diff --git a/backend/src/adapters/request/anthropic.ts b/backend/src/adapters/request/anthropic.ts
index 10e1e99..7db6ffb 100644
--- a/backend/src/adapters/request/anthropic.ts
+++ b/backend/src/adapters/request/anthropic.ts
@@ -70,10 +70,12 @@ interface AnthropicRequest {
 }
 
 // =============================================================================
-// Known Fields (for extracting extra body)
+// Mapped Fields (fields explicitly extracted by parse(), excluded from extraParams)
+// All other fields (e.g. metadata) are passed through via extraParams
+// to the upstream provider.
 // =============================================================================
 
-const KNOWN_FIELDS = new Set([
+const MAPPED_FIELDS = new Set([
   "model",
   "messages",
   "system",
@@ -85,7 +87,6 @@ const KNOWN_FIELDS = new Set([
   "stop_sequences",
   "tools",
   "tool_choice",
-  "metadata",
 ]);
 
 // =============================================================================
@@ -343,7 +344,7 @@ export const anthropicRequestAdapter: RequestAdapter<AnthropicRequest> = {
     let hasExtra = false;
 
     for (const [key, value] of Object.entries(body)) {
-      if (!KNOWN_FIELDS.has(key)) {
+      if (!MAPPED_FIELDS.has(key)) {
         extra[key] = value;
         hasExtra = true;
       }
diff --git a/backend/src/adapters/request/openai-chat.ts b/backend/src/adapters/request/openai-chat.ts
index ef6839f..4a5a188 100644
--- a/backend/src/adapters/request/openai-chat.ts
+++ b/backend/src/adapters/request/openai-chat.ts
@@ -112,30 +112,22 @@ interface OpenAIChatRequest {
 }
 
 // =============================================================================
-// Known Fields (for extracting extra body)
+// Mapped Fields (fields explicitly extracted by parse(), excluded from extraParams)
+// All other fields (e.g. response_format, presence_penalty, seed, etc.)
+// are passed through via extraParams to the upstream provider.
 // =============================================================================
 
-const KNOWN_FIELDS = new Set([
+const MAPPED_FIELDS = new Set([
   "model",
   "messages",
   "max_tokens",
   "max_completion_tokens",
   "temperature",
   "top_p",
-  "n",
   "stream",
-  "stream_options",
   "stop",
   "tools",
   "tool_choice",
-  "presence_penalty",
-  "frequency_penalty",
-  "logit_bias",
-  "logprobs",
-  "top_logprobs",
-  "user",
-  "seed",
-  "response_format",
 ]);
 
 // =============================================================================
@@ -340,7 +332,7 @@ export const openaiChatRequestAdapter: RequestAdapter<OpenAIChatRequest> = {
     let hasExtra = false;
 
     for (const [key, value] of Object.entries(body)) {
-      if (!KNOWN_FIELDS.has(key)) {
+      if (!MAPPED_FIELDS.has(key)) {
         extra[key] = value;
         hasExtra = true;
       }
diff --git a/backend/src/adapters/request/openai-response.ts b/backend/src/adapters/request/openai-response.ts
index 296ed7e..777d83f 100644
--- a/backend/src/adapters/request/openai-response.ts
+++ b/backend/src/adapters/request/openai-response.ts
@@ -109,24 +109,21 @@ interface ResponseApiRequest {
 }
 
 // =============================================================================
-// Known Fields (for extracting extra body)
+// Mapped Fields (fields explicitly extracted by parse(), excluded from extraParams)
+// All other fields (e.g. modalities, parallel_tool_calls, store, metadata, etc.)
+// are passed through via extraParams to the upstream provider.
 // =============================================================================
 
-const KNOWN_FIELDS = new Set([
+const MAPPED_FIELDS = new Set([
   "model",
   "input",
   "instructions",
-  "modalities",
   "max_output_tokens",
   "temperature",
   "top_p",
   "stream",
   "tools",
   "tool_choice",
-  "parallel_tool_calls",
-  "previous_response_id",
-  "store",
-  "metadata",
 ]);
 
 // =============================================================================
@@ -298,7 +295,7 @@ export const openaiResponseRequestAdapter: RequestAdapter<ResponseApiRequest> =
       let hasExtra = false;
 
       for (const [key, value] of Object.entries(body)) {
-        if (!KNOWN_FIELDS.has(key)) {
+        if (!MAPPED_FIELDS.has(key)) {
           extra[key] = value;
           hasExtra = true;
         }
diff --git a/backend/src/adapters/response/openai-chat.ts b/backend/src/adapters/response/openai-chat.ts
index e6a4660..2fa81ea 100644
--- a/backend/src/adapters/response/openai-chat.ts
+++ b/backend/src/adapters/response/openai-chat.ts
@@ -35,6 +35,7 @@ interface OpenAIChatChoice {
 interface OpenAIChatMessage {
   role: "assistant";
   content: string | null;
+  reasoning_content?: string | null;
   tool_calls?: OpenAIToolCall[];
   refusal?: string | null;
 }
@@ -113,9 +114,11 @@ function convertStopReason(stopReason: StopReason): string | null {
 }
 
 /**
- * Extract text content from internal content blocks
+ * Extract text and reasoning content from internal content blocks
  */
-function extractTextContent(content: InternalContentBlock[]): string {
+function extractContent(
+  content: InternalContentBlock[],
+): { text: string; reasoning?: string } {
   const textParts: string[] = [];
   const thinkingParts: string[] = [];
 
@@ -127,14 +130,10 @@ function extractTextContent(content: InternalContentBlock[]): string {
     }
   }
 
-  // Prepend thinking content wrapped in <think> tags if present
-  let result = "";
-  if (thinkingParts.length > 0) {
-    result += `<think>${thinkingParts.join("")}</think>\n`;
-  }
-  result += textParts.join("");
-
-  return result;
+  return {
+    text: textParts.join(""),
+    reasoning: thinkingParts.length > 0 ? thinkingParts.join("") : undefined,
+  };
 }
 
 /**
@@ -168,7 +167,8 @@ export const openaiChatResponseAdapter: ResponseAdapter<OpenAIChatCompletion> =
     format: "openai-chat",
 
     serialize(response: InternalResponse): OpenAIChatCompletion {
-      const content = extractTextContent(response.content);
+      const { text: content, reasoning: reasoningContent } =
+        extractContent(response.content);
       const toolCalls = convertToolCalls(response.content);
 
       return {
@@ -182,6 +182,7 @@ export const openaiChatResponseAdapter: ResponseAdapter<OpenAIChatCompletion> =
             message: {
               role: "assistant",
               content: content || null,
+              ...(reasoningContent !== undefined && { reasoning_content: reasoningContent }),
               tool_calls: toolCalls,
             },
             finish_reason: convertStopReason(response.stopReason),