diff --git a/changelog.d/fixes/9168-streamed-responses-tool-null.plan.md b/changelog.d/fixes/9168-streamed-responses-tool-null.plan.md
new file mode 100644
index 0000000000..d847e0af94
--- /dev/null
+++ b/changelog.d/fixes/9168-streamed-responses-tool-null.plan.md
@@ -0,0 +1 @@
+- fix(translator): buffer and normalize upstream tool-call argument deltas so optional null values are stripped before reaching the client (#9168)
diff --git a/open-sse/translator/response/openai-responses.ts b/open-sse/translator/response/openai-responses.ts
index 4533b30958..d459350d35 100644
--- a/open-sse/translator/response/openai-responses.ts
+++ b/open-sse/translator/response/openai-responses.ts
@@ -874,6 +874,7 @@ function openaiResponsesToOpenAIResponseStream(chunk, state) {
     if (state.currentToolCallId) state.toolCallIdsSeen.add(state.currentToolCallId);
 
     const toolName = normalizeToolName(item.name);
+    state.currentToolName = toolName; // track for schema lookup at done time
     if (!toolName) {
       // Some Responses providers briefly emit placeholder/empty tool names.
       // Defer emission until output_item.done in case the final name is populated there.
@@ -919,26 +920,9 @@ function openaiResponsesToOpenAIResponseStream(chunk, state) {
     state.currentToolCallArgsBuffer = (state.currentToolCallArgsBuffer || "") + argsDelta;
     if (state.currentToolCallDeferred) return null;
 
-    return {
-      id: state.chatId,
-      object: "chat.completion.chunk",
-      created: state.created,
-      model: state.model || "gpt-4",
-      choices: [
-        {
-          index: 0,
-          delta: {
-            tool_calls: [
-              {
-                index: state.toolCallIndex,
-                function: { arguments: argsDelta },
-              },
-            ],
-          },
-          finish_reason: null,
-        },
-      ],
-    };
+    // #9168: buffer arguments until output_item.done for schema-aware null normalization
+    // Previously emitted raw null values for optional enum fields (e.g. isolation: null).
+    return null;
   }
 
   // Function call done — emit args chunk from item.arguments when no deltas were received,
@@ -1011,6 +995,35 @@ function openaiResponsesToOpenAIResponseStream(chunk, state) {
     if (item.arguments != null && !buffered) {
       const argsToEmit = stripEmptyOptionalToolArgs(item.arguments, toolName, toolSchema);
 
+      const argsStr = typeof argsToEmit === "string" ? argsToEmit : JSON.stringify(argsToEmit);
+      if (argsStr) {
+        return {
+          id: state.chatId,
+          object: "chat.completion.chunk",
+          created: state.created,
+          model: state.model || "gpt-4",
+          choices: [
+            {
+              index: 0,
+              delta: {
+                tool_calls: [
+                  {
+                    index: currentIndex,
+                    function: { arguments: argsStr },
+                  },
+                ],
+              },
+              finish_reason: null,
+            },
+          ],
+        };
+      }
+    } else if (buffered) {
+      // #9168: deltas were buffered — normalize against the original client schema
+      // and emit the cleaned arguments once, stripping optional null values that
+      // would otherwise reach the client raw.
+      const argsToEmit = stripEmptyOptionalToolArgs(buffered, toolName, toolSchema);
+
       const argsStr = typeof argsToEmit === "string" ? argsToEmit : JSON.stringify(argsToEmit);
       if (argsStr) {
         return {
diff --git a/tests/unit/responses-chat-assistant-role-first-chunk.test.ts b/tests/unit/responses-chat-assistant-role-first-chunk.test.ts
index de243d6c2d..ee012d97b4 100644
--- a/tests/unit/responses-chat-assistant-role-first-chunk.test.ts
+++ b/tests/unit/responses-chat-assistant-role-first-chunk.test.ts
@@ -40,13 +40,21 @@ test("Responses->Chat: first tool_call chunk announces role=assistant", () => {
   );
   assert.equal(first.choices[0].delta.tool_calls[0].function.name, "get_weather");
 
-  // Subsequent argument deltas must NOT repeat the role announcement.
+  // #9168: arguments deltas are buffered until output_item.done for schema normalization.
   const next = openaiResponsesToOpenAIResponse(
     { type: "response.function_call_arguments.delta", delta: '{"x":1}' },
     state
   );
-  assert.ok(next, "should emit a chunk for arguments.delta");
-  assert.equal(next.choices[0].delta.role, undefined, "only the first delta announces the role");
+  assert.equal(next, null, "arguments delta should buffer until output_item.done");
+
+  // The args are emitted at output_item.done, and the role is not re-announced.
+  const done = openaiResponsesToOpenAIResponse(
+    { type: "response.output_item.done", item: { type: "function_call", call_id: "call_abc", name: "get_weather" } },
+    state
+  );
+  assert.ok(done, "should emit a chunk for output_item.done");
+  assert.equal(done.choices[0].delta.role, undefined, "role announcement already happened on first chunk");
+  assert.equal(done.choices[0].delta.tool_calls[0].function.arguments, '{"x":1}');
 });
 
 test("Responses->Chat: first text chunk announces role=assistant", () => {
diff --git a/tests/unit/translator-resp-openai-responses-completed-synthesis.test.ts b/tests/unit/translator-resp-openai-responses-completed-synthesis.test.ts
index b7f1436096..fbbf3d6324 100644
--- a/tests/unit/translator-resp-openai-responses-completed-synthesis.test.ts
+++ b/tests/unit/translator-resp-openai-responses-completed-synthesis.test.ts
@@ -203,7 +203,8 @@ test("Responses -> OpenAI: incremental tool call events + response.completed sna
     },
     state
   );
-  assert.ok(args, "should emit args delta chunk");
+  // #9168: arguments deltas are buffered until output_item.done for schema normalization
+  assert.equal(args, null, "args delta should buffer until output_item.done");
 
   openaiResponsesToOpenAIResponse(
     {
diff --git a/tests/unit/translator-resp-openai-responses.test.ts b/tests/unit/translator-resp-openai-responses.test.ts
index 3239412464..3eeef460b8 100644
--- a/tests/unit/translator-resp-openai-responses.test.ts
+++ b/tests/unit/translator-resp-openai-responses.test.ts
@@ -474,7 +474,7 @@ test("Responses -> OpenAI: tool-call delta, reasoning delta and completed usage
     },
     state
   );
-  openaiResponsesToOpenAIResponse(
+  const done = openaiResponsesToOpenAIResponse(
     {
       type: "response.output_item.done",
       item: { type: "function_call", call_id: "call_2", name: "weather" },
@@ -497,7 +497,10 @@ test("Responses -> OpenAI: tool-call delta, reasoning delta and completed usage
   );
 
   assert.equal(added.choices[0].delta.tool_calls[0].function.name, "weather");
-  assert.equal(args.choices[0].delta.tool_calls[0].function.arguments, '{"city":"SP"}');
+  // #9168: function_call_arguments.delta is buffered and returns null;
+  // arguments are emitted by output_item.done instead.
+  assert.equal(args, null);
+  assert.equal(done.choices[0].delta.tool_calls[0].function.arguments, '{"city":"SP"}');
   assert.equal(reasoning.choices[0].delta.reasoning_content, "Need weather info.");
   assert.equal(completed.choices[0].finish_reason, "tool_calls");
   const comp = completed as {