diff --git a/go-runtime/compile/build.go b/go-runtime/compile/build.go
index 4c3d0bef73..eb426057a3 100644
--- a/go-runtime/compile/build.go
+++ b/go-runtime/compile/build.go
@@ -59,13 +59,12 @@ type mainModuleContext struct {
 	TypesCtx           typesFileContext
 }
 
-func (c mainModuleContext) withImports(mainModuleImport string) mainModuleContext {
+func (c *mainModuleContext) withImports(mainModuleImport string) {
 	c.MainCtx.Imports = c.generateMainImports()
 	c.TypesCtx.Imports = c.generateTypesImports(mainModuleImport)
-	return c
 }
 
-func (c mainModuleContext) generateMainImports() []string {
+func (c *mainModuleContext) generateMainImports() []string {
 	imports := sets.NewSet[string]()
 	imports.Add(`"context"`)
 	imports.Add(`"github.com/TBD54566975/ftl/backend/protos/xyz/block/ftl/v1/ftlv1connect"`)
@@ -90,7 +89,7 @@ func (c mainModuleContext) generateMainImports() []string {
 	return imports.ToSlice()
 }
 
-func (c mainModuleContext) generateTypesImports(mainModuleImport string) []string {
+func (c *mainModuleContext) generateTypesImports(mainModuleImport string) []string {
 	imports := sets.NewSet[string]()
 	if len(c.TypesCtx.SumTypes) > 0 || len(c.TypesCtx.ExternalTypes) > 0 {
 		imports.Add(`"github.com/TBD54566975/ftl/go-runtime/ftl/reflection"`)
@@ -563,7 +562,7 @@ func buildMainModuleContext(sch *schema.Schema, result extract.Result, goModVers
 
 func (b *mainModuleContextBuilder) build(goModVersion, ftlVersion, projectName string,
 	sharedModulesPaths []string, replacements []*modfile.Replace) (mainModuleContext, error) {
-	ctx := mainModuleContext{
+	ctx := &mainModuleContext{
 		GoVersion:          goModVersion,
 		FTLVersion:         ftlVersion,
 		Name:               b.mainModule.Name,
@@ -582,8 +581,53 @@ func (b *mainModuleContextBuilder) build(goModVersion, ftlVersion, projectName s
 	}
 
 	visited := sets.NewSet[string]()
-	err := schema.Visit(b.mainModule, func(node schema.Node, next func() error) error {
-		maybeGoType, isLocal, err := b.getGoType(b.mainModule, node)
+	err := b.visit(ctx, b.mainModule, b.mainModule, visited)
+	if err != nil {
+		return mainModuleContext{}, err
+	}
+
+	slices.SortFunc(ctx.MainCtx.SumTypes, func(a, b goSumType) int {
+		return strings.Compare(a.TypeName(), b.TypeName())
+	})
+	slices.SortFunc(ctx.TypesCtx.SumTypes, func(a, b goSumType) int {
+		return strings.Compare(a.TypeName(), b.TypeName())
+	})
+
+	ctx.TypesCtx.MainModulePkg = b.mainModule.Name
+	mainModuleImport := fmt.Sprintf("ftl/%s", b.mainModule.Name)
+	if alias, ok := b.imports[mainModuleImport]; ok {
+		mainModuleImport = fmt.Sprintf("%s %q", alias, mainModuleImport)
+		ctx.TypesCtx.MainModulePkg = alias
+	}
+	ctx.withImports(mainModuleImport)
+	return *ctx, nil
+}
+
+func (b *mainModuleContextBuilder) visit(
+	ctx *mainModuleContext,
+	module *schema.Module,
+	node schema.Node,
+	visited sets.Set[string],
+) error {
+	err := schema.Visit(node, func(node schema.Node, next func() error) error {
+		if ref, ok := node.(*schema.Ref); ok {
+			maybeResolved, maybeModule := b.sch.ResolveWithModule(ref)
+			resolved, ok := maybeResolved.Get()
+			if !ok {
+				return next()
+			}
+			m, ok := maybeModule.Get()
+			if !ok {
+				return next()
+			}
+			err := b.visit(ctx, m, resolved, visited)
+			if err != nil {
+				return fmt.Errorf("failed to visit children of %s: %w", ref, err)
+			}
+			return next()
+		}
+
+		maybeGoType, isLocal, err := b.getGoType(module, node)
 		if err != nil {
 			return err
 		}
@@ -611,40 +655,14 @@ func (b *mainModuleContextBuilder) build(goModVersion, ftlVersion, projectName s
 		return next()
 	})
 	if err != nil {
-		return mainModuleContext{}, fmt.Errorf("failed to build main module context: %w", err)
-	}
-
-	slices.SortFunc(ctx.MainCtx.SumTypes, func(a, b goSumType) int {
-		return strings.Compare(a.TypeName(), b.TypeName())
-	})
-	slices.SortFunc(ctx.TypesCtx.SumTypes, func(a, b goSumType) int {
-		return strings.Compare(a.TypeName(), b.TypeName())
-	})
-
-	ctx.TypesCtx.MainModulePkg = b.mainModule.Name
-	mainModuleImport := fmt.Sprintf("ftl/%s", b.mainModule.Name)
-	if alias, ok := b.imports[mainModuleImport]; ok {
-		mainModuleImport = fmt.Sprintf("%s %q", alias, mainModuleImport)
-		ctx.TypesCtx.MainModulePkg = alias
+		return fmt.Errorf("failed to build main module context: %w", err)
 	}
-	return ctx.withImports(mainModuleImport), nil
+	return nil
 }
 
 func (b *mainModuleContextBuilder) getGoType(module *schema.Module, node schema.Node) (gotype optional.Option[goType], isLocal bool, err error) {
 	isLocal = b.visitingMainModule(module.Name)
 	switch n := node.(type) {
-	case *schema.Ref:
-		maybeResolved, maybeModule := b.sch.ResolveWithModule(n)
-		resolved, ok := maybeResolved.Get()
-		if !ok {
-			return optional.None[goType](), isLocal, nil
-		}
-		m, ok := maybeModule.Get()
-		if !ok {
-			return optional.None[goType](), isLocal, nil
-		}
-		return b.getGoType(m, resolved)
-
 	case *schema.Verb:
 		if !isLocal {
 			return optional.None[goType](), false, nil
diff --git a/internal/buildengine/testdata/alpha/types.ftl.go b/internal/buildengine/testdata/alpha/types.ftl.go
index 8a98b8c467..93ec7a961f 100644
--- a/internal/buildengine/testdata/alpha/types.ftl.go
+++ b/internal/buildengine/testdata/alpha/types.ftl.go
@@ -5,6 +5,7 @@ import (
 	"context"
 	ftlother "ftl/other"
 	"github.com/TBD54566975/ftl/go-runtime/ftl/reflection"
+	lib "github.com/TBD54566975/ftl/go-runtime/schema/testdata"
 	"github.com/TBD54566975/ftl/go-runtime/server"
 )
 
@@ -12,6 +13,7 @@ type EchoClient func(context.Context, EchoRequest) (EchoResponse, error)
 
 func init() {
 	reflection.Register(
+		reflection.ExternalType(*new(lib.AnotherNonFTLType)),
 		reflection.ProvideResourcesForVerb(
 			Echo,
 			server.VerbClient[ftlother.EchoClient, ftlother.EchoRequest, ftlother.EchoResponse](),
diff --git a/internal/buildengine/testdata/type_registry_main.go b/internal/buildengine/testdata/type_registry_main.go
index 2283fb995e..d14b8fd770 100644
--- a/internal/buildengine/testdata/type_registry_main.go
+++ b/internal/buildengine/testdata/type_registry_main.go
@@ -14,6 +14,10 @@ import (
 
 func init() {
 	reflection.Register(
+		reflection.SumType[ftlanother.SecondTypeEnum](
+			*new(ftlanother.One),
+			*new(ftlanother.Two),
+		),
 		reflection.SumType[ftlanother.TypeEnum](
 			*new(ftlanother.A),
 			*new(ftlanother.B),