diff --git a/src/functions.ts b/src/functions.ts
index 60e9cc7..42d5a5d 100644
--- a/src/functions.ts
+++ b/src/functions.ts
@@ -56,7 +56,7 @@ export function isAnyOfProp(prop: Prop): prop is AnyOfProp {
 /** When OpenAI use functions in the prompt, they format them as TypeScript
  * definitions rather than OpenAPI JSON schemas. This function converts the JSON
  * schemas into TypeScript definitions. */
-export function formatFunctionDefinitions(functions: FunctionDef[]) {
+export function formatFunctionDefinitions(functions: FunctionDef[]): string {
 	const lines = ["namespace functions {", ""]
 	for (const f of functions) {
 		if (f.description) {
diff --git a/src/token-counts.ts b/src/token-counts.ts
index 6ce790d..70c4fa1 100644
--- a/src/token-counts.ts
+++ b/src/token-counts.ts
@@ -122,7 +122,7 @@ export function messageTokensEstimate(message: Message): number {
  * @param funcs An array of OpenAI function definitions
  * @returns An estimate for the number of tokens the function definitions will use
  */
-export function functionsTokensEstimate(funcs: FunctionDef[]) {
+export function functionsTokensEstimate(funcs: FunctionDef[]): number {
 	const promptDefinitions = formatFunctionDefinitions(funcs)
 	let tokens = stringTokens(promptDefinitions)
 	tokens += 9 // Add nine per completion
diff --git a/tsconfig.json b/tsconfig.json
index 72366af..7558d28 100644
--- a/tsconfig.json
+++ b/tsconfig.json
@@ -31,6 +31,7 @@
 		/* Interop Constraints */
 		"isolatedModules": true,
 		"verbatimModuleSyntax": true,
+		"isolatedDeclarations": true,
 		"forceConsistentCasingInFileNames": true,
 
 		/* Type Checking */