diff --git a/packages/shell/bash-local/src/index.ts b/packages/shell/bash-local/src/index.ts
index ab021be141..0ab4c03964 100644
--- a/packages/shell/bash-local/src/index.ts
+++ b/packages/shell/bash-local/src/index.ts
@@ -285,7 +285,13 @@ export class LocalBashExecutor extends ShellExecutor {
       }, (error: unknown) => {
         // Background provider failures settle as killed and surface through the read path.
         proc.status = 'killed'
-        providerFailureNote = `subprocess failed before reporting an outcome: ${String(error)}`
+        let detail = 'unprintable provider failure'
+        try {
+          detail = String(error)
+        } catch {
+          // Provider-owned rejection values cannot make ShellProcess.done reject.
+        }
+        providerFailureNote = `subprocess failed before reporting an outcome: ${detail}`
         this.onProcessDone(proc, providerFailureNote, true, error)
       }),
       readOutput: (): ShellProcessRead => {
diff --git a/packages/shell/bash-local/tests/executor.spec.ts b/packages/shell/bash-local/tests/executor.spec.ts
index 7668012964..9cc63475da 100644
--- a/packages/shell/bash-local/tests/executor.spec.ts
+++ b/packages/shell/bash-local/tests/executor.spec.ts
@@ -322,6 +322,32 @@ describe('LocalBashExecutor.start (background process handles)', () => {
     expect(proc.readOutput().delta).toBe('')
   })
 
+  it('settles an unprintable provider rejection instead of rejecting done', async () => {
+    const { ctx, bash } = await setup()
+    const emptyReader: SubprocessOutputReader = {
+      readFrom: () => ({ text: '', nextOffset: 0, lossy: false }),
+    }
+    const providerError = new Error('unprintable provider error')
+    Object.defineProperty(providerError, Symbol.toPrimitive, {
+      value: () => { throw new Error('provider formatting must not escape') },
+    })
+    vi.spyOn(ctx.subprocess, 'spawn').mockReturnValue({
+      stdin: undefined,
+      stdout: undefined,
+      stderr: undefined,
+      collected: { stdout: emptyReader, stderr: emptyReader },
+      done: Promise.reject(providerError),
+      terminate: vi.fn(),
+      waitForExit: async () => true,
+    } satisfies SubprocessHandle)
+
+    const proc = bash.start(bash.resolve({ command: 'true' }))
+    await expect(proc.done).resolves.toBeUndefined()
+    expect(proc.status).toBe('killed')
+    expect(proc.readOutput().delta).toContain('unprintable provider failure')
+    expect(proc.readOutput().delta).toBe('')
+  })
+
   it('an asynchronous creation failure settles as killed with a stage-neutral note', async () => {
     const { bash } = await setup()
     const proc = bash.start(bash.resolve({ command: 'true', workdir: '/nonexistent-dsh' }))
diff --git a/packages/shell/pwsh-local/src/index.ts b/packages/shell/pwsh-local/src/index.ts
index 92770a2742..99d7d1564b 100644
--- a/packages/shell/pwsh-local/src/index.ts
+++ b/packages/shell/pwsh-local/src/index.ts
@@ -314,7 +314,13 @@ export class PwshLocalExecutor extends ShellExecutor {
       }, (error: unknown) => {
         // Background provider failures settle as killed and surface through the read path.
         proc.status = 'killed'
-        providerFailureNote = `subprocess failed before reporting an outcome: ${String(error)}`
+        let detail = 'unprintable provider failure'
+        try {
+          detail = String(error)
+        } catch {
+          // Provider-owned rejection values cannot make ShellProcess.done reject.
+        }
+        providerFailureNote = `subprocess failed before reporting an outcome: ${detail}`
         this.onProcessDone(proc, providerFailureNote, true, error)
       }),
       readOutput: (): ShellProcessRead => {
diff --git a/packages/shell/pwsh-local/tests/executor.spec.ts b/packages/shell/pwsh-local/tests/executor.spec.ts
index 341826fa5a..3997ac5787 100644
--- a/packages/shell/pwsh-local/tests/executor.spec.ts
+++ b/packages/shell/pwsh-local/tests/executor.spec.ts
@@ -212,6 +212,23 @@ describe('spawn construction (pure, every platform)', () => {
     expect(output).not.toContain('spawn failed:')
     expect(proc.readOutput().delta).toBe('')
   })
+
+  it('settles an unprintable provider rejection instead of rejecting done', async () => {
+    const ctx = new Context()
+    const subprocess = new CapturingSubprocessRuntime(ctx)
+    await ctx.plugin(PwshLocalExecutor)
+    const providerError = new Error('unprintable provider error')
+    Object.defineProperty(providerError, Symbol.toPrimitive, {
+      value: () => { throw new Error('provider formatting must not escape') },
+    })
+    subprocess.done = Promise.reject(providerError)
+
+    const proc = ctx.shell.start(ctx.shell.resolve({ command: 'Write-Output maybe-ran' }))
+    await expect(proc.done).resolves.toBeUndefined()
+    expect(proc.status).toBe('killed')
+    expect(proc.readOutput().delta).toContain('unprintable provider failure')
+    expect(proc.readOutput().delta).toBe('')
+  })
 })
 
 describe.skipIf(!hasPwsh)('PwshLocalExecutor.run', () => {
diff --git a/packages/subprocess/subprocess-local/src/linux-scope.ts b/packages/subprocess/subprocess-local/src/linux-scope.ts
index 80cd06c781..7352e55623 100644
--- a/packages/subprocess/subprocess-local/src/linux-scope.ts
+++ b/packages/subprocess/subprocess-local/src/linux-scope.ts
@@ -156,7 +156,7 @@ interface DirectRange {
 }
 
 class SystemdScopeOwner implements BoundProcessOwner {
-  private establishment: 'pending' | 'established' | 'never-created' = 'pending'
+  private establishment: 'pending' | 'established' = 'pending'
   private stopped = false
   private observation: Promise<void> | undefined
   private killFailure: Error | undefined
@@ -229,7 +229,6 @@ class SystemdScopeOwner implements BoundProcessOwner {
     this.observeRequestConsumption()
     if (this.establishment === 'established') return false
     if (!this.direct.running() && existsSync(this.files.requestPath)) {
-      this.establishment = 'never-created'
       return false
     }
     if (this.killFailure !== undefined) throw this.killFailure
diff --git a/packages/subprocess/subprocess-local/src/spawn.ts b/packages/subprocess/subprocess-local/src/spawn.ts
index b17787a08e..b203ea4460 100644
--- a/packages/subprocess/subprocess-local/src/spawn.ts
+++ b/packages/subprocess/subprocess-local/src/spawn.ts
@@ -344,7 +344,13 @@ export function validateSubprocessSpec(spec: SubprocessSpawnSpec): void {
     throw new Error(`subprocess graceMs must be a positive finite number no greater than ${MAX_TIMER_DELAY_MS}`)
   }
   if (spec.signal?.aborted) {
-    throw new Error(`aborted before spawn: ${String(spec.signal.reason ?? 'aborted')}`)
+    let reason = 'aborted'
+    try {
+      reason = String(spec.signal.reason ?? reason)
+    } catch {
+      // Arbitrary caller-owned reasons cannot escape the stable Error boundary.
+    }
+    throw new Error(`aborted before spawn: ${reason}`)
   }
   const [program] = spec.argv
   if (program === undefined || program.length === 0) {
diff --git a/packages/subprocess/subprocess-local/src/windows-job.ts b/packages/subprocess/subprocess-local/src/windows-job.ts
index a2ef3262c1..d6d89b53de 100644
--- a/packages/subprocess/subprocess-local/src/windows-job.ts
+++ b/packages/subprocess/subprocess-local/src/windows-job.ts
@@ -141,7 +141,6 @@ export function launchWindowsJob(
   const rangeExit = Promise.withResolvers<void>()
   let directResultType: WindowsRunnerResult['type'] | undefined
   let runnerSpawned = false
-  let runnerNeverCreated = false
   const failInfrastructure = (error: unknown): void => {
     direct.reject(error)
     rangeExit.reject(error)
@@ -193,7 +192,6 @@ export function launchWindowsJob(
   })
   child.once('error', (error) => {
     if (!runnerSpawned) {
-      runnerNeverCreated = true
       direct.reject(error)
       rangeExit.resolve()
       return
@@ -201,7 +199,7 @@ export function launchWindowsJob(
     failInfrastructure(error)
   })
   child.once('close', (exitCode, signal) => {
-    if (runnerNeverCreated) return
+    if (!runnerSpawned) return
     const clean = exitCode === 0 && signal === null && directResultType !== undefined
     if (clean) {
       rangeExit.resolve()
diff --git a/packages/subprocess/subprocess-local/tests/spawn.spec.ts b/packages/subprocess/subprocess-local/tests/spawn.spec.ts
index f268fd16e2..5916790b3b 100644
--- a/packages/subprocess/subprocess-local/tests/spawn.spec.ts
+++ b/packages/subprocess/subprocess-local/tests/spawn.spec.ts
@@ -323,6 +323,15 @@ describe('spawnSubprocess', () => {
       expect(() => { validateSubprocessSpec(spec('echo hi', { signal: controller.signal })) })
         .toThrow(new Error(message))
     }
+
+    const controller = new AbortController()
+    controller.abort({
+      [Symbol.toPrimitive]() {
+        throw new Error('reason formatting must not escape')
+      },
+    })
+    expect(() => { validateSubprocessSpec(spec('echo hi', { signal: controller.signal })) })
+      .toThrow(new Error('aborted before spawn: aborted'))
   })
 
   it('rejects with a spawn error for a nonexistent cwd', async () => {