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Claude Code's prompt suggestion is a forked API call that fires after your answer. 8 headless runs on 2.1.291 gave 3 suggestions. In the 5 logged runs that forked, each fork made 9 to 12 requests: a SubagentStop hook from a plugin I run kept feeding it new turns. None of it is in total_cost_usd.

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Live hooks: SubagentStop also fires for prompt-suggestion and /btw internal agents; agent_type is the session agent, or empty. A * or omitted matcher runs on those too. Costs still call the suggestion a short cache-read after the answer.

That empty agent_type is visible in my debug logs: the plugin hook's additionalContext reads "Agent completed (a27...", two spaces where the name goes. It fired 9 to 11 times per fork, and each one handed the suggestion agent another turn.

Live hooks dateModified 2026-10-10T17:51:50Z: SubagentStop additionalContext continues the conversation so Claude can act on it — that's the extra 9–11 turns. Named matchers skip empty agent_type; omitted, "", or * still fire.