diff --git a/plugins/muse-provider/server/children.ts b/plugins/muse-provider/server/children.ts
index 69e0f4a855..7b8e11632e 100644
--- a/plugins/muse-provider/server/children.ts
+++ b/plugins/muse-provider/server/children.ts
@@ -21,6 +21,7 @@ export class Children {
     private readonly parentId: string,
     private readonly cwd: string,
     private readonly emit: (event: ProviderEvent) => void,
+    private readonly subsessions: boolean,
   ) {}
 
   async update(item: WireItem): Promise<void> {
@@ -34,6 +35,10 @@ export class Children {
       )
     )
       return;
+    // Emitting a child session.opened with parentSessionId is only valid when the
+    // parent session negotiated the session.subsession capability; the daemon
+    // fails the provider connection otherwise.
+    if (!this.subsessions) return;
     const id = item.childSessionId;
     if (!this.children.has(id)) {
       const { session } = await this.host.request(
diff --git a/plugins/muse-provider/server/provider.ts b/plugins/muse-provider/server/provider.ts
index 8d9a3ddfea..ce2518cbc2 100644
--- a/plugins/muse-provider/server/provider.ts
+++ b/plugins/muse-provider/server/provider.ts
@@ -27,6 +27,7 @@ const capabilities = [
   "session.persistence",
   "permission",
   "session.list",
+  "session.subsession",
 ] as const;
 
 export function createMuseProvider(usage: Usage): ProviderRegistration {
diff --git a/plugins/muse-provider/server/session.ts b/plugins/muse-provider/server/session.ts
index 31066c5b04..6a5a690102 100644
--- a/plugins/muse-provider/server/session.ts
+++ b/plugins/muse-provider/server/session.ts
@@ -142,7 +142,13 @@ export class Session {
     const nativeId = response.session.sessionId;
     this.liveState = {
       nativeId,
-      children: new Children(this.host, this.options.id, this.config.cwd, this.options.emit),
+      children: new Children(
+        this.host,
+        this.options.id,
+        this.config.cwd,
+        this.options.emit,
+        this.options.capabilities.includes("session.subsession"),
+      ),
       timeline: new Timeline(
         this.host,
         nativeId,
diff --git a/plugins/muse-provider/test/provider.test.ts b/plugins/muse-provider/test/provider.test.ts
index 4868634d70..dae8029245 100644
--- a/plugins/muse-provider/test/provider.test.ts
+++ b/plugins/muse-provider/test/provider.test.ts
@@ -45,6 +45,7 @@ async function harness(
   scenario = "text-reasoning",
   env: Record<string, string> = {},
   providerOptions?: Record<string, unknown>,
+  connectCapabilities: readonly string[] = PROVIDER_CAPABILITIES,
 ) {
   const root = await mkdtemp(path.join(os.tmpdir(), "muse-provider-test-"));
   roots.push(root);
@@ -62,7 +63,7 @@ async function harness(
   const connection = await provider.connect({
     launch,
     versions: [1],
-    capabilities: PROVIDER_CAPABILITIES,
+    capabilities: connectCapabilities,
   });
   connections.push(connection);
   const events: ProviderEvent[] = [];
@@ -1192,6 +1193,20 @@ for (const variant of ["tool", "dedicated", "workflow"]) {
     ).toHaveLength(1);
   });
 }
+
+test("subagent children are not projected when session.subsession is not negotiated", async () => {
+  const h = await harness(
+    "subagent",
+    {},
+    undefined,
+    PROVIDER_CAPABILITIES.filter((capability) => capability !== "session.subsession"),
+  );
+  await h.open();
+  await h.prompt();
+  await h.wait((e) => e.type === "session.turn" && e.state === "completed");
+  expect(h.events.filter((e) => e.type === "session.opened" && e.parentSessionId)).toHaveLength(0);
+  expect((await h.recorded()).filter((f) => f.method === "session/read")).toEqual([]);
+});
 test("gap backfill repairs a missing completion without duplicating item revisions", async () => {
   const h = await harness("text-reasoning", { MUSE_TEST_GAP: "1" });
   await h.open();