diff --git a/Makefile b/Makefile
index 84a569a..5d87481 100644
--- a/Makefile
+++ b/Makefile
@@ -4,18 +4,23 @@
 # build step. `make test` is the full gate. `make remint` rebuilds the seed after a
 # source change.
 
-.PHONY: test ci values corpus unit smoke buildsmoke staticnativesmoke selfhost sci cts certify ffi transient infer wp devirt fieldread numwp fieldnum protoret narrow directlink numeric inline shakesmoke remint joltc joltc-release joltc-debug joltcsmoke
+.PHONY: test ci values corpus unit smoke buildsmoke staticnativesmoke selfhost sci cts certify ffi transient infer wp devirt fieldread numwp fieldnum protoret narrow directlink numeric inline shakesmoke remint joltc joltc-release joltc-debug joltcsmoke submodules
+
+# Every target needs the vendored submodules; fail with the fix, not a load error.
+submodules:
+	@test -f vendor/irregex/irregex.scm || { \
+	  echo "vendor submodules missing; run: git submodule update --init --recursive"; exit 1; }
 
 # Full gate (dev machine). Includes the self-host byte-fixpoint, which only holds
 # on the same Chez that minted the seed.
-test: selfhost ci
+test: submodules selfhost ci
 	@echo "OK: all gates passed"
 
 # CI gate: behavior only. The checked-in seed is a minted artifact (like a
 # lockfile) — it RUNS correctly on any Chez, but `selfhost` rebuilds it and a
 # different Chez version may emit byte-different (gensym/order) output, so the
 # byte-fixpoint is a dev-machine check, not a CI one (jolt-8479).
-ci: values corpus unit smoke buildsmoke staticnativesmoke sci cts ffi transient infer wp devirt fieldread numwp fieldnum protoret narrow directlink numeric inline certify
+ci: submodules values corpus unit smoke buildsmoke staticnativesmoke sci cts ffi transient infer wp devirt fieldread numwp fieldnum protoret narrow directlink numeric inline certify
 	@echo "OK: CI gates passed"
 
 # Self-host fixpoint: bootstrap.ss rebuild == checked-in seed.
diff --git a/README.md b/README.md
index 81319c7..16c1f50 100644
--- a/README.md
+++ b/README.md
@@ -9,8 +9,12 @@ runs it on Chez. The compiler is self-hosted: it is written in Clojure
 
 ## Install
 
-Grab the self-contained `joltc` binary (Linux/macOS) — it bundles the runtime,
-compiler, and standard library, so there is nothing else to install.
+Grab the self-contained `joltc` binary (Linux/macOS/Windows) — it bundles the
+runtime, compiler, and standard library, so there is nothing else to install.
+Download the binary archive for your platform from the
+[releases page](https://github.com/jolt-lang/jolt/releases) (`joltc-<ver>-<platform>.tar.gz`,
+or the `.zip` on Windows). The "Source code" archives GitHub attaches to every
+release are not binaries — see [Build](#build) before using one.
 
 With Homebrew:
 
@@ -45,6 +49,18 @@ cd jolt
 bin/joltc -e '(+ 1 2)'        # => 3
 ```
 
+The `--recurse-submodules` matters: jolt vendors its regex engine and test
+suites as git submodules. In a checkout that's missing them (a plain
+`git clone`, or after pulling a commit that adds one), fetch them with:
+
+```bash
+git submodule update --init --recursive
+```
+
+Note that GitHub's auto-generated "Source code (zip/tar.gz)" archives on the
+releases page do **not** contain submodules, so they can't run or build —
+clone the repo instead (or grab a prebuilt binary from the same page).
+
 After changing a compiler source — the reader (`host/chez/reader.ss`), the
 analyzer/IR/backend (`jolt-core/jolt/*.clj`), or the `clojure.core` overlay
 (`jolt-core/clojure/core/*.clj`) — re-mint the seed:
diff --git a/host/chez/cli.ss b/host/chez/cli.ss
index 79c9a54..9e580c1 100644
--- a/host/chez/cli.ss
+++ b/host/chez/cli.ss
@@ -11,6 +11,26 @@
 
 (define cli-args (cdr (command-line)))   ; drop the script name
 
+;; Fail early and actionably when the vendored submodules aren't checked out —
+;; a plain `git clone` or GitHub's auto-generated "Source code" release archive
+;; lacks them, and the raw failure ("load failed for vendor/irregex/irregex.scm")
+;; doesn't say how to fix it. (The self-contained joltc binary embeds these and
+;; never runs this file.)
+(unless (file-exists? "vendor/irregex/irregex.scm")
+  (display "jolt: vendor submodules are missing (vendor/irregex).
+" (current-error-port))
+  (display "GitHub's 'Source code' release archives don't include submodules.
+" (current-error-port))
+  (display "Clone the repo instead:
+" (current-error-port))
+  (display "  git clone --recurse-submodules https://github.com/jolt-lang/jolt.git
+" (current-error-port))
+  (display "or, in an existing checkout:
+" (current-error-port))
+  (display "  git submodule update --init --recursive
+" (current-error-port))
+  (exit 1))
+
 (load "host/chez/rt.ss")
 (set-chez-ns! "clojure.core")
 (load "host/chez/seed/prelude.ss")