What causes "npm pack: failed to build: %w" in gohugoio/hugo?

Inspect the module package files (the walk covers _jsconfig mounts of package.json/package.hugo.json) and validate each as JSON.

gohugoio/hugo · error

npm pack: failed to build

Error message

npm pack: failed to build: %w

What it means

In npm.Pack (modules/npm/package_builder.go:175). After processing all collected package entries via b.Add, if the builder accumulated any error in b.err it is wrapped as 'failed to build'. This is the aggregate/catch-all for a parse failure that occurred while adding a module's package file during the walk phase.

Solutions

Inspect the module package files (the walk covers _jsconfig mounts of package.json/package.hugo.json) and validate each as JSON.
Run 'hugo mod clean' and 'hugo mod tidy' to refresh modules.
Temporarily set the offending module's usePackageJSON to 'never' to skip it.

Example fix

// before: a theme module ships
{ "name": "theme", "dependencies": { "x": } }
// -> 'npm pack: failed to build: invalid character '}' ...'

// after: fix the theme's package.json (or upgrade the theme)
{ "name": "theme", "dependencies": { "x": "^1.0" } }

Defensive patterns

Strategy: validation

Validate before calling

// validate every module's package.json/package.hugo.json as JSON
for _, m := range modules {
    for _, name := range []string{"package.json", "package.hugo.json"} {
        p := filepath.Join(m.Dir(), name)
        if data, err := os.ReadFile(p); err == nil {
            var v any
            if err := json.Unmarshal(data, &v); err != nil {
                return fmt.Errorf("%s: %w", p, err)
            }
        }
    }
}

Prevention

Validate module package files after 'hugo mod tidy'.
Set usePackageJSON='never' on modules you don't want merged.
Report broken package files to the module/theme author.

When it happens

Trigger: Any module package file (package.json or package.hugo.json mounted under _jsconfig) fails json.Unmarshal inside b.Add -> b.unmarshal, and the error surfaces here rather than at a specific earlier check.

Common situations: A theme/module ships a malformed package.json that passes the existence checks but fails JSON parsing; a stale module cache with a half-written package.json; a module whose package.hugo.json was hand-edited incorrectly.

Related errors

AI-assisted analysis of gohugoio/hugo@52c9bd7908 (2026-08-09). Data as JSON: /api/errors/e169a862347ed134. Report an issue: GitHub.

Appendix: source

Thrown at modules/npm/package_builder.go:175

	// Process collected entries: for each module, prefer package.hugo.json
	// over package.json at the root level. Workspace package.json files are always processed.
	for _, e := range entries {
		m := e.info.Meta()
		// Skip root-level package.json if this module has package.hugo.json.
		if !e.isHugoJSON && e.isRootLevel && modulesWithHugoJSON[m.ModulePath()] {
			continue
		}

		f, err := m.Open()
		if err != nil {
			return fmt.Errorf("npm pack: failed to open package file: %w", err)
		}
		b.Add(m.ModulePath(), f)
		f.Close()
	}

	if b.Err() != nil {
		return fmt.Errorf("npm pack: failed to build: %w", b.Err())
	}

	// 4. Build the autogenerated workspace package.json.
	// Exclude deps already defined by the project itself — they don't
	// need to be duplicated in the workspace and it simplifies maintenance.
	moduleDeps := make(map[string]any)
	moduleDepsComments := make(map[string]any)
	for k, v := range b.dependencies {
		if b.dependenciesComments[k] != "project" {
			moduleDeps[k] = v
			moduleDepsComments[k] = b.dependenciesComments[k]
		}
	}
	moduleDevDeps := make(map[string]any)
	moduleDevDepsComments := make(map[string]any)
	for k, v := range b.devDependencies {
		if b.devDependenciesComments[k] != "project" {
			moduleDevDeps[k] = v

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