diff --git a/test/config.test.ts b/test/config.test.ts
new file mode 100644
index 0000000..3e4e3fb
--- /dev/null
+++ b/test/config.test.ts
@@ -0,0 +1,40 @@
+import { test, expect, describe } from "bun:test";
+import { loadConfig } from "../src/config";
+
+describe("config", () => {
+  test("loadConfig env variables override config file and defaults", async () => {
+    const origBase = process.env.GAI_API_BASE;
+    const origModel = process.env.GAI_MODEL;
+
+    process.env.GAI_API_BASE = "https://custom.api.com/v1";
+    process.env.GAI_MODEL = "custom-model";
+
+    const config = await loadConfig();
+
+    expect(config.apiBase).toBe("https://custom.api.com/v1");
+    expect(config.model).toBe("custom-model");
+
+    if (origBase) process.env.GAI_API_BASE = origBase;
+    else delete process.env.GAI_API_BASE;
+    if (origModel) process.env.GAI_MODEL = origModel;
+    else delete process.env.GAI_MODEL;
+  });
+
+  test("loadConfig reads from environment variables", async () => {
+    const origBase = process.env.GAI_API_BASE;
+    const origModel = process.env.GAI_MODEL;
+
+    process.env.GAI_API_BASE = "https://api.deepseek.com/v1";
+    process.env.GAI_MODEL = "deepseek-chat";
+
+    const config = await loadConfig();
+
+    expect(config.apiBase).toBe("https://api.deepseek.com/v1");
+    expect(config.model).toBe("deepseek-chat");
+
+    if (origBase) process.env.GAI_API_BASE = origBase;
+    else delete process.env.GAI_API_BASE;
+    if (origModel) process.env.GAI_MODEL = origModel;
+    else delete process.env.GAI_MODEL;
+  });
+});
diff --git a/test/prompt.test.ts b/test/prompt.test.ts
new file mode 100644
index 0000000..032fb05
--- /dev/null
+++ b/test/prompt.test.ts
@@ -0,0 +1,64 @@
+import { test, expect, describe } from "bun:test";
+import { buildPrompt, SYSTEM_PROMPT } from "../src/prompt";
+import type { ProjectContext } from "../src/types";
+
+describe("prompt", () => {
+  test("SYSTEM_PROMPT contains Conventional Commits format", () => {
+    expect(SYSTEM_PROMPT).toContain("<type>(<scope>)");
+    expect(SYSTEM_PROMPT).toContain("feat");
+    expect(SYSTEM_PROMPT).toContain("fix");
+  });
+
+  test("buildPrompt with full context", () => {
+    const ctx: ProjectContext = {
+      readme: "A test project",
+      packageDescription: "Test package",
+      structure: "src/, tests/",
+      recentCommits: ["abc123 feat: initial commit"],
+      diff: "+new line\n-old line",
+    };
+
+    const prompt = buildPrompt(ctx);
+
+    expect(prompt).toContain("## Project Context");
+    expect(prompt).toContain("Test package");
+    expect(prompt).toContain("src/, tests/");
+    expect(prompt).toContain("A test project");
+    expect(prompt).toContain("## Recent Commits");
+    expect(prompt).toContain("abc123 feat: initial commit");
+    expect(prompt).toContain("## Staged Changes");
+    expect(prompt).toContain("+new line");
+    expect(prompt).toContain("Generate a commit message");
+  });
+
+  test("buildPrompt with minimal context (only diff)", () => {
+    const ctx: ProjectContext = {
+      readme: null,
+      packageDescription: null,
+      structure: null,
+      recentCommits: [],
+      diff: "+added code",
+    };
+
+    const prompt = buildPrompt(ctx);
+
+    expect(prompt).not.toContain("## Project Context");
+    expect(prompt).not.toContain("## Recent Commits");
+    expect(prompt).toContain("## Staged Changes");
+    expect(prompt).toContain("+added code");
+  });
+
+  test("buildPrompt includes diff as-is (truncation is caller's responsibility)", () => {
+    const diff = "+short diff content";
+    const ctx: ProjectContext = {
+      readme: null,
+      packageDescription: null,
+      structure: null,
+      recentCommits: [],
+      diff,
+    };
+
+    const prompt = buildPrompt(ctx);
+    expect(prompt).toContain(diff);
+  });
+});