diff --git a/packages/@glimmer/interfaces/index.d.ts b/packages/@glimmer/interfaces/index.d.ts
index 44d2854abf..4126dc5a10 100644
--- a/packages/@glimmer/interfaces/index.d.ts
+++ b/packages/@glimmer/interfaces/index.d.ts
@@ -1,27 +1,27 @@
-import * as WireFormat from './lib/compile/wire-format/api';
+import * as WireFormat from './lib/compile/wire-format/api.js';
 
-export * from './lib/array';
-export * from './lib/compile';
-export * from './lib/components';
-export * from './lib/content';
-export * from './lib/core';
-export * from './lib/curry';
-export * from './lib/dom/attributes';
-export * from './lib/dom/bounds';
-export * from './lib/dom/changes';
-export * from './lib/dom/simple';
-export * from './lib/dom/tree-construction';
-export * from './lib/managers';
-export * from './lib/program';
-export * from './lib/references';
-export * from './lib/runtime';
-export * from './lib/runtime/vm';
-export * from './lib/serialize';
-export * from './lib/stack';
-export * from './lib/tags';
-export * from './lib/template';
-export * from './lib/tier1/symbol-table';
-export * from './lib/type-utils';
-export * from './lib/vm-opcodes';
+export * from './lib/array.js';
+export * from './lib/compile/index.js';
+export * from './lib/components.js';
+export * from './lib/content.js';
+export * from './lib/core.js';
+export * from './lib/curry.js';
+export * from './lib/dom/attributes.js';
+export * from './lib/dom/bounds.js';
+export * from './lib/dom/changes.js';
+export * from './lib/dom/simple.js';
+export * from './lib/dom/tree-construction.js';
+export * from './lib/managers.js';
+export * from './lib/program.js';
+export * from './lib/references.js';
+export * from './lib/runtime.js';
+export * from './lib/runtime/vm.js';
+export * from './lib/serialize.js';
+export * from './lib/stack.js';
+export * from './lib/tags.js';
+export * from './lib/template.js';
+export * from './lib/tier1/symbol-table.js';
+export * from './lib/type-utils.js';
+export * from './lib/vm-opcodes.js';
 
 export { WireFormat };
diff --git a/packages/@glimmer/interfaces/lib/array.d.ts b/packages/@glimmer/interfaces/lib/array.d.ts
index a79ae59599..ee0fb403e8 100644
--- a/packages/@glimmer/interfaces/lib/array.d.ts
+++ b/packages/@glimmer/interfaces/lib/array.d.ts
@@ -1,4 +1,4 @@
-import type { Optional } from './core';
+import type { Optional } from './core.js';
 
 export type PresentArray<T> = [T, ...T[]];
 export type OptionalArray<T> = Optional<PresentArray<T>>;
diff --git a/packages/@glimmer/interfaces/lib/compile/encoder.ts b/packages/@glimmer/interfaces/lib/compile/encoder.ts
index 6330e29c3b..b4d3a06376 100644
--- a/packages/@glimmer/interfaces/lib/compile/encoder.ts
+++ b/packages/@glimmer/interfaces/lib/compile/encoder.ts
@@ -1,10 +1,10 @@
-import type { CompileTimeComponent } from '../..';
-import type { Nullable } from '../core';
-import type { CompileTimeConstants } from '../program';
-import type { HandleResult, NamedBlocks } from '../template';
-import type { VmMachineOp as MachineOp, VmOp as Op } from '../vm-opcodes';
-import type { SingleBuilderOperand } from './operands';
-import type * as WireFormat from './wire-format/api';
+import type { CompileTimeComponent } from '../../index.js';
+import type { Nullable } from '../core.js';
+import type { CompileTimeConstants } from '../program.js';
+import type { HandleResult, NamedBlocks } from '../template.js';
+import type { VmMachineOp as MachineOp, VmOp as Op } from '../vm-opcodes.js';
+import type { SingleBuilderOperand } from './operands.js';
+import type * as WireFormat from './wire-format/api.js';
 
 // These values are used in the same space as standard opcodes, so we need to
 // start them at a higher value to prevent collisions
diff --git a/packages/@glimmer/interfaces/lib/compile/index.d.ts b/packages/@glimmer/interfaces/lib/compile/index.d.ts
index b666903c99..d0f1a020eb 100644
--- a/packages/@glimmer/interfaces/lib/compile/index.d.ts
+++ b/packages/@glimmer/interfaces/lib/compile/index.d.ts
@@ -1,4 +1,4 @@
-export * from './encoder';
-export * from './instruction-encoder';
-export * from './operands';
-export * from './wire-format/api';
+export * from './encoder.js';
+export * from './instruction-encoder.js';
+export * from './operands.js';
+export * from './wire-format/api.js';
diff --git a/packages/@glimmer/interfaces/lib/compile/instruction-encoder.d.ts b/packages/@glimmer/interfaces/lib/compile/instruction-encoder.d.ts
index 278b817a7b..6c6287458f 100644
--- a/packages/@glimmer/interfaces/lib/compile/instruction-encoder.d.ts
+++ b/packages/@glimmer/interfaces/lib/compile/instruction-encoder.d.ts
@@ -1,6 +1,6 @@
-import type { CompilerBuffer } from '../template';
-import type { VmMachineOp, VmOp } from '../vm-opcodes';
-import type { Operand } from './operands';
+import type { CompilerBuffer } from '../template.js';
+import type { VmMachineOp, VmOp } from '../vm-opcodes.js';
+import type { Operand } from './operands.js';
 
 export type ARG_SHIFT = 8;
 export type MAX_SIZE = 0x7fffffff;
diff --git a/packages/@glimmer/interfaces/lib/compile/operands.d.ts b/packages/@glimmer/interfaces/lib/compile/operands.d.ts
index aa07e5d826..e7c359da04 100644
--- a/packages/@glimmer/interfaces/lib/compile/operands.d.ts
+++ b/packages/@glimmer/interfaces/lib/compile/operands.d.ts
@@ -1,6 +1,6 @@
-import type { CompilableTemplate } from '../template';
-import type { SymbolTable } from '../tier1/symbol-table';
-import type * as WireFormat from './wire-format/api';
+import type { CompilableTemplate } from '../template.js';
+import type { SymbolTable } from '../tier1/symbol-table.js';
+import type * as WireFormat from './wire-format/api.js';
 
 export type LabelOperandType = 1;
 export type IsStrictModeOperandType = 2;
diff --git a/packages/@glimmer/interfaces/lib/compile/wire-format/api.d.ts b/packages/@glimmer/interfaces/lib/compile/wire-format/api.d.ts
index ec4699ce60..83048652b9 100644
--- a/packages/@glimmer/interfaces/lib/compile/wire-format/api.d.ts
+++ b/packages/@glimmer/interfaces/lib/compile/wire-format/api.d.ts
@@ -1,6 +1,6 @@
-import type { PresentArray } from '../../array';
-import type { Nullable } from '../../core';
-import type { CurriedType } from '../../curry';
+import type { PresentArray } from '../../array.js';
+import type { Nullable } from '../../core.js';
+import type { CurriedType } from '../../curry.js';
 import type {
   AppendOpcode,
   AttrOpcode,
@@ -51,10 +51,10 @@ import type {
   WithDynamicVarsOpcode,
   WithOpcode,
   YieldOpcode,
-} from './opcodes';
+} from './opcodes.js';
 
-export * from './opcodes';
-export * from './resolution';
+export * from './opcodes.js';
+export * from './resolution.js';
 
 export type TupleSyntax = Statement | TupleExpression;
 
diff --git a/packages/@glimmer/interfaces/lib/components.d.ts b/packages/@glimmer/interfaces/lib/components.d.ts
index 1dbd99c7bf..9838e1e798 100644
--- a/packages/@glimmer/interfaces/lib/components.d.ts
+++ b/packages/@glimmer/interfaces/lib/components.d.ts
@@ -1,9 +1,9 @@
-import type { Dict } from './core';
-import type { InternalComponentManager } from './managers';
-import type { Reference } from './references';
-import type { ScopeSlot } from './runtime';
-import type { CompilableProgram } from './template';
-import type { ProgramSymbolTable } from './tier1/symbol-table';
+import type { Dict } from './core.js';
+import type { InternalComponentManager } from './managers.js';
+import type { Reference } from './references.js';
+import type { ScopeSlot } from './runtime.js';
+import type { CompilableProgram } from './template.js';
+import type { ProgramSymbolTable } from './tier1/symbol-table.js';
 
 export type ComponentDefinitionState = object;
 export type ComponentInstanceState = unknown;
diff --git a/packages/@glimmer/interfaces/lib/dom/attributes.d.ts b/packages/@glimmer/interfaces/lib/dom/attributes.d.ts
index e5836f2d68..370671a937 100644
--- a/packages/@glimmer/interfaces/lib/dom/attributes.d.ts
+++ b/packages/@glimmer/interfaces/lib/dom/attributes.d.ts
@@ -1,8 +1,8 @@
-import type { Maybe, Nullable } from '../core';
-import type { ElementOperations, Environment, ModifierInstance } from '../runtime';
-import type { Stack } from '../stack';
-import type { Bounds, Cursor } from './bounds';
-import type { GlimmerTreeChanges, GlimmerTreeConstruction } from './changes';
+import type { Maybe, Nullable } from '../core.js';
+import type { ElementOperations, Environment, ModifierInstance } from '../runtime.js';
+import type { Stack } from '../stack.js';
+import type { Bounds, Cursor } from './bounds.js';
+import type { GlimmerTreeChanges, GlimmerTreeConstruction } from './changes.js';
 import type {
   AttrNamespace,
   SimpleComment,
@@ -10,7 +10,7 @@ import type {
   SimpleElement,
   SimpleNode,
   SimpleText,
-} from './simple';
+} from './simple.js';
 
 export interface LiveBlock extends Bounds {
   openElement(element: SimpleElement): void;
diff --git a/packages/@glimmer/interfaces/lib/dom/bounds.d.ts b/packages/@glimmer/interfaces/lib/dom/bounds.d.ts
index bae04d9a3a..e1c97f3fe5 100644
--- a/packages/@glimmer/interfaces/lib/dom/bounds.d.ts
+++ b/packages/@glimmer/interfaces/lib/dom/bounds.d.ts
@@ -1,5 +1,5 @@
-import type { Nullable } from '../core';
-import type { SimpleElement, SimpleNode } from './simple';
+import type { Nullable } from '../core.js';
+import type { SimpleElement, SimpleNode } from './simple.js';
 
 export interface Bounds {
   // a method to future-proof for wormholing; may not be needed ultimately
diff --git a/packages/@glimmer/interfaces/lib/dom/changes.d.ts b/packages/@glimmer/interfaces/lib/dom/changes.d.ts
index 48e8a043ec..a847cbeeb3 100644
--- a/packages/@glimmer/interfaces/lib/dom/changes.d.ts
+++ b/packages/@glimmer/interfaces/lib/dom/changes.d.ts
@@ -1,6 +1,6 @@
-import type { Nullable } from '../core';
-import type { Bounds } from './bounds';
-import type { Namespace, SimpleComment, SimpleElement, SimpleNode, SimpleText } from './simple';
+import type { Nullable } from '../core.js';
+import type { Bounds } from './bounds.js';
+import type { Namespace, SimpleComment, SimpleElement, SimpleNode, SimpleText } from './simple.js';
 
 export interface GlimmerDOMOperations {
   createElement(tag: string, context?: SimpleElement): SimpleElement;
diff --git a/packages/@glimmer/interfaces/lib/dom/tree-construction.d.ts b/packages/@glimmer/interfaces/lib/dom/tree-construction.d.ts
index 2b55703892..75570d944d 100644
--- a/packages/@glimmer/interfaces/lib/dom/tree-construction.d.ts
+++ b/packages/@glimmer/interfaces/lib/dom/tree-construction.d.ts
@@ -1,4 +1,4 @@
-import type { Namespace, SimpleDocumentFragment, SimpleElement, SimpleNode } from './simple';
+import type { Namespace, SimpleDocumentFragment, SimpleElement, SimpleNode } from './simple.js';
 
 export type NodeToken = number;
 
diff --git a/packages/@glimmer/interfaces/lib/managers.d.ts b/packages/@glimmer/interfaces/lib/managers.d.ts
index 813403eb50..66d86b2fdb 100644
--- a/packages/@glimmer/interfaces/lib/managers.d.ts
+++ b/packages/@glimmer/interfaces/lib/managers.d.ts
@@ -1,5 +1,5 @@
-export * from './managers/capabilities';
-export * from './managers/component';
-export * from './managers/helper';
-export * from './managers/internal';
-export * from './managers/modifier';
+export * from './managers/capabilities.js';
+export * from './managers/component.js';
+export * from './managers/helper.js';
+export * from './managers/internal.js';
+export * from './managers/modifier.js';
diff --git a/packages/@glimmer/interfaces/lib/managers/component.d.ts b/packages/@glimmer/interfaces/lib/managers/component.d.ts
index 02447deade..815015632e 100644
--- a/packages/@glimmer/interfaces/lib/managers/component.d.ts
+++ b/packages/@glimmer/interfaces/lib/managers/component.d.ts
@@ -1,5 +1,5 @@
-import type { Arguments } from '../runtime';
-import type { Capabilities } from './capabilities';
+import type { Arguments } from '../runtime.js';
+import type { Capabilities } from './capabilities.js';
 
 export interface ComponentCapabilitiesVersions {
   '3.4': {
diff --git a/packages/@glimmer/interfaces/lib/managers/helper.d.ts b/packages/@glimmer/interfaces/lib/managers/helper.d.ts
index 8d5953b45a..773439adfe 100644
--- a/packages/@glimmer/interfaces/lib/managers/helper.d.ts
+++ b/packages/@glimmer/interfaces/lib/managers/helper.d.ts
@@ -1,5 +1,5 @@
-import type { Arguments } from '../runtime';
-import type { Capabilities } from './capabilities';
+import type { Arguments } from '../runtime.js';
+import type { Capabilities } from './capabilities.js';
 
 export interface HelperCapabilitiesVersions {
   '3.23': {
diff --git a/packages/@glimmer/interfaces/lib/managers/internal.d.ts b/packages/@glimmer/interfaces/lib/managers/internal.d.ts
index 41ecc5efad..4b7315b7ab 100644
--- a/packages/@glimmer/interfaces/lib/managers/internal.d.ts
+++ b/packages/@glimmer/interfaces/lib/managers/internal.d.ts
@@ -1,3 +1,3 @@
-export * from './internal/component';
-export * from './internal/helper';
-export * from './internal/modifier';
+export * from './internal/component.js';
+export * from './internal/helper.js';
+export * from './internal/modifier.js';
diff --git a/packages/@glimmer/interfaces/lib/managers/internal/component.d.ts b/packages/@glimmer/interfaces/lib/managers/internal/component.d.ts
index 5c072c7552..3ac36eb4b9 100644
--- a/packages/@glimmer/interfaces/lib/managers/internal/component.d.ts
+++ b/packages/@glimmer/interfaces/lib/managers/internal/component.d.ts
@@ -1,17 +1,17 @@
-import type { ComponentInstanceState, PreparedArguments } from '../../components';
-import type { Destroyable, Nullable } from '../../core';
-import type { Bounds } from '../../dom/bounds';
-import type { SimpleElement } from '../../dom/simple';
-import type { Reference } from '../../references';
-import type { Owner } from '../../runtime';
-import type { CapturedArguments, VMArguments } from '../../runtime/arguments';
-import type { RenderNode } from '../../runtime/debug-render-tree';
-import type { ElementOperations } from '../../runtime/element';
-import type { Environment } from '../../runtime/environment';
-import type { DynamicScope } from '../../runtime/scope';
-import type { RuntimeResolver } from '../../serialize';
-import type { CompilableProgram } from '../../template';
-import type { ProgramSymbolTable } from '../../tier1/symbol-table';
+import type { ComponentInstanceState, PreparedArguments } from '../../components.js';
+import type { Destroyable, Nullable } from '../../core.js';
+import type { Bounds } from '../../dom/bounds.js';
+import type { SimpleElement } from '../../dom/simple.js';
+import type { Reference } from '../../references.js';
+import type { Owner } from '../../runtime.js';
+import type { CapturedArguments, VMArguments } from '../../runtime/arguments.js';
+import type { RenderNode } from '../../runtime/debug-render-tree.js';
+import type { ElementOperations } from '../../runtime/element.js';
+import type { Environment } from '../../runtime/environment.js';
+import type { DynamicScope } from '../../runtime/scope.js';
+import type { RuntimeResolver } from '../../serialize.js';
+import type { CompilableProgram } from '../../template.js';
+import type { ProgramSymbolTable } from '../../tier1/symbol-table.js';
 
 /**
  * Describes the capabilities of a particular component. The capabilities are
diff --git a/packages/@glimmer/interfaces/lib/managers/internal/helper.d.ts b/packages/@glimmer/interfaces/lib/managers/internal/helper.d.ts
index 7b32e77272..cf5dd35fc1 100644
--- a/packages/@glimmer/interfaces/lib/managers/internal/helper.d.ts
+++ b/packages/@glimmer/interfaces/lib/managers/internal/helper.d.ts
@@ -1,5 +1,5 @@
-import type { Helper, HelperDefinitionState, Owner } from '../../runtime';
-import type { HelperManager } from '../helper';
+import type { Helper, HelperDefinitionState, Owner } from '../../runtime.js';
+import type { HelperManager } from '../helper.js';
 
 export interface InternalHelperManager<TOwner extends Owner> {
   getDelegateFor(owner: TOwner | undefined): HelperManager<unknown>;
diff --git a/packages/@glimmer/interfaces/lib/managers/internal/modifier.d.ts b/packages/@glimmer/interfaces/lib/managers/internal/modifier.d.ts
index 48d6685efd..4907d9048f 100644
--- a/packages/@glimmer/interfaces/lib/managers/internal/modifier.d.ts
+++ b/packages/@glimmer/interfaces/lib/managers/internal/modifier.d.ts
@@ -1,10 +1,10 @@
 import type { SimpleElement } from '@simple-dom/interface';
 
-import type { Destroyable } from '../../core';
-import type { Owner } from '../../runtime';
-import type { CapturedArguments } from '../../runtime/arguments';
-import type { ModifierDefinitionState, ModifierInstanceState } from '../../runtime/modifier';
-import type { UpdatableTag } from '../../tags';
+import type { Destroyable } from '../../core.js';
+import type { Owner } from '../../runtime.js';
+import type { CapturedArguments } from '../../runtime/arguments.js';
+import type { ModifierDefinitionState, ModifierInstanceState } from '../../runtime/modifier.js';
+import type { UpdatableTag } from '../../tags.js';
 
 export interface InternalModifierManager<
   TModifierInstanceState = ModifierInstanceState,
diff --git a/packages/@glimmer/interfaces/lib/managers/modifier.d.ts b/packages/@glimmer/interfaces/lib/managers/modifier.d.ts
index 21f046c992..1c078905d2 100644
--- a/packages/@glimmer/interfaces/lib/managers/modifier.d.ts
+++ b/packages/@glimmer/interfaces/lib/managers/modifier.d.ts
@@ -1,5 +1,5 @@
-import type { Arguments } from '../runtime';
-import type { Capabilities } from './capabilities';
+import type { Arguments } from '../runtime.js';
+import type { Capabilities } from './capabilities.js';
 
 export interface ModifierCapabilitiesVersions {
   // passes factoryFor(...).class to `.createModifier`
diff --git a/packages/@glimmer/interfaces/lib/program.d.ts b/packages/@glimmer/interfaces/lib/program.d.ts
index 8d9a889ed5..8af484bf5b 100644
--- a/packages/@glimmer/interfaces/lib/program.d.ts
+++ b/packages/@glimmer/interfaces/lib/program.d.ts
@@ -1,10 +1,10 @@
-import type { Encoder } from './compile';
-import type { ComponentDefinition, ComponentDefinitionState } from './components';
-import type { HelperDefinitionState } from './runtime';
-import type { ModifierDefinitionState } from './runtime/modifier';
-import type { CompileTimeResolver, ResolvedComponentDefinition } from './serialize';
-import type { ContainingMetadata, STDLib, Template } from './template';
-import type { SomeVmOp, VmMachineOp, VmOp } from './vm-opcodes';
+import type { Encoder } from './compile/index.js';
+import type { ComponentDefinition, ComponentDefinitionState } from './components.js';
+import type { HelperDefinitionState } from './runtime.js';
+import type { ModifierDefinitionState } from './runtime/modifier.js';
+import type { CompileTimeResolver, ResolvedComponentDefinition } from './serialize.js';
+import type { ContainingMetadata, STDLib, Template } from './template.js';
+import type { SomeVmOp, VmMachineOp, VmOp } from './vm-opcodes.js';
 
 export type CreateRuntimeOp = (heap: CompileTimeHeap) => RuntimeOp;
 
diff --git a/packages/@glimmer/interfaces/lib/references.d.ts b/packages/@glimmer/interfaces/lib/references.d.ts
index c54fe2dc69..c25708f21c 100644
--- a/packages/@glimmer/interfaces/lib/references.d.ts
+++ b/packages/@glimmer/interfaces/lib/references.d.ts
@@ -1,4 +1,4 @@
-import type { Nullable } from './core';
+import type { Nullable } from './core.js';
 
 export type ConstantReference = 0;
 export type ComputeReference = 1;
diff --git a/packages/@glimmer/interfaces/lib/runtime.d.ts b/packages/@glimmer/interfaces/lib/runtime.d.ts
index 58ac633f73..33a51844ad 100644
--- a/packages/@glimmer/interfaces/lib/runtime.d.ts
+++ b/packages/@glimmer/interfaces/lib/runtime.d.ts
@@ -1,10 +1,10 @@
-export * from './runtime/arguments';
-export * from './runtime/debug-render-tree';
-export * from './runtime/element';
-export * from './runtime/environment';
-export * from './runtime/helper';
-export * from './runtime/modifier';
-export * from './runtime/owner';
-export * from './runtime/render';
-export * from './runtime/runtime';
-export * from './runtime/scope';
+export * from './runtime/arguments.js';
+export * from './runtime/debug-render-tree.js';
+export * from './runtime/element.js';
+export * from './runtime/environment.js';
+export * from './runtime/helper.js';
+export * from './runtime/modifier.js';
+export * from './runtime/owner.js';
+export * from './runtime/render.js';
+export * from './runtime/runtime.js';
+export * from './runtime/scope.js';
diff --git a/packages/@glimmer/interfaces/lib/runtime/arguments.d.ts b/packages/@glimmer/interfaces/lib/runtime/arguments.d.ts
index 4b0f267082..e2e2397426 100644
--- a/packages/@glimmer/interfaces/lib/runtime/arguments.d.ts
+++ b/packages/@glimmer/interfaces/lib/runtime/arguments.d.ts
@@ -1,6 +1,6 @@
-import type { Nullable } from '../core';
-import type { Reference } from '../references';
-import type { ScopeBlock } from './scope';
+import type { Nullable } from '../core.js';
+import type { Reference } from '../references.js';
+import type { ScopeBlock } from './scope.js';
 
 declare const CAPTURED_ARGS: unique symbol;
 
diff --git a/packages/@glimmer/interfaces/lib/runtime/debug-render-tree.d.ts b/packages/@glimmer/interfaces/lib/runtime/debug-render-tree.d.ts
index f50bf99222..f9f8ae5943 100644
--- a/packages/@glimmer/interfaces/lib/runtime/debug-render-tree.d.ts
+++ b/packages/@glimmer/interfaces/lib/runtime/debug-render-tree.d.ts
@@ -1,7 +1,7 @@
 import type { SimpleElement, SimpleNode } from '@simple-dom/interface';
 
-import type { Bounds } from '../dom/bounds';
-import type { Arguments, CapturedArguments } from './arguments';
+import type { Bounds } from '../dom/bounds.js';
+import type { Arguments, CapturedArguments } from './arguments.js';
 
 export type RenderNodeType = 'outlet' | 'engine' | 'route-template' | 'component';
 
diff --git a/packages/@glimmer/interfaces/lib/runtime/element.d.ts b/packages/@glimmer/interfaces/lib/runtime/element.d.ts
index 1fd03fbe22..deb14f0f97 100644
--- a/packages/@glimmer/interfaces/lib/runtime/element.d.ts
+++ b/packages/@glimmer/interfaces/lib/runtime/element.d.ts
@@ -1,5 +1,5 @@
-import type { Nullable } from '../core';
-import type { Reference } from '../references';
+import type { Nullable } from '../core.js';
+import type { Reference } from '../references.js';
 
 export interface ElementOperations {
   setAttribute(
diff --git a/packages/@glimmer/interfaces/lib/runtime/environment.d.ts b/packages/@glimmer/interfaces/lib/runtime/environment.d.ts
index cfa5d83385..da75e09070 100644
--- a/packages/@glimmer/interfaces/lib/runtime/environment.d.ts
+++ b/packages/@glimmer/interfaces/lib/runtime/environment.d.ts
@@ -4,12 +4,12 @@ import type {
   ComponentDefinitionState,
   ComponentInstance,
   ComponentInstanceState,
-} from '../components';
-import type { Nullable } from '../core';
-import type { GlimmerTreeChanges, GlimmerTreeConstruction } from '../dom/changes';
-import type { WithCreateInstance } from '../managers';
-import type { DebugRenderTree } from './debug-render-tree';
-import type { ModifierInstance } from './modifier';
+} from '../components.js';
+import type { Nullable } from '../core.js';
+import type { GlimmerTreeChanges, GlimmerTreeConstruction } from '../dom/changes.js';
+import type { WithCreateInstance } from '../managers.js';
+import type { DebugRenderTree } from './debug-render-tree.js';
+import type { ModifierInstance } from './modifier.js';
 
 export interface EnvironmentOptions {
   document?: SimpleDocument;
diff --git a/packages/@glimmer/interfaces/lib/runtime/helper.d.ts b/packages/@glimmer/interfaces/lib/runtime/helper.d.ts
index f4edafcbfa..e0993e90fe 100644
--- a/packages/@glimmer/interfaces/lib/runtime/helper.d.ts
+++ b/packages/@glimmer/interfaces/lib/runtime/helper.d.ts
@@ -1,7 +1,7 @@
-import type { Reference } from '../references';
-import type { CapturedArguments } from './arguments';
-import type { Owner } from './owner';
-import type { DynamicScope } from './scope';
+import type { Reference } from '../references.js';
+import type { CapturedArguments } from './arguments.js';
+import type { Owner } from './owner.js';
+import type { DynamicScope } from './scope.js';
 
 export type HelperDefinitionState = object;
 
diff --git a/packages/@glimmer/interfaces/lib/runtime/modifier.d.ts b/packages/@glimmer/interfaces/lib/runtime/modifier.d.ts
index a42e922039..d9857afb1f 100644
--- a/packages/@glimmer/interfaces/lib/runtime/modifier.d.ts
+++ b/packages/@glimmer/interfaces/lib/runtime/modifier.d.ts
@@ -1,4 +1,4 @@
-import type { InternalModifierManager } from '../managers';
+import type { InternalModifierManager } from '../managers.js';
 
 export type ModifierDefinitionState = object;
 export type ModifierInstanceState = unknown;
diff --git a/packages/@glimmer/interfaces/lib/runtime/render.d.ts b/packages/@glimmer/interfaces/lib/runtime/render.d.ts
index 22708f89f1..bd8ad0a202 100644
--- a/packages/@glimmer/interfaces/lib/runtime/render.d.ts
+++ b/packages/@glimmer/interfaces/lib/runtime/render.d.ts
@@ -1,8 +1,8 @@
 import type { SimpleElement, SimpleNode } from '@simple-dom/interface';
 
-import type { RichIteratorResult } from '../core';
-import type { Bounds } from '../dom/bounds';
-import type { Environment } from './environment';
+import type { RichIteratorResult } from '../core.js';
+import type { Bounds } from '../dom/bounds.js';
+import type { Environment } from './environment.js';
 
 export interface ExceptionHandler {
   handleException(): void;
diff --git a/packages/@glimmer/interfaces/lib/runtime/runtime.d.ts b/packages/@glimmer/interfaces/lib/runtime/runtime.d.ts
index a3e9cf188f..a91a3a6cdb 100644
--- a/packages/@glimmer/interfaces/lib/runtime/runtime.d.ts
+++ b/packages/@glimmer/interfaces/lib/runtime/runtime.d.ts
@@ -1,6 +1,11 @@
-import type { ResolutionTimeConstants, RuntimeConstants, RuntimeHeap, RuntimeOp } from '../program';
-import type { RuntimeResolver } from '../serialize';
-import type { Environment } from './environment';
+import type {
+  ResolutionTimeConstants,
+  RuntimeConstants,
+  RuntimeHeap,
+  RuntimeOp,
+} from '../program.js';
+import type { RuntimeResolver } from '../serialize.js';
+import type { Environment } from './environment.js';
 
 /**
   The Runtime is the set of static structures that contain the compiled
diff --git a/packages/@glimmer/interfaces/lib/runtime/scope.d.ts b/packages/@glimmer/interfaces/lib/runtime/scope.d.ts
index b3a8292c1e..bfbc950fa2 100644
--- a/packages/@glimmer/interfaces/lib/runtime/scope.d.ts
+++ b/packages/@glimmer/interfaces/lib/runtime/scope.d.ts
@@ -1,8 +1,8 @@
-import type { Dict, Nullable } from '../core';
-import type { Reference } from '../references';
-import type { CompilableBlock } from '../template';
-import type { BlockSymbolTable } from '../tier1/symbol-table';
-import type { Owner } from './owner';
+import type { Dict, Nullable } from '../core.js';
+import type { Reference } from '../references.js';
+import type { CompilableBlock } from '../template.js';
+import type { BlockSymbolTable } from '../tier1/symbol-table.js';
+import type { Owner } from './owner.js';
 
 export type Block = CompilableBlock | number;
 
diff --git a/packages/@glimmer/interfaces/lib/runtime/vm.d.ts b/packages/@glimmer/interfaces/lib/runtime/vm.d.ts
index f62b4ffb84..a9e99a99a9 100644
--- a/packages/@glimmer/interfaces/lib/runtime/vm.d.ts
+++ b/packages/@glimmer/interfaces/lib/runtime/vm.d.ts
@@ -1,10 +1,10 @@
-import type { Destroyable } from '../core';
-import type { GlimmerTreeChanges } from '../dom/changes';
-import type { Reference } from '../references';
-import type { Environment } from './environment';
-import type { Owner } from './owner';
-import type { ExceptionHandler } from './render';
-import type { DynamicScope } from './scope';
+import type { Destroyable } from '../core.js';
+import type { GlimmerTreeChanges } from '../dom/changes.js';
+import type { Reference } from '../references.js';
+import type { Environment } from './environment.js';
+import type { Owner } from './owner.js';
+import type { ExceptionHandler } from './render.js';
+import type { DynamicScope } from './scope.js';
 /**
  * This is used in the Glimmer Embedding API. In particular, embeddings
  * provide helpers through the `CompileTimeLookup` interface, and the
diff --git a/packages/@glimmer/interfaces/lib/serialize.d.ts b/packages/@glimmer/interfaces/lib/serialize.d.ts
index 487489046a..b40be8868a 100644
--- a/packages/@glimmer/interfaces/lib/serialize.d.ts
+++ b/packages/@glimmer/interfaces/lib/serialize.d.ts
@@ -53,11 +53,11 @@ import type {
   CapabilityMask,
   ComponentDefinitionState,
   ComponentInstanceState,
-} from './components';
-import type { Nullable } from './core';
-import type { InternalComponentManager } from './managers';
-import type { HelperDefinitionState, ModifierDefinitionState, Owner } from './runtime';
-import type { CompilableProgram, Template } from './template';
+} from './components.js';
+import type { Nullable } from './core.js';
+import type { InternalComponentManager } from './managers.js';
+import type { HelperDefinitionState, ModifierDefinitionState, Owner } from './runtime.js';
+import type { CompilableProgram, Template } from './template.js';
 
 export interface CompileTimeComponent {
   handle: number;
diff --git a/packages/@glimmer/interfaces/lib/stack.d.ts b/packages/@glimmer/interfaces/lib/stack.d.ts
index 3dfd730390..bcb67e0b76 100644
--- a/packages/@glimmer/interfaces/lib/stack.d.ts
+++ b/packages/@glimmer/interfaces/lib/stack.d.ts
@@ -1,4 +1,4 @@
-import type { Nullable } from './core';
+import type { Nullable } from './core.js';
 
 export interface Stack<T> {
   current: Nullable<T>;
diff --git a/packages/@glimmer/interfaces/lib/template.d.ts b/packages/@glimmer/interfaces/lib/template.d.ts
index 86325ec94a..edf7610508 100644
--- a/packages/@glimmer/interfaces/lib/template.d.ts
+++ b/packages/@glimmer/interfaces/lib/template.d.ts
@@ -1,11 +1,11 @@
-import type { PresentArray } from './array';
-import type { Operand, SerializedInlineBlock, SerializedTemplateBlock } from './compile';
-import type { EncoderError } from './compile/encoder';
-import type { Nullable } from './core';
-import type { InternalComponentCapabilities } from './managers/internal/component';
-import type { CompileTimeCompilationContext, ConstantPool, SerializedHeap } from './program';
-import type { Owner } from './runtime';
-import type { BlockSymbolTable, ProgramSymbolTable, SymbolTable } from './tier1/symbol-table';
+import type { PresentArray } from './array.js';
+import type { EncoderError } from './compile/encoder.js';
+import type { Operand, SerializedInlineBlock, SerializedTemplateBlock } from './compile/index.js';
+import type { Nullable } from './core.js';
+import type { InternalComponentCapabilities } from './managers/internal/component.js';
+import type { CompileTimeCompilationContext, ConstantPool, SerializedHeap } from './program.js';
+import type { Owner } from './runtime.js';
+import type { BlockSymbolTable, ProgramSymbolTable, SymbolTable } from './tier1/symbol-table.js';
 
 export interface CompilableProgram extends CompilableTemplate<ProgramSymbolTable> {
   moduleName: string;
diff --git a/packages/@glimmer/interfaces/package.json b/packages/@glimmer/interfaces/package.json
index c343a8dd98..183f60682b 100644
--- a/packages/@glimmer/interfaces/package.json
+++ b/packages/@glimmer/interfaces/package.json
@@ -17,7 +17,7 @@
   "scripts": {
     "test:lint": "eslint .",
     "test:publint": "publint",
-    "test:types": "tsc --noEmit -p ../tsconfig.json"
+    "test:types": "tsc --noEmit"
   },
   "dependencies": {
     "@simple-dom/interface": "^1.4.0"
diff --git a/packages/@glimmer/interfaces/tsconfig.json b/packages/@glimmer/interfaces/tsconfig.json
new file mode 100644
index 0000000000..2103b085d9
--- /dev/null
+++ b/packages/@glimmer/interfaces/tsconfig.json
@@ -0,0 +1,8 @@
+{
+  "extends": ["../tsconfig.json"],
+  "compilerOptions": {
+    "rootDir": ".",
+    "moduleResolution": "Node16"
+  },
+  "include": ["index.d.ts", "lib"]
+}