diff --git a/src/cli.ts b/src/cli.ts
index 12ab2fd..dd2fad4 100755
--- a/src/cli.ts
+++ b/src/cli.ts
@@ -315,7 +315,7 @@ program
 program
   .command("diff")
   .argument("<branch>", "branch name")
-  .description("Show unstaged changes in a session's worktree")
+  .description("Show uncommitted changes, or full branch diff vs main")
   .action(async (branch: string) => {
     const root = await git.repoRoot()
     const session = await state.getSession(root, branch)
@@ -324,11 +324,29 @@ program
       process.exit(1)
     }
 
-    const result = await $`git -C ${session.worktree} diff`.nothrow()
-    if (result.exitCode !== 0) {
-      console.error("git diff failed")
+    // Check for uncommitted changes (staged + unstaged)
+    const status = await $`git -C ${session.worktree} status --porcelain`.nothrow().quiet()
+    if (status.exitCode !== 0) {
+      console.error("git status failed")
       process.exit(1)
     }
+
+    if (status.text().trim().length > 0) {
+      // Show uncommitted changes (both staged and unstaged)
+      const result = await $`git -C ${session.worktree} diff HEAD`.nothrow()
+      if (result.exitCode !== 0) {
+        // HEAD may not exist yet (no commits); fall back to showing all tracked + untracked
+        await $`git -C ${session.worktree} diff`.nothrow()
+      }
+    } else {
+      // No uncommitted changes — show full branch diff vs main
+      const main = await git.mainBranch(root)
+      const result = await $`git -C ${session.worktree} diff ${main}...${branch}`.nothrow()
+      if (result.exitCode !== 0) {
+        console.error("git diff failed")
+        process.exit(1)
+      }
+    }
   })
 
 // ── sandlot vm ───────────────────────────────────────────────────────
diff --git a/src/git.ts b/src/git.ts
index 5196985..69a869d 100644
--- a/src/git.ts
+++ b/src/git.ts
@@ -112,3 +112,10 @@ export async function commitMerge(cwd: string): Promise<void> {
 export async function abortMerge(cwd: string): Promise<void> {
   await $`git merge --abort`.cwd(cwd).nothrow().quiet()
 }
+
+/** Detect the main branch name (main or master). */
+export async function mainBranch(cwd?: string): Promise<string> {
+  const dir = cwd ?? "."
+  const result = await $`git rev-parse --verify --quiet refs/heads/main`.cwd(dir).nothrow().quiet()
+  return result.exitCode === 0 ? "main" : "master"
+}