🪙 feat: Context-projection backend endpoint

POST /api/endpoints/context-projection → resolveContextProjection (packages/api):
reconstructs the viewed branch (parent-chain walk from messageId), resolves the
agent config (instructions/provider/model/maxContextTokens), reuses LibreChat's
stored per-message tokenCounts as the index map (no re-tokenizing), and calls
the agents SDK projectAgentContextUsage — no model call. Thin controller injects
db.getMessages/db.getAgent; route mirrors /token-config.

First cut targets message-windowing accuracy; tool-schema tokens are deferred to
a follow-up that reuses the full initializeAgent path.

api/server/controllers/ContextProjectionController.js

@@ -0,0 +1,31 @@
+const { logger } = require('@librechat/data-schemas');
+const { resolveContextProjection } = require('@librechat/api');
+const db = require('~/models');
+
+/**
+ * Returns a server-side context-usage projection for the viewed branch + config
+ * (agents SDK, no model call) — powers the gauge for snapshot-less branches and
+ * after a model/window switch. Resolution lives in `@librechat/api`; this
+ * controller only injects request-scoped model accessors.
+ * @param {ServerRequest} req
+ * @param {ServerResponse} res
+ */
+async function contextProjectionController(req, res) {
+  try {
+    const params = req.body ?? {};
+    if (!params.conversationId || !params.messageId) {
+      res.json(null);
+      return;
+    }
+    const projection = await resolveContextProjection(
+      { getMessages: db.getMessages, getAgent: db.getAgent },
+      params,
+    );
+    res.json(projection ?? null);
+  } catch (error) {
+    logger.error('[contextProjectionController]', error);
+    res.status(500).json({ error: 'Failed to resolve context projection' });
+  }
+}
+
+module.exports = contextProjectionController;

api/server/routes/endpoints.js

@@ -3,10 +3,12 @@ const requireJwtAuth = require('~/server/middleware/requireJwtAuth');
 const configMiddleware = require('~/server/middleware/config/app');
 const endpointController = require('~/server/controllers/EndpointController');
 const tokenConfigController = require('~/server/controllers/TokenConfigController');
+const contextProjectionController = require('~/server/controllers/ContextProjectionController');
 
 const router = express.Router();
 /** Auth required for role/tenant-scoped endpoint config resolution. */
 router.get('/', requireJwtAuth, endpointController);
 router.get('/token-config', requireJwtAuth, configMiddleware, tokenConfigController);
+router.post('/context-projection', requireJwtAuth, configMiddleware, contextProjectionController);
 
 module.exports = router;