diff --git a/.diffex/extensions/prompt-url-widget.ts b/.diffex/extensions/prompt-url-widget.ts
index b2de3f94b..0fb462138 100644
--- a/.diffex/extensions/prompt-url-widget.ts
+++ b/.diffex/extensions/prompt-url-widget.ts
@@ -217,9 +217,9 @@ export default function promptUrlWidgetExtension(pi: ExtensionAPI) {
 		});
 	};
 
-	pi.on("before_agent_start", async (event, ctx) => {
-		if (!ctx.hasUI) return;
-		const match = extractPromptMatch(event.prompt);
+	pi.on("message_start", async (event, ctx) => {
+		if (!ctx.hasUI || event.message.role !== "user") return;
+		const match = extractPromptMatch(getUserText(event.message.content));
 		if (!match) {
 			return;
 		}
@@ -231,7 +231,7 @@ export default function promptUrlWidgetExtension(pi: ExtensionAPI) {
 		rebuildFromSession(ctx);
 	});
 
-	const getUserText = (content: string | { type: string; text?: string }[] | undefined): string => {
+	function getUserText(content: string | { type: string; text?: string }[] | undefined): string {
 		if (!content) return "";
 		if (typeof content === "string") return content;
 		return (
@@ -240,7 +240,7 @@ export default function promptUrlWidgetExtension(pi: ExtensionAPI) {
 				.map((block) => block.text)
 				.join("\n") ?? ""
 		);
-	};
+	}
 
 	const rebuildFromSession = (ctx: ExtensionContext) => {
 		if (!ctx.hasUI) return;
diff --git a/packages/coding-agent/test/suite/subagent-registration.test.ts b/packages/coding-agent/test/suite/subagent-registration.test.ts
index 3c0f0d3da..9ab6ca8f4 100644
--- a/packages/coding-agent/test/suite/subagent-registration.test.ts
+++ b/packages/coding-agent/test/suite/subagent-registration.test.ts
@@ -41,7 +41,7 @@ const INCOMPATIBLE_PARENT_HOOKS = [
 	"message_end",
 ] as const;
 type IncompatibleParentHook = (typeof INCOMPATIBLE_PARENT_HOOKS)[number];
-type ObservationParentHook = "agent_start" | "after_provider_response" | "tool_execution_start";
+type ObservationParentHook = "agent_start" | "after_provider_response" | "message_start" | "tool_execution_start";
 
 function registerParentHook(api: ExtensionAPI, hook: IncompatibleParentHook | ObservationParentHook): void {
 	switch (hook) {
@@ -81,6 +81,9 @@ function registerParentHook(api: ExtensionAPI, hook: IncompatibleParentHook | Ob
 		case "after_provider_response":
 			api.on("after_provider_response", () => {});
 			break;
+		case "message_start":
+			api.on("message_start", () => {});
+			break;
 		case "tool_execution_start":
 			api.on("tool_execution_start", () => {});
 			break;
@@ -389,6 +392,7 @@ describe("root subagent tool registration", () => {
 		const observed = await createPolicyRoot(harness, [
 			"agent_start",
 			"after_provider_response",
+			"message_start",
 			"tool_execution_start",
 		]);
 		await getActiveTool(observed.session, "spawn_agent").execute("spawn", { name: "worker", task: "task" });