apple/pkl · error · PklException

noResourceReaderRegistered

noResourceReaderRegistered

Error message

noResourceReaderRegistered

What it means

This evalError is thrown when Pkl code tries to read a resource from a local (file-backed) project dependency whose URI scheme has no registered ResourceReader in the ResourceManager. ResourceReaders resolves the dependency to a local URI, performs a security check, then asks the resource manager for a reader; a null reader means the scheme is unsupported (e.g. no reader registered for 'file' or a custom scheme). It is a configuration/evaluation-environment problem, not a data problem.

Source

Thrown at pkl-core/src/main/java/org/pkl/core/resource/ResourceReaders.java:556

    @Override
    public String getUriScheme() {
      return "projectpackage";
    }

    @Override
    public Optional<Object> read(URI uri)
        throws IOException, URISyntaxException, SecurityManagerException {
      var assetUri = new PackageAssetUri(uri);
      var dependency = getProjectDepsResolver().getResolvedDependency(assetUri.getPackageUri());
      var local = getLocalUri(dependency, assetUri);
      if (local != null) {
        var resourceManager = VmContext.get(null).getResourceManager();
        var securityManager = VmContext.get(null).getSecurityManager();
        securityManager.checkReadResource(local);
        var reader = resourceManager.getResourceReader(local);
        if (reader == null) {
          throw new VmExceptionBuilder()
              .evalError("noResourceReaderRegistered", uri.getScheme())
              .build();
        }
        return resourceManager.doRead(reader, local, null);
      }
      var remoteDep = (Dependency.RemoteDependency) dependency;
      var bytes = getPackageResolver().getBytes(assetUri, true, remoteDep.getChecksums());
      return Optional.of(new Resource(uri, bytes));
    }

    @Override
    public boolean hasHierarchicalUris() {
      return true;
    }

    @Override
    public boolean isGlobbable() {
      return true;

View on GitHub (pinned to f3efcbfc9b)

Solutions

  1. Register a ResourceReader for the missing scheme when building the evaluator (e.g. EvaluatorBuilder.resourceReaders / ResourceManager registration).
  2. Check the URI scheme of the resource being read; if it is a custom scheme, add the matching reader or switch the resource to a supported scheme (e.g. file/https).
  3. If embedding Pkl, verify the same reader set used by the CLI is attached to your EvaluatorBuilder.

Example fix

// before (embedded eval, no readers)
EvaluatorBuilder.withDefault();
// after
EvaluatorBuilder.withDefault()
    .resourceReaders(new FileResourceReader());
Defensive patterns

Strategy: validation

Validate before calling

// in host code, before eval
var supported = Set.of("file", "https", "env", "projectpackage");
var scheme = URI.create(resourceUri).getScheme();
if (scheme == null || !supported.contains(scheme)) throw new IllegalArgumentException("no resource reader for scheme: " + scheme);

Prevention

When it happens

Trigger: Calling `read(...)` on a resource whose resolved dependency URI has a scheme with no reader registered; occurs in the read path when getResourceReader(local) returns null right after securityManager.checkReadResource(local) succeeds.

Common situations: Running Pkl embedded with a custom evaluator that omitted a resource reader (e.g. file or a custom scheme reader); reading package resources under a scheme the host environment does not support; security manager allows the read but no reader plugin is installed.

Understand the failure class

Background: "not installed", "pip install", "required for": how missing-dependency errors surface across open-source libraries — this error's family across 34 libraries.

Related errors

AI-assisted analysis of apple/pkl@f3efcbfc9b (2026-09-08). Data as JSON: /api/errors/1ef78be95cec3654. Report an issue: GitHub.