diff --git a/cli/src/commands/datatable_migrations.ts b/cli/src/commands/datatable_migrations.ts
index 0a565e821a..193f57e220 100644
--- a/cli/src/commands/datatable_migrations.ts
+++ b/cli/src/commands/datatable_migrations.ts
@@ -105,6 +105,69 @@ export async function rollbackMigrations(
   }
 }
 
+/**
+ * Validate the on-disk migration files for the given data tables (or all of
+ * them when `datatables` is omitted). Returns a list of human-readable problems;
+ * an empty list means the migrations are well-formed. Two states are invalid:
+ *  - two up (or two down) files sharing the same timestamp, which collide on the
+ *    `(datatable, timestamp)` identity used to upsert; and
+ *  - a `.down.sql` with no matching `.up.sql` (an up file is mandatory).
+ */
+export function validateLocalMigrations(datatables?: Set<string>): string[] {
+  const errors: string[] = [];
+  const root = path.join(process.cwd(), MIGRATIONS_DIR);
+  if (!fs.existsSync(root)) return errors;
+
+  for (const datatable of fs.readdirSync(root)) {
+    if (datatables && !datatables.has(datatable)) continue;
+    const dtDir = path.join(root, datatable);
+    if (!fs.statSync(dtDir).isDirectory()) continue;
+
+    const upNamesByTs = new Map<number, string[]>();
+    const downNamesByTs = new Map<number, string[]>();
+    const upBases = new Set<string>();
+    const downBases: { ts: number; name: string }[] = [];
+
+    for (const file of fs.readdirSync(dtDir)) {
+      const m = file.match(/^(\d+)_(.*)\.(up|down)\.sql$/);
+      if (!m) continue;
+      const ts = Number(m[1]);
+      const name = m[2];
+      if (m[3] === "up") {
+        (upNamesByTs.get(ts) ?? upNamesByTs.set(ts, []).get(ts)!).push(name);
+        upBases.add(`${ts}_${name}`);
+      } else {
+        (downNamesByTs.get(ts) ?? downNamesByTs.set(ts, []).get(ts)!).push(name);
+        downBases.push({ ts, name });
+      }
+    }
+
+    for (const [ts, names] of upNamesByTs) {
+      if (names.length > 1) {
+        errors.push(
+          `${datatable}: ${names.length} up migrations share timestamp ${ts} (${names.join(", ")})`,
+        );
+      }
+    }
+    for (const [ts, names] of downNamesByTs) {
+      if (names.length > 1) {
+        errors.push(
+          `${datatable}: ${names.length} down migrations share timestamp ${ts} (${names.join(", ")})`,
+        );
+      }
+    }
+    for (const d of downBases) {
+      if (!upBases.has(`${d.ts}_${d.name}`)) {
+        errors.push(
+          `${datatable}: ${d.ts}_${d.name}.down.sql has no matching ${d.ts}_${d.name}.up.sql`,
+        );
+      }
+    }
+  }
+
+  return errors;
+}
+
 /**
  * Sync a single migration to the workspace based on the current on-disk state of
  * its `<datatable>/<timestamp>_<name>.up.sql` file: upsert it when the up file
diff --git a/cli/src/commands/sync/sync.ts b/cli/src/commands/sync/sync.ts
index 1c8cdba315..b7e70ae8df 100644
--- a/cli/src/commands/sync/sync.ts
+++ b/cli/src/commands/sync/sync.ts
@@ -32,6 +32,7 @@ import { pullSharedUi, pushSharedUi } from "../shared_ui.ts";
 import {
   pushMigrationFromDisk,
   offerToRunNewMigrations,
+  validateLocalMigrations,
 } from "../datatable_migrations.ts";
 
 import {
@@ -4194,6 +4195,24 @@ export async function push(
       ));
     }
 
+    // Reject malformed datatable migrations (duplicate timestamps, orphan downs)
+    // before touching the remote, scanning only the data tables in this push.
+    const migrationDatatables = new Set(
+      changes
+        .map((c) => parseDatatableMigrationPath(c.path)?.datatable)
+        .filter((d): d is string => !!d),
+    );
+    if (migrationDatatables.size > 0) {
+      const migrationErrors = validateLocalMigrations(migrationDatatables);
+      if (migrationErrors.length > 0) {
+        log.error(
+          "Invalid datatable migrations, aborting push:\n" +
+            migrationErrors.map((e) => `  - ${e}`).join("\n"),
+        );
+        process.exit(1);
+      }
+    }
+
     if (
       !opts.yes &&
       !(await Confirm.prompt({