diff --git a/src/agents/subagent-spawn.ts b/src/agents/subagent-spawn.ts
index d75a8717a22..5a5a177c64d 100644
--- a/src/agents/subagent-spawn.ts
+++ b/src/agents/subagent-spawn.ts
@@ -685,6 +685,7 @@ export async function spawnSubagentDirect(
         // Best-effort cleanup only.
       }
     }
+    let emitLifecycleHooks = false;
     if (threadBindingReady) {
       const hasEndedHook = hookRunner?.hasHooks("subagent_ended") === true;
       let endedHookEmitted = false;
@@ -712,21 +713,22 @@ export async function spawnSubagentDirect(
           // Spawn should still return an actionable error even if cleanup hooks fail.
         }
       }
-      // Always delete the provisional child session after a failed spawn attempt.
-      // If we already emitted subagent_ended above, suppress a duplicate lifecycle hook.
-      try {
-        await callGateway({
-          method: "sessions.delete",
-          params: {
-            key: childSessionKey,
-            deleteTranscript: true,
-            emitLifecycleHooks: !endedHookEmitted,
-          },
-          timeoutMs: 10_000,
-        });
-      } catch {
-        // Best-effort only.
-      }
+      emitLifecycleHooks = !endedHookEmitted;
+    }
+    // Always delete the provisional child session after a failed spawn attempt.
+    // If we already emitted subagent_ended above, suppress a duplicate lifecycle hook.
+    try {
+      await callGateway({
+        method: "sessions.delete",
+        params: {
+          key: childSessionKey,
+          deleteTranscript: true,
+          emitLifecycleHooks,
+        },
+        timeoutMs: 10_000,
+      });
+    } catch {
+      // Best-effort only.
     }
     const messageText = summarizeError(err);
     return {
diff --git a/src/agents/subagent-spawn.workspace.test.ts b/src/agents/subagent-spawn.workspace.test.ts
index 13796c9d03e..dcf0e079b01 100644
--- a/src/agents/subagent-spawn.workspace.test.ts
+++ b/src/agents/subagent-spawn.workspace.test.ts
@@ -231,4 +231,60 @@ describe("spawnSubagentDirect workspace inheritance", () => {
       expectedWorkspaceDir: "/tmp/requester-workspace",
     });
   });
+
+  it("deletes the provisional child session when a non-thread subagent start fails", async () => {
+    hoisted.callGatewayMock.mockImplementation(async (request: {
+      method?: string;
+      params?: { key?: string; deleteTranscript?: boolean; emitLifecycleHooks?: boolean };
+    }) => {
+      if (request.method === "sessions.patch") {
+        return { ok: true };
+      }
+      if (request.method === "agent") {
+        throw new Error("spawn startup failed");
+      }
+      if (request.method === "sessions.delete") {
+        return { ok: true };
+      }
+      return {};
+    });
+
+    const result = await spawnSubagentDirect(
+      {
+        task: "fail after provisional session creation",
+      },
+      {
+        agentSessionKey: "agent:main:main",
+        agentChannel: "discord",
+        agentAccountId: "acct-1",
+        agentTo: "user-1",
+        workspaceDir: "/tmp/requester-workspace",
+      },
+    );
+
+    expect(result).toMatchObject({
+      status: "error",
+      error: "spawn startup failed",
+    });
+    expect(result.childSessionKey).toMatch(/^agent:main:subagent:/);
+    expect(hoisted.registerSubagentRunMock).not.toHaveBeenCalled();
+
+    const deleteCall = hoisted.callGatewayMock.mock.calls.find(
+      ([request]) => (request as { method?: string }).method === "sessions.delete",
+    )?.[0] as
+      | {
+          params?: {
+            key?: string;
+            deleteTranscript?: boolean;
+            emitLifecycleHooks?: boolean;
+          };
+        }
+      | undefined;
+
+    expect(deleteCall?.params).toMatchObject({
+      key: result.childSessionKey,
+      deleteTranscript: true,
+      emitLifecycleHooks: false,
+    });
+  });
 });