diff --git a/open-sse/handlers/sseParser.ts b/open-sse/handlers/sseParser.ts
index e1e6136127..4a9626ac7b 100644
--- a/open-sse/handlers/sseParser.ts
+++ b/open-sse/handlers/sseParser.ts
@@ -52,6 +52,10 @@ export function parseSSEToOpenAIResponse(rawSSE, fallbackModel) {
     if (typeof delta.reasoning_content === "string" && delta.reasoning_content.length > 0) {
       reasoningParts.push(delta.reasoning_content);
     }
+    // Normalize `reasoning` alias (NVIDIA kimi-k2.5 etc.)
+    if (typeof delta.reasoning === "string" && delta.reasoning.length > 0 && !delta.reasoning_content) {
+      reasoningParts.push(delta.reasoning);
+    }
 
     // T18: Accumulate tool calls correctly across streamed chunks
     if (delta.tool_calls) {
diff --git a/open-sse/utils/stream.ts b/open-sse/utils/stream.ts
index 4c30fe3782..6c274663d7 100644
--- a/open-sse/utils/stream.ts
+++ b/open-sse/utils/stream.ts
@@ -314,6 +314,12 @@ export function createSSEStream(options: StreamOptions = {}) {
 
                   const delta = parsed.choices?.[0]?.delta;
 
+                  // Normalize `reasoning` alias → `reasoning_content` (NVIDIA kimi-k2.5 etc.)
+                  if (delta?.reasoning && typeof delta.reasoning === "string" && !delta.reasoning_content) {
+                    delta.reasoning_content = delta.reasoning;
+                    delete delta.reasoning;
+                  }
+
                   // Extract <think> tags from streaming content
                   if (delta?.content && typeof delta.content === "string") {
                     const { content, thinking } = extractThinkingFromContent(delta.content);
@@ -323,6 +329,14 @@ export function createSSEStream(options: StreamOptions = {}) {
                     }
                   }
 
+                  // If reasoning was normalized (reasoning → reasoning_content) or <think>
+                  // tags were extracted, force re-serialization so the client sees
+                  // the standard `reasoning_content` field instead of the raw provider line.
+                  if (delta?.reasoning_content && !injectedUsage) {
+                    output = `data: ${JSON.stringify(parsed)}\n`;
+                    injectedUsage = true;
+                  }
+
                   // T18: Track if we saw tool calls & accumulate for call log
                   if (delta?.tool_calls && delta.tool_calls.length > 0) {
                     passthroughHasToolCalls = true;
@@ -483,6 +497,16 @@ export function createSSEStream(options: StreamOptions = {}) {
               if (state?.accumulatedContent !== undefined) state.accumulatedContent += r;
             }
           }
+          // Normalize `reasoning` alias → `reasoning_content` (NVIDIA kimi-k2.5 etc.)
+          if (parsed.choices?.[0]?.delta?.reasoning && !parsed.choices?.[0]?.delta?.reasoning_content) {
+            const r = parsed.choices[0].delta.reasoning;
+            if (typeof r === "string") {
+              parsed.choices[0].delta.reasoning_content = r;
+              delete parsed.choices[0].delta.reasoning;
+              totalContentLength += r.length;
+              if (state?.accumulatedContent !== undefined) state.accumulatedContent += r;
+            }
+          }
 
           // Gemini format - may have multiple parts
           if (parsed.candidates?.[0]?.content?.parts) {
diff --git a/open-sse/utils/streamPayloadCollector.ts b/open-sse/utils/streamPayloadCollector.ts
index 961dfc46ba..427770fa08 100644
--- a/open-sse/utils/streamPayloadCollector.ts
+++ b/open-sse/utils/streamPayloadCollector.ts
@@ -157,6 +157,10 @@ function buildOpenAISummary(events: StructuredSSEEvent[], fallbackModel?: string
     if (typeof delta.reasoning_content === "string" && delta.reasoning_content.length > 0) {
       reasoningParts.push(delta.reasoning_content);
     }
+    // Normalize `reasoning` alias (NVIDIA kimi-k2.5 etc.)
+    if (typeof delta.reasoning === "string" && delta.reasoning.length > 0 && !delta.reasoning_content) {
+      reasoningParts.push(delta.reasoning);
+    }
 
     if (Array.isArray(delta.tool_calls)) {
       for (const item of delta.tool_calls) {