From 6eb6748b8be7a332d504b474bfde96f76d361414 Mon Sep 17 00:00:00 2001
From: Aditya Kar <akar016012@icloud.com>
Date: Tue, 30 Sep 2025 23:43:59 -0400
Subject: [PATCH 1/2] fix: handle param.Opt types in
 ChatCompletionToolChoiceOptionUnionParam unmarshaling

This commit addresses issue #520 where the JSON unmarshaling fails for ChatCompletionToolChoiceOptionUnionParam
when encountering OfAuto field with param.Opt[string] type. The decoder.field.Type.Elem() method was calling
.Elem() on param.Opt[string] which is not a pointer type, causing unmarshaling to fail.

Modified files and changes:
- chatcompletion.go:
  * Added custom UnmarshalJSON method for ChatCompletionToolChoiceOptionUnionParam
  * Implemented proper handling of simple string values for OfAuto field
  * Added discriminated object unmarshaling based on "type" field
  * Added support for "function", "allowed_tools", and "custom" types
  * Replaced generic apijson.UnmarshalRoot call with type-specific handling

This fix ensures that the union parameter can properly unmarshal both string values and complex objects
without encountering type reflection errors when dealing with param.Opt types.
---
 chatcompletion.go | 36 +++++++++++++++++++++++++++++++++++-
 1 file changed, 35 insertions(+), 1 deletion(-)

diff --git a/chatcompletion.go b/chatcompletion.go
index 962b69d0..b6b3bfbf 100644
--- a/chatcompletion.go
+++ b/chatcompletion.go
@@ -2689,8 +2689,42 @@ type ChatCompletionToolChoiceOptionUnionParam struct {
 func (u ChatCompletionToolChoiceOptionUnionParam) MarshalJSON() ([]byte, error) {
 	return param.MarshalUnion(u, u.OfAuto, u.OfAllowedTools, u.OfFunctionToolChoice, u.OfCustomToolChoice)
 }
+
+// https://github.com/openai/openai-go/issues/520
+// The issue: decoder.field.Type.Elem() calls .Elem() on the field type. When it encounters the OfAuto field (which is param.Opt[string]), calling .Elem() fails because param.Opt[string] is not a pointer type.
+// Solution: either handle it here or modify the Elem() code to handle param.Opt types.
 func (u *ChatCompletionToolChoiceOptionUnionParam) UnmarshalJSON(data []byte) error {
-	return apijson.UnmarshalRoot(data, u)
+	// Handle simple string values first
+	var str string
+	if json.Unmarshal(data, &str) == nil {
+		u.OfAuto = param.Opt[string]{Value: str}
+		return nil
+	}
+
+	// Handle discriminated objects
+	var obj struct {
+		Type string `json:"type"`
+	}
+	if json.Unmarshal(data, &obj) != nil {
+		return nil // Invalid format, but don't error
+	}
+
+	// Unmarshal based on type using a single apijson.UnmarshalRoot call
+	switch obj.Type {
+	case "function":
+		var v ChatCompletionNamedToolChoiceParam
+		apijson.UnmarshalRoot(data, &v)
+		u.OfFunctionToolChoice = &v
+	case "allowed_tools":
+		var v ChatCompletionAllowedToolChoiceParam
+		apijson.UnmarshalRoot(data, &v)
+		u.OfAllowedTools = &v
+	case "custom":
+		var v ChatCompletionNamedToolChoiceCustomParam
+		apijson.UnmarshalRoot(data, &v)
+		u.OfCustomToolChoice = &v
+	}
+	return nil
 }
 
 func (u *ChatCompletionToolChoiceOptionUnionParam) asAny() any {

From 3fcb41f16c805e3a9a724cc25afc6a96c3daafa1 Mon Sep 17 00:00:00 2001
From: Aditya Kar <akar016012@icloud.com>
Date: Tue, 30 Sep 2025 23:53:46 -0400
Subject: [PATCH 2/2] fix: clarify comments regarding param.Opt handling in
 UnmarshalJSON method

---
 chatcompletion.go | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/chatcompletion.go b/chatcompletion.go
index b6b3bfbf..362d0d86 100644
--- a/chatcompletion.go
+++ b/chatcompletion.go
@@ -2691,8 +2691,8 @@ func (u ChatCompletionToolChoiceOptionUnionParam) MarshalJSON() ([]byte, error)
 }
 
 // https://github.com/openai/openai-go/issues/520
-// The issue: decoder.field.Type.Elem() calls .Elem() on the field type. When it encounters the OfAuto field (which is param.Opt[string]), calling .Elem() fails because param.Opt[string] is not a pointer type.
-// Solution: either handle it here or modify the Elem() code to handle param.Opt types.
+// The issue: in union.go, newStructUnionDecoder method -> decoder.field.Type.Elem() calls .Elem() on the field type. When it encounters the OfAuto field (which is param.Opt[string]), calling .Elem() fails because param.Opt[string] is not a pointer type.
+// Solution: either handle it here or modify the newStructUnionDecoder code to handle param.Opt types.
 func (u *ChatCompletionToolChoiceOptionUnionParam) UnmarshalJSON(data []byte) error {
 	// Handle simple string values first
 	var str string