T06

Python works when run by hand but crashes under Task Scheduler or redirection

Symptom

A script that runs fine in a terminal crashes with UnicodeEncodeError when its output is redirected to a file or it runs from Task Scheduler. Printing a check mark (U+2713) is enough.

Check it in 30 seconds

Paste into Windows PowerShell 5.1. It only creates files under your %TEMP% folder. If the last line says REPRODUCED, it happens on your machine.

$d = Join-Path $env:TEMP ('cc-trap-' + (Get-Date -Format 'yyyyMMddHHmmss')); New-Item -ItemType Directory $d -Force | Out-Null
$pyv = & python -c "import sys; print('%d.%d.%d' % sys.version_info[:3])" 2>$null
if (-not $pyv) { 'CANNOT CHECK (python not found, or only the Microsoft Store stub: install Python from python.org)' } else {
"python $pyv"
[IO.File]::WriteAllText("$d\dash.py", 'print("a" + chr(0x2014) + chr(0x2713) + "b")')
python "$d\dash.py" > "$d\out.txt" 2> "$d\err.txt"; $e = $LASTEXITCODE
$err = [IO.File]::ReadAllText("$d\err.txt")
"exit=$e, UnicodeEncodeError=$($err -match 'UnicodeEncodeError')"
if ($e -ne 0 -and $err -match 'UnicodeEncodeError') { 'REPRODUCED' } else { 'NOT REPRODUCED' }
}

Why it happens

On Windows, Python writes Unicode directly to a console, but encodes with the ANSI code page (1252, 932 ...) when output goes to a file or pipe. Characters outside that code page crash it. Python 3.15 (released 2026-10-09) made UTF-8 the default (PEP 686), so this is not expected on 3.15 or later (the author tested 3.13.7, not 3.15; the check prints your Python version first).

Fix

Set PYTHONIOENCODING=utf-8 on the sending side. In the app itself, call sys.stdout.reconfigure(encoding='utf-8') when stdout is not a terminal.

$env:PYTHONIOENCODING = 'utf-8'

On the author's machine

UnicodeEncodeError, exit 1

Author's machine: Windows 11 / Windows PowerShell 5.1.26100 / Python 3.13.7 / Claude Code 2.1.304, Japanese Windows (code page 932). Checked 2026-10-20 to 22.

If you searched for

UnicodeEncodeError charmap codec can't encode character
python works manually fails in task scheduler
python redirect output unicode error windows

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