diff --git a/src/taskgraph/transforms/task.py b/src/taskgraph/transforms/task.py
index d2383635..5b8de43d 100644
--- a/src/taskgraph/transforms/task.py
+++ b/src/taskgraph/transforms/task.py
@@ -16,11 +16,14 @@
 from dataclasses import dataclass
 from typing import Callable, Literal, Optional, Union
 
+import voluptuous
+
 from taskgraph.transforms.base import TransformSequence
 from taskgraph.util.hash import hash_path
 from taskgraph.util.keyed_by import evaluate_keyed_by
 from taskgraph.util.schema import (
     IndexSchema,
+    LegacySchema,
     OptimizationType,
     Schema,
     TaskPriority,
@@ -194,6 +197,14 @@ class PayloadBuilder:
 
 
 def payload_builder(name, schema):
+    if isinstance(schema, dict):
+        schema = LegacySchema(
+            {
+                voluptuous.Required("implementation"): name,
+                voluptuous.Optional("os"): str,
+            }
+        ).extend(schema)
+
     def wrap(func):
         assert name not in payload_builders, f"duplicate payload builder name {name}"
         payload_builders[name] = PayloadBuilder(schema, func)
diff --git a/src/taskgraph/util/schema.py b/src/taskgraph/util/schema.py
index 0afdb001..29df7429 100644
--- a/src/taskgraph/util/schema.py
+++ b/src/taskgraph/util/schema.py
@@ -48,6 +48,9 @@ def validate_schema(schema, obj, msg_prefix):
         # Handle plain Python types (e.g. str, int) via msgspec.convert
         elif isinstance(schema, type):
             msgspec.convert(obj, schema)
+        # Handle plain dict schemas (e.g. from downstream payload builders)
+        elif isinstance(schema, dict):
+            voluptuous.Schema(schema)(obj)
         else:
             raise TypeError(f"Unsupported schema type: {type(schema)}")
     except (
diff --git a/test/test_transforms_task.py b/test/test_transforms_task.py
index ad5697f4..3d49e799 100644
--- a/test/test_transforms_task.py
+++ b/test/test_transforms_task.py
@@ -7,6 +7,7 @@
 from pprint import pprint
 
 import pytest
+import voluptuous
 from pytest_taskgraph import FakeParameters
 
 from taskgraph.transforms import task
@@ -966,3 +967,36 @@ def test_task_priority(run_transform, graph_config, test_task):
         assert task_dict["task"]["priority"] == priority
     else:
         assert task_dict["task"]["priority"] == graph_config["task-priority"]
+
+
+@pytest.fixture
+def dict_schema_builder():
+    @task.payload_builder("test-builder", schema={"command": [str]})
+    def _builder(config, task, task_def):
+        pass
+
+    yield task.payload_builders["test-builder"].schema
+    task.payload_builders.pop("test-builder", None)
+
+
+@pytest.mark.parametrize(
+    "payload",
+    (
+        {"implementation": "test-builder", "command": ["echo"]},
+        {"implementation": "test-builder", "command": ["echo"], "os": "linux"},
+    ),
+)
+def test_dict_schema_accepts_valid_payload(dict_schema_builder, payload):
+    dict_schema_builder(payload)
+
+
+@pytest.mark.parametrize(
+    "payload",
+    (
+        {"implementation": "wrong-name", "command": ["echo"]},
+        {"command": ["echo"]},
+    ),
+)
+def test_dict_schema_rejects_invalid_payload(dict_schema_builder, payload):
+    with pytest.raises(voluptuous.MultipleInvalid):
+        dict_schema_builder(payload)
diff --git a/test/test_util_schema.py b/test/test_util_schema.py
index 4da2e58a..b3701a89 100644
--- a/test/test_util_schema.py
+++ b/test/test_util_schema.py
@@ -271,6 +271,18 @@ def test_index_schema_accepts_all_fields(self):
         )
 
 
+class TestValidateSchemaDictHandler(unittest.TestCase):
+    """validate_schema must accept plain dict schemas passed
+    by downstream payload builders without raising TypeError."""
+
+    def test_dict_schema_valid(self):
+        validate_schema({"name": str, "count": int}, {"name": "a", "count": 1}, "pfx")
+
+    def test_dict_schema_invalid(self):
+        with self.assertRaises(Exception):
+            validate_schema({"name": str}, {"name": 123}, "pfx")
+
+
 def test_optionally_keyed_by():
     typ = optionally_keyed_by("foo", str, use_msgspec=True)
     assert msgspec.convert("baz", typ) == "baz"