diff --git a/server/index.ts b/server/index.ts
index e0318b1..97ce54c 100644
--- a/server/index.ts
+++ b/server/index.ts
@@ -31,9 +31,12 @@ function err(message: string, status: number) {
 const TASK_FIELDS = ["title", "notes", "status", "project_id", "context", "waiting_for", "due_date", "defer_date", "priority", "estimate_min", "sort_order"];
 const PROJECT_FIELDS = ["name", "status", "notes", "sort_order"];
 
+// Only these may be cleared to NULL; notes/title are NOT NULL and keep "".
+const NULLABLE = new Set(["project_id", "context", "waiting_for", "due_date", "defer_date", "estimate_min"]);
+
 function pick(body: Record<string, unknown>, fields: string[]) {
   const out: Record<string, unknown> = {};
-  for (const f of fields) if (f in body) out[f] = body[f] === "" ? null : body[f];
+  for (const f of fields) if (f in body) out[f] = body[f] === "" && NULLABLE.has(f) ? null : body[f];
   return out;
 }
 
@@ -166,6 +169,10 @@ Bun.serve({
   fetch: (req) =>
     handle(req).catch((e) => {
       console.error(e);
+      // Constraint violations are bad input, not server faults — 400 so
+      // offline clients drop the op instead of retrying it forever.
+      const errno = (e as { errno?: string }).errno ?? "";
+      if (errno.startsWith("23")) return err(`constraint violation: ${(e as Error).message}`, 400);
       return err("internal error", 500);
     }),
 });