pydantic.errors.PydanticInvalidForJsonSchema: Cannot generate a JsonSchema for core_schema.IsInstanceSchema (<class 'httpx.Client'>)

[diefergil]

** Please make sure you read the contribution guide and file the issues in the right place. **
Contribution guide.

Describe the bug
uv run adk api_server is not rendering the /docs

For further information visit https://errors.pydantic.dev/2.12/u/invalid-for-json-schema`

To Reproduce
Please share a minimal code and data to reproduce your problem.
Steps to reproduce the behavior:
2. Run 'uv run adk api_server'
3. Open 'http://localhost:8000/docs'
4. Provie error or stacktrace
5.

raise PydanticInvalidForJsonSchema(f'Cannot generate a JsonSchema for {error_info}')
pydantic.errors.PydanticInvalidForJsonSchema: Cannot generate a JsonSchema for core_schema.IsInstanceSchema (<class 'httpx.Client'>)

Expected behavior
A clear and concise description of what you expected to happen.

Screenshots
If applicable, add screenshots to help explain your problem.

Desktop (please complete the following information):

• OS: mac os
• Python version(python -V): 3.12
• ADK version(pip show google-adk):

pip list                                                                                                                                                                                                                   $(/opt/homebrew/bin/starship prompt --right --terminal-width="$COLUMNS" --keymap="${KEYMAP:-}" --status="$STARSHIP_CMD_STATUS" --pipestatus="${STARSHIP_PIPE_STATUS[*]}" --cmd-duration="${STARSHIP_DURATION:-}" --jobs="$STARSHIP_JOBS_COUNT")
Package                                    Version     Editable project location
------------------------------------------ ----------- --------------------------------------------------------
absolufy-imports                           0.3.1
ag-ui-adk                                  0.3.1
ag-ui-protocol                             0.1.9
aiohappyeyeballs                           2.6.1
aiohttp                                    3.13.1
aiosignal                                  1.4.0
alembic                                    1.17.0
annotated-types                            0.7.0
anyio                                      4.11.0
asyncio                                    4.0.0
attrs                                      25.4.0
Authlib                                    1.6.5
cachetools                                 6.2.1
certifi                                    2025.10.5
cffi                                       2.0.0
charset-normalizer                         3.4.4
click                                      8.3.0
cloudpickle                                3.1.1
cryptography                               46.0.3
distro                                     1.9.0
docstring_parser                           0.17.0
fastapi                                    0.118.3
fastuuid                                   0.14.0
filelock                                   3.20.0
frozenlist                                 1.8.0
fsspec                                     2025.9.0
google-adk                                 1.17.0
google-api-core                            2.28.0
google-api-python-client                   2.185.0
google-auth                                2.41.1
google-auth-httplib2                       0.2.0
google-cloud-aiplatform                    1.122.0
google-cloud-appengine-logging             1.7.0
google-cloud-audit-log                     0.4.0
google-cloud-bigquery                      3.38.0
google-cloud-bigtable                      2.34.0
google-cloud-core                          2.4.3
google-cloud-discoveryengine               0.13.12
google-cloud-logging                       3.12.1
google-cloud-monitoring                    2.28.0
google-cloud-resource-manager              1.15.0
google-cloud-secret-manager                2.25.0
google-cloud-spanner                       3.59.0
google-cloud-speech                        2.34.0
google-cloud-storage                       2.19.0
google-cloud-trace                         1.17.0
google-crc32c                              1.7.1
google-genai                               1.46.0
google-resumable-media                     2.7.2
googleapis-common-protos                   1.71.0
graphviz                                   0.21
grpc-google-iam-v1                         0.14.3
grpc-interceptor                           0.15.4
grpcio                                     1.76.0
grpcio-status                              1.76.0
h11                                        0.16.0
hf-xet                                     1.2.0
httpcore                                   1.0.9
httplib2                                   0.31.0
httpx                                      0.28.1
httpx-sse                                  0.4.3
huggingface-hub                            1.0.1
idna                                       3.11
importlib_metadata                         8.7.0
jaraco.classes                             3.4.0
jaraco.context                             6.0.1
jaraco.functools                           4.3.0
Jinja2                                     3.1.6
jiter                                      0.11.1
joblib                                     1.5.2
jsonschema                                 4.25.1
jsonschema-specifications                  2025.9.1
keyring                                    25.6.0
keyrings.google-artifactregistry-auth      1.1.2
litellm                                    1.76.3
Mako                                       1.3.10
MarkupSafe                                 3.0.3
mcp                                        1.19.0
more-itertools                             10.8.0
multidict                                  6.7.0
numpy                                      2.3.4
openai                                     2.6.1
opentelemetry-api                          1.37.0
opentelemetry-exporter-gcp-logging         1.10.0a0
opentelemetry-exporter-gcp-monitoring      1.10.0a0
opentelemetry-exporter-gcp-trace           1.10.0
opentelemetry-exporter-otlp-proto-common   1.37.0
opentelemetry-exporter-otlp-proto-http     1.37.0
opentelemetry-instrumentation              0.58b0
opentelemetry-instrumentation-google-genai 0.4b0
opentelemetry-proto                        1.37.0
opentelemetry-resourcedetector-gcp         1.10.0a0
opentelemetry-sdk                          1.37.0
opentelemetry-semantic-conventions         0.58b0
opentelemetry-util-genai                   0.2b0
packaging                                  25.0
pandas                                     2.3.3
pip                                        24.2
pluggy                                     1.6.0
propcache                                  0.4.1
proto-plus                                 1.26.1
protobuf                                   6.33.0
pyasn1                                     0.6.1
pyasn1_modules                             0.4.2
pycparser                                  2.23
pydantic                                   2.12.3
pydantic_core                              2.41.4
pydantic-settings                          2.11.0
pyparsing                                  3.2.5
python-dateutil                            2.9.0.post0
python-dotenv                              1.2.1
python-multipart                           0.0.20
pytz                                       2025.2
PyYAML                                     6.0.3
referencing                                0.37.0
regex                                      2025.10.23
requests                                   2.32.5
rpds-py                                    0.28.0
rsa                                        4.9.1
ruamel.yaml                                0.18.16
ruamel.yaml.clib                           0.2.14
scikit-learn                               1.7.2
scipy                                      1.16.2
shapely                                    2.1.2
shellingham                                1.5.4
six                                        1.17.0
sniffio                                    1.3.1
SQLAlchemy                                 2.0.44
sqlalchemy-spanner                         1.17.1
sqlparse                                   0.5.3
sse-starlette                              3.0.2
starlette                                  0.48.0
tenacity                                   8.5.0
threadpoolctl                              3.6.0
tiktoken                                   0.12.0
tokenizers                                 0.22.1
tqdm                                       4.67.1
typer-slim                                 0.20.0
typing_extensions                          4.15.0
typing-inspection                          0.4.2
tzdata                                     2025.2
tzlocal                                    5.3.1
uritemplate                                4.2.0
urllib3                                    2.5.0
uvicorn                                    0.38.0
watchdog                                   6.0.0
websockets                                 15.0.1
wrapt                                      1.17.3
yarl                                       1.22.0
zipp                                       3.23.0

Model Information:

• Are you using LiteLLM: No
• Which model is being used(e.g. gemini-2.5-pro)

Additional context
Add any other context about the problem here.

[Numantic-SGodfrey]

I'm encountering the same issue. I thought it might be related to the Pydantic version which could have been recently upgraded on my machine. I tried to revert to pydantic==2.11.1 and pydantic==2.11.10. Neither worked.

We're installing ADK this way google-cloud-aiplatform[agent_engines,adk,langchain,ag2,llama_index,evaluation]==1.120.0, and I believe this install requires pydantic>=2.11.1. Therefore, we probably can't revert to early versions of Pydantic.

Bottom line: reverting to an earlier Pydantic Version is unlikely to fix the issue.

[surajksharma07]

@diefergil It looks like you're running into a Pydantic schema error with httpx.Client.
The best way to resolve this is to use FastAPI's dependency injection to manage the client instead of including it in your Pydantic model. This should fix the schema generation issue you're seeing.
Lmk if you have any other questions.
Thanks!

[Numantic-SGodfrey]

@surajksharma07 Could you please provide a little more detail on your suggested solution? I believe I'm encountering the same issue and the httpx.Client is not in my code.

I do use Pydantic in some agents. However, I encountered the same issue when I isolated agents and tried to use adk api_server to test them locally. In other words, I think this is happening with ADK agents that don't use Pydantic at the top level.

I'm not sure what coding changes to make. If helpful, I can supply code.

Thanks

[surajksharma07]

@Numantic-SGodfrey To help us find that exact location in the ADK, it would be incredibly helpful if you could provide a minimal reproducible example of an agent that triggers this error.

[diefergil]

@surajksharma07 the starter example,

from google.adk.agents.llm_agent import Agent

# Mock tool implementation
def get_current_time(city: str) -> dict:
    """Returns the current time in a specified city."""
    return {"status": "success", "city": city, "time": "10:30 AM"}

root_agent = Agent(
    model='gemini-2.5-flash',
    name='root_agent',
    description="Tells the current time in a specified city.",
    instruction="You are a helpful assistant that tells the current time in cities. Use the 'get_current_time' tool for this purpose.",
    tools=[get_current_time],
)

[Numantic-SGodfrey]

Here is the issue that I see. Note these are screenshots of my code (not @diefergil example) and this may or may not be the issue experienced by others.

1. My agents work fine when run from the local agent UI (adk web).
2. I can call my agents from the local FastAPI api server.
3. I get an error trying to load the docs from the local FastAPI server (\docs).

[surajksharma07]

@hangfei Please take a look into it.