diff --git a/CHANGELOG.md b/CHANGELOG.md
index 258db3f1..9a3e1e5f 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -4,6 +4,9 @@
 ### Added
 - **chat**: add reasoning effort, max completion tokens, store options for reasoning model support (#415) (thanks @Taewan-P)
 
+### Added
+- **chat**: feat(chat): add reasoningContent support for both ChatMessage and ChatDelta
+
 ## 4.0.0
 > Published 01 Feb 2025
 
diff --git a/openai-core/src/commonMain/kotlin/com.aallam.openai.api/chat/ChatDelta.kt b/openai-core/src/commonMain/kotlin/com.aallam.openai.api/chat/ChatDelta.kt
index db448ebe..40540c1e 100644
--- a/openai-core/src/commonMain/kotlin/com.aallam.openai.api/chat/ChatDelta.kt
+++ b/openai-core/src/commonMain/kotlin/com.aallam.openai.api/chat/ChatDelta.kt
@@ -18,6 +18,11 @@ public data class ChatDelta(
      */
     @SerialName("content") val content: String? = null,
 
+    /**
+     * The reasoning contents of the message.
+     */
+    @SerialName("reasoning_content") val reasoningContent: String? = null,
+
     /**
      * The name and arguments of a function that should be called, as generated by the model.
      */
diff --git a/openai-core/src/commonMain/kotlin/com.aallam.openai.api/chat/ChatMessage.kt b/openai-core/src/commonMain/kotlin/com.aallam.openai.api/chat/ChatMessage.kt
index 1a76e031..9d0f778a 100644
--- a/openai-core/src/commonMain/kotlin/com.aallam.openai.api/chat/ChatMessage.kt
+++ b/openai-core/src/commonMain/kotlin/com.aallam.openai.api/chat/ChatMessage.kt
@@ -23,6 +23,12 @@ public data class ChatMessage(
      */
     @SerialName("content") public val messageContent: Content? = null,
 
+    /**
+     * The model's reasoning content, providing intermediate logical steps or justification for the response.
+     * This field is typically populated by advanced models supporting reasoning transparency.
+     */
+    @SerialName("reasoning_content") public val messageReasoningContent: Content? = null,
+
     /**
      * The author's name of this message.
      * [name] is required if the role is `[ChatRole.Function], and it should be the name of the function whose response is
@@ -103,6 +109,12 @@ public data class ChatMessage(
             else -> error("Content is not text")
         }
 
+    val reasoningContent: String?
+        get() = when (messageReasoningContent) {
+            is TextContent? -> messageReasoningContent?.content
+            else -> error("Content is not text")
+        }
+
     @Suppress("FunctionName")
     public companion object {