diff --git a/src/lib/dotenv-parse.test.ts b/src/lib/dotenv-parse.test.ts
new file mode 100644
index 0000000..03d560c
--- /dev/null
+++ b/src/lib/dotenv-parse.test.ts
@@ -0,0 +1,103 @@
+import { describe, expect, it } from "vitest";
+import { appendDotenvKey, parseDotenv, removeDotenvKey } from "./dotenv-parse";
+
+describe("parseDotenv", () => {
+  it("ignores blank lines and # comments", () => {
+    expect(parseDotenv("\n  \n# ignored\nFOO=bar\n")).toEqual([
+      { key: "FOO", value: "bar" },
+    ]);
+  });
+
+  it("strips optional single or double quotes from values", () => {
+    expect(parseDotenv(`A="x"\nB='y z'`)).toEqual([
+      { key: "A", value: "x" },
+      { key: "B", value: "y z" },
+    ]);
+  });
+
+  it("handles CRLF line endings", () => {
+    expect(parseDotenv("A=1\r\nB=2")).toEqual([
+      { key: "A", value: "1" },
+      { key: "B", value: "2" },
+    ]);
+  });
+
+  it("skips lines without = or with empty key", () => {
+    expect(parseDotenv("noequals\n=noval\nOK=1")).toEqual([
+      { key: "OK", value: "1" },
+    ]);
+  });
+});
+
+describe("removeDotenvKey", () => {
+  it("removes assignment lines for the key and keeps comments", () => {
+    const input = "# keep\nSECRET=x\nOTHER=1\nSECRET=y\n";
+    expect(removeDotenvKey(input, "SECRET")).toBe("# keep\nOTHER=1\n");
+  });
+});
+
+describe("appendDotenvKey", () => {
+  it("appends a line with a trailing newline when content is non-empty", () => {
+    const r = appendDotenvKey("A=1", "B", "2");
+    expect(r).toEqual({ ok: true, content: "A=1\nB=2" });
+  });
+
+  it("uses unquoted form when safe", () => {
+    expect(appendDotenvKey("", "K", "v")).toEqual({
+      ok: true,
+      content: "K=v",
+    });
+  });
+
+  it("double-quotes values with spaces and round-trips through parseDotenv", () => {
+    const r = appendDotenvKey("", "K", "a b");
+    expect(r.ok).toBe(true);
+    if (r.ok) {
+      expect(r.content).toBe('K="a b"');
+      expect(parseDotenv(r.content)).toEqual([{ key: "K", value: "a b" }]);
+    }
+  });
+
+  it("escapes embedded double quotes in formatted output", () => {
+    const r = appendDotenvKey("", "K", 'say "hi"');
+    expect(r.ok).toBe(true);
+    if (r.ok) {
+      expect(r.content).toBe('K="say \\"hi\\""');
+    }
+  });
+
+  it("rejects empty key, = in key, # prefix, and newlines in key or value", () => {
+    expect(appendDotenvKey("", "  ", "x")).toMatchObject({
+      ok: false,
+      error: "Enter a variable name",
+    });
+    expect(appendDotenvKey("", "A=B", "x")).toMatchObject({
+      ok: false,
+      error: 'Key cannot contain "="',
+    });
+    expect(appendDotenvKey("", "#X", "1")).toMatchObject({
+      ok: false,
+      error: "Key cannot start with #",
+    });
+    expect(appendDotenvKey("", "bad\nkey", "1")).toMatchObject({
+      ok: false,
+      error: "Key cannot span lines",
+    });
+    expect(appendDotenvKey("", "K", "a\nb")).toMatchObject({
+      ok: false,
+      error: "Value cannot contain line breaks",
+    });
+  });
+
+  it("rejects duplicate keys according to parseDotenv", () => {
+    expect(appendDotenvKey("FOO=1", "FOO", "2")).toMatchObject({
+      ok: false,
+      error: '"FOO" is already defined',
+    });
+  });
+
+  it("trims trailing whitespace on existing content before appending", () => {
+    const r = appendDotenvKey("A=1\n\n  \n", "B", "2");
+    expect(r).toEqual({ ok: true, content: "A=1\nB=2" });
+  });
+});