When stdin is unreadable (e.g., the process was launched without a proper stdin pipe), Claude Code now emits a clear warning and continues rather than failing silently:
"Warning: stdin is unreadable (<reason>), proceeding without piped input. If you piped input, it was not received — pass it as a prompt argument, or check that the process launching Claude Code wires stdin to a pipe or /dev/null."
Evidence
Stdin error handler (search for "pass it as a prompt argument, or check that the process launching Claude Code wires stdin to a pipe or /dev/null")
Strings lifted out of the shipped bundle, so the claim above can be checked against them.