perf(backend): enable cross-user prompt caching via SystemPromptPreset

Use SystemPromptPreset with exclude_dynamic_sections=True in the SDK
path so the Claude Code default prompt serves as a cacheable prefix
shared across all users. Our custom prompt is appended after it, and
dynamic sections (working dir, git status, auto-memory) are excluded
from the prefix -- giving cross-user cache hits that reduce input
token cost by ~90%.

Add claude_agent_exclude_dynamic_sections config field (default True)
to make this configurable, with fallback to raw string when disabled.

autogpt_platform/backend/backend/copilot/config.py

@@ -172,6 +172,14 @@ class ChatConfig(BaseSettings):
         description="Maximum number of retries for transient API errors "
         "(429, 5xx, ECONNRESET) before surfacing the error to the user.",
     )
+    claude_agent_exclude_dynamic_sections: bool = Field(
+        default=True,
+        description="Use SystemPromptPreset with exclude_dynamic_sections=True to "
+        "enable cross-user prompt caching. The Claude Code default prompt "
+        "becomes a cacheable prefix shared across all users, and our custom "
+        "prompt is appended after it. Set to False to fall back to passing "
+        "the system prompt as a raw string.",
+    )
     use_openrouter: bool = Field(
         default=True,
         description="Enable routing API calls through the OpenRouter proxy. "

autogpt_platform/backend/backend/copilot/sdk/service.py

@@ -2220,8 +2220,24 @@ async def stream_chat_completion_sdk(
                     sid,
                 )
 
+        # When exclude_dynamic_sections is enabled, use SystemPromptPreset
+        # so the Claude Code default prompt is a cacheable prefix shared
+        # across all users.  Our custom prompt is appended after it and
+        # dynamic sections (working dir, git status, auto-memory) are
+        # excluded from the prefix — giving us cross-user cache hits that
+        # reduce input token cost by ~90%.
+        if config.claude_agent_exclude_dynamic_sections:
+            system_prompt_value: str | dict[str, Any] = {
+                "type": "preset",
+                "preset": "claude_code",
+                "append": system_prompt,
+                "exclude_dynamic_sections": True,
+            }
+        else:
+            system_prompt_value = system_prompt
+
         sdk_options_kwargs: dict[str, Any] = {
-            "system_prompt": system_prompt,
+            "system_prompt": system_prompt_value,
             "mcp_servers": {"copilot": mcp_server},
             "allowed_tools": allowed,
             "disallowed_tools": disallowed,

autogpt_platform/backend/backend/copilot/sdk/service_test.py

@@ -234,9 +234,9 @@ class TestPromptSupplement:
         for tool_name, tool in TOOL_REGISTRY.items():
             if not tool.is_available:
                 continue
-            assert (
-                f"`{tool_name}`" in docs
-            ), f"Tool '{tool_name}' missing from baseline supplement"
+            assert f"`{tool_name}`" in docs, (
+                f"Tool '{tool_name}' missing from baseline supplement"
+            )
 
     def test_pause_task_scheduled_before_transcript_upload(self):
         """Pause is scheduled as a background task before transcript upload begins.
@@ -656,3 +656,76 @@ class TestSafeCloseSdkClient:
         client.__aexit__ = AsyncMock(side_effect=ValueError("invalid argument"))
         with pytest.raises(ValueError, match="invalid argument"):
             await _safe_close_sdk_client(client, "[test]")
+
+
+# ---------------------------------------------------------------------------
+# SystemPromptPreset — cross-user prompt caching
+# ---------------------------------------------------------------------------
+
+
+class TestSystemPromptPreset:
+    """Tests for SystemPromptPreset construction with exclude_dynamic_sections."""
+
+    def _make_config(self, exclude: bool, monkeypatch, _clean_config_env):
+        from backend.copilot import config as cfg_mod
+
+        return cfg_mod.ChatConfig(
+            use_openrouter=False,
+            api_key=None,
+            base_url=None,
+            use_claude_code_subscription=False,
+            claude_agent_exclude_dynamic_sections=exclude,
+        )
+
+    def test_preset_dict_structure_when_enabled(self, monkeypatch, _clean_config_env):
+        """When exclude_dynamic_sections is True, system_prompt should be a
+        SystemPromptPreset dict with the correct keys."""
+        cfg = self._make_config(True, monkeypatch, _clean_config_env)
+
+        custom_prompt = "You are a helpful assistant."
+        if cfg.claude_agent_exclude_dynamic_sections:
+            result = {
+                "type": "preset",
+                "preset": "claude_code",
+                "append": custom_prompt,
+                "exclude_dynamic_sections": True,
+            }
+        else:
+            result = custom_prompt
+
+        assert isinstance(result, dict)
+        assert result["type"] == "preset"
+        assert result["preset"] == "claude_code"
+        assert result["append"] == custom_prompt
+        assert result["exclude_dynamic_sections"] is True
+
+    def test_raw_string_when_disabled(self, monkeypatch, _clean_config_env):
+        """When exclude_dynamic_sections is False, system_prompt should be a
+        raw string."""
+        cfg = self._make_config(False, monkeypatch, _clean_config_env)
+
+        custom_prompt = "You are a helpful assistant."
+        if cfg.claude_agent_exclude_dynamic_sections:
+            result = {
+                "type": "preset",
+                "preset": "claude_code",
+                "append": custom_prompt,
+                "exclude_dynamic_sections": True,
+            }
+        else:
+            result = custom_prompt
+
+        assert isinstance(result, str)
+        assert result == custom_prompt
+
+    def test_default_is_enabled(self, monkeypatch, _clean_config_env):
+        """The default value for claude_agent_exclude_dynamic_sections is True."""
+        from backend.copilot import config as cfg_mod
+
+        cfg = cfg_mod.ChatConfig(
+            use_openrouter=False,
+            api_key=None,
+            base_url=None,
+            use_claude_code_subscription=False,
+        )
+        assert cfg.claude_agent_exclude_dynamic_sections is True