fix(session): preserve structured plugin messages

Request-only suffixes join after AI SDK prompt lowering. Keep appends with non-text parts in the normal conversion path so files and tool content retain upstream lowering semantics.

Author: Sean Smith

packages/opencode/src/session/message-v2.ts

@@ -447,9 +447,22 @@ export const toModelMessagesSplitEffect = Effect.fnUntraced(function* (
     return { messages: yield* convert(filtered), tail: [] }
   }
 
+  // The AI SDK lowers ModelMessage into a provider prompt before middleware runs. The suffix joins
+  // inside that middleware, so only text (plus the step boundary that converts into text messages)
+  // is already in the same shape on both sides of that lowering. Keep richer plugin appends in the
+  // main array rather than risk changing files, tool results, or other structured content in flight.
+  const tail = filtered.slice(split)
+  if (
+    !tail.every((message) =>
+      message.parts.every((part) => part.type === "step-start" || (part.type === "text" && part.text !== "")),
+    )
+  ) {
+    return { messages: yield* convert(filtered), tail: [] }
+  }
+
   return {
     messages: yield* convert(filtered.slice(0, split)),
-    tail: yield* convert(filtered.slice(split)),
+    tail: yield* convert(tail),
   }
 })
 

packages/opencode/test/session/message-v2.test.ts

@@ -1794,7 +1794,7 @@ describe("session.message-v2 request-only partition", () => {
   })
 
   test("keeps every provider message expanded from a request-only source in the suffix", async () => {
-    const assistantID = "m-tail-tool"
+    const assistantID = "m-tail-split"
     const input: SessionV1.WithParts[] = [
       {
         info: userInfo("m-durable"),
@@ -1803,32 +1803,71 @@ describe("session.message-v2 request-only partition", () => {
       {
         info: assistantInfo(assistantID, "m-durable"),
         parts: [
-          { ...basePart(assistantID, "text"), type: "text", text: "request-only answer" },
+          { ...basePart(assistantID, "first"), type: "text", text: "request-only first" },
+          { ...basePart(assistantID, "step"), type: "step-start" },
+          { ...basePart(assistantID, "second"), type: "text", text: "request-only second" },
+        ] as SessionV1.Part[],
+      },
+    ]
+
+    const output = await Effect.runPromise(
+      MessageV2.toModelMessagesSplitEffect(input, model, { requestOnlyTailCount: 1 }),
+    )
+    expect(output.messages).toEqual([{ role: "user", content: [{ type: "text", text: "durable" }] }])
+    expect(output.tail.map((message) => message.role)).toEqual(["assistant", "assistant"])
+    expect(JSON.stringify(output.tail)).toContain("request-only first")
+    expect(JSON.stringify(output.tail)).toContain("request-only second")
+  })
+
+  test("keeps structured request-only content in the normal AI SDK conversion path", async () => {
+    const input: SessionV1.WithParts[] = [
+      {
+        info: userInfo("m-durable"),
+        parts: [{ ...basePart("m-durable", "durable"), type: "text", text: "durable" }] as SessionV1.Part[],
+      },
+      {
+        info: userInfo("m-tail-file"),
+        parts: [
           {
-            ...basePart(assistantID, "tool"),
-            type: "tool",
-            callID: "call-request-only",
-            tool: "probe",
-            state: {
-              status: "completed",
-              input: { value: 1 },
-              output: "request-only output",
-              title: "Probe",
-              metadata: {},
-              time: { start: 0, end: 1 },
-            },
+            ...basePart("m-tail-file", "file"),
+            type: "file",
+            mime: "image/png",
+            filename: "image.png",
+            url: "https://example.com/image.png",
           },
         ] as SessionV1.Part[],
       },
     ]
 
+    const bulk = await MessageV2.toModelMessages(input, model)
     const output = await Effect.runPromise(
       MessageV2.toModelMessagesSplitEffect(input, model, { requestOnlyTailCount: 1 }),
     )
-    expect(output.messages).toEqual([{ role: "user", content: [{ type: "text", text: "durable" }] }])
-    expect(output.tail.map((message) => message.role)).toEqual(["assistant", "tool"])
-    expect(JSON.stringify(output.tail)).toContain("request-only answer")
-    expect(JSON.stringify(output.tail)).toContain("request-only output")
+
+    expect(output.messages).toEqual(bulk)
+    expect(output.tail).toEqual([])
+  })
+
+  test("keeps empty request-only text in the normal AI SDK conversion path", async () => {
+    const assistantID = "m-tail-empty"
+    const input: SessionV1.WithParts[] = [
+      {
+        info: userInfo("m-durable"),
+        parts: [{ ...basePart("m-durable", "durable"), type: "text", text: "durable" }] as SessionV1.Part[],
+      },
+      {
+        info: assistantInfo(assistantID, "m-durable"),
+        parts: [{ ...basePart(assistantID, "empty"), type: "text", text: "" }] as SessionV1.Part[],
+      },
+    ]
+
+    const bulk = await MessageV2.toModelMessages(input, model)
+    const output = await Effect.runPromise(
+      MessageV2.toModelMessagesSplitEffect(input, model, { requestOnlyTailCount: 1 }),
+    )
+
+    expect(output.messages).toEqual(bulk)
+    expect(output.tail).toEqual([])
   })
 })