diff --git a/specification/cognitiveservices/OpenAI.Inference/models/completions/chat_messages.tsp b/specification/cognitiveservices/OpenAI.Inference/models/completions/chat_messages.tsp
index 788e0e490f63..b323ce3d38f3 100644
--- a/specification/cognitiveservices/OpenAI.Inference/models/completions/chat_messages.tsp
+++ b/specification/cognitiveservices/OpenAI.Inference/models/completions/chat_messages.tsp
@@ -13,6 +13,16 @@ using TypeSpec.Versioning;
 
 namespace Azure.OpenAI;
 
+#suppress "@azure-tools/typespec-autorest/union-unsupported" "External API shape is defined in OpenAPI 3.0 as oneOf."
+@removed(ServiceApiVersions.v2024_08_01_Preview)
+@doc("The contents of the user message, with available input types varying by selected model.")
+union ChatMessageContent {
+  string,
+
+  @added(ServiceApiVersions.v2024_02_15_Preview)
+  ChatMessageContentItem[],
+}
+
 @added(ServiceApiVersions.v2024_02_15_Preview)
 @doc("An abstract representation of a structured content item within a chat message.")
 @discriminator("type")
@@ -121,7 +131,7 @@ model ChatRequestUserMessage extends ChatRequestMessage {
 
   #suppress "@azure-tools/typespec-autorest/union-unsupported" "OpenAPI v2 support deferred"
   @doc("The contents of the user message, with available input types varying by selected model.")
-  @typeChangedFrom(ServiceApiVersions.v2024_08_01_Preview, string | ChatMessageContentItem[])
+  @typeChangedFrom(ServiceApiVersions.v2024_08_01_Preview, ChatMessageContent)
   content: string | ChatMessageTextContentItem[] | ChatMessageImageContentItem[];
 
   @doc("An optional name for the participant.")
diff --git a/specification/cognitiveservices/data-plane/AzureOpenAI/inference/preview/2023-06-01-preview/generated.json b/specification/cognitiveservices/data-plane/AzureOpenAI/inference/preview/2023-06-01-preview/generated.json
index e256ef0ca371..b0b758392aa2 100644
--- a/specification/cognitiveservices/data-plane/AzureOpenAI/inference/preview/2023-06-01-preview/generated.json
+++ b/specification/cognitiveservices/data-plane/AzureOpenAI/inference/preview/2023-06-01-preview/generated.json
@@ -588,6 +588,10 @@
         "messages"
       ]
     },
+    "ChatMessageContent": {
+      "type": "string",
+      "x-nullable": false
+    },
     "ChatRequestAssistantMessage": {
       "type": "object",
       "description": "A request chat message representing response or action from the assistant.",
@@ -652,6 +656,7 @@
       "description": "A request chat message representing user input to the assistant.",
       "properties": {
         "content": {
+          "$ref": "#/definitions/ChatMessageContent",
           "description": "The contents of the user message, with available input types varying by selected model."
         },
         "name": {
diff --git a/specification/cognitiveservices/data-plane/AzureOpenAI/inference/preview/2023-07-01-preview/generated.json b/specification/cognitiveservices/data-plane/AzureOpenAI/inference/preview/2023-07-01-preview/generated.json
index 0cc4f303cbe5..4b4b45c51f65 100644
--- a/specification/cognitiveservices/data-plane/AzureOpenAI/inference/preview/2023-07-01-preview/generated.json
+++ b/specification/cognitiveservices/data-plane/AzureOpenAI/inference/preview/2023-07-01-preview/generated.json
@@ -600,6 +600,10 @@
         "messages"
       ]
     },
+    "ChatMessageContent": {
+      "type": "string",
+      "x-nullable": false
+    },
     "ChatRequestAssistantMessage": {
       "type": "object",
       "description": "A request chat message representing response or action from the assistant.",
@@ -694,6 +698,7 @@
       "description": "A request chat message representing user input to the assistant.",
       "properties": {
         "content": {
+          "$ref": "#/definitions/ChatMessageContent",
           "description": "The contents of the user message, with available input types varying by selected model."
         },
         "name": {
diff --git a/specification/cognitiveservices/data-plane/AzureOpenAI/inference/preview/2024-02-15-preview/generated.json b/specification/cognitiveservices/data-plane/AzureOpenAI/inference/preview/2024-02-15-preview/generated.json
index 8068385a503b..0b8e2f430fee 100644
--- a/specification/cognitiveservices/data-plane/AzureOpenAI/inference/preview/2024-02-15-preview/generated.json
+++ b/specification/cognitiveservices/data-plane/AzureOpenAI/inference/preview/2024-02-15-preview/generated.json
@@ -2326,6 +2326,7 @@
         "type"
       ]
     },
+    "ChatMessageContent": {},
     "ChatRequestAssistantMessage": {
       "type": "object",
       "description": "A request chat message representing response or action from the assistant.",
@@ -2454,6 +2455,7 @@
       "description": "A request chat message representing user input to the assistant.",
       "properties": {
         "content": {
+          "$ref": "#/definitions/ChatMessageContent",
           "description": "The contents of the user message, with available input types varying by selected model."
         },
         "name": {
diff --git a/specification/cognitiveservices/data-plane/AzureOpenAI/inference/preview/2024-03-01-preview/generated.json b/specification/cognitiveservices/data-plane/AzureOpenAI/inference/preview/2024-03-01-preview/generated.json
index 604908670166..3ffa78d976d5 100644
--- a/specification/cognitiveservices/data-plane/AzureOpenAI/inference/preview/2024-03-01-preview/generated.json
+++ b/specification/cognitiveservices/data-plane/AzureOpenAI/inference/preview/2024-03-01-preview/generated.json
@@ -2326,6 +2326,7 @@
         "type"
       ]
     },
+    "ChatMessageContent": {},
     "ChatRequestAssistantMessage": {
       "type": "object",
       "description": "A request chat message representing response or action from the assistant.",
@@ -2454,6 +2455,7 @@
       "description": "A request chat message representing user input to the assistant.",
       "properties": {
         "content": {
+          "$ref": "#/definitions/ChatMessageContent",
           "description": "The contents of the user message, with available input types varying by selected model."
         },
         "name": {
diff --git a/specification/cognitiveservices/data-plane/AzureOpenAI/inference/preview/2024-04-01-preview/generated.json b/specification/cognitiveservices/data-plane/AzureOpenAI/inference/preview/2024-04-01-preview/generated.json
index d25b3a41e46a..11c47686fb64 100644
--- a/specification/cognitiveservices/data-plane/AzureOpenAI/inference/preview/2024-04-01-preview/generated.json
+++ b/specification/cognitiveservices/data-plane/AzureOpenAI/inference/preview/2024-04-01-preview/generated.json
@@ -2462,6 +2462,7 @@
         "type"
       ]
     },
+    "ChatMessageContent": {},
     "ChatRequestAssistantMessage": {
       "type": "object",
       "description": "A request chat message representing response or action from the assistant.",
@@ -2590,6 +2591,7 @@
       "description": "A request chat message representing user input to the assistant.",
       "properties": {
         "content": {
+          "$ref": "#/definitions/ChatMessageContent",
           "description": "The contents of the user message, with available input types varying by selected model."
         },
         "name": {
diff --git a/specification/cognitiveservices/data-plane/AzureOpenAI/inference/preview/2024-05-01-preview/generated.json b/specification/cognitiveservices/data-plane/AzureOpenAI/inference/preview/2024-05-01-preview/generated.json
index 8fb501e9453d..2f3fecf5277a 100644
--- a/specification/cognitiveservices/data-plane/AzureOpenAI/inference/preview/2024-05-01-preview/generated.json
+++ b/specification/cognitiveservices/data-plane/AzureOpenAI/inference/preview/2024-05-01-preview/generated.json
@@ -2616,6 +2616,7 @@
         "type"
       ]
     },
+    "ChatMessageContent": {},
     "ChatRequestAssistantMessage": {
       "type": "object",
       "description": "A request chat message representing response or action from the assistant.",
@@ -2744,6 +2745,7 @@
       "description": "A request chat message representing user input to the assistant.",
       "properties": {
         "content": {
+          "$ref": "#/definitions/ChatMessageContent",
           "description": "The contents of the user message, with available input types varying by selected model."
         },
         "name": {
diff --git a/specification/cognitiveservices/data-plane/AzureOpenAI/inference/preview/2024-07-01-preview/generated.json b/specification/cognitiveservices/data-plane/AzureOpenAI/inference/preview/2024-07-01-preview/generated.json
index 3faefd7bf412..2850dc5290bf 100644
--- a/specification/cognitiveservices/data-plane/AzureOpenAI/inference/preview/2024-07-01-preview/generated.json
+++ b/specification/cognitiveservices/data-plane/AzureOpenAI/inference/preview/2024-07-01-preview/generated.json
@@ -3293,6 +3293,7 @@
         "type"
       ]
     },
+    "ChatMessageContent": {},
     "ChatRequestAssistantMessage": {
       "type": "object",
       "description": "A request chat message representing response or action from the assistant.",
@@ -3421,6 +3422,7 @@
       "description": "A request chat message representing user input to the assistant.",
       "properties": {
         "content": {
+          "$ref": "#/definitions/ChatMessageContent",
           "description": "The contents of the user message, with available input types varying by selected model."
         },
         "name": {
diff --git a/specification/cognitiveservices/data-plane/AzureOpenAI/inference/stable/2023-05-15/generated.json b/specification/cognitiveservices/data-plane/AzureOpenAI/inference/stable/2023-05-15/generated.json
index b121b443c361..83a2a7aa2d12 100644
--- a/specification/cognitiveservices/data-plane/AzureOpenAI/inference/stable/2023-05-15/generated.json
+++ b/specification/cognitiveservices/data-plane/AzureOpenAI/inference/stable/2023-05-15/generated.json
@@ -405,6 +405,10 @@
         "messages"
       ]
     },
+    "ChatMessageContent": {
+      "type": "string",
+      "x-nullable": false
+    },
     "ChatRequestAssistantMessage": {
       "type": "object",
       "description": "A request chat message representing response or action from the assistant.",
@@ -469,6 +473,7 @@
       "description": "A request chat message representing user input to the assistant.",
       "properties": {
         "content": {
+          "$ref": "#/definitions/ChatMessageContent",
           "description": "The contents of the user message, with available input types varying by selected model."
         },
         "name": {
diff --git a/specification/cognitiveservices/data-plane/AzureOpenAI/inference/stable/2024-02-01/generated.json b/specification/cognitiveservices/data-plane/AzureOpenAI/inference/stable/2024-02-01/generated.json
index f219b028b2e0..2cd199818a7b 100644
--- a/specification/cognitiveservices/data-plane/AzureOpenAI/inference/stable/2024-02-01/generated.json
+++ b/specification/cognitiveservices/data-plane/AzureOpenAI/inference/stable/2024-02-01/generated.json
@@ -600,6 +600,10 @@
         "messages"
       ]
     },
+    "ChatMessageContent": {
+      "type": "string",
+      "x-nullable": false
+    },
     "ChatRequestAssistantMessage": {
       "type": "object",
       "description": "A request chat message representing response or action from the assistant.",
@@ -694,6 +698,7 @@
       "description": "A request chat message representing user input to the assistant.",
       "properties": {
         "content": {
+          "$ref": "#/definitions/ChatMessageContent",
           "description": "The contents of the user message, with available input types varying by selected model."
         },
         "name": {
diff --git a/specification/cognitiveservices/data-plane/AzureOpenAI/inference/stable/2024-06-01/generated.json b/specification/cognitiveservices/data-plane/AzureOpenAI/inference/stable/2024-06-01/generated.json
index 1582b5c7d135..823f294c8fb1 100644
--- a/specification/cognitiveservices/data-plane/AzureOpenAI/inference/stable/2024-06-01/generated.json
+++ b/specification/cognitiveservices/data-plane/AzureOpenAI/inference/stable/2024-06-01/generated.json
@@ -2616,6 +2616,7 @@
         "type"
       ]
     },
+    "ChatMessageContent": {},
     "ChatRequestAssistantMessage": {
       "type": "object",
       "description": "A request chat message representing response or action from the assistant.",
@@ -2744,6 +2745,7 @@
       "description": "A request chat message representing user input to the assistant.",
       "properties": {
         "content": {
+          "$ref": "#/definitions/ChatMessageContent",
           "description": "The contents of the user message, with available input types varying by selected model."
         },
         "name": {