diff --git a/src/agents/run_state.py b/src/agents/run_state.py
index 1f5fb2f9a..dc0c3144c 100644
--- a/src/agents/run_state.py
+++ b/src/agents/run_state.py
@@ -94,8 +94,13 @@
 ContextSerializer = Callable[[Any], Mapping[str, Any]]
 ContextDeserializer = Callable[[Mapping[str, Any]], Any]
 
-# Schema version for serialization compatibility
-CURRENT_SCHEMA_VERSION = "1.0"
+# RunState schema policy.
+# 1. Bump CURRENT_SCHEMA_VERSION when serialized shape/semantics change.
+# 2. Keep older readable versions in SUPPORTED_SCHEMA_VERSIONS for backward reads.
+# 3. to_json() always emits CURRENT_SCHEMA_VERSION.
+# 4. Forward compatibility is intentionally fail-fast (older SDKs reject newer versions).
+CURRENT_SCHEMA_VERSION = "1.1"
+SUPPORTED_SCHEMA_VERSIONS = frozenset({"1.0", CURRENT_SCHEMA_VERSION})
 
 _FUNCTION_OUTPUT_ADAPTER: TypeAdapter[FunctionCallOutput] = TypeAdapter(FunctionCallOutput)
 _COMPUTER_OUTPUT_ADAPTER: TypeAdapter[ComputerCallOutput] = TypeAdapter(ComputerCallOutput)
@@ -1894,10 +1899,12 @@ async def _build_run_state_from_json(
     schema_version = state_json.get("$schemaVersion")
     if not schema_version:
         raise UserError("Run state is missing schema version")
-    if schema_version != CURRENT_SCHEMA_VERSION:
+    if schema_version not in SUPPORTED_SCHEMA_VERSIONS:
+        supported_versions = ", ".join(sorted(SUPPORTED_SCHEMA_VERSIONS))
         raise UserError(
             f"Run state schema version {schema_version} is not supported. "
-            f"Please use version {CURRENT_SCHEMA_VERSION}"
+            f"Supported versions are: {supported_versions}. "
+            f"New snapshots are written as version {CURRENT_SCHEMA_VERSION}."
         )
 
     agent_map = _build_agent_map(initial_agent)
diff --git a/tests/test_run_state.py b/tests/test_run_state.py
index ef5e9091f..c819d62bd 100644
--- a/tests/test_run_state.py
+++ b/tests/test_run_state.py
@@ -61,6 +61,7 @@
 )
 from agents.run_state import (
     CURRENT_SCHEMA_VERSION,
+    SUPPORTED_SCHEMA_VERSIONS,
     RunState,
     _build_agent_map,
     _deserialize_items,
@@ -286,11 +287,13 @@ async def test_throws_error_if_schema_version_is_missing_or_invalid(self):
             await RunState.from_string(agent, str_data)
 
         json_data["$schemaVersion"] = "0.1"
+        supported_versions = ", ".join(sorted(SUPPORTED_SCHEMA_VERSIONS))
         with pytest.raises(
             Exception,
             match=(
                 f"Run state schema version 0.1 is not supported. "
-                f"Please use version {CURRENT_SCHEMA_VERSION}"
+                f"Supported versions are: {supported_versions}. "
+                f"New snapshots are written as version {CURRENT_SCHEMA_VERSION}."
             ),
         ):
             await RunState.from_string(agent, json.dumps(json_data))
@@ -3330,6 +3333,31 @@ async def test_from_json_unsupported_schema_version(self):
         with pytest.raises(UserError, match="Run state schema version 2.0 is not supported"):
             await RunState.from_json(agent, state_json)
 
+    @pytest.mark.asyncio
+    async def test_from_json_accepts_previous_schema_version(self):
+        """Test that from_json accepts a previous, explicitly supported schema version."""
+        agent = Agent(name="TestAgent")
+        state_json = {
+            "$schemaVersion": "1.0",
+            "original_input": "test",
+            "current_agent": {"name": "TestAgent"},
+            "context": {
+                "context": {"foo": "bar"},
+                "usage": {"requests": 0, "input_tokens": 0, "output_tokens": 0, "total_tokens": 0},
+                "approvals": {},
+            },
+            "max_turns": 3,
+            "current_turn": 0,
+            "model_responses": [],
+            "generated_items": [],
+        }
+
+        restored = await RunState.from_json(agent, state_json)
+        assert restored._current_agent is not None
+        assert restored._current_agent.name == "TestAgent"
+        assert restored._context is not None
+        assert restored._context.context == {"foo": "bar"}
+
     @pytest.mark.asyncio
     async def test_from_json_agent_not_found(self):
         """Test that from_json raises error when agent is not found in agent map."""