diff --git a/packages/opencode/src/session/prompt.ts b/packages/opencode/src/session/prompt.ts
index 6248ce455c..5d79bff049 100644
--- a/packages/opencode/src/session/prompt.ts
+++ b/packages/opencode/src/session/prompt.ts
@@ -1755,12 +1755,25 @@ NOTE: At any point in time through this workflow you should feel free to ask the
             sessionID,
           }
           yield* sessions.updateMessage(msg)
-          const handle = yield* processor.create({
-            assistantMessage: msg,
-            sessionID,
-            model,
+
+          const finalizeInterruptedAssistant = Effect.gen(function* () {
+            if (msg.time.completed) return
+            msg.error ??= MessageV2.fromError(new DOMException("Aborted", "AbortError"), {
+              providerID: msg.providerID,
+              aborted: true,
+            })
+            msg.time.completed = Date.now()
+            yield* sessions.updateMessage(msg)
           })
 
+          const handle = yield* processor
+            .create({
+              assistantMessage: msg,
+              sessionID,
+              model,
+            })
+            .pipe(Effect.onInterrupt(() => finalizeInterruptedAssistant))
+
           const outcome: "break" | "continue" = yield* Effect.gen(function* () {
             const lastUserMsg = msgs.findLast((m) => m.info.role === "user")
             const bypassAgentCheck = lastUserMsg?.parts.some((p) => p.type === "agent") ?? false
@@ -1859,7 +1872,10 @@ NOTE: At any point in time through this workflow you should feel free to ask the
               })
             }
             return "continue" as const
-          }).pipe(Effect.ensuring(instruction.clear(handle.message.id)))
+          }).pipe(
+            Effect.ensuring(instruction.clear(handle.message.id)),
+            Effect.onInterrupt(() => finalizeInterruptedAssistant),
+          )
           if (outcome === "break") break
           continue
         }
diff --git a/packages/opencode/test/session/prompt.test.ts b/packages/opencode/test/session/prompt.test.ts
index 9ad2fbe1f6..9ac038aa66 100644
--- a/packages/opencode/test/session/prompt.test.ts
+++ b/packages/opencode/test/session/prompt.test.ts
@@ -152,7 +152,16 @@ const lsp = Layer.succeed(
 const status = SessionStatus.layer.pipe(Layer.provideMerge(Bus.layer))
 const run = SessionRunState.layer.pipe(Layer.provide(status))
 const infra = Layer.mergeAll(NodeFileSystem.layer, CrossSpawnSpawner.defaultLayer)
-function makeHttp() {
+
+const processorCreateStarted: Array<() => void> = []
+const blockingProcessor = Layer.succeed(
+  SessionProcessor.Service,
+  SessionProcessor.Service.of({
+    create: () => Effect.sync(() => processorCreateStarted.shift()?.()).pipe(Effect.andThen(Effect.never)),
+  }),
+)
+
+function makeHttp(input?: { processor?: "blocking" }) {
   const deps = Layer.mergeAll(
     Session.defaultLayer,
     Snapshot.defaultLayer,
@@ -186,12 +195,15 @@ function makeHttp() {
     Layer.provideMerge(deps),
   )
   const trunc = Truncate.layer.pipe(Layer.provideMerge(deps))
-  const proc = SessionProcessor.layer.pipe(
-    Layer.provide(summary),
-    Layer.provide(Image.defaultLayer),
-    Layer.provide(RuntimeFlags.layer({ experimentalEventSystem: true })),
-    Layer.provideMerge(deps),
-  )
+  const proc =
+    input?.processor === "blocking"
+      ? blockingProcessor
+      : SessionProcessor.layer.pipe(
+          Layer.provide(summary),
+          Layer.provide(Image.defaultLayer),
+          Layer.provide(RuntimeFlags.layer({ experimentalEventSystem: true })),
+          Layer.provideMerge(deps),
+        )
   const compact = SessionCompaction.layer.pipe(
     Layer.provide(RuntimeFlags.layer({ experimentalEventSystem: true })),
     Layer.provideMerge(proc),
@@ -218,6 +230,7 @@ function makeHttp() {
 }
 
 const it = testEffect(makeHttp())
+const race = testEffect(makeHttp({ processor: "blocking" }))
 const unix = process.platform !== "win32" ? it.instance : it.instance.skip
 
 // Config that registers a custom "test" provider with a "test-model" model
@@ -341,6 +354,14 @@ const deferredAsPromise = <A>(deferred: Deferred.Deferred<A>): PromiseLike<A> =>
   },
 })
 
+function defer<T>() {
+  let resolve!: (value: T | PromiseLike<T>) => void
+  const promise = new Promise<T>((done) => {
+    resolve = done
+  })
+  return { promise, resolve }
+}
+
 const succeedVoid = (deferred: Deferred.Deferred<void>) => {
   Effect.runSync(Deferred.succeed(deferred, void 0).pipe(Effect.ignore))
 }
@@ -896,6 +917,96 @@ it.instance(
   3_000,
 )
 
+race.instance(
+  "finalizes assistant when cancelled before processor creation completes",
+  () =>
+    Effect.gen(function* () {
+      yield* useServerConfig(providerCfg)
+      processorCreateStarted.length = 0
+      yield* Effect.addFinalizer(() =>
+        Effect.sync(() => {
+          processorCreateStarted.length = 0
+        }),
+      )
+
+      const prompt = yield* SessionPrompt.Service
+      const sessions = yield* Session.Service
+      const chat = yield* sessions.create({ title: "Processor creation race" })
+
+      yield* prompt.prompt({
+        sessionID: chat.id,
+        agent: "build",
+        noReply: true,
+        parts: [{ type: "text", text: "first" }],
+      })
+
+      const firstCreate = defer<void>()
+      processorCreateStarted.push(firstCreate.resolve)
+      const first = yield* prompt.loop({ sessionID: chat.id }).pipe(Effect.forkChild)
+      yield* Effect.promise(() => firstCreate.promise)
+
+      yield* prompt.cancel(chat.id)
+      const firstExit = yield* Fiber.await(first)
+      expect(Exit.isSuccess(firstExit)).toBe(true)
+
+      let messages = yield* sessions.messages({ sessionID: chat.id })
+      const firstInterrupted = messages.at(-1)
+      expect(firstInterrupted?.info.role).toBe("assistant")
+      expect(firstInterrupted?.parts).toHaveLength(0)
+      if (firstInterrupted?.info.role === "assistant") {
+        expect(firstInterrupted.info.finish).toBeUndefined()
+        expect(firstInterrupted.info.time.completed).toBeNumber()
+        expect(firstInterrupted.info.error?.name).toBe("MessageAbortedError")
+      }
+
+      yield* prompt.prompt({
+        sessionID: chat.id,
+        agent: "build",
+        noReply: true,
+        parts: [{ type: "text", text: "second" }],
+      })
+
+      const secondCreate = defer<void>()
+      processorCreateStarted.push(secondCreate.resolve)
+      const second = yield* prompt.loop({ sessionID: chat.id }).pipe(Effect.forkChild)
+      yield* Effect.promise(() => secondCreate.promise)
+
+      yield* prompt.cancel(chat.id)
+      const secondExit = yield* Fiber.await(second)
+      expect(Exit.isSuccess(secondExit)).toBe(true)
+
+      messages = yield* sessions.messages({ sessionID: chat.id })
+      const poisonMessages = messages.filter(
+        (message) =>
+          message.info.role === "assistant" &&
+          message.parts.length === 0 &&
+          !message.info.finish &&
+          !message.info.time.completed &&
+          !message.info.error,
+      )
+      expect(poisonMessages).toHaveLength(0)
+
+      const interruptedMessages = messages.filter(
+        (message) =>
+          message.info.role === "assistant" &&
+          message.parts.length === 0 &&
+          message.info.time.completed &&
+          message.info.error?.name === "MessageAbortedError",
+      )
+      expect(interruptedMessages).toHaveLength(2)
+
+      const lastUser = messages.at(-2)
+      const lastAssistant = messages.at(-1)
+      expect(lastUser?.info.role).toBe("user")
+      expect(lastAssistant?.info.role).toBe("assistant")
+      if (lastUser?.info.role === "user" && lastAssistant?.info.role === "assistant") {
+        expect(lastAssistant.info.parentID).toBe(lastUser?.info.id)
+      }
+    }),
+  { git: true },
+  3_000,
+)
+
 it.instance(
   "cancel finalizes subtask tool state",
   () =>