diff --git a/packages/llm/src/protocols/gemini.ts b/packages/llm/src/protocols/gemini.ts
index c4bb9476a49d..f80bdf4b2083 100644
--- a/packages/llm/src/protocols/gemini.ts
+++ b/packages/llm/src/protocols/gemini.ts
@@ -19,6 +19,7 @@ import {
 } from "../schema"
 import { JsonObject, optionalArray, ProviderShared } from "./shared"
 import { GeminiToolSchema } from "./utils/gemini-tool-schema"
+import { unflattenArgs } from "./utils/unflatten-args"
 import { Lifecycle } from "./utils/lifecycle"
 import { ToolSchemaProjection } from "./utils/tool-schema"
 
@@ -439,7 +440,7 @@ const step = (state: ParserState, event: GeminiEvent) => {
     }
 
     if ("functionCall" in part) {
-      const input = part.functionCall.args
+      const input = unflattenArgs(part.functionCall.args)
       const id = `tool_${nextToolCallId++}`
       lifecycle = Lifecycle.reasoningEnd(
         lifecycle,
diff --git a/packages/llm/src/protocols/utils/unflatten-args.ts b/packages/llm/src/protocols/utils/unflatten-args.ts
new file mode 100644
index 000000000000..7407c2546bac
--- /dev/null
+++ b/packages/llm/src/protocols/utils/unflatten-args.ts
@@ -0,0 +1,67 @@
+/**
+ * Unflattens dot-bracket notation keys into nested objects/arrays.
+ *
+ * Gemini models sometimes return tool call arguments in flattened form, e.g.
+ * `{ "questions[0].header": "Auth", "questions[0].multiSelect": false }` instead
+ * of the nested `{ questions: [{ header: "Auth", multiSelect: false }] }` that
+ * downstream schema validation expects.
+ */
+export function unflattenArgs(args: Record<string, unknown> | null | undefined): Record<string, unknown> | null | undefined {
+  if (!args || typeof args !== "object") return args
+  const keys = Object.keys(args)
+  if (keys.length === 0) return args
+
+  // Fast-path: if no key contains '[', the args are already nested.
+  const needsUnflatten = keys.some((k) => k.includes("["))
+  if (!needsUnflatten) return args
+
+  const result: Record<string, unknown> = Object.create(null)
+  for (const key of keys) {
+    const tokens = tokenize(key)
+    if (tokens.length > 0) setNested(result, tokens, args[key])
+  }
+  return result
+}
+
+/** Parse a flat key like "a[0].b.c[1]" into tokens: ["a", 0, "b", "c", 1] */
+function tokenize(key: string): Array<string | number> {
+  const tokens: Array<string | number> = []
+  let i = 0
+  while (i < key.length) {
+    if (key[i] === "[") {
+      // bracket segment
+      const end = key.indexOf("]", i)
+      if (end === -1) break // malformed key, stop parsing
+      const inner = key.slice(i + 1, end)
+      tokens.push(/^\d+$/.test(inner) ? Number(inner) : inner)
+      i = end + 1
+      if (key[i] === ".") i++ // skip trailing dot
+    } else {
+      // dot-delimited segment
+      let end = i
+      while (end < key.length && key[end] !== "." && key[end] !== "[") end++
+      tokens.push(key.slice(i, end))
+      i = end
+      if (key[i] === ".") i++ // skip dot
+    }
+  }
+  return tokens
+}
+
+const BANNED_KEYS = new Set(["__proto__", "constructor", "prototype"])
+
+function setNested(obj: Record<string, unknown>, tokens: Array<string | number>, value: unknown): void {
+  let current: any = obj
+  for (let i = 0; i < tokens.length - 1; i++) {
+    const token = tokens[i]
+    if (typeof token === "string" && BANNED_KEYS.has(token)) return
+    const next = tokens[i + 1]
+    if (current[token as any] == null) {
+      current[token as any] = typeof next === "number" ? [] : Object.create(null)
+    }
+    current = current[token as any]
+  }
+  const last = tokens[tokens.length - 1]
+  if (typeof last === "string" && BANNED_KEYS.has(last)) return
+  current[last as any] = value
+}
diff --git a/packages/llm/test/unflatten-args.test.ts b/packages/llm/test/unflatten-args.test.ts
new file mode 100644
index 000000000000..88e91acbcd78
--- /dev/null
+++ b/packages/llm/test/unflatten-args.test.ts
@@ -0,0 +1,89 @@
+import { describe, expect, test } from "bun:test"
+import { unflattenArgs } from "../src/protocols/utils/unflatten-args"
+
+describe("unflattenArgs", () => {
+  test("returns null/undefined as-is", () => {
+    expect(unflattenArgs(null)).toBe(null)
+    expect(unflattenArgs(undefined)).toBe(undefined)
+  })
+
+  test("returns empty object as-is", () => {
+    const obj = {}
+    expect(unflattenArgs(obj)).toBe(obj)
+  })
+
+  test("returns already-nested object unchanged", () => {
+    const obj = { name: "hello", nested: { a: 1 } }
+    expect(unflattenArgs(obj)).toBe(obj)
+  })
+
+  test("passes through dot-only keys (no brackets)", () => {
+    const obj = { "a.b": "val" }
+    expect(unflattenArgs(obj)).toBe(obj)
+  })
+
+  test("unflattens bracket notation", () => {
+    expect(unflattenArgs({ "a[0]": "val" })).toEqual({ a: ["val"] })
+  })
+
+  test("unflattens deep mixed notation", () => {
+    expect(
+      unflattenArgs({ "questions[0].header": "Auth" }),
+    ).toEqual({ questions: [{ header: "Auth" }] })
+  })
+
+  test("unflattens multiple array items", () => {
+    expect(
+      unflattenArgs({
+        "a[0].x": 1,
+        "a[1].x": 2,
+      }),
+    ).toEqual({ a: [{ x: 1 }, { x: 2 }] })
+  })
+
+  test("handles the issue #35105 example", () => {
+    const flat = {
+      "questions[0].question": "Which auth method?",
+      "questions[0].header": "Auth",
+      "questions[0].options[0].label": "OAuth",
+      "questions[0].options[0].description": "Use OAuth",
+      "questions[0].options[1].label": "JWT",
+      "questions[0].options[1].description": "Use JWT",
+      "questions[0].multiSelect": false,
+    }
+    expect(unflattenArgs(flat)).toEqual({
+      questions: [
+        {
+          question: "Which auth method?",
+          header: "Auth",
+          options: [
+            { label: "OAuth", description: "Use OAuth" },
+            { label: "JWT", description: "Use JWT" },
+          ],
+          multiSelect: false,
+        },
+      ],
+    })
+  })
+
+  test("preserves non-bracket keys alongside bracket keys", () => {
+    expect(
+      unflattenArgs({
+        plain: "yes",
+        "arr[0]": "val",
+      }),
+    ).toEqual({ plain: "yes", arr: ["val"] })
+  })
+
+  test("handles malformed key with missing closing bracket", () => {
+    // Should not hang — partial token is ignored
+    const result = unflattenArgs({ "a[0": "val" })
+    expect(result).toBeDefined()
+  })
+
+  test("rejects prototype pollution attempts", () => {
+    const result = unflattenArgs({ "__proto__[0]": "evil" }) as any
+    expect(({} as any).constructor).toBeDefined() // Object.prototype untouched
+    expect(result["__proto__"] ?? undefined).toBeUndefined()
+  })
+})