diff --git a/.changeset/fuzzy-uppish-capybara.md b/.changeset/fuzzy-uppish-capybara.md
new file mode 100644
index 00000000..256c81fd
--- /dev/null
+++ b/.changeset/fuzzy-uppish-capybara.md
@@ -0,0 +1,5 @@
+---
+"stagehand": patch
+---
+
+Properly serialize os on browserbase session create params
diff --git a/stagehand/config.py b/stagehand/config.py
index e69faa20..2135c7c4 100644
--- a/stagehand/config.py
+++ b/stagehand/config.py
@@ -1,7 +1,6 @@
 import os
-from typing import Any, Callable, Literal, Optional, Union
+from typing import Any, Callable, Literal, Optional
 
-from browserbase.types import SessionCreateParams as BrowserbaseSessionCreateParams
 from pydantic import BaseModel, ConfigDict, Field, field_validator
 
 from stagehand.schemas import AvailableModel
@@ -71,9 +70,7 @@ class StagehandConfig(BaseModel):
         alias="domSettleTimeoutMs",
         description="Timeout for DOM to settle (in ms)",
     )
-    browserbase_session_create_params: Optional[
-        Union[BrowserbaseSessionCreateParams, dict[str, Any]]
-    ] = Field(
+    browserbase_session_create_params: Optional[dict[str, Any]] = Field(
         None,
         alias="browserbaseSessionCreateParams",
         description="Browserbase session create params",
@@ -87,7 +84,9 @@ class StagehandConfig(BaseModel):
         description="Session ID for resuming Browserbase sessions",
     )
     model_name: Optional[str] = Field(
-        AvailableModel.GPT_4O, alias="modelName", description="Name of the model to use"
+        AvailableModel.GPT_4_1_MINI,
+        alias="modelName",
+        description="Name of the model to use",
     )
     self_heal: Optional[bool] = Field(
         True, alias="selfHeal", description="Enable self-healing functionality"
@@ -124,11 +123,25 @@ class StagehandConfig(BaseModel):
     @classmethod
     def validate_browserbase_params(cls, v, info):
         """Validate and convert browserbase session create params."""
-        if isinstance(v, dict) and "project_id" not in v:
-            values = info.data
-            project_id = values.get("project_id") or values.get("projectId")
-            if project_id:
-                v = {**v, "project_id": project_id}
+        if isinstance(v, dict):
+            # Make a copy to avoid mutating the original
+            v = dict(v)
+
+            # Add project_id if not present
+            if "project_id" not in v:
+                values = info.data
+                project_id = values.get("project_id") or values.get("projectId")
+                if project_id:
+                    v["project_id"] = project_id
+
+            # Handle browser_settings
+            if "browser_settings" in v:
+                # Make a copy of browser_settings to avoid mutating
+                v["browser_settings"] = dict(v["browser_settings"])
+
+                # Normalize 'os' key to lowercase
+                if "os" in v["browser_settings"]:
+                    v["browser_settings"]["os"] = v["browser_settings"]["os"].lower()
         return v
 
     def with_overrides(self, **overrides) -> "StagehandConfig":
diff --git a/stagehand/schemas.py b/stagehand/schemas.py
index 3cb0d54e..de8448b2 100644
--- a/stagehand/schemas.py
+++ b/stagehand/schemas.py
@@ -15,6 +15,7 @@
 class AvailableModel(str, Enum):
     GPT_4O = "gpt-4o"
     GPT_4O_MINI = "gpt-4o-mini"
+    GPT_4_1_MINI = "gpt-4.1-mini"
     CLAUDE_3_5_SONNET_LATEST = "claude-3-5-sonnet-latest"
     CLAUDE_3_7_SONNET_LATEST = "claude-3-7-sonnet-latest"
     COMPUTER_USE_PREVIEW = "computer-use-preview"