diff --git a/server/src/mcp/index.ts b/server/src/mcp/index.ts
index 0ac801d4c..9334c888b 100644
--- a/server/src/mcp/index.ts
+++ b/server/src/mcp/index.ts
@@ -1,8 +1,8 @@
 import { readEnv } from '../app-config';
 import { sessions } from './sessionManager';
-import { revokeUserSessions, revokeUserSessionsForClient } from './sessionManager';
+import { invalidateMcpSessions, revokeUserSessions, revokeUserSessionsForClient } from './sessionManager';
 
-export { revokeUserSessions, revokeUserSessionsForClient };
+export { invalidateMcpSessions, revokeUserSessions, revokeUserSessionsForClient };
 
 /**
  * Process-wide MCP state: the tuning knobs, the per-user rate-limit window,
@@ -64,16 +64,6 @@ const sessionSweepInterval = setInterval(() => {
 // Prevent the interval from keeping the process alive if nothing else is running
 sessionSweepInterval.unref();
 
-/** Invalidate all active MCP sessions (call when addon state changes so sessions re-create with updated tools). */
-export function invalidateMcpSessions(): void {
-  for (const [sid, session] of sessions) {
-    try { session.server.close(); } catch { /* ignore */ }
-    try { session.transport.close(); } catch { /* ignore */ }
-    sessions.delete(sid);
-  }
-  console.log('[MCP] All sessions invalidated due to addon state change');
-}
-
 /** Close all active MCP sessions (call during graceful shutdown). */
 export function closeMcpSessions(): void {
   clearInterval(sessionSweepInterval);
diff --git a/server/src/mcp/sessionManager.ts b/server/src/mcp/sessionManager.ts
index f71b15f39..a0e260d83 100644
--- a/server/src/mcp/sessionManager.ts
+++ b/server/src/mcp/sessionManager.ts
@@ -42,6 +42,26 @@ export function revokeUserSessionsForClient(userId: number, clientId: string): v
   }
 }
 
+/**
+ * Terminate every active MCP session, so the next request re-initializes and
+ * picks up a changed tool surface.
+ *
+ * Lives here rather than in the barrel because callers outside src/mcp/ need
+ * it: index.ts evaluates readEnv().mcp at module scope and installs the sweep
+ * interval, so importing the barrel from a domain module drags both into every
+ * test file that mocks app-config with a partial export set. This module has
+ * no module-scope side effects.
+ *
+ * Unconditional by design. The advertised surface is per-session (it depends on
+ * the caller's scopes and grants), so there is no ctx-free set to diff, and
+ * tools/list_changed is not an alternative: no event store is configured and
+ * the surface is frozen at initialize anyway.
+ */
+export function invalidateMcpSessions(): void {
+  for (const [sid, session] of sessions) closeSession(sid, session);
+  console.log('[MCP] All sessions invalidated after a tool-surface change');
+}
+
 /**
  * Close the least-recently-active session for a user so a new one can take its slot,
  * and return its id (null when the user has no sessions).
diff --git a/server/tests/unit/mcp/sessionManager.test.ts b/server/tests/unit/mcp/sessionManager.test.ts
index b451aaaf4..934c54d9c 100644
--- a/server/tests/unit/mcp/sessionManager.test.ts
+++ b/server/tests/unit/mcp/sessionManager.test.ts
@@ -1,9 +1,10 @@
 /**
- * Unit tests for MCP sessionManager — SESS-001 to SESS-016.
- * Covers revokeUserSessions, revokeUserSessionsForClient and evictOldestSessionForUser.
+ * Unit tests for MCP sessionManager — SESS-001 to SESS-020.
+ * Covers revokeUserSessions, revokeUserSessionsForClient, evictOldestSessionForUser
+ * and invalidateMcpSessions.
  */
 import { describe, it, expect, vi, beforeEach } from 'vitest';
-import { sessions, revokeUserSessions, revokeUserSessionsForClient, evictOldestSessionForUser, McpSession } from '../../../src/mcp/sessionManager';
+import { sessions, revokeUserSessions, revokeUserSessionsForClient, evictOldestSessionForUser, invalidateMcpSessions, McpSession } from '../../../src/mcp/sessionManager';
 
 function makeSession(overrides: Partial<McpSession> = {}): McpSession {
   return {
@@ -179,3 +180,42 @@ describe('evictOldestSessionForUser', () => {
     expect(sessions.has('sid-1')).toBe(false);
   });
 });
+
+describe('invalidateMcpSessions', () => {
+  it('SESS-017: drops every session regardless of user or client', () => {
+    sessions.set('sid-1', makeSession({ userId: 1, clientId: 'client-a' }));
+    sessions.set('sid-2', makeSession({ userId: 2, clientId: null }));
+
+    invalidateMcpSessions();
+
+    expect(sessions.size).toBe(0);
+  });
+
+  it('SESS-018: closes both halves of every session', () => {
+    const a = makeSession({ userId: 1 });
+    const b = makeSession({ userId: 2 });
+    sessions.set('sid-1', a);
+    sessions.set('sid-2', b);
+
+    invalidateMcpSessions();
+
+    expect(a.server.close).toHaveBeenCalledOnce();
+    expect(a.transport.close).toHaveBeenCalledOnce();
+    expect(b.server.close).toHaveBeenCalledOnce();
+    expect(b.transport.close).toHaveBeenCalledOnce();
+  });
+
+  it('SESS-019: still empties the map when close() throws', () => {
+    const s = makeSession({ userId: 1 });
+    (s.server.close as ReturnType<typeof vi.fn>).mockImplementation(() => { throw new Error('close failed'); });
+    sessions.set('sid-1', s);
+
+    expect(() => invalidateMcpSessions()).not.toThrow();
+    expect(sessions.size).toBe(0);
+  });
+
+  it('SESS-020: is a no-op on an empty map', () => {
+    expect(() => invalidateMcpSessions()).not.toThrow();
+    expect(sessions.size).toBe(0);
+  });
+});