API reference

Base64: Decode

Decode Base64 to UTF-8 text.

GET POST /api/base64/decode MCP tool base64_decode

Accepts either alphabet, with or without padding, and ignores whitespace. mode does not change decoding.

Browser version (runs locally): Base64.

Try it
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POST /api/base64/decode with a JSON body

Parameters

Parameter Type Default Description
input required string — Text to convert. As a text/plain POST body it is used verbatim.
mode standard | url standard standard: RFC 4648 §4 — A–Z a–z 0–9 + / with = padding. url: RFC 4648 §5 — - and _ instead of + and /, no padding (JWT style)

Also accepted on every endpoint: format (json | text). Parameters may come from the query string, a JSON object body, an urlencoded or multipart form, or (for input) a text/plain body.

Global parameters
Parameter Type Default Description
format json | text json json returns the documented object. text returns only the primary value as text/plain (see "Text output").

Response 200 application/json

Field Type Description
operation "decode" The direction performed.
mode "standard" | "url" The mode used (the default if none was given).
output string The converted text.

Text output (format=text, text/plain): The converted text only.

Errors

Errors are JSON: {"error": "<message>", "code": "<code>", "param": "<name>"?}.

Status Code When
422 unprocessable_input The input is not Base64, or the decoded bytes are not valid UTF-8 (binary data).
400 missing_parameter A required parameter is absent.
400 unknown_parameter A parameter name is not in the table above (typos are rejected, not ignored).
400 invalid_parameter A value has the wrong type, is out of range, or is not an allowed value.
413 payload_too_large The request body exceeds 1 MiB.
405 method_not_allowed A method other than GET, POST or OPTIONS.

Behaviour

  • Idempotent: the same arguments always return the same result.
  • Requires a client ID and secret (authentication). Each call is recorded as one metadata row against your client ID; inputs and outputs are never stored or logged. Responses carry cache-control: no-store.
  • CORS: any origin may call this endpoint from a browser, using your credentials.

Examples

Responses below were produced by running each request through this code at build time. Long strings are shortened.

Decode a JWT header

curl 'https://util.caruana.me/api/base64/decode?input=eyJhbGciOiJIUzI1NiJ9&mode=url'

Response 200

{
  "operation": "decode",
  "mode": "url",
  "output": "{\"alg\":\"HS256\"}"
}
Same call as an MCP tools/call
{
  "jsonrpc": "2.0",
  "id": 1,
  "method": "tools/call",
  "params": {
    "name": "base64_decode",
    "arguments": {
      "input": "eyJhbGciOiJIUzI1NiJ9",
      "mode": "url"
    }
  }
}