fix: stabilize system prompt across gateway turns for cache hits

Two changes to prevent unnecessary Anthropic prompt cache misses in the
gateway, where a fresh AIAgent is created per user message:

1. Reuse stored system prompt for continuing sessions:
   When conversation_history is non-empty, load the system prompt from
   the session DB instead of rebuilding from disk. The model already has
   updated memory in its conversation history (it wrote it!), so
   re-reading memory from disk produces a different system prompt that
   breaks the cache prefix.

2. Stabilize Honcho context per session:
   - Only prefetch Honcho context on the first turn (empty history)
   - Bake Honcho context into the cached system prompt and store to DB
   - Remove the per-turn Honcho injection from the API call loop

   This ensures the system message is identical across all turns in a
   session. Previously, re-fetching Honcho could return different context
   on each turn, changing the system message and invalidating the cache.

Both changes preserve the existing behavior for compression (which
invalidates the prompt and rebuilds from scratch) and for the CLI
(where the same AIAgent persists and the cached prompt is already
stable across turns).

Tests: 2556 passed (6 new)

run_agent.py

@@ -3092,9 +3092,14 @@ class AIAgent:
             )
             self._iters_since_skill = 0
 
-        # Honcho prefetch: retrieve user context for system prompt injection
+        # Honcho prefetch: retrieve user context for system prompt injection.
+        # Only on the FIRST turn of a session (empty history).  On subsequent
+        # turns the model already has all prior context in its conversation
+        # history, and the Honcho context is baked into the stored system
+        # prompt — re-fetching it would change the system message and break
+        # Anthropic prompt caching.
         self._honcho_context = ""
-        if self._honcho and self._honcho_session_key:
+        if self._honcho and self._honcho_session_key and not conversation_history:
             try:
                 self._honcho_context = self._honcho_prefetch(user_message)
             except Exception as e:
@@ -3112,14 +3117,42 @@ class AIAgent:
         # Built once on first call, reused for all subsequent calls.
         # Only rebuilt after context compression events (which invalidate
         # the cache and reload memory from disk).
+        #
+        # For continuing sessions (gateway creates a fresh AIAgent per
+        # message), we load the stored system prompt from the session DB
+        # instead of rebuilding.  Rebuilding would pick up memory changes
+        # from disk that the model already knows about (it wrote them!),
+        # producing a different system prompt and breaking the Anthropic
+        # prefix cache.
         if self._cached_system_prompt is None:
-            self._cached_system_prompt = self._build_system_prompt(system_message)
-            # Store the system prompt snapshot in SQLite
-            if self._session_db:
+            stored_prompt = None
+            if conversation_history and self._session_db:
                 try:
-                    self._session_db.update_system_prompt(self.session_id, self._cached_system_prompt)
-                except Exception as e:
-                    logger.debug("Session DB update_system_prompt failed: %s", e)
+                    session_row = self._session_db.get_session(self.session_id)
+                    if session_row:
+                        stored_prompt = session_row.get("system_prompt") or None
+                except Exception:
+                    pass  # Fall through to build fresh
+
+            if stored_prompt:
+                # Continuing session — reuse the exact system prompt from
+                # the previous turn so the Anthropic cache prefix matches.
+                self._cached_system_prompt = stored_prompt
+            else:
+                # First turn of a new session — build from scratch.
+                self._cached_system_prompt = self._build_system_prompt(system_message)
+                # Bake Honcho context into the prompt so it's stable for
+                # the entire session (not re-fetched per turn).
+                if self._honcho_context:
+                    self._cached_system_prompt = (
+                        self._cached_system_prompt + "\n\n" + self._honcho_context
+                    ).strip()
+                # Store the system prompt snapshot in SQLite
+                if self._session_db:
+                    try:
+                        self._session_db.update_system_prompt(self.session_id, self._cached_system_prompt)
+                    except Exception as e:
+                        logger.debug("Session DB update_system_prompt failed: %s", e)
 
         active_system_prompt = self._cached_system_prompt
 
@@ -3244,11 +3277,13 @@ class AIAgent:
             # Build the final system message: cached prompt + ephemeral system prompt.
             # The ephemeral part is appended here (not baked into the cached prompt)
             # so it stays out of the session DB and logs.
+            # Note: Honcho context is baked into _cached_system_prompt on the first
+            # turn and stored in the session DB, so it does NOT need to be injected
+            # here.  This keeps the system message identical across all turns in a
+            # session, maximizing Anthropic prompt cache hits.
             effective_system = active_system_prompt or ""
             if self.ephemeral_system_prompt:
                 effective_system = (effective_system + "\n\n" + self.ephemeral_system_prompt).strip()
-            if self._honcho_context:
-                effective_system = (effective_system + "\n\n" + self._honcho_context).strip()
             if effective_system:
                 api_messages = [{"role": "system", "content": effective_system}] + api_messages