diff --git a/AGENTS.md b/AGENTS.md
index 22febd9db3c..8bc6860c642 100644
--- a/AGENTS.md
+++ b/AGENTS.md
@@ -8,6 +8,7 @@
 * Avoid global state at (almost) all cost.
 * This is a project with a long history; assume that a similiar problem has been solved before, look hard for helper functions before creating new ones.
 * In tests, use `qt` matchers (e.g. `b.Assert(err, qt.ErrorMatches, ...)`) instead of raw `if`/`t.Fatal` checks.
+* Brevity is good. This applies to code, comments and commit messages. Don't write a novel.
 * Use `./check.sh ./somepackage/...` when iterating.
 * Use `./check.sh` when you're done.
 
diff --git a/hugolib/hugo_modules_test.go b/hugolib/hugo_modules_test.go
index cc0bde639f4..450e4cbce88 100644
--- a/hugolib/hugo_modules_test.go
+++ b/hugolib/hugo_modules_test.go
@@ -267,6 +267,29 @@ Home: {{ .Title }}|{{ .Content }}|
 	b.AssertFileContent("public/index.html", "Home: |<h1 id=\"hello-world\">Hello World</h1>\n|")
 }
 
+// https://github.com/gohugoio/hugo/issues/14543
+func TestModulePrivateRepo(t *testing.T) {
+	t.Parallel()
+
+	files := `
+-- hugo.toml --
+baseURL = "https://example.com/"
+[module]
+[[module.imports]]
+path = "github.com/bep/this-is-a-non-existing-repo"
+version = "main"
+[[module.imports.mounts]]
+source = "content"
+target = "content"
+-- layouts/all.html --
+Title: {{ .Title }}|
+List: {{ .Title }}
+`
+
+	b, err := TestE(t, files, TestOptOsFs())
+	b.Assert(err, qt.ErrorMatches, `(?s).*mod download.*invalid version.*repository.*`)
+}
+
 // https://github.com/gohugoio/hugo/issues/6299
 func TestSiteWithGoModButNoModules(t *testing.T) {
 	t.Parallel()
diff --git a/modules/client.go b/modules/client.go
index dbad4d35cb8..968d1050014 100644
--- a/modules/client.go
+++ b/modules/client.go
@@ -458,6 +458,11 @@ func (c *Client) downloadModuleVersion(path, version string) (*goModule, error)
 	b := &bytes.Buffer{}
 	err := c.runGo(context.Background(), b, args...)
 	if err != nil {
+		// The -json flag makes Go output error details as JSON to stdout
+		// even on failure. Try to extract the error message from the JSON output.
+		if jsonErr := extractGoModDownloadError(b.Bytes()); jsonErr != "" {
+			return nil, fmt.Errorf("failed to download module %s@%s: %s: %s", path, version, err, jsonErr)
+		}
 		return nil, fmt.Errorf("failed to download module %s@%s: %w", path, version, err)
 	}
 
@@ -965,6 +970,18 @@ type goModuleError struct {
 	Err string // the error itself
 }
 
+func extractGoModDownloadError(b []byte) string {
+	// go mod download -json outputs Error as a plain string,
+	// unlike go list -m -json which uses {"Err": "..."}.
+	var m struct {
+		Error string
+	}
+	if err := json.Unmarshal(b, &m); err == nil && m.Error != "" {
+		return m.Error
+	}
+	return ""
+}
+
 type goModules []*goModule
 
 type modSum struct {