diff --git a/open-sse/translator/response/openai-responses.ts b/open-sse/translator/response/openai-responses.ts
index 69116139ed..4d255d40be 100644
--- a/open-sse/translator/response/openai-responses.ts
+++ b/open-sse/translator/response/openai-responses.ts
@@ -1155,7 +1155,12 @@ function openaiResponsesToOpenAIResponseStream(chunk, state) {
       // Keyed by index, not insertion order — readers that need call order for
       // parallel calls closed out of order should sort by this key rather than
       // relying on Map iteration order.
+      // Responses→Claude uses this same shared map for Claude block lifecycle
+      // state. Preserve those fields when adding the completed-call summary;
+      // replacing the entry makes the arguments chunk look like a new unnamed
+      // tool and emits a duplicate empty content_block_start.
       state.toolCalls.set(currentIndex, {
+        ...state.toolCalls.get(currentIndex),
         id: callId,
         index: currentIndex,
         type: "function",
diff --git a/tests/unit/codex-claude-empty-tool-use.test.ts b/tests/unit/codex-claude-empty-tool-use.test.ts
new file mode 100644
index 0000000000..6d0d948a51
--- /dev/null
+++ b/tests/unit/codex-claude-empty-tool-use.test.ts
@@ -0,0 +1,133 @@
+import test from "node:test";
+import assert from "node:assert/strict";
+
+import { FORMATS } from "../../open-sse/translator/formats.ts";
+import { createSSETransformStreamWithLogger } from "../../open-sse/utils/stream.ts";
+
+function sse(type: string, data: Record<string, unknown>): string {
+  return `event: ${type}\ndata: ${JSON.stringify({ type, ...data })}\n\n`;
+}
+
+async function translateCodexToolCall(rawSse: string): Promise<Record<string, unknown>[]> {
+  const transform = createSSETransformStreamWithLogger(
+    FORMATS.OPENAI_RESPONSES,
+    FORMATS.CLAUDE,
+    "codex",
+    null,
+    null,
+    "gpt-5.6-sol",
+    "connection-codex-tool",
+    { model: "gpt-5.6-sol", stream: true },
+    null,
+    null,
+    null
+  );
+  const writer = transform.writable.getWriter();
+  const reader = transform.readable.getReader();
+  const readAll = (async () => {
+    const decoder = new TextDecoder();
+    let output = "";
+    for (;;) {
+      const { done, value } = await reader.read();
+      if (done) break;
+      output += decoder.decode(value, { stream: true });
+    }
+    output += decoder.decode();
+    return output;
+  })();
+
+  await writer.write(new TextEncoder().encode(rawSse));
+  await writer.close();
+
+  return (await readAll)
+    .split(/\r?\n/)
+    .filter((line) => line.startsWith("data:"))
+    .map((line) => line.slice(5).trim())
+    .filter((payload) => payload && payload !== "[DONE]")
+    .map((payload) => JSON.parse(payload) as Record<string, unknown>);
+}
+
+test("Codex Responses tool call emits exactly one named Claude tool_use block", async () => {
+  const callId = "call_codex_claude_1";
+  const itemId = "fc_codex_claude_1";
+  const raw = [
+    sse("response.created", {
+      sequence_number: 0,
+      response: { id: "resp_codex_claude_1", status: "in_progress", model: "gpt-5.6-sol" },
+    }),
+    sse("response.output_item.added", {
+      sequence_number: 1,
+      output_index: 0,
+      item: {
+        id: itemId,
+        type: "function_call",
+        call_id: callId,
+        name: "check_status",
+        arguments: "",
+        status: "in_progress",
+      },
+    }),
+    sse("response.function_call_arguments.delta", {
+      sequence_number: 2,
+      item_id: itemId,
+      output_index: 0,
+      delta: '{"value":"ok"}',
+    }),
+    sse("response.function_call_arguments.done", {
+      sequence_number: 3,
+      item_id: itemId,
+      output_index: 0,
+      arguments: '{"value":"ok"}',
+    }),
+    sse("response.output_item.done", {
+      sequence_number: 4,
+      output_index: 0,
+      item: {
+        id: itemId,
+        type: "function_call",
+        call_id: callId,
+        name: "check_status",
+        arguments: '{"value":"ok"}',
+        status: "completed",
+      },
+    }),
+    sse("response.completed", {
+      sequence_number: 5,
+      response: {
+        id: "resp_codex_claude_1",
+        status: "completed",
+        model: "gpt-5.6-sol",
+        output: [
+          {
+            id: itemId,
+            type: "function_call",
+            call_id: callId,
+            name: "check_status",
+            arguments: '{"value":"ok"}',
+            status: "completed",
+          },
+        ],
+        usage: { input_tokens: 10, output_tokens: 5, total_tokens: 15 },
+      },
+    }),
+  ].join("");
+
+  const events = await translateCodexToolCall(raw);
+  const starts = events.filter((event) => event.type === "content_block_start") as Array<{
+    index?: number;
+    content_block?: { type?: string; id?: string; name?: string };
+  }>;
+  const toolStarts = starts.filter((event) => event.content_block?.type === "tool_use");
+
+  assert.equal(toolStarts.length, 1, "must not append a duplicate empty tool_use block");
+  assert.deepEqual(toolStarts[0], {
+    type: "content_block_start",
+    index: 0,
+    content_block: {
+      type: "tool_use",
+      id: callId,
+      name: "check_status",
+      input: {},
+    },
+  });
+});