diff --git a/test/settings.test.ts b/test/settings.test.ts
new file mode 100644
index 0000000..fe505d9
--- /dev/null
+++ b/test/settings.test.ts
@@ -0,0 +1,161 @@
+import { mkdtempSync, readFileSync, rmSync, writeFileSync } from "fs";
+import { tmpdir } from "os";
+import { join } from "path";
+import { afterEach, describe, expect, it } from "vitest";
+import {
+	DEFAULT_SKILL_REINJECT_SETTINGS,
+	SKILL_REINJECT_SETTINGS_KEY,
+	createDefaultSettings,
+	effectiveEnabled,
+	effectiveIntegration,
+	mergeSkillReinjectIntoSettingsFile,
+	mergeSkillReinjectSettings,
+	parseSkillReinjectPartial,
+} from "../src/settings";
+
+const tempDirs: string[] = [];
+
+afterEach(() => {
+	for (const dir of tempDirs.splice(0)) {
+		rmSync(dir, { recursive: true, force: true });
+	}
+});
+
+function tempSettingsPath(): string {
+	const dir = mkdtempSync(join(tmpdir(), "pi-skill-reinject-settings-"));
+	tempDirs.push(dir);
+	return join(dir, "settings.json");
+}
+
+describe("createDefaultSettings", () => {
+	it("matches SPEC §7.3 defaults", () => {
+		expect(createDefaultSettings()).toEqual(DEFAULT_SKILL_REINJECT_SETTINGS);
+	});
+});
+
+describe("parseSkillReinjectPartial", () => {
+	it("ignores invalid shapes and fields", () => {
+		expect(parseSkillReinjectPartial(null)).toEqual({});
+		expect(parseSkillReinjectPartial([])).toEqual({});
+		expect(
+			parseSkillReinjectPartial({
+				enabled: "yes",
+				autoCompactIntegration: "bogus",
+				extra: true,
+			}),
+		).toEqual({});
+	});
+
+	it("keeps valid fields only", () => {
+		expect(
+			parseSkillReinjectPartial({
+				enabled: true,
+				suffix: "custom",
+			}),
+		).toEqual({ enabled: true, suffix: "custom" });
+	});
+});
+
+describe("mergeSkillReinjectSettings", () => {
+	it("applies global then project overrides on defaults", () => {
+		expect(
+			mergeSkillReinjectSettings({ enabled: true }, { trackReadPaths: false }),
+		).toEqual({
+			...DEFAULT_SKILL_REINJECT_SETTINGS,
+			enabled: true,
+			trackReadPaths: false,
+		});
+	});
+
+	it("lets project win over global for the same field", () => {
+		expect(
+			mergeSkillReinjectSettings({ enabled: true }, { enabled: false }),
+		).toEqual({
+			...DEFAULT_SKILL_REINJECT_SETTINGS,
+			enabled: false,
+		});
+	});
+});
+
+describe("mergeSkillReinjectIntoSettingsFile", () => {
+	it("merges skillReinject fields across successive writes", () => {
+		const path = tempSettingsPath();
+		mergeSkillReinjectIntoSettingsFile(path, { enabled: true });
+		mergeSkillReinjectIntoSettingsFile(path, { triggerTurn: true });
+
+		const saved = JSON.parse(readFileSync(path, "utf8")) as Record<string, unknown>;
+		expect(saved[SKILL_REINJECT_SETTINGS_KEY]).toEqual({
+			enabled: true,
+			triggerTurn: true,
+		});
+	});
+
+	it("preserves existing top-level keys", () => {
+		const path = tempSettingsPath();
+		writeFileSync(
+			path,
+			`${JSON.stringify({ theme: "light", compaction: { enabled: false } }, null, 2)}\n`,
+			"utf8",
+		);
+		mergeSkillReinjectIntoSettingsFile(path, { enabled: true, suffix: "x" });
+
+		const saved = JSON.parse(readFileSync(path, "utf8")) as Record<string, unknown>;
+		expect(saved.theme).toBe("light");
+		expect(saved.compaction).toEqual({ enabled: false });
+		expect(saved[SKILL_REINJECT_SETTINGS_KEY]).toEqual({
+			enabled: true,
+			suffix: "x",
+		});
+	});
+});
+
+describe("effectiveEnabled", () => {
+	const settings = createDefaultSettings();
+
+	it("uses session override when set", () => {
+		expect(effectiveEnabled(true, settings)).toBe(true);
+		expect(effectiveEnabled(false, { ...settings, enabled: true })).toBe(false);
+	});
+
+	it("falls back to global enabled when override is null", () => {
+		expect(effectiveEnabled(null, settings)).toBe(false);
+		expect(effectiveEnabled(null, { ...settings, enabled: true })).toBe(true);
+	});
+});
+
+describe("effectiveIntegration", () => {
+	const base = createDefaultSettings();
+
+	it("honors explicit defer and immediate", () => {
+		expect(
+			effectiveIntegration({ ...base, autoCompactIntegration: "defer" }, false),
+		).toBe("defer");
+		expect(
+			effectiveIntegration({ ...base, autoCompactIntegration: "immediate" }, true),
+		).toBe("immediate");
+	});
+
+	it("uses triggerTurn when integration is off", () => {
+		expect(
+			effectiveIntegration({ ...base, autoCompactIntegration: "off", triggerTurn: false }, true),
+		).toBe("defer");
+		expect(
+			effectiveIntegration({ ...base, autoCompactIntegration: "off", triggerTurn: true }, true),
+		).toBe("immediate");
+	});
+
+	it("defers when auto mode detects pi-auto-compact", () => {
+		expect(effectiveIntegration(base, true)).toBe("defer");
+	});
+
+	it("uses triggerTurn in auto mode without pi-auto-compact", () => {
+		expect(effectiveIntegration({ ...base, triggerTurn: false }, false)).toBe("defer");
+		expect(effectiveIntegration({ ...base, triggerTurn: true }, false)).toBe("immediate");
+	});
+
+	it("prefers session integration override", () => {
+		expect(
+			effectiveIntegration({ ...base, autoCompactIntegration: "immediate" }, false, "defer"),
+		).toBe("defer");
+	});
+});