diff --git a/CONTRIBUTING.md b/CONTRIBUTING.md
index 5168cdeaf..bd0f1e206 100644
--- a/CONTRIBUTING.md
+++ b/CONTRIBUTING.md
@@ -23,6 +23,7 @@ you would make is not already covered.
 - [A note on AI-assisted contributions](#a-note-on-ai-assisted-contributions)
 - [Use the Search, Luke](#use-the-search-luke)
 - [Commit Guidelines](#commit-guidelines)
+  - [Plugin And Theme Bootstrap Compatibility](#plugin-and-theme-bootstrap-compatibility)
   - [Format](#format)
   - [Style](#style)
 - [Volunteer](#volunteer)
@@ -176,6 +177,24 @@ specification. The automatic changelog tool uses these to automatically generate
 a changelog based on the commit messages. Here's a guide to writing a commit message
 to allow this:
 
+### Plugin And Theme Bootstrap Compatibility
+
+Oh My Zsh now provides a shared bootstrap entrypoint at `lib/bootstrap.zsh` for plugin-manager setups that don't
+source `oh-my-zsh.sh`.
+
+When working on plugins/themes:
+
+- assume bootstrap has already run in manager-driven setups
+- rely on bootstrap guarantees (`ZSH*` defaults, cache/completions setup, required `fpath` entries)
+- avoid adding per-plugin/per-theme copies of common setup unless truly plugin-specific
+
+Common patterns we should migrate away from over time:
+
+- repeated cache setup logic (`mkdir -p "$ZSH_CACHE_DIR/completions"`)
+- repeated base `fpath` bootstrapping for OMZ core/custom directories
+
+If your plugin/theme still needs custom initialization, keep it scoped to that plugin/theme behavior only.
+
 ### Format
 
 ```
diff --git a/README.md b/README.md
index d7f143397..af63bc16c 100644
--- a/README.md
+++ b/README.md
@@ -46,6 +46,7 @@ Twitter), and join us on [Discord](https://discord.gg/ohmyzsh).
     - [Manual Installation](#manual-installation)
   - [Installation Problems](#installation-problems)
   - [Custom Plugins And Themes](#custom-plugins-and-themes)
+  - [Using Plugin Managers Without `oh-my-zsh.sh`](#using-plugin-managers-without-oh-my-zshsh)
   - [Enable GNU ls In macOS And FreeBSD Systems](#enable-gnu-ls-in-macos-and-freebsd-systems)
   - [Skip Aliases](#skip-aliases)
   - [Async git prompt](#async-git-prompt)
@@ -349,6 +350,54 @@ If you have many functions that go well together, you can put them as a `XYZ.plu
 If you would like to override the functionality of a plugin distributed with Oh My Zsh, create a plugin of the
 same name in the `custom/plugins/` directory and it will be loaded instead of the one in `plugins/`.
 
+### Using Plugin Managers Without `oh-my-zsh.sh`
+
+Some plugin managers load individual OMZ plugins/themes directly and do not source `oh-my-zsh.sh`.
+For those setups, source this bootstrap entrypoint before loading any OMZ plugin or theme:
+
+```zsh
+source "$ZSH/lib/bootstrap.zsh"
+```
+
+#### Bootstrap compatibility contract
+
+`lib/bootstrap.zsh` is idempotent and guarantees:
+
+- `ZSH`, `ZSH_CUSTOM`, and `ZSH_CACHE_DIR` defaults
+- writable cache fallback to `${XDG_CACHE_HOME:-$HOME/.cache}/oh-my-zsh`
+- `$ZSH_CACHE_DIR/completions` directory creation
+- required OMZ `fpath` entries for plugins/themes:
+  - `$ZSH/functions`
+  - `$ZSH/completions`
+  - `$ZSH_CUSTOM/functions`
+  - `$ZSH_CUSTOM/completions`
+  - `$ZSH_CACHE_DIR/completions`
+- public signal: `OMZ_IS_BOOTSTRAPPED=true`
+
+What still requires `oh-my-zsh.sh`:
+
+- update checks
+- plugin auto-discovery from the `plugins=(...)` list
+- `compinit`/`compfix` orchestration and zcompdump metadata refresh
+- alias filtering via `:omz:*` zstyles during file sourcing
+- final full-init signal: `OMZ_IS_LOADED=true`
+
+#### Plugin manager hook guidance
+
+Run bootstrap once, before any OMZ plugin/theme load:
+
+- **Antigen:** add a one-time pre-bundle hook to source `lib/bootstrap.zsh`.
+- **Zinit:** use a pre-load/`atinit` hook for OMZ snippets/plugins.
+- **zplug:** use a load hook (`hook-load`) before OMZ plugin/theme sourcing.
+- **zgen / zulu:** source `lib/bootstrap.zsh` immediately before OMZ plugin/theme load lines.
+
+Compatibility matrix:
+
+| Load mode | `OMZ_IS_BOOTSTRAPPED` | `OMZ_IS_LOADED` | Suitable for |
+| :-- | :--: | :--: | :-- |
+| `source $ZSH/oh-my-zsh.sh` | ✅ | ✅ | Full OMZ framework features |
+| `source $ZSH/lib/bootstrap.zsh` + manager-loaded OMZ plugins/themes | ✅ | ❌ | Plugin/theme prerequisites only |
+
 ### Enable GNU ls In macOS And FreeBSD Systems
 
 <a name="enable-gnu-ls"></a>
diff --git a/lib/bootstrap.zsh b/lib/bootstrap.zsh
new file mode 100644
index 000000000..c08022b61
--- /dev/null
+++ b/lib/bootstrap.zsh
@@ -0,0 +1,44 @@
+#
+# Shared Oh My Zsh bootstrap entrypoint.
+#
+# This file can be sourced by plugin managers that don't source `oh-my-zsh.sh`.
+# It is safe to source multiple times.
+#
+
+# Keep source path for callers that invoke omz_bootstrap again later.
+typeset -g _OMZ_BOOTSTRAP_SOURCE="${${(%):-%x}:a}"
+
+omz_bootstrap() {
+  # If ZSH is not defined, infer from this file location.
+  [[ -n "${ZSH:-}" ]] || export ZSH="${_OMZ_BOOTSTRAP_SOURCE:h:h}"
+
+  # Set ZSH_CUSTOM to the path where custom config files and plugins exist.
+  [[ -n "${ZSH_CUSTOM:-}" ]] || ZSH_CUSTOM="$ZSH/custom"
+
+  # Set cache directory.
+  [[ -n "${ZSH_CACHE_DIR:-}" ]] || ZSH_CACHE_DIR="$ZSH/cache"
+
+  # Ensure cache dir is writable, otherwise fallback to a HOME-based cache.
+  if [[ ! -w "$ZSH_CACHE_DIR" ]]; then
+    ZSH_CACHE_DIR="${XDG_CACHE_HOME:-$HOME/.cache}/oh-my-zsh"
+  fi
+
+  # Create cache and completions dir.
+  command mkdir -p "$ZSH_CACHE_DIR/completions"
+
+  # Add required OMZ search paths.
+  local dir
+  for dir in \
+    "$ZSH/functions" \
+    "$ZSH/completions" \
+    "$ZSH_CUSTOM/functions" \
+    "$ZSH_CUSTOM/completions" \
+    "$ZSH_CACHE_DIR/completions"; do
+    (( ${fpath[(Ie)$dir]} )) || fpath=("$dir" $fpath)
+  done
+
+  # Public signal: OMZ bootstrap has completed.
+  typeset -g OMZ_IS_BOOTSTRAPPED=true
+}
+
+omz_bootstrap "$@"
diff --git a/lib/tests/bootstrap.test.zsh b/lib/tests/bootstrap.test.zsh
new file mode 100644
index 000000000..baf1cb919
--- /dev/null
+++ b/lib/tests/bootstrap.test.zsh
@@ -0,0 +1,88 @@
+#!/usr/bin/zsh -df
+
+set -u
+
+bootstrap_file="${0:A:h:h}/bootstrap.zsh"
+
+_assert() {
+  local condition="$1" message="$2"
+  if ! eval "$condition"; then
+    print -u2 "\e[31mError\e[0m: $message"
+    return 1
+  fi
+}
+
+test_bootstrap_sets_defaults_and_paths() {
+  local tmp==(:)
+  mkdir -p "$tmp/ohmyzsh"/{functions,completions,cache} "$tmp/ohmyzsh/custom"/{functions,completions}
+
+  (
+    set -e
+    export ZSH="$tmp/ohmyzsh"
+    unset ZSH_CUSTOM ZSH_CACHE_DIR OMZ_IS_BOOTSTRAPPED
+    fpath=()
+    source "$bootstrap_file"
+
+    _assert '[[ "$ZSH_CUSTOM" == "'"$tmp/ohmyzsh/custom"'" ]]' "ZSH_CUSTOM default should be set"
+    _assert '[[ "$ZSH_CACHE_DIR" == "'"$tmp/ohmyzsh/cache"'" ]]' "ZSH_CACHE_DIR default should be set"
+    _assert '[[ -d "'"$tmp/ohmyzsh/cache/completions"'" ]]' "completions dir should be created"
+    _assert '[[ "${fpath[(Ie)'"$tmp/ohmyzsh/functions"']}" -gt 0 ]]' "fpath should include OMZ functions dir"
+    _assert '[[ "${fpath[(Ie)'"$tmp/ohmyzsh/completions"']}" -gt 0 ]]' "fpath should include OMZ completions dir"
+    _assert '[[ "${fpath[(Ie)'"$tmp/ohmyzsh/custom/functions"']}" -gt 0 ]]' "fpath should include custom functions dir"
+    _assert '[[ "${fpath[(Ie)'"$tmp/ohmyzsh/custom/completions"']}" -gt 0 ]]' "fpath should include custom completions dir"
+    _assert '[[ "${fpath[(Ie)'"$tmp/ohmyzsh/cache/completions"']}" -gt 0 ]]' "fpath should include cache completions dir"
+    _assert '[[ "$OMZ_IS_BOOTSTRAPPED" == true ]]' "bootstrap signal should be set"
+  )
+}
+
+test_bootstrap_is_idempotent() {
+  local tmp==(:)
+  mkdir -p "$tmp/ohmyzsh"/{functions,completions,cache} "$tmp/ohmyzsh/custom"/{functions,completions}
+
+  (
+    set -e
+    export ZSH="$tmp/ohmyzsh"
+    unset ZSH_CUSTOM ZSH_CACHE_DIR OMZ_IS_BOOTSTRAPPED
+    fpath=()
+    source "$bootstrap_file"
+    source "$bootstrap_file"
+    omz_bootstrap
+
+    _assert '[[ ${#fpath} -eq ${#${(u)fpath}} ]]' "fpath entries should not duplicate after repeated bootstrap"
+    _assert '[[ "$OMZ_IS_BOOTSTRAPPED" == true ]]' "bootstrap signal should remain true"
+  )
+}
+
+test_bootstrap_uses_writable_cache_fallback() {
+  local tmp==(:)
+  mkdir -p "$tmp/ohmyzsh"/{functions,completions} "$tmp/ohmyzsh/custom"/{functions,completions}
+  mkdir -p "$tmp/no-write" "$tmp/xdg-cache"
+  chmod 0555 "$tmp/no-write"
+
+  (
+    set -e
+    export ZSH="$tmp/ohmyzsh"
+    export XDG_CACHE_HOME="$tmp/xdg-cache"
+    export ZSH_CACHE_DIR="$tmp/no-write"
+    unset ZSH_CUSTOM OMZ_IS_BOOTSTRAPPED
+    fpath=()
+    source "$bootstrap_file"
+
+    _assert '[[ "$ZSH_CACHE_DIR" == "'"$tmp/xdg-cache/oh-my-zsh"'" ]]' "cache dir should fallback when not writable"
+    _assert '[[ -d "'"$tmp/xdg-cache/oh-my-zsh/completions"'" ]]' "fallback completions dir should be created"
+    _assert '[[ "${fpath[(Ie)'"$tmp/xdg-cache/oh-my-zsh/completions"']}" -gt 0 ]]' "fpath should include fallback completions dir"
+  )
+}
+
+tests=(
+  test_bootstrap_sets_defaults_and_paths
+  test_bootstrap_is_idempotent
+  test_bootstrap_uses_writable_cache_fallback
+)
+
+for test_name in $tests; do
+  print -u2 "Test: $test_name"
+  "$test_name" || exit 1
+  print -u2 "\e[32mSuccess\e[0m"
+  print -u2 ""
+done
diff --git a/oh-my-zsh.sh b/oh-my-zsh.sh
index b4e95e0ff..6538f8400 100644
--- a/oh-my-zsh.sh
+++ b/oh-my-zsh.sh
@@ -50,31 +50,14 @@ unset -f omz_f
 # If ZSH is not defined, use the current script's directory.
 [[ -n "$ZSH" ]] || export ZSH="${${(%):-%x}:a:h}"
 
-# Set ZSH_CUSTOM to the path where your custom config files
-# and plugins exists, or else we will use the default custom/
-[[ -n "$ZSH_CUSTOM" ]] || ZSH_CUSTOM="$ZSH/custom"
-
-# Set ZSH_CACHE_DIR to the path where cache files should be created
-# or else we will use the default cache/
-[[ -n "$ZSH_CACHE_DIR" ]] || ZSH_CACHE_DIR="$ZSH/cache"
-
-# Make sure $ZSH_CACHE_DIR is writable, otherwise use a directory in $HOME
-if [[ ! -w "$ZSH_CACHE_DIR" ]]; then
-  ZSH_CACHE_DIR="${XDG_CACHE_HOME:-$HOME/.cache}/oh-my-zsh"
-fi
-
-# Create cache and completions dir and add to $fpath
-mkdir -p "$ZSH_CACHE_DIR/completions"
-(( ${fpath[(Ie)$ZSH_CACHE_DIR/completions]} )) || fpath=("$ZSH_CACHE_DIR/completions" $fpath)
+# Shared, manager-agnostic bootstrap.
+source "$ZSH/lib/bootstrap.zsh"
 
 # Check for updates on initial load...
 source "$ZSH/tools/check_for_upgrade.sh"
 
 # Initializes Oh My Zsh
 
-# add a function path
-fpath=($ZSH/{functions,completions} $ZSH_CUSTOM/{functions,completions} $fpath)
-
 # Load all stock functions (from $fpath files) called below.
 autoload -U compaudit compinit zrecompile
 
@@ -234,3 +217,6 @@ fi
 
 # set completion colors to be the same as `ls`, after theme has been loaded
 [[ -z "$LS_COLORS" ]] || zstyle ':completion:*' list-colors "${(s.:.)LS_COLORS}"
+
+# Public signal: full Oh My Zsh init script has completed.
+typeset -g OMZ_IS_LOADED=true