perf[core]: remove unnecessary model validators (#31238)

* Remove unnecessary cast of id -> str (can do with a field setting)
* Remove unnecessary `set_text` model validator (can be done with a
computed field - though we had to make some changes to the `Generation`
class to make this possible

Before: ~2.4s

Blue circles represent time spent in custom validators :(

<img width="1337" alt="Screenshot 2025-05-14 at 10 10 12 AM"
src="https://github.com/user-attachments/assets/bb4f477f-4ee3-4870-ae93-14ca7f197d55"
/>


After: ~2.2s

<img width="1344" alt="Screenshot 2025-05-14 at 10 11 03 AM"
src="https://github.com/user-attachments/assets/99f97d80-49de-462f-856f-9e7e8662adbc"
/>

We still want to optimize the backwards compatible tool calls model
validator, though I think this might involve breaking changes, so wanted
to separate that into a different PR. This is circled in green.

libs/core/langchain_core/documents/base.py

@@ -8,7 +8,7 @@ from io import BufferedReader, BytesIO
 from pathlib import Path, PurePath
 from typing import TYPE_CHECKING, Any, Literal, Optional, Union, cast
 
-from pydantic import ConfigDict, Field, field_validator, model_validator
+from pydantic import ConfigDict, Field, model_validator
 
 from langchain_core.load.serializable import Serializable
 
@@ -33,7 +33,7 @@ class BaseMedia(Serializable):
     # The ID field is optional at the moment.
     # It will likely become required in a future major release after
     # it has been adopted by enough vectorstore implementations.
-    id: Optional[str] = None
+    id: Optional[str] = Field(default=None, coerce_numbers_to_str=True)
     """An optional identifier for the document.
 
     Ideally this should be unique across the document collection and formatted
@@ -45,17 +45,6 @@ class BaseMedia(Serializable):
     metadata: dict = Field(default_factory=dict)
     """Arbitrary metadata associated with the content."""
 
-    @field_validator("id", mode="before")
-    def cast_id_to_str(cls, id_value: Any) -> Optional[str]:
-        """Coerce the id field to a string.
-
-        Args:
-            id_value: The id value to coerce.
-        """
-        if id_value is not None:
-            return str(id_value)
-        return id_value
-
 
 class Blob(BaseMedia):
     """Blob represents raw data by either reference or value.

libs/core/langchain_core/messages/ai.py

@@ -194,6 +194,7 @@ class AIMessage(BaseMessage):
             "invalid_tool_calls": self.invalid_tool_calls,
         }
 
+    # TODO: remove this logic if possible, reducing breaking nature of changes
     @model_validator(mode="before")
     @classmethod
     def _backwards_compat_tool_calls(cls, values: dict) -> Any:

libs/core/langchain_core/messages/base.py

@@ -4,7 +4,7 @@ from __future__ import annotations
 
 from typing import TYPE_CHECKING, Any, Optional, Union, cast
 
-from pydantic import ConfigDict, Field, field_validator
+from pydantic import ConfigDict, Field
 
 from langchain_core.load.serializable import Serializable
 from langchain_core.utils import get_bolded_text
@@ -52,7 +52,7 @@ class BaseMessage(Serializable):
     model implementation.
     """
 
-    id: Optional[str] = None
+    id: Optional[str] = Field(default=None, coerce_numbers_to_str=True)
     """An optional unique identifier for the message. This should ideally be
     provided by the provider/model which created the message."""
 
@@ -60,13 +60,6 @@ class BaseMessage(Serializable):
         extra="allow",
     )
 
-    @field_validator("id", mode="before")
-    def cast_id_to_str(cls, id_value: Any) -> Optional[str]:
-        """Coerce the id field to a string."""
-        if id_value is not None:
-            return str(id_value)
-        return id_value
-
     def __init__(
         self, content: Union[str, list[Union[str, dict]]], **kwargs: Any
     ) -> None:

libs/core/langchain_core/outputs/chat_generation.py

@@ -4,8 +4,7 @@ from __future__ import annotations
 
 from typing import Literal, Union
 
-from pydantic import model_validator
-from typing_extensions import Self
+from pydantic import computed_field
 
 from langchain_core.messages import BaseMessage, BaseMessageChunk
 from langchain_core.outputs.generation import Generation
@@ -26,48 +25,30 @@ class ChatGeneration(Generation):
     via callbacks).
     """
 
-    text: str = ""
-    """*SHOULD NOT BE SET DIRECTLY* The text contents of the output message."""
     message: BaseMessage
     """The message output by the chat model."""
-    # Override type to be ChatGeneration, ignore mypy error as this is intentional
+
     type: Literal["ChatGeneration"] = "ChatGeneration"  # type: ignore[assignment]
     """Type is used exclusively for serialization purposes."""
 
-    @model_validator(mode="after")
-    def set_text(self) -> Self:
-        """Set the text attribute to be the contents of the message.
-
-        Args:
-            values: The values of the object.
-
-        Returns:
-            The values of the object with the text attribute set.
-
-        Raises:
-            ValueError: If the message is not a string or a list.
-        """
-        try:
-            text = ""
+    @computed_field  # type: ignore[prop-decorator]
+    @property
+    def text(self) -> str:
+        """Set the text attribute to be the contents of the message."""
+        text_ = ""
         if isinstance(self.message.content, str):
-                text = self.message.content
+            text_ = self.message.content
         # Assumes text in content blocks in OpenAI format.
         # Uses first text block.
         elif isinstance(self.message.content, list):
             for block in self.message.content:
                 if isinstance(block, str):
-                        text = block
+                    text_ = block
                     break
                 if isinstance(block, dict) and "text" in block:
-                        text = block["text"]
+                    text_ = block["text"]
                     break
-            else:
-                pass
-            self.text = text
-        except (KeyError, AttributeError) as e:
-            msg = "Error while initializing ChatGeneration"
-            raise ValueError(msg) from e
-        return self
+        return text_
 
 
 class ChatGenerationChunk(ChatGeneration):
@@ -78,7 +59,7 @@ class ChatGenerationChunk(ChatGeneration):
 
     message: BaseMessageChunk
     """The message chunk output by the chat model."""
-    # Override type to be ChatGeneration, ignore mypy error as this is intentional
+
     type: Literal["ChatGenerationChunk"] = "ChatGenerationChunk"  # type: ignore[assignment]
     """Type is used exclusively for serialization purposes."""
 

libs/core/langchain_core/outputs/generation.py

@@ -4,6 +4,8 @@ from __future__ import annotations
 
 from typing import Any, Literal, Optional
 
+from pydantic import computed_field
+
 from langchain_core.load import Serializable
 from langchain_core.utils._merge import merge_dicts
 
@@ -24,14 +26,30 @@ class Generation(Serializable):
     for more information.
     """
 
-    text: str
-    """Generated text output."""
+    def __init__(
+        self,
+        text: str = "",
+        generation_info: Optional[dict[str, Any]] = None,
+        **kwargs: Any,
+    ):
+        """Initialize a Generation."""
+        super().__init__(generation_info=generation_info, **kwargs)
+        self._text = text
+
+    # workaround for ChatGeneration so that we can use a computed field to populate
+    # the text field from the message content (parent class needs to have a property)
+    @computed_field  # type: ignore[prop-decorator]
+    @property
+    def text(self) -> str:
+        """The text contents of the output."""
+        return self._text
 
     generation_info: Optional[dict[str, Any]] = None
     """Raw response from the provider.
 
     May include things like the reason for finishing or token log probabilities.
     """
+
     type: Literal["Generation"] = "Generation"
     """Type is used exclusively for serialization purposes.
     Set to "Generation" for this class."""
@@ -53,6 +71,16 @@ class Generation(Serializable):
 class GenerationChunk(Generation):
     """Generation chunk, which can be concatenated with other Generation chunks."""
 
+    def __init__(
+        self,
+        text: str = "",
+        generation_info: Optional[dict[str, Any]] = None,
+        **kwargs: Any,
+    ):
+        """Initialize a GenerationChunk."""
+        super().__init__(text=text, generation_info=generation_info, **kwargs)
+        self._text = text
+
     def __add__(self, other: GenerationChunk) -> GenerationChunk:
         """Concatenate two GenerationChunks."""
         if isinstance(other, GenerationChunk):