diff --git a/packages/cli/src/commands/sandbox/exec.ts b/packages/cli/src/commands/sandbox/exec.ts
index a5c62df0..5d9231d4 100644
--- a/packages/cli/src/commands/sandbox/exec.ts
+++ b/packages/cli/src/commands/sandbox/exec.ts
@@ -2,7 +2,12 @@
  * Execute a command in a running sandbox.
  */
 
-import { Sandbox, CommandExitError, SandboxNotFoundError } from '@abox-dev/sdk'
+import {
+  Sandbox,
+  CommandExitError,
+  CommandNotFoundError,
+  SandboxNotFoundError,
+} from '@abox-dev/sdk'
 import * as commander from 'commander'
 
 import { ensureAPIKey } from '../../api'
@@ -182,7 +187,10 @@ async function sendStdin(sandbox: Sandbox, pid: number): Promise<void> {
       try {
         await sandbox.commands.sendStdin(pid, chunk)
       } catch (err) {
-        if (err instanceof SandboxNotFoundError) {
+        if (
+          err instanceof CommandNotFoundError ||
+          err instanceof SandboxNotFoundError
+        ) {
           processExited = true
           console.error(
             'agentbox: Remote command exited before stdin could be delivered.'
@@ -203,7 +211,10 @@ async function sendStdin(sandbox: Sandbox, pid: number): Promise<void> {
   try {
     await sandbox.commands.closeStdin(pid)
   } catch (err) {
-    if (err instanceof SandboxNotFoundError) {
+    if (
+      err instanceof CommandNotFoundError ||
+      err instanceof SandboxNotFoundError
+    ) {
       // Process already exited — EOF is moot.
       return
     }
diff --git a/packages/cli/tests/commands/sandbox/exec_close_stdin.test.ts b/packages/cli/tests/commands/sandbox/exec_close_stdin.test.ts
index 6bb41f54..a19bab16 100644
--- a/packages/cli/tests/commands/sandbox/exec_close_stdin.test.ts
+++ b/packages/cli/tests/commands/sandbox/exec_close_stdin.test.ts
@@ -36,6 +36,7 @@ vi.mock('@abox-dev/sdk', () => {
   }
 
   class SandboxNotFoundError extends Error {}
+  class CommandNotFoundError extends Error {}
 
   return {
     Sandbox: {
@@ -43,6 +44,7 @@ vi.mock('@abox-dev/sdk', () => {
     },
     CommandExitError,
     SandboxNotFoundError,
+    CommandNotFoundError,
   }
 })
 
@@ -131,10 +133,10 @@ describe('sandbox exec closeStdin handling', () => {
     expect(exitSpy).toHaveBeenCalledWith(1)
   })
 
-  test('keeps SandboxNotFoundError from closeStdin non-fatal', async () => {
-    const { SandboxNotFoundError } = await import('@abox-dev/sdk')
+  test('keeps CommandNotFoundError from closeStdin non-fatal', async () => {
+    const { CommandNotFoundError } = await import('@abox-dev/sdk')
     mocks.closeStdin.mockRejectedValue(
-      new SandboxNotFoundError('already exited')
+      new CommandNotFoundError('already exited')
     )
 
     const exitSpy = vi
@@ -153,10 +155,10 @@ describe('sandbox exec closeStdin handling', () => {
     expect(exitSpy).toHaveBeenCalledWith(0)
   })
 
-  test('stops stdin streaming after SandboxNotFoundError from sendStdin', async () => {
-    const { SandboxNotFoundError } = await import('@abox-dev/sdk')
+  test('stops stdin streaming after CommandNotFoundError from sendStdin', async () => {
+    const { CommandNotFoundError } = await import('@abox-dev/sdk')
     mocks.sendStdin.mockRejectedValueOnce(
-      new SandboxNotFoundError('already exited')
+      new CommandNotFoundError('already exited')
     )
     mocks.streamStdinChunks.mockImplementation(
       async (
diff --git a/packages/cli/tests/commands/sandbox/exec_passthrough.test.ts b/packages/cli/tests/commands/sandbox/exec_passthrough.test.ts
index d02d8f1f..5f2418ed 100644
--- a/packages/cli/tests/commands/sandbox/exec_passthrough.test.ts
+++ b/packages/cli/tests/commands/sandbox/exec_passthrough.test.ts
@@ -26,6 +26,7 @@ vi.mock('@abox-dev/sdk', () => {
   }
 
   class SandboxNotFoundError extends Error {}
+  class CommandNotFoundError extends Error {}
 
   return {
     Sandbox: {
@@ -33,6 +34,7 @@ vi.mock('@abox-dev/sdk', () => {
     },
     CommandExitError,
     SandboxNotFoundError,
+    CommandNotFoundError,
   }
 })
 
diff --git a/packages/cli/tests/commands/sandbox/exec_stdin_flag.test.ts b/packages/cli/tests/commands/sandbox/exec_stdin_flag.test.ts
index 9c46020d..544ee276 100644
--- a/packages/cli/tests/commands/sandbox/exec_stdin_flag.test.ts
+++ b/packages/cli/tests/commands/sandbox/exec_stdin_flag.test.ts
@@ -32,6 +32,7 @@ vi.mock('@abox-dev/sdk', () => {
   }
 
   class SandboxNotFoundError extends Error {}
+  class CommandNotFoundError extends Error {}
 
   return {
     Sandbox: {
@@ -39,6 +40,7 @@ vi.mock('@abox-dev/sdk', () => {
     },
     CommandExitError,
     SandboxNotFoundError,
+    CommandNotFoundError,
   }
 })
 
diff --git a/packages/js-sdk/src/errors.ts b/packages/js-sdk/src/errors.ts
index 79954452..9b8e1fc2 100644
--- a/packages/js-sdk/src/errors.ts
+++ b/packages/js-sdk/src/errors.ts
@@ -78,6 +78,16 @@ export class SandboxNotFoundError extends SandboxError {
   }
 }
 
+/**
+ * Thrown when a command process has already exited or its PID does not exist.
+ */
+export class CommandNotFoundError extends SandboxError {
+  constructor(message: string) {
+    super(message)
+    this.name = 'CommandNotFoundError'
+  }
+}
+
 /**
  * Thrown when authentication fails.
  */
diff --git a/packages/js-sdk/src/index.ts b/packages/js-sdk/src/index.ts
index a2f0664a..ecbfb2d1 100644
--- a/packages/js-sdk/src/index.ts
+++ b/packages/js-sdk/src/index.ts
@@ -10,6 +10,7 @@ export {
   NotEnoughSpaceError,
   SandboxError,
   SandboxNotFoundError,
+  CommandNotFoundError,
   TemplateError,
   TimeoutError,
   RateLimitError,
diff --git a/packages/js-sdk/src/sandbox/commands/index.ts b/packages/js-sdk/src/sandbox/commands/index.ts
index bbc92241..f85955dc 100644
--- a/packages/js-sdk/src/sandbox/commands/index.ts
+++ b/packages/js-sdk/src/sandbox/commands/index.ts
@@ -30,7 +30,7 @@ import {
   SandboxHealthCheck,
 } from '../../envd/rpc'
 import { ENVD_COMMANDS_STDIN, ENVD_ENVD_CLOSE } from '../../envd/versions'
-import { SandboxError } from '../../errors'
+import { CommandNotFoundError, SandboxError } from '../../errors'
 import { CommandHandle, CommandResult } from './commandHandle'
 export { Pty } from './pty'
 
@@ -228,7 +228,9 @@ export class Commands {
         }
       )
     } catch (err) {
-      throw await handleRpcErrorWithHealthCheck(err, this.checkHealth)
+      throw await handleRpcErrorWithHealthCheck(err, this.checkHealth, {
+        [Code.NotFound]: (message) => new CommandNotFoundError(message),
+      })
     }
   }
 
@@ -265,7 +267,9 @@ export class Commands {
         }
       )
     } catch (err) {
-      throw await handleRpcErrorWithHealthCheck(err, this.checkHealth)
+      throw await handleRpcErrorWithHealthCheck(err, this.checkHealth, {
+        [Code.NotFound]: (message) => new CommandNotFoundError(message),
+      })
     }
   }
 
diff --git a/packages/js-sdk/tests/envd/commandProcessNotFound.test.ts b/packages/js-sdk/tests/envd/commandProcessNotFound.test.ts
new file mode 100644
index 00000000..1b6db782
--- /dev/null
+++ b/packages/js-sdk/tests/envd/commandProcessNotFound.test.ts
@@ -0,0 +1,32 @@
+import { Code, ConnectError } from '@connectrpc/connect'
+import { describe, expect, it, vi } from 'vitest'
+
+import { CommandNotFoundError } from '../../src/errors'
+import { Commands } from '../../src/sandbox/commands'
+
+function commandsWithMissingProcess(): Commands {
+  const missing = () =>
+    Promise.reject(new ConnectError('process not found', Code.NotFound))
+  const commands = Object.create(Commands.prototype) as Commands
+  Object.assign(commands, {
+    rpc: { sendInput: vi.fn(missing), closeStdin: vi.fn(missing) },
+    envdVersion: '0.6.15',
+    connectionConfig: { getSignal: () => undefined },
+    checkHealth: async () => true,
+  })
+  return commands
+}
+
+describe('command process disappearance', () => {
+  it('identifies a missing PID while sending stdin', async () => {
+    await expect(
+      commandsWithMissingProcess().sendStdin(1234, 'input')
+    ).rejects.toBeInstanceOf(CommandNotFoundError)
+  })
+
+  it('identifies a missing PID while closing stdin', async () => {
+    await expect(
+      commandsWithMissingProcess().closeStdin(1234)
+    ).rejects.toBeInstanceOf(CommandNotFoundError)
+  })
+})