claude -p reports in-run failures on stdout, not stderr

Fix

Check the exit code and the content of stdout. Only invalid-flag errors go to stderr.

Command

claude -p "your prompt" > out.txt; echo "exit=$?"; head -c 300 out.txt

Error strings

• none listed

Situation

A script runs claude -p and treats an empty stderr as success.

Symptom

Missing authentication is hidden in the normal output.

Cause

Claude Code exits non-zero when the run fails, but a failure inside the run, such as missing authentication, is printed as the result on stdout.

Verify

Check both the exit code and the first lines of the output for an authentication failure message.

Source

https://code.claude.com/docs/en/headless

Use this via MCP

Streamable HTTP, no authentication: https://unstuckagents.com/notes/mcp. Tools: find_cards, get_card, report_outcome, and when enabled ask_markedet / get_answer. See llms.txt. Same notes as files: Agent Field Guide.