diff --git a/src/servers/api/chat/websocket.ts b/src/servers/api/chat/websocket.ts
index d3942fa6..81522b19 100644
--- a/src/servers/api/chat/websocket.ts
+++ b/src/servers/api/chat/websocket.ts
@@ -41,6 +41,8 @@ import { resolveHomeDir } from '@@/user-home';
 import { renameClaudeSession, type ChatIdentity } from './claude-sessions';
 import { claudeLoginState } from '@@/os-user-claude';
 import { mkdirSync, existsSync } from 'node:fs';
+import { snapshotPlugins } from '@@/plugins/mount';
+import { writeAtomic } from '@@/write-atomic';
 import { logger } from './logger';
 import { isSuperAdmin } from '@@/super-admin';
 import { snapshotClaudeUsage } from './usage-store';
@@ -203,6 +205,54 @@ async function resolveEmailCwd(ownerEmail: string, accountEmail?: string): Promi
   return dir;
 }
 
+/**
+ * Put the plugin's own `CLAUDE.md` where its chats will run.
+ *
+ * ── Why a file and not a prompt prefix ──
+ *
+ * A plugin used to say what its chats should know through `promptPrefix`, which rides every message. That
+ * has three costs and the owner hit all of them: it is paid per turn forever, it is INVISIBLE as a design
+ * artefact because it lives in a TypeScript constant, and it is visible in the worst place — the owner
+ * sent three words and watched a paragraph of preamble go out attached to them.
+ *
+ * Claude Code reads `CLAUDE.md` from its working directory at session start. So the same words, written
+ * once into the directory these chats already run in, cost nothing per turn, can be opened and edited by
+ * the person they are about, and are the mechanism `docs/commander-directory.md` already settled on for
+ * exactly this problem.
+ *
+ * ── Copied, not read in place, and why that is not a contradiction ──
+ *
+ * `docs/plugin-capabilities.md` argues at length that a plugin's capabilities are READ where they live and
+ * never copied. That rule is about files the PLATFORM reads, where a copy goes stale and survives an
+ * uninstall. This one is read by Claude Code, from a cwd, so it has to exist there — no amount of
+ * indirection changes where that program looks.
+ *
+ * The staleness the copy rule warns about is answered by rewriting it on every resolve rather than once at
+ * install: the file on disk is always this version of the plugin's, and a `plugins:update` needs no
+ * republish step to take effect. It is a few kilobytes written when a chat starts.
+ *
+ * Best-effort throughout. A plugin with no brief is the ordinary case, and a chat that cannot be given one
+ * still runs — it simply knows less, which is where every chat was before this existed.
+ */
+async function installPluginChatBrief(appName: string, dir: string): Promise<void> {
+  try {
+    const { states } = await snapshotPlugins();
+    const state = states.find((s) => s.plugin.appName === appName && s.install?.enabled);
+    if (!state) return;
+
+    const source = join(state.plugin.dir, 'chat', 'CLAUDE.md');
+    if (!existsSync(source)) return;
+
+    const target = join(dir, 'CLAUDE.md');
+    const next = await Bun.file(source).text();
+    // Skip the write when nothing changed, so a directory's mtimes do not churn on every chat start.
+    if (existsSync(target) && (await Bun.file(target).text()) === next) return;
+    await writeAtomic(target, next);
+  } catch (err) {
+    console.error(`[chat] could not install ${appName}'s chat brief:`, err instanceof Error ? err.message : err);
+  }
+}
+
 // The working directory a chat turn runs in, by context: email → the account dir; /chat → a chosen
 // pwd or the caller's own home; everything else (browser/project/dashboard) → the given cwd.
 async function resolveChatCwd(
@@ -282,6 +332,7 @@ async function resolveChatCwd(
   if (msg.context === 'plugin' && msg.contextId) {
     const dir = getPluginChatsDir(email, msg.contextId);
     mkdirSync(dir, { recursive: true });
+    await installPluginChatBrief(msg.contextId, dir);
     return dir;
   }