diff --git a/bot/shared/services/observe/observe-draft.service.js b/bot/shared/services/observe/observe-draft.service.js
index 0918c04d..a3c5d561 100644
--- a/bot/shared/services/observe/observe-draft.service.js
+++ b/bot/shared/services/observe/observe-draft.service.js
@@ -170,9 +170,16 @@ async function applyObserverEdits(sessionId, edits) {
       for (const [prefix, field] of [['ev_', 'evidence_sw'], ['imp_', 'improvement_sw']]) {
         const val = edits[`${prefix}${f}`];
         if (typeof val === 'string') {
-          const origText = String(orig[field] || '').slice(0, PREFILL_TEXT_CAP);
-          if (val !== origText && val !== String(orig[field] || '')) textChanged += 1;
-          if (val !== origText) ind[field] = val;
+          const full = String(orig[field] || '');
+          const shown = full.slice(0, PREFILL_TEXT_CAP);
+          // bd-2218: the leader only ever saw `shown`. Anything past the cap
+          // never reached the screen, so writing their edit verbatim deletes
+          // text they had no chance to review — and nothing in the Flow hints
+          // there was more, so neither they nor the teacher can catch it. An
+          // edit may shorten what was reviewed; it must not touch what wasn't.
+          const unseen = full.slice(PREFILL_TEXT_CAP);
+          if (val !== shown && val !== full) textChanged += 1;
+          if (val !== shown) ind[field] = unseen ? `${val}${unseen}` : val;
         }
       }
     });
diff --git a/bot/tests/observe/observe-evidence-unseen-tail.test.js b/bot/tests/observe/observe-evidence-unseen-tail.test.js
new file mode 100644
index 00000000..cd21f24f
--- /dev/null
+++ b/bot/tests/observe/observe-evidence-unseen-tail.test.js
@@ -0,0 +1,99 @@
+/**
+ * bd-2218 — an observer edit must never silently delete evidence the observer
+ * never saw.
+ *
+ * The Flow prefills each Evidence box with only the first PREFILL_TEXT_CAP
+ * characters of the AI draft. When the leader edits that box, the value coming
+ * back is capped by construction. Writing it verbatim over the original throws
+ * away everything past the cap — and nothing on screen ever showed the leader
+ * that there was more, so the loss is invisible to the only person who could
+ * catch it. That truncated text is what feeds the teacher's report.
+ *
+ * Measured on NIETE prod (2026-07-21): 9 evidence notes across 5 of the 13
+ * recorded observations already run past 600 characters (longest 681), and 156
+ * coach-edited fields sit at exactly 300 — the fingerprint of the earlier cap
+ * landing in v2. Raising 300 -> 600 moved the threshold; it left the mechanism.
+ */
+
+const mockDb = { row: null };
+const mockSingle = jest.fn(() => Promise.resolve(
+  mockDb.row ? { data: mockDb.row, error: null } : { data: null, error: { message: 'not found' } }));
+const mockUpdateEq = jest.fn().mockResolvedValue({ data: null, error: null });
+const mockUpdate = jest.fn((patch) => {
+  if (mockDb.row) mockDb.row = { ...mockDb.row, ...patch };
+  return { eq: mockUpdateEq };
+});
+function mockMakeChain() {
+  const chain = {};
+  for (const m of ['select', 'eq', 'neq', 'order']) chain[m] = jest.fn(() => chain);
+  chain.single = mockSingle;
+  chain.limit = jest.fn().mockResolvedValue({ data: [], error: null });
+  chain.update = mockUpdate;
+  return chain;
+}
+jest.mock('../../shared/config/supabase', () => ({ from: jest.fn(() => mockMakeChain()) }));
+
+const { applyObserverEdits } = require('../../shared/services/observe/observe-draft.service');
+
+const SID = 'sess-bd2218';
+const CAP = 600;
+
+// A draft evidence note longer than the cap, with a marker in the tail so we
+// can assert on the exact part the leader never saw.
+const TAIL = ' …and then she asked the class to justify the grouping [24:10].';
+const LONG_EVIDENCE = 'E'.repeat(CAP + 40) + TAIL;
+const SHORT_IMPROVEMENT = 'Ask one open question and wait three seconds.';
+
+function seedSession() {
+  const draft = () => ({
+    framework: 'fico',
+    domains: {
+      lesson_plan_fidelity: {
+        domain_max: 20,
+        indicators: [
+          { id: 'B1', name: 'Instructional Clarity', score: 2, evidence_sw: LONG_EVIDENCE, improvement_sw: SHORT_IMPROVEMENT },
+        ],
+      },
+    },
+  });
+  mockDb.row = {
+    id: SID,
+    user_id: 'u-1',
+    observer_user_id: 'o-1',
+    autofill_analysis_data: draft(),
+    analysis_data: draft(),
+    users: { phone_number: '923001234567', first_name: 'Warda', preferred_language: 'en' },
+  };
+}
+
+const b1 = () => mockDb.row.analysis_data.domains.lesson_plan_fidelity.indicators[0];
+
+beforeEach(() => {
+  jest.clearAllMocks();
+  seedSession();
+});
+
+describe('bd-2218 — observer edits never delete unseen evidence', () => {
+  test('editing a capped Evidence box keeps the text past the cap', async () => {
+    // What the Flow actually showed the leader, with one word changed by them.
+    const shown = LONG_EVIDENCE.slice(0, CAP);
+    const edited = `${shown.slice(0, CAP - 20)}CHANGED BY OBSERVER`;
+
+    await applyObserverEdits(SID, { ev_B1: edited });
+
+    const saved = b1().evidence_sw;
+    expect(saved).toContain('CHANGED BY OBSERVER'); // the edit is honoured
+    expect(saved).toContain(TAIL);                  // and the unseen tail survives
+    expect(saved.length).toBeGreaterThan(CAP);
+  });
+
+  test('leaving a capped Evidence box untouched keeps the full original', async () => {
+    await applyObserverEdits(SID, { ev_B1: LONG_EVIDENCE.slice(0, CAP) });
+    expect(b1().evidence_sw).toBe(LONG_EVIDENCE);
+  });
+
+  test('a field shorter than the cap is replaced exactly, with no tail bolted on', async () => {
+    await applyObserverEdits(SID, { imp_B1: 'Wait five seconds instead of three.' });
+    expect(b1().improvement_sw).toBe('Wait five seconds instead of three.');
+  });
+});