diff --git a/platform/api/src/learning-routes.js b/platform/api/src/learning-routes.js
index aae3b1b..0191ff6 100644
--- a/platform/api/src/learning-routes.js
+++ b/platform/api/src/learning-routes.js
@@ -156,6 +156,31 @@ module.exports = function registerLearningRoutes(app, deps) {
     );
     const definition = rows[0];
     if (!definition) return res.status(404).json({ error: 'not_found' });
+
+    // Phase 6B §10 — a known Phase 3B gap, documented in docs/PHASE6A_ROOT_COURSE_FOUNDATION.md
+    // §10: this route only ever checked the definition existed/was active. GET /api/capstone
+    // computes and displays lock state correctly and the UI never exposes a "start" action while
+    // locked, but student UI state is not the enforcement boundary — a direct POST here could
+    // start a capstone session before its prerequisite stages were actually complete. Fixed by
+    // binding to the SAME gate /api/capstone already derives from the curriculum path's own
+    // capstone stage (computeCurriculumState), rather than re-parsing definition.unlock_rule
+    // independently — one computation of "is the capstone unlocked," not two that could drift.
+    // Only a definition the path actually names as ITS capstone is gated this way; an ordinary
+    // exam (no capstone stage names it) is unaffected — see e2e-phase6b.js for both cases.
+    const path = curriculum.loadCurriculumPath();
+    const capstoneStage = path?.stages.find((s) => s.type === 'capstone' && s.capstone_assessment_id === definition.assessment_definition_id);
+    if (capstoneStage) {
+      const progress = await loadProgress(pool, req.studentId);
+      const capstoneAchievement = await getCapstoneAchievement(pool, req.studentId);
+      const state = computeCurriculumState(path, progress, capstoneAchievement ? 'completed' : undefined);
+      if (state.capstone?.status === 'locked') {
+        return res.status(403).json({
+          error: 'locked', message: 'Complete the required stages before starting this assessment.',
+          blocking_stage_ids: state.capstone.blocking_stage_ids,
+        });
+      }
+    }
+
     const cohortId = await studentCohortId(pool, req.studentId);
     try {
       const session = await assessmentsLib.startSession(pool, { definition, studentId: req.studentId, cohortId });
diff --git a/platform/test/e2e-phase5b.js b/platform/test/e2e-phase5b.js
index cc4d430..9b135cb 100644
--- a/platform/test/e2e-phase5b.js
+++ b/platform/test/e2e-phase5b.js
@@ -38,7 +38,14 @@ const WORKLOADS = ['deploy-nginx', 'scale-deployment', 'rolling-update-task', 'l
 const NETWORKING = ['services-mcq', 'expose-service-basics'];
 const CONFIGURATION = ['configmap-volume', 'resource-limits-task', 'env-vars-configmap', 'secrets-basics', 'pod-env-vars-inline'];
 const TROUBLESHOOTING = ['extract-logs-basics', 'kubectl-cp-basics', 'exec-command-basics', 'node-cordon-basics', 'pod-node-assignment-mcq'];
-const ALL_NON_CAPSTONE = [...FOUNDATION, ...WORKLOADS, ...NETWORKING, ...CONFIGURATION, ...TROUBLESHOOTING];
+// Phase 6B §10 — REAL_OPS was missing from this list entirely, which this test never noticed
+// because POST /api/assessments/:id/start didn't check the capstone's unlock_rule server-side
+// before Phase 6B's fix (docs/PHASE6A_ROOT_COURSE_FOUNDATION.md §10). Now that the fix actually
+// enforces "all 6 stages complete," a genuine capstone attempt needs all 6, real-ops included —
+// completing only 5 of 6 is not a legitimately-unlocked capstone and should not (and now does
+// not) be accepted. Found by testing.
+const REAL_OPS = ['rootnode-runaway-monitor', 'broken-deployment', 'rootnode-probe-port-mismatch', 'rootnode-enforce-limits'];
+const ALL_NON_CAPSTONE = [...FOUNDATION, ...WORKLOADS, ...NETWORKING, ...CONFIGURATION, ...TROUBLESHOOTING, ...REAL_OPS];
 
 let passed = 0, failed = 0;
 async function test(name, fn) {