diff --git a/src/__tests__/args.test.ts b/src/__tests__/args.test.ts
new file mode 100644
index 0000000..bc862a8
--- /dev/null
+++ b/src/__tests__/args.test.ts
@@ -0,0 +1,171 @@
+import { describe, test, expect } from "bun:test";
+import { parseArgs, applyPositionals } from "../args";
+import { defaultOptions, type Options } from "../types";
+
+describe("parseArgs", () => {
+  test("no args → defaults", () => {
+    const { options, help, version } = parseArgs([]);
+    expect(help).toBe(false);
+    expect(version).toBe(false);
+    expect(options.dist).toBe("uniform");
+    expect(options.count).toBe(1);
+    expect(options.decimals).toBe(0);
+    expect(options.min).toBe(0);
+    expect(options.max).toBe(100);
+  });
+
+  test("-h / --help", () => {
+    expect(parseArgs(["-h"]).help).toBe(true);
+    expect(parseArgs(["--help"]).help).toBe(true);
+  });
+
+  test("-V / --version", () => {
+    expect(parseArgs(["-V"]).version).toBe(true);
+    expect(parseArgs(["--version"]).version).toBe(true);
+  });
+
+  test("-c / --count", () => {
+    expect(parseArgs(["-c", "5"]).options.count).toBe(5);
+    expect(parseArgs(["--count", "10"]).options.count).toBe(10);
+  });
+
+  test("-c with invalid value throws", () => {
+    expect(() => parseArgs(["-c", "0"])).toThrow();
+    expect(() => parseArgs(["-c", "-1"])).toThrow();
+    expect(() => parseArgs(["-c", "1000001"])).toThrow();
+  });
+
+  test("-f with no arg defaults to 2", () => {
+    expect(parseArgs(["-f"]).options.decimals).toBe(2);
+  });
+
+  test("-f with space-separated arg", () => {
+    expect(parseArgs(["-f", "3"]).options.decimals).toBe(3);
+    expect(parseArgs(["-f", "0"]).options.decimals).toBe(0);
+  });
+
+  test("-fN combined form", () => {
+    expect(parseArgs(["-f1"]).options.decimals).toBe(1);
+    expect(parseArgs(["-f5"]).options.decimals).toBe(5);
+    expect(parseArgs(["-f0"]).options.decimals).toBe(0);
+  });
+
+  test("--float with no arg defaults to 2", () => {
+    expect(parseArgs(["--float"]).options.decimals).toBe(2);
+  });
+
+  test("--float with arg", () => {
+    expect(parseArgs(["--float", "4"]).options.decimals).toBe(4);
+  });
+
+  test("-d / --dist", () => {
+    expect(parseArgs(["-d", "normal"]).options.dist).toBe("normal");
+    expect(parseArgs(["--dist", "poisson"]).options.dist).toBe("poisson");
+  });
+
+  test("-d with invalid dist throws", () => {
+    expect(() => parseArgs(["-d", "invalid"])).toThrow();
+  });
+
+  test("positional args for uniform", () => {
+    const { options } = parseArgs(["10"]);
+    expect(options.max).toBe(10);
+    expect(options.min).toBe(0);
+  });
+
+  test("positional args for uniform with two values", () => {
+    const { options } = parseArgs(["10", "20"]);
+    expect(options.min).toBe(10);
+    expect(options.max).toBe(20);
+  });
+
+  test("positional args for normal", () => {
+    const { options } = parseArgs(["-d", "normal", "100", "15"]);
+    expect(options.mean).toBe(100);
+    expect(options.stddev).toBe(15);
+  });
+
+  test("-- separator passes everything as positional", () => {
+    const { options } = parseArgs(["--", "-10", "-5"]);
+    expect(options.min).toBe(-10);
+    expect(options.max).toBe(-5);
+  });
+
+  test("-- separator stops flag parsing", () => {
+    // numbers after -- become positional
+    const { options } = parseArgs(["--", "5"]);
+    expect(options.max).toBe(5);
+    // non-numeric after -- still throws
+    expect(() => parseArgs(["--", "-h"])).toThrow("not a number");
+  });
+
+  test("unknown flag throws", () => {
+    expect(() => parseArgs(["--unknown"])).toThrow("unknown option");
+  });
+});
+
+describe("applyPositionals", () => {
+  function makeOpts(dist: string, overrides?: Partial<Options>): Options {
+    return { ...defaultOptions(dist as Options["dist"]), dist: dist as Options["dist"], ...overrides };
+  }
+
+  test("uniform: 0 args leaves defaults", () => {
+    const o = makeOpts("uniform");
+    applyPositionals(o, []);
+    expect(o.min).toBe(0);
+    expect(o.max).toBe(100);
+  });
+
+  test("uniform: 1 arg sets max", () => {
+    const o = makeOpts("uniform");
+    applyPositionals(o, ["50"]);
+    expect(o.max).toBe(50);
+  });
+
+  test("uniform: 2 args sets min and max", () => {
+    const o = makeOpts("uniform");
+    applyPositionals(o, ["10", "20"]);
+    expect(o.min).toBe(10);
+    expect(o.max).toBe(20);
+  });
+
+  test("uniform: min > max throws", () => {
+    const o = makeOpts("uniform");
+    expect(() => applyPositionals(o, ["10", "5"])).toThrow("min");
+  });
+
+  test("uniform: too many args throws", () => {
+    const o = makeOpts("uniform");
+    expect(() => applyPositionals(o, ["1", "2", "3"])).toThrow("uniform expects");
+  });
+
+  test("normal: stddev <= 0 throws", () => {
+    const o = makeOpts("normal");
+    expect(() => applyPositionals(o, ["0", "-1"])).toThrow("stddev");
+  });
+
+  test("binomial: prob out of range throws", () => {
+    const o = makeOpts("binomial");
+    expect(() => applyPositionals(o, ["10", "1.5"])).toThrow("prob");
+  });
+
+  test("poisson: lambda <= 0 throws", () => {
+    const o = makeOpts("poisson");
+    expect(() => applyPositionals(o, ["0"])).toThrow("lambda");
+  });
+
+  test("poisson: too many args throws", () => {
+    const o = makeOpts("poisson");
+    expect(() => applyPositionals(o, ["1", "2"])).toThrow("poisson expects");
+  });
+
+  test("exponential: lambda <= 0 throws", () => {
+    const o = makeOpts("exponential");
+    expect(() => applyPositionals(o, ["0"])).toThrow("lambda");
+  });
+
+  test("hypergeometric: K > N throws", () => {
+    const o = makeOpts("hypergeometric");
+    expect(() => applyPositionals(o, ["100", "150", "10"])).toThrow("K");
+  });
+});
diff --git a/src/__tests__/dist.test.ts b/src/__tests__/dist.test.ts
new file mode 100644
index 0000000..3a62c77
--- /dev/null
+++ b/src/__tests__/dist.test.ts
@@ -0,0 +1,214 @@
+import { describe, test, expect } from "bun:test";
+import { generate } from "../dist";
+import { type Options, type Dist, defaultOptions } from "../types";
+
+function opts(overrides: Partial<Options> & { dist: Dist }): Options {
+  return { ...defaultOptions(overrides.dist), ...overrides };
+}
+
+describe("generate", () => {
+  test("uniform produces values in [min, max] for decimals=0 (inclusive)", () => {
+    const n = 2000;
+    const o = opts({ dist: "uniform", min: 10, max: 20, count: n, decimals: 0 });
+    const values = [...generate(o)];
+    expect(values.length).toBe(n);
+    for (const v of values) {
+      expect(v).toBeGreaterThanOrEqual(10);
+      expect(v).toBeLessThanOrEqual(20);
+      expect(Number.isInteger(v)).toBe(true);
+    }
+    // All values in [10,20] must appear at least once with high probability
+    const seen = new Set(values);
+    for (let i = 10; i <= 20; i++) expect(seen.has(i)).toBe(true);
+  });
+
+  test("uniform produces values in [min, max) for decimals>0", () => {
+    const o = opts({ dist: "uniform", min: 0, max: 1, count: 500, decimals: 4 });
+    const values = [...generate(o)];
+    for (const v of values) {
+      expect(v).toBeGreaterThanOrEqual(0);
+      expect(v).toBeLessThan(1);
+    }
+  });
+
+  test("uniform negative range produces inclusive values", () => {
+    const o = opts({ dist: "uniform", min: -10, max: -5, count: 100, decimals: 0 });
+    const values = [...generate(o)];
+    for (const v of values) {
+      expect(v).toBeGreaterThanOrEqual(-10);
+      expect(v).toBeLessThanOrEqual(-5);
+      expect(Number.isInteger(v)).toBe(true);
+    }
+  });
+
+  test("uniform single-value range", () => {
+    const o = opts({ dist: "uniform", min: 7, max: 7, count: 50, decimals: 0 });
+    const values = [...generate(o)];
+    for (const v of values) expect(v).toBe(7);
+  });
+
+  test("normal produces values around mean", () => {
+    const o = opts({ dist: "normal", mean: 100, stddev: 15, count: 2000 });
+    const values = [...generate(o)];
+    const avg = values.reduce((a, b) => a + b, 0) / values.length;
+    // Mean should be within ~3 std errors of 100
+    expect(avg).toBeGreaterThan(98);
+    expect(avg).toBeLessThan(102);
+  });
+
+  test("normal with decimals=0 produces integers via Math.round", () => {
+    const o = opts({ dist: "normal", mean: 0, stddev: 1, count: 500, decimals: 0 });
+    const values = [...generate(o)];
+    for (const v of values) {
+      expect(Number.isInteger(v)).toBe(true);
+    }
+  });
+
+  test("binomial produces values in [0, n]", () => {
+    const o = opts({ dist: "binomial", trials: 10, prob: 0.5, count: 500 });
+    const values = [...generate(o)];
+    for (const v of values) {
+      expect(v).toBeGreaterThanOrEqual(0);
+      expect(v).toBeLessThanOrEqual(10);
+      expect(Number.isInteger(v)).toBe(true);
+    }
+  });
+
+  test("binomial p=0 always returns 0", () => {
+    const o = opts({ dist: "binomial", trials: 10, prob: 0, count: 50 });
+    for (const v of generate(o)) expect(v).toBe(0);
+  });
+
+  test("binomial p=1 always returns n", () => {
+    const o = opts({ dist: "binomial", trials: 10, prob: 1, count: 50 });
+    for (const v of generate(o)) expect(v).toBe(10);
+  });
+
+  test("binomial trials=0 always returns 0", () => {
+    const o = opts({ dist: "binomial", trials: 0, prob: 0.5, count: 50 });
+    for (const v of generate(o)) expect(v).toBe(0);
+  });
+
+  test("binomial large n uses Normal approximation", () => {
+    const o = opts({ dist: "binomial", trials: 20000, prob: 0.5, count: 200 });
+    const values = [...generate(o)];
+    const avg = values.reduce((a, b) => a + b, 0) / values.length;
+    expect(avg).toBeGreaterThan(9500);
+    expect(avg).toBeLessThan(10500);
+    for (const v of values) {
+      expect(v).toBeGreaterThanOrEqual(0);
+      expect(v).toBeLessThanOrEqual(20000);
+    }
+  });
+
+  test("poisson produces non-negative integers", () => {
+    const o = opts({ dist: "poisson", lambda: 5, count: 500 });
+    const values = [...generate(o)];
+    for (const v of values) {
+      expect(v).toBeGreaterThanOrEqual(0);
+      expect(Number.isInteger(v)).toBe(true);
+    }
+  });
+
+  test("poisson large lambda uses Normal approximation", () => {
+    const o = opts({ dist: "poisson", lambda: 20000, count: 200 });
+    const values = [...generate(o)];
+    const avg = values.reduce((a, b) => a + b, 0) / values.length;
+    expect(avg).toBeGreaterThan(19000);
+    expect(avg).toBeLessThan(21000);
+    for (const v of values) {
+      expect(v).toBeGreaterThanOrEqual(0);
+      expect(Number.isInteger(v)).toBe(true);
+    }
+  });
+
+  test("exponential produces non-negative values", () => {
+    // decimals=0: Math.round can make very small values become 0
+    const o = opts({ dist: "exponential", lambda: 1, count: 500 });
+    for (const v of generate(o)) {
+      expect(v).toBeGreaterThanOrEqual(0);
+    }
+  });
+
+  test("exponential mean approximates 1/lambda", () => {
+    const lambda = 2;
+    const o = opts({ dist: "exponential", lambda, count: 2000 });
+    const values = [...generate(o)];
+    const avg = values.reduce((a, b) => a + b, 0) / values.length;
+    expect(avg).toBeGreaterThan(0.35);
+    expect(avg).toBeLessThan(0.65);
+  });
+
+  test("hypergeometric produces values in [0, min(K, n)]", () => {
+    const o = opts({ dist: "hypergeometric", popSize: 100, successes: 30, draws: 10, count: 500 });
+    const values = [...generate(o)];
+    for (const v of values) {
+      expect(v).toBeGreaterThanOrEqual(0);
+      expect(v).toBeLessThanOrEqual(10);
+      expect(Number.isInteger(v)).toBe(true);
+    }
+  });
+
+  test("hypergeometric K=0 always returns 0", () => {
+    const o = opts({ dist: "hypergeometric", popSize: 100, successes: 0, draws: 10, count: 50 });
+    for (const v of generate(o)) expect(v).toBe(0);
+  });
+
+  test("hypergeometric draws=0 always returns 0", () => {
+    const o = opts({ dist: "hypergeometric", popSize: 100, successes: 50, draws: 0, count: 50 });
+    for (const v of generate(o)) expect(v).toBe(0);
+  });
+
+  test("generator yields exactly count values", () => {
+    for (const count of [1, 5, 100]) {
+      const o = opts({ dist: "uniform", count });
+      expect([...generate(o)].length).toBe(count);
+    }
+  });
+
+  // validateOptions defense-in-depth tests
+  test("throws on count < 1", () => {
+    const o = opts({ dist: "uniform", count: 0 });
+    expect(() => [...generate(o)]).toThrow("invalid count");
+  });
+
+  test("throws on negative decimals", () => {
+    const o = opts({ dist: "uniform", decimals: -1 });
+    expect(() => [...generate(o)]).toThrow("decimals must be 0–100");
+  });
+
+  test("throws on decimals > 100", () => {
+    const o = opts({ dist: "uniform", decimals: 101 });
+    expect(() => [...generate(o)]).toThrow("decimals must be 0–100");
+  });
+
+  test("throws on min > max", () => {
+    const o = opts({ dist: "uniform", min: 10, max: 5 });
+    expect(() => [...generate(o)]).toThrow("min");
+  });
+
+  test("throws on stddev <= 0", () => {
+    const o = opts({ dist: "normal", stddev: 0 });
+    expect(() => [...generate(o)]).toThrow("stddev");
+  });
+
+  test("throws on prob out of range", () => {
+    const o = opts({ dist: "binomial", prob: 1.5 });
+    expect(() => [...generate(o)]).toThrow("prob");
+  });
+
+  test("throws on lambda <= 0 for poisson", () => {
+    const o = opts({ dist: "poisson", lambda: 0 });
+    expect(() => [...generate(o)]).toThrow("lambda");
+  });
+
+  test("throws on lambda <= 0 for exponential", () => {
+    const o = opts({ dist: "exponential", lambda: -1 });
+    expect(() => [...generate(o)]).toThrow("lambda");
+  });
+
+  test("throws on hypergeometric K > N", () => {
+    const o = opts({ dist: "hypergeometric", popSize: 100, successes: 150, draws: 10 });
+    expect(() => [...generate(o)]).toThrow("K");
+  });
+});
diff --git a/src/__tests__/stdin.test.ts b/src/__tests__/stdin.test.ts
new file mode 100644
index 0000000..be881fd
--- /dev/null
+++ b/src/__tests__/stdin.test.ts
@@ -0,0 +1,52 @@
+import { describe, test, expect } from "bun:test";
+import { parseStdinNumbers } from "../stdin";
+
+describe("parseStdinNumbers", () => {
+  test("empty string returns []", () => {
+    expect(parseStdinNumbers("")).toEqual([]);
+  });
+
+  test("whitespace-only returns []", () => {
+    expect(parseStdinNumbers("   \n\t  ")).toEqual([]);
+  });
+
+  test("single number", () => {
+    expect(parseStdinNumbers("42")).toEqual([42]);
+  });
+
+  test("multiple numbers", () => {
+    expect(parseStdinNumbers("5 2")).toEqual([5, 2]);
+  });
+
+  test("numbers separated by newlines", () => {
+    expect(parseStdinNumbers("5\n2\n3")).toEqual([5, 2, 3]);
+  });
+
+  test("numbers separated by tabs and multiple spaces", () => {
+    expect(parseStdinNumbers("5\t2   3")).toEqual([5, 2, 3]);
+  });
+
+  test("leading and trailing whitespace is trimmed", () => {
+    expect(parseStdinNumbers("  10 20  \n")).toEqual([10, 20]);
+  });
+
+  test("negative numbers", () => {
+    expect(parseStdinNumbers("-1 -0.5")).toEqual([-1, -0.5]);
+  });
+
+  test("float numbers", () => {
+    expect(parseStdinNumbers("3.14 2.718")).toEqual([3.14, 2.718]);
+  });
+
+  test("non-numeric input returns []", () => {
+    expect(parseStdinNumbers("abc")).toEqual([]);
+  });
+
+  test("mixed numeric and non-numeric returns []", () => {
+    expect(parseStdinNumbers("5 abc 10")).toEqual([]);
+  });
+
+  test("empty string after trim returns []", () => {
+    expect(parseStdinNumbers("  ")).toEqual([]);
+  });
+});