diff --git a/01_funccall.ipynb b/01_funccall.ipynb
index b717242..c29a603 100644
--- a/01_funccall.ipynb
+++ b/01_funccall.ipynb
@@ -353,7 +353,9 @@
     "        args = get_args(t)\n",
     "        if not args: return {'type': 'array', 'items': {}}\n",
     "        if args[-1] is Ellipsis: return {'type': 'array', 'items': _handle_type(args[0], defs)}\n",
-    "        return {'type': 'array', 'prefixItems': [_handle_type(a, defs) for a in args]}\n",
+    "        prefix = [_handle_type(a, defs) for a in args]\n",
+    "        items = prefix[0] if all(p == prefix[0] for p in prefix) else {'anyOf': prefix}\n",
+    "        return {'type': 'array', 'prefixItems': prefix, 'items': items, 'minItems': len(args), 'maxItems': len(args)}\n",
     "    if ot in (list, set):\n",
     "        args = get_args(t)\n",
     "        schema = {'type': 'array', 'items': _handle_type(args[0], defs) if args else {}}\n",
@@ -388,6 +390,14 @@
     "_handle_type(int, None), _handle_type(Path, None)"
    ]
   },
+  {
+   "cell_type": "markdown",
+   "id": "e669cbc1",
+   "metadata": {},
+   "source": [
+    "Fixed-length tuples (e.g. `tuple[int, str]`) are mapped to a JSON Schema array with `prefixItems` for per-position types, which is accepted by Anthropic. OpenAI/Gemini require `items` so this is added, along with `minItems`/`maxItems` to enforce the fixed length. If all positions share the same type, `items` is that type directly; otherwise it's an `anyOf` of the unique types."
+   ]
+  },
   {
    "cell_type": "code",
    "execution_count": null,
@@ -398,7 +408,11 @@
      "data": {
       "text/plain": [
        "({'type': 'array', 'items': {}},\n",
-       " {'type': 'array', 'prefixItems': [{'type': 'string'}]},\n",
+       " {'type': 'array',\n",
+       "  'prefixItems': [{'type': 'string'}],\n",
+       "  'items': {'type': 'string'},\n",
+       "  'minItems': 1,\n",
+       "  'maxItems': 1},\n",
        " {'type': 'array', 'items': {'type': 'string'}, 'uniqueItems': True})"
       ]
      },
@@ -571,7 +585,7 @@
    "source": [
     "# Test primitive types in containers\n",
     "test_eq(_handle_type(list[int], defs), {'type': 'array', 'items': {'type': 'integer'}})\n",
-    "test_eq(_handle_type(tuple[str], defs), {'type': 'array', 'prefixItems': [{'type': 'string'}]})\n",
+    "test_eq(_handle_type(tuple[str], defs), {'type': 'array', 'prefixItems': [{'type': 'string'}], 'items': {'type': 'string'}, 'minItems': 1, 'maxItems': 1})\n",
     "test_eq(_handle_type(set[str], defs), dict(type='array', items={'type': 'string'}, uniqueItems=True))\n",
     "test_eq(_handle_type(dict[str,bool], defs), {'type': 'object', 'additionalProperties': {'type': 'boolean'}})"
    ]
@@ -736,7 +750,10 @@
        "  'properties': {'o': {'description': 'the o', 'type': 'object'},\n",
        "   'q': {'description': '',\n",
        "    'type': 'array',\n",
-       "    'prefixItems': [{'type': 'integer'}, {'type': 'string'}]},\n",
+       "    'prefixItems': [{'type': 'integer'}, {'type': 'string'}],\n",
+       "    'items': {'anyOf': [{'type': 'integer'}, {'type': 'string'}]},\n",
+       "    'minItems': 2,\n",
+       "    'maxItems': 2},\n",
        "   'p': {'description': '',\n",
        "    'default': 'a',\n",
        "    'anyOf': [{'type': 'string'},\n",
@@ -756,7 +773,7 @@
     "test_eq(s['name'], 'f')\n",
     "inpp = s['input_schema']['properties']\n",
     "test_eq(inpp['o'], {'type': 'object', 'description': 'the o'})\n",
-    "test_eq(inpp['q'], dict(type='array', description='', prefixItems=[{'type': 'integer'}, {'type': 'string'}]))\n",
+    "test_eq(inpp['q'], dict(type='array', description='', prefixItems=[{'type': 'integer'}, {'type': 'string'}], items=dict(anyOf=[{'type': 'integer'}, {'type': 'string'}]), minItems=2, maxItems=2))\n",
     "test_eq(inpp['p'], dict(description='', default='a', anyOf=[{'type': 'string'}, {'type': 'array', 'items': {'type': 'string'}}]))\n",
     "s"
    ]
@@ -1172,7 +1189,10 @@
        "  'properties': {'opt_tup': {'description': '',\n",
        "    'default': None,\n",
        "    'anyOf': [{'type': 'array',\n",
-       "      'prefixItems': [{'type': 'integer'}, {'type': 'integer'}]},\n",
+       "      'prefixItems': [{'type': 'integer'}, {'type': 'integer'}],\n",
+       "      'items': {'type': 'integer'},\n",
+       "      'minItems': 2,\n",
+       "      'maxItems': 2},\n",
        "     {'type': 'string'},\n",
        "     {'type': 'integer'}]}}}}"
       ]
@@ -1215,7 +1235,10 @@
        "  'properties': {'opt_tup': {'description': '',\n",
        "    'default': None,\n",
        "    'anyOf': [{'type': 'array',\n",
-       "      'prefixItems': [{'type': 'integer'}, {'type': 'integer'}]},\n",
+       "      'prefixItems': [{'type': 'integer'}, {'type': 'integer'}],\n",
+       "      'items': {'type': 'integer'},\n",
+       "      'minItems': 2,\n",
+       "      'maxItems': 2},\n",
        "     {'type': 'string'},\n",
        "     {'type': 'integer'}]}}}}"
       ]
@@ -1257,7 +1280,10 @@
        "  'properties': {'opt_tup': {'description': '',\n",
        "    'default': None,\n",
        "    'anyOf': [{'type': 'array',\n",
-       "      'prefixItems': [{'type': 'integer'}, {'type': 'integer'}]},\n",
+       "      'prefixItems': [{'type': 'integer'}, {'type': 'integer'}],\n",
+       "      'items': {'type': 'integer'},\n",
+       "      'minItems': 2,\n",
+       "      'maxItems': 2},\n",
        "     {'type': 'null'}]}}}}"
       ]
      },
@@ -1620,7 +1646,26 @@
    "execution_count": null,
    "id": "b2c66a73",
    "metadata": {},
-   "outputs": [],
+   "outputs": [
+    {
+     "data": {
+      "text/plain": [
+       "{'name': '_cust_type',\n",
+       " 'description': 'Mandatory docstring',\n",
+       " 'input_schema': {'type': 'object',\n",
+       "  'properties': {'a': {'description': '',\n",
+       "    'anyOf': [{'type': 'string'},\n",
+       "     {'type': 'array', 'items': {'type': 'string'}}]}},\n",
+       "  'required': ['a']}}"
+      ]
+     },
+     "execution_count": null,
+     "metadata": {
+      "__type": "dict"
+     },
+     "output_type": "execute_result"
+    }
+   ],
    "source": [
     "Cmd = str | list[str]\n",
     "\n",
@@ -1856,14 +1901,14 @@
      "output_type": "stream",
      "text": [
       "Traceback (most recent call last):\n",
-      "  File \"/var/folders/51/b2_szf2945n072c0vj2cyty40000gn/T/ipykernel_38179/3422083997.py\", line 13, in python\n",
+      "  File \"/var/folders/wm/y9k35r7n7q56mvx2wnndd0880000gp/T/ipykernel_61561/3422083997.py\", line 13, in python\n",
       "    try: return _run(code, glb, loc)\n",
       "                ^^^^^^^^^^^^^^^^^^^^\n",
-      "  File \"/var/folders/51/b2_szf2945n072c0vj2cyty40000gn/T/ipykernel_38179/2387754473.py\", line 17, in _run\n",
+      "  File \"/var/folders/wm/y9k35r7n7q56mvx2wnndd0880000gp/T/ipykernel_61561/2387754473.py\", line 17, in _run\n",
       "    try: exec(compiled_code, glb, loc)\n",
       "         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^\n",
       "  File \"<ast>\", line 1, in <module>\n",
-      "  File \"/var/folders/51/b2_szf2945n072c0vj2cyty40000gn/T/ipykernel_38179/3422083997.py\", line 8, in handler\n",
+      "  File \"/var/folders/wm/y9k35r7n7q56mvx2wnndd0880000gp/T/ipykernel_61561/3422083997.py\", line 8, in handler\n",
       "    def handler(*args): raise TimeoutError()\n",
       "                        ^^^^^^^^^^^^^^^^^^^^\n",
       "TimeoutError\n",
@@ -2771,7 +2816,10 @@
    ]
   }
  ],
- "metadata": {},
+ "metadata": {
+  "solveit_dialog_mode": "learning",
+  "solveit_ver": 2
+ },
  "nbformat": 4,
  "nbformat_minor": 5
 }
diff --git a/toolslm/funccall.py b/toolslm/funccall.py
index 986477d..3e61bca 100644
--- a/toolslm/funccall.py
+++ b/toolslm/funccall.py
@@ -58,7 +58,9 @@ def _handle_type(t, defs):
         args = get_args(t)
         if not args: return {'type': 'array', 'items': {}}
         if args[-1] is Ellipsis: return {'type': 'array', 'items': _handle_type(args[0], defs)}
-        return {'type': 'array', 'prefixItems': [_handle_type(a, defs) for a in args]}
+        prefix = [_handle_type(a, defs) for a in args]
+        items = prefix[0] if all(p == prefix[0] for p in prefix) else {'anyOf': prefix}
+        return {'type': 'array', 'prefixItems': prefix, 'items': items, 'minItems': len(args), 'maxItems': len(args)}
     if ot in (list, set):
         args = get_args(t)
         schema = {'type': 'array', 'items': _handle_type(args[0], defs) if args else {}}