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Value not declarable with JSON Schema (Pydantic V2) #200

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opened 2026-02-16 00:19:20 -05:00 by yindo · 9 comments
yindo commented 2026-02-16 00:19:20 -05:00
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Originally created by @JP-Ellis on GitHub (Jun 19, 2024).

Summary

I am unable to get LangServe to work nicely with a Pydantic V2 model.

Background

I understand that LangChain and LangServe are undergoing an upgrade from Pydantic V1 to V2. I was under the impression that LangServe supports V2 models, with the one caveat that OpenAPI docs aren't presently generated.

Example

Here is a simple reproducible example of the error, along with all installed dependencies and the error logs. The TL;DR is:

  • Dependencies:
    • langchain-core==0.2.9
    • langserve==0.2.2
    • pydantic==2.7.4
  • Error: 
    raise ValueError(f'Value not declarable with JSON Schema, field: {field}')
ValueError: Value not declarable with JSON Schema, field: name='__root__' type=Optional[ExampleInput] required=False default=None
Virtual Environment 
❯ which python
/Users/JP-Ellis/tmp/.venv/bin/python

❯ python --version
Python 3.12.4

❯ uv pip install fastapi langchain_core langserve pydantic sse_starlette
Resolved 55 packages in 36ms
Installed 55 packages in 44ms
 + annotated-types==0.7.0
 + anyio==4.4.0
 + attrs==23.2.0
 + certifi==2024.6.2
 + charset-normalizer==3.3.2
 + click==8.1.7
 + dnspython==2.6.1
 + email-validator==2.1.2
 + fastapi==0.111.0
 + fastapi-cli==0.0.4
 + h11==0.14.0
 + httpcore==1.0.5
 + httptools==0.6.1
 + httpx==0.27.0
 + idna==3.7
 + jinja2==3.1.4
 + jsonpatch==1.33
 + jsonpointer==3.0.0
 + jsonschema==4.22.0
 + jsonschema-specifications==2023.12.1
 + langchain-core==0.2.9
 + langserve==0.2.2
 + langsmith==0.1.81
 + markdown-it-py==3.0.0
 + markupsafe==2.1.5
 + mdurl==0.1.2
 + orjson==3.10.5
 + packaging==24.1
 + pydantic==2.7.4
 + pydantic-core==2.18.4
 + pygments==2.18.0
 + pyproject-toml==0.0.10
 + python-dotenv==1.0.1
 + python-multipart==0.0.9
 + pyyaml==6.0.1
 + referencing==0.35.1
 + requests==2.32.3
 + rich==13.7.1
 + rpds-py==0.18.1
 + setuptools==70.1.0
 + shellingham==1.5.4
 + sniffio==1.3.1
 + sse-starlette==2.1.2
 + starlette==0.37.2
 + tenacity==8.4.1
 + toml==0.10.2
 + typer==0.12.3
 + typing-extensions==4.12.2
 + ujson==5.10.0
 + urllib3==2.2.2
 + uvicorn==0.30.1
 + uvloop==0.19.0
 + watchfiles==0.22.0
 + websockets==12.0
 + wheel==0.43.0
example.py 
from fastapi import APIRouter
from langchain_core.runnables import Runnable, RunnableConfig
from langserve import add_routes
from pydantic import BaseModel

router = APIRouter()


class ExampleInput(BaseModel):
    foo: str


class ExampleOutput(BaseModel):
    bar: str


class ExampleGenerator(Runnable[ExampleInput, ExampleOutput]):
    def invoke(
        self, input: ExampleInput, config: RunnableConfig | None = None
    ) -> ExampleOutput:
        return ExampleOutput(bar=f"Received foo={input.foo}")


add_routes(
    router,
    ExampleGenerator(),
    path="/example",
)
Error logs 
❯ python example.py
Traceback (most recent call last):
  File "/Users/JP-Ellis/tmp/example.py", line 26, in <module>
    add_routes(
  File "/Users/JP-Ellis/tmp/.venv/lib/python3.12/site-packages/langserve/server.py", line 443, in add_routes
    api_handler = APIHandler(
                  ^^^^^^^^^^^
  File "/Users/JP-Ellis/tmp/.venv/lib/python3.12/site-packages/langserve/api_handler.py", line 667, in __init__
    input_type_ = _resolve_model(
                  ^^^^^^^^^^^^^^^
  File "/Users/JP-Ellis/tmp/.venv/lib/python3.12/site-packages/langserve/api_handler.py", line 338, in _resolve_model
    hash_ = model.schema_json()
            ^^^^^^^^^^^^^^^^^^^
  File "/Users/JP-Ellis/tmp/.venv/lib/python3.12/site-packages/pydantic/v1/main.py", line 675, in schema_json
    cls.schema(by_alias=by_alias, ref_template=ref_template), default=pydantic_encoder, **dumps_kwargs
    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/JP-Ellis/tmp/.venv/lib/python3.12/site-packages/pydantic/v1/main.py", line 664, in schema
    s = model_schema(cls, by_alias=by_alias, ref_template=ref_template)
        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/JP-Ellis/tmp/.venv/lib/python3.12/site-packages/pydantic/v1/schema.py", line 188, in model_schema
    m_schema, m_definitions, nested_models = model_process_schema(
                                             ^^^^^^^^^^^^^^^^^^^^^
  File "/Users/JP-Ellis/tmp/.venv/lib/python3.12/site-packages/pydantic/v1/schema.py", line 581, in model_process_schema
    m_schema, m_definitions, nested_models = model_type_schema(
                                             ^^^^^^^^^^^^^^^^^^
  File "/Users/JP-Ellis/tmp/.venv/lib/python3.12/site-packages/pydantic/v1/schema.py", line 622, in model_type_schema
    f_schema, f_definitions, f_nested_models = field_schema(
                                               ^^^^^^^^^^^^^
  File "/Users/JP-Ellis/tmp/.venv/lib/python3.12/site-packages/pydantic/v1/schema.py", line 255, in field_schema
    f_schema, f_definitions, f_nested_models = field_type_schema(
                                               ^^^^^^^^^^^^^^^^^^
  File "/Users/JP-Ellis/tmp/.venv/lib/python3.12/site-packages/pydantic/v1/schema.py", line 527, in field_type_schema
    f_schema, f_definitions, f_nested_models = field_singleton_schema(
                                               ^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/JP-Ellis/tmp/.venv/lib/python3.12/site-packages/pydantic/v1/schema.py", line 951, in field_singleton_schema
    raise ValueError(f'Value not declarable with JSON Schema, field: {field}')
ValueError: Value not declarable with JSON Schema, field: name='__root__' type=Optional[ExampleInput] required=False default=None
Originally created by @JP-Ellis on GitHub (Jun 19, 2024). ## Summary I am unable to get LangServe to work nicely with a Pydantic V2 model. ## Background I understand that LangChain and LangServe are undergoing an upgrade from Pydantic V1 to V2. I was under the impression that LangServe supports V2 models, with the one caveat that OpenAPI docs aren't presently generated. ## Example Here is a simple reproducible example of the error, along with all installed dependencies and the error logs. The TL;DR is: - Dependencies: + `langchain-core==0.2.9` + `langserve==0.2.2` + `pydantic==2.7.4` - Error: ``` raise ValueError(f'Value not declarable with JSON Schema, field: {field}') ValueError: Value not declarable with JSON Schema, field: name='__root__' type=Optional[ExampleInput] required=False default=None ``` <details> <summary>Virtual Environment</summary> ```shell ❯ which python /Users/JP-Ellis/tmp/.venv/bin/python ❯ python --version Python 3.12.4 ❯ uv pip install fastapi langchain_core langserve pydantic sse_starlette Resolved 55 packages in 36ms Installed 55 packages in 44ms + annotated-types==0.7.0 + anyio==4.4.0 + attrs==23.2.0 + certifi==2024.6.2 + charset-normalizer==3.3.2 + click==8.1.7 + dnspython==2.6.1 + email-validator==2.1.2 + fastapi==0.111.0 + fastapi-cli==0.0.4 + h11==0.14.0 + httpcore==1.0.5 + httptools==0.6.1 + httpx==0.27.0 + idna==3.7 + jinja2==3.1.4 + jsonpatch==1.33 + jsonpointer==3.0.0 + jsonschema==4.22.0 + jsonschema-specifications==2023.12.1 + langchain-core==0.2.9 + langserve==0.2.2 + langsmith==0.1.81 + markdown-it-py==3.0.0 + markupsafe==2.1.5 + mdurl==0.1.2 + orjson==3.10.5 + packaging==24.1 + pydantic==2.7.4 + pydantic-core==2.18.4 + pygments==2.18.0 + pyproject-toml==0.0.10 + python-dotenv==1.0.1 + python-multipart==0.0.9 + pyyaml==6.0.1 + referencing==0.35.1 + requests==2.32.3 + rich==13.7.1 + rpds-py==0.18.1 + setuptools==70.1.0 + shellingham==1.5.4 + sniffio==1.3.1 + sse-starlette==2.1.2 + starlette==0.37.2 + tenacity==8.4.1 + toml==0.10.2 + typer==0.12.3 + typing-extensions==4.12.2 + ujson==5.10.0 + urllib3==2.2.2 + uvicorn==0.30.1 + uvloop==0.19.0 + watchfiles==0.22.0 + websockets==12.0 + wheel==0.43.0 ``` </details> <details> <summary><code>example.py</code></summary> ```python from fastapi import APIRouter from langchain_core.runnables import Runnable, RunnableConfig from langserve import add_routes from pydantic import BaseModel router = APIRouter() class ExampleInput(BaseModel): foo: str class ExampleOutput(BaseModel): bar: str class ExampleGenerator(Runnable[ExampleInput, ExampleOutput]): def invoke( self, input: ExampleInput, config: RunnableConfig | None = None ) -> ExampleOutput: return ExampleOutput(bar=f"Received foo={input.foo}") add_routes( router, ExampleGenerator(), path="/example", ) ``` </details> <details> <summary>Error logs</summary> ```text ❯ python example.py Traceback (most recent call last): File "/Users/JP-Ellis/tmp/example.py", line 26, in <module> add_routes( File "/Users/JP-Ellis/tmp/.venv/lib/python3.12/site-packages/langserve/server.py", line 443, in add_routes api_handler = APIHandler( ^^^^^^^^^^^ File "/Users/JP-Ellis/tmp/.venv/lib/python3.12/site-packages/langserve/api_handler.py", line 667, in __init__ input_type_ = _resolve_model( ^^^^^^^^^^^^^^^ File "/Users/JP-Ellis/tmp/.venv/lib/python3.12/site-packages/langserve/api_handler.py", line 338, in _resolve_model hash_ = model.schema_json() ^^^^^^^^^^^^^^^^^^^ File "/Users/JP-Ellis/tmp/.venv/lib/python3.12/site-packages/pydantic/v1/main.py", line 675, in schema_json cls.schema(by_alias=by_alias, ref_template=ref_template), default=pydantic_encoder, **dumps_kwargs ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ File "/Users/JP-Ellis/tmp/.venv/lib/python3.12/site-packages/pydantic/v1/main.py", line 664, in schema s = model_schema(cls, by_alias=by_alias, ref_template=ref_template) ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ File "/Users/JP-Ellis/tmp/.venv/lib/python3.12/site-packages/pydantic/v1/schema.py", line 188, in model_schema m_schema, m_definitions, nested_models = model_process_schema( ^^^^^^^^^^^^^^^^^^^^^ File "/Users/JP-Ellis/tmp/.venv/lib/python3.12/site-packages/pydantic/v1/schema.py", line 581, in model_process_schema m_schema, m_definitions, nested_models = model_type_schema( ^^^^^^^^^^^^^^^^^^ File "/Users/JP-Ellis/tmp/.venv/lib/python3.12/site-packages/pydantic/v1/schema.py", line 622, in model_type_schema f_schema, f_definitions, f_nested_models = field_schema( ^^^^^^^^^^^^^ File "/Users/JP-Ellis/tmp/.venv/lib/python3.12/site-packages/pydantic/v1/schema.py", line 255, in field_schema f_schema, f_definitions, f_nested_models = field_type_schema( ^^^^^^^^^^^^^^^^^^ File "/Users/JP-Ellis/tmp/.venv/lib/python3.12/site-packages/pydantic/v1/schema.py", line 527, in field_type_schema f_schema, f_definitions, f_nested_models = field_singleton_schema( ^^^^^^^^^^^^^^^^^^^^^^^ File "/Users/JP-Ellis/tmp/.venv/lib/python3.12/site-packages/pydantic/v1/schema.py", line 951, in field_singleton_schema raise ValueError(f'Value not declarable with JSON Schema, field: {field}') ValueError: Value not declarable with JSON Schema, field: name='__root__' type=Optional[ExampleInput] required=False default=None ``` </details>
yindo commented 2026-02-16 00:19:21 -05:00
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@eyurtsev commented on GitHub (Jun 20, 2024):

LangChain does not currently support pydantic 2 models. If you're using pydantic 2, you need to use the pydantic.v1 namespace

Do this in your code;

from pydantic.v1 import BaseModel
@eyurtsev commented on GitHub (Jun 20, 2024): LangChain does not currently support pydantic 2 models. If you're using pydantic 2, you need to use the pydantic.v1 namespace Do this in your code; ```python from pydantic.v1 import BaseModel ```
yindo commented 2026-02-16 00:19:21 -05:00
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@JP-Ellis commented on GitHub (Jun 26, 2024):

Is there a tracking issue for when LangServe will support Pydantic 2? Or is there a way to use the pydantic.v1 namespace in Pydantic V2 installations?

I'm asking because I have found that the most recent release of LangChain no longer support Pydantic V1 at all due to this commit which introduced the following change to the langchain-core dependency:

pydantic = [
  { version = ">=1,<3", python =  "<3.12.4" },
  { version = "^2.7.4", python = ">=3.12.4" },
]
@JP-Ellis commented on GitHub (Jun 26, 2024): Is there a tracking issue for when LangServe will support Pydantic 2? Or is there a way to use the `pydantic.v1` namespace in Pydantic V2 installations? I'm asking because I have found that the most recent release of LangChain no longer support Pydantic V1 at all due to [this commit](https://github.com/langchain-ai/langchain/commit/093ae04d58371a7df1a5b217847d671299dd4449) which introduced the following change to the `langchain-core` dependency: ```toml pydantic = [ { version = ">=1,<3", python = "<3.12.4" }, { version = "^2.7.4", python = ">=3.12.4" }, ] ```
yindo commented 2026-02-16 00:19:21 -05:00
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@eyurtsev commented on GitHub (Jun 26, 2024):

Use pydantic.v1 right now with pydantic 2. We first need to migrate langchain to support pydantic 2 proper (rather than from the v1 namespace).

@eyurtsev commented on GitHub (Jun 26, 2024): Use pydantic.v1 right now with pydantic 2. We first need to migrate langchain to support pydantic 2 proper (rather than from the v1 namespace).
yindo commented 2026-02-16 00:19:21 -05:00
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@JP-Ellis commented on GitHub (Jun 26, 2024):

Unfortunately, using the pydantic.v1 namespace does not work for me at the moment, due to some mix up between V1 and V2 functions deep in dependencies which I would rather avoid monkey-patching. Happy to raise another issue to track that specifically if you want.

@JP-Ellis commented on GitHub (Jun 26, 2024): Unfortunately, using the `pydantic.v1` namespace does _not_ work for me at the moment, due to some mix up between V1 and V2 functions deep in dependencies which I would rather avoid monkey-patching. Happy to raise another issue to track that specifically if you want.
yindo commented 2026-02-16 00:19:21 -05:00
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@eyurtsev commented on GitHub (Jun 27, 2024):

Is it coming from langchain dependencies or another dependency?

@eyurtsev commented on GitHub (Jun 27, 2024): Is it coming from langchain dependencies or another dependency?
yindo commented 2026-02-16 00:19:22 -05:00
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@JP-Ellis commented on GitHub (Jul 3, 2024):

In my case, it comes from the underlying FastAPI dependency. I am getting the same error as reported in:

Let me see if I can generate a minimal example within LangChain's context.

EDIT: On investigating the issue, I have confirmed it is identical to the example documented above. This does raise another question though: if FastAPI doesn't support Pydantic V1 models, how is LangServe able to avoid this very issue?

Having said all this, I am happy for you to close this issue (or keep it open for visibility, until Pydantic V2 is fully supported?)

@JP-Ellis commented on GitHub (Jul 3, 2024): In my case, it comes from the underlying `FastAPI` dependency. I am getting the same error as reported in: - [ ] tiangolo/fastapi#10360 Let me see if I can generate a minimal example within LangChain's context. EDIT: On investigating the issue, I have confirmed it is identical to the example documented above. This does raise another question though: if FastAPI doesn't support Pydantic V1 models, how is LangServe able to avoid this very issue? Having said all this, I am happy for you to close this issue (or keep it open for visibility, until Pydantic V2 is fully supported?)
yindo commented 2026-02-16 00:19:22 -05:00
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@codekiln commented on GitHub (Aug 4, 2024):

Just now saw this issue, which is very similar to the one I just filed - https://github.com/langchain-ai/langserve/issues/725. In my case, the only reason I upgraded was that I need to update langchain-core, which was holding back many of my other lang* libraries. In order to do that, I needed to update pydantic to 2.x. I tried converting all models to from langchain.pydantic_v1 import ..., but then I discovered FastAPI was incompatible with having 2.x installed and using 1.x models; specifically, I was getting fastapi basemodel.validate() takes 2 positional arguments but 3 were given in my middleware on every request when I tried that.

@codekiln commented on GitHub (Aug 4, 2024): Just now saw this issue, which is very similar to the one I just filed - https://github.com/langchain-ai/langserve/issues/725. In my case, the only reason I upgraded was that I need to update `langchain-core`, which was holding back many of my other `lang*` libraries. In order to do that, I needed to update pydantic to `2.x`. I tried converting all models to `from langchain.pydantic_v1 import ...`, but then I discovered FastAPI was incompatible with having 2.x installed and using 1.x models; specifically, I was getting `fastapi basemodel.validate() takes 2 positional arguments but 3 were given` in my middleware on every request when I tried that.
yindo commented 2026-02-16 00:19:22 -05:00
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@eyurtsev commented on GitHub (Aug 7, 2024):

Hi this is a documented limitation currently. Please hold off until LangChain 0.3.x. If you want to use pydantic proper 2. You can use pydantic.v1 namespace but FastAPI does not support it.

https://github.com/langchain-ai/langserve?tab=readme-ov-file#pydantic
https://github.com/langchain-ai/langserve?tab=readme-ov-file#limitations
https://python.langchain.com/v0.2/docs/how_to/pydantic_compatibility/#4-langserve-cannot-generate-openapi-docs-if-running-pydantic-2

@eyurtsev commented on GitHub (Aug 7, 2024): Hi this is a documented limitation currently. Please hold off until LangChain 0.3.x. If you want to use pydantic proper 2. You can use pydantic.v1 namespace but FastAPI does not support it. https://github.com/langchain-ai/langserve?tab=readme-ov-file#pydantic https://github.com/langchain-ai/langserve?tab=readme-ov-file#limitations https://python.langchain.com/v0.2/docs/how_to/pydantic_compatibility/#4-langserve-cannot-generate-openapi-docs-if-running-pydantic-2
yindo commented 2026-02-16 00:19:22 -05:00
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@Abd-elr4hman commented on GitHub (Aug 19, 2024):

how can one help with moving langchain to pydantic v2 ? can you point me towards a good starting point to get started ?
@eyurtsev

@Abd-elr4hman commented on GitHub (Aug 19, 2024): how can one help with moving langchain to pydantic v2 ? can you point me towards a good starting point to get started ? @eyurtsev
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Reference: langchain-ai/langserve#200
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