diff --git a/src/core/migrate.ts b/src/core/migrate.ts
index 6f7c9aa20..fceb81bd7 100644
--- a/src/core/migrate.ts
+++ b/src/core/migrate.ts
@@ -5326,6 +5326,44 @@ export const MIGRATIONS: Migration[] = [
       DELETE FROM query_cache;
     `,
   },
+  {
+    version: 119,
+    name: 'op_checkpoints_completed_keys_array_check',
+    // v0.42.x — make the op_checkpoints scalar-corruption class structurally
+    // impossible. completed_keys is JSONB and the loader runs
+    // jsonb_array_elements_text(completed_keys); a non-array (scalar) value
+    // makes that throw "cannot extract elements from a scalar", which takes
+    // down the whole UNION load (including the valid op_checkpoint_paths child
+    // rows) and loses all checkpoint progress for that key. No current writer
+    // can produce a scalar, but an older binary / external script / future bug
+    // could — the CHECK is a DB-enforced, always-on guard (the correct pattern
+    // vs a migration verify-hook, which would not run on already-stamped
+    // brains). LOCK first so an out-of-band scalar write can't land between the
+    // repair and the ADD CONSTRAINT (no-op on single-connection PGLite). The
+    // repair resets any pre-existing scalar to '[]'; op_checkpoint_paths child
+    // rows are the append-only source of truth, so the reset loses nothing.
+    // Mirrored in src/schema.sql, src/core/pglite-schema.ts, and the generated
+    // src/core/schema-embedded.ts so fresh installs carry the same CHECK.
+    idempotent: true,
+    sql: `
+      LOCK TABLE op_checkpoints IN SHARE ROW EXCLUSIVE MODE;
+
+      UPDATE op_checkpoints
+         SET completed_keys = '[]'::jsonb, updated_at = now()
+       WHERE jsonb_typeof(completed_keys) <> 'array';
+
+      DO $$
+      BEGIN
+        IF NOT EXISTS (
+          SELECT 1 FROM pg_constraint WHERE conname = 'op_checkpoints_completed_keys_array'
+        ) THEN
+          ALTER TABLE op_checkpoints
+            ADD CONSTRAINT op_checkpoints_completed_keys_array
+            CHECK (jsonb_typeof(completed_keys) = 'array');
+        END IF;
+      END $$;
+    `,
+  },
 ];
 
 export const LATEST_VERSION = MIGRATIONS.length > 0
diff --git a/src/core/op-checkpoint.ts b/src/core/op-checkpoint.ts
index a338d20b6..f1c0c1e2e 100644
--- a/src/core/op-checkpoint.ts
+++ b/src/core/op-checkpoint.ts
@@ -122,18 +122,40 @@ export async function loadOpCheckpoint(
     // dedupes, so we skip a server-side dedup sort over up to 204K rows on every
     // resume. `jsonb_array_elements_text` expands the legacy array server-side,
     // which also removes the old postgres.js-vs-PGLite string/array handling.
-    const rows = await engine.executeRaw<{ ckey: unknown }>(
-      `SELECT path AS ckey FROM op_checkpoint_paths
+    //
+    // v0.42.x (BUG 3 guard): the legacy arm is gated on
+    // `jsonb_typeof(completed_keys) = 'array'`. Without it a non-array (scalar)
+    // parent row makes jsonb_array_elements_text throw "cannot extract elements
+    // from a scalar", which kills the WHOLE union — including the valid child
+    // rows — and loses all checkpoint progress for the key. Skipping the scalar
+    // keeps the child rows; the third arm flags the corruption so we log it once
+    // (migration v119's CHECK makes this impossible going forward; a hit implies
+    // schema drift / disabled constraint / an out-of-band writer).
+    const rows = await engine.executeRaw<{ ckey: unknown; corrupt: number }>(
+      `SELECT path AS ckey, 0 AS corrupt FROM op_checkpoint_paths
          WHERE op = $1 AND fingerprint = $2
        UNION ALL
-       SELECT jsonb_array_elements_text(completed_keys) AS ckey FROM op_checkpoints
-         WHERE op = $1 AND fingerprint = $2`,
+       SELECT jsonb_array_elements_text(completed_keys) AS ckey, 0 AS corrupt FROM op_checkpoints
+         WHERE op = $1 AND fingerprint = $2 AND jsonb_typeof(completed_keys) = 'array'
+       UNION ALL
+       SELECT NULL AS ckey, 1 AS corrupt FROM op_checkpoints
+         WHERE op = $1 AND fingerprint = $2 AND jsonb_typeof(completed_keys) <> 'array'`,
       [key.op, key.fingerprint],
     );
     const set = new Set<string>();
+    let corruptParent = false;
     for (const r of rows) {
+      if (Number(r.corrupt) === 1) {
+        corruptParent = true;
+        continue;
+      }
       if (typeof r.ckey === 'string') set.add(r.ckey);
     }
+    if (corruptParent) {
+      console.error(
+        `[op-checkpoint] WARNING: op_checkpoints.completed_keys for (${key.op}, ${key.fingerprint}) is a non-array (scalar) and was skipped to protect the load — child op_checkpoint_paths rows still applied. This implies schema drift, a disabled CHECK constraint, or an out-of-band writer.`,
+      );
+    }
     return [...set];
   } catch (e) {
     console.error(`[op-checkpoint] load failed (${key.op}, ${key.fingerprint}):`, (e as Error).message);
diff --git a/src/core/pglite-schema.ts b/src/core/pglite-schema.ts
index 17705f206..1279ce910 100644
--- a/src/core/pglite-schema.ts
+++ b/src/core/pglite-schema.ts
@@ -932,7 +932,11 @@ CREATE TABLE IF NOT EXISTS oauth_codes (
 CREATE TABLE IF NOT EXISTS op_checkpoints (
   op             TEXT NOT NULL,
   fingerprint    TEXT NOT NULL,
-  completed_keys JSONB NOT NULL DEFAULT '[]'::jsonb,
+  -- v0.42.x: must be a JSONB array. The loader runs jsonb_array_elements_text
+  -- over it; a scalar would throw and wipe the whole checkpoint load. CHECK is
+  -- the DB-enforced always-on guard (mirrors migration v119).
+  completed_keys JSONB NOT NULL DEFAULT '[]'::jsonb
+    CONSTRAINT op_checkpoints_completed_keys_array CHECK (jsonb_typeof(completed_keys) = 'array'),
   updated_at     TIMESTAMPTZ NOT NULL DEFAULT now(),
   PRIMARY KEY (op, fingerprint)
 );
diff --git a/src/core/schema-embedded.ts b/src/core/schema-embedded.ts
index fc65638bd..d0e2c51ef 100644
--- a/src/core/schema-embedded.ts
+++ b/src/core/schema-embedded.ts
@@ -698,7 +698,11 @@ CREATE INDEX IF NOT EXISTS idx_mcp_log_agent_time ON mcp_request_log(agent_name,
 CREATE TABLE IF NOT EXISTS op_checkpoints (
   op             TEXT NOT NULL,
   fingerprint    TEXT NOT NULL,
-  completed_keys JSONB NOT NULL DEFAULT '[]'::jsonb,
+  -- v0.42.x: must be a JSONB array. The loader runs jsonb_array_elements_text
+  -- over it; a scalar would throw and wipe the whole checkpoint load. CHECK is
+  -- the DB-enforced always-on guard (mirrors migration v119).
+  completed_keys JSONB NOT NULL DEFAULT '[]'::jsonb
+    CONSTRAINT op_checkpoints_completed_keys_array CHECK (jsonb_typeof(completed_keys) = 'array'),
   updated_at     TIMESTAMPTZ NOT NULL DEFAULT now(),
   PRIMARY KEY (op, fingerprint)
 );
diff --git a/src/schema.sql b/src/schema.sql
index 0ea4a6b09..e02bc7f02 100644
--- a/src/schema.sql
+++ b/src/schema.sql
@@ -694,7 +694,11 @@ CREATE INDEX IF NOT EXISTS idx_mcp_log_agent_time ON mcp_request_log(agent_name,
 CREATE TABLE IF NOT EXISTS op_checkpoints (
   op             TEXT NOT NULL,
   fingerprint    TEXT NOT NULL,
-  completed_keys JSONB NOT NULL DEFAULT '[]'::jsonb,
+  -- v0.42.x: must be a JSONB array. The loader runs jsonb_array_elements_text
+  -- over it; a scalar would throw and wipe the whole checkpoint load. CHECK is
+  -- the DB-enforced always-on guard (mirrors migration v119).
+  completed_keys JSONB NOT NULL DEFAULT '[]'::jsonb
+    CONSTRAINT op_checkpoints_completed_keys_array CHECK (jsonb_typeof(completed_keys) = 'array'),
   updated_at     TIMESTAMPTZ NOT NULL DEFAULT now(),
   PRIMARY KEY (op, fingerprint)
 );
diff --git a/test/op-checkpoint.test.ts b/test/op-checkpoint.test.ts
index 6c821e38c..b5fe46127 100644
--- a/test/op-checkpoint.test.ts
+++ b/test/op-checkpoint.test.ts
@@ -243,6 +243,82 @@ describe('resumeFilter (pure)', () => {
   });
 });
 
+describe('BUG 3: completed_keys array-shape guard (v119 CHECK + defensive loader)', () => {
+  const CONSTRAINT = 'op_checkpoints_completed_keys_array';
+
+  test('CHECK rejects a scalar completed_keys write — exactly one constraint (no blob+migration dupe)', async () => {
+    // Fresh PGLite install: the schema blob ships the NAMED inline CHECK and
+    // migration v119's IF NOT EXISTS skips re-adding it. Exactly one constraint.
+    const c = await engine.executeRaw<{ n: number }>(
+      `SELECT count(*)::int AS n FROM pg_constraint WHERE conname = $1`,
+      [CONSTRAINT],
+    );
+    expect(Number(c[0].n)).toBe(1);
+
+    let threw = false;
+    try {
+      await engine.executeRaw(
+        `INSERT INTO op_checkpoints (op, fingerprint, completed_keys, updated_at)
+         VALUES ('embed', 'fp-reject', '"not-an-array"'::jsonb, now())`,
+      );
+    } catch {
+      threw = true;
+    }
+    expect(threw).toBe(true);
+  });
+
+  test('loader survives a scalar parent: returns child rows (not []), does not throw', async () => {
+    const key = { op: 'sync', fingerprint: 'fp-scalar-survive' };
+    // Bypass the CHECK to simulate pre-migration / out-of-band corruption.
+    await engine.executeRaw(`ALTER TABLE op_checkpoints DROP CONSTRAINT ${CONSTRAINT}`);
+    try {
+      await engine.executeRaw(
+        `INSERT INTO op_checkpoints (op, fingerprint, completed_keys, updated_at)
+         VALUES ('sync', 'fp-scalar-survive', '"corrupt-scalar"'::jsonb, now())`,
+      );
+      await engine.executeRaw(
+        `INSERT INTO op_checkpoint_paths (op, fingerprint, path)
+         VALUES ('sync', 'fp-scalar-survive', 'child-a.md')`,
+      );
+      // Pre-guard, jsonb_array_elements_text on the scalar threw and the catch
+      // returned [] — losing child-a.md. The typeof guard skips the scalar so
+      // the valid child survives.
+      const loaded = await loadOpCheckpoint(engine, key);
+      expect(loaded).toEqual(['child-a.md']);
+    } finally {
+      await engine.executeRaw(
+        `UPDATE op_checkpoints SET completed_keys = '[]'::jsonb WHERE jsonb_typeof(completed_keys) <> 'array'`,
+      );
+      await engine.executeRaw(
+        `ALTER TABLE op_checkpoints ADD CONSTRAINT ${CONSTRAINT} CHECK (jsonb_typeof(completed_keys) = 'array')`,
+      );
+    }
+  });
+
+  test('v119 repair converts a scalar parent to an empty array', async () => {
+    await engine.executeRaw(`ALTER TABLE op_checkpoints DROP CONSTRAINT ${CONSTRAINT}`);
+    try {
+      await engine.executeRaw(
+        `INSERT INTO op_checkpoints (op, fingerprint, completed_keys, updated_at)
+         VALUES ('embed', 'fp-repair', '"scalar"'::jsonb, now())`,
+      );
+      // Migration v119's repair statement.
+      await engine.executeRaw(
+        `UPDATE op_checkpoints SET completed_keys = '[]'::jsonb, updated_at = now()
+         WHERE jsonb_typeof(completed_keys) <> 'array'`,
+      );
+      const typ = await engine.executeRaw<{ t: string }>(
+        `SELECT jsonb_typeof(completed_keys) AS t FROM op_checkpoints WHERE op = 'embed' AND fingerprint = 'fp-repair'`,
+      );
+      expect(typ[0].t).toBe('array');
+    } finally {
+      await engine.executeRaw(
+        `ALTER TABLE op_checkpoints ADD CONSTRAINT ${CONSTRAINT} CHECK (jsonb_typeof(completed_keys) = 'array')`,
+      );
+    }
+  });
+});
+
 describe('purgeStaleCheckpoints', () => {
   test('no stale rows: returns 0', async () => {
     await recordCompleted(engine, { op: 'embed', fingerprint: 'fresh' }, ['x']);