diff --git a/src/opencode_ai/resources/session.py b/src/opencode_ai/resources/session.py
index 7e50413..201a312 100644
--- a/src/opencode_ai/resources/session.py
+++ b/src/opencode_ai/resources/session.py
@@ -192,9 +192,11 @@ class SessionResource(SyncAPIResource):
             f"/session/{id}/message",
             body=maybe_transform(
                 {
-                    "model_id": model_id,
+                    "model": {
+                        "model_id": model_id,
+                        "provider_id": provider_id,
+                    },
                     "parts": parts,
-                    "provider_id": provider_id,
                     "message_id": message_id,
                     "mode": mode,
                     "system": system,
@@ -243,8 +245,10 @@ class SessionResource(SyncAPIResource):
             body=maybe_transform(
                 {
                     "message_id": message_id,
-                    "model_id": model_id,
-                    "provider_id": provider_id,
+                    "model": {
+                        "model_id": model_id,
+                        "provider_id": provider_id,
+                    },
                 },
                 session_init_params.SessionInitParams,
             ),
@@ -637,9 +641,11 @@ class AsyncSessionResource(AsyncAPIResource):
             f"/session/{id}/message",
             body=await async_maybe_transform(
                 {
-                    "model_id": model_id,
+                    "model": {
+                        "model_id": model_id,
+                        "provider_id": provider_id,
+                    },
                     "parts": parts,
-                    "provider_id": provider_id,
                     "message_id": message_id,
                     "mode": mode,
                     "system": system,
@@ -688,8 +694,10 @@ class AsyncSessionResource(AsyncAPIResource):
             body=await async_maybe_transform(
                 {
                     "message_id": message_id,
-                    "model_id": model_id,
-                    "provider_id": provider_id,
+                    "model": {
+                        "model_id": model_id,
+                        "provider_id": provider_id,
+                    },
                 },
                 session_init_params.SessionInitParams,
             ),
diff --git a/src/opencode_ai/types/session_chat_params.py b/src/opencode_ai/types/session_chat_params.py
index e827282..10f4bcd 100644
--- a/src/opencode_ai/types/session_chat_params.py
+++ b/src/opencode_ai/types/session_chat_params.py
@@ -9,16 +9,22 @@ from .._utils import PropertyInfo
 from .file_part_input_param import FilePartInputParam
 from .text_part_input_param import TextPartInputParam
 
-__all__ = ["SessionChatParams", "Part"]
+__all__ = ["ModelParam", "SessionChatParams", "Part"]
+
+
+class ModelParam(TypedDict, total=False):
+    """Model selection parameters nested under the ``model`` key."""
+
+    model_id: Required[Annotated[str, PropertyInfo(alias="modelID")]]
+
+    provider_id: Required[Annotated[str, PropertyInfo(alias="providerID")]]
 
 
 class SessionChatParams(TypedDict, total=False):
-    model_id: Required[Annotated[str, PropertyInfo(alias="modelID")]]
+    model: Required[ModelParam]
 
     parts: Required[Iterable[Part]]
 
-    provider_id: Required[Annotated[str, PropertyInfo(alias="providerID")]]
-
     message_id: Annotated[str, PropertyInfo(alias="messageID")]
 
     mode: str
diff --git a/src/opencode_ai/types/session_init_params.py b/src/opencode_ai/types/session_init_params.py
index 8d6d0d8..9e0b554 100644
--- a/src/opencode_ai/types/session_init_params.py
+++ b/src/opencode_ai/types/session_init_params.py
@@ -5,6 +5,7 @@ from __future__ import annotations
 from typing_extensions import Required, Annotated, TypedDict
 
 from .._utils import PropertyInfo
+from .session_chat_params import ModelParam
 
 __all__ = ["SessionInitParams"]
 
@@ -12,6 +13,4 @@ __all__ = ["SessionInitParams"]
 class SessionInitParams(TypedDict, total=False):
     message_id: Required[Annotated[str, PropertyInfo(alias="messageID")]]
 
-    model_id: Required[Annotated[str, PropertyInfo(alias="modelID")]]
-
-    provider_id: Required[Annotated[str, PropertyInfo(alias="providerID")]]
+    model: Required[ModelParam]