Impossible to resume agents throught fastapi endpoint without "new_message"

Describe the bug
Unable to resume agents without specifying "new_message"

To Reproduce

Create a new session

> curl -X POST -H "Content-Type: application/json" http://localhost:8000/apps/my_app/users/user/sessions
{"id":"a0eccf39-5499-4990-8b09-14a336a3c14e","appName":"my_app","userId":"user","state":{},"events":[],"lastUpdateTime":1767879087.399263}

Start an invocation

> curl -X POST -H "Content-Type: application/json" -H "Accept: text/event-stream" -H "Cache-Control: no-cache" http://localhost:8000/run_sse --data '{"app_name": "my_app", "user_id": "user", "session_id": "a0eccf39-5499-4990-8b09-14a336a3c14e", "new_message": {"role": "user", "parts": [{"text": "Hello !"}]}}'
...
data: {...,"invocationId":"e-8679c93c-6a64-4669-a35a-d05c018cd40a", ...}

Cut the server during the invocation
Restart the adk server
Try to resume the invocation using

> curl -X POST -H "Content-Type: application/json" -H "Accept: text/event-stream" -H "Cache-Control: no-cache" http://localhost:8000/run_sse --data '{"app_name": "my_app", "user_id": "user", "session_id": "a0eccf39-5499-4990-8b09-14a336a3c14e", "invocation_id": "e-8679c93c-6a64-4669-a35a-d05c018cd40a"}'
{"detail":[{"type":"missing","loc":["body","newMessage"],"msg":"Field required","input":{"app_name":"my_app","user_id":"user","session_id":"a0eccf39-5499-4990-8b09-14a336a3c14e","invocation_id":"e-8679c93c-6a64-4669-a35a-d05c018cd40a"}}]}%

Expected behavior
Should resume invocation with given id as stated in the documentation: https://google.github.io/adk-docs/runtime/resume/#resume-a-stopped-workflow.

Desktop (please complete the following information):

OS: Linux
Python version(python -V): 3.10.12
ADK version(pip show google-adk): 1.21.0

Additional context
I think we only need to make Optional "newMessage" in the pydantic model here:

adk-python/src/google/adk/cli/adk_web_server.py

Line 205 in a0b2aac

new_message: types.Content

I'd like to help fix this FastAPI session resume issue. I've been working on the ADK adapters and would like to ensure the session state flow is working correctly for web deployments. Can I take this on?

@wuliang229

Hi @wuliang229, I see you've self-assigned this and moved it to core. I was planning to look into the FastAPI side of this flow. If there are specific parts of the fix you'd like to delegate, or if you'd like me to help with testing the FastAPI integration once the core fix is ready, please let me know!
Otherwise, I'll look for another open issue to help with. Thanks!

@Armadindon

@Armadindon Thanks for reporting!
Making new_message optional in the FastAPI request model fixes this issue. Currently, FastAPI requires new_message because it is defined as a mandatory field, so resume requests that only provide invocation_id fail validation before reaching the resume logic. Resume flows do not represent a new user input, so they should not require a message. Defining new_message as Optional[types.Content] = None allows FastAPI to accept resume requests and correctly pass them to the existing resume implementation, which already works as documented.
Thank you.

@Akshat8510

@Akshat8510 Hi, feel free to raise any PRs that you think would fix this and any other issues that you find yourself interested in solving. Please do follow the ADK contribution guide. The ADK team will promptly review the PRs.

Thanks a lot ! 🙏 Didn't have time to do the change this weekend :)

@Armadindon

Hi @Armadindon , I’ve raised PR #4185 to resolve this. It makes new_message optional in the request model and updates the runner logic to handle resumption gracefully. Ready for review!

How Do you add resumability config to fast_api_app wrapped agent?

Currently there's no flag / config Variable in
[adk-python]/[src]/[google]/[adk]/[cli]/fast_api.py
to add this :

def get_fast_api_app(
    *,
    agents_dir: str,
    session_service_uri: Optional[str] = None,
    session_db_kwargs: Optional[Mapping[str, Any]] = None,
    artifact_service_uri: Optional[str] = None,
    memory_service_uri: Optional[str] = None,
    use_local_storage: bool = True,
    eval_storage_uri: Optional[str] = None,
    allow_origins: Optional[list[str]] = None,
    web: bool,
    a2a: bool = False,
    host: str = "127.0.0.1",
    port: int = 8000,
    url_prefix: Optional[str] = None,
    trace_to_cloud: bool = False,
    otel_to_cloud: bool = False,
    reload_agents: bool = False,
    lifespan: Optional[Lifespan[FastAPI]] = None,
    extra_plugins: Optional[list[str]] = None,
    logo_text: Optional[str] = None,
    logo_image_url: Optional[str] = None,
) -> FastAPI:

@veevortexiq

That’s a great point, @veevortexiq. You're right that get_fast_api_app currently lacks a direct flag to pass resumability_config.

My PR (#4185) specifically addresses the bug where the API schema itself was blocking resume requests (the 422 error), which was the first major hurdle

Regarding your point: currently, resumability is typically defined within the Agent/App definition itself before it’s wrapped. However, adding a flag to get_fast_api_app to make this more accessible via the CLI/Web wrapper is a great candidate for a follow-up feature request. I’d be happy to look into that once this blocker is cleared!

@Akshat8510

Thank you @Akshat8510

fix(web): allow session resume without new message

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Impossible to resume agents throught fastapi endpoint without "new_message" #4100

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Impossible to resume agents throught fastapi endpoint without "new_message" #4100

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Akshat8510 commented on Jan 9, 2026

Akshat8510 commented on Jan 10, 2026

klateefa commented on Jan 12, 2026

wuliang229 commented on Jan 12, 2026

Armadindon commented on Jan 13, 2026

Akshat8510 commented on Jan 20, 2026

veevortexiq commented on Jan 21, 2026

Akshat8510 commented on Jan 21, 2026

Akshat8510 commented on Jan 21, 2026

Akshat8510 commented on Jan 21, 2026

veevortexiq commented on Jan 21, 2026

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