diff --git a/functions/_agents_repo_slug.fish b/functions/_agents_repo_slug.fish
new file mode 100644
index 0000000..6825d2a
--- /dev/null
+++ b/functions/_agents_repo_slug.fish
@@ -0,0 +1,69 @@
+# Copyright (C) 2026 Rootiest
+# SPDX-License-Identifier: AGPL-3.0-or-later
+
+# SYNOPSIS
+#   _agents_repo_slug <dir>
+#
+# DESCRIPTION
+#   Derives the vault slug for a project directory. Prefers the normalized
+#   git remote URL so the same project keys identically from any clone on
+#   any machine; falls back to a path-derived key when no remote exists.
+#
+#   Normalization strips the scheme, userinfo, and a numeric port, rewrites
+#   scp-form host:path to host/path, drops a trailing .git, lowercases, and
+#   maps every character outside [a-z0-9._-] to a dash. These all yield
+#   git.rootiest.dev-rootiest-fish-config:
+#
+#     https://git.rootiest.dev/rootiest/fish-config.git
+#     git@git.rootiest.dev:rootiest/fish-config.git
+#     ssh://git@git.rootiest.dev:22/rootiest/fish-config.git
+#
+#   With no remote the slug is local-<basename>-<8 hex of sha256(realpath)>.
+#   That key is machine-dependent by construction and is best-effort only;
+#   agents-vault --adopt rebinds such an entry by hand.
+#
+# ARGUMENTS
+#   dir  Absolute path to the project directory
+#
+# EXIT STATUS
+#   0  Slug printed
+#   1  No directory argument given
+#
+# RETURNS
+#   The slug, one line on stdout.
+#
+# EXAMPLE
+#   set -l slug (_agents_repo_slug /home/user/myproject)
+function _agents_repo_slug --argument-names dir
+    test -n "$dir"; or return 1
+
+    set -l url (git -C "$dir" remote get-url origin 2>/dev/null)
+    if test -z "$url"
+        set -l remotes (git -C "$dir" remote 2>/dev/null)
+        if test (count $remotes) -gt 0
+            set url (git -C "$dir" remote get-url $remotes[1] 2>/dev/null)
+        end
+    end
+
+    if test -n "$url"
+        set -l s $url
+        # Order matters: the port must go before the scp-form rewrite, or
+        # ssh://host:22/a/b becomes host/22/a/b and diverges from the
+        # https slug for the same repository.
+        set s (string replace -r '^[A-Za-z][A-Za-z0-9+.-]*://' '' -- $s)
+        set s (string replace -r '^[^@/]+@' '' -- $s)
+        set s (string replace -r '^([^/:]+):[0-9]+/' '$1/' -- $s)
+        set s (string replace -r '^([^/:]+):' '$1/' -- $s)
+        set s (string replace -r '\.git$' '' -- $s)
+        set s (string replace -r '/+$' '' -- $s)
+        set s (string lower -- $s)
+        set s (string replace -ra '[^a-z0-9._-]' '-' -- $s)
+        printf '%s\n' $s
+        return 0
+    end
+
+    set -l rp (path resolve "$dir")
+    set -l base (string lower -- (path basename "$rp"))
+    set -l digest (printf '%s' "$rp" | sha256sum | string split -f1 ' ')
+    printf 'local-%s-%s\n' "$base" (string sub -l 8 -- "$digest")
+end
diff --git a/tests/test-agents-vault.fish b/tests/test-agents-vault.fish
new file mode 100644
index 0000000..8de37ba
--- /dev/null
+++ b/tests/test-agents-vault.fish
@@ -0,0 +1,88 @@
+#!/usr/bin/env fish
+# Copyright (C) 2026 Rootiest
+# SPDX-License-Identifier: AGPL-3.0-or-later
+#
+# Hermetic tests for the agent memory vault helpers. Every test builds its
+# own throwaway git repos and directories under mktemp; nothing touches the
+# live vault, ~/.claude, or this checkout.
+#
+# Usage: fish tests/test-agents-vault.fish
+
+set -l here (realpath (dirname (status filename)))
+set -g repo_root (realpath $here/..)
+set -p fish_function_path $repo_root/functions
+
+set -g TESTS_RUN 0
+set -g TESTS_FAILED 0
+set -g TMPDIRS
+
+function check --argument-names label want got
+    set -g TESTS_RUN (math $TESTS_RUN + 1)
+    if test "$want" = "$got"
+        echo "  PASS  $label"
+    else
+        echo "  FAIL  $label"
+        echo "        want: $want"
+        echo "        got:  $got"
+        set -g TESTS_FAILED (math $TESTS_FAILED + 1)
+    end
+end
+
+# Create a throwaway git repo, optionally with an origin remote.
+function new_repo --argument-names url
+    set -l d (mktemp -d)
+    set -ga TMPDIRS $d
+    git -C $d init -q
+    git -C $d config user.email t@t
+    git -C $d config user.name t
+    git -C $d config commit.gpgsign false
+    git -C $d config core.hooksPath /dev/null
+    test -n "$url"; and git -C $d remote add origin $url
+    printf '%s\n' $d
+end
+
+function cleanup
+    for d in $TMPDIRS
+        test -n "$d"; and rm -rf $d
+    end
+end
+
+#   ─────────────────────────── slug derivation ───────────────────────────
+echo "== _agents_repo_slug =="
+
+set -l want git.rootiest.dev-rootiest-fish-config
+
+set -l r (new_repo https://git.rootiest.dev/rootiest/fish-config.git)
+check "https form" $want (_agents_repo_slug $r)
+
+set r (new_repo git@git.rootiest.dev:rootiest/fish-config.git)
+check "scp form" $want (_agents_repo_slug $r)
+
+set r (new_repo ssh://git@git.rootiest.dev:22/rootiest/fish-config.git)
+check "ssh form with port" $want (_agents_repo_slug $r)
+
+set r (new_repo https://git.rootiest.dev/rootiest/fish-config)
+check "no .git suffix" $want (_agents_repo_slug $r)
+
+set r (new_repo HTTPS://Git.Rootiest.DEV/Rootiest/Fish-Config.git)
+check "case folded" $want (_agents_repo_slug $r)
+
+# Remote-less: local- prefix, stable across runs, distinct per path.
+set -l n1 (new_repo)
+set -l s1 (_agents_repo_slug $n1)
+set -l s2 (_agents_repo_slug $n1)
+check "local slug is stable" $s1 $s2
+check "local slug is prefixed" true (string match -q 'local-*' -- $s1; and echo true; or echo false)
+
+set -l n2 (new_repo)
+check "local slugs differ by path" false (test "$s1" = (_agents_repo_slug $n2); and echo true; or echo false)
+
+# A non-origin remote is used when origin is absent.
+set -l r3 (new_repo)
+git -C $r3 remote add upstream https://git.rootiest.dev/rootiest/fish-config.git
+check "falls back to first remote" $want (_agents_repo_slug $r3)
+
+cleanup
+echo ""
+echo (math $TESTS_RUN - $TESTS_FAILED)"/$TESTS_RUN passed"
+exit $TESTS_FAILED