diff --git a/src/git.ts b/src/git.ts
new file mode 100644
index 0000000..8f5f69b
--- /dev/null
+++ b/src/git.ts
@@ -0,0 +1,87 @@
+import { exec } from "child_process"
+import { promisify } from "util"
+
+const execAsync = promisify(exec)
+
+async function isGitInstalled(): Promise<boolean> {
+  try {
+    await execAsync("git --version")
+    return true
+  } catch {
+    return false
+  }
+}
+
+async function isGitRepo(cwd: string): Promise<boolean> {
+  try {
+    await execAsync("git rev-parse --git-dir", { cwd })
+    return true
+  } catch {
+    return false
+  }
+}
+
+async function hasCommits(cwd: string): Promise<boolean> {
+  try {
+    await execAsync("git rev-parse HEAD", { cwd })
+    return true
+  } catch {
+    return false
+  }
+}
+
+export async function getGitDiff(cwd: string, stagedOnly = false): Promise<string> {
+  if (!(await isGitInstalled())) {
+    throw new Error("Git is not installed")
+  }
+  if (!(await isGitRepo(cwd))) {
+    throw new Error("Not a git repository")
+  }
+
+  let diff = ""
+
+  if (await hasCommits(cwd)) {
+    const { stdout: staged } = await execAsync("git --no-pager diff --staged --diff-filter=d", { cwd })
+    diff = staged.trim()
+  }
+
+  if (!stagedOnly && !diff) {
+    const { stdout: unstaged } = await execAsync("git --no-pager diff HEAD --diff-filter=d", { cwd })
+    diff = unstaged.trim()
+  }
+
+  // New repo with no commits yet — show everything that's staged/tracked
+  if (!diff) {
+    const { stdout: newRepo } = await execAsync("git --no-pager diff --cached --diff-filter=d", { cwd })
+    diff = newRepo.trim()
+  }
+
+  // Include untracked (new) files when not limiting to staged only
+  if (!stagedOnly) {
+    const { stdout: untrackedList } = await execAsync("git ls-files --others --exclude-standard", { cwd })
+    const untrackedFiles = untrackedList.trim().split("\n").filter(Boolean)
+    for (const file of untrackedFiles) {
+      // git diff --no-index exits with code 1 when files differ — that's normal
+      const result = await execAsync(
+        `git --no-pager diff --no-index -- /dev/null ${JSON.stringify(file)}`,
+        { cwd },
+      ).catch((e: any) => ({ stdout: e.stdout as string | undefined }))
+      if (result.stdout) diff += (diff ? "\n" : "") + result.stdout
+    }
+    diff = diff.trim()
+  }
+
+  if (!diff) {
+    throw new Error("No changes found to generate a commit message from")
+  }
+
+  return diff
+}
+
+export async function getGitDiffStagedFirst(cwd: string): Promise<string> {
+  try {
+    return await getGitDiff(cwd, true)
+  } catch {
+    return await getGitDiff(cwd, false)
+  }
+}