perf(prompt): load requested skills after a cache point (#1382)

Siblings differ only in the skills they were spawned with, but those came
first in <specialized_knowledge>, so their prompts diverged at 39%. Shared
skills and the catalog now come first, and the requested skills follow a
cache point, so siblings share 93%.

The extra system message takes a fourth Claude breakpoint, so the Bedrock
tool_config one goes: the first system breakpoint already covers the tools.

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

strix/agents/prompt.py

@@ -102,6 +102,16 @@ def render_system_prompt(
             ),
         )
 
+        shared = {
+            name.split("/")[-1]
+            for name in _resolve_skills(
+                requested=None,
+                scan_mode=scan_mode,
+                is_whitebox=is_whitebox,
+                is_root=is_root,
+                is_diff_scoped=is_diff_scoped,
+            )
+        }
         skills_to_load = _resolve_skills(
             requested=skills,
             scan_mode=scan_mode,
@@ -112,8 +122,11 @@ def render_system_prompt(
         skill_content = load_skills(skills_to_load)
         env.globals["get_skill"] = lambda name: skill_content.get(name, "")
 
+        # Skills every agent of this kind loads come first, so siblings share them
+        # as a cached prefix; the ones the caller asked for vary and go after.
         rendered = env.get_template("system_prompt.jinja").render(
-            loaded_skill_names=list(skill_content.keys()),
+            shared_skill_names=[name for name in skill_content if name in shared],
+            requested_skill_names=[name for name in skill_content if name not in shared],
             available_skills=get_available_skills(),
             interactive=interactive,
             is_root=is_root,

strix/agents/prompts/system_prompt.jinja

@@ -493,9 +493,9 @@ Directories:
 Default user: pentester (sudo available)
 </environment>
 
-{% if loaded_skill_names %}
+{% if shared_skill_names %}
 <specialized_knowledge>
-{% for skill_name in loaded_skill_names %}
+{% for skill_name in shared_skill_names %}
 <{{ skill_name }}>
 {{ get_skill(skill_name) }}
 </{{ skill_name }}>
@@ -505,7 +505,7 @@ Default user: pentester (sudo available)
 
 {% if available_skills %}
 <available_skills>
-On-demand specialist skills. Spawn a specialist via `create_agent(skills=[...])`, or pull guidance inline for yourself via `load_skill(skills=[...])`. Anything wrapped in `<specialized_knowledge>` above is already loaded for you.
+On-demand specialist skills. Spawn a specialist via `create_agent(skills=[...])`, or pull guidance inline for yourself via `load_skill(skills=[...])`. Anything wrapped in `<specialized_knowledge>` is already loaded for you.
 
 {% for category, skills in available_skills | dictsort -%}
 {% for skill in skills -%}
@@ -515,6 +515,17 @@ On-demand specialist skills. Spawn a specialist via `create_agent(skills=[...])`
 </available_skills>
 {% endif %}
 
+{% if requested_skill_names %}
+<cache_point>
+<specialized_knowledge>
+{% for skill_name in requested_skill_names %}
+<{{ skill_name }}>
+{{ get_skill(skill_name) }}
+</{{ skill_name }}>
+{% endfor %}
+</specialized_knowledge>
+{% endif %}
+
 {% if include_scope %}
 {% include "scope.jinja" %}
 {% endif %}

strix/core/inputs.py

@@ -312,11 +312,12 @@ def _reasoning_settings(effort: ReasoningEffort) -> ModelSettings:
 def _prompt_cache_extra_args(model_name: str) -> dict[str, Any] | None:
     """LiteLLM ``cache_control_injection_points`` for Claude prompt caching.
 
-    System prompt + rolling last-message breakpoint everywhere; ``tool_config``
-    only on Bedrock Converse (the only route whose LiteLLM transform consumes
-    it — elsewhere it leaks onto the wire and native Anthropic 400s). Unmapped
-    Bedrock models get no points at all: Bedrock rejects the passed-through
-    field outright.
+    A breakpoint on each system message, plus a rolling last-message one. The
+    system prompt is split into up to three messages, which with the last
+    message uses all four breakpoints Claude allows. There is none on
+    ``tool_config``: the tools come before the system prompt, so its first
+    breakpoint caches them too. Unmapped Bedrock models get no points at all:
+    Bedrock rejects the passed-through field outright.
 
     The field is LiteLLM's own, consumed by its transform, so it only goes to
     routes LiteLLM serves. A bare ``claude-...`` name is served by the SDK's
@@ -328,11 +329,12 @@ def _prompt_cache_extra_args(model_name: str) -> dict[str, Any] | None:
     if is_bedrock_route(model_name) and not bedrock_route_supports_prompt_caching(model_name):
         return None
 
-    points: list[dict[str, Any]] = [{"location": "message", "role": "system"}]
-    if is_bedrock_route(model_name):
-        points.append({"location": "tool_config"})
-    points.append({"location": "message", "index": -1})
-    return {"cache_control_injection_points": points}
+    return {
+        "cache_control_injection_points": [
+            {"location": "message", "role": "system"},
+            {"location": "message", "index": -1},
+        ]
+    }
 
 
 def child_initial_input(

tests/test_inputs.py

@@ -70,7 +70,6 @@ def _cache_points(model_name: str) -> Any:
 def test_make_model_settings_enables_prompt_cache_for_bedrock_claude() -> None:
     assert _cache_points("bedrock/global.anthropic.claude-opus-4-8") == [
         {"location": "message", "role": "system"},
-        {"location": "tool_config"},
         {"location": "message", "index": -1},
     ]
 

tests/test_runner_root_prompt.py

@@ -6,6 +6,7 @@ flow through to the root agent's ``build_strix_agent`` call.
 
 from __future__ import annotations
 
+import os
 import types
 from typing import Any
 
@@ -279,6 +280,16 @@ def test_scope_is_rendered_once_at_the_end_of_the_prompt() -> None:
     assert prompt.index("</available_skills>") < prompt.index("SYSTEM-VERIFIED SCOPE")
 
 
+def test_requested_skills_follow_the_shared_prefix() -> None:
+    xss = render_system_prompt(skills=["xss"], include_scope=False)
+    sqli = render_system_prompt(skills=["sql_injection"], include_scope=False)
+
+    shared = os.path.commonprefix([xss, sqli])
+    assert "</available_skills>" in shared
+    assert shared.count("<cache_point>") == 1
+    assert "<xss>" in xss.split("<cache_point>")[1]
+
+
 def test_scope_is_sent_as_its_own_system_message_on_cache_point_routes() -> None:
     settings = make_model_settings(None, model_name="anthropic/claude-sonnet-5-5")
     prompt = render_system_prompt(