diff --git a/open-sse/transformer/responsesTransformer.ts b/open-sse/transformer/responsesTransformer.ts
index e98c4d98db..a9c0db949e 100644
--- a/open-sse/transformer/responsesTransformer.ts
+++ b/open-sse/transformer/responsesTransformer.ts
@@ -209,6 +209,12 @@ export function createResponsesApiTransformStream(
     funcItemTypes: {},
     funcArgsDone: {},
     funcItemDone: {},
+    // Cached at first computation (see toolCallOutputIndexBase) so every
+    // added/delta/done event for a given tool call — including ones emitted
+    // later from the finish_reason handler or flush(), where the reasoning/
+    // message state used to derive the base is no longer meaningful to
+    // recompute — shares exactly the same output_index.
+    funcOutputIndex: {} as Record<string, number>,
     completedOutputItems: [] as Array<{
       output_index: number;
       item: Record<string, unknown>;
@@ -380,6 +386,27 @@ export function createResponsesApiTransformStream(
     }
   };
 
+  // Tool calls sit after reasoning (if any) AND after a text message (if one
+  // was actually emitted this turn). The provider's own tool_calls[].index is
+  // scoped only to the tool_calls array and legitimately restarts at 0 — using
+  // it directly as the Responses API output_index collides with whatever
+  // reasoning/message item already claimed that slot, and a client that
+  // tracks response items by output_index silently drops the tool call.
+  //
+  // Computed once per tcIdx (from the chunk's own choice index, `chunkIdx`)
+  // and cached in state.funcOutputIndex so every added/delta/done event for
+  // that call — including ones emitted later from the finish_reason handler
+  // or flush(), which have no fresh chunk/reasoning/message state to
+  // recompute from — shares exactly the same output_index.
+  const computeToolCallOutputIndex = (chunkIdx, tcIdx) => {
+    if (state.funcOutputIndex[tcIdx] === undefined) {
+      const msgIdx = state.reasoningId ? state.reasoningIndex + 1 : chunkIdx;
+      const base = state.msgItemAdded[msgIdx] ? msgIdx + 1 : msgIdx;
+      state.funcOutputIndex[tcIdx] = base + normalizeOutputIndex(tcIdx);
+    }
+    return state.funcOutputIndex[tcIdx];
+  };
+
   const emitToolCallAdded = (controller, idx) => {
     if (state.funcItemAdded[idx] || !state.funcCallIds[idx]) return false;
 
@@ -390,7 +417,7 @@ export function createResponsesApiTransformStream(
 
     emit(controller, "response.output_item.added", {
       type: "response.output_item.added",
-      output_index: idx,
+      output_index: state.funcOutputIndex[idx],
       item: {
         id: `fc_${state.funcCallIds[idx]}`,
         type: itemType,
@@ -406,7 +433,7 @@ export function createResponsesApiTransformStream(
   const closeToolCall = (controller, idx, recordAsCompleted = true) => {
     const callId = state.funcCallIds[idx];
     if (callId && !state.funcItemDone[idx]) {
-      const normalizedIndex = normalizeOutputIndex(idx);
+      const normalizedIndex = state.funcOutputIndex[idx];
       let args = state.funcArgsBuf[idx] || "{}";
       const toolName = state.funcNames[idx] || "";
       emitToolCallAdded(controller, idx);
@@ -750,6 +777,7 @@ export function createResponsesApiTransformStream(
 
             for (const tc of delta.tool_calls) {
               const tcIdx = tc.index ?? 0;
+              const outputIndex = computeToolCallOutputIndex(idx, tcIdx);
               const newCallId = tc.id;
               const funcName = tc.function?.name;
 
@@ -765,6 +793,10 @@ export function createResponsesApiTransformStream(
                 delete state.funcItemTypes[tcIdx];
                 delete state.funcArgsDone[tcIdx];
                 delete state.funcItemDone[tcIdx];
+                // Deliberately keep funcOutputIndex[tcIdx]: the replacement call
+                // reuses the same positional slot, so it should keep the same
+                // output_index rather than recomputing (which could drift if
+                // msgItemAdded state shifted mid-turn).
               }
 
               if (funcName) state.funcNames[tcIdx] = funcName;
@@ -786,7 +818,7 @@ export function createResponsesApiTransformStream(
                   emit(controller, "response.function_call_arguments.delta", {
                     type: "response.function_call_arguments.delta",
                     item_id: `fc_${state.funcCallIds[tcIdx]}`,
-                    output_index: tcIdx,
+                    output_index: outputIndex,
                     delta: state.funcArgsBuf[tcIdx],
                   });
                 }
@@ -825,7 +857,7 @@ export function createResponsesApiTransformStream(
                   emit(controller, "response.function_call_arguments.delta", {
                     type: "response.function_call_arguments.delta",
                     item_id: `fc_${refCallId}`,
-                    output_index: tcIdx,
+                    output_index: outputIndex,
                     delta: emittedDelta,
                   });
                 }
diff --git a/tests/unit/responses-transformer-tool-call-reasoning-collision.test.ts b/tests/unit/responses-transformer-tool-call-reasoning-collision.test.ts
new file mode 100644
index 0000000000..0568247c4e
--- /dev/null
+++ b/tests/unit/responses-transformer-tool-call-reasoning-collision.test.ts
@@ -0,0 +1,158 @@
+/**
+ * Regression test for a tool call landing on the same `output_index` as a
+ * preceding reasoning item in the Responses API stream.
+ *
+ * `emitToolCallAdded`/`closeToolCall` in responsesTransformer.ts used the
+ * provider's raw Chat Completions `tool_calls[].index` directly as the
+ * Responses API `output_index`. That index is scoped only to the tool_calls
+ * array and legitimately restarts at 0 for the first tool call — but by the
+ * time a tool call arrives, a reasoning item (and/or a text message) may
+ * already have claimed output_index 0 (and 1). A client that tracks response
+ * items by output_index (as the Responses API spec expects) then sees the
+ * tool call's added/delta/done events land on an index it already marked
+ * complete, and silently drops the tool call — producing an "incomplete
+ * turn" that never dispatches the tool.
+ *
+ * Reported live: OpenClaw on combo `default` -> opencode-zen/big-pickle,
+ * a reasoning block immediately followed by a function call in the same
+ * turn (no text message in between).
+ */
+
+import test from "node:test";
+import assert from "node:assert/strict";
+
+const { createResponsesApiTransformStream } =
+  await import("../../open-sse/transformer/responsesTransformer.ts");
+
+const encoder = new TextEncoder();
+const decoder = new TextDecoder();
+
+async function runTransformStream(chunks) {
+  const stream = createResponsesApiTransformStream();
+  const writer = stream.writable.getWriter();
+  const reader = stream.readable.getReader();
+
+  const output = [];
+  const readerTask = (async () => {
+    while (true) {
+      const { value, done } = await reader.read();
+      if (done) break;
+      output.push(decoder.decode(value));
+    }
+  })();
+
+  for (const chunk of chunks) {
+    await writer.write(encoder.encode(chunk));
+  }
+  await writer.close();
+  await readerTask;
+
+  return output.join("");
+}
+
+function parseSseOutput(output) {
+  return output
+    .trim()
+    .split("\n\n")
+    .map((entry) => {
+      const lines = entry.split("\n");
+      const eventLine = lines.find((line) => line.startsWith("event: "));
+      const dataLine = lines.find((line) => line.startsWith("data: "));
+      return {
+        event: eventLine ? eventLine.slice("event: ".length) : null,
+        data: dataLine ? dataLine.slice("data: ".length) : null,
+      };
+    });
+}
+
+test("tool call immediately after reasoning must not collide on output_index", async () => {
+  const output = await runTransformStream([
+    // Reasoning content — claims output_index 0.
+    `data: {"id":"chatcmpl-collide","choices":[{"index":0,"delta":{"reasoning_content":"thinking..."}}]}\n\n`,
+    // A tool call starts. The provider scopes tool_calls[].index to 0 for the
+    // first (and only) call here, same as reasoning's own output_index.
+    `data: {"id":"chatcmpl-collide","choices":[{"index":0,"delta":{"tool_calls":[{"index":0,"id":"call_1","function":{"name":"run","arguments":""}}]}}]}\n\n`,
+    `data: {"id":"chatcmpl-collide","choices":[{"index":0,"delta":{"tool_calls":[{"index":0,"function":{"arguments":"{\\"cmd\\":\\"ls\\"}"}}]}}]}\n\n`,
+    `data: {"id":"chatcmpl-collide","choices":[{"index":0,"delta":{},"finish_reason":"tool_calls"}]}\n\n`,
+  ]);
+
+  const events = parseSseOutput(output);
+
+  const reasoningAdded = events.find(
+    (e) => e.event === "response.output_item.added" && JSON.parse(e.data).item.type === "reasoning"
+  );
+  const toolCallAdded = events.find(
+    (e) =>
+      e.event === "response.output_item.added" && JSON.parse(e.data).item.type === "function_call"
+  );
+
+  assert.ok(reasoningAdded, "reasoning output_item.added must be emitted");
+  assert.ok(toolCallAdded, "function_call output_item.added must be emitted");
+
+  const reasoningIndex = JSON.parse(reasoningAdded.data).output_index;
+  const toolCallIndex = JSON.parse(toolCallAdded.data).output_index;
+
+  assert.notEqual(
+    toolCallIndex,
+    reasoningIndex,
+    `function_call output_index (${toolCallIndex}) must not collide with reasoning's output_index (${reasoningIndex})`
+  );
+
+  // All function_call-related events for this call must share one consistent
+  // output_index across added/delta/done — a client tracking by output_index
+  // must be able to follow the whole lifecycle at a single index.
+  const funcCallEvents = events.filter((e) => {
+    if (
+      e.event !== "response.function_call_arguments.delta" &&
+      e.event !== "response.function_call_arguments.done" &&
+      e.event !== "response.output_item.done"
+    ) {
+      return false;
+    }
+    if (!e.data) return false;
+    const parsed = JSON.parse(e.data);
+    return e.event !== "response.output_item.done" || parsed.item?.type === "function_call";
+  });
+  assert.ok(funcCallEvents.length > 0, "expected function_call lifecycle events");
+  for (const e of funcCallEvents) {
+    assert.equal(
+      JSON.parse(e.data).output_index,
+      toolCallIndex,
+      `event ${e.event} must use the same output_index as the tool call's added event`
+    );
+  }
+
+  // response.completed output must contain both items at distinct indices.
+  const completed = JSON.parse(events.find((e) => e.event === "response.completed").data).response;
+  const reasoningItem = completed.output.find((item) => item.type === "reasoning");
+  const funcItem = completed.output.find((item) => item.type === "function_call");
+  assert.ok(reasoningItem, "completed output must include the reasoning item");
+  assert.ok(funcItem, "completed output must include the function_call item");
+  assert.equal(funcItem.call_id, "call_1");
+  assert.equal(funcItem.arguments, '{"cmd":"ls"}');
+});
+
+test("multiple tool calls after reasoning use sequential output_index values, none colliding with reasoning", async () => {
+  const output = await runTransformStream([
+    `data: {"id":"chatcmpl-multi","choices":[{"index":0,"delta":{"reasoning_content":"planning"}}]}\n\n`,
+    `data: {"id":"chatcmpl-multi","choices":[{"index":0,"delta":{"tool_calls":[{"index":0,"id":"call_a","function":{"name":"first","arguments":"{}"}}]}}]}\n\n`,
+    `data: {"id":"chatcmpl-multi","choices":[{"index":0,"delta":{"tool_calls":[{"index":1,"id":"call_b","function":{"name":"second","arguments":"{}"}}]}}]}\n\n`,
+    `data: {"id":"chatcmpl-multi","choices":[{"index":0,"delta":{},"finish_reason":"tool_calls"}]}\n\n`,
+  ]);
+
+  const events = parseSseOutput(output);
+  const addedEvents = events.filter((e) => e.event === "response.output_item.added");
+  const indexByType = addedEvents.map((e) => {
+    const parsed = JSON.parse(e.data);
+    return { type: parsed.item.type, output_index: parsed.output_index };
+  });
+
+  const reasoningIdx = indexByType.find((i) => i.type === "reasoning").output_index;
+  const funcIndices = indexByType.filter((i) => i.type === "function_call").map((i) => i.output_index);
+
+  assert.equal(funcIndices.length, 2);
+  assert.ok(new Set(funcIndices).size === 2, "the two tool calls must not share an output_index");
+  for (const fi of funcIndices) {
+    assert.notEqual(fi, reasoningIdx, "no tool call may collide with the reasoning output_index");
+  }
+});