diff --git a/libs/core/langchain_core/messages/utils.py b/libs/core/langchain_core/messages/utils.py
index f78d877e595..8059280044f 100644
--- a/libs/core/langchain_core/messages/utils.py
+++ b/libs/core/langchain_core/messages/utils.py
@@ -29,7 +29,7 @@ from typing import (
     overload,
 )
 
-from pydantic import Discriminator, Field
+from pydantic import Discriminator, Field, Tag
 from typing_extensions import Annotated
 
 from langchain_core.messages.ai import AIMessage, AIMessageChunk
@@ -49,22 +49,26 @@ if TYPE_CHECKING:
     from langchain_core.runnables.base import Runnable
 
 
+def _get_type(v: Any) -> str:
+    return v.type
+
+
 AnyMessage = Annotated[
     Union[
-        AIMessage,
-        HumanMessage,
-        ChatMessage,
-        SystemMessage,
-        FunctionMessage,
-        ToolMessage,
-        AIMessageChunk,
-        HumanMessageChunk,
-        ChatMessageChunk,
-        SystemMessageChunk,
-        FunctionMessageChunk,
-        ToolMessageChunk,
+        Annotated[AIMessage, Tag(tag="ai")],
+        Annotated[HumanMessage, Tag(tag="human")],
+        Annotated[ChatMessage, Tag(tag="chat")],
+        Annotated[SystemMessage, Tag(tag="system")],
+        Annotated[FunctionMessage, Tag(tag="function")],
+        Annotated[ToolMessage, Tag(tag="tool")],
+        Annotated[AIMessageChunk, Tag(tag="AIMessageChunk")],
+        Annotated[HumanMessageChunk, Tag(tag="HumanMessageChunk")],
+        Annotated[ChatMessageChunk, Tag(tag="ChatMessageChunk")],
+        Annotated[SystemMessageChunk, Tag(tag="SystemMessageChunk")],
+        Annotated[FunctionMessageChunk, Tag(tag="FunctionMessageChunk")],
+        Annotated[ToolMessageChunk, Tag(tag="ToolMessageChunk")],
     ],
-    Field(discriminator=Discriminator("type")),
+    Field(discriminator=Discriminator(_get_type)),
 ]
 
 
diff --git a/libs/core/poetry.lock b/libs/core/poetry.lock
index 40ba83b9c64..c31d1a76e3f 100644
--- a/libs/core/poetry.lock
+++ b/libs/core/poetry.lock
@@ -1,4 +1,4 @@
-# This file is automatically @generated by Poetry 1.6.1 and should not be changed by hand.
+# This file is automatically @generated by Poetry 1.8.2 and should not be changed by hand.
 
 [[package]]
 name = "annotated-types"
@@ -278,37 +278,6 @@ files = [
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-    {file = "cffi-1.17.1-cp313-cp313-macosx_11_0_arm64.whl", hash = "sha256:0984a4925a435b1da406122d4d7968dd861c1385afe3b45ba82b750f229811e2"},
-    {file = "cffi-1.17.1-cp313-cp313-manylinux_2_12_i686.manylinux2010_i686.manylinux_2_17_i686.manylinux2014_i686.whl", hash = "sha256:d01b12eeeb4427d3110de311e1774046ad344f5b1a7403101878976ecd7a10f3"},
-    {file = "cffi-1.17.1-cp313-cp313-manylinux_2_17_aarch64.manylinux2014_aarch64.whl", hash = "sha256:706510fe141c86a69c8ddc029c7910003a17353970cff3b904ff0686a5927683"},
-    {file = "cffi-1.17.1-cp313-cp313-manylinux_2_17_ppc64le.manylinux2014_ppc64le.whl", hash = "sha256:de55b766c7aa2e2a3092c51e0483d700341182f08e67c63630d5b6f200bb28e5"},
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-    {file = "cffi-1.17.1-cp39-cp39-win32.whl", hash = "sha256:e31ae45bc2e29f6b2abd0de1cc3b9d5205aa847cafaecb8af1476a609a2f6eb7"},
-    {file = "cffi-1.17.1-cp39-cp39-win_amd64.whl", hash = "sha256:d016c76bdd850f3c626af19b0542c9677ba156e4ee4fccfdd7848803533ef662"},
-    {file = "cffi-1.17.1.tar.gz", hash = "sha256:1c39c6016c32bc48dd54561950ebd6836e1670f2ae46128f67cf49e789c52824"},
 ]
 
 [package.dependencies]
@@ -3070,4 +3039,4 @@ type = ["pytest-mypy"]
 [metadata]
 lock-version = "2.0"
 python-versions = ">=3.9,<4.0"
-content-hash = "f6f4881a490558c029e567af38ee5c6f3c2e050e96edd6fc3e841a75901a9225"
+content-hash = "0e0eec209d5652d6476b0d71393415eea2917f38235ad6e0b09ffd97f5b2ce87"
diff --git a/libs/core/pyproject.toml b/libs/core/pyproject.toml
index c3a06e70a0d..ac38bfad986 100644
--- a/libs/core/pyproject.toml
+++ b/libs/core/pyproject.toml
@@ -37,7 +37,7 @@ jsonpatch = "^1.33"
 PyYAML = ">=5.3"
 packaging = ">=23.2,<25"
 typing-extensions = ">=4.7"
-pydantic = "^2.9"
+pydantic = "^2.7.4"
 
 [tool.poetry.extras]
 
diff --git a/libs/core/tests/unit_tests/prompts/__snapshots__/test_chat.ambr b/libs/core/tests/unit_tests/prompts/__snapshots__/test_chat.ambr
index 7c3c2ffb456..11cec3e3b2e 100644
--- a/libs/core/tests/unit_tests/prompts/__snapshots__/test_chat.ambr
+++ b/libs/core/tests/unit_tests/prompts/__snapshots__/test_chat.ambr
@@ -1349,23 +1349,6 @@
       'history': dict({
         'default': None,
         'items': dict({
-          'discriminator': dict({
-            'mapping': dict({
-              'AIMessageChunk': '#/$defs/AIMessageChunk',
-              'ChatMessageChunk': '#/$defs/ChatMessageChunk',
-              'FunctionMessageChunk': '#/$defs/FunctionMessageChunk',
-              'HumanMessageChunk': '#/$defs/HumanMessageChunk',
-              'SystemMessageChunk': '#/$defs/SystemMessageChunk',
-              'ToolMessageChunk': '#/$defs/ToolMessageChunk',
-              'ai': '#/$defs/AIMessage',
-              'chat': '#/$defs/ChatMessage',
-              'function': '#/$defs/FunctionMessage',
-              'human': '#/$defs/HumanMessage',
-              'system': '#/$defs/SystemMessage',
-              'tool': '#/$defs/ToolMessage',
-            }),
-            'propertyName': 'type',
-          }),
           'oneOf': list([
             dict({
               '$ref': '#/$defs/AIMessage',
@@ -2769,23 +2752,6 @@
     'properties': dict({
       'history': dict({
         'items': dict({
-          'discriminator': dict({
-            'mapping': dict({
-              'AIMessageChunk': '#/$defs/AIMessageChunk',
-              'ChatMessageChunk': '#/$defs/ChatMessageChunk',
-              'FunctionMessageChunk': '#/$defs/FunctionMessageChunk',
-              'HumanMessageChunk': '#/$defs/HumanMessageChunk',
-              'SystemMessageChunk': '#/$defs/SystemMessageChunk',
-              'ToolMessageChunk': '#/$defs/ToolMessageChunk',
-              'ai': '#/$defs/AIMessage',
-              'chat': '#/$defs/ChatMessage',
-              'function': '#/$defs/FunctionMessage',
-              'human': '#/$defs/HumanMessage',
-              'system': '#/$defs/SystemMessage',
-              'tool': '#/$defs/ToolMessage',
-            }),
-            'propertyName': 'type',
-          }),
           'oneOf': list([
             dict({
               '$ref': '#/$defs/AIMessage',
diff --git a/libs/core/tests/unit_tests/runnables/__snapshots__/test_graph.ambr b/libs/core/tests/unit_tests/runnables/__snapshots__/test_graph.ambr
index 34929ad3abd..bf14546547a 100644
--- a/libs/core/tests/unit_tests/runnables/__snapshots__/test_graph.ambr
+++ b/libs/core/tests/unit_tests/runnables/__snapshots__/test_graph.ambr
@@ -1710,23 +1710,6 @@
               'type': 'string',
             }),
             dict({
-              'discriminator': dict({
-                'mapping': dict({
-                  'AIMessageChunk': '#/$defs/AIMessageChunk',
-                  'ChatMessageChunk': '#/$defs/ChatMessageChunk',
-                  'FunctionMessageChunk': '#/$defs/FunctionMessageChunk',
-                  'HumanMessageChunk': '#/$defs/HumanMessageChunk',
-                  'SystemMessageChunk': '#/$defs/SystemMessageChunk',
-                  'ToolMessageChunk': '#/$defs/ToolMessageChunk',
-                  'ai': '#/$defs/AIMessage',
-                  'chat': '#/$defs/ChatMessage',
-                  'function': '#/$defs/FunctionMessage',
-                  'human': '#/$defs/HumanMessage',
-                  'system': '#/$defs/SystemMessage',
-                  'tool': '#/$defs/ToolMessage',
-                }),
-                'propertyName': 'type',
-              }),
               'oneOf': list([
                 dict({
                   '$ref': '#/$defs/AIMessage',
diff --git a/libs/core/tests/unit_tests/runnables/__snapshots__/test_runnable.ambr b/libs/core/tests/unit_tests/runnables/__snapshots__/test_runnable.ambr
index bee55716e86..23bc73ab3a8 100644
--- a/libs/core/tests/unit_tests/runnables/__snapshots__/test_runnable.ambr
+++ b/libs/core/tests/unit_tests/runnables/__snapshots__/test_runnable.ambr
@@ -6855,23 +6855,6 @@
     'properties': dict({
       'history': dict({
         'items': dict({
-          'discriminator': dict({
-            'mapping': dict({
-              'AIMessageChunk': '#/$defs/AIMessageChunk',
-              'ChatMessageChunk': '#/$defs/ChatMessageChunk',
-              'FunctionMessageChunk': '#/$defs/FunctionMessageChunk',
-              'HumanMessageChunk': '#/$defs/HumanMessageChunk',
-              'SystemMessageChunk': '#/$defs/SystemMessageChunk',
-              'ToolMessageChunk': '#/$defs/ToolMessageChunk',
-              'ai': '#/$defs/AIMessage',
-              'chat': '#/$defs/ChatMessage',
-              'function': '#/$defs/FunctionMessage',
-              'human': '#/$defs/HumanMessage',
-              'system': '#/$defs/SystemMessage',
-              'tool': '#/$defs/ToolMessage',
-            }),
-            'propertyName': 'type',
-          }),
           'oneOf': list([
             dict({
               '$ref': '#/$defs/AIMessage',
@@ -7320,23 +7303,6 @@
         'properties': dict({
           'messages': dict({
             'items': dict({
-              'discriminator': dict({
-                'mapping': dict({
-                  'AIMessageChunk': '#/$defs/AIMessageChunk',
-                  'ChatMessageChunk': '#/$defs/ChatMessageChunk',
-                  'FunctionMessageChunk': '#/$defs/FunctionMessageChunk',
-                  'HumanMessageChunk': '#/$defs/HumanMessageChunk',
-                  'SystemMessageChunk': '#/$defs/SystemMessageChunk',
-                  'ToolMessageChunk': '#/$defs/ToolMessageChunk',
-                  'ai': '#/$defs/AIMessage',
-                  'chat': '#/$defs/ChatMessage',
-                  'function': '#/$defs/FunctionMessage',
-                  'human': '#/$defs/HumanMessage',
-                  'system': '#/$defs/SystemMessage',
-                  'tool': '#/$defs/ToolMessage',
-                }),
-                'propertyName': 'type',
-              }),
               'oneOf': list([
                 dict({
                   '$ref': '#/$defs/AIMessage',
@@ -8399,23 +8365,6 @@
       }),
       dict({
         'items': dict({
-          'discriminator': dict({
-            'mapping': dict({
-              'AIMessageChunk': '#/definitions/AIMessageChunk',
-              'ChatMessageChunk': '#/definitions/ChatMessageChunk',
-              'FunctionMessageChunk': '#/definitions/FunctionMessageChunk',
-              'HumanMessageChunk': '#/definitions/HumanMessageChunk',
-              'SystemMessageChunk': '#/definitions/SystemMessageChunk',
-              'ToolMessageChunk': '#/definitions/ToolMessageChunk',
-              'ai': '#/definitions/AIMessage',
-              'chat': '#/definitions/ChatMessage',
-              'function': '#/definitions/FunctionMessage',
-              'human': '#/definitions/HumanMessage',
-              'system': '#/definitions/SystemMessage',
-              'tool': '#/definitions/ToolMessage',
-            }),
-            'propertyName': 'type',
-          }),
           'oneOf': list([
             dict({
               '$ref': '#/definitions/AIMessage',
@@ -8854,23 +8803,6 @@
         'properties': dict({
           'messages': dict({
             'items': dict({
-              'discriminator': dict({
-                'mapping': dict({
-                  'AIMessageChunk': '#/definitions/AIMessageChunk',
-                  'ChatMessageChunk': '#/definitions/ChatMessageChunk',
-                  'FunctionMessageChunk': '#/definitions/FunctionMessageChunk',
-                  'HumanMessageChunk': '#/definitions/HumanMessageChunk',
-                  'SystemMessageChunk': '#/definitions/SystemMessageChunk',
-                  'ToolMessageChunk': '#/definitions/ToolMessageChunk',
-                  'ai': '#/definitions/AIMessage',
-                  'chat': '#/definitions/ChatMessage',
-                  'function': '#/definitions/FunctionMessage',
-                  'human': '#/definitions/HumanMessage',
-                  'system': '#/definitions/SystemMessage',
-                  'tool': '#/definitions/ToolMessage',
-                }),
-                'propertyName': 'type',
-              }),
               'oneOf': list([
                 dict({
                   '$ref': '#/definitions/AIMessage',
@@ -11253,23 +11185,6 @@
         'type': 'object',
       }),
     }),
-    'discriminator': dict({
-      'mapping': dict({
-        'AIMessageChunk': '#/definitions/AIMessageChunk',
-        'ChatMessageChunk': '#/definitions/ChatMessageChunk',
-        'FunctionMessageChunk': '#/definitions/FunctionMessageChunk',
-        'HumanMessageChunk': '#/definitions/HumanMessageChunk',
-        'SystemMessageChunk': '#/definitions/SystemMessageChunk',
-        'ToolMessageChunk': '#/definitions/ToolMessageChunk',
-        'ai': '#/definitions/AIMessage',
-        'chat': '#/definitions/ChatMessage',
-        'function': '#/definitions/FunctionMessage',
-        'human': '#/definitions/HumanMessage',
-        'system': '#/definitions/SystemMessage',
-        'tool': '#/definitions/ToolMessage',
-      }),
-      'propertyName': 'type',
-    }),
     'oneOf': list([
       dict({
         '$ref': '#/definitions/AIMessage',
@@ -11325,23 +11240,6 @@
       }),
       dict({
         'items': dict({
-          'discriminator': dict({
-            'mapping': dict({
-              'AIMessageChunk': '#/definitions/AIMessageChunk',
-              'ChatMessageChunk': '#/definitions/ChatMessageChunk',
-              'FunctionMessageChunk': '#/definitions/FunctionMessageChunk',
-              'HumanMessageChunk': '#/definitions/HumanMessageChunk',
-              'SystemMessageChunk': '#/definitions/SystemMessageChunk',
-              'ToolMessageChunk': '#/definitions/ToolMessageChunk',
-              'ai': '#/definitions/AIMessage',
-              'chat': '#/definitions/ChatMessage',
-              'function': '#/definitions/FunctionMessage',
-              'human': '#/definitions/HumanMessage',
-              'system': '#/definitions/SystemMessage',
-              'tool': '#/definitions/ToolMessage',
-            }),
-            'propertyName': 'type',
-          }),
           'oneOf': list([
             dict({
               '$ref': '#/definitions/AIMessage',
@@ -11780,23 +11678,6 @@
         'properties': dict({
           'messages': dict({
             'items': dict({
-              'discriminator': dict({
-                'mapping': dict({
-                  'AIMessageChunk': '#/definitions/AIMessageChunk',
-                  'ChatMessageChunk': '#/definitions/ChatMessageChunk',
-                  'FunctionMessageChunk': '#/definitions/FunctionMessageChunk',
-                  'HumanMessageChunk': '#/definitions/HumanMessageChunk',
-                  'SystemMessageChunk': '#/definitions/SystemMessageChunk',
-                  'ToolMessageChunk': '#/definitions/ToolMessageChunk',
-                  'ai': '#/definitions/AIMessage',
-                  'chat': '#/definitions/ChatMessage',
-                  'function': '#/definitions/FunctionMessage',
-                  'human': '#/definitions/HumanMessage',
-                  'system': '#/definitions/SystemMessage',
-                  'tool': '#/definitions/ToolMessage',
-                }),
-                'propertyName': 'type',
-              }),
               'oneOf': list([
                 dict({
                   '$ref': '#/definitions/AIMessage',
@@ -12842,23 +12723,6 @@
         'type': 'string',
       }),
       dict({
-        'discriminator': dict({
-          'mapping': dict({
-            'AIMessageChunk': '#/definitions/AIMessageChunk',
-            'ChatMessageChunk': '#/definitions/ChatMessageChunk',
-            'FunctionMessageChunk': '#/definitions/FunctionMessageChunk',
-            'HumanMessageChunk': '#/definitions/HumanMessageChunk',
-            'SystemMessageChunk': '#/definitions/SystemMessageChunk',
-            'ToolMessageChunk': '#/definitions/ToolMessageChunk',
-            'ai': '#/definitions/AIMessage',
-            'chat': '#/definitions/ChatMessage',
-            'function': '#/definitions/FunctionMessage',
-            'human': '#/definitions/HumanMessage',
-            'system': '#/definitions/SystemMessage',
-            'tool': '#/definitions/ToolMessage',
-          }),
-          'propertyName': 'type',
-        }),
         'oneOf': list([
           dict({
             '$ref': '#/definitions/AIMessage',
@@ -14642,23 +14506,6 @@
         'properties': dict({
           'messages': dict({
             'items': dict({
-              'discriminator': dict({
-                'mapping': dict({
-                  'AIMessageChunk': '#/definitions/AIMessageChunk',
-                  'ChatMessageChunk': '#/definitions/ChatMessageChunk',
-                  'FunctionMessageChunk': '#/definitions/FunctionMessageChunk',
-                  'HumanMessageChunk': '#/definitions/HumanMessageChunk',
-                  'SystemMessageChunk': '#/definitions/SystemMessageChunk',
-                  'ToolMessageChunk': '#/definitions/ToolMessageChunk',
-                  'ai': '#/definitions/AIMessage',
-                  'chat': '#/definitions/ChatMessage',
-                  'function': '#/definitions/FunctionMessage',
-                  'human': '#/definitions/HumanMessage',
-                  'system': '#/definitions/SystemMessage',
-                  'tool': '#/definitions/ToolMessage',
-                }),
-                'propertyName': 'type',
-              }),
               'oneOf': list([
                 dict({
                   '$ref': '#/definitions/AIMessage',
@@ -16118,23 +15965,6 @@
         'properties': dict({
           'messages': dict({
             'items': dict({
-              'discriminator': dict({
-                'mapping': dict({
-                  'AIMessageChunk': '#/definitions/AIMessageChunk',
-                  'ChatMessageChunk': '#/definitions/ChatMessageChunk',
-                  'FunctionMessageChunk': '#/definitions/FunctionMessageChunk',
-                  'HumanMessageChunk': '#/definitions/HumanMessageChunk',
-                  'SystemMessageChunk': '#/definitions/SystemMessageChunk',
-                  'ToolMessageChunk': '#/definitions/ToolMessageChunk',
-                  'ai': '#/definitions/AIMessage',
-                  'chat': '#/definitions/ChatMessage',
-                  'function': '#/definitions/FunctionMessage',
-                  'human': '#/definitions/HumanMessage',
-                  'system': '#/definitions/SystemMessage',
-                  'tool': '#/definitions/ToolMessage',
-                }),
-                'propertyName': 'type',
-              }),
               'oneOf': list([
                 dict({
                   '$ref': '#/definitions/AIMessage',