diff --git a/.release-please-manifest.json b/.release-please-manifest.json
index 2a8f4ff..3e9af1b 100644
--- a/.release-please-manifest.json
+++ b/.release-please-manifest.json
@@ -1,3 +1,3 @@
 {
-  ".": "1.3.0"
+  ".": "1.4.0"
 }
\ No newline at end of file
diff --git a/.stats.yml b/.stats.yml
index b6f1d02..fdb7ef3 100644
--- a/.stats.yml
+++ b/.stats.yml
@@ -1,4 +1,4 @@
 configured_endpoints: 97
-openapi_spec_url: https://storage.googleapis.com/stainless-sdk-openapi-specs/openai%2Fopenai-0205acb1015d29b2312a48526734c0399f93026d4fe2dff5c7768f566e333fd2.yml
-openapi_spec_hash: 1772cc9056c2f6dfb2a4e9cb77ee6343
+openapi_spec_url: https://storage.googleapis.com/stainless-sdk-openapi-specs/openai%2Fopenai-4865dda2b62927bd141cbc85f81be3d88602f103e2c581e15eb1caded3e3aaa2.yml
+openapi_spec_hash: 7d14a9b23ef4ac93ea46d629601b6f6b
 config_hash: ed1e6b3c5f93d12b80d31167f55c557c
diff --git a/CHANGELOG.md b/CHANGELOG.md
index 5f78495..520b714 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,5 +1,18 @@
 # Changelog
 
+## 1.4.0 (2025-06-09)
+
+Full Changelog: [v1.3.0...v1.4.0](https://github.com/openai/openai-go/compare/v1.3.0...v1.4.0)
+
+### Features
+
+* **client:** allow overriding unions ([27c6299](https://github.com/openai/openai-go/commit/27c6299cb4ac275c6542b5691d81b795e65eeff6))
+
+
+### Bug Fixes
+
+* **client:** cast to raw message when converting to params ([a3282b0](https://github.com/openai/openai-go/commit/a3282b01a8d9a2c0cd04f24b298bf2ffcd160ebd))
+
 ## 1.3.0 (2025-06-03)
 
 Full Changelog: [v1.2.1...v1.3.0](https://github.com/openai/openai-go/compare/v1.2.1...v1.3.0)
diff --git a/README.md b/README.md
index 7911bb5..33d706f 100644
--- a/README.md
+++ b/README.md
@@ -26,7 +26,7 @@ Or to pin the version:
 <!-- x-release-please-start-version -->
 
 ```sh
-go get -u 'github.com/openai/openai-go@v1.3.0'
+go get -u 'github.com/openai/openai-go@v1.4.0'
 ```
 
 <!-- x-release-please-end -->
diff --git a/audiotranscription.go b/audiotranscription.go
index 2511d28..33b6478 100644
--- a/audiotranscription.go
+++ b/audiotranscription.go
@@ -398,7 +398,7 @@ type AudioTranscriptionNewParamsChunkingStrategyUnion struct {
 }
 
 func (u AudioTranscriptionNewParamsChunkingStrategyUnion) MarshalJSON() ([]byte, error) {
-	return param.MarshalUnion[AudioTranscriptionNewParamsChunkingStrategyUnion](u.OfAuto, u.OfAudioTranscriptionNewsChunkingStrategyVadConfig)
+	return param.MarshalUnion(u, u.OfAuto, u.OfAudioTranscriptionNewsChunkingStrategyVadConfig)
 }
 func (u *AudioTranscriptionNewParamsChunkingStrategyUnion) UnmarshalJSON(data []byte) error {
 	return apijson.UnmarshalRoot(data, u)
diff --git a/betaassistant.go b/betaassistant.go
index 6adbfa9..69cb0fa 100644
--- a/betaassistant.go
+++ b/betaassistant.go
@@ -1407,7 +1407,7 @@ func (r *AssistantToolUnion) UnmarshalJSON(data []byte) error {
 // be used at the last possible moment before sending a request. Test for this with
 // AssistantToolUnionParam.Overrides()
 func (r AssistantToolUnion) ToParam() AssistantToolUnionParam {
-	return param.Override[AssistantToolUnionParam](r.RawJSON())
+	return param.Override[AssistantToolUnionParam](json.RawMessage(r.RawJSON()))
 }
 
 func AssistantToolParamOfFunction(function shared.FunctionDefinitionParam) AssistantToolUnionParam {
@@ -1427,7 +1427,7 @@ type AssistantToolUnionParam struct {
 }
 
 func (u AssistantToolUnionParam) MarshalJSON() ([]byte, error) {
-	return param.MarshalUnion[AssistantToolUnionParam](u.OfCodeInterpreter, u.OfFileSearch, u.OfFunction)
+	return param.MarshalUnion(u, u.OfCodeInterpreter, u.OfFileSearch, u.OfFunction)
 }
 func (u *AssistantToolUnionParam) UnmarshalJSON(data []byte) error {
 	return apijson.UnmarshalRoot(data, u)
@@ -1504,7 +1504,7 @@ func (r *CodeInterpreterTool) UnmarshalJSON(data []byte) error {
 // be used at the last possible moment before sending a request. Test for this with
 // CodeInterpreterToolParam.Overrides()
 func (r CodeInterpreterTool) ToParam() CodeInterpreterToolParam {
-	return param.Override[CodeInterpreterToolParam](r.RawJSON())
+	return param.Override[CodeInterpreterToolParam](json.RawMessage(r.RawJSON()))
 }
 
 func NewCodeInterpreterToolParam() CodeInterpreterToolParam {
@@ -1555,7 +1555,7 @@ func (r *FileSearchTool) UnmarshalJSON(data []byte) error {
 // be used at the last possible moment before sending a request. Test for this with
 // FileSearchToolParam.Overrides()
 func (r FileSearchTool) ToParam() FileSearchToolParam {
-	return param.Override[FileSearchToolParam](r.RawJSON())
+	return param.Override[FileSearchToolParam](json.RawMessage(r.RawJSON()))
 }
 
 // Overrides for the file search tool.
@@ -1728,7 +1728,7 @@ func (r *FunctionTool) UnmarshalJSON(data []byte) error {
 // be used at the last possible moment before sending a request. Test for this with
 // FunctionToolParam.Overrides()
 func (r FunctionTool) ToParam() FunctionToolParam {
-	return param.Override[FunctionToolParam](r.RawJSON())
+	return param.Override[FunctionToolParam](json.RawMessage(r.RawJSON()))
 }
 
 // The properties Function, Type are required.
@@ -1922,7 +1922,7 @@ type BetaAssistantNewParamsToolResourcesFileSearchVectorStoreChunkingStrategyUni
 }
 
 func (u BetaAssistantNewParamsToolResourcesFileSearchVectorStoreChunkingStrategyUnion) MarshalJSON() ([]byte, error) {
-	return param.MarshalUnion[BetaAssistantNewParamsToolResourcesFileSearchVectorStoreChunkingStrategyUnion](u.OfAuto, u.OfStatic)
+	return param.MarshalUnion(u, u.OfAuto, u.OfStatic)
 }
 func (u *BetaAssistantNewParamsToolResourcesFileSearchVectorStoreChunkingStrategyUnion) UnmarshalJSON(data []byte) error {
 	return apijson.UnmarshalRoot(data, u)
diff --git a/betathread.go b/betathread.go
index df8da48..7e351bf 100644
--- a/betathread.go
+++ b/betathread.go
@@ -185,7 +185,7 @@ func (r *AssistantResponseFormatOptionUnion) UnmarshalJSON(data []byte) error {
 // be used at the last possible moment before sending a request. Test for this with
 // AssistantResponseFormatOptionUnionParam.Overrides()
 func (r AssistantResponseFormatOptionUnion) ToParam() AssistantResponseFormatOptionUnionParam {
-	return param.Override[AssistantResponseFormatOptionUnionParam](r.RawJSON())
+	return param.Override[AssistantResponseFormatOptionUnionParam](json.RawMessage(r.RawJSON()))
 }
 
 func AssistantResponseFormatOptionParamOfAuto() AssistantResponseFormatOptionUnionParam {
@@ -211,7 +211,7 @@ type AssistantResponseFormatOptionUnionParam struct {
 }
 
 func (u AssistantResponseFormatOptionUnionParam) MarshalJSON() ([]byte, error) {
-	return param.MarshalUnion[AssistantResponseFormatOptionUnionParam](u.OfAuto, u.OfText, u.OfJSONObject, u.OfJSONSchema)
+	return param.MarshalUnion(u, u.OfAuto, u.OfText, u.OfJSONObject, u.OfJSONSchema)
 }
 func (u *AssistantResponseFormatOptionUnionParam) UnmarshalJSON(data []byte) error {
 	return apijson.UnmarshalRoot(data, u)
@@ -279,7 +279,7 @@ func (r *AssistantToolChoice) UnmarshalJSON(data []byte) error {
 // be used at the last possible moment before sending a request. Test for this with
 // AssistantToolChoiceParam.Overrides()
 func (r AssistantToolChoice) ToParam() AssistantToolChoiceParam {
-	return param.Override[AssistantToolChoiceParam](r.RawJSON())
+	return param.Override[AssistantToolChoiceParam](json.RawMessage(r.RawJSON()))
 }
 
 // The type of the tool. If type is `function`, the function name must be set
@@ -336,7 +336,7 @@ func (r *AssistantToolChoiceFunction) UnmarshalJSON(data []byte) error {
 // be used at the last possible moment before sending a request. Test for this with
 // AssistantToolChoiceFunctionParam.Overrides()
 func (r AssistantToolChoiceFunction) ToParam() AssistantToolChoiceFunctionParam {
-	return param.Override[AssistantToolChoiceFunctionParam](r.RawJSON())
+	return param.Override[AssistantToolChoiceFunctionParam](json.RawMessage(r.RawJSON()))
 }
 
 // The property Name is required.
@@ -400,7 +400,7 @@ func (r *AssistantToolChoiceOptionUnion) UnmarshalJSON(data []byte) error {
 // be used at the last possible moment before sending a request. Test for this with
 // AssistantToolChoiceOptionUnionParam.Overrides()
 func (r AssistantToolChoiceOptionUnion) ToParam() AssistantToolChoiceOptionUnionParam {
-	return param.Override[AssistantToolChoiceOptionUnionParam](r.RawJSON())
+	return param.Override[AssistantToolChoiceOptionUnionParam](json.RawMessage(r.RawJSON()))
 }
 
 // `none` means the model will not call any tools and instead generates a message.
@@ -432,7 +432,7 @@ type AssistantToolChoiceOptionUnionParam struct {
 }
 
 func (u AssistantToolChoiceOptionUnionParam) MarshalJSON() ([]byte, error) {
-	return param.MarshalUnion[AssistantToolChoiceOptionUnionParam](u.OfAuto, u.OfAssistantToolChoice)
+	return param.MarshalUnion(u, u.OfAuto, u.OfAssistantToolChoice)
 }
 func (u *AssistantToolChoiceOptionUnionParam) UnmarshalJSON(data []byte) error {
 	return apijson.UnmarshalRoot(data, u)
@@ -643,7 +643,7 @@ type BetaThreadNewParamsMessageContentUnion struct {
 }
 
 func (u BetaThreadNewParamsMessageContentUnion) MarshalJSON() ([]byte, error) {
-	return param.MarshalUnion[BetaThreadNewParamsMessageContentUnion](u.OfString, u.OfArrayOfContentParts)
+	return param.MarshalUnion(u, u.OfString, u.OfArrayOfContentParts)
 }
 func (u *BetaThreadNewParamsMessageContentUnion) UnmarshalJSON(data []byte) error {
 	return apijson.UnmarshalRoot(data, u)
@@ -684,7 +684,7 @@ type BetaThreadNewParamsMessageAttachmentToolUnion struct {
 }
 
 func (u BetaThreadNewParamsMessageAttachmentToolUnion) MarshalJSON() ([]byte, error) {
-	return param.MarshalUnion[BetaThreadNewParamsMessageAttachmentToolUnion](u.OfCodeInterpreter, u.OfFileSearch)
+	return param.MarshalUnion(u, u.OfCodeInterpreter, u.OfFileSearch)
 }
 func (u *BetaThreadNewParamsMessageAttachmentToolUnion) UnmarshalJSON(data []byte) error {
 	return apijson.UnmarshalRoot(data, u)
@@ -831,7 +831,7 @@ type BetaThreadNewParamsToolResourcesFileSearchVectorStoreChunkingStrategyUnion
 }
 
 func (u BetaThreadNewParamsToolResourcesFileSearchVectorStoreChunkingStrategyUnion) MarshalJSON() ([]byte, error) {
-	return param.MarshalUnion[BetaThreadNewParamsToolResourcesFileSearchVectorStoreChunkingStrategyUnion](u.OfAuto, u.OfStatic)
+	return param.MarshalUnion(u, u.OfAuto, u.OfStatic)
 }
 func (u *BetaThreadNewParamsToolResourcesFileSearchVectorStoreChunkingStrategyUnion) UnmarshalJSON(data []byte) error {
 	return apijson.UnmarshalRoot(data, u)
@@ -1188,7 +1188,7 @@ type BetaThreadNewAndRunParamsThreadMessageContentUnion struct {
 }
 
 func (u BetaThreadNewAndRunParamsThreadMessageContentUnion) MarshalJSON() ([]byte, error) {
-	return param.MarshalUnion[BetaThreadNewAndRunParamsThreadMessageContentUnion](u.OfString, u.OfArrayOfContentParts)
+	return param.MarshalUnion(u, u.OfString, u.OfArrayOfContentParts)
 }
 func (u *BetaThreadNewAndRunParamsThreadMessageContentUnion) UnmarshalJSON(data []byte) error {
 	return apijson.UnmarshalRoot(data, u)
@@ -1229,7 +1229,7 @@ type BetaThreadNewAndRunParamsThreadMessageAttachmentToolUnion struct {
 }
 
 func (u BetaThreadNewAndRunParamsThreadMessageAttachmentToolUnion) MarshalJSON() ([]byte, error) {
-	return param.MarshalUnion[BetaThreadNewAndRunParamsThreadMessageAttachmentToolUnion](u.OfCodeInterpreter, u.OfFileSearch)
+	return param.MarshalUnion(u, u.OfCodeInterpreter, u.OfFileSearch)
 }
 func (u *BetaThreadNewAndRunParamsThreadMessageAttachmentToolUnion) UnmarshalJSON(data []byte) error {
 	return apijson.UnmarshalRoot(data, u)
@@ -1376,7 +1376,7 @@ type BetaThreadNewAndRunParamsThreadToolResourcesFileSearchVectorStoreChunkingSt
 }
 
 func (u BetaThreadNewAndRunParamsThreadToolResourcesFileSearchVectorStoreChunkingStrategyUnion) MarshalJSON() ([]byte, error) {
-	return param.MarshalUnion[BetaThreadNewAndRunParamsThreadToolResourcesFileSearchVectorStoreChunkingStrategyUnion](u.OfAuto, u.OfStatic)
+	return param.MarshalUnion(u, u.OfAuto, u.OfStatic)
 }
 func (u *BetaThreadNewAndRunParamsThreadToolResourcesFileSearchVectorStoreChunkingStrategyUnion) UnmarshalJSON(data []byte) error {
 	return apijson.UnmarshalRoot(data, u)
diff --git a/betathreadmessage.go b/betathreadmessage.go
index f02b393..3078e0a 100644
--- a/betathreadmessage.go
+++ b/betathreadmessage.go
@@ -511,7 +511,7 @@ func (r *ImageFile) UnmarshalJSON(data []byte) error {
 // be used at the last possible moment before sending a request. Test for this with
 // ImageFileParam.Overrides()
 func (r ImageFile) ToParam() ImageFileParam {
-	return param.Override[ImageFileParam](r.RawJSON())
+	return param.Override[ImageFileParam](json.RawMessage(r.RawJSON()))
 }
 
 // Specifies the detail level of the image if specified by the user. `low` uses
@@ -573,7 +573,7 @@ func (r *ImageFileContentBlock) UnmarshalJSON(data []byte) error {
 // be used at the last possible moment before sending a request. Test for this with
 // ImageFileContentBlockParam.Overrides()
 func (r ImageFileContentBlock) ToParam() ImageFileContentBlockParam {
-	return param.Override[ImageFileContentBlockParam](r.RawJSON())
+	return param.Override[ImageFileContentBlockParam](json.RawMessage(r.RawJSON()))
 }
 
 // References an image [File](https://platform.openai.com/docs/api-reference/files)
@@ -686,7 +686,7 @@ func (r *ImageURL) UnmarshalJSON(data []byte) error {
 // be used at the last possible moment before sending a request. Test for this with
 // ImageURLParam.Overrides()
 func (r ImageURL) ToParam() ImageURLParam {
-	return param.Override[ImageURLParam](r.RawJSON())
+	return param.Override[ImageURLParam](json.RawMessage(r.RawJSON()))
 }
 
 // Specifies the detail level of the image. `low` uses fewer tokens, you can opt in
@@ -746,7 +746,7 @@ func (r *ImageURLContentBlock) UnmarshalJSON(data []byte) error {
 // be used at the last possible moment before sending a request. Test for this with
 // ImageURLContentBlockParam.Overrides()
 func (r ImageURLContentBlock) ToParam() ImageURLContentBlockParam {
-	return param.Override[ImageURLContentBlockParam](r.RawJSON())
+	return param.Override[ImageURLContentBlockParam](json.RawMessage(r.RawJSON()))
 }
 
 // References an image URL in the content of a message.
@@ -1216,7 +1216,7 @@ type MessageContentPartParamUnion struct {
 }
 
 func (u MessageContentPartParamUnion) MarshalJSON() ([]byte, error) {
-	return param.MarshalUnion[MessageContentPartParamUnion](u.OfImageFile, u.OfImageURL, u.OfText)
+	return param.MarshalUnion(u, u.OfImageFile, u.OfImageURL, u.OfText)
 }
 func (u *MessageContentPartParamUnion) UnmarshalJSON(data []byte) error {
 	return apijson.UnmarshalRoot(data, u)
@@ -1541,7 +1541,7 @@ type BetaThreadMessageNewParamsContentUnion struct {
 }
 
 func (u BetaThreadMessageNewParamsContentUnion) MarshalJSON() ([]byte, error) {
-	return param.MarshalUnion[BetaThreadMessageNewParamsContentUnion](u.OfString, u.OfArrayOfContentParts)
+	return param.MarshalUnion(u, u.OfString, u.OfArrayOfContentParts)
 }
 func (u *BetaThreadMessageNewParamsContentUnion) UnmarshalJSON(data []byte) error {
 	return apijson.UnmarshalRoot(data, u)
@@ -1595,7 +1595,7 @@ type BetaThreadMessageNewParamsAttachmentToolUnion struct {
 }
 
 func (u BetaThreadMessageNewParamsAttachmentToolUnion) MarshalJSON() ([]byte, error) {
-	return param.MarshalUnion[BetaThreadMessageNewParamsAttachmentToolUnion](u.OfCodeInterpreter, u.OfFileSearch)
+	return param.MarshalUnion(u, u.OfCodeInterpreter, u.OfFileSearch)
 }
 func (u *BetaThreadMessageNewParamsAttachmentToolUnion) UnmarshalJSON(data []byte) error {
 	return apijson.UnmarshalRoot(data, u)
diff --git a/betathreadrun.go b/betathreadrun.go
index 32c235a..7d7e116 100644
--- a/betathreadrun.go
+++ b/betathreadrun.go
@@ -741,7 +741,7 @@ type BetaThreadRunNewParamsAdditionalMessageContentUnion struct {
 }
 
 func (u BetaThreadRunNewParamsAdditionalMessageContentUnion) MarshalJSON() ([]byte, error) {
-	return param.MarshalUnion[BetaThreadRunNewParamsAdditionalMessageContentUnion](u.OfString, u.OfArrayOfContentParts)
+	return param.MarshalUnion(u, u.OfString, u.OfArrayOfContentParts)
 }
 func (u *BetaThreadRunNewParamsAdditionalMessageContentUnion) UnmarshalJSON(data []byte) error {
 	return apijson.UnmarshalRoot(data, u)
@@ -782,7 +782,7 @@ type BetaThreadRunNewParamsAdditionalMessageAttachmentToolUnion struct {
 }
 
 func (u BetaThreadRunNewParamsAdditionalMessageAttachmentToolUnion) MarshalJSON() ([]byte, error) {
-	return param.MarshalUnion[BetaThreadRunNewParamsAdditionalMessageAttachmentToolUnion](u.OfCodeInterpreter, u.OfFileSearch)
+	return param.MarshalUnion(u, u.OfCodeInterpreter, u.OfFileSearch)
 }
 func (u *BetaThreadRunNewParamsAdditionalMessageAttachmentToolUnion) UnmarshalJSON(data []byte) error {
 	return apijson.UnmarshalRoot(data, u)
diff --git a/chatcompletion.go b/chatcompletion.go
index 58ac887..98680ec 100644
--- a/chatcompletion.go
+++ b/chatcompletion.go
@@ -4,6 +4,7 @@ package openai
 
 import (
 	"context"
+	"encoding/json"
 	"errors"
 	"fmt"
 	"net/http"
@@ -366,7 +367,7 @@ type ChatCompletionAssistantMessageParamContentUnion struct {
 }
 
 func (u ChatCompletionAssistantMessageParamContentUnion) MarshalJSON() ([]byte, error) {
-	return param.MarshalUnion[ChatCompletionAssistantMessageParamContentUnion](u.OfString, u.OfArrayOfContentParts)
+	return param.MarshalUnion(u, u.OfString, u.OfArrayOfContentParts)
 }
 func (u *ChatCompletionAssistantMessageParamContentUnion) UnmarshalJSON(data []byte) error {
 	return apijson.UnmarshalRoot(data, u)
@@ -391,7 +392,7 @@ type ChatCompletionAssistantMessageParamContentArrayOfContentPartUnion struct {
 }
 
 func (u ChatCompletionAssistantMessageParamContentArrayOfContentPartUnion) MarshalJSON() ([]byte, error) {
-	return param.MarshalUnion[ChatCompletionAssistantMessageParamContentArrayOfContentPartUnion](u.OfText, u.OfRefusal)
+	return param.MarshalUnion(u, u.OfText, u.OfRefusal)
 }
 func (u *ChatCompletionAssistantMessageParamContentArrayOfContentPartUnion) UnmarshalJSON(data []byte) error {
 	return apijson.UnmarshalRoot(data, u)
@@ -847,7 +848,7 @@ type ChatCompletionContentPartUnionParam struct {
 }
 
 func (u ChatCompletionContentPartUnionParam) MarshalJSON() ([]byte, error) {
-	return param.MarshalUnion[ChatCompletionContentPartUnionParam](u.OfText, u.OfImageURL, u.OfInputAudio, u.OfFile)
+	return param.MarshalUnion(u, u.OfText, u.OfImageURL, u.OfInputAudio, u.OfFile)
 }
 func (u *ChatCompletionContentPartUnionParam) UnmarshalJSON(data []byte) error {
 	return apijson.UnmarshalRoot(data, u)
@@ -1153,7 +1154,7 @@ type ChatCompletionDeveloperMessageParamContentUnion struct {
 }
 
 func (u ChatCompletionDeveloperMessageParamContentUnion) MarshalJSON() ([]byte, error) {
-	return param.MarshalUnion[ChatCompletionDeveloperMessageParamContentUnion](u.OfString, u.OfArrayOfContentParts)
+	return param.MarshalUnion(u, u.OfString, u.OfArrayOfContentParts)
 }
 func (u *ChatCompletionDeveloperMessageParamContentUnion) UnmarshalJSON(data []byte) error {
 	return apijson.UnmarshalRoot(data, u)
@@ -1439,7 +1440,7 @@ type ChatCompletionMessageParamUnion struct {
 }
 
 func (u ChatCompletionMessageParamUnion) MarshalJSON() ([]byte, error) {
-	return param.MarshalUnion[ChatCompletionMessageParamUnion](u.OfDeveloper,
+	return param.MarshalUnion(u, u.OfDeveloper,
 		u.OfSystem,
 		u.OfUser,
 		u.OfAssistant,
@@ -1620,7 +1621,7 @@ func (r *ChatCompletionMessageToolCall) UnmarshalJSON(data []byte) error {
 // be used at the last possible moment before sending a request. Test for this with
 // ChatCompletionMessageToolCallParam.Overrides()
 func (r ChatCompletionMessageToolCall) ToParam() ChatCompletionMessageToolCallParam {
-	return param.Override[ChatCompletionMessageToolCallParam](r.RawJSON())
+	return param.Override[ChatCompletionMessageToolCallParam](json.RawMessage(r.RawJSON()))
 }
 
 // The function that the model called.
@@ -1761,7 +1762,7 @@ type ChatCompletionPredictionContentContentUnionParam struct {
 }
 
 func (u ChatCompletionPredictionContentContentUnionParam) MarshalJSON() ([]byte, error) {
-	return param.MarshalUnion[ChatCompletionPredictionContentContentUnionParam](u.OfString, u.OfArrayOfContentParts)
+	return param.MarshalUnion(u, u.OfString, u.OfArrayOfContentParts)
 }
 func (u *ChatCompletionPredictionContentContentUnionParam) UnmarshalJSON(data []byte) error {
 	return apijson.UnmarshalRoot(data, u)
@@ -1852,7 +1853,7 @@ type ChatCompletionSystemMessageParamContentUnion struct {
 }
 
 func (u ChatCompletionSystemMessageParamContentUnion) MarshalJSON() ([]byte, error) {
-	return param.MarshalUnion[ChatCompletionSystemMessageParamContentUnion](u.OfString, u.OfArrayOfContentParts)
+	return param.MarshalUnion(u, u.OfString, u.OfArrayOfContentParts)
 }
 func (u *ChatCompletionSystemMessageParamContentUnion) UnmarshalJSON(data []byte) error {
 	return apijson.UnmarshalRoot(data, u)
@@ -1963,7 +1964,7 @@ type ChatCompletionToolChoiceOptionUnionParam struct {
 }
 
 func (u ChatCompletionToolChoiceOptionUnionParam) MarshalJSON() ([]byte, error) {
-	return param.MarshalUnion[ChatCompletionToolChoiceOptionUnionParam](u.OfAuto, u.OfChatCompletionNamedToolChoice)
+	return param.MarshalUnion(u, u.OfAuto, u.OfChatCompletionNamedToolChoice)
 }
 func (u *ChatCompletionToolChoiceOptionUnionParam) UnmarshalJSON(data []byte) error {
 	return apijson.UnmarshalRoot(data, u)
@@ -2020,7 +2021,7 @@ type ChatCompletionToolMessageParamContentUnion struct {
 }
 
 func (u ChatCompletionToolMessageParamContentUnion) MarshalJSON() ([]byte, error) {
-	return param.MarshalUnion[ChatCompletionToolMessageParamContentUnion](u.OfString, u.OfArrayOfContentParts)
+	return param.MarshalUnion(u, u.OfString, u.OfArrayOfContentParts)
 }
 func (u *ChatCompletionToolMessageParamContentUnion) UnmarshalJSON(data []byte) error {
 	return apijson.UnmarshalRoot(data, u)
@@ -2070,7 +2071,7 @@ type ChatCompletionUserMessageParamContentUnion struct {
 }
 
 func (u ChatCompletionUserMessageParamContentUnion) MarshalJSON() ([]byte, error) {
-	return param.MarshalUnion[ChatCompletionUserMessageParamContentUnion](u.OfString, u.OfArrayOfContentParts)
+	return param.MarshalUnion(u, u.OfString, u.OfArrayOfContentParts)
 }
 func (u *ChatCompletionUserMessageParamContentUnion) UnmarshalJSON(data []byte) error {
 	return apijson.UnmarshalRoot(data, u)
@@ -2302,7 +2303,7 @@ type ChatCompletionNewParamsFunctionCallUnion struct {
 }
 
 func (u ChatCompletionNewParamsFunctionCallUnion) MarshalJSON() ([]byte, error) {
-	return param.MarshalUnion[ChatCompletionNewParamsFunctionCallUnion](u.OfFunctionCallMode, u.OfFunctionCallOption)
+	return param.MarshalUnion(u, u.OfFunctionCallMode, u.OfFunctionCallOption)
 }
 func (u *ChatCompletionNewParamsFunctionCallUnion) UnmarshalJSON(data []byte) error {
 	return apijson.UnmarshalRoot(data, u)
@@ -2367,7 +2368,7 @@ type ChatCompletionNewParamsResponseFormatUnion struct {
 }
 
 func (u ChatCompletionNewParamsResponseFormatUnion) MarshalJSON() ([]byte, error) {
-	return param.MarshalUnion[ChatCompletionNewParamsResponseFormatUnion](u.OfText, u.OfJSONSchema, u.OfJSONObject)
+	return param.MarshalUnion(u, u.OfText, u.OfJSONSchema, u.OfJSONObject)
 }
 func (u *ChatCompletionNewParamsResponseFormatUnion) UnmarshalJSON(data []byte) error {
 	return apijson.UnmarshalRoot(data, u)
@@ -2439,7 +2440,7 @@ type ChatCompletionNewParamsStopUnion struct {
 }
 
 func (u ChatCompletionNewParamsStopUnion) MarshalJSON() ([]byte, error) {
-	return param.MarshalUnion[ChatCompletionNewParamsStopUnion](u.OfString, u.OfStringArray)
+	return param.MarshalUnion(u, u.OfString, u.OfStringArray)
 }
 func (u *ChatCompletionNewParamsStopUnion) UnmarshalJSON(data []byte) error {
 	return apijson.UnmarshalRoot(data, u)
diff --git a/completion.go b/completion.go
index 67fa357..72b2c51 100644
--- a/completion.go
+++ b/completion.go
@@ -381,7 +381,7 @@ type CompletionNewParamsPromptUnion struct {
 }
 
 func (u CompletionNewParamsPromptUnion) MarshalJSON() ([]byte, error) {
-	return param.MarshalUnion[CompletionNewParamsPromptUnion](u.OfString, u.OfArrayOfStrings, u.OfArrayOfTokens, u.OfArrayOfTokenArrays)
+	return param.MarshalUnion(u, u.OfString, u.OfArrayOfStrings, u.OfArrayOfTokens, u.OfArrayOfTokenArrays)
 }
 func (u *CompletionNewParamsPromptUnion) UnmarshalJSON(data []byte) error {
 	return apijson.UnmarshalRoot(data, u)
@@ -410,7 +410,7 @@ type CompletionNewParamsStopUnion struct {
 }
 
 func (u CompletionNewParamsStopUnion) MarshalJSON() ([]byte, error) {
-	return param.MarshalUnion[CompletionNewParamsStopUnion](u.OfString, u.OfStringArray)
+	return param.MarshalUnion(u, u.OfString, u.OfStringArray)
 }
 func (u *CompletionNewParamsStopUnion) UnmarshalJSON(data []byte) error {
 	return apijson.UnmarshalRoot(data, u)
diff --git a/embedding.go b/embedding.go
index 6a19d05..f7650f5 100644
--- a/embedding.go
+++ b/embedding.go
@@ -174,7 +174,7 @@ type EmbeddingNewParamsInputUnion struct {
 }
 
 func (u EmbeddingNewParamsInputUnion) MarshalJSON() ([]byte, error) {
-	return param.MarshalUnion[EmbeddingNewParamsInputUnion](u.OfString, u.OfArrayOfStrings, u.OfArrayOfTokens, u.OfArrayOfTokenArrays)
+	return param.MarshalUnion(u, u.OfString, u.OfArrayOfStrings, u.OfArrayOfTokens, u.OfArrayOfTokenArrays)
 }
 func (u *EmbeddingNewParamsInputUnion) UnmarshalJSON(data []byte) error {
 	return apijson.UnmarshalRoot(data, u)
diff --git a/finetuningalphagrader.go b/finetuningalphagrader.go
index ff50e99..4920583 100644
--- a/finetuningalphagrader.go
+++ b/finetuningalphagrader.go
@@ -301,7 +301,7 @@ type FineTuningAlphaGraderRunParamsGraderUnion struct {
 }
 
 func (u FineTuningAlphaGraderRunParamsGraderUnion) MarshalJSON() ([]byte, error) {
-	return param.MarshalUnion[FineTuningAlphaGraderRunParamsGraderUnion](u.OfStringCheck,
+	return param.MarshalUnion(u, u.OfStringCheck,
 		u.OfTextSimilarity,
 		u.OfPython,
 		u.OfScoreModel,
@@ -505,7 +505,7 @@ type FineTuningAlphaGraderValidateParamsGraderUnion struct {
 }
 
 func (u FineTuningAlphaGraderValidateParamsGraderUnion) MarshalJSON() ([]byte, error) {
-	return param.MarshalUnion[FineTuningAlphaGraderValidateParamsGraderUnion](u.OfStringCheckGrader,
+	return param.MarshalUnion(u, u.OfStringCheckGrader,
 		u.OfTextSimilarityGrader,
 		u.OfPythonGrader,
 		u.OfScoreModelGrader,
diff --git a/finetuningjob.go b/finetuningjob.go
index 42e1992..43ce46f 100644
--- a/finetuningjob.go
+++ b/finetuningjob.go
@@ -688,7 +688,7 @@ type FineTuningJobNewParamsHyperparametersBatchSizeUnion struct {
 }
 
 func (u FineTuningJobNewParamsHyperparametersBatchSizeUnion) MarshalJSON() ([]byte, error) {
-	return param.MarshalUnion[FineTuningJobNewParamsHyperparametersBatchSizeUnion](u.OfAuto, u.OfInt)
+	return param.MarshalUnion(u, u.OfAuto, u.OfInt)
 }
 func (u *FineTuningJobNewParamsHyperparametersBatchSizeUnion) UnmarshalJSON(data []byte) error {
 	return apijson.UnmarshalRoot(data, u)
@@ -714,7 +714,7 @@ type FineTuningJobNewParamsHyperparametersLearningRateMultiplierUnion struct {
 }
 
 func (u FineTuningJobNewParamsHyperparametersLearningRateMultiplierUnion) MarshalJSON() ([]byte, error) {
-	return param.MarshalUnion[FineTuningJobNewParamsHyperparametersLearningRateMultiplierUnion](u.OfAuto, u.OfFloat)
+	return param.MarshalUnion(u, u.OfAuto, u.OfFloat)
 }
 func (u *FineTuningJobNewParamsHyperparametersLearningRateMultiplierUnion) UnmarshalJSON(data []byte) error {
 	return apijson.UnmarshalRoot(data, u)
@@ -740,7 +740,7 @@ type FineTuningJobNewParamsHyperparametersNEpochsUnion struct {
 }
 
 func (u FineTuningJobNewParamsHyperparametersNEpochsUnion) MarshalJSON() ([]byte, error) {
-	return param.MarshalUnion[FineTuningJobNewParamsHyperparametersNEpochsUnion](u.OfAuto, u.OfInt)
+	return param.MarshalUnion(u, u.OfAuto, u.OfInt)
 }
 func (u *FineTuningJobNewParamsHyperparametersNEpochsUnion) UnmarshalJSON(data []byte) error {
 	return apijson.UnmarshalRoot(data, u)
diff --git a/finetuningmethod.go b/finetuningmethod.go
index a7fa95c..b315a9d 100644
--- a/finetuningmethod.go
+++ b/finetuningmethod.go
@@ -68,7 +68,7 @@ func (r *DpoHyperparametersResp) UnmarshalJSON(data []byte) error {
 // be used at the last possible moment before sending a request. Test for this with
 // DpoHyperparameters.Overrides()
 func (r DpoHyperparametersResp) ToParam() DpoHyperparameters {
-	return param.Override[DpoHyperparameters](r.RawJSON())
+	return param.Override[DpoHyperparameters](json.RawMessage(r.RawJSON()))
 }
 
 // DpoHyperparametersBatchSizeUnionResp contains all possible properties and values
@@ -255,7 +255,7 @@ type DpoHyperparametersBatchSizeUnion struct {
 }
 
 func (u DpoHyperparametersBatchSizeUnion) MarshalJSON() ([]byte, error) {
-	return param.MarshalUnion[DpoHyperparametersBatchSizeUnion](u.OfAuto, u.OfInt)
+	return param.MarshalUnion(u, u.OfAuto, u.OfInt)
 }
 func (u *DpoHyperparametersBatchSizeUnion) UnmarshalJSON(data []byte) error {
 	return apijson.UnmarshalRoot(data, u)
@@ -281,7 +281,7 @@ type DpoHyperparametersBetaUnion struct {
 }
 
 func (u DpoHyperparametersBetaUnion) MarshalJSON() ([]byte, error) {
-	return param.MarshalUnion[DpoHyperparametersBetaUnion](u.OfAuto, u.OfFloat)
+	return param.MarshalUnion(u, u.OfAuto, u.OfFloat)
 }
 func (u *DpoHyperparametersBetaUnion) UnmarshalJSON(data []byte) error {
 	return apijson.UnmarshalRoot(data, u)
@@ -307,7 +307,7 @@ type DpoHyperparametersLearningRateMultiplierUnion struct {
 }
 
 func (u DpoHyperparametersLearningRateMultiplierUnion) MarshalJSON() ([]byte, error) {
-	return param.MarshalUnion[DpoHyperparametersLearningRateMultiplierUnion](u.OfAuto, u.OfFloat)
+	return param.MarshalUnion(u, u.OfAuto, u.OfFloat)
 }
 func (u *DpoHyperparametersLearningRateMultiplierUnion) UnmarshalJSON(data []byte) error {
 	return apijson.UnmarshalRoot(data, u)
@@ -333,7 +333,7 @@ type DpoHyperparametersNEpochsUnion struct {
 }
 
 func (u DpoHyperparametersNEpochsUnion) MarshalJSON() ([]byte, error) {
-	return param.MarshalUnion[DpoHyperparametersNEpochsUnion](u.OfAuto, u.OfInt)
+	return param.MarshalUnion(u, u.OfAuto, u.OfInt)
 }
 func (u *DpoHyperparametersNEpochsUnion) UnmarshalJSON(data []byte) error {
 	return apijson.UnmarshalRoot(data, u)
@@ -372,7 +372,7 @@ func (r *DpoMethod) UnmarshalJSON(data []byte) error {
 // be used at the last possible moment before sending a request. Test for this with
 // DpoMethodParam.Overrides()
 func (r DpoMethod) ToParam() DpoMethodParam {
-	return param.Override[DpoMethodParam](r.RawJSON())
+	return param.Override[DpoMethodParam](json.RawMessage(r.RawJSON()))
 }
 
 // Configuration for the DPO fine-tuning method.
@@ -438,7 +438,7 @@ func (r *ReinforcementHyperparametersResp) UnmarshalJSON(data []byte) error {
 // be used at the last possible moment before sending a request. Test for this with
 // ReinforcementHyperparameters.Overrides()
 func (r ReinforcementHyperparametersResp) ToParam() ReinforcementHyperparameters {
-	return param.Override[ReinforcementHyperparameters](r.RawJSON())
+	return param.Override[ReinforcementHyperparameters](json.RawMessage(r.RawJSON()))
 }
 
 // ReinforcementHyperparametersBatchSizeUnionResp contains all possible properties
@@ -718,7 +718,7 @@ type ReinforcementHyperparametersBatchSizeUnion struct {
 }
 
 func (u ReinforcementHyperparametersBatchSizeUnion) MarshalJSON() ([]byte, error) {
-	return param.MarshalUnion[ReinforcementHyperparametersBatchSizeUnion](u.OfAuto, u.OfInt)
+	return param.MarshalUnion(u, u.OfAuto, u.OfInt)
 }
 func (u *ReinforcementHyperparametersBatchSizeUnion) UnmarshalJSON(data []byte) error {
 	return apijson.UnmarshalRoot(data, u)
@@ -744,7 +744,7 @@ type ReinforcementHyperparametersComputeMultiplierUnion struct {
 }
 
 func (u ReinforcementHyperparametersComputeMultiplierUnion) MarshalJSON() ([]byte, error) {
-	return param.MarshalUnion[ReinforcementHyperparametersComputeMultiplierUnion](u.OfAuto, u.OfFloat)
+	return param.MarshalUnion(u, u.OfAuto, u.OfFloat)
 }
 func (u *ReinforcementHyperparametersComputeMultiplierUnion) UnmarshalJSON(data []byte) error {
 	return apijson.UnmarshalRoot(data, u)
@@ -770,7 +770,7 @@ type ReinforcementHyperparametersEvalIntervalUnion struct {
 }
 
 func (u ReinforcementHyperparametersEvalIntervalUnion) MarshalJSON() ([]byte, error) {
-	return param.MarshalUnion[ReinforcementHyperparametersEvalIntervalUnion](u.OfAuto, u.OfInt)
+	return param.MarshalUnion(u, u.OfAuto, u.OfInt)
 }
 func (u *ReinforcementHyperparametersEvalIntervalUnion) UnmarshalJSON(data []byte) error {
 	return apijson.UnmarshalRoot(data, u)
@@ -796,7 +796,7 @@ type ReinforcementHyperparametersEvalSamplesUnion struct {
 }
 
 func (u ReinforcementHyperparametersEvalSamplesUnion) MarshalJSON() ([]byte, error) {
-	return param.MarshalUnion[ReinforcementHyperparametersEvalSamplesUnion](u.OfAuto, u.OfInt)
+	return param.MarshalUnion(u, u.OfAuto, u.OfInt)
 }
 func (u *ReinforcementHyperparametersEvalSamplesUnion) UnmarshalJSON(data []byte) error {
 	return apijson.UnmarshalRoot(data, u)
@@ -822,7 +822,7 @@ type ReinforcementHyperparametersLearningRateMultiplierUnion struct {
 }
 
 func (u ReinforcementHyperparametersLearningRateMultiplierUnion) MarshalJSON() ([]byte, error) {
-	return param.MarshalUnion[ReinforcementHyperparametersLearningRateMultiplierUnion](u.OfAuto, u.OfFloat)
+	return param.MarshalUnion(u, u.OfAuto, u.OfFloat)
 }
 func (u *ReinforcementHyperparametersLearningRateMultiplierUnion) UnmarshalJSON(data []byte) error {
 	return apijson.UnmarshalRoot(data, u)
@@ -848,7 +848,7 @@ type ReinforcementHyperparametersNEpochsUnion struct {
 }
 
 func (u ReinforcementHyperparametersNEpochsUnion) MarshalJSON() ([]byte, error) {
-	return param.MarshalUnion[ReinforcementHyperparametersNEpochsUnion](u.OfAuto, u.OfInt)
+	return param.MarshalUnion(u, u.OfAuto, u.OfInt)
 }
 func (u *ReinforcementHyperparametersNEpochsUnion) UnmarshalJSON(data []byte) error {
 	return apijson.UnmarshalRoot(data, u)
@@ -890,7 +890,7 @@ func (r *ReinforcementMethod) UnmarshalJSON(data []byte) error {
 // be used at the last possible moment before sending a request. Test for this with
 // ReinforcementMethodParam.Overrides()
 func (r ReinforcementMethod) ToParam() ReinforcementMethodParam {
-	return param.Override[ReinforcementMethodParam](r.RawJSON())
+	return param.Override[ReinforcementMethodParam](json.RawMessage(r.RawJSON()))
 }
 
 // ReinforcementMethodGraderUnion contains all possible properties and values from
@@ -1030,7 +1030,7 @@ type ReinforcementMethodGraderUnionParam struct {
 }
 
 func (u ReinforcementMethodGraderUnionParam) MarshalJSON() ([]byte, error) {
-	return param.MarshalUnion[ReinforcementMethodGraderUnionParam](u.OfStringCheckGrader,
+	return param.MarshalUnion(u, u.OfStringCheckGrader,
 		u.OfTextSimilarityGrader,
 		u.OfPythonGrader,
 		u.OfScoreModelGrader,
@@ -1230,7 +1230,7 @@ func (r *SupervisedHyperparametersResp) UnmarshalJSON(data []byte) error {
 // be used at the last possible moment before sending a request. Test for this with
 // SupervisedHyperparameters.Overrides()
 func (r SupervisedHyperparametersResp) ToParam() SupervisedHyperparameters {
-	return param.Override[SupervisedHyperparameters](r.RawJSON())
+	return param.Override[SupervisedHyperparameters](json.RawMessage(r.RawJSON()))
 }
 
 // SupervisedHyperparametersBatchSizeUnionResp contains all possible properties and
@@ -1377,7 +1377,7 @@ type SupervisedHyperparametersBatchSizeUnion struct {
 }
 
 func (u SupervisedHyperparametersBatchSizeUnion) MarshalJSON() ([]byte, error) {
-	return param.MarshalUnion[SupervisedHyperparametersBatchSizeUnion](u.OfAuto, u.OfInt)
+	return param.MarshalUnion(u, u.OfAuto, u.OfInt)
 }
 func (u *SupervisedHyperparametersBatchSizeUnion) UnmarshalJSON(data []byte) error {
 	return apijson.UnmarshalRoot(data, u)
@@ -1403,7 +1403,7 @@ type SupervisedHyperparametersLearningRateMultiplierUnion struct {
 }
 
 func (u SupervisedHyperparametersLearningRateMultiplierUnion) MarshalJSON() ([]byte, error) {
-	return param.MarshalUnion[SupervisedHyperparametersLearningRateMultiplierUnion](u.OfAuto, u.OfFloat)
+	return param.MarshalUnion(u, u.OfAuto, u.OfFloat)
 }
 func (u *SupervisedHyperparametersLearningRateMultiplierUnion) UnmarshalJSON(data []byte) error {
 	return apijson.UnmarshalRoot(data, u)
@@ -1429,7 +1429,7 @@ type SupervisedHyperparametersNEpochsUnion struct {
 }
 
 func (u SupervisedHyperparametersNEpochsUnion) MarshalJSON() ([]byte, error) {
-	return param.MarshalUnion[SupervisedHyperparametersNEpochsUnion](u.OfAuto, u.OfInt)
+	return param.MarshalUnion(u, u.OfAuto, u.OfInt)
 }
 func (u *SupervisedHyperparametersNEpochsUnion) UnmarshalJSON(data []byte) error {
 	return apijson.UnmarshalRoot(data, u)
@@ -1468,7 +1468,7 @@ func (r *SupervisedMethod) UnmarshalJSON(data []byte) error {
 // be used at the last possible moment before sending a request. Test for this with
 // SupervisedMethodParam.Overrides()
 func (r SupervisedMethod) ToParam() SupervisedMethodParam {
-	return param.Override[SupervisedMethodParam](r.RawJSON())
+	return param.Override[SupervisedMethodParam](json.RawMessage(r.RawJSON()))
 }
 
 // Configuration for the supervised fine-tuning method.
diff --git a/gradergradermodel.go b/gradergradermodel.go
index e4a3e8e..e27a51e 100644
--- a/gradergradermodel.go
+++ b/gradergradermodel.go
@@ -71,7 +71,7 @@ func (r *LabelModelGrader) UnmarshalJSON(data []byte) error {
 // be used at the last possible moment before sending a request. Test for this with
 // LabelModelGraderParam.Overrides()
 func (r LabelModelGrader) ToParam() LabelModelGraderParam {
-	return param.Override[LabelModelGraderParam](r.RawJSON())
+	return param.Override[LabelModelGraderParam](json.RawMessage(r.RawJSON()))
 }
 
 // A message input to the model with a role indicating instruction following
@@ -250,7 +250,7 @@ type LabelModelGraderInputContentUnionParam struct {
 }
 
 func (u LabelModelGraderInputContentUnionParam) MarshalJSON() ([]byte, error) {
-	return param.MarshalUnion[LabelModelGraderInputContentUnionParam](u.OfString, u.OfInputText, u.OfOutputText)
+	return param.MarshalUnion(u, u.OfString, u.OfInputText, u.OfOutputText)
 }
 func (u *LabelModelGraderInputContentUnionParam) UnmarshalJSON(data []byte) error {
 	return apijson.UnmarshalRoot(data, u)
@@ -343,7 +343,7 @@ func (r *MultiGrader) UnmarshalJSON(data []byte) error {
 // be used at the last possible moment before sending a request. Test for this with
 // MultiGraderParam.Overrides()
 func (r MultiGrader) ToParam() MultiGraderParam {
-	return param.Override[MultiGraderParam](r.RawJSON())
+	return param.Override[MultiGraderParam](json.RawMessage(r.RawJSON()))
 }
 
 // MultiGraderGradersUnion contains all possible properties and values from
@@ -496,7 +496,7 @@ type MultiGraderGradersUnionParam struct {
 }
 
 func (u MultiGraderGradersUnionParam) MarshalJSON() ([]byte, error) {
-	return param.MarshalUnion[MultiGraderGradersUnionParam](u.OfStringCheckGrader,
+	return param.MarshalUnion(u, u.OfStringCheckGrader,
 		u.OfTextSimilarityGrader,
 		u.OfPythonGrader,
 		u.OfScoreModelGrader,
@@ -701,7 +701,7 @@ func (r *PythonGrader) UnmarshalJSON(data []byte) error {
 // be used at the last possible moment before sending a request. Test for this with
 // PythonGraderParam.Overrides()
 func (r PythonGrader) ToParam() PythonGraderParam {
-	return param.Override[PythonGraderParam](r.RawJSON())
+	return param.Override[PythonGraderParam](json.RawMessage(r.RawJSON()))
 }
 
 // A PythonGrader object that runs a python script on the input.
@@ -768,7 +768,7 @@ func (r *ScoreModelGrader) UnmarshalJSON(data []byte) error {
 // be used at the last possible moment before sending a request. Test for this with
 // ScoreModelGraderParam.Overrides()
 func (r ScoreModelGrader) ToParam() ScoreModelGraderParam {
-	return param.Override[ScoreModelGraderParam](r.RawJSON())
+	return param.Override[ScoreModelGraderParam](json.RawMessage(r.RawJSON()))
 }
 
 // A message input to the model with a role indicating instruction following
@@ -947,7 +947,7 @@ type ScoreModelGraderInputContentUnionParam struct {
 }
 
 func (u ScoreModelGraderInputContentUnionParam) MarshalJSON() ([]byte, error) {
-	return param.MarshalUnion[ScoreModelGraderInputContentUnionParam](u.OfString, u.OfInputText, u.OfOutputText)
+	return param.MarshalUnion(u, u.OfString, u.OfInputText, u.OfOutputText)
 }
 func (u *ScoreModelGraderInputContentUnionParam) UnmarshalJSON(data []byte) error {
 	return apijson.UnmarshalRoot(data, u)
@@ -1044,7 +1044,7 @@ func (r *StringCheckGrader) UnmarshalJSON(data []byte) error {
 // be used at the last possible moment before sending a request. Test for this with
 // StringCheckGraderParam.Overrides()
 func (r StringCheckGrader) ToParam() StringCheckGraderParam {
-	return param.Override[StringCheckGraderParam](r.RawJSON())
+	return param.Override[StringCheckGraderParam](json.RawMessage(r.RawJSON()))
 }
 
 // The string check operation to perform. One of `eq`, `ne`, `like`, or `ilike`.
@@ -1127,7 +1127,7 @@ func (r *TextSimilarityGrader) UnmarshalJSON(data []byte) error {
 // be used at the last possible moment before sending a request. Test for this with
 // TextSimilarityGraderParam.Overrides()
 func (r TextSimilarityGrader) ToParam() TextSimilarityGraderParam {
-	return param.Override[TextSimilarityGraderParam](r.RawJSON())
+	return param.Override[TextSimilarityGraderParam](json.RawMessage(r.RawJSON()))
 }
 
 // The evaluation metric to use. One of `fuzzy_match`, `bleu`, `gleu`, `meteor`,
diff --git a/image.go b/image.go
index f5ec0d6..b2f296b 100644
--- a/image.go
+++ b/image.go
@@ -322,7 +322,7 @@ type ImageEditParamsImageUnion struct {
 }
 
 func (u ImageEditParamsImageUnion) MarshalJSON() ([]byte, error) {
-	return param.MarshalUnion[ImageEditParamsImageUnion](u.OfFile, u.OfFileArray)
+	return param.MarshalUnion(u, u.OfFile, u.OfFileArray)
 }
 func (u *ImageEditParamsImageUnion) UnmarshalJSON(data []byte) error {
 	return apijson.UnmarshalRoot(data, u)
diff --git a/internal/version.go b/internal/version.go
index 83c436e..02bfc38 100644
--- a/internal/version.go
+++ b/internal/version.go
@@ -2,4 +2,4 @@
 
 package internal
 
-const PackageVersion = "1.3.0" // x-release-please-version
+const PackageVersion = "1.4.0" // x-release-please-version
diff --git a/moderation.go b/moderation.go
index 0036387..ee88345 100644
--- a/moderation.go
+++ b/moderation.go
@@ -342,7 +342,7 @@ type ModerationMultiModalInputUnionParam struct {
 }
 
 func (u ModerationMultiModalInputUnionParam) MarshalJSON() ([]byte, error) {
-	return param.MarshalUnion[ModerationMultiModalInputUnionParam](u.OfImageURL, u.OfText)
+	return param.MarshalUnion(u, u.OfImageURL, u.OfText)
 }
 func (u *ModerationMultiModalInputUnionParam) UnmarshalJSON(data []byte) error {
 	return apijson.UnmarshalRoot(data, u)
@@ -467,7 +467,7 @@ type ModerationNewParamsInputUnion struct {
 }
 
 func (u ModerationNewParamsInputUnion) MarshalJSON() ([]byte, error) {
-	return param.MarshalUnion[ModerationNewParamsInputUnion](u.OfString, u.OfStringArray, u.OfModerationMultiModalArray)
+	return param.MarshalUnion(u, u.OfString, u.OfStringArray, u.OfModerationMultiModalArray)
 }
 func (u *ModerationNewParamsInputUnion) UnmarshalJSON(data []byte) error {
 	return apijson.UnmarshalRoot(data, u)
diff --git a/packages/param/encoder.go b/packages/param/encoder.go
index 479222c..222fb75 100644
--- a/packages/param/encoder.go
+++ b/packages/param/encoder.go
@@ -68,7 +68,7 @@ func MarshalWithExtras[T ParamStruct, R any](f T, underlying any, extras map[str
 // MarshalUnion uses a shimmed 'encoding/json' from Go 1.24, to support the 'omitzero' tag
 //
 // Stability for the API of MarshalUnion is not guaranteed.
-func MarshalUnion[T any](variants ...any) ([]byte, error) {
+func MarshalUnion[T ParamStruct](metadata T, variants ...any) ([]byte, error) {
 	nPresent := 0
 	presentIdx := -1
 	for i, variant := range variants {
@@ -78,6 +78,9 @@ func MarshalUnion[T any](variants ...any) ([]byte, error) {
 		}
 	}
 	if nPresent == 0 || presentIdx == -1 {
+		if ovr, ok := metadata.Overrides(); ok {
+			return shimjson.Marshal(ovr)
+		}
 		return []byte(`null`), nil
 	} else if nPresent > 1 {
 		return nil, &json.MarshalerError{
diff --git a/packages/param/encoder_test.go b/packages/param/encoder_test.go
index 81a4b49..247fe64 100644
--- a/packages/param/encoder_test.go
+++ b/packages/param/encoder_test.go
@@ -218,10 +218,11 @@ func TestExtraFieldsForceOmitted(t *testing.T) {
 type UnionWithDates struct {
 	OfDate param.Opt[time.Time]
 	OfTime param.Opt[time.Time]
+	param.APIUnion
 }
 
 func (r UnionWithDates) MarshalJSON() (data []byte, err error) {
-	return param.MarshalUnion[UnionWithDates](param.EncodedAsDate(r.OfDate), r.OfTime)
+	return param.MarshalUnion(r, param.EncodedAsDate(r.OfDate), r.OfTime)
 }
 
 func TestUnionDateMarshal(t *testing.T) {
@@ -324,3 +325,41 @@ func TestOptionalInterfaceAssignability(t *testing.T) {
 
 	notOpt.implOpt() // silence the warning
 }
+
+type PrimitiveUnion struct {
+	OfString param.Opt[string]
+	OfInt    param.Opt[int]
+	param.APIUnion
+}
+
+func (p PrimitiveUnion) MarshalJSON() (data []byte, err error) {
+	return param.MarshalUnion(p, p.OfString, p.OfInt)
+}
+
+func TestOverriddenUnion(t *testing.T) {
+	tests := map[string]struct {
+		value    PrimitiveUnion
+		expected string
+	}{
+		"string": {
+			param.Override[PrimitiveUnion](json.RawMessage(`"hello"`)),
+			`"hello"`,
+		},
+		"int": {
+			param.Override[PrimitiveUnion](json.RawMessage(`42`)),
+			`42`,
+		},
+	}
+
+	for name, test := range tests {
+		t.Run(name, func(t *testing.T) {
+			b, err := json.Marshal(test.value)
+			if err != nil {
+				t.Fatalf("didn't expect error %v, expected %s", err, test.expected)
+			}
+			if string(b) != test.expected {
+				t.Fatalf("expected %s, received %s", test.expected, string(b))
+			}
+		})
+	}
+}
diff --git a/responses/response.go b/responses/response.go
index 28e8002..ac4770b 100644
--- a/responses/response.go
+++ b/responses/response.go
@@ -177,7 +177,7 @@ func (r *ComputerTool) UnmarshalJSON(data []byte) error {
 // be used at the last possible moment before sending a request. Test for this with
 // ComputerToolParam.Overrides()
 func (r ComputerTool) ToParam() ComputerToolParam {
-	return param.Override[ComputerToolParam](r.RawJSON())
+	return param.Override[ComputerToolParam](json.RawMessage(r.RawJSON()))
 }
 
 // The type of computer environment to control.
@@ -261,7 +261,7 @@ type EasyInputMessageContentUnionParam struct {
 }
 
 func (u EasyInputMessageContentUnionParam) MarshalJSON() ([]byte, error) {
-	return param.MarshalUnion[EasyInputMessageContentUnionParam](u.OfString, u.OfInputItemContentList)
+	return param.MarshalUnion(u, u.OfString, u.OfInputItemContentList)
 }
 func (u *EasyInputMessageContentUnionParam) UnmarshalJSON(data []byte) error {
 	return apijson.UnmarshalRoot(data, u)
@@ -333,7 +333,7 @@ func (r *FileSearchTool) UnmarshalJSON(data []byte) error {
 // be used at the last possible moment before sending a request. Test for this with
 // FileSearchToolParam.Overrides()
 func (r FileSearchTool) ToParam() FileSearchToolParam {
-	return param.Override[FileSearchToolParam](r.RawJSON())
+	return param.Override[FileSearchToolParam](json.RawMessage(r.RawJSON()))
 }
 
 // FileSearchToolFiltersUnion contains all possible properties and values from
@@ -439,7 +439,7 @@ type FileSearchToolFiltersUnionParam struct {
 }
 
 func (u FileSearchToolFiltersUnionParam) MarshalJSON() ([]byte, error) {
-	return param.MarshalUnion[FileSearchToolFiltersUnionParam](u.OfComparisonFilter, u.OfCompoundFilter)
+	return param.MarshalUnion(u, u.OfComparisonFilter, u.OfCompoundFilter)
 }
 func (u *FileSearchToolFiltersUnionParam) UnmarshalJSON(data []byte) error {
 	return apijson.UnmarshalRoot(data, u)
@@ -554,7 +554,7 @@ func (r *FunctionTool) UnmarshalJSON(data []byte) error {
 // be used at the last possible moment before sending a request. Test for this with
 // FunctionToolParam.Overrides()
 func (r FunctionTool) ToParam() FunctionToolParam {
-	return param.Override[FunctionToolParam](r.RawJSON())
+	return param.Override[FunctionToolParam](json.RawMessage(r.RawJSON()))
 }
 
 // Defines a function in your own code the model can choose to call. Learn more
@@ -1142,7 +1142,7 @@ func (r *ResponseCodeInterpreterToolCall) UnmarshalJSON(data []byte) error {
 // be used at the last possible moment before sending a request. Test for this with
 // ResponseCodeInterpreterToolCallParam.Overrides()
 func (r ResponseCodeInterpreterToolCall) ToParam() ResponseCodeInterpreterToolCallParam {
-	return param.Override[ResponseCodeInterpreterToolCallParam](r.RawJSON())
+	return param.Override[ResponseCodeInterpreterToolCallParam](json.RawMessage(r.RawJSON()))
 }
 
 // ResponseCodeInterpreterToolCallResultUnion contains all possible properties and
@@ -1325,7 +1325,7 @@ type ResponseCodeInterpreterToolCallResultUnionParam struct {
 }
 
 func (u ResponseCodeInterpreterToolCallResultUnionParam) MarshalJSON() ([]byte, error) {
-	return param.MarshalUnion[ResponseCodeInterpreterToolCallResultUnionParam](u.OfLogs, u.OfFiles)
+	return param.MarshalUnion(u, u.OfLogs, u.OfFiles)
 }
 func (u *ResponseCodeInterpreterToolCallResultUnionParam) UnmarshalJSON(data []byte) error {
 	return apijson.UnmarshalRoot(data, u)
@@ -1503,7 +1503,7 @@ func (r *ResponseComputerToolCall) UnmarshalJSON(data []byte) error {
 // be used at the last possible moment before sending a request. Test for this with
 // ResponseComputerToolCallParam.Overrides()
 func (r ResponseComputerToolCall) ToParam() ResponseComputerToolCallParam {
-	return param.Override[ResponseComputerToolCallParam](r.RawJSON())
+	return param.Override[ResponseComputerToolCallParam](json.RawMessage(r.RawJSON()))
 }
 
 // ResponseComputerToolCallActionUnion contains all possible properties and values
@@ -2000,7 +2000,7 @@ type ResponseComputerToolCallActionUnionParam struct {
 }
 
 func (u ResponseComputerToolCallActionUnionParam) MarshalJSON() ([]byte, error) {
-	return param.MarshalUnion[ResponseComputerToolCallActionUnionParam](u.OfClick,
+	return param.MarshalUnion(u, u.OfClick,
 		u.OfDoubleClick,
 		u.OfDrag,
 		u.OfKeypress,
@@ -2534,7 +2534,7 @@ func (r *ResponseComputerToolCallOutputScreenshot) UnmarshalJSON(data []byte) er
 // be used at the last possible moment before sending a request. Test for this with
 // ResponseComputerToolCallOutputScreenshotParam.Overrides()
 func (r ResponseComputerToolCallOutputScreenshot) ToParam() ResponseComputerToolCallOutputScreenshotParam {
-	return param.Override[ResponseComputerToolCallOutputScreenshotParam](r.RawJSON())
+	return param.Override[ResponseComputerToolCallOutputScreenshotParam](json.RawMessage(r.RawJSON()))
 }
 
 // A computer screenshot image used with the computer use tool.
@@ -3027,7 +3027,7 @@ func (r *ResponseFileSearchToolCall) UnmarshalJSON(data []byte) error {
 // be used at the last possible moment before sending a request. Test for this with
 // ResponseFileSearchToolCallParam.Overrides()
 func (r ResponseFileSearchToolCall) ToParam() ResponseFileSearchToolCallParam {
-	return param.Override[ResponseFileSearchToolCallParam](r.RawJSON())
+	return param.Override[ResponseFileSearchToolCallParam](json.RawMessage(r.RawJSON()))
 }
 
 // The status of the file search tool call. One of `in_progress`, `searching`,
@@ -3188,7 +3188,7 @@ type ResponseFileSearchToolCallResultAttributeUnionParam struct {
 }
 
 func (u ResponseFileSearchToolCallResultAttributeUnionParam) MarshalJSON() ([]byte, error) {
-	return param.MarshalUnion[ResponseFileSearchToolCallResultAttributeUnionParam](u.OfString, u.OfFloat, u.OfBool)
+	return param.MarshalUnion(u, u.OfString, u.OfFloat, u.OfBool)
 }
 func (u *ResponseFileSearchToolCallResultAttributeUnionParam) UnmarshalJSON(data []byte) error {
 	return apijson.UnmarshalRoot(data, u)
@@ -3292,7 +3292,7 @@ func (r *ResponseFormatTextConfigUnion) UnmarshalJSON(data []byte) error {
 // be used at the last possible moment before sending a request. Test for this with
 // ResponseFormatTextConfigUnionParam.Overrides()
 func (r ResponseFormatTextConfigUnion) ToParam() ResponseFormatTextConfigUnionParam {
-	return param.Override[ResponseFormatTextConfigUnionParam](r.RawJSON())
+	return param.Override[ResponseFormatTextConfigUnionParam](json.RawMessage(r.RawJSON()))
 }
 
 func ResponseFormatTextConfigParamOfJSONSchema(name string, schema map[string]any) ResponseFormatTextConfigUnionParam {
@@ -3313,7 +3313,7 @@ type ResponseFormatTextConfigUnionParam struct {
 }
 
 func (u ResponseFormatTextConfigUnionParam) MarshalJSON() ([]byte, error) {
-	return param.MarshalUnion[ResponseFormatTextConfigUnionParam](u.OfText, u.OfJSONSchema, u.OfJSONObject)
+	return param.MarshalUnion(u, u.OfText, u.OfJSONSchema, u.OfJSONObject)
 }
 func (u *ResponseFormatTextConfigUnionParam) UnmarshalJSON(data []byte) error {
 	return apijson.UnmarshalRoot(data, u)
@@ -3429,7 +3429,7 @@ func (r *ResponseFormatTextJSONSchemaConfig) UnmarshalJSON(data []byte) error {
 // be used at the last possible moment before sending a request. Test for this with
 // ResponseFormatTextJSONSchemaConfigParam.Overrides()
 func (r ResponseFormatTextJSONSchemaConfig) ToParam() ResponseFormatTextJSONSchemaConfigParam {
-	return param.Override[ResponseFormatTextJSONSchemaConfigParam](r.RawJSON())
+	return param.Override[ResponseFormatTextJSONSchemaConfigParam](json.RawMessage(r.RawJSON()))
 }
 
 // JSON Schema response format. Used to generate structured JSON responses. Learn
@@ -3572,7 +3572,7 @@ func (r *ResponseFunctionToolCall) UnmarshalJSON(data []byte) error {
 // be used at the last possible moment before sending a request. Test for this with
 // ResponseFunctionToolCallParam.Overrides()
 func (r ResponseFunctionToolCall) ToParam() ResponseFunctionToolCallParam {
-	return param.Override[ResponseFunctionToolCallParam](r.RawJSON())
+	return param.Override[ResponseFunctionToolCallParam](json.RawMessage(r.RawJSON()))
 }
 
 // The status of the item. One of `in_progress`, `completed`, or `incomplete`.
@@ -3717,7 +3717,7 @@ func (r *ResponseFunctionWebSearch) UnmarshalJSON(data []byte) error {
 // be used at the last possible moment before sending a request. Test for this with
 // ResponseFunctionWebSearchParam.Overrides()
 func (r ResponseFunctionWebSearch) ToParam() ResponseFunctionWebSearchParam {
-	return param.Override[ResponseFunctionWebSearchParam](r.RawJSON())
+	return param.Override[ResponseFunctionWebSearchParam](json.RawMessage(r.RawJSON()))
 }
 
 // The status of the web search tool call.
@@ -4042,7 +4042,7 @@ func (r *ResponseInputContentUnion) UnmarshalJSON(data []byte) error {
 // be used at the last possible moment before sending a request. Test for this with
 // ResponseInputContentUnionParam.Overrides()
 func (r ResponseInputContentUnion) ToParam() ResponseInputContentUnionParam {
-	return param.Override[ResponseInputContentUnionParam](r.RawJSON())
+	return param.Override[ResponseInputContentUnionParam](json.RawMessage(r.RawJSON()))
 }
 
 func ResponseInputContentParamOfInputText(text string) ResponseInputContentUnionParam {
@@ -4068,7 +4068,7 @@ type ResponseInputContentUnionParam struct {
 }
 
 func (u ResponseInputContentUnionParam) MarshalJSON() ([]byte, error) {
-	return param.MarshalUnion[ResponseInputContentUnionParam](u.OfInputText, u.OfInputImage, u.OfInputFile)
+	return param.MarshalUnion(u, u.OfInputText, u.OfInputImage, u.OfInputFile)
 }
 func (u *ResponseInputContentUnionParam) UnmarshalJSON(data []byte) error {
 	return apijson.UnmarshalRoot(data, u)
@@ -4189,7 +4189,7 @@ func (r *ResponseInputFile) UnmarshalJSON(data []byte) error {
 // be used at the last possible moment before sending a request. Test for this with
 // ResponseInputFileParam.Overrides()
 func (r ResponseInputFile) ToParam() ResponseInputFileParam {
-	return param.Override[ResponseInputFileParam](r.RawJSON())
+	return param.Override[ResponseInputFileParam](json.RawMessage(r.RawJSON()))
 }
 
 // A file input to the model.
@@ -4255,7 +4255,7 @@ func (r *ResponseInputImage) UnmarshalJSON(data []byte) error {
 // be used at the last possible moment before sending a request. Test for this with
 // ResponseInputImageParam.Overrides()
 func (r ResponseInputImage) ToParam() ResponseInputImageParam {
-	return param.Override[ResponseInputImageParam](r.RawJSON())
+	return param.Override[ResponseInputImageParam](json.RawMessage(r.RawJSON()))
 }
 
 // The detail level of the image to be sent to the model. One of `high`, `low`, or
@@ -4432,7 +4432,7 @@ type ResponseInputItemUnionParam struct {
 }
 
 func (u ResponseInputItemUnionParam) MarshalJSON() ([]byte, error) {
-	return param.MarshalUnion[ResponseInputItemUnionParam](u.OfMessage,
+	return param.MarshalUnion(u, u.OfMessage,
 		u.OfInputMessage,
 		u.OfOutputMessage,
 		u.OfFileSearchCall,
@@ -5599,7 +5599,7 @@ func (r *ResponseInputText) UnmarshalJSON(data []byte) error {
 // be used at the last possible moment before sending a request. Test for this with
 // ResponseInputTextParam.Overrides()
 func (r ResponseInputText) ToParam() ResponseInputTextParam {
-	return param.Override[ResponseInputTextParam](r.RawJSON())
+	return param.Override[ResponseInputTextParam](json.RawMessage(r.RawJSON()))
 }
 
 // A text input to the model.
@@ -7095,7 +7095,7 @@ func (r *ResponseOutputMessage) UnmarshalJSON(data []byte) error {
 // be used at the last possible moment before sending a request. Test for this with
 // ResponseOutputMessageParam.Overrides()
 func (r ResponseOutputMessage) ToParam() ResponseOutputMessageParam {
-	return param.Override[ResponseOutputMessageParam](r.RawJSON())
+	return param.Override[ResponseOutputMessageParam](json.RawMessage(r.RawJSON()))
 }
 
 // ResponseOutputMessageContentUnion contains all possible properties and values
@@ -7223,7 +7223,7 @@ type ResponseOutputMessageContentUnionParam struct {
 }
 
 func (u ResponseOutputMessageContentUnionParam) MarshalJSON() ([]byte, error) {
-	return param.MarshalUnion[ResponseOutputMessageContentUnionParam](u.OfOutputText, u.OfRefusal)
+	return param.MarshalUnion(u, u.OfOutputText, u.OfRefusal)
 }
 func (u *ResponseOutputMessageContentUnionParam) UnmarshalJSON(data []byte) error {
 	return apijson.UnmarshalRoot(data, u)
@@ -7315,7 +7315,7 @@ func (r *ResponseOutputRefusal) UnmarshalJSON(data []byte) error {
 // be used at the last possible moment before sending a request. Test for this with
 // ResponseOutputRefusalParam.Overrides()
 func (r ResponseOutputRefusal) ToParam() ResponseOutputRefusalParam {
-	return param.Override[ResponseOutputRefusalParam](r.RawJSON())
+	return param.Override[ResponseOutputRefusalParam](json.RawMessage(r.RawJSON()))
 }
 
 // A refusal from the model.
@@ -7371,7 +7371,7 @@ func (r *ResponseOutputText) UnmarshalJSON(data []byte) error {
 // be used at the last possible moment before sending a request. Test for this with
 // ResponseOutputTextParam.Overrides()
 func (r ResponseOutputText) ToParam() ResponseOutputTextParam {
-	return param.Override[ResponseOutputTextParam](r.RawJSON())
+	return param.Override[ResponseOutputTextParam](json.RawMessage(r.RawJSON()))
 }
 
 // ResponseOutputTextAnnotationUnion contains all possible properties and values
@@ -7661,7 +7661,7 @@ type ResponseOutputTextAnnotationUnionParam struct {
 }
 
 func (u ResponseOutputTextAnnotationUnionParam) MarshalJSON() ([]byte, error) {
-	return param.MarshalUnion[ResponseOutputTextAnnotationUnionParam](u.OfFileCitation, u.OfURLCitation, u.OfContainerFileCitation, u.OfFilePath)
+	return param.MarshalUnion(u, u.OfFileCitation, u.OfURLCitation, u.OfContainerFileCitation, u.OfFilePath)
 }
 func (u *ResponseOutputTextAnnotationUnionParam) UnmarshalJSON(data []byte) error {
 	return apijson.UnmarshalRoot(data, u)
@@ -8077,7 +8077,7 @@ func (r *ResponseReasoningItem) UnmarshalJSON(data []byte) error {
 // be used at the last possible moment before sending a request. Test for this with
 // ResponseReasoningItemParam.Overrides()
 func (r ResponseReasoningItem) ToParam() ResponseReasoningItemParam {
-	return param.Override[ResponseReasoningItemParam](r.RawJSON())
+	return param.Override[ResponseReasoningItemParam](json.RawMessage(r.RawJSON()))
 }
 
 type ResponseReasoningItemSummary struct {
@@ -9240,7 +9240,7 @@ func (r *ResponseTextConfig) UnmarshalJSON(data []byte) error {
 // be used at the last possible moment before sending a request. Test for this with
 // ResponseTextConfigParam.Overrides()
 func (r ResponseTextConfig) ToParam() ResponseTextConfigParam {
-	return param.Override[ResponseTextConfigParam](r.RawJSON())
+	return param.Override[ResponseTextConfigParam](json.RawMessage(r.RawJSON()))
 }
 
 // Configuration options for a text response from the model. Can be plain text or
@@ -9643,7 +9643,7 @@ func (r *ToolUnion) UnmarshalJSON(data []byte) error {
 // be used at the last possible moment before sending a request. Test for this with
 // ToolUnionParam.Overrides()
 func (r ToolUnion) ToParam() ToolUnionParam {
-	return param.Override[ToolUnionParam](r.RawJSON())
+	return param.Override[ToolUnionParam](json.RawMessage(r.RawJSON()))
 }
 
 // Give the model access to additional tools via remote Model Context Protocol
@@ -10093,7 +10093,7 @@ type ToolUnionParam struct {
 }
 
 func (u ToolUnionParam) MarshalJSON() ([]byte, error) {
-	return param.MarshalUnion[ToolUnionParam](u.OfFunction,
+	return param.MarshalUnion(u, u.OfFunction,
 		u.OfFileSearch,
 		u.OfWebSearchPreview,
 		u.OfComputerUsePreview,
@@ -10430,7 +10430,7 @@ type ToolMcpAllowedToolsUnionParam struct {
 }
 
 func (u ToolMcpAllowedToolsUnionParam) MarshalJSON() ([]byte, error) {
-	return param.MarshalUnion[ToolMcpAllowedToolsUnionParam](u.OfMcpAllowedTools, u.OfMcpAllowedToolsFilter)
+	return param.MarshalUnion(u, u.OfMcpAllowedTools, u.OfMcpAllowedToolsFilter)
 }
 func (u *ToolMcpAllowedToolsUnionParam) UnmarshalJSON(data []byte) error {
 	return apijson.UnmarshalRoot(data, u)
@@ -10472,7 +10472,7 @@ type ToolMcpRequireApprovalUnionParam struct {
 }
 
 func (u ToolMcpRequireApprovalUnionParam) MarshalJSON() ([]byte, error) {
-	return param.MarshalUnion[ToolMcpRequireApprovalUnionParam](u.OfMcpToolApprovalFilter, u.OfMcpToolApprovalSetting)
+	return param.MarshalUnion(u, u.OfMcpToolApprovalFilter, u.OfMcpToolApprovalSetting)
 }
 func (u *ToolMcpRequireApprovalUnionParam) UnmarshalJSON(data []byte) error {
 	return apijson.UnmarshalRoot(data, u)
@@ -10565,7 +10565,7 @@ type ToolCodeInterpreterContainerUnionParam struct {
 }
 
 func (u ToolCodeInterpreterContainerUnionParam) MarshalJSON() ([]byte, error) {
-	return param.MarshalUnion[ToolCodeInterpreterContainerUnionParam](u.OfString, u.OfCodeInterpreterContainerAuto)
+	return param.MarshalUnion(u, u.OfString, u.OfCodeInterpreterContainerAuto)
 }
 func (u *ToolCodeInterpreterContainerUnionParam) UnmarshalJSON(data []byte) error {
 	return apijson.UnmarshalRoot(data, u)
@@ -10746,7 +10746,7 @@ func (r *ToolChoiceFunction) UnmarshalJSON(data []byte) error {
 // be used at the last possible moment before sending a request. Test for this with
 // ToolChoiceFunctionParam.Overrides()
 func (r ToolChoiceFunction) ToParam() ToolChoiceFunctionParam {
-	return param.Override[ToolChoiceFunctionParam](r.RawJSON())
+	return param.Override[ToolChoiceFunctionParam](json.RawMessage(r.RawJSON()))
 }
 
 // Use this option to force the model to call a specific function.
@@ -10824,7 +10824,7 @@ func (r *ToolChoiceTypes) UnmarshalJSON(data []byte) error {
 // be used at the last possible moment before sending a request. Test for this with
 // ToolChoiceTypesParam.Overrides()
 func (r ToolChoiceTypes) ToParam() ToolChoiceTypesParam {
-	return param.Override[ToolChoiceTypesParam](r.RawJSON())
+	return param.Override[ToolChoiceTypesParam](json.RawMessage(r.RawJSON()))
 }
 
 // The type of hosted tool the model should to use. Learn more about
@@ -10919,7 +10919,7 @@ func (r *WebSearchTool) UnmarshalJSON(data []byte) error {
 // be used at the last possible moment before sending a request. Test for this with
 // WebSearchToolParam.Overrides()
 func (r WebSearchTool) ToParam() WebSearchToolParam {
-	return param.Override[WebSearchToolParam](r.RawJSON())
+	return param.Override[WebSearchToolParam](json.RawMessage(r.RawJSON()))
 }
 
 // The type of the web search tool. One of `web_search_preview` or
@@ -11189,7 +11189,7 @@ type ResponseNewParamsInputUnion struct {
 }
 
 func (u ResponseNewParamsInputUnion) MarshalJSON() ([]byte, error) {
-	return param.MarshalUnion[ResponseNewParamsInputUnion](u.OfString, u.OfInputItemList)
+	return param.MarshalUnion(u, u.OfString, u.OfInputItemList)
 }
 func (u *ResponseNewParamsInputUnion) UnmarshalJSON(data []byte) error {
 	return apijson.UnmarshalRoot(data, u)
@@ -11241,7 +11241,7 @@ type ResponseNewParamsToolChoiceUnion struct {
 }
 
 func (u ResponseNewParamsToolChoiceUnion) MarshalJSON() ([]byte, error) {
-	return param.MarshalUnion[ResponseNewParamsToolChoiceUnion](u.OfToolChoiceMode, u.OfHostedTool, u.OfFunctionTool)
+	return param.MarshalUnion(u, u.OfToolChoiceMode, u.OfHostedTool, u.OfFunctionTool)
 }
 func (u *ResponseNewParamsToolChoiceUnion) UnmarshalJSON(data []byte) error {
 	return apijson.UnmarshalRoot(data, u)
diff --git a/shared/shared.go b/shared/shared.go
index b5b1887..077e25c 100644
--- a/shared/shared.go
+++ b/shared/shared.go
@@ -119,7 +119,7 @@ func (r *ComparisonFilter) UnmarshalJSON(data []byte) error {
 // be used at the last possible moment before sending a request. Test for this with
 // ComparisonFilterParam.Overrides()
 func (r ComparisonFilter) ToParam() ComparisonFilterParam {
-	return param.Override[ComparisonFilterParam](r.RawJSON())
+	return param.Override[ComparisonFilterParam](json.RawMessage(r.RawJSON()))
 }
 
 // Specifies the comparison operator: `eq`, `ne`, `gt`, `gte`, `lt`, `lte`.
@@ -228,7 +228,7 @@ type ComparisonFilterValueUnionParam struct {
 }
 
 func (u ComparisonFilterValueUnionParam) MarshalJSON() ([]byte, error) {
-	return param.MarshalUnion[ComparisonFilterValueUnionParam](u.OfString, u.OfFloat, u.OfBool)
+	return param.MarshalUnion(u, u.OfString, u.OfFloat, u.OfBool)
 }
 func (u *ComparisonFilterValueUnionParam) UnmarshalJSON(data []byte) error {
 	return apijson.UnmarshalRoot(data, u)
@@ -275,7 +275,7 @@ func (r *CompoundFilter) UnmarshalJSON(data []byte) error {
 // be used at the last possible moment before sending a request. Test for this with
 // CompoundFilterParam.Overrides()
 func (r CompoundFilter) ToParam() CompoundFilterParam {
-	return param.Override[CompoundFilterParam](r.RawJSON())
+	return param.Override[CompoundFilterParam](json.RawMessage(r.RawJSON()))
 }
 
 // Type of operation: `and` or `or`.
@@ -374,7 +374,7 @@ func (r *FunctionDefinition) UnmarshalJSON(data []byte) error {
 // be used at the last possible moment before sending a request. Test for this with
 // FunctionDefinitionParam.Overrides()
 func (r FunctionDefinition) ToParam() FunctionDefinitionParam {
-	return param.Override[FunctionDefinitionParam](r.RawJSON())
+	return param.Override[FunctionDefinitionParam](json.RawMessage(r.RawJSON()))
 }
 
 // The property Name is required.
@@ -466,7 +466,7 @@ func (r *Reasoning) UnmarshalJSON(data []byte) error {
 // be used at the last possible moment before sending a request. Test for this with
 // ReasoningParam.Overrides()
 func (r Reasoning) ToParam() ReasoningParam {
-	return param.Override[ReasoningParam](r.RawJSON())
+	return param.Override[ReasoningParam](json.RawMessage(r.RawJSON()))
 }
 
 // **Deprecated:** use `summary` instead.
@@ -577,7 +577,7 @@ func (ResponseFormatJSONObject) ImplResponseFormatTextConfigUnion() {}
 // be used at the last possible moment before sending a request. Test for this with
 // ResponseFormatJSONObjectParam.Overrides()
 func (r ResponseFormatJSONObject) ToParam() ResponseFormatJSONObjectParam {
-	return param.Override[ResponseFormatJSONObjectParam](r.RawJSON())
+	return param.Override[ResponseFormatJSONObjectParam](json.RawMessage(r.RawJSON()))
 }
 
 func NewResponseFormatJSONObjectParam() ResponseFormatJSONObjectParam {
@@ -636,7 +636,7 @@ func (r *ResponseFormatJSONSchema) UnmarshalJSON(data []byte) error {
 // be used at the last possible moment before sending a request. Test for this with
 // ResponseFormatJSONSchemaParam.Overrides()
 func (r ResponseFormatJSONSchema) ToParam() ResponseFormatJSONSchemaParam {
-	return param.Override[ResponseFormatJSONSchemaParam](r.RawJSON())
+	return param.Override[ResponseFormatJSONSchemaParam](json.RawMessage(r.RawJSON()))
 }
 
 // Structured Outputs configuration options, including a JSON Schema.
@@ -752,7 +752,7 @@ func (ResponseFormatText) ImplResponseFormatTextConfigUnion() {}
 // be used at the last possible moment before sending a request. Test for this with
 // ResponseFormatTextParam.Overrides()
 func (r ResponseFormatText) ToParam() ResponseFormatTextParam {
-	return param.Override[ResponseFormatTextParam](r.RawJSON())
+	return param.Override[ResponseFormatTextParam](json.RawMessage(r.RawJSON()))
 }
 
 func NewResponseFormatTextParam() ResponseFormatTextParam {
diff --git a/vectorstore.go b/vectorstore.go
index 9a69e03..9ee6f54 100644
--- a/vectorstore.go
+++ b/vectorstore.go
@@ -248,7 +248,7 @@ type FileChunkingStrategyParamUnion struct {
 }
 
 func (u FileChunkingStrategyParamUnion) MarshalJSON() ([]byte, error) {
-	return param.MarshalUnion[FileChunkingStrategyParamUnion](u.OfAuto, u.OfStatic)
+	return param.MarshalUnion(u, u.OfAuto, u.OfStatic)
 }
 func (u *FileChunkingStrategyParamUnion) UnmarshalJSON(data []byte) error {
 	return apijson.UnmarshalRoot(data, u)
@@ -339,7 +339,7 @@ func (r *StaticFileChunkingStrategy) UnmarshalJSON(data []byte) error {
 // be used at the last possible moment before sending a request. Test for this with
 // StaticFileChunkingStrategyParam.Overrides()
 func (r StaticFileChunkingStrategy) ToParam() StaticFileChunkingStrategyParam {
-	return param.Override[StaticFileChunkingStrategyParam](r.RawJSON())
+	return param.Override[StaticFileChunkingStrategyParam](json.RawMessage(r.RawJSON()))
 }
 
 // The properties ChunkOverlapTokens, MaxChunkSizeTokens are required.
@@ -808,7 +808,7 @@ type VectorStoreSearchParamsQueryUnion struct {
 }
 
 func (u VectorStoreSearchParamsQueryUnion) MarshalJSON() ([]byte, error) {
-	return param.MarshalUnion[VectorStoreSearchParamsQueryUnion](u.OfString, u.OfStringArray)
+	return param.MarshalUnion(u, u.OfString, u.OfStringArray)
 }
 func (u *VectorStoreSearchParamsQueryUnion) UnmarshalJSON(data []byte) error {
 	return apijson.UnmarshalRoot(data, u)
@@ -833,7 +833,7 @@ type VectorStoreSearchParamsFiltersUnion struct {
 }
 
 func (u VectorStoreSearchParamsFiltersUnion) MarshalJSON() ([]byte, error) {
-	return param.MarshalUnion[VectorStoreSearchParamsFiltersUnion](u.OfComparisonFilter, u.OfCompoundFilter)
+	return param.MarshalUnion(u, u.OfComparisonFilter, u.OfCompoundFilter)
 }
 func (u *VectorStoreSearchParamsFiltersUnion) UnmarshalJSON(data []byte) error {
 	return apijson.UnmarshalRoot(data, u)
diff --git a/vectorstorefile.go b/vectorstorefile.go
index bc04983..fedbb34 100644
--- a/vectorstorefile.go
+++ b/vectorstorefile.go
@@ -416,7 +416,7 @@ type VectorStoreFileNewParamsAttributeUnion struct {
 }
 
 func (u VectorStoreFileNewParamsAttributeUnion) MarshalJSON() ([]byte, error) {
-	return param.MarshalUnion[VectorStoreFileNewParamsAttributeUnion](u.OfString, u.OfFloat, u.OfBool)
+	return param.MarshalUnion(u, u.OfString, u.OfFloat, u.OfBool)
 }
 func (u *VectorStoreFileNewParamsAttributeUnion) UnmarshalJSON(data []byte) error {
 	return apijson.UnmarshalRoot(data, u)
@@ -462,7 +462,7 @@ type VectorStoreFileUpdateParamsAttributeUnion struct {
 }
 
 func (u VectorStoreFileUpdateParamsAttributeUnion) MarshalJSON() ([]byte, error) {
-	return param.MarshalUnion[VectorStoreFileUpdateParamsAttributeUnion](u.OfString, u.OfFloat, u.OfBool)
+	return param.MarshalUnion(u, u.OfString, u.OfFloat, u.OfBool)
 }
 func (u *VectorStoreFileUpdateParamsAttributeUnion) UnmarshalJSON(data []byte) error {
 	return apijson.UnmarshalRoot(data, u)
diff --git a/vectorstorefilebatch.go b/vectorstorefilebatch.go
index b9d3a15..7618cf5 100644
--- a/vectorstorefilebatch.go
+++ b/vectorstorefilebatch.go
@@ -292,7 +292,7 @@ type VectorStoreFileBatchNewParamsAttributeUnion struct {
 }
 
 func (u VectorStoreFileBatchNewParamsAttributeUnion) MarshalJSON() ([]byte, error) {
-	return param.MarshalUnion[VectorStoreFileBatchNewParamsAttributeUnion](u.OfString, u.OfFloat, u.OfBool)
+	return param.MarshalUnion(u, u.OfString, u.OfFloat, u.OfBool)
 }
 func (u *VectorStoreFileBatchNewParamsAttributeUnion) UnmarshalJSON(data []byte) error {
 	return apijson.UnmarshalRoot(data, u)