From f57f68e1c8aca3a7e784979fcf3413583980e61c Mon Sep 17 00:00:00 2001
From: Kit Langton <kit.langton@gmail.com>
Date: Wed, 15 Jul 2026 16:38:17 -0400
Subject: [PATCH] fix(session): reject empty assistant responses

---
 packages/opencode/src/session/prompt.ts       | 30 ++++++++++++-
 packages/opencode/test/session/prompt.test.ts | 42 +++++++++++++++++++
 2 files changed, 71 insertions(+), 1 deletion(-)

diff --git a/packages/opencode/src/session/prompt.ts b/packages/opencode/src/session/prompt.ts
index 8b025cc2f0..e8c8949f37 100644
--- a/packages/opencode/src/session/prompt.ts
+++ b/packages/opencode/src/session/prompt.ts
@@ -1083,6 +1083,7 @@ const layer = Layer.effect(
         const ctx = yield* InstanceState.context
         let structured: unknown
         let step = 0
+        let retriedEmptyResponse = false
         const session = yield* sessions.get(sessionID).pipe(Effect.orDie)
 
         while (true) {
@@ -1107,12 +1108,17 @@ const layer = Layer.effect(
             lastAssistantMsg?.parts.some(
               (part) => part.type === "tool" && !part.metadata?.providerExecuted && !isOrphanedInterruptedTool(part),
             ) ?? false
+          const hasVisibleOutput =
+            lastAssistantMsg?.parts.some(
+              (part) => part.type === "tool" || (part.type === "text" && part.text.trim().length > 0),
+            ) ?? false
 
           if (
             lastAssistant?.finish &&
             !["tool-calls"].includes(lastAssistant.finish) &&
             !hasToolCalls &&
-            lastUser.id < lastAssistant.id
+            lastUser.id < lastAssistant.id &&
+            !(retriedEmptyResponse && !hasVisibleOutput)
           ) {
             const orphan = lastAssistantMsg?.parts.find(
               (part): part is SessionV1.ToolPart => part.type === "tool" && isOrphanedInterruptedTool(part),
@@ -1314,6 +1320,28 @@ const layer = Layer.effect(
                 yield* sessions.updateMessage(handle.message)
                 return "break" as const
               }
+
+              const parts = yield* MessageV2.parts(handle.message.id).pipe(
+                Effect.provideService(Database.Service, database),
+              )
+              const hasVisibleOutput = parts.some(
+                (part) => part.type === "tool" || (part.type === "text" && part.text.trim().length > 0),
+              )
+              if (!hasVisibleOutput) {
+                if (!retriedEmptyResponse) {
+                  retriedEmptyResponse = true
+                  yield* Effect.logWarning("retrying empty assistant response", {
+                    "session.id": sessionID,
+                    messageID: handle.message.id,
+                  })
+                  return "continue" as const
+                }
+                const error = new NamedError.Unknown({ message: "Model returned an empty response after retry" })
+                handle.message.error = error.toObject()
+                yield* sessions.updateMessage(handle.message)
+                yield* events.publish(Session.Event.Error, { sessionID, error: handle.message.error })
+                throw error
+              }
             }
 
             if (result === "stop") return "break" as const
diff --git a/packages/opencode/test/session/prompt.test.ts b/packages/opencode/test/session/prompt.test.ts
index 420bd49324..03c6044b7f 100644
--- a/packages/opencode/test/session/prompt.test.ts
+++ b/packages/opencode/test/session/prompt.test.ts
@@ -2201,6 +2201,48 @@ it.instance("does not loop empty assistant turns for a simple reply", () =>
   }),
 )
 
+it.instance("retries a reasoning-only assistant response once", () =>
+  Effect.gen(function* () {
+    const { llm } = yield* useServerConfig(providerCfg)
+    const prompt = yield* SessionPrompt.Service
+    const sessions = yield* Session.Service
+    const session = yield* sessions.create({ title: "Empty response retry" })
+
+    yield* llm.push(reply().reason("thinking").stop(), reply().text("done").stop())
+
+    const result = yield* prompt.prompt({
+      sessionID: session.id,
+      agent: "build",
+      parts: [{ type: "text", text: "Answer me" }],
+    })
+
+    expect(yield* llm.calls).toBe(2)
+    expect(result.parts.some((part) => part.type === "text" && part.text === "done")).toBe(true)
+  }),
+)
+
+it.instance("fails after two reasoning-only assistant responses", () =>
+  Effect.gen(function* () {
+    const { llm } = yield* useServerConfig(providerCfg)
+    const prompt = yield* SessionPrompt.Service
+    const sessions = yield* Session.Service
+    const session = yield* sessions.create({ title: "Empty response failure" })
+
+    yield* llm.push(reply().reason("thinking").stop(), reply().reason("still thinking").stop())
+
+    const exit = yield* prompt
+      .prompt({
+        sessionID: session.id,
+        agent: "build",
+        parts: [{ type: "text", text: "Answer me" }],
+      })
+      .pipe(Effect.exit)
+
+    expect(yield* llm.calls).toBe(2)
+    expect(Exit.isFailure(exit)).toBe(true)
+  }),
+)
+
 it.instance("records aborted errors when prompt is cancelled mid-stream", () =>
   Effect.gen(function* () {
     const { llm } = yield* useServerConfig(providerCfg)