diff --git a/src/__test__/utils.test-d.ts b/src/__test__/utils.test-d.ts
new file mode 100644
index 0000000..ff52acc
--- /dev/null
+++ b/src/__test__/utils.test-d.ts
@@ -0,0 +1,26 @@
+import { describe, expectTypeOf, it } from "vitest";
+import { allHeaderFields, headerField } from "../utils.ts";
+
+describe("headerField", () => {
+  it("has correct return type", () => {
+    expectTypeOf(
+      headerField("Some-Header", "Addresses")
+    ).toEqualTypeOf<`header:Some-Header:asAddresses`>();
+
+    expectTypeOf(
+      headerField("From", "Raw")
+    ).toEqualTypeOf<`header:From:asRaw`>();
+  });
+});
+
+describe("allHeaderFields", () => {
+  it("has correct return type", () => {
+    expectTypeOf(
+      allHeaderFields("Some-Header", "Addresses")
+    ).toEqualTypeOf<`header:Some-Header:asAddresses:all`>();
+
+    expectTypeOf(
+      allHeaderFields("From", "Raw")
+    ).toEqualTypeOf<`header:From:asRaw:all`>();
+  });
+});
diff --git a/src/__test__/utils.test.ts b/src/__test__/utils.test.ts
new file mode 100644
index 0000000..c2666fa
--- /dev/null
+++ b/src/__test__/utils.test.ts
@@ -0,0 +1,18 @@
+import { describe, expect, it } from "vitest";
+import { allHeaderFields, headerField } from "../utils.ts";
+
+describe("headerField", () => {
+  it("passes name and form arguments into string", () => {
+    expect(headerField("Some-Header", "Addresses")).toBe(
+      "header:Some-Header:asAddresses"
+    );
+  });
+});
+
+describe("allHeaderFields", () => {
+  it("applies correct suffix", async () => {
+    expect(allHeaderFields("Some-Header", "Addresses")).toBe(
+      "header:Some-Header:asAddresses:all"
+    );
+  });
+});
diff --git a/tsconfig.build.json b/tsconfig.build.json
new file mode 100644
index 0000000..703c917
--- /dev/null
+++ b/tsconfig.build.json
@@ -0,0 +1,5 @@
+{
+  "extends": "./tsconfig.json",
+  "include": ["src/**/*"],
+  "exclude": ["src/**/__test__"]
+}
diff --git a/tsconfig.json b/tsconfig.json
index 8c50101..1344c5a 100644
--- a/tsconfig.json
+++ b/tsconfig.json
@@ -12,7 +12,10 @@
     "declaration": true,
     "outDir": "dist",
     "declarationMap": true,
-    "noUnusedLocals": true
+    "noUnusedLocals": true,
+    "types": [
+      "vitest/importMeta"
+    ]
   },
   "include": ["src/**/*"]
 }
diff --git a/tsup.config.js b/tsup.config.js
index 16c7426..28177b3 100644
--- a/tsup.config.js
+++ b/tsup.config.js
@@ -11,6 +11,6 @@ export default defineConfig({
   target: ["es2018"],
   sourcemap: true,
   async onSuccess() {
-    execSync("tsc");
+    execSync("tsc --project tsconfig.build.json");
   }
 });
diff --git a/vitest.config.ts b/vitest.config.ts
index 691b3ff..354e0f4 100644
--- a/vitest.config.ts
+++ b/vitest.config.ts
@@ -3,7 +3,8 @@ import { defineConfig } from "vitest/config";
 export default defineConfig({
   test: {
     typecheck: {
-      enabled: true
-    }
-  }
+      enabled: true,
+    },
+    includeSource: ["src/**/*.ts"],
+  },
 });