diff --git a/packages/opencode/src/effect/runner.ts b/packages/opencode/src/effect/runner.ts
index e03cbffb11..bbc85309e8 100644
--- a/packages/opencode/src/effect/runner.ts
+++ b/packages/opencode/src/effect/runner.ts
@@ -4,7 +4,7 @@ export interface Runner<A, E = never> {
   readonly state: State<A, E>
   readonly busy: boolean
   readonly ensureRunning: (work: Effect.Effect<A, E>) => Effect.Effect<A, E>
-  readonly startShell: (work: Effect.Effect<A, E>) => Effect.Effect<A, E>
+  readonly startShell: (work: Effect.Effect<A, E>, ready?: Deferred.Deferred<void>) => Effect.Effect<A, E>
   readonly cancel: Effect.Effect<void>
 }
 
@@ -19,6 +19,7 @@ interface RunHandle<A, E> {
 interface ShellHandle<A, E> {
   id: number
   cancelled: Deferred.Deferred<void>
+  ready?: Deferred.Deferred<void>
   fiber: Fiber.Fiber<A, E>
 }
 
@@ -106,6 +107,7 @@ export const make = <A, E = never>(
 
   const stopShell = (shell: ShellHandle<A, E>) =>
     Effect.gen(function* () {
+      if (shell.ready) yield* Deferred.await(shell.ready).pipe(Effect.exit, Effect.asVoid)
       yield* Deferred.succeed(shell.cancelled, undefined).pipe(Effect.asVoid)
       yield* Fiber.interrupt(shell.fiber)
     })
@@ -135,7 +137,7 @@ export const make = <A, E = never>(
       }),
     ).pipe(Effect.flatten)
 
-  const startShell = (work: Effect.Effect<A, E>) =>
+  const startShell = (work: Effect.Effect<A, E>, ready?: Deferred.Deferred<void>) =>
     SynchronizedRef.modifyEffect(
       ref,
       Effect.fnUntraced(function* (st) {
@@ -152,12 +154,12 @@ export const make = <A, E = never>(
         const id = next()
         const cancelled = yield* Deferred.make<void>()
         const fiber = yield* work.pipe(Effect.ensuring(finishShell(id)), Effect.forkChild)
-        const shell = { id, cancelled, fiber } satisfies ShellHandle<A, E>
+        const shell = { id, cancelled, ready, fiber } satisfies ShellHandle<A, E>
         return [
           Effect.gen(function* () {
             const exit = yield* Fiber.await(fiber)
             if (Exit.isSuccess(exit)) return exit.value
-            if ((yield* Deferred.isDone(cancelled)) || Cause.hasInterruptsOnly(exit.cause)) {
+            if (Cause.hasInterruptsOnly(exit.cause) || ((yield* Deferred.isDone(cancelled)) && !Cause.hasDies(exit.cause))) {
               if (onInterrupt) return yield* onInterrupt
               return yield* Effect.die(new Cancelled())
             }
@@ -192,8 +194,8 @@ export const make = <A, E = never>(
       case "ShellThenRun":
         return [
           Effect.gen(function* () {
-            yield* Deferred.fail(st.run.done, new Cancelled()).pipe(Effect.asVoid)
             yield* stopShell(st.shell)
+            yield* Deferred.fail(st.run.done, new Cancelled()).pipe(Effect.asVoid)
             yield* idleIfCurrent()
           }),
           { _tag: "Idle" } as const,
diff --git a/packages/opencode/src/session/prompt.ts b/packages/opencode/src/session/prompt.ts
index a94f520c30..2f359610a3 100644
--- a/packages/opencode/src/session/prompt.ts
+++ b/packages/opencode/src/session/prompt.ts
@@ -45,7 +45,7 @@ import { AppFileSystem } from "@opencode-ai/core/filesystem"
 import { Truncate } from "@/tool/truncate"
 import { decodeDataUrl } from "@/util/data-url"
 import { Process } from "@/util/process"
-import { Cause, Effect, Exit, Layer, Option, Scope, Context, Schema } from "effect"
+import { Cause, Deferred, Effect, Exit, Layer, Option, Scope, Context, Schema } from "effect"
 import { zod } from "@/util/effect-zod"
 import { withStatics } from "@/util/schema"
 import * as EffectLogger from "@opencode-ai/core/effect/logger"
@@ -720,9 +720,10 @@ NOTE: At any point in time through this workflow you should feel free to ask the
       } satisfies MessageV2.TextPart)
     })
 
-    const shellImpl = Effect.fn("SessionPrompt.shellImpl")(function* (input: ShellInput) {
+    const shellImpl = Effect.fn("SessionPrompt.shellImpl")(function* (input: ShellInput, ready?: Deferred.Deferred<void>) {
       return yield* Effect.uninterruptibleMask((restore) =>
         Effect.gen(function* () {
+          const markReady = ready ? Deferred.succeed(ready, undefined).pipe(Effect.asVoid) : Effect.void
           const { msg, part, cwd } = yield* Effect.gen(function* () {
             const ctx = yield* InstanceState.context
             const session = yield* sessions.get(input.sessionID)
@@ -786,8 +787,9 @@ NOTE: At any point in time through this workflow you should feel free to ask the
               },
             }
             yield* sessions.updatePart(part)
+            yield* markReady
             return { msg, part, cwd: ctx.directory }
-          })
+          }).pipe(Effect.onExit(() => markReady))
 
           const cfg = yield* config.get()
           const sh = Shell.preferred(cfg.shell)
@@ -1508,7 +1510,8 @@ NOTE: At any point in time through this workflow you should feel free to ask the
 
     const shell: (input: ShellInput) => Effect.Effect<MessageV2.WithParts> = Effect.fn("SessionPrompt.shell")(
       function* (input: ShellInput) {
-        return yield* state.startShell(input.sessionID, lastAssistant(input.sessionID), shellImpl(input))
+        const ready = yield* Deferred.make<void>()
+        return yield* state.startShell(input.sessionID, lastAssistant(input.sessionID), shellImpl(input, ready), ready)
       },
     )
 
diff --git a/packages/opencode/src/session/run-state.ts b/packages/opencode/src/session/run-state.ts
index 4b210d63d7..5ad95c9c7d 100644
--- a/packages/opencode/src/session/run-state.ts
+++ b/packages/opencode/src/session/run-state.ts
@@ -1,6 +1,6 @@
 import { InstanceState } from "@/effect/instance-state"
 import { Runner } from "@/effect/runner"
-import { Effect, Layer, Scope, Context } from "effect"
+import { Deferred, Effect, Layer, Scope, Context } from "effect"
 import * as Session from "./session"
 import { MessageV2 } from "./message-v2"
 import { SessionID } from "./schema"
@@ -18,6 +18,7 @@ export interface Interface {
     sessionID: SessionID,
     onInterrupt: Effect.Effect<MessageV2.WithParts>,
     work: Effect.Effect<MessageV2.WithParts>,
+    ready?: Deferred.Deferred<void>,
   ) => Effect.Effect<MessageV2.WithParts>
 }
 
@@ -95,8 +96,9 @@ export const layer = Layer.effect(
       sessionID: SessionID,
       onInterrupt: Effect.Effect<MessageV2.WithParts>,
       work: Effect.Effect<MessageV2.WithParts>,
+      ready?: Deferred.Deferred<void>,
     ) {
-      return yield* (yield* runner(sessionID, onInterrupt)).startShell(work)
+      return yield* (yield* runner(sessionID, onInterrupt)).startShell(work, ready)
     })
 
     return Service.of({ assertNotBusy, cancel, ensureRunning, startShell })
diff --git a/packages/opencode/test/effect/runner.test.ts b/packages/opencode/test/effect/runner.test.ts
index 4b0fbc1b51..ee99050a8c 100644
--- a/packages/opencode/test/effect/runner.test.ts
+++ b/packages/opencode/test/effect/runner.test.ts
@@ -334,6 +334,22 @@ describe("Runner", () => {
     }),
   )
 
+  it.live(
+    "cancel does not mask shell defects",
+    Effect.gen(function* () {
+      const s = yield* Scope.Scope
+      const runner = Runner.make<string>(s, { onInterrupt: Effect.succeed("interrupted") })
+
+      const sh = yield* runner
+        .startShell(Effect.never.pipe(Effect.ensuring(Effect.die("boom")), Effect.as("ignored")))
+        .pipe(Effect.forkChild)
+      yield* Effect.sleep("10 millis")
+
+      yield* runner.cancel
+      expect(Exit.isFailure(yield* Fiber.await(sh))).toBe(true)
+    }),
+  )
+
   // --- shell→run handoff ---
 
   it.live(
diff --git a/packages/opencode/test/session/prompt.test.ts b/packages/opencode/test/session/prompt.test.ts
index 422c1400c9..5330569401 100644
--- a/packages/opencode/test/session/prompt.test.ts
+++ b/packages/opencode/test/session/prompt.test.ts
@@ -1470,6 +1470,10 @@ unix(
 
           const exit = yield* Fiber.await(loop)
           expect(Exit.isSuccess(exit)).toBe(true)
+          if (Exit.isSuccess(exit)) {
+            const tool = completedTool(exit.value.parts)
+            expect(tool?.state.output).toContain("User aborted the command")
+          }
 
           yield* Fiber.await(sh)
         }),