diff --git a/src/inbox-item-persistence.ts b/src/inbox-item-persistence.ts
index 44b2f87..bd65e47 100644
--- a/src/inbox-item-persistence.ts
+++ b/src/inbox-item-persistence.ts
@@ -21,6 +21,14 @@ const ACTIONS_REQUIRING_SPHERES: readonly string[] = [
   "someday-file",
 ];
 
+function flattenToSingleLine(text: string): string {
+  return text
+    .split(/\r?\n/)
+    .map((line) => line.trim())
+    .filter((line) => line.length > 0)
+    .join(" ");
+}
+
 export class InboxItemPersistenceService {
   constructor(
     private readonly writer: FileWriter,
@@ -59,7 +67,9 @@ export class InboxItemPersistenceService {
       finalNextActions = [item.original];
     }
 
-    return finalNextActions;
+    // Each next action must occupy a single line in the destination file, so
+    // collapse any multi-line content (e.g. a captured note used as the action).
+    return finalNextActions.map(flattenToSingleLine);
   }
 
   private validateFinalNextActions(item: EditableItem, finalNextActions: string[]): void {
diff --git a/tests/inbox-item-persistence.test.ts b/tests/inbox-item-persistence.test.ts
index bd4fe47..56a37fd 100644
--- a/tests/inbox-item-persistence.test.ts
+++ b/tests/inbox-item-persistence.test.ts
@@ -115,6 +115,22 @@ describe("InboxItemPersistenceService", () => {
     expect(writerMocks.addToNextActionsFile).not.toHaveBeenCalled();
   });
 
+  it("flattens multi-line note content into a single-line next action", async () => {
+    const item: EditableItem = {
+      original: "Call dentist\nabout the crown that fell out",
+      isAIProcessed: false,
+      selectedAction: "next-actions-file",
+      selectedSpheres: ["personal"],
+      sourceNoteLink: "[[Captured note|source]]",
+    };
+
+    await service.persist(item);
+
+    expect(writerMocks.addToNextActionsFile).toHaveBeenCalledTimes(1);
+    const actionsArg = writerMocks.addToNextActionsFile.mock.calls[0][0];
+    expect(actionsArg).toEqual(["Call dentist about the crown that fell out"]);
+  });
+
   it("skips persistence work for trash items", async () => {
     const item: EditableItem = {
       original: "Duplicate",