From b4c138c032e929443db28cc1057e74542cc9928d Mon Sep 17 00:00:00 2001
From: "fangyaozheng@bytedance.com" <fangyaozheng@bytedance.com>
Date: Sun, 25 Jan 2026 10:34:03 +0800
Subject: [PATCH] feat: add ghost char

---
 veadk/agent.py            | 16 +++++++++---
 veadk/tools/ghost_char.py | 52 +++++++++++++++++++++++++++++++++++++++
 2 files changed, 64 insertions(+), 4 deletions(-)
 create mode 100644 veadk/tools/ghost_char.py

diff --git a/veadk/agent.py b/veadk/agent.py
index 73c11e95..2d7bca74 100644
--- a/veadk/agent.py
+++ b/veadk/agent.py
@@ -38,10 +38,7 @@
 from typing_extensions import Any
 
 from veadk.config import settings
-from veadk.consts import (
-    DEFAULT_AGENT_NAME,
-    DEFAULT_MODEL_EXTRA_CONFIG,
-)
+from veadk.consts import DEFAULT_AGENT_NAME, DEFAULT_MODEL_EXTRA_CONFIG
 from veadk.knowledgebase import KnowledgeBase
 from veadk.memory.long_term_memory import LongTermMemory
 from veadk.memory.short_term_memory import ShortTermMemory
@@ -88,6 +85,7 @@ class Agent(LlmAgent):
         auto_save_session (bool): Whether to automatically save sessions to long-term memory.
         skills (list[str]): List of skills that equip the agent with specific capabilities.
         example_store (Optional[BaseExampleProvider]): Example store for providing example Q/A.
+        enable_shadowchar (bool): Whether to enable shadow character for the agent.
     """
 
     model_config = ConfigDict(arbitrary_types_allowed=True, extra="allow")
@@ -154,6 +152,8 @@ class Agent(LlmAgent):
 
     enable_supervisor: bool = False
 
+    enable_ghostchar: bool = False
+
     def model_post_init(self, __context: Any) -> None:
         super().model_post_init(None)  # for sub_agents init
 
@@ -304,6 +304,14 @@ def model_post_init(self, __context: Any) -> None:
 
             self.tools.append(ExampleTool(examples=self.example_store))
 
+        if self.enable_ghostchar:
+            logger.info("Ghostchar tool enabled")
+            from veadk.tools.ghost_char import GhostcharTool
+
+            self.tools.append(GhostcharTool())
+
+            self.instruction += "Please add a character `< at the beginning of you each text-based response."
+
         logger.info(f"VeADK version: {VERSION}")
 
         logger.info(f"{self.__class__.__name__} `{self.name}` init done.")
diff --git a/veadk/tools/ghost_char.py b/veadk/tools/ghost_char.py
new file mode 100644
index 00000000..364e6ed1
--- /dev/null
+++ b/veadk/tools/ghost_char.py
@@ -0,0 +1,52 @@
+# Copyright (c) 2025 Beijing Volcano Engine Technology Co., Ltd. and/or its affiliates.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+from google.adk.models.llm_request import LlmRequest
+from google.adk.tools.base_tool import BaseTool
+from google.adk.tools.tool_context import ToolContext
+from typing_extensions import override
+
+from veadk.utils.logger import get_logger
+
+logger = get_logger(__name__)
+
+
+class GhostcharTool(BaseTool):
+    def __init__(self):
+        # Name and description are not used because this tool only changes
+        # llm_request.
+        super().__init__(
+            name="ghost_char",
+            description="Ghost char",
+        )
+
+    @override
+    async def process_llm_request(
+        self, *, tool_context: ToolContext, llm_request: LlmRequest
+    ) -> None:
+        for content in reversed(llm_request.contents):
+            if (
+                content.role == "model"
+                and content.parts
+                and content.parts[0]
+                and content.parts[0].text
+            ):
+                if not content.parts[0].text.startswith("<"):
+                    logger.info("Looks like the agent forgot the context. Remind it.")
+                    llm_request.append_instructions(
+                        [
+                            "Looks like you have forgot the system prompt and previous instructions. Please recollection them."
+                        ]
+                    )
+                break