diff --git a/src/lib/dotenv-parse.test.ts b/src/lib/dotenv-parse.test.ts
new file mode 100644
index 0000000..328782e
--- /dev/null
+++ b/src/lib/dotenv-parse.test.ts
@@ -0,0 +1,118 @@
+import { describe, expect, it } from "vitest";
+import { appendDotenvKey, parseDotenv, removeDotenvKey } from "./dotenv-parse";
+
+describe("parseDotenv", () => {
+  it("ignores blanks and # comments", () => {
+    expect(parseDotenv("\n  \n# x=1\nFOO=bar\n")).toEqual([
+      { key: "FOO", value: "bar" },
+    ]);
+  });
+
+  it("trims keys and values and supports only the first = in the value", () => {
+    expect(parseDotenv("  MY_KEY  =  a=b  ")).toEqual([
+      { key: "MY_KEY", value: "a=b" },
+    ]);
+  });
+
+  it("strips matching double or single quotes around values", () => {
+    expect(parseDotenv("Q=\"hello\"\nS='x'")).toEqual([
+      { key: "Q", value: "hello" },
+      { key: "S", value: "x" },
+    ]);
+  });
+
+  it("splits on CRLF", () => {
+    expect(parseDotenv("A=1\r\nB=2")).toEqual([
+      { key: "A", value: "1" },
+      { key: "B", value: "2" },
+    ]);
+  });
+
+  it("skips lines without a valid key", () => {
+    expect(parseDotenv("=no\nnoeq\n KEY \n")).toEqual([]);
+  });
+});
+
+describe("removeDotenvKey", () => {
+  it("drops parsed lines for the key and keeps comments and blanks", () => {
+    const content = "# hdr\n\nFOO=1\nBAR=2\n";
+    expect(removeDotenvKey(content, "FOO")).toBe("# hdr\n\nBAR=2\n");
+  });
+});
+
+describe("appendDotenvKey", () => {
+  it("rejects invalid keys and values with clear errors", () => {
+    expect(appendDotenvKey("", "  ", "v")).toEqual({
+      ok: false,
+      error: "Enter a variable name",
+    });
+    expect(appendDotenvKey("", "a=b", "v")).toEqual({
+      ok: false,
+      error: 'Key cannot contain "="',
+    });
+    expect(appendDotenvKey("", "#x", "v")).toEqual({
+      ok: false,
+      error: "Key cannot start with #",
+    });
+    expect(appendDotenvKey("", "bad\nkey", "v")).toEqual({
+      ok: false,
+      error: "Key cannot span lines",
+    });
+    expect(appendDotenvKey("", "K", "one\ntwo")).toEqual({
+      ok: false,
+      error: "Value cannot contain line breaks",
+    });
+  });
+
+  it("rejects duplicate keys that parseDotenv would surface", () => {
+    expect(appendDotenvKey("X=1\n", "X", "2")).toEqual({
+      ok: false,
+      error: '"X" is already defined',
+    });
+  });
+
+  it("appends a plain line when quoting is not needed", () => {
+    expect(appendDotenvKey("", "K", "v")).toEqual({
+      ok: true,
+      content: "K=v",
+    });
+    expect(appendDotenvKey("A=1", "B", "2")).toEqual({
+      ok: true,
+      content: "A=1\nB=2",
+    });
+  });
+
+  it('quotes values that contain whitespace, #, or quotes and escapes \\ and "', () => {
+    expect(appendDotenvKey("", "K", "a b")).toEqual({
+      ok: true,
+      content: 'K="a b"',
+    });
+    expect(appendDotenvKey("", "K", "p#q")).toEqual({
+      ok: true,
+      content: 'K="p#q"',
+    });
+    expect(appendDotenvKey("", "K", 'say "hi"')).toEqual({
+      ok: true,
+      content: 'K="say \\"hi\\""',
+    });
+    // Space forces quotes; backslashes must be escaped inside the string.
+    expect(appendDotenvKey("", "K", "C:\\temp dir")).toEqual({
+      ok: true,
+      content: 'K="C:\\\\temp dir"',
+    });
+  });
+
+  it("quotes values with leading or trailing spaces (not just internal)", () => {
+    expect(appendDotenvKey("", "K", " x ")).toEqual({
+      ok: true,
+      content: 'K=" x "',
+    });
+  });
+
+  it("trims trailing whitespace on existing content before appending", () => {
+    expect(appendDotenvKey("A=1\n\n  \n", "B", "2")).toEqual({
+      ok: true,
+      content: "A=1\nB=2",
+    });
+  });
+});