chore(desktop): hide custom instructions from task timeline

packages/ui/src/features/canvas/components/ActivityTimeline.test.tsx

@@ -128,4 +128,37 @@ describe("ActivityTimeline", () => {
     expect(screen.getByText("Saved workspace context")).toBeVisible();
     expect(useThreadNavigationStore.getState().scrollRequests).toEqual({});
   });
+
+  it("hides injected custom instructions from conversation previews", () => {
+    renderTimeline(true, [
+      {
+        type: "user_message",
+        id: "custom-instructions-message",
+        content:
+          "Review this PR\n\n<user_custom_instructions>\nThe user has saved custom instructions that apply to all of their tasks. Follow them.\n\nNever update an existing PR description.\n</user_custom_instructions>",
+        timestamp: Date.parse("2026-07-17T09:05:00Z"),
+      },
+    ]);
+
+    expect(screen.getByText("Review this PR")).toBeInTheDocument();
+    expect(screen.queryByText(/user_custom_instructions/)).toBeNull();
+    expect(
+      screen.queryByText("Never update an existing PR description."),
+    ).toBeNull();
+  });
+
+  it("shows user-authored custom-instruction tag examples", () => {
+    renderTimeline(true, [
+      {
+        type: "user_message",
+        id: "literal-custom-instructions-message",
+        content:
+          "Render this example: <user_custom_instructions>be terse</user_custom_instructions>",
+        timestamp: Date.parse("2026-07-17T09:05:00Z"),
+      },
+    ]);
+
+    expect(screen.getByText(/user_custom_instructions/)).toBeInTheDocument();
+    expect(screen.getByText(/be terse/)).toBeInTheDocument();
+  });
 });

packages/ui/src/features/canvas/components/ActivityTimeline.tsx

@@ -34,6 +34,7 @@ import { ThreadTimestamp } from "@posthog/ui/features/canvas/components/ThreadTi
 import { userDisplayName } from "@posthog/ui/features/canvas/utils/userDisplay";
 import type { buildConversationItems } from "@posthog/ui/features/sessions/components/buildConversationItems";
 import { extractChannelContext } from "@posthog/ui/features/sessions/components/session-update/channelContext";
+import { extractCustomInstructions } from "@posthog/ui/features/sessions/components/session-update/customInstructions";
 import { useThreadNavigationStore } from "@posthog/ui/features/sessions/threadNavigationStore";
 import { Fragment, type KeyboardEvent, type ReactNode, useMemo } from "react";
 
@@ -83,7 +84,12 @@ function UserMessageRow({
     () => extractChannelContext(content),
     [content],
   );
-  const displayContent = channelContext?.stripped ?? content;
+  const afterChannelContext = channelContext?.stripped ?? content;
+  const customInstructions = useMemo(
+    () => extractCustomInstructions(afterChannelContext),
+    [afterChannelContext],
+  );
+  const displayContent = customInstructions?.stripped ?? afterChannelContext;
   // The row itself is the hit target. `ThreadItem` renders an <article>, which a
   // <button> may not wrap and which can't become one (quill's primitive takes no
   // `render`), so it carries the button role and its own key handling.

packages/ui/src/features/sessions/components/chat-thread/composerPromptRecall.test.ts

@@ -186,7 +186,7 @@ describe("resolvePromptRecall", () => {
     {
       name: "a custom instructions block",
       content:
-        "fix the bug\n\n<user_custom_instructions>be terse</user_custom_instructions>",
+        "fix the bug\n\n<user_custom_instructions>\nThe user has saved custom instructions that apply to all of their tasks. Follow them.\n\nbe terse\n</user_custom_instructions>",
     },
     {
       name: "a trailing attachment summary",
@@ -195,7 +195,7 @@ describe("resolvePromptRecall", () => {
     {
       name: "several injected blocks at once",
       content:
-        '<channel_context channel="growth">CONTEXT.md body</channel_context>\n\nfix the bug\n\n<user_custom_instructions>be terse</user_custom_instructions>',
+        '<channel_context channel="growth">CONTEXT.md body</channel_context>\n\nfix the bug\n\n<user_custom_instructions>\nThe user has saved custom instructions that apply to all of their tasks. Follow them.\n\nbe terse\n</user_custom_instructions>',
     },
   ])("strips $name from the recalled text", ({ content }) => {
     expect(resolvePromptRecall([{ id: "m1", content }], null, -1)).toEqual({

packages/ui/src/features/sessions/components/session-update/customInstructions.test.ts

@@ -22,9 +22,17 @@ describe("extractCustomInstructions", () => {
 
   it("strips the element even when it is the only content", () => {
     const result = extractCustomInstructions(
-      "<user_custom_instructions>\nbody\n</user_custom_instructions>",
+      "<user_custom_instructions>\nThe user has saved custom instructions that apply to all of their tasks. Follow them.\n\nbody\n</user_custom_instructions>",
     );
-    expect(result?.body).toBe("body");
+    expect(result?.body).toContain("body");
     expect(result?.stripped).toBe("");
   });
+
+  it("preserves user-authored custom-instruction tags", () => {
+    const content =
+      "Render this example: <user_custom_instructions>be terse</user_custom_instructions>";
+
+    expect(extractCustomInstructions(content)).toBeNull();
+    expect(hasCustomInstructions(content)).toBe(false);
+  });
 });

packages/ui/src/features/sessions/components/session-update/customInstructions.ts

@@ -7,8 +7,12 @@
 //
 // The body shown is exactly what was sent in the prompt — parsed from the stored
 // event, never re-read from the (possibly newer) live setting.
+// The tag alone is not enough to identify injected metadata: users may include
+// the same XML in examples they want displayed verbatim. Match the fixed
+// preamble emitted by buildCustomInstructionsText so only system-generated
+// blocks are hidden.
 const CUSTOM_INSTRUCTIONS_REGEX =
-  /<user_custom_instructions\b[^>]*>([\s\S]*?)<\/user_custom_instructions>/;
+  /<user_custom_instructions\b[^>]*>(\r?\nThe user has saved custom instructions that apply to all of their tasks\. Follow them\.\r?\n\r?\n[\s\S]*?)<\/user_custom_instructions>/;
 
 export function hasCustomInstructions(content: string): boolean {
   return CUSTOM_INSTRUCTIONS_REGEX.test(content);