diff --git a/src/lib/invariant.ts b/src/lib/invariant.ts
new file mode 100644
index 0000000..a49ebd6
--- /dev/null
+++ b/src/lib/invariant.ts
@@ -0,0 +1,113 @@
+/**
+ * Invariant assertion helpers for runtime validation.
+ *
+ * These utilities provide type-safe assertions that narrow types and
+ * throw descriptive errors when invariants are violated.
+ */
+
+/**
+ * Custom error class for invariant violations.
+ * Provides a distinct error type that can be caught and identified.
+ */
+export class InvariantError extends Error {
+  constructor(message: string) {
+    super(message);
+    this.name = "InvariantError";
+
+    // Maintains proper stack trace in V8 environments (Node, Chrome)
+    if (Error.captureStackTrace) {
+      Error.captureStackTrace(this, InvariantError);
+    }
+  }
+}
+
+/**
+ * Message can be a string or a function that returns a string.
+ * Using a function allows for lazy evaluation of expensive messages.
+ */
+export type InvariantMessage = string | (() => string);
+
+/**
+ * Assert that a condition is truthy. Throws InvariantError if false.
+ *
+ * This function acts as a type guard - TypeScript will narrow the type
+ * after the assertion. Useful for null/undefined checks.
+ *
+ * @param condition - The condition to check (must be truthy)
+ * @param message - Error message or function returning message
+ * @throws InvariantError if condition is falsy
+ *
+ * @example
+ * ```ts
+ * const user: User | null = getUser();
+ * invariant(user !== null, "User must be logged in");
+ * // TypeScript now knows user is User, not null
+ * console.log(user.name);
+ * ```
+ *
+ * @example
+ * ```ts
+ * // Lazy message evaluation for performance
+ * invariant(isValid, () => `Invalid state: ${JSON.stringify(state)}`);
+ * ```
+ */
+export function invariant(
+  condition: unknown,
+  message: InvariantMessage
+): asserts condition {
+  if (!condition) {
+    const errorMessage = typeof message === "function" ? message() : message;
+    throw new InvariantError(errorMessage);
+  }
+}
+
+/**
+ * Assert that a value should never be reached (exhaustive type checking).
+ *
+ * Use this in the default case of switch statements to ensure all
+ * union members are handled. TypeScript will error at compile time
+ * if a case is missing.
+ *
+ * @param value - The value that should be of type `never`
+ * @returns never - This function always throws
+ * @throws InvariantError with a description of the unexpected value
+ *
+ * @example
+ * ```ts
+ * type Status = "pending" | "complete" | "failed";
+ *
+ * function handleStatus(status: Status): string {
+ *   switch (status) {
+ *     case "pending": return "Waiting...";
+ *     case "complete": return "Done!";
+ *     case "failed": return "Error occurred";
+ *     default:
+ *       // If a new status is added, TypeScript will error here
+ *       return assertNever(status);
+ *   }
+ * }
+ * ```
+ */
+export function assertNever(value: never): never {
+  const description = formatValue(value);
+  throw new InvariantError(`Unexpected value: ${description}`);
+}
+
+/**
+ * Format a value for display in error messages.
+ * Handles objects, arrays, and primitives.
+ */
+function formatValue(value: unknown): string {
+  if (value === null) return "null";
+  if (value === undefined) return "undefined";
+
+  if (typeof value === "object") {
+    try {
+      return JSON.stringify(value);
+    } catch {
+      return String(value);
+    }
+  }
+
+  return String(value);
+}
diff --git a/tests/lib/invariant.test.ts b/tests/lib/invariant.test.ts
new file mode 100644
index 0000000..22f0b1f
--- /dev/null
+++ b/tests/lib/invariant.test.ts
@@ -0,0 +1,143 @@
+import { describe, it, expect } from "vitest";
+import {
+  invariant,
+  assertNever,
+  InvariantError,
+} from "@/lib/invariant";
+
+describe("InvariantError", () => {
+  it("is an instance of Error", () => {
+    const error = new InvariantError("test message");
+    expect(error).toBeInstanceOf(Error);
+  });
+
+  it("has the correct name", () => {
+    const error = new InvariantError("test message");
+    expect(error.name).toBe("InvariantError");
+  });
+
+  it("preserves the message", () => {
+    const error = new InvariantError("something went wrong");
+    expect(error.message).toBe("something went wrong");
+  });
+
+  it("has a stack trace", () => {
+    const error = new InvariantError("test");
+    expect(error.stack).toBeDefined();
+  });
+});
+
+describe("invariant", () => {
+  it("does not throw when condition is true", () => {
+    expect(() => invariant(true, "should not throw")).not.toThrow();
+  });
+
+  it("does not throw for truthy values", () => {
+    expect(() => invariant(1, "truthy number")).not.toThrow();
+    expect(() => invariant("non-empty", "truthy string")).not.toThrow();
+    expect(() => invariant({}, "empty object is truthy")).not.toThrow();
+    expect(() => invariant([], "empty array is truthy")).not.toThrow();
+  });
+
+  it("throws InvariantError when condition is false", () => {
+    expect(() => invariant(false, "invariant violated")).toThrow(
+      InvariantError
+    );
+  });
+
+  it("throws InvariantError for falsy values", () => {
+    expect(() => invariant(null, "null check")).toThrow(InvariantError);
+    expect(() => invariant(undefined, "undefined check")).toThrow(
+      InvariantError
+    );
+    expect(() => invariant(0, "zero check")).toThrow(InvariantError);
+    expect(() => invariant("", "empty string check")).toThrow(InvariantError);
+  });
+
+  it("includes the message in the error", () => {
+    try {
+      invariant(false, "Expected value to be defined");
+    } catch (e) {
+      expect(e).toBeInstanceOf(InvariantError);
+      expect((e as InvariantError).message).toBe("Expected value to be defined");
+    }
+  });
+
+  it("works as a type guard narrowing null/undefined", () => {
+    const maybeValue: string | null = "hello";
+    invariant(maybeValue !== null, "value should not be null");
+    // TypeScript should now know maybeValue is string
+    const length: number = maybeValue.length;
+    expect(length).toBe(5);
+  });
+
+  it("works with a lazy message function", () => {
+    const expensiveMessage = () => `Computed at ${Date.now()}`;
+    expect(() => invariant(false, expensiveMessage)).toThrow(InvariantError);
+  });
+
+  it("only evaluates lazy message when condition fails", () => {
+    let called = false;
+    const lazyMessage = () => {
+      called = true;
+      return "computed message";
+    };
+
+    invariant(true, lazyMessage);
+    expect(called).toBe(false);
+
+    try {
+      invariant(false, lazyMessage);
+    } catch {
+      // expected
+    }
+    expect(called).toBe(true);
+  });
+});
+
+describe("assertNever", () => {
+  it("throws InvariantError when called", () => {
+    // We need to force TypeScript to let us call this
+    const invalid = "unexpected" as never;
+    expect(() => assertNever(invalid)).toThrow(InvariantError);
+  });
+
+  it("includes the unexpected value in the error message", () => {
+    const invalid = "unexpected_value" as never;
+    try {
+      assertNever(invalid);
+    } catch (e) {
+      expect(e).toBeInstanceOf(InvariantError);
+      expect((e as InvariantError).message).toContain("unexpected_value");
+    }
+  });
+
+  it("handles object values in error message", () => {
+    const invalid = { type: "unknown" } as never;
+    try {
+      assertNever(invalid);
+    } catch (e) {
+      expect(e).toBeInstanceOf(InvariantError);
+      expect((e as InvariantError).message).toContain("unknown");
+    }
+  });
+
+  it("is useful for exhaustive switch statements", () => {
+    type Status = "pending" | "complete";
+
+    function handleStatus(status: Status): string {
+      switch (status) {
+        case "pending":
+          return "waiting";
+        case "complete":
+          return "done";
+        default:
+          // If we add a new status, TypeScript will error here
+          return assertNever(status);
+      }
+    }
+
+    expect(handleStatus("pending")).toBe("waiting");
+    expect(handleStatus("complete")).toBe("done");
+  });
+});