diff --git a/packages/bash/bash-local/src/index.ts b/packages/bash/bash-local/src/index.ts
index 6ad655fcb6..4ed49be02a 100644
--- a/packages/bash/bash-local/src/index.ts
+++ b/packages/bash/bash-local/src/index.ts
@@ -247,12 +247,12 @@ export class LocalBashExecutor extends BashExecutor {
         }
         proc.exitCode = outcome.exitCode
         proc.signal = outcome.signal
-        this.onProcessDone(proc, collected.stderr.readFrom(0).text)
+        this.onProcessDone(proc, collected.stderr.readFrom(0).text, false)
       }, (error: unknown) => {
         // Background spawn failures settle as killed and surface through the read path.
         proc.status = 'killed'
         spawnFailureNote = `spawn failed: ${String(error)}`
-        this.onProcessDone(proc, spawnFailureNote, error)
+        this.onProcessDone(proc, spawnFailureNote, true, error)
       }),
       readOutput: (): BashProcessRead => {
         const out = collected.stdout.readFrom(stdoutOffset)
@@ -292,9 +292,10 @@ export class LocalBashExecutor extends BashExecutor {
    * empty.
    * @param _proc - the settled process handle.
    * @param _stderr - the process's retained stderr tail used by subclasses for settlement classification.
-   * @param _spawnError - the original spawn rejection; absent after a process successfully started.
+   * @param _spawnFailed - whether the subprocess promise rejected before a process started.
+   * @param _spawnError - the original spawn rejection reason, which may itself be undefined.
    */
-  protected onProcessDone(_proc: BashProcess, _stderr: string, _spawnError?: unknown): void {}
+  protected onProcessDone(_proc: BashProcess, _stderr: string, _spawnFailed: boolean, _spawnError?: unknown): void {}
 }
 
 export default LocalBashExecutor
diff --git a/packages/bash/bash-sandbox/src/index.ts b/packages/bash/bash-sandbox/src/index.ts
index 142ab80b1f..8de2369a26 100644
--- a/packages/bash/bash-sandbox/src/index.ts
+++ b/packages/bash/bash-sandbox/src/index.ts
@@ -124,13 +124,13 @@ export class SandboxBashExecutor extends LocalBashExecutor {
    * Stamp per-process sandbox facts before `done` settles. Full-access processes
    * have no facts; signal deaths are not denials.
    */
-  protected override onProcessDone(proc: BashProcess, stderr: string, spawnError?: unknown): void {
+  protected override onProcessDone(proc: BashProcess, stderr: string, spawnFailed: boolean, spawnError?: unknown): void {
     const facts = this.processFacts.get(proc)
     if (facts !== undefined) {
       this.processFacts.delete(proc)
       // A rejected spawn never started the confined launch. Otherwise runner
       // failure outranks denial because its diagnostics may contain denial terms.
-      const runnerFailed = spawnError !== undefined
+      const runnerFailed = spawnFailed
         || classifyRunnerFailure(proc.exitCode, stderr, facts.runnerFailureRules) !== undefined
       proc.sandbox = {
         mode: facts.mode,
@@ -139,7 +139,7 @@ export class SandboxBashExecutor extends LocalBashExecutor {
         ...(runnerFailed ? { runnerFailed } : {}),
       }
     }
-    super.onProcessDone(proc, stderr, spawnError)
+    super.onProcessDone(proc, stderr, spawnFailed, spawnError)
   }
 
   /**
diff --git a/packages/bash/bash-sandbox/tests/sandbox.spec.ts b/packages/bash/bash-sandbox/tests/sandbox.spec.ts
index 10943b167a..6328d96db3 100644
--- a/packages/bash/bash-sandbox/tests/sandbox.spec.ts
+++ b/packages/bash/bash-sandbox/tests/sandbox.spec.ts
@@ -16,6 +16,7 @@ import type { ConfinedArgv, SandboxExecutionPolicy, SandboxMode, SandboxPolicy }
 import { SandboxPolicyService } from '@deepseek-ai/dsh-sandbox-policy'
 import { SandboxBashExecutor } from '@deepseek-ai/dsh-bash-sandbox'
 import LocalSubprocessService from '@deepseek-ai/dsh-subprocess-local'
+import type { SubprocessHandle, SubprocessOutputReader } from '@deepseek-ai/dsh-subprocess'
 import { classifyDenial, classifyRunnerFailure } from '../src/helpers.ts'
 import type { Config } from '@deepseek-ai/dsh-bash-sandbox'
 
@@ -372,6 +373,36 @@ describe('background sandbox facts', () => {
     expect(accounting.size).toBe(0)
   })
 
+  it('classifies a spawn rejection whose reason is undefined', async () => {
+    const { ctx, bash } = await setup()
+    const emptyReader: SubprocessOutputReader = {
+      readFrom: () => ({ text: '', nextOffset: 0, lossy: false }),
+    }
+    vi.spyOn(ctx.subprocess, 'spawn').mockReturnValue({
+      pid: -1,
+      stdin: undefined,
+      stdout: undefined,
+      stderr: undefined,
+      collected: { stdout: emptyReader, stderr: emptyReader },
+      // Arbitrary subprocess providers can reject without a value; that edge is the point of this test.
+      // oxlint-disable-next-line typescript/prefer-promise-reject-errors
+      done: Promise.reject(undefined),
+      terminate: vi.fn(),
+      waitForExit: async () => true,
+    } satisfies SubprocessHandle)
+
+    const task = bash.start(bash.resolve({ command: 'true' }))
+    await task.done
+
+    expect(task.readOutput().delta).toContain('spawn failed: undefined')
+    expect(task.sandbox).toEqual({
+      mode: 'read-only',
+      denied: false,
+      enforcement: 'full',
+      runnerFailed: true,
+    })
+  })
+
   it('stamps a settled denial: nonzero exit + permission stderr under a confined mode', async () => {
     const { bash } = await setup()
     const task = bash.start(bash.resolve({ command: 'echo "x: Permission denied" >&2; exit 1' }))