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

2 files changed, 62 insertions, 25 deletions

diff --git a/packages/opencode/src/session/prompt.ts b/packages/opencode/src/session/prompt.ts
index 5890a4a73..43ad9a09d 100644
--- a/packages/opencode/src/session/prompt.ts
+++ b/packages/opencode/src/session/prompt.ts
@@ -974,17 +974,22 @@ export namespace SessionPrompt {
     }
     using _ = defer(() => InstructionPrompt.clear(info.id))
 
+    type Draft<T> = T extends MessageV2.Part ? Omit<T, "id"> & { id?: string } : never
+    const assign = (part: Draft<MessageV2.Part>): MessageV2.Part => ({
+      ...part,
+      id: part.id ?? Identifier.ascending("part"),
+    })
+
     const parts = await Promise.all(
-      input.parts.map(async (part): Promise<MessageV2.Part[]> => {
+      input.parts.map(async (part): Promise<Draft<MessageV2.Part>[]> => {
         if (part.type === "file") {
           // before checking the protocol we check if this is an mcp resource because it needs special handling
           if (part.source?.type === "resource") {
             const { clientName, uri } = part.source
             log.info("mcp resource", { clientName, uri, mime: part.mime })
 
-            const pieces: MessageV2.Part[] = [
+            const pieces: Draft<MessageV2.Part>[] = [
               {
-                id: Identifier.ascending("part"),
                 messageID: info.id,
                 sessionID: input.sessionID,
                 type: "text",
@@ -1007,7 +1012,6 @@ export namespace SessionPrompt {
               for (const content of contents) {
                 if ("text" in content && content.text) {
                   pieces.push({
-                    id: Identifier.ascending("part"),
                     messageID: info.id,
                     sessionID: input.sessionID,
                     type: "text",
@@ -1018,7 +1022,6 @@ export namespace SessionPrompt {
                   // Handle binary content if needed
                   const mimeType = "mimeType" in content ? content.mimeType : part.mime
                   pieces.push({
-                    id: Identifier.ascending("part"),
                     messageID: info.id,
                     sessionID: input.sessionID,
                     type: "text",
@@ -1030,7 +1033,6 @@ export namespace SessionPrompt {
 
               pieces.push({
                 ...part,
-                id: part.id ?? Identifier.ascending("part"),
                 messageID: info.id,
                 sessionID: input.sessionID,
               })
@@ -1038,7 +1040,6 @@ export namespace SessionPrompt {
               log.error("failed to read MCP resource", { error, clientName, uri })
               const message = error instanceof Error ? error.message : String(error)
               pieces.push({
-                id: Identifier.ascending("part"),
                 messageID: info.id,
                 sessionID: input.sessionID,
                 type: "text",
@@ -1055,7 +1056,6 @@ export namespace SessionPrompt {
               if (part.mime === "text/plain") {
                 return [
                   {
-                    id: Identifier.ascending("part"),
                     messageID: info.id,
                     sessionID: input.sessionID,
                     type: "text",
@@ -1063,7 +1063,6 @@ export namespace SessionPrompt {
                     text: `Called the Read tool with the following input: ${JSON.stringify({ filePath: part.filename })}`,
                   },
                   {
-                    id: Identifier.ascending("part"),
                     messageID: info.id,
                     sessionID: input.sessionID,
                     type: "text",
@@ -1072,7 +1071,6 @@ export namespace SessionPrompt {
                   },
                   {
                     ...part,
-                    id: part.id ?? Identifier.ascending("part"),
                     messageID: info.id,
                     sessionID: input.sessionID,
                   },
@@ -1129,9 +1127,8 @@ export namespace SessionPrompt {
                 }
                 const args = { filePath: filepath, offset, limit }
 
-                const pieces: MessageV2.Part[] = [
+                const pieces: Draft<MessageV2.Part>[] = [
                   {
-                    id: Identifier.ascending("part"),
                     messageID: info.id,
                     sessionID: input.sessionID,
                     type: "text",
@@ -1155,7 +1152,6 @@ export namespace SessionPrompt {
                     }
                     const result = await t.execute(args, readCtx)
                     pieces.push({
-                      id: Identifier.ascending("part"),
                       messageID: info.id,
                       sessionID: input.sessionID,
                       type: "text",
@@ -1166,7 +1162,6 @@ export namespace SessionPrompt {
                       pieces.push(
                         ...result.attachments.map((attachment) => ({
                           ...attachment,
-                          id: Identifier.ascending("part"),
                           synthetic: true,
                           filename: attachment.filename ?? part.filename,
                           messageID: info.id,
@@ -1176,7 +1171,6 @@ export namespace SessionPrompt {
                     } else {
                       pieces.push({
                         ...part,
-                        id: part.id ?? Identifier.ascending("part"),
                         messageID: info.id,
                         sessionID: input.sessionID,
                       })
@@ -1192,7 +1186,6 @@ export namespace SessionPrompt {
                       }).toObject(),
                     })
                     pieces.push({
-                      id: Identifier.ascending("part"),
                       messageID: info.id,
                       sessionID: input.sessionID,
                       type: "text",
@@ -1219,7 +1212,6 @@ export namespace SessionPrompt {
                 const result = await ReadTool.init().then((t) => t.execute(args, listCtx))
                 return [
                   {
-                    id: Identifier.ascending("part"),
                     messageID: info.id,
                     sessionID: input.sessionID,
                     type: "text",
@@ -1227,7 +1219,6 @@ export namespace SessionPrompt {
                     text: `Called the Read tool with the following input: ${JSON.stringify(args)}`,
                   },
                   {
-                    id: Identifier.ascending("part"),
                     messageID: info.id,
                     sessionID: input.sessionID,
                     type: "text",
@@ -1236,7 +1227,6 @@ export namespace SessionPrompt {
                   },
                   {
                     ...part,
-                    id: part.id ?? Identifier.ascending("part"),
                     messageID: info.id,
                     sessionID: input.sessionID,
                   },
@@ -1247,7 +1237,6 @@ export namespace SessionPrompt {
               FileTime.read(input.sessionID, filepath)
               return [
                 {
-                  id: Identifier.ascending("part"),
                   messageID: info.id,
                   sessionID: input.sessionID,
                   type: "text",
@@ -1255,7 +1244,7 @@ export namespace SessionPrompt {
                   synthetic: true,
                 },
                 {
-                  id: part.id ?? Identifier.ascending("part"),
+                  id: part.id,
                   messageID: info.id,
                   sessionID: input.sessionID,
                   type: "file",
@@ -1274,13 +1263,11 @@ export namespace SessionPrompt {
           const hint = perm.action === "deny" ? " . Invoked by user; guaranteed to exist." : ""
           return [
             {
-              id: Identifier.ascending("part"),
               ...part,
               messageID: info.id,
               sessionID: input.sessionID,
             },
             {
-              id: Identifier.ascending("part"),
               messageID: info.id,
               sessionID: input.sessionID,
               type: "text",
@@ -1297,14 +1284,13 @@ export namespace SessionPrompt {
 
         return [
           {
-            id: Identifier.ascending("part"),
             ...part,
             messageID: info.id,
             sessionID: input.sessionID,
           },
         ]
       }),
-    ).then((x) => x.flat())
+    ).then((x) => x.flat().map(assign))
 
     await Plugin.trigger(
       "chat.message",
diff --git a/packages/opencode/test/session/prompt-missing-file.test.ts b/packages/opencode/test/session/prompt-missing-file.test.ts
index 081847c67..c3f52f56c 100644
--- a/packages/opencode/test/session/prompt-missing-file.test.ts
+++ b/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)
+      },
+    })
+  })
 })