From 71ebf2f84c18761ef8b9e29dbff86559fe178690 Mon Sep 17 00:00:00 2001
From: Sami Jawhar <sami@trajectorylabs.net>
Date: Mon, 27 Apr 2026 02:25:49 +0000
Subject: [PATCH] fix(session): preserve agent and model on async prompt

When a prompt comes in without explicit agent/model fields (the common
case for prompt_async), createUserMessage was falling back to the
config's default agent and that agent's default model. This caused the
session to silently switch agents/models mid-conversation.

Fix: read the most recent user message in the session and prefer its
agent and model as the fallback before going to defaults. Both fields
fall through together because the user's previous choice was a coherent
pairing.

Replaces the dropped fix/prompt-async-agent-preserve branch with a
minimal version. Loop continuation guard and 30-min timeout from the
original branch are intentionally omitted; can be added later if
needed.
---
 packages/opencode/src/session/prompt.ts       | 10 ++++--
 packages/opencode/test/session/prompt.test.ts | 36 +++++++++++++++++++
 2 files changed, 44 insertions(+), 2 deletions(-)

diff --git a/packages/opencode/src/session/prompt.ts b/packages/opencode/src/session/prompt.ts
index 22fe4d81cd40..0ac920be37f2 100644
--- a/packages/opencode/src/session/prompt.ts
+++ b/packages/opencode/src/session/prompt.ts
@@ -691,7 +691,13 @@ export const layer = Layer.effect(
     })
 
     const createUserMessage = Effect.fn("SessionPrompt.createUserMessage")(function* (input: PromptInput) {
-      const agentName = input.agent
+      const prev = input.agent
+        ? undefined
+        : (yield* MessageV2.filterCompactedEffect(input.sessionID)).findLast(
+            (m): m is MessageV2.WithParts & { info: MessageV2.User } =>
+              m.info.role === "user" && !!m.info.agent,
+          )
+      const agentName = input.agent ?? prev?.info.agent
       const ag = agentName ? yield* agents.get(agentName) : yield* agents.defaultInfo()
       if (!ag) {
         const available = (yield* agents.list()).filter((a) => !a.hidden).map((a) => a.name)
@@ -708,7 +714,7 @@ export const layer = Layer.effect(
           .where(eq(SessionTable.id, input.sessionID))
           .get(),
       )
-      const model = input.model ?? ag.model ?? (yield* currentModel(input.sessionID))
+      const model = input.model ?? prev?.info.model ?? ag.model ?? (yield* currentModel(input.sessionID))
       const same = ag.model && model.providerID === ag.model.providerID && model.modelID === ag.model.modelID
       const full =
         !input.variant && ag.variant && same
diff --git a/packages/opencode/test/session/prompt.test.ts b/packages/opencode/test/session/prompt.test.ts
index 4c4647457814..ceedd1a105f7 100644
--- a/packages/opencode/test/session/prompt.test.ts
+++ b/packages/opencode/test/session/prompt.test.ts
@@ -2187,6 +2187,41 @@ it.instance(
 
 // Agent variant
 
+noLLMServer.instance(
+  "prompt without agent and model preserves current session agent and model",
+  () =>
+    Effect.gen(function* () {
+      const prompt = yield* SessionPrompt.Service
+      const sessions = yield* Session.Service
+      const session = yield* sessions.create({})
+
+      yield* prompt.prompt({
+        sessionID: session.id,
+        agent: "build",
+        model: ref,
+        noReply: true,
+        parts: [{ type: "text", text: "hello" }],
+      })
+
+      const next = yield* prompt.prompt({
+        sessionID: session.id,
+        noReply: true,
+        parts: [{ type: "text", text: "hello again" }],
+      })
+      if (next.info.role !== "user") throw new Error("expected user message")
+      expect(next.info.agent).toBe("build")
+      expect(next.info.model).toEqual(ref)
+
+      yield* sessions.remove(session.id)
+    }),
+  {
+    config: {
+      ...cfg,
+      default_agent: "plan",
+    },
+  },
+)
+
 noLLMServer.instance(
   "applies agent variant only when using agent model",
   () =>
@@ -2256,6 +2291,7 @@ noLLMServer.instance(
   },
 )
 
+
 // Agent / command resolution errors
 
 noLLMServer.instance(