Harness spec 3.1.0-draft: the prefix stays still

Measured on 2026-11-05 against a local model's prompt cache: a harness
that rebuilds its system prompt for every request re-reads the whole
session whenever any part changes (a date at midnight: 16,912 tokens,
99 % -> 0 % cached, 139 s). context.md §10: the system prompt is made
once per session and resent; no clock in it; mid-session changes go as
a <session-note> on the next user message (loop.json context.note); the
tool list is fixed for the session, a requires that comes and goes
answers unavailable instead (plan_submit gains requires: plan); §7: the
compaction summary is the next turn of the same conversation, not a
separate request with a pasted transcript; cached tokens are recorded.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

harness/tools/plan_submit.json

@@ -6,6 +6,10 @@
   "exclusive": true,
   "block": "tool",
   "purpose": false,
+  "requires": [
+    "plan"
+  ],
+  "unavailable": "Not in plan mode: plan_submit is how a plan made in plan mode is put to the person for approval. Carry on with the work; keep the plan current with plan_update.",
   "aliases": {
     "cli": [
       "plan_exit"