diff --git a/packages/app/src/plugins/composer-items/model.test.ts b/packages/app/src/plugins/composer-items/model.test.ts
index 4e5f3dee18..e75585e36f 100644
--- a/packages/app/src/plugins/composer-items/model.test.ts
+++ b/packages/app/src/plugins/composer-items/model.test.ts
@@ -205,6 +205,9 @@ describe("toPluginComposer", () => {
 
     expect(() => composer.setPrompt("a")).toThrow("The composer is locked");
     expect(() => composer.insertText("b")).toThrow("The composer is locked");
+    expect(() => composer.addAttachment({ sourceId: "issues", item: issue })).toThrow(
+      "The composer is locked",
+    );
     await expect(composer.setProvider("codex")).rejects.toThrow("The composer is locked");
     await expect(composer.setModel("gpt")).rejects.toThrow("The composer is locked");
     await expect(composer.setMode("plan")).rejects.toThrow("The composer is locked");
@@ -231,6 +234,12 @@ describe("toPluginComposer", () => {
     input.isLocked = () => lock.locked;
 
     expect(() => composer.insertText("late")).toThrow("The composer is locked");
+    expect(() => composer.addAttachment({ sourceId: "issues", item: issue })).toThrow(
+      "The composer is locked",
+    );
+    expect(() => composer.removeAttachment({ sourceId: "issues", itemId: issue.id })).toThrow(
+      "The composer is locked",
+    );
     await expect(composer.setOptions({ effort: "low" })).rejects.toThrow("The composer is locked");
   });
 
diff --git a/packages/app/src/plugins/composer-items/model.ts b/packages/app/src/plugins/composer-items/model.ts
index f4c502a996..e2d8f173e7 100644
--- a/packages/app/src/plugins/composer-items/model.ts
+++ b/packages/app/src/plugins/composer-items/model.ts
@@ -68,6 +68,7 @@ export function toPluginComposer(input: {
       composerInput.insertText(text);
     },
     addAttachment({ sourceId, item }) {
+      assertUnlocked(composerInput.isLocked);
       const source = plugin.attachmentSources.find((candidate) => candidate.id === sourceId);
       if (!source) throw new Error(`Unknown attachment source: ${sourceId}`);
       const attachment = createPluginResourceAttachment(
@@ -86,6 +87,7 @@ export function toPluginComposer(input: {
       );
     },
     removeAttachment({ sourceId, itemId }) {
+      assertUnlocked(composerInput.isLocked);
       composerInput.changeAttachments((current) =>
         current.filter((candidate) => !isPluginAttachment(candidate, plugin.id, sourceId, itemId)),
       );