diff --git a/open-sse/transformer/responsesTransformer.ts b/open-sse/transformer/responsesTransformer.ts
index 16c33cb73f..602adcdd1c 100644
--- a/open-sse/transformer/responsesTransformer.ts
+++ b/open-sse/transformer/responsesTransformer.ts
@@ -98,6 +98,7 @@ export function createResponsesApiTransformStream(logger = null) {
     funcItemDone: {},
     buffer: "",
     completedSent: false,
+    usage: null,
   };
 
   const encoder = new TextEncoder();
@@ -249,16 +250,52 @@ export function createResponsesApiTransformStream(logger = null) {
   const sendCompleted = (controller) => {
     if (!state.completedSent) {
       state.completedSent = true;
+
+      // Build output from accumulated state
+      const output = [];
+      if (state.reasoningId) {
+        output.push({
+          id: state.reasoningId,
+          type: "reasoning",
+          summary: [{ type: "summary_text", text: state.reasoningBuf }],
+        });
+      }
+      for (const idx in state.msgItemAdded) {
+        output.push({
+          id: `msg_${state.responseId}_${idx}`,
+          type: "message",
+          role: "assistant",
+          content: [{ type: "output_text", annotations: [], text: state.msgTextBuf[idx] || "" }],
+        });
+      }
+      for (const idx in state.funcCallIds) {
+        const callId = state.funcCallIds[idx];
+        output.push({
+          id: `fc_${callId}`,
+          type: "function_call",
+          call_id: callId,
+          name: state.funcNames[idx] || "",
+          arguments: state.funcArgsBuf[idx] || "{}",
+        });
+      }
+
+      const response: Record<string, unknown> = {
+        id: state.responseId,
+        object: "response",
+        created_at: state.created,
+        status: "completed",
+        background: false,
+        error: null,
+        output,
+      };
+
+      if (state.usage) {
+        response.usage = state.usage;
+      }
+
       emit(controller, "response.completed", {
         type: "response.completed",
-        response: {
-          id: state.responseId,
-          object: "response",
-          created_at: state.created,
-          status: "completed",
-          background: false,
-          error: null,
-        },
+        response,
       });
     }
   };
@@ -288,7 +325,12 @@ export function createResponsesApiTransformStream(logger = null) {
           continue;
         }
 
-        if (!parsed.choices?.length) continue;
+        if (!parsed.choices?.length) {
+          if (parsed.usage) {
+            state.usage = parsed.usage;
+          }
+          continue;
+        }
 
         const choice = parsed.choices[0];
         const idx = choice.index || 0;
diff --git a/open-sse/translator/response/openai-responses.ts b/open-sse/translator/response/openai-responses.ts
index b22b9f5f46..41e40e4f66 100644
--- a/open-sse/translator/response/openai-responses.ts
+++ b/open-sse/translator/response/openai-responses.ts
@@ -14,7 +14,13 @@ export function openaiToOpenAIResponsesResponse(chunk, state) {
     return flushEvents(state);
   }
 
-  if (!chunk.choices?.length) return [];
+  if (!chunk.choices?.length) {
+    // Capture usage from usage-only chunks (stream_options.include_usage)
+    if (chunk.usage) {
+      state.usage = chunk.usage;
+    }
+    return [];
+  }
 
   const events = [];
   const nextSeq = () => ++state.seq;
@@ -334,16 +340,52 @@ function closeToolCall(state, emit, idx) {
 function sendCompleted(state, emit) {
   if (!state.completedSent) {
     state.completedSent = true;
+
+    // Build output from accumulated state
+    const output = [];
+    if (state.reasoningId) {
+      output.push({
+        id: state.reasoningId,
+        type: "reasoning",
+        summary: [{ type: "summary_text", text: state.reasoningBuf }],
+      });
+    }
+    for (const idx in state.msgItemAdded) {
+      output.push({
+        id: `msg_${state.responseId}_${idx}`,
+        type: "message",
+        role: "assistant",
+        content: [{ type: "output_text", annotations: [], text: state.msgTextBuf[idx] || "" }],
+      });
+    }
+    for (const idx in state.funcCallIds) {
+      const callId = state.funcCallIds[idx];
+      output.push({
+        id: `fc_${callId}`,
+        type: "function_call",
+        call_id: callId,
+        name: state.funcNames[idx] || "",
+        arguments: state.funcArgsBuf[idx] || "{}",
+      });
+    }
+
+    const response: Record<string, unknown> = {
+      id: state.responseId,
+      object: "response",
+      created_at: state.created,
+      status: "completed",
+      background: false,
+      error: null,
+      output,
+    };
+
+    if (state.usage) {
+      response.usage = state.usage;
+    }
+
     emit("response.completed", {
       type: "response.completed",
-      response: {
-        id: state.responseId,
-        object: "response",
-        created_at: state.created,
-        status: "completed",
-        background: false,
-        error: null,
-      },
+      response,
     });
   }
 }
diff --git a/tests/unit/responses-translation-fixes.test.mjs b/tests/unit/responses-translation-fixes.test.mjs
index e3f0850219..20c827f96a 100644
--- a/tests/unit/responses-translation-fixes.test.mjs
+++ b/tests/unit/responses-translation-fixes.test.mjs
@@ -341,3 +341,50 @@ test("Chat→Responses: deprecated function role message converted to function_c
   const fcItem = result.input.find((i) => i.type === "function_call");
   assert.equal(fcOutput.call_id, fcItem.call_id);
 });
+
+const { openaiToOpenAIResponsesResponse } = await import(
+  "../../open-sse/translator/response/openai-responses.ts"
+);
+const { initState } = await import("../../open-sse/translator/index.ts");
+const { FORMATS } = await import("../../open-sse/translator/formats.ts");
+
+test("Chat→Responses streaming: usage-only chunk is captured (not dropped)", () => {
+  const state = initState(FORMATS.OPENAI_RESPONSES);
+
+  // First chunk with content
+  const chunk1 = { choices: [{ index: 0, delta: { content: "hello" }, finish_reason: null }], id: "c1" };
+  openaiToOpenAIResponsesResponse(chunk1, state);
+
+  // Usage-only chunk (empty choices, has usage)
+  const usageChunk = {
+    choices: [],
+    usage: { prompt_tokens: 10, completion_tokens: 5, total_tokens: 15 },
+  };
+  const usageEvents = openaiToOpenAIResponsesResponse(usageChunk, state);
+  assert.ok(Array.isArray(usageEvents));
+
+  // Finish chunk
+  const finishChunk = { choices: [{ index: 0, delta: {}, finish_reason: "stop" }] };
+  const finishEvents = openaiToOpenAIResponsesResponse(finishChunk, state);
+  const completedEvent = finishEvents.find((e) => e.event === "response.completed");
+  assert.ok(completedEvent, "should have completed event");
+  assert.ok(completedEvent.data.response.usage, "completed event should include usage");
+  assert.equal(completedEvent.data.response.usage.prompt_tokens, 10);
+});
+
+test("Chat→Responses streaming: completed event includes accumulated output", () => {
+  const state = initState(FORMATS.OPENAI_RESPONSES);
+
+  // Text content
+  const chunk = { choices: [{ index: 0, delta: { content: "hello world" }, finish_reason: null }], id: "c1" };
+  openaiToOpenAIResponsesResponse(chunk, state);
+
+  // Finish
+  const finishChunk = { choices: [{ index: 0, delta: {}, finish_reason: "stop" }] };
+  const events = openaiToOpenAIResponsesResponse(finishChunk, state);
+  const completedEvent = events.find((e) => e.event === "response.completed");
+  assert.ok(completedEvent.data.response.output, "completed should have output");
+  assert.ok(completedEvent.data.response.output.length > 0, "output should not be empty");
+  const msgOutput = completedEvent.data.response.output.find((o) => o.type === "message");
+  assert.ok(msgOutput, "should have message output item");
+});