feat(http): honor explicit external base URL in BaseURL

When _external_base_url is set in the request context it dictates the
origin (scheme+host+port); the proxy path prefix is still appended.

Refs #10482

Assisted-by: Claude:claude-opus-4-8
Signed-off-by: Ettore Di Giacinto <mudler@localai.io>

core/http/middleware/baseurl.go

@@ -55,6 +55,14 @@ func BasePathPrefix(c echo.Context) string {
 // The returned URL is guaranteed to end with `/`.
 // The method should be used in conjunction with the StripPathPrefix middleware.
 func BaseURL(c echo.Context) string {
+	// An explicit external base URL (LOCALAI_BASE_URL) is authoritative for
+	// the origin. The proxy-derived path prefix is still appended so a
+	// reverse-proxy mount point keeps working. Trailing slashes are
+	// normalized via BasePathPrefix, which always starts and ends with "/".
+	if ext, ok := c.Get("_external_base_url").(string); ok && ext != "" {
+		return strings.TrimRight(ext, "/") + BasePathPrefix(c)
+	}
+
 	fwdProto, fwdHost := parseForwarded(c.Request().Header.Get("Forwarded"))
 
 	scheme := "http"

core/http/middleware/baseurl_test.go

@@ -180,4 +180,51 @@ var _ = Describe("BaseURL", func() {
 			Expect(actualURL).To(Equal("https://xfh.example/"))
 		})
 	})
+
+	Context("explicit external base URL override", func() {
+		It("uses the configured origin over conflicting forwarded headers", func() {
+			app := echo.New()
+			actualURL := ""
+			app.GET("/x", func(c echo.Context) error {
+				c.Set("_external_base_url", "https://192.168.0.13:34567")
+				actualURL = BaseURL(c)
+				return nil
+			})
+			req := httptest.NewRequest("GET", "/x", nil)
+			req.Header.Set("X-Forwarded-Proto", "http")
+			req.Header.Set("X-Forwarded-Host", "internal:8080")
+			rec := httptest.NewRecorder()
+			app.ServeHTTP(rec, req)
+			Expect(actualURL).To(Equal("https://192.168.0.13:34567/"))
+		})
+
+		It("combines the configured origin with a detected path prefix", func() {
+			app := echo.New()
+			actualURL := ""
+			app.GET("/hello", func(c echo.Context) error {
+				c.Set("_original_path", "/localai/hello")
+				c.Set("_external_base_url", "https://ext.example")
+				actualURL = BaseURL(c)
+				return nil
+			})
+			req := httptest.NewRequest("GET", "/hello", nil)
+			rec := httptest.NewRecorder()
+			app.ServeHTTP(rec, req)
+			Expect(actualURL).To(Equal("https://ext.example/localai/"))
+		})
+
+		It("ignores an empty override", func() {
+			app := echo.New()
+			actualURL := ""
+			app.GET("/x", func(c echo.Context) error {
+				c.Set("_external_base_url", "")
+				actualURL = BaseURL(c)
+				return nil
+			})
+			req := httptest.NewRequest("GET", "/x", nil)
+			rec := httptest.NewRecorder()
+			app.ServeHTTP(rec, req)
+			Expect(actualURL).To(Equal("http://example.com/"))
+		})
+	})
 })