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/loop.json

@@ -62,7 +62,7 @@
     "lifecycle": "A subagent's own background commands stop when it ends, and its report says so. A session's jobs stop when the session is deleted. A front end asked to exit while jobs run asks the person first; one with nobody to ask waits for the jobs the prompt started, then stops the rest."
   },
   "context": {
-    "about": "Context hygiene (spec 3.0.0, context.md): every measure acts when a result is inserted, and nothing already sent to the model is ever rewritten — changing earlier context throws away the prompt cache from that point. conformance/context.json gives the texts.",
+    "about": "Context hygiene (spec 3.0.0, the prefix from 3.1.0; context.md): every measure acts when a result is inserted, and nothing already sent to the model is ever rewritten — changing earlier context throws away the prompt cache from that point. conformance/context.json gives the texts.",
     "ids": {
       "about": "Every tool result gets an id: `#` and its number in the session, counting from 1, in lowercase hex, at least 4 digits (`#0001`, `#000a`, `#10000`) — unique in the session and never reused (not after a compaction either), so `recall` finds any of them. It is the first thing in the result, in brackets.",
       "digits": 4
@@ -105,6 +105,18 @@
       "line": "Read a file again for its content; recall shows a result by its id.",
       "rows": 50
     },
-    "reasoning": "Earlier turns' reasoning is left to the model's chat template: the harness sends the history as it does, neither stripping reasoning the template keeps nor adding what it drops."
+    "reasoning": "Earlier turns' reasoning is left to the model's chat template: the harness sends the history as it does, neither stripping reasoning the template keeps nor adding what it drops.",
+    "note": {
+      "about": "A note on the next user message (context.md §10, rule 3): what changed since the system prompt was made, after the person's words. Lines in this order, each only when it applies.",
+      "open": "<session-note>",
+      "close": "</session-note>",
+      "date": "Today is {{date}}.",
+      "mode": "The permission mode is now {{mode}}:\n{{mode_text}}",
+      "plan": "The person changed the plan; it is now:\n{{plan}}",
+      "memory": "The person changed what is remembered; it is now:\n{{memory}}",
+      "instructions": "{{path}} changed; it now reads:\n{{text}}",
+      "personality": "The person chose a different personality:\n{{personality}}",
+      "mcp": "Instructions from an MCP server that connected since the session began:\n{{instructions}}"
+    }
   }
 }