diff --git a/lib/clean-start.sh b/lib/clean-start.sh
new file mode 100644
index 0000000..545035b
--- /dev/null
+++ b/lib/clean-start.sh
@@ -0,0 +1,18 @@
+#!/usr/bin/env bash
+# =============================================================================
+# lib/clean-start.sh — single source of truth for the pre-session reset.
+# Caller must already be INSIDE the genome checkout.
+# Aligns the working tree to origin/<base>. Never force-pushes a shared branch.
+# Tolerates a missing remote branch (first-setup scenario).
+# NOTE: sourced library — no `set -euo pipefail` (would leak into the caller).
+# =============================================================================
+
+clean_start() {
+  local base="${INGEST_BASE:-main}"
+  git fetch -q origin || return 1
+  git switch -q "$base" 2>/dev/null || git checkout -q -b "$base" || return 1
+  if git ls-remote --exit-code --heads origin "$base" >/dev/null 2>&1; then
+    git reset -q --hard "origin/${base}" || return 1
+  fi
+  git clean -q -fd || return 1
+}
diff --git a/tests/clean-start.bats b/tests/clean-start.bats
new file mode 100644
index 0000000..2be2326
--- /dev/null
+++ b/tests/clean-start.bats
@@ -0,0 +1,18 @@
+#!/usr/bin/env bats
+setup() {
+  load 'helpers'
+  source "${LIB_DIR}/clean-start.sh" 2>/dev/null || source "${REPO_ROOT}/lib/clean-start.sh"
+}
+@test "clean_start: aligns to origin/base, reverts tracked edits, removes untracked" {
+  G="$(make_fixture_genome)"; cd "$G"
+  echo "from origin" >> wiki/index.md
+  git add -A && git commit -q -m "origin ahead" && git push -q
+  git reset --hard HEAD~1                 # local BEHIND origin/main
+  echo "local junk" >> wiki/log.md        # tracked edit, uncommitted
+  echo "scratch" > scratch.txt            # genuinely untracked
+  INGEST_BASE="main" clean_start
+  git diff --quiet origin/main            # aligned to origin
+  grep -q "from origin" wiki/index.md     # forwarded to origin state
+  ! grep -q "local junk" wiki/log.md      # tracked edit reverted
+  [ ! -f scratch.txt ]                    # untracked removed
+}
diff --git a/tests/permissions.bats b/tests/permissions.bats
index ebe9888..b77c30f 100644
--- a/tests/permissions.bats
+++ b/tests/permissions.bats
@@ -29,7 +29,7 @@ EXECUTABLES=(
 
 # Librerie sourced: NON devono essere eseguibili.
 LIBRARIES=(
-  lib/lint.sh lib/output.sh lib/deps.sh lib/git-crypt.sh lib/scaffold.sh lib/structure.sh
+  lib/lint.sh lib/output.sh lib/deps.sh lib/git-crypt.sh lib/scaffold.sh lib/structure.sh lib/clean-start.sh
   providers/forgejo.sh providers/github.sh
   registry.sh globals.env
 )