fix(model): report backend crash diagnostics (#11532)

Unexpected runtime exits only reported an exit code, which hid the backend diagnostic. Include the final non-empty stderr line when one exists.

Assisted-by: Codex:gpt-5

Co-authored-by: localai-org-maint-bot <306269227+localai-org-maint-bot@users.noreply.github.com>

pkg/model/process.go

@@ -350,6 +350,9 @@ func (ml *ModelLoader) startProcess(grpcProcess, id string, serverAddress string
 		} else {
 			// A stop we didn't initiate — a SIGSEGV from a missing shared
 			// library, a Python ImportError, an OOM kill, an unexpected self-exit.
+			if diagnostic := lastNonEmptyLine(grpcControlProcess.StderrPath(), startupStderrTailBytes); diagnostic != "" {
+				fields = append(fields, "stderr", diagnostic)
+			}
 			xlog.Warn("Backend process exited unexpectedly", fields...)
 		}
 	}()

pkg/model/process_exit_test.go

@@ -0,0 +1,38 @@
+package model
+
+import (
+	"bytes"
+	"log/slog"
+	"os"
+	"path/filepath"
+
+	"github.com/mudler/LocalAI/pkg/system"
+	"github.com/mudler/xlog"
+	. "github.com/onsi/ginkgo/v2"
+	. "github.com/onsi/gomega"
+)
+
+var _ = Describe("backend process exit diagnostics", func() {
+	It("includes the exit code and final stderr line for an unexpected exit", func() {
+		tmpDir := GinkgoT().TempDir()
+		backendPath := filepath.Join(tmpDir, "failing-backend")
+		Expect(os.WriteFile(backendPath, []byte("#!/bin/sh\necho 'first diagnostic' >&2\necho 'fatal metal pipeline error' >&2\nexit 42\n"), 0o700)).To(Succeed())
+
+		captured := &bytes.Buffer{}
+		handler := slog.NewTextHandler(captured, &slog.HandlerOptions{Level: slog.LevelWarn})
+		xlog.SetLogger(xlog.NewLoggerWithHandler(handler, xlog.LogLevelWarn))
+		DeferCleanup(func() {
+			xlog.SetLogger(xlog.NewLogger(xlog.LogLevel("info"), "text"))
+		})
+
+		loader := NewModelLoader(&system.SystemState{Model: system.Model{ModelsPath: tmpDir}})
+		process, err := loader.startProcess(backendPath, "test-model", "127.0.0.1:65535")
+		Expect(err).ToNot(HaveOccurred())
+		Eventually(process.Done()).Should(BeClosed())
+		Eventually(captured.String).Should(And(
+			ContainSubstring("Backend process exited unexpectedly"),
+			ContainSubstring("exitCode=42"),
+			ContainSubstring(`stderr="fatal metal pipeline error"`),
+		))
+	})
+})