--- a/src/config.ts
+++ b/src/config.ts
@@ -44,6 +44,12 @@
   "count-tokens-ms": number;
 }
 
+export interface ThinkingConfig {
+  /** Default reasoning effort when the client sends no reasoning_effort.
+   *  Accepts: none | minimal | low | medium | high | xhigh (default: medium) */
+  "default-effort": string;
+}
+
 export interface StatsConfig {
   /** Default true. Set false to disable per-request stats recording entirely. */
   enabled: boolean;
@@ -58,6 +64,7 @@
   "api-keys": Set<string>;
   "body-limit": string;
   cloaking: CloakingConfig;
+  thinking: ThinkingConfig;
   timeouts: TimeoutConfig;
   stats: StatsConfig;
   debug: DebugMode;
@@ -78,6 +85,9 @@
     "cli-version": "2.1.88",
     entrypoint: "cli",
   },
+  thinking: {
+    "default-effort": "medium",
+  },
   timeouts: {
     "messages-ms": 120000,
     "stream-messages-ms": 600000,
--- a/src/handlers/openai.ts
+++ b/src/handlers/openai.ts
@@ -540,7 +540,13 @@
       const structured =
         body.response_format?.type === "json_object" ||
         body.response_format?.type === "json_schema";
-      const translatedBody = openaiToAnthropic(body);
+      // Inject default thinking effort when the client hasn't specified one.
+      const effort = config.thinking?.["default-effort"] ?? "medium";
+      const bodyWithThinking =
+        body.reasoning_effort || effort === "none"
+          ? body
+          : { ...body, reasoning_effort: effort };
+      const translatedBody = openaiToAnthropic(bodyWithThinking);
 
       if (isDebugLevel(config.debug, "verbose")) {
         console.log(
--- a/src/upstream/anthropic-api.ts
+++ b/src/upstream/anthropic-api.ts
@@ -18,17 +18,17 @@
 
   if (isHaiku) {
     if (structured) {
-      return "oauth-2025-04-20,interleaved-thinking-2025-05-14,redact-thinking-2026-02-12,context-management-2025-06-27,prompt-caching-scope-2026-01-05,structured-outputs-2025-12-15";
+      return "oauth-2025-04-20,interleaved-thinking-2025-05-14,context-management-2025-06-27,prompt-caching-scope-2026-01-05,structured-outputs-2025-12-15";
     } else {
-      return "oauth-2025-04-20,interleaved-thinking-2025-05-14,redact-thinking-2026-02-12,context-management-2025-06-27,prompt-caching-scope-2026-01-05,claude-code-20250219";
+      return "oauth-2025-04-20,interleaved-thinking-2025-05-14,context-management-2025-06-27,prompt-caching-scope-2026-01-05,claude-code-20250219";
     }
   }
 
   if (structured) {
-    return "claude-code-20250219,oauth-2025-04-20,interleaved-thinking-2025-05-14,redact-thinking-2026-02-12,context-management-2025-06-27,prompt-caching-scope-2026-01-05,advanced-tool-use-2025-11-20,effort-2025-11-24,structured-outputs-2025-12-15";
+    return "claude-code-20250219,oauth-2025-04-20,interleaved-thinking-2025-05-14,context-management-2025-06-27,prompt-caching-scope-2026-01-05,advanced-tool-use-2025-11-20,effort-2025-11-24,structured-outputs-2025-12-15";
   }
 
-  return "claude-code-20250219,oauth-2025-04-20,interleaved-thinking-2025-05-14,redact-thinking-2026-02-12,context-management-2025-06-27,prompt-caching-scope-2026-01-05,advanced-tool-use-2025-11-20,effort-2025-11-24";
+  return "claude-code-20250219,oauth-2025-04-20,interleaved-thinking-2025-05-14,context-management-2025-06-27,prompt-caching-scope-2026-01-05,advanced-tool-use-2025-11-20,effort-2025-11-24";
 }
 
 /**
--- a/src/upstream/translator.ts
+++ b/src/upstream/translator.ts
@@ -9,13 +9,14 @@
   inputTokens: number,
   outputTokens: number,
   cachedTokens: number,
+  reasoningTokens: number = 0,
 ): any {
   return {
     prompt_tokens: inputTokens,
     completion_tokens: outputTokens,
     total_tokens: inputTokens + outputTokens,
     prompt_tokens_details: { cached_tokens: cachedTokens },
-    completion_tokens_details: { reasoning_tokens: 0 },
+    completion_tokens_details: { reasoning_tokens: reasoningTokens },
   };
 }
 
@@ -281,6 +282,7 @@
 
 export function anthropicToOpenai(anthropicResp: any, model: string): any {
   let textContent = "";
+  let reasoningContent = "";
   const toolCalls: any[] = [];
 
   if (Array.isArray(anthropicResp.content)) {
@@ -288,7 +290,7 @@
       if (block.type === "text") {
         textContent += block.text;
       } else if (block.type === "thinking" && block.thinking) {
-        // thinking blocks not exposed in chat completions response
+        reasoningContent += block.thinking;
       } else if (block.type === "tool_use") {
         toolCalls.push({
           id: block.id,
@@ -303,10 +305,12 @@
   }
 
   const message: any = { role: "assistant", content: textContent || null };
+  if (reasoningContent) message.reasoning_content = reasoningContent;
   if (toolCalls.length) message.tool_calls = toolCalls;
 
   const inputTokens = anthropicResp.usage?.input_tokens || 0;
   const outputTokens = anthropicResp.usage?.output_tokens || 0;
+  const reasoningTokens = anthropicResp.usage?.output_tokens_details?.thinking_tokens || 0;
 
   return {
     id: `chatcmpl-${uuidv4()}`,
@@ -325,6 +329,7 @@
       inputTokens,
       outputTokens,
       anthropicResp.usage?.cache_read_input_tokens || 0,
+      reasoningTokens,
     ),
   };
 }