denoland/deno · error · TypeError

Piped stdin is not supported for this function, use 'Deno.Co

Error message

Piped stdin is not supported for this function, use 'Deno.Command.spawn()' instead

What it means

Deno.Command.prototype.output() rejects stdin: "piped" with a TypeError because output() resolves only after the child exits, leaving no way to write to a piped stdin. To feed a process input you must keep a live ChildProcess from spawn() and use its .stdin WritableStream.

Source

Thrown at ext/process/40_process.js:802

      }
      return result.stderr;
    },
  };
  return output;
}

class Command {
  #command;
  #options;

  constructor(command, options) {
    this.#command = command;
    this.#options = options;
  }

  output() {
    if (this.#options?.stdin === "piped") {
      throw new TypeError(
        "Piped stdin is not supported for this function, use 'Deno.Command.spawn()' instead",
      );
    }
    return spawnInner(this.#command, this.#options);
  }

  outputSync() {
    if (this.#options?.stdin === "piped") {
      throw new TypeError(
        "Piped stdin is not supported for this function, use 'Deno.Command.spawn()' instead",
      );
    }
    return spawnSyncInner(this.#command, this.#options);
  }

  spawn() {
    const options = {
      __proto__: null,

View on GitHub (pinned to 9ad36f7a2c)

Solutions

Call cmd.spawn() instead of cmd.output(), write to child.stdin, then await child.output() for the collected result
Drop stdin: "piped" (use "inherit"/"null") if the child needs no input
Split your options: one preset without piped stdin for output(), one with it for spawn()

Example fix

// before
const cmd = new Deno.Command("cat", { stdin: "piped" });
const out = await cmd.output(); // TypeError

// after
const cmd = new Deno.Command("cat", { stdin: "piped", stdout: "piped" });
const child = cmd.spawn();
const w = child.stdin.getWriter();
await w.write(new TextEncoder().encode("hello"));
await w.close();
const out = await child.output();

Defensive patterns

Strategy: validation

Validate before calling

function runCommand(cmd: Deno.Command, wantsStdin: boolean) {
  if (wantsStdin) {
    const child = cmd.spawn();
    return { child, write: child.stdin.getWriter().write.bind(child.stdin.getWriter()) };
  }
  return { out: cmd.output() };
}

Type guard

const needsInteractiveStdin = (o: Deno.CommandOptions | undefined): boolean => o?.stdin === "piped";

Prevention

Decide per call: spawn() for interactive stdin, output() otherwise
Never put stdin: "piped" into shared option presets used by output()
Write to child.stdin, close the writer, then await child.output() for the result

When it happens

Trigger: const cmd = new Deno.Command("cat", { stdin: "piped" }); await cmd.output(); — also any shared options object with stdin: "piped" passed to a Command whose .output() is later called.

Common situations: Building one Command/options object and wanting both interactive writes and a final result; porting from APIs where exec accepted stdin; wrappers that offer either output() or spawn() behind the same options.

Related errors

AI-assisted analysis of denoland/deno@9ad36f7a2c (2026-08-20). Data as JSON: /api/errors/ca4c339b33b2c67e. Report an issue: GitHub.