diff --git a/packages/templates/src/templates/standard-agent-append-instructions.md b/packages/templates/src/templates/standard-agent-append-instructions.md
index 687c4db5c57..eebc47f08da 100644
--- a/packages/templates/src/templates/standard-agent-append-instructions.md
+++ b/packages/templates/src/templates/standard-agent-append-instructions.md
@@ -11,6 +11,7 @@ You are working inside bb, an agentic IDE for managing coding agents in projects
 - Prefer bare `bb` on PATH. When `BB_CLI` is set, official `bb` entrypoints re-exec to that absolute binary; you can also invoke `"$BB_CLI"` directly.
 - Run `bb status` to see the current project, thread, and environment.
 - Run `bb guide` for BB concepts and `bb guide <chapter>` for command details.
+- Do not message or coordinate other BB threads unless the user explicitly asks you to.
 - Use `bb thread ...` when you need to create, inspect, message, wait for, or coordinate other BB threads.
 - Reference a BB thread as `@thread:thr_abc123`, substituting its actual ID, so bb renders the correct project-aware link. Do not construct thread URLs manually.
 - Use Markdown links for files, artifacts, and URLs you want the user to open; bb is a visual IDE and renders them as clickable links.
diff --git a/packages/templates/test/templates.test.ts b/packages/templates/test/templates.test.ts
index b09ce2fc53c..c1ff8280a69 100644
--- a/packages/templates/test/templates.test.ts
+++ b/packages/templates/test/templates.test.ts
@@ -55,6 +55,12 @@ describe("@bb/templates", () => {
 
     expect(rendered).toContain("You are working inside bb");
     expect(rendered).toContain("agentic IDE");
+    expect(rendered).toContain(
+      "Do not message or coordinate other BB threads unless the user explicitly asks you to.",
+    );
+    expect(rendered).toContain(
+      "Use `bb thread ...` when you need to create, inspect, message, wait for, or coordinate other BB threads.",
+    );
     expect(rendered).toContain("Reference a BB thread as `@thread:thr_abc123`");
     expect(rendered).toContain("Do not construct thread URLs manually");
     expect(rendered).not.toContain(
diff --git a/plugins/bb-guide/skills/bb-cli/references/thread-operation.md b/plugins/bb-guide/skills/bb-cli/references/thread-operation.md
index 9fd86ad40f0..e839f0c8a98 100644
--- a/plugins/bb-guide/skills/bb-cli/references/thread-operation.md
+++ b/plugins/bb-guide/skills/bb-cli/references/thread-operation.md
@@ -2,6 +2,9 @@
 
 ## Coordinating Work
 
+Do not message or coordinate other BB threads unless the user explicitly asks you
+to. Sharing a project or workspace is not permission.
+
 - Use one clear owner per task.
 - Spawn independent tasks separately when parallel work is useful.
 - Let threads work after spawning. Do not poll with shell sleeps, repeated log