Sem: New primitive for code understanding – not LSPs, but entities on top of Git

git diff

diff --git a/src/auth/login.ts b/src/auth/login.ts
index 4a2b1c0..8f3d2e1 100644
--- a/src/auth/login.ts
+++ b/src/auth/login.ts
@@ -12,6 +12,18 @@
+export function validateToken(token: string) {
+  const decoded = jwt.verify(token, SECRET);
+  if (!decoded.exp || decoded.exp < Date.now()) {
+    throw new TokenExpiredError();
+  }
+  return decoded;
+}
+
@@ -24,8 +36,10 @@
 export async function authenticateUser(
-  const user = await db.findUser(email);
-  if (!user) return null;
+  const user = await db.findUser(email);
+  if (!user) throw new UserNotFoundError();
+  await rateLimiter.check(email);
@@ -45,12 +59,0 @@
-export function legacyAuth(user, pass) {
-  return db.query('SELECT * FROM users
-    WHERE email = ? AND password = ?',
-    [user, pass]);
-}

sem diff

┌─ src/auth/login.ts ────────────────
│
│  ⊕ function  validateToken    [added]
│  ∆ function  authenticateUser [modified]
│  ⊖ function  legacyAuth       [deleted]
│
└────────────────────────────────────

3 entities changed across 1 file

sem diff

What changed? Entity-level diffs with rename detection, structural hashing, and word-level inline highlights.

│  ⊕ function  validateToken    [added]
│  ∆ function  authenticateUser [modified]
│  ⊖ function  legacyAuth       [deleted]

sem blame

Who changed it? Per-entity blame showing the last commit that touched each function, class, or method.

│  ⊕ render_inline_diff  a1a6fbf  Rohan  04-03
│  ⊕ format_terminal     a1a6fbf  Rohan  04-03

sem impact

What breaks? Cross-file dependency graph shows every entity that depends on a given function, plus affected tests.

⊕ function authenticateUser
  → depends on:  db.findUser, rateLimiter
  ← used by:    loginRoute, authMiddleware
  ! 42 entities transitively affected

sem log

How did it evolve? Git history for a single entity. See every commit that touched a specific function.

│  ae576ab  Rohan  02-05  added
│  a105183  Rohan  02-08  modified (logic)
│  a1a6fbf  Rohan  04-03  modified (logic)

sem entities

What's under a path? Lists every function, class, method, and type with line ranges.

entities: src/auth/login.ts
  function validateToken     (L12:24)
  function authenticateUser  (L26:45)
  interface AuthConfig       (L47:52)

sem context

Smart context for AI. Token-budgeted context window: the entity, its dependencies, and dependents. Fits in any LLM prompt.

context for authenticateUser (budget: 8000)
  target:          ~705 tokens
  dependencies:    ~256 tokens
  dependents:      ~812 tokens

$ brew install sem-cli
$ sem setup
✓ Created wrapper script
✓ Set git config --global diff.external = sem
✓ Pre-commit hook installed

Done! Running git diff in any repo will now use sem.
  To revert, run: sem unsetup