feat(ai-chat): per-MCP-server instructions in the agent system prompt (#180)

Admins can now give each EXTERNAL MCP server a free-text instruction ("how/
when to use this server's tools") that the agent receives in its SYSTEM
PROMPT next to the tool descriptions — porting the built-in SERVER_INSTRUCTIONS
idea to admin-configured servers. Trusted, admin-authored text (like a system
prompt); NON-secret, so unlike headersEnc it IS returned in views/forms.

- Migration: nullable `instructions text` on ai_mcp_servers (old rows = null =
  no guidance). Table type + repo insert/update (blank/whitespace -> null via
  blankToNull). DTO `@MaxLength(4000)`. Service threads it through
  McpServerView/toView.
- mcp-clients: `McpServerInstruction { serverName, toolPrefix, instructions }`
  threaded through the toolset/cache/lease. Guidance is built ONLY for a server
  that actually connected AND contributed >=1 callable tool (the allowlist may
  filter all of them out) AND has non-blank text — so a guide never appears for
  tools the agent cannot call. Cached with the toolset, so an edit is picked up
  next turn via the existing CRUD cache invalidation.
- System prompt: `buildMcpToolingBlock` renders an <mcp_tooling> block INSIDE
  the safety sandwich (after context, before the trailing SAFETY_FRAMEWORK) so
  it informs tool choice but cannot override the rules; each section is headed
  by the server's `prefix_*` namespace. Empty/blank -> block omitted. The
  caller (ai-chat.service) now builds the external toolset BEFORE the prompt and
  passes external.instructions; client-handle lifecycle (close-once) unchanged.
- Client: instructions field in types + a Textarea (autosize, maxLength 4000)
  in the MCP-server form with a namespace-prefix hint; i18n (en/ru).

Tests across every layer (prompt block placement + both SAFETY copies; view
blank->null; buildEntry includes guidance only for connected+>=1-tool+non-blank;
DTO MaxLength; repo + integration round-trip; service wiring). Delegated impl
reviewed (APPROVE); applied the import-type follow-up.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

apps/client/src/features/workspace/components/settings/components/ai-mcp-server-form.tsx

@@ -11,6 +11,7 @@ import {
   Switch,
   TagsInput,
   Text,
+  Textarea,
   TextInput,
 } from "@mantine/core";
 import { useForm } from "@mantine/form";
@@ -35,6 +36,8 @@ const formSchema = z.object({
   // Write-only secret buffer. Empty string means "do not change" (unless cleared).
   authHeader: z.string(),
   toolAllowlist: z.array(z.string()),
+  // Admin-authored prompt guidance (#180). Capped to mirror the DTO MaxLength.
+  instructions: z.string().max(4000),
   enabled: z.boolean(),
 });
 
@@ -63,6 +66,7 @@ function buildInitialValues(server?: IAiMcpServer): FormValues {
     toolAllowlist: Array.isArray(server?.toolAllowlist)
       ? server.toolAllowlist
       : [],
+    instructions: server?.instructions ?? "",
     enabled: server?.enabled ?? true,
   };
 }
@@ -124,6 +128,8 @@ export default function AiMcpServerForm({
         transport: values.transport,
         url: values.url,
         toolAllowlist: values.toolAllowlist,
+        // Always sent: a blank value clears the stored guidance (server -> null).
+        instructions: values.instructions,
         enabled: values.enabled,
       };
       // Only attach headers when set or explicitly cleared (omit => unchanged).
@@ -135,6 +141,8 @@ export default function AiMcpServerForm({
         transport: values.transport,
         url: values.url,
         toolAllowlist: values.toolAllowlist,
+        // Blank => server stores null (no guidance).
+        instructions: values.instructions,
         enabled: values.enabled,
       };
       // On create, only a typed value matters (no prior stored headers).
@@ -158,10 +166,7 @@ export default function AiMcpServerForm({
 
   return (
     <Stack>
-      <TextInput
-        label={t("Server name")}
-        {...form.getInputProps("name")}
-      />
+      <TextInput label={t("Server name")} {...form.getInputProps("name")} />
 
       <Select
         label={t("Transport")}
@@ -177,7 +182,7 @@ export default function AiMcpServerForm({
         // Clarify that the value is sent verbatim as the Authorization header,
         // so the user supplies the full scheme (no implicit Bearer prefix).
         description={t(
-          "Sent verbatim as the value of the Authorization header (e.g. \"Bearer <token>\" or \"Basic <base64>\").",
+          'Sent verbatim as the value of the Authorization header (e.g. "Bearer <token>" or "Basic <base64>").',
         )}
         // Placeholder hints whether headers are stored; the value is never shown.
         placeholder={hasHeaders ? t("•••• set") : ""}
@@ -208,6 +213,20 @@ export default function AiMcpServerForm({
         {...form.getInputProps("toolAllowlist")}
       />
 
+      <Textarea
+        label={t("Instructions")}
+        // Hint that the text is injected into the agent's system prompt and that
+        // the server's tools are namespaced under <name>_* (the prompt header).
+        description={t(
+          "Optional guidance for the agent on how and when to use this server's tools. Injected into the system prompt. The server's tools are namespaced as \"<server name>_*\".",
+        )}
+        autosize
+        minRows={2}
+        maxRows={8}
+        maxLength={4000}
+        {...form.getInputProps("instructions")}
+      />
+
       <Switch
         label={t("Enabled")}
         checked={form.values.enabled}

apps/client/src/features/workspace/services/ai-mcp-server-service.ts

@@ -14,6 +14,9 @@ export interface IAiMcpServer {
   enabled: boolean;
   toolAllowlist: string[] | null;
   hasHeaders: boolean;
+  // Admin-authored guidance injected into the agent system prompt (#180).
+  // NON-secret, so it IS returned. Null when no guidance is configured.
+  instructions: string | null;
 }
 
 // Create payload. `headers` is write-only: omit => no auth headers.
@@ -25,6 +28,8 @@ export interface IAiMcpServerCreate {
   // never returned.
   headers?: Record<string, string>;
   toolAllowlist?: string[];
+  // Admin-authored prompt guidance (#180). Blank => stored as null.
+  instructions?: string;
   enabled?: boolean;
 }
 
@@ -39,6 +44,8 @@ export interface IAiMcpServerUpdate {
   url?: string;
   headers?: Record<string, string>;
   toolAllowlist?: string[];
+  // Admin-authored prompt guidance (#180). Absent => unchanged; blank => cleared.
+  instructions?: string;
   enabled?: boolean;
 }