Two envelopes, one hook

Three stdin handlers in run-hook.mjs can start a hook. Two of them check whether it already ran. The third set the flag and never read it. Turn both dials on to see what reached stdout.

Stdin conditions

timeout fired first

stdin delivered 0 bytes in 100ms

A slow producer loses the race (#3415). The timeout runs the hook against an empty payload.

payload over the cap

stdin exceeded 512KB

An image paste (#620). The truncation branch runs the hook again.

The asymmetry, in one table

handler	line	guards on stdinClosed?
process.stdin.on('end')	:442	yes, if (!stdinClosed)
process.stdin.on('error')	:459	yes, if (!stdinClosed)
process.stdin.on('data')	:421	NO: sets it, never reads it

Measured across six stdin shapes. Only the two that conjoin a post-timeout arrival with an over-cap payload double-emit, including the multi-chunk variant. Each condition alone was always fine, which is why three green tests sat on top of this.

Why it surfaced now

The defect predates CC 2.1.248. What that release changed is the consequence:

CC version	what happens to `{...}{...}`
2.1.247 and earlier	silently treated as plain text
2.1.248 onward	reported hook error, result discarded

Neither is good. On UserPromptSubmit and SessionStart the plain-text channel is the context channel, so the old behaviour injected a doubled envelope as prompt text. The new behaviour at least says so out loud.