diff --git a/packages/server-utils/src/ai/openai/streaming.ts b/packages/server-utils/src/ai/openai/streaming.ts
index 0828e755e4cb..c706111fce97 100644
--- a/packages/server-utils/src/ai/openai/streaming.ts
+++ b/packages/server-utils/src/ai/openai/streaming.ts
@@ -37,7 +37,9 @@ interface StreamingState {
    * Accumulated tool calls from Chat Completion streaming, indexed by tool call index.
    * @see https://platform.openai.com/docs/guides/function-calling?api-mode=chat#streaming
    */
-  chatCompletionToolCalls: Record<number, ChatCompletionToolCall>;
+  chatCompletionToolCalls: Record<number | string, ChatCompletionToolCall>;
+  /** Key of the latest tool call, which unindexed argument fragments continue. */
+  lastChatCompletionToolCallKey?: number | string;
   /**
    * Accumulated function calls from Responses API streaming.
    * @see https://platform.openai.com/docs/guides/function-calling?api-mode=responses#streaming
@@ -56,8 +58,8 @@ interface StreamingState {
  */
 function processChatCompletionToolCalls(toolCalls: ChatCompletionToolCall[], state: StreamingState): void {
   for (const toolCall of toolCalls) {
-    const index = toolCall.index;
-    if (index === undefined || !toolCall.function) continue;
+    if (!toolCall.function) continue;
+    const index = getToolCallKey(toolCall, state);
 
     // Initialize tool call if this is the first chunk for this index
     if (!(index in state.chatCompletionToolCalls)) {
@@ -75,9 +77,33 @@ function processChatCompletionToolCalls(toolCalls: ChatCompletionToolCall[], sta
         existingToolCall.function.arguments += toolCall.function.arguments;
       }
     }
+    state.lastChatCompletionToolCallKey = index;
   }
 }
 
+/**
+ * OpenAI-compatible providers such as Gemini's OpenAI endpoint send tool calls without an `index`.
+ * Those are matched by a known `id`; a new `id` or a function name opens a new call, and anything
+ * else continues the latest call.
+ */
+function getToolCallKey(toolCall: ChatCompletionToolCall, state: StreamingState): number | string {
+  if (toolCall.index !== undefined) {
+    return toolCall.index;
+  }
+  if (toolCall.id) {
+    const known = Object.keys(state.chatCompletionToolCalls).find(
+      key => state.chatCompletionToolCalls[key]?.id === toolCall.id,
+    );
+    if (known !== undefined) {
+      return known;
+    }
+  }
+  if (toolCall.id || toolCall.function?.name || state.lastChatCompletionToolCallKey === undefined) {
+    return `no-index-${Object.keys(state.chatCompletionToolCalls).length}`;
+  }
+  return state.lastChatCompletionToolCallKey;
+}
+
 /**
  * Processes a single OpenAI ChatCompletionChunk event, updating the streaming state.
  *
diff --git a/packages/server-utils/test/ai/lib/tracing/openai-streaming-tool-calls.test.ts b/packages/server-utils/test/ai/lib/tracing/openai-streaming-tool-calls.test.ts
new file mode 100644
index 000000000000..f68a09e41546
--- /dev/null
+++ b/packages/server-utils/test/ai/lib/tracing/openai-streaming-tool-calls.test.ts
@@ -0,0 +1,103 @@
+import { describe, expect, it } from 'vitest';
+import type { Span } from '@sentry/core';
+import { GEN_AI_RESPONSE_TOOL_CALLS } from '@sentry/conventions/attributes';
+import { instrumentStream } from '../../../../src/ai/openai/streaming';
+import type { ChatCompletionChunk, ChatCompletionToolCall } from '../../../../src/ai/openai/types';
+
+function createMockSpan(): { span: Span; attributes: Record<string, unknown> } {
+  const attributes: Record<string, unknown> = {};
+  let isEnded = false;
+  const span = {
+    isRecording: () => !isEnded,
+    setAttribute: (key: string, value: unknown) => {
+      attributes[key] = value;
+    },
+    setAttributes: (attrs: Record<string, unknown>) => {
+      Object.assign(attributes, attrs);
+    },
+    setStatus: () => {},
+    end: () => {
+      isEnded = true;
+    },
+  } as unknown as Span;
+  return { span, attributes };
+}
+
+function chunkWith(toolCalls: Array<Partial<ChatCompletionToolCall>>): ChatCompletionChunk {
+  return {
+    id: 'chatcmpl-1',
+    object: 'chat.completion.chunk',
+    model: 'gemini-test',
+    choices: [{ index: 0, delta: { tool_calls: toolCalls } }],
+  } as unknown as ChatCompletionChunk;
+}
+
+async function recordedToolCalls(chunks: ChatCompletionChunk[]): Promise<Array<Record<string, unknown>>> {
+  const { span, attributes } = createMockSpan();
+  async function* stream(): AsyncGenerator<ChatCompletionChunk> {
+    yield* chunks;
+  }
+  for await (const _ of instrumentStream(stream(), span, true)) {
+    void _;
+  }
+  return JSON.parse(attributes[GEN_AI_RESPONSE_TOOL_CALLS] as string) as Array<Record<string, unknown>>;
+}
+
+describe('instrumentStream (openai chat completion tool calls)', () => {
+  it('keeps whole tool calls that carry an id and no index', async () => {
+    const toolCalls = await recordedToolCalls([
+      chunkWith([
+        { id: 'call_a', type: 'function', function: { name: 'lookup', arguments: '{"q":"a"}' } },
+        { id: 'call_b', type: 'function', function: { name: 'search', arguments: '{"q":"b"}' } },
+      ]),
+    ]);
+
+    expect(toolCalls).toEqual([
+      { id: 'call_a', type: 'function', function: { name: 'lookup', arguments: '{"q":"a"}' } },
+      { id: 'call_b', type: 'function', function: { name: 'search', arguments: '{"q":"b"}' } },
+    ]);
+  });
+
+  it('joins argument fragments by id, then onto the latest call when there is no id', async () => {
+    const toolCalls = await recordedToolCalls([
+      chunkWith([{ id: 'call_a', type: 'function', function: { name: 'lookup', arguments: '{"q":' } }]),
+      chunkWith([{ id: 'call_a', function: { name: '', arguments: '"a"' } }]),
+      chunkWith([{ function: { name: '', arguments: '}' } }]),
+    ]);
+
+    expect(toolCalls).toEqual([
+      { id: 'call_a', type: 'function', function: { name: 'lookup', arguments: '{"q":"a"}' } },
+    ]);
+  });
+
+  it('keeps named calls that have neither id nor index', async () => {
+    const toolCalls = await recordedToolCalls([
+      chunkWith([{ function: { name: 'lookup', arguments: '{}' } }]),
+      chunkWith([{ function: { name: 'search', arguments: '{}' } }]),
+    ]);
+
+    expect(toolCalls.map(call => (call.function as { name: string }).name)).toEqual(['lookup', 'search']);
+  });
+
+  it('does not let a later real index overwrite an earlier call without one', async () => {
+    const toolCalls = await recordedToolCalls([
+      chunkWith([{ id: 'call_b', type: 'function', function: { name: 'search', arguments: '{}' } }]),
+      chunkWith([{ index: 0, id: 'call_a', function: { name: 'lookup', arguments: '{}' } }]),
+    ]);
+
+    expect(toolCalls.map(call => call.id).sort()).toEqual(['call_a', 'call_b']);
+  });
+
+  it('still groups indexed deltas by index', async () => {
+    const toolCalls = await recordedToolCalls([
+      chunkWith([{ index: 1, id: 'call_b', function: { name: 'search', arguments: '{' } }]),
+      chunkWith([{ index: 0, id: 'call_a', function: { name: 'lookup', arguments: '{}' } }]),
+      chunkWith([{ index: 1, function: { name: '', arguments: '}' } }]),
+    ]);
+
+    expect(toolCalls).toEqual([
+      expect.objectContaining({ id: 'call_a', function: { name: 'lookup', arguments: '{}' } }),
+      expect.objectContaining({ id: 'call_b', function: { name: 'search', arguments: '{}' } }),
+    ]);
+  });
+});