diff --git a/test/expand.test.ts b/test/expand.test.ts
new file mode 100644
index 0000000..013aebd
--- /dev/null
+++ b/test/expand.test.ts
@@ -0,0 +1,117 @@
+import { mkdirSync, mkdtempSync, rmSync, writeFileSync } from "fs";
+import { tmpdir } from "os";
+import { join } from "path";
+import { afterEach, describe, expect, it } from "vitest";
+import {
+	appendSuffix,
+	expandSkill,
+	formatBlock,
+	readSkillBody,
+	type SkillBlockMeta,
+} from "../src/expand";
+
+const tempDirs: string[] = [];
+
+afterEach(() => {
+	for (const dir of tempDirs.splice(0)) {
+		rmSync(dir, { recursive: true, force: true });
+	}
+});
+
+function writeSkillMd(dir: string, name: string, content: string): SkillBlockMeta {
+	const baseDir = join(dir, name);
+	const filePath = join(baseDir, "SKILL.md");
+	mkdirSync(baseDir, { recursive: true });
+	writeFileSync(filePath, content, "utf-8");
+	return { name, filePath, baseDir };
+}
+
+describe("readSkillBody", () => {
+	it("strips YAML frontmatter and trims body", () => {
+		const dir = mkdtempSync(join(tmpdir(), "pi-skill-reinject-expand-"));
+		tempDirs.push(dir);
+		const meta = writeSkillMd(
+			dir,
+			"demo-skill",
+			`---
+name: demo-skill
+description: Demo
+---
+
+# Instructions
+
+Do the thing.
+`,
+		);
+
+		expect(readSkillBody(meta.filePath)).toBe("# Instructions\n\nDo the thing.");
+	});
+
+	it("returns trimmed body when frontmatter is absent", () => {
+		const dir = mkdtempSync(join(tmpdir(), "pi-skill-reinject-expand-"));
+		tempDirs.push(dir);
+		const meta = writeSkillMd(dir, "plain", "  hello world  \n");
+
+		expect(readSkillBody(meta.filePath)).toBe("hello world");
+	});
+});
+
+describe("formatBlock", () => {
+	it("embeds name, location, baseDir, and body", () => {
+		const meta: SkillBlockMeta = {
+			name: "brave-search",
+			filePath: "/home/user/.pi/skills/brave-search/SKILL.md",
+			baseDir: "/home/user/.pi/skills/brave-search",
+		};
+
+		expect(formatBlock(meta, "Search the web.")).toBe(
+			`<skill name="brave-search" location="/home/user/.pi/skills/brave-search/SKILL.md">
+References are relative to /home/user/.pi/skills/brave-search.
+
+Search the web.
+</skill>`,
+		);
+	});
+});
+
+describe("appendSuffix", () => {
+	it("appends suffix after a blank line", () => {
+		const block = "<skill>body</skill>";
+		expect(appendSuffix(block, "[skill-reinject] note")).toBe(
+			`${block}\n\n[skill-reinject] note`,
+		);
+	});
+
+	it("returns block unchanged for empty or whitespace suffix", () => {
+		const block = "<skill>body</skill>";
+		expect(appendSuffix(block, undefined)).toBe(block);
+		expect(appendSuffix(block, "   ")).toBe(block);
+	});
+});
+
+describe("expandSkill", () => {
+	it("produces full injectable text with optional suffix", () => {
+		const dir = mkdtempSync(join(tmpdir(), "pi-skill-reinject-expand-"));
+		tempDirs.push(dir);
+		const meta = writeSkillMd(
+			dir,
+			"pdf-tools",
+			`---
+name: pdf-tools
+---
+
+Extract pages from PDFs.
+`,
+		);
+
+		const withoutSuffix = expandSkill(meta);
+		expect(withoutSuffix).toContain('name="pdf-tools"');
+		expect(withoutSuffix).toContain(`location="${meta.filePath}"`);
+		expect(withoutSuffix).toContain(`References are relative to ${meta.baseDir}.`);
+		expect(withoutSuffix).toContain("Extract pages from PDFs.");
+
+		expect(expandSkill(meta, "[skill-reinject] Re-applied.")).toBe(
+			`${withoutSuffix}\n\n[skill-reinject] Re-applied.`,
+		);
+	});
+});