apple/pkl · error · RendererException

Values of type `Bytes` cannot be rendered as JSON. Value: %s

Error message

Values of type `Bytes` cannot be rendered as JSON. Value: %s

What it means

JsonRenderer throws this RendererException when asked to render Pkl Bytes (a byte array) as JSON, because raw binary has no standard JSON encoding. The value's byte[] is included in the message via its string form.

Source

Thrown at pkl-core/src/main/java/org/pkl/core/JsonRenderer.java:107

        throw new UncheckedIOException(e);
      }
    }

    @Override
    public void visitDuration(Duration value) {
      throw new RendererException(
          String.format("Values of type `Duration` cannot be rendered as JSON. Value: %s", value));
    }

    @Override
    public void visitDataSize(DataSize value) {
      throw new RendererException(
          String.format("Values of type `DataSize` cannot be rendered as JSON. Value: %s", value));
    }

    @Override
    public void visitBytes(byte[] value) {
      throw new RendererException(
          String.format(
              "Values of type `Bytes` cannot be rendered as JSON. Value: %s", (Object) value));
    }

    // Pair coming from the runtime can never admit null (in-language null is represented as PNull)
    @SuppressWarnings("DataFlowIssue")
    @Override
    public void visitPair(Pair<?, ?> value) {
      try {
        writer.beginArray();
        visit(value.getFirst());
        visit(value.getSecond());
        writer.endArray();
      } catch (IOException e) {
        throw new UncheckedIOException(e);
      }
    }

View on GitHub (pinned to f3efcbfc9b)

Solutions

1. Encode the Bytes as base64 (or hex) text before rendering
2. Use prop type String/Listing<String> with an explicit encoding instead of Bytes
3. Exclude binary fields from the JSON output

Example fix

// before
prop key: Bytes = ...
// after
prop keyBase64: String = key.base64Encoded

if (value instanceof byte[] || "Bytes".equals(value.getClass().getSimpleName())) {
  throw new IllegalArgumentException("encode Bytes (e.g. base64) before JSON rendering");
}

boolean isJsonRenderable(Object v) {
  return !(v instanceof Duration || v instanceof DataSize || v instanceof byte[]);
}

try {
  renderer.render(value);
} catch (RendererException e) {
  LOG.error("JSON rendering unsupported: {}", e.getMessage());
}

When it happens

Trigger: Rendering to JSON any property that evaluates to Bytes (e.g. `data = new Bytes { ... }` or base64 output of binary resources) via visitBytes.

Common situations: Modules embedding binary blobs — keys, certificates, hashes, or file contents — rendered to JSON for consumption by other tools.

Understand the failure class

Background: "JSON serialization failed", "not JSON serializable", "Failed to serialize": why JSON marshaling errors happen and how to fix them — this error's family across 46 libraries.

Related errors

• Values of type `Duration` cannot be rendered as JSON. Value:
• Values of type `DataSize` cannot be rendered as JSON. Value:
• cannotRenderValue
• JavaType token must be parameterized.
• Failed to convert `pkl.base#String` to `java.net.URI`.

AI-assisted analysis of apple/pkl@f3efcbfc9b (2026-09-08). Data as JSON: /api/errors/60f2cd0c761c4f07. Report an issue: GitHub.