# agentation-context — UserPromptSubmit (in dispatcher) → SessionStart

Interactive playground for M104 PR-B (follow-up to the #1234 audit). Same pattern as PR-A (#1825): toggle the `.mcp.json` state and the turn count to see the envelopes side-by-side.

.mcp.json state
Turns simulated

## before Inside prompt/unified-dispatcher

fires file I/O session cost

Wired as `runOnce: true` inside the UPS dispatcher. The runOnce flag is a per-session file probe — runs every prompt, file-flag short-circuits after turn 1. The reminder still re-injects until the flag lands.

## after SessionStart (cached prefix)

fires file I/O session cost

Standalone SessionStart entry (async, 5s). One `.mcp.json` read at session boot; reminder pins to cached prompt prefix. No flag-file dance. PR #1822 made this safe by allow-listing `SessionStart` in the output-guard.