diff --git a/skills/build-seller-agent/SKILL.md b/skills/build-seller-agent/SKILL.md
index 2e1b0328..252b273d 100644
--- a/skills/build-seller-agent/SKILL.md
+++ b/skills/build-seller-agent/SKILL.md
@@ -168,7 +168,7 @@ class MySeller(ADCPHandler):
 Sellers emit webhooks to notify buyers asynchronously. AdCP 3.0 uses RFC 9421 HTTP Message Signatures — use `adcp.webhooks.WebhookSender` (one-call helper) or `sign_webhook` for lower-level control.
 
 **When to emit:**
-- Async-approval media buy transitions (`pending_activation` → `active`, or `→ rejected`)
+- Async-approval media buy transitions (`pending_creatives` → `pending_start` → `active`, or `→ rejected`)
 - Artifact-ready notifications after long-running operations
 - Delivery reports the buyer subscribed to via `reporting_webhook` on `create_media_buy`
 
diff --git a/src/adcp/server/helpers.py b/src/adcp/server/helpers.py
index 0e6f80f1..0ffda133 100644
--- a/src/adcp/server/helpers.py
+++ b/src/adcp/server/helpers.py
@@ -141,7 +141,15 @@ def adcp_error(
 # Actions are operations available via update_media_buy for each status.
 # Public constant — servers can inspect, test against, or extend this.
 MEDIA_BUY_STATE_MACHINE: dict[str, list[str]] = {
-    "pending_activation": [
+    "pending_creatives": [
+        "cancel",
+        "update_budget",
+        "update_dates",
+        "update_packages",
+        "add_packages",
+        "sync_creatives",
+    ],
+    "pending_start": [
         "cancel",
         "update_budget",
         "update_dates",
@@ -164,7 +172,18 @@ def adcp_error(
 
 
 def valid_actions_for_status(status: str) -> list[str]:
-    """Get valid buyer actions for a media buy status."""
+    """Get valid buyer actions for a media buy status.
+
+    Returns the list of ``update_media_buy`` actions available to a buyer for
+    the given status string. Returns ``[]`` for terminal statuses and for any
+    unrecognized status string.
+
+    Valid statuses per ``enums/media-buy-status.json``:
+    ``pending_creatives``, ``pending_start``, ``active``, ``paused``,
+    ``completed``, ``rejected``, ``canceled``.
+
+    Inspect or extend :data:`MEDIA_BUY_STATE_MACHINE` to add custom actions.
+    """
     return list(MEDIA_BUY_STATE_MACHINE.get(status, []))
 
 
diff --git a/tests/test_server_helpers.py b/tests/test_server_helpers.py
index 0df53a3a..5947acfd 100644
--- a/tests/test_server_helpers.py
+++ b/tests/test_server_helpers.py
@@ -85,11 +85,41 @@ def test_terminal_statuses_empty(self) -> None:
         for status in ("completed", "rejected", "canceled"):
             assert valid_actions_for_status(status) == []
 
-    def test_pending_activation_allows_cancel(self) -> None:
-        actions = valid_actions_for_status("pending_activation")
+    def test_pending_start_allows_cancel(self) -> None:
+        actions = valid_actions_for_status("pending_start")
         assert "cancel" in actions
         assert "update_packages" in actions
 
+    def test_pending_creatives_allows_sync_creatives(self) -> None:
+        actions = valid_actions_for_status("pending_creatives")
+        assert "sync_creatives" in actions
+        assert "cancel" in actions
+
+    def test_pending_activation_is_not_recognized(self) -> None:
+        # AdCP v3 renamed pending_activation to pending_creatives + pending_start.
+        # Lock the rename so a copy-paste from old docs / old SDK code returns
+        # an empty action list rather than silently matching a stale entry.
+        from adcp.server.helpers import MEDIA_BUY_STATE_MACHINE
+
+        assert valid_actions_for_status("pending_activation") == []
+        assert "pending_activation" not in MEDIA_BUY_STATE_MACHINE
+
+    def test_state_machine_keys_match_spec_enum(self) -> None:
+        # Keys must exactly match enums/media-buy-status.json. Terminal
+        # statuses are present with empty action lists. Guards against
+        # future silent drift between the spec enum and the dispatcher.
+        from adcp.server.helpers import MEDIA_BUY_STATE_MACHINE
+
+        assert set(MEDIA_BUY_STATE_MACHINE) == {
+            "pending_creatives",
+            "pending_start",
+            "active",
+            "paused",
+            "completed",
+            "rejected",
+            "canceled",
+        }
+
     def test_unknown_status_empty(self) -> None:
         assert valid_actions_for_status("nonexistent") == []