diff --git a/packages/opencode/src/server/instance/session.ts b/packages/opencode/src/server/instance/session.ts
index 0b54dd240..af3898c6b 100644
--- a/packages/opencode/src/server/instance/session.ts
+++ b/packages/opencode/src/server/instance/session.ts
@@ -1331,6 +1331,17 @@ export const SessionRoutes = lazy(() =>
         await AppRuntime.runPromise(
           Effect.gen(function* () {
             const sessions = yield* Session.Service
+            // Surface the route's declared 404 instead of silently succeeding:
+            // deleting a part that does not exist is a not-found, not a no-op.
+            // The check lives in the route, not Session.removePart, because
+            // removePart is also called internally by the message processor,
+            // which must stay a tolerant no-op for an already-gone part.
+            const part = yield* sessions.getPart({
+              sessionID: params.sessionID,
+              messageID: params.messageID,
+              partID: params.partID,
+            })
+            if (!part) throw new NotFoundError({ message: `Part not found: ${params.partID}` })
             yield* sessions.removePart({
               sessionID: params.sessionID,
               messageID: params.messageID,
diff --git a/packages/opencode/test/server/session-messages.test.ts b/packages/opencode/test/server/session-messages.test.ts
index f7fdfd2a8..c05ee6f4b 100644
--- a/packages/opencode/test/server/session-messages.test.ts
+++ b/packages/opencode/test/server/session-messages.test.ts
@@ -416,6 +416,14 @@ describe("session messages endpoint", () => {
           expect(await remove.json()).toBe(true)
           expect((await svc.messages({ sessionID: session.id }))[0].parts).toHaveLength(0)
 
+          // The route's declared 404 is now real: deleting an already-removed
+          // part surfaces NotFoundError instead of silently succeeding.
+          const missing = await app.request(`/session/${session.id}/message/${messageID}/part/${partID}`, {
+            method: "DELETE",
+          })
+          expect(missing.status).toBe(404)
+          expect((await missing.json()).name).toBe("NotFoundError")
+
           await svc.remove(session.id)
         },
       }),