core: keep message part order stable when files resolve asynchronously (#13915)

packages/opencode/test/session/prompt-missing-file.test.ts

@@ -2,6 +2,7 @@ import path from "path"
 import { describe, expect, test } from "bun:test"
 import { Instance } from "../../src/project/instance"
 import { Session } from "../../src/session"
+import { MessageV2 } from "../../src/session/message-v2"
 import { SessionPrompt } from "../../src/session/prompt"
 import { tmpdir } from "../fixture/fixture"
 
@@ -50,4 +51,54 @@ describe("session.prompt missing file", () => {
       },
     })
   })
+
+  test("keeps stored part order stable when file resolution is async", async () => {
+    await using tmp = await tmpdir({
+      git: true,
+      config: {
+        agent: {
+          build: {
+            model: "openai/gpt-5.2",
+          },
+        },
+      },
+    })
+
+    await Instance.provide({
+      directory: tmp.path,
+      fn: async () => {
+        const session = await Session.create({})
+
+        const missing = path.join(tmp.path, "still-missing.ts")
+        const msg = await SessionPrompt.prompt({
+          sessionID: session.id,
+          agent: "build",
+          noReply: true,
+          parts: [
+            {
+              type: "file",
+              mime: "text/plain",
+              url: `file://${missing}`,
+              filename: "still-missing.ts",
+            },
+            { type: "text", text: "after-file" },
+          ],
+        })
+
+        if (msg.info.role !== "user") throw new Error("expected user message")
+
+        const stored = await MessageV2.get({
+          sessionID: session.id,
+          messageID: msg.info.id,
+        })
+        const text = stored.parts.filter((part) => part.type === "text").map((part) => part.text)
+
+        expect(text[0]?.startsWith("Called the Read tool with the following input:")).toBe(true)
+        expect(text[1]?.includes("Read tool failed to read")).toBe(true)
+        expect(text[2]).toBe("after-file")
+
+        await Session.remove(session.id)
+      },
+    })
+  })
 })