diff --git a/client_test.go b/client_test.go
index 321971445..bf2c9f094 100644
--- a/client_test.go
+++ b/client_test.go
@@ -276,10 +276,10 @@ func TestClientReturnsRequestBuilderErrors(t *testing.T) {
 
 	testCases := []TestCase{
 		{"CreateCompletion", func() (any, error) {
-			return client.CreateCompletion(ctx, CompletionRequest{Prompt: "testing"})
+			return client.CreateCompletion(ctx, CompletionRequest{Model: GPT3Dot5TurboInstruct, Prompt: "testing"})
 		}},
 		{"CreateCompletionStream", func() (any, error) {
-			return client.CreateCompletionStream(ctx, CompletionRequest{Prompt: ""})
+			return client.CreateCompletionStream(ctx, CompletionRequest{Model: GPT3Dot5TurboInstruct, Prompt: ""})
 		}},
 		{"CreateChatCompletion", func() (any, error) {
 			return client.CreateChatCompletion(ctx, ChatCompletionRequest{Model: GPT3Dot5Turbo})
@@ -475,11 +475,11 @@ func TestClientReturnsRequestBuilderErrorsAddition(t *testing.T) {
 	client := NewClientWithConfig(config)
 	client.requestBuilder = &failingRequestBuilder{}
 	ctx := context.Background()
-	_, err := client.CreateCompletion(ctx, CompletionRequest{Prompt: 1})
+	_, err := client.CreateCompletion(ctx, CompletionRequest{Model: GPT3Dot5TurboInstruct, Prompt: 1})
 	if !errors.Is(err, ErrCompletionRequestPromptTypeNotSupported) {
 		t.Fatalf("Did not return error when request builder failed: %v", err)
 	}
-	_, err = client.CreateCompletionStream(ctx, CompletionRequest{Prompt: 1})
+	_, err = client.CreateCompletionStream(ctx, CompletionRequest{Model: GPT3Dot5TurboInstruct, Prompt: 1})
 	if !errors.Is(err, ErrCompletionRequestPromptTypeNotSupported) {
 		t.Fatalf("Did not return error when request builder failed: %v", err)
 	}
diff --git a/completion.go b/completion.go
index 9c3a64dd5..71c8a7df2 100644
--- a/completion.go
+++ b/completion.go
@@ -5,6 +5,11 @@ import (
 	"net/http"
 )
 
+// Endpoint constants.
+const (
+	completionsEndpoint = "/completions"
+)
+
 // GPT3 Defines the models provided by OpenAI to use when generating
 // completions from OpenAI.
 // GPT3 Models are designed for text-based tasks. For code-specific
@@ -95,75 +100,61 @@ const (
 	CodexCodeDavinci001 = "code-davinci-001"
 )
 
-var disabledModelsForEndpoints = map[string]map[string]bool{
-	"/completions": {
-		O1Mini:                  true,
-		O1Mini20240912:          true,
-		O1Preview:               true,
-		O1Preview20240912:       true,
-		O3Mini:                  true,
-		O3Mini20250131:          true,
-		O4Mini:                  true,
-		O4Mini2020416:           true,
-		O3:                      true,
-		O320250416:              true,
-		GPT3Dot5Turbo:           true,
-		GPT3Dot5Turbo0301:       true,
-		GPT3Dot5Turbo0613:       true,
-		GPT3Dot5Turbo1106:       true,
-		GPT3Dot5Turbo0125:       true,
-		GPT3Dot5Turbo16K:        true,
-		GPT3Dot5Turbo16K0613:    true,
-		GPT4:                    true,
-		GPT4Dot5Preview:         true,
-		GPT4Dot5Preview20250227: true,
-		GPT4o:                   true,
-		GPT4o20240513:           true,
-		GPT4o20240806:           true,
-		GPT4o20241120:           true,
-		GPT4oLatest:             true,
-		GPT4oMini:               true,
-		GPT4oMini20240718:       true,
-		GPT4TurboPreview:        true,
-		GPT4VisionPreview:       true,
-		GPT4Turbo1106:           true,
-		GPT4Turbo0125:           true,
-		GPT4Turbo:               true,
-		GPT4Turbo20240409:       true,
-		GPT40314:                true,
-		GPT40613:                true,
-		GPT432K:                 true,
-		GPT432K0314:             true,
-		GPT432K0613:             true,
-		O1:                      true,
-		GPT4Dot1:                true,
-		GPT4Dot120250414:        true,
-		GPT4Dot1Mini:            true,
-		GPT4Dot1Mini20250414:    true,
-		GPT4Dot1Nano:            true,
-		GPT4Dot1Nano20250414:    true,
-	},
-	chatCompletionsSuffix: {
-		CodexCodeDavinci002:     true,
-		CodexCodeCushman001:     true,
-		CodexCodeDavinci001:     true,
-		GPT3TextDavinci003:      true,
-		GPT3TextDavinci002:      true,
-		GPT3TextCurie001:        true,
-		GPT3TextBabbage001:      true,
-		GPT3TextAda001:          true,
-		GPT3TextDavinci001:      true,
-		GPT3DavinciInstructBeta: true,
-		GPT3Davinci:             true,
-		GPT3CurieInstructBeta:   true,
-		GPT3Curie:               true,
-		GPT3Ada:                 true,
-		GPT3Babbage:             true,
-	},
+// This map lists models that are *not* supported by the chat completions endpoint.
+// We maintain this deny list because most new models *are* supported by chat completions.
+// It is simpler to list the exceptions rather than updating an allow list
+// each time a new model is released and supported.
+var disabledModelsForChatCompletionsEndpoint = map[string]bool{
+	CodexCodeCushman001:     true,
+	CodexCodeDavinci001:     true,
+	CodexCodeDavinci002:     true,
+	GPT3Ada:                 true,
+	GPT3Babbage:             true,
+	GPT3Curie:               true,
+	GPT3CurieInstructBeta:   true,
+	GPT3Davinci:             true,
+	GPT3DavinciInstructBeta: true,
+	GPT3TextAda001:          true,
+	GPT3TextBabbage001:      true,
+	GPT3TextCurie001:        true,
+	GPT3TextDavinci001:      true,
+	GPT3TextDavinci002:      true,
+	GPT3TextDavinci003:      true,
+}
+
+// This map lists models that *are* supported by the deprecated completions endpoint.
+// We maintain this allow list because the completions endpoint is deprecated and most new models
+// are *not* supported by it. It is simpler to list the older models that are supported
+// rather than updating a deny list each time a new model is released and *not* supported.
+var enabledModelsForDeprecatedCompletionsEndpoint = map[string]bool{
+	CodexCodeCushman001:     true,
+	CodexCodeDavinci001:     true,
+	CodexCodeDavinci002:     true,
+	GPT3Ada:                 true,
+	GPT3Ada002:              true,
+	GPT3Babbage:             true,
+	GPT3Babbage002:          true,
+	GPT3Curie:               true,
+	GPT3Curie002:            true,
+	GPT3CurieInstructBeta:   true,
+	GPT3Davinci:             true,
+	GPT3Davinci002:          true,
+	GPT3DavinciInstructBeta: true,
+	GPT3Dot5TurboInstruct:   true,
+	GPT3TextAda001:          true,
+	GPT3TextBabbage001:      true,
+	GPT3TextCurie001:        true,
+	GPT3TextDavinci001:      true,
+	GPT3TextDavinci002:      true,
+	GPT3TextDavinci003:      true,
 }
 
 func checkEndpointSupportsModel(endpoint, model string) bool {
-	return !disabledModelsForEndpoints[endpoint][model]
+	if endpoint == completionsEndpoint {
+		return enabledModelsForDeprecatedCompletionsEndpoint[model]
+	}
+
+	return !disabledModelsForChatCompletionsEndpoint[model]
 }
 
 func checkPromptType(prompt any) bool {
@@ -261,7 +252,7 @@ func (c *Client) CreateCompletion(
 		return
 	}
 
-	urlSuffix := "/completions"
+	urlSuffix := completionsEndpoint
 	if !checkEndpointSupportsModel(urlSuffix, request.Model) {
 		err = ErrCompletionUnsupportedModel
 		return