Internal compiler error: An unhandled panic occurred in the Encore compiler: unknown builtin type Error

[sacenox]

The error output:

❯ encore test ./...

── Internal compiler error ────────────────────────────────────────────────────────────────[E0001]──

An unhandled panic occurred in the Encore compiler: unknown builtin type Error

This is a bug in Encore and should not have occurred. Please report this issue to the Encore team
either on Github at https://github.com/encoredev/encore/issues/new and include this error.

╭─[Stack Trace]
├─▶ v2builder.BuilderImpl.Parse.func1.1 encr.dev/v2/v2builder/v2builder.go:51
├─▶ legacymeta.(*builder).builtinType   encr.dev/v2/app/legacymeta/schema.go:61
├─▶ legacymeta.(*builder).schemaType    encr.dev/v2/app/legacymeta/schema.go:72
├─▶ legacymeta.(*builder).schemaType    encr.dev/v2/app/legacymeta/schema.go:130
├─▶ legacymeta.(*builder).structField   encr.dev/v2/app/legacymeta/schema.go:163
├─▶ legacymeta.(*builder).schemaType    encr.dev/v2/app/legacymeta/schema.go:92
╰─[... remaining frames omitted ...]

Versions:

❯ encore version
encore version v1.52.3

❯ go version
go version go1.25.1 darwin/arm64

Environment:

MacOS Sequoia 15.6.1 (Apple M4)

Details

I was able to find the code change that caused this on our end:

type Redacted struct {
	Error        *error  `json:"error,omitempty"`
	(more stuff here)
}

type RedactedCollection struct {
	Results []Redacted
}

RedactedCollection is a return type for a private encore endpoint. Changing the error pointer to a string pointer bypasses the error.

Steps to reproduce:

• Create a private encore endpoint that returns a struct with a member representing an array of structs, which have a member that is a pointer to an error. Example using the structs represented above (roughly, sorry about all the redaction):

//encore:api private
func (n *RedactedServiceName) RedactedEndpointName(ctx context.Context, req RedactedResquest) (*RedactedCollection, error) { ... }

• Add a test that invokes this private endpoint.
• Observe the compiler error.

[eandre]

error values cannot be encoded/decoded as request/response values directly, as they are interfaces. So it is correct that this doesn't compile, but the error message should be more useful and explain this.

The fix is to use something like option.Option[string] (see docs) to represent the optional error.

[sacenox]

@eandre Thank you, I'll take a look at the docs, feel free to close this.