adk web build_graph crashes with LiteLlm model due to non-serializable LiteLLMClient

[LukasKH]

Describe the Bug

When an ADK app uses google.adk.models.lite_llm.LiteLlm as the root agent model, adk web loads the app successfully but the dev builder endpoint crashes when rendering the graph.

The failing endpoint is:

GET /dev/build_graph/{app_name}

The crash happens because LiteLlm contains an internal llm_client: LiteLLMClient field, and ADK's builder serialization path calls model_dump() on the model object. That includes the LiteLLMClient instance, which Pydantic cannot serialize.

Steps to Reproduce

1. Install google-adk with LiteLLM support.
2. Create a minimal agent like this:

from google.adk.agents import Agent
from google.adk.models.lite_llm import LiteLlm

root_agent = Agent(
    name="root_agent",
    model=LiteLlm(model="openrouter/openai/gpt-5.4-mini"),
    instruction="You are a helpful assistant.",
)

3. Run:

adk web

4. Open the dev UI or request:

http://127.0.0.1:8000/dev/build_graph/src

Expected Behavior

/dev/build_graph/{app_name} should return the serialized graph JSON and the dev UI should render normally.

Observed Behavior

/dev/build_graph/{app_name} returns 500 Internal Server Error.

Traceback:

pydantic_core._pydantic_core.PydanticSerializationError: Unable to serialize unknown type: <class 'google.adk.models.lite_llm.LiteLLMClient'>

In my local repro, LiteLlm.model_dump(mode="python", exclude_none=True) includes:

{
    "model": "openrouter/openai/gpt-5.4-mini",
    "llm_client": <google.adk.models.lite_llm.LiteLLMClient object at ...>,
}

Root Cause

The builder endpoint in google/adk/cli/adk_web_server.py serializes agent fields like this:

elif hasattr(value, "model_dump"):
    agent_dict[field_name] = value.model_dump(mode="python", exclude_none=True)

For LiteLlm, that includes llm_client, which is not serializable.

LiteLlm currently defines:

llm_client: LiteLLMClient = Field(default_factory=LiteLLMClient)

so the model shape itself is not safe for this serialization path.

Environment Details

• ADK Library Version: 1.31.0
• Desktop OS: Windows 11
• Python Version: 3.12

Model Information

• Are you using LiteLLM: Yes
• Which model is being used: openrouter/openai/gpt-5.4-mini

Additional Context

The app itself imports and loads correctly. The failure is specific to the ADK Web builder serialization path.

As a local workaround, excluding llm_client from Pydantic serialization fixes the issue, e.g. by marking the field exclude=True in a subclass of LiteLlm.

That suggests the fix likely belongs either in:

1. LiteLlm itself (exclude llm_client from serialization), or
2. the /dev/build_graph serializer (special-case model objects / use a safer fallback path for non-serializable fields).

[ammarfitwalla]

Hi, I'd like to work on this.

I've gone through the root cause -- the fix likely sits in LiteLlm itself, marking llm_client with Field(exclude=True) so it's dropped from model_dump() output before the builder serialization path sees it. Option 2 (fallback in the serializer) could also work as a safety net.

Could you assign this to me? Happy to have a PR up shortly.