? 4 : 8;
    const hex = source.slice(index, index + width);
    if (!new RegExp(`^[0-9A-Fa-f]{${width}}$`).test(hex)) {
      throw new Error(`Invalid TOML Unicode escape \\${escape}${hex}`);
    }
    const codePoint = Number.parseInt(hex, 16);
    if (codePoint > 0x10ffff || (codePoint >= 0xd800 && codePoint <= 0xdfff)) {
      throw new Error(`Invalid TOML Unicode code point U+${hex}`);
    }
    value += String.fromCodePoint(codePoint);
    index += width;
  }
  throw new Error("Unterminated TOML basic string in MCP args");
}

function parseTomlLiteralString(source: string, start: number): TomlStringToken {

View on GitHub (pinned to edc9eeb77b)

Solutions

1. Double the backslash for literal backslashes: "C:\\Users\\me"
2. Use only supported escapes: \n \t \r \f \b \" \\ or \uXXXX / \UXXXXXXXX
3. For regexes, use a literal single-quoted TOML string ('...') where backslashes are literal

Example fix

// before
args = ["--path", "C:\Users\me"]
// after
args = ["--path", "C:\\Users\\me"]

const ALLOWED = ['b','t','n','f','r','"','\\\\','u','U'];
const re = /\\([^btnfr"\\uU])/g;
if (re.test(str)) throw new Error('Unsupported escape: ' + re.exec(str)?.[1]);

function hasOnlySupportedTomlEscapes(s: string): boolean {
  return !/\\[^btnfr"\\uU]/.test(s);
}

catch (e) { if (/Unsupported TOML escape/.test(e.message)) { /* rewrite escapes or use literal string */ } throw e; }

When it happens

Trigger: A double-quoted args string containing an escape like "\x41", "\d+", or a Windows-style path written as "C:\Users" (where \U is treated as a unicode escape start or fails).

Common situations: Writing regex patterns or Windows paths in args without doubling backslashes; copy-pasting JSON/JS escape syntax into TOML; using \x hex escapes that TOML doesn't have.

Related errors

• Multiline strings are not supported in MCP args
• Invalid TOML Unicode escape \\${escape}${hex}
• Invalid TOML Unicode code point U+${hex}
• Expected a TOML array for MCP args
• MCP args must be a TOML array containing only strings

AI-assisted analysis of upstash/context7@edc9eeb77b (2026-08-28). Data as JSON: /api/errors/8c170a8235dfde3c. Report an issue: GitHub.