fix(cache): kill long-lived prefix layout — system prompt is now byte-static within a session (#24778)

The long-lived prefix-cache layout split the system prompt into stable/
context/volatile blocks and re-derived them on every API call. The
volatile tier (timestamp + memory snapshot + USER profile) ticks per
turn, so the system message bytes mutated mid-conversation and broke
upstream prompt caches (OpenRouter, Nous Portal, Anthropic).

Diagnosed via live wire-format diffing: an 8-turn conversation showed
OLD layout flipping system block[1] sha mid-session at the minute
boundary, dropping cached_tokens to 0 on that turn (cumulative
66.6% vs 83.3% for the single-block layout). Hermes invariant:
history (system + all but the last 1-2 messages) must be static.

Fix: drop the long-lived layout entirely. Single layout everywhere —
system_and_3 with one cached system string built once on first turn,
replayed verbatim on every subsequent turn. Loses cross-session 1h
prefix caching for Claude (the feature that motivated the split), but
within-session caching now actually works on every provider.

Removed:
- run_agent.py: _use_long_lived_prefix_cache flag, _long_lived_cache_ttl,
  _supports_long_lived_anthropic_cache method, the long-lived branch in
  run_conversation, mark_tools_for_long_lived_cache call site
- agent/prompt_caching.py: apply_anthropic_cache_control_long_lived,
  mark_tools_for_long_lived_cache, _mark_system_stable_block helper
- hermes_cli/config.py: prompt_caching.long_lived_prefix and
  prompt_caching.long_lived_ttl config keys
- tests/agent/test_prompt_caching_live.py (entire file)
- tests/agent/test_prompt_caching.py: TestMarkToolsForLongLivedCache,
  TestApplyAnthropicCacheControlLongLived
- tests/run_agent/test_anthropic_prompt_cache_policy.py:
  TestSupportsLongLivedAnthropicCache

Targeted tests: 62/62 pass.

agent/anthropic_adapter.py

@@ -1305,9 +1305,8 @@ def convert_tools_to_anthropic(tools: List[Dict]) -> List[Dict]:
             ),
         }
         # Forward cache_control marker when present on the OpenAI-format
-        # tool dict (set by ``mark_tools_for_long_lived_cache``). Anthropic's
-        # tools array supports cache_control on the last tool to cache the
-        # entire schema cross-session.
+        # tool dict. Anthropic's tools array supports cache_control on the
+        # last tool to cache the entire schema cross-session.
         cache_control = t.get("cache_control")
         if isinstance(cache_control, dict):
             anthropic_tool["cache_control"] = dict(cache_control)

agent/prompt_caching.py

@@ -1,25 +1,15 @@
-"""Anthropic prompt caching strategies.
+"""Anthropic prompt caching strategy.
 
-Two layouts:
-
-* ``system_and_3`` (default, used everywhere except the long-lived path):
-  4 cache_control breakpoints — system prompt + last 3 non-system messages.
-  All at the same TTL (5m or 1h). Reduces input token costs by ~75% on
-  multi-turn conversations within a single session.
-
-* ``prefix_and_2`` (Claude on Anthropic / OpenRouter / Nous Portal):
-  4 breakpoints split across two TTL tiers — tools[-1] (1h) +
-  stable system prefix (1h) + last 2 non-system messages (5m). The
-  long-lived prefix is byte-stable across sessions for a given user
-  config, so every fresh session reads the cached system+tools instead
-  of re-paying for them. Within-session rolling window shrinks from 3
-  messages to 2 to free the breakpoint budget.
+Single layout: ``system_and_3``. 4 cache_control breakpoints — system
+prompt + last 3 non-system messages, all at the same TTL (5m or 1h).
+Reduces input token costs by ~75% on multi-turn conversations within a
+single session.
 
 Pure functions -- no class state, no AIAgent dependency.
 """
 
 import copy
-from typing import Any, Dict, List, Optional
+from typing import Any, Dict, List
 
 
 def _apply_cache_marker(msg: dict, cache_marker: dict, native_anthropic: bool = False) -> None:
@@ -87,115 +77,3 @@ def apply_anthropic_cache_control(
         _apply_cache_marker(messages[idx], marker, native_anthropic=native_anthropic)
 
     return messages
-
-
-def _mark_system_stable_block(
-    messages: List[Dict[str, Any]],
-    long_lived_marker: Dict[str, str],
-) -> bool:
-    """Mark the *first* content block of the system message with the 1h marker.
-
-    The system message is expected to have been split into multiple content
-    blocks beforehand by the caller — block[0] is the cross-session-stable
-    prefix, subsequent blocks carry context files + volatile suffix.
-    Falls back to marking the whole system message as a single block when
-    the message hasn't been split (preserves correctness on the fallback path).
-
-    Returns True when a marker was placed.
-    """
-    if not messages or messages[0].get("role") != "system":
-        return False
-
-    sys_msg = messages[0]
-    content = sys_msg.get("content")
-
-    # Already a list of blocks → mark the first block.
-    if isinstance(content, list) and content:
-        first = content[0]
-        if isinstance(first, dict):
-            first["cache_control"] = long_lived_marker
-            return True
-        return False
-
-    # String content (no split) → cannot place a stable-prefix breakpoint
-    # without changing the byte content.  Caller is responsible for
-    # splitting; if they didn't, fall through to envelope marker so we still
-    # cache *something* for this turn.
-    if isinstance(content, str) and content:
-        sys_msg["content"] = [
-            {"type": "text", "text": content, "cache_control": long_lived_marker}
-        ]
-        return True
-
-    return False
-
-
-def apply_anthropic_cache_control_long_lived(
-    api_messages: List[Dict[str, Any]],
-    long_lived_ttl: str = "1h",
-    rolling_ttl: str = "5m",
-    native_anthropic: bool = False,
-) -> List[Dict[str, Any]]:
-    """Apply prefix_and_2 caching: long-lived stable prefix + rolling window.
-
-    Layout (4 breakpoints total):
-      * Stable system prefix (block[0]) → ``long_lived_ttl`` TTL
-      * Last 2 non-system messages → ``rolling_ttl`` TTL each
-
-    NOTE: this function does NOT mark the tools array. Tools cache_control
-    is attached separately (see ``mark_tools_for_long_lived_cache``) because
-    tools live outside the messages list in the API payload.
-
-    The caller MUST have split the system message into ordered content
-    blocks where block[0] is the cross-session-stable portion. If the system
-    message is still a single string, it is wrapped into a single block and
-    marked — this is correct, just less effective (the volatile suffix is
-    not isolated, so the prefix invalidates per-session).
-
-    Returns:
-        Deep copy of messages with cache_control breakpoints injected.
-    """
-    messages = copy.deepcopy(api_messages)
-    if not messages:
-        return messages
-
-    long_marker = _build_marker(long_lived_ttl)
-    rolling_marker = _build_marker(rolling_ttl)
-
-    placed_prefix = _mark_system_stable_block(messages, long_marker)
-
-    # Reserve 1 breakpoint for the system prefix (when placed); spend the
-    # remaining 3 on the rolling tail.  Anthropic max is 4 total —
-    # tools[-1] (when marked) consumes the 4th, so we cap rolling at 2 here.
-    rolling_budget = 2 if placed_prefix else 3
-    non_sys = [i for i in range(len(messages)) if messages[i].get("role") != "system"]
-    for idx in non_sys[-rolling_budget:]:
-        _apply_cache_marker(messages[idx], rolling_marker, native_anthropic=native_anthropic)
-
-    return messages
-
-
-def mark_tools_for_long_lived_cache(
-    tools: Optional[List[Dict[str, Any]]],
-    long_lived_ttl: str = "1h",
-) -> Optional[List[Dict[str, Any]]]:
-    """Attach cache_control to the last tool in the OpenAI-format tools list.
-
-    Anthropic prefix-cache order is ``tools → system → messages``.  Marking
-    the last tool dict caches the entire tools array (Anthropic's docs:
-    "the marker is placed on the last block you want included in the cached
-    prefix").  Marker is preserved across the OpenAI-wire boundary on
-    OpenRouter and Nous Portal (which proxies to OpenRouter); on native
-    Anthropic the marker is forwarded by ``convert_tools_to_anthropic``.
-
-    Returns a deep copy of the tools list with the marker attached, or the
-    input unchanged when tools is empty/None.  Pure function — does not
-    mutate the input.
-    """
-    if not tools:
-        return tools
-    out = copy.deepcopy(tools)
-    last = out[-1]
-    if isinstance(last, dict):
-        last["cache_control"] = _build_marker(long_lived_ttl)
-    return out