diff --git a/tools/buildsys/src/args.rs b/tools/buildsys/src/args.rs
index c1eb63776..fcf4c4937 100644
--- a/tools/buildsys/src/args.rs
+++ b/tools/buildsys/src/args.rs
@@ -15,8 +15,9 @@ use url::Url;
 /// variable changes. The build type is represented with bit flags so that we can easily list
 /// multiple build types for a single variable. See `[BuildType]` and `[rerun_for_envs]` below to
 /// see how this list is used.
-const REBUILD_VARS: [(&str, u8); 12] = [
+const REBUILD_VARS: [(&str, u8); 13] = [
     ("BUILDSYS_ARCH", PACKAGE | VARIANT),
+    ("BUILDSYS_EPOCH", PACKAGE | VARIANT),
     ("BUILDSYS_NAME", VARIANT),
     ("BUILDSYS_OUTPUT_DIR", VARIANT),
     ("BUILDSYS_PACKAGES_DIR", PACKAGE),
diff --git a/twoliter/src/cargo_make.rs b/twoliter/src/cargo_make.rs
index 63115e9ad..5a266ceef 100644
--- a/twoliter/src/cargo_make.rs
+++ b/twoliter/src/cargo_make.rs
@@ -1,4 +1,4 @@
-use crate::common::exec_log;
+use crate::common::{exec_log, BUILDSYS_EPOCH};
 use crate::docker::ImageUri;
 use crate::project::Project;
 use anyhow::{bail, Result};
@@ -53,7 +53,9 @@ impl CargoMake {
     /// definition in `Twoliter.toml`.
     pub(crate) fn new(project: &Project) -> Result<Self> {
         let sdk = require_sdk(project)?;
-        Ok(Self::default().env("TLPRIVATE_SDK_IMAGE", sdk))
+        Ok(Self::default()
+            .env("TLPRIVATE_SDK_IMAGE", sdk)
+            .env("BUILDSYS_EPOCH", BUILDSYS_EPOCH.to_string()))
     }
 
     /// Specify the path to the `Makefile.toml` for the `cargo make` command
@@ -189,10 +191,11 @@ const ENV_VARS: [&str; 23] = [
     "no_proxy",
 ];
 
-const DISALLOWED_SDK_VARS: [&str; 3] = [
+const DISALLOWED_ENV_VARS: [&str; 4] = [
     "BUILDSYS_SDK_NAME",
     "BUILDSYS_SDK_VERSION",
     "BUILDSYS_REGISTRY",
+    "BUILDSYS_EPOCH",
 ];
 
 /// Returns `true` if `key` is an environment variable that needs to be passed to `cargo make`.
@@ -208,11 +211,8 @@ fn is_build_system_env(key: impl AsRef<str>) -> bool {
 }
 
 fn check_for_disallowed_var(key: &str) -> Result<()> {
-    if DISALLOWED_SDK_VARS.contains(&key) {
-        bail!(
-            "The environment variable '{}' can no longer be used. Specify the SDK in Twoliter.toml",
-            key
-        )
+    if DISALLOWED_ENV_VARS.contains(&key) {
+        bail!("The environment variable '{}' can not be used.", key)
     }
     Ok(())
 }
@@ -242,4 +242,6 @@ fn test_is_build_system_env() {
 fn test_check_for_disallowed_var() {
     assert!(check_for_disallowed_var("BUILDSYS_REGISTRY").is_err());
     assert!(check_for_disallowed_var("BUILDSYS_PRETTY_NAME").is_ok());
+    assert!(check_for_disallowed_var("BUILDSYS_EPOCH").is_err());
+    assert!(check_for_disallowed_var("BUILDSYS_FOO").is_ok());
 }
diff --git a/twoliter/src/common.rs b/twoliter/src/common.rs
index 24c45ec33..2175868cb 100644
--- a/twoliter/src/common.rs
+++ b/twoliter/src/common.rs
@@ -2,6 +2,12 @@ use anyhow::{ensure, Context, Result};
 use log::{self, debug, LevelFilter};
 use tokio::process::Command;
 
+/// This is passed as an environment variable to Buildsys. Buildsys tells Cargo to watch this
+/// environment variable for changes. So if we have a breaking change to the way Buildsys and/or
+/// Twoliter function, we can increment this so that we know users will rebuild after updating
+/// Twoliter.
+pub(crate) const BUILDSYS_EPOCH: u32 = 1;
+
 /// Run a `tokio::process::Command` and return a `Result` letting us know whether or not it worked.
 /// Pipes stdout/stderr when logging `LevelFilter` is more verbose than `Warn`.
 pub(crate) async fn exec_log(cmd: &mut Command) -> Result<()> {