feat(tests): enhance bash script validation in platform tests

- Added assertions to ensure the presence and order of `set -e` and `set +e` in the generated bash script when a system prompt is included.
- Updated the test for the scenario without a prompt to confirm that `set -e` is not present, ensuring correct script behavior.
- Improved documentation in the `buildBashScript` function to clarify the use of `set -e` for error handling during script execution.

agent/sandbox/v2/claude/platform.go

@@ -68,6 +68,11 @@ func (b *posixBase) ListDirCmd(dir string) []string {
 }
 
 // buildBashScript is the shared bash script builder for macOS/Linux.
+//
+// When a system prompt is present, the script uses `set -e` to ensure that
+// any failure in directory creation or prompt file writing aborts the entire
+// script before Claude CLI is launched. This prevents silent fallback to
+// running without a system prompt.
 func (b *posixBase) buildBashScript(in scriptInput, xauthCmd string) string {
 	var s strings.Builder
 
@@ -76,10 +81,12 @@ func (b *posixBase) buildBashScript(in scriptInput, xauthCmd string) string {
 	}
 
 	if in.systemPrompt != "" {
+		s.WriteString("set -e\n")
 		s.WriteString(fmt.Sprintf("mkdir -p \"$(dirname %q)\"\n", in.promptFile))
 		s.WriteString(fmt.Sprintf("cat << 'PROMPTEOF' > %s\n", in.promptFile))
 		s.WriteString(in.systemPrompt)
 		s.WriteString("\nPROMPTEOF\n")
+		s.WriteString("set +e\n")
 		in.args = append(in.args, "--append-system-prompt-file", in.promptFile)
 	}
 

agent/sandbox/v2/claude/platform_test.go

@@ -82,6 +82,7 @@ func TestPosixBase_BuildBashScript_NoPrompt(t *testing.T) {
 	assert.Contains(t, script, "--verbose")
 	assert.Contains(t, script, "INPUTEOF")
 	assert.NotContains(t, script, "PROMPTEOF")
+	assert.NotContains(t, script, "set -e", "set -e should not be present when no prompt is written")
 }
 
 func TestPosixBase_BuildBashScript_WithPrompt(t *testing.T) {
@@ -98,6 +99,16 @@ func TestPosixBase_BuildBashScript_WithPrompt(t *testing.T) {
 	assert.Contains(t, script, "PROMPTEOF")
 	assert.Contains(t, script, "You are a helpful assistant.")
 	assert.Contains(t, script, "--append-system-prompt-file")
+
+	promptIdx := strings.Index(script, "PROMPTEOF")
+	claudeIdx := strings.Index(script, "claude -p")
+	assert.True(t, strings.Contains(script, "set -e"), "script should enable set -e before prompt write")
+	assert.True(t, strings.Contains(script, "set +e"), "script should disable set -e before claude command")
+	setEIdx := strings.Index(script, "set -e")
+	setNoEIdx := strings.Index(script, "set +e")
+	assert.Less(t, setEIdx, promptIdx, "set -e should come before PROMPTEOF")
+	assert.Less(t, promptIdx, setNoEIdx, "set +e should come after PROMPTEOF")
+	assert.Less(t, setNoEIdx, claudeIdx, "set +e should come before claude -p")
 }
 
 func TestPosixBase_BuildBashScript_WithXauth(t *testing.T) {

agent/sandbox/v2/claude/session.go

@@ -64,7 +64,27 @@ func (s *session) runStream(handler message.StreamFunc) (completed bool, err err
 		return true, nil
 	}
 
-	return false, s.waitForExit(parseErr)
+	exitErr := s.waitForExit(parseErr)
+	if exitErr != nil {
+		if handler != nil {
+			handler(message.ChunkError, []byte(exitErr.Error()))
+		}
+		return false, exitErr
+	}
+
+	s.stderrMu.Lock()
+	stderrStr := strings.TrimSpace(s.stderr.String())
+	s.stderrMu.Unlock()
+	if stderrStr != "" {
+		s.logger.Warn("claude exited with code 0 but stream incomplete and stderr present: %s", stderrStr)
+		errMsg := fmt.Errorf("claude CLI setup failed: %s", stderrStr)
+		if handler != nil {
+			handler(message.ChunkError, []byte(errMsg.Error()))
+		}
+		return false, errMsg
+	}
+
+	return false, nil
 }
 
 // collectStderr reads stderr in a background goroutine.