diff --git a/CHANGELOG.adoc b/CHANGELOG.adoc
index dd0306f..d58a20f 100644
--- a/CHANGELOG.adoc
+++ b/CHANGELOG.adoc
@@ -7,6 +7,7 @@
 - dtmt: split `build` into `build` and `package`
 - dtmt: implement deploying built bundles
 - dtmm: indicate when a deployment is necessary
+- dtmm: check for Steam game update before deployment
 
 === Fixed
 
diff --git a/crates/dtmm/src/controller/app.rs b/crates/dtmm/src/controller/app.rs
index aba2442..c2f5962 100644
--- a/crates/dtmm/src/controller/app.rs
+++ b/crates/dtmm/src/controller/app.rs
@@ -8,7 +8,6 @@ use color_eyre::{Help, Result};
 use druid::im::Vector;
 use druid::FileInfo;
 use dtmt_shared::ModConfig;
-use serde::Deserialize;
 use tokio::fs::{self, DirEntry};
 use tokio::runtime::Runtime;
 use tokio_stream::wrappers::ReadDirStream;
@@ -18,6 +17,8 @@ use zip::ZipArchive;
 use crate::state::{ModInfo, PackageInfo, State};
 use crate::util::config::{ConfigSerialize, LoadOrderEntry};
 
+use super::read_sjson_file;
+
 #[tracing::instrument(skip(state))]
 pub(crate) async fn import_mod(state: State, info: FileInfo) -> Result<ModInfo> {
     let data = fs::read(&info.path)
@@ -144,16 +145,6 @@ pub(crate) async fn save_settings(state: State) -> Result<()> {
         })
 }
 
-async fn read_sjson_file<P, T>(path: P) -> Result<T>
-where
-    T: for<'a> Deserialize<'a>,
-    P: AsRef<Path> + std::fmt::Debug,
-{
-    let buf = fs::read(path).await.wrap_err("failed to read file")?;
-    let data = String::from_utf8(buf).wrap_err("invalid UTF8")?;
-    serde_sjson::from_str(&data).wrap_err("failed to deserialize")
-}
-
 #[tracing::instrument(skip_all,fields(
     name = ?res.as_ref().map(|entry| entry.file_name())
 ))]
diff --git a/crates/dtmm/src/controller/game.rs b/crates/dtmm/src/controller/game.rs
index cbb1b02..f504b3a 100644
--- a/crates/dtmm/src/controller/game.rs
+++ b/crates/dtmm/src/controller/game.rs
@@ -5,7 +5,7 @@ use std::str::FromStr;
 use std::sync::Arc;
 
 use color_eyre::eyre::Context;
-use color_eyre::{eyre, Help, Result};
+use color_eyre::{eyre, Help, Report, Result};
 use futures::stream;
 use futures::StreamExt;
 use path_slash::PathBufExt;
@@ -21,6 +21,7 @@ use tokio::fs;
 use tokio::io::AsyncWriteExt;
 use tracing::Instrument;
 
+use super::read_sjson_file;
 use crate::state::{PackageInfo, State};
 
 const MOD_BUNDLE_NAME: &str = "packages/mods";
@@ -32,7 +33,7 @@ const MOD_DATA_SCRIPT: &str = "scripts/mods/mod_data";
 const SETTINGS_FILE_PATH: &str = "application_settings/settings_common.ini";
 const DEPLOYMENT_DATA_PATH: &str = "dtmm-deployment.sjson";
 
-#[derive(Serialize, Deserialize)]
+#[derive(Debug, Serialize, Deserialize)]
 struct DeploymentData {
     bundles: Vec<String>,
     #[serde(with = "time::serde::iso8601")]
@@ -535,6 +536,42 @@ pub(crate) async fn deploy_mods(state: State) -> Result<()> {
         }
     }
 
+    let (game_info, deployment_info) = tokio::try_join!(
+        async {
+            tokio::task::spawn_blocking(dtmt_shared::collect_game_info)
+                .await
+                .map_err(Report::new)
+        },
+        async {
+            let path = state.game_dir.join(DEPLOYMENT_DATA_PATH);
+            match read_sjson_file::<_, DeploymentData>(path)
+                .await
+            {
+                Ok(data) => Ok(Some(data)),
+                Err(err) => {
+                    if let Some(err) = err.downcast_ref::<std::io::Error>() && err.kind() == ErrorKind::NotFound {
+                        Ok(None)
+                    } else {
+                        Err(err).wrap_err("failed to read deployment data")
+                    }
+                }
+            }
+        }
+    )
+    .wrap_err("failed to gather deployment information")?;
+
+    let game_info = game_info.wrap_err("failed to collect Steam info")?;
+
+    tracing::debug!(?game_info, ?deployment_info);
+
+    if deployment_info
+        .as_ref()
+        .map(|i| game_info.last_updated > i.timestamp)
+        .unwrap_or(false)
+    {
+        eyre::bail!("Game was updated since last mod deployment. Please reset first.");
+    }
+
     tracing::info!(
         "Deploying {} mods to {}",
         state.mods.len(),
diff --git a/crates/dtmm/src/controller/mod.rs b/crates/dtmm/src/controller/mod.rs
new file mode 100644
index 0000000..f7f250d
--- /dev/null
+++ b/crates/dtmm/src/controller/mod.rs
@@ -0,0 +1,23 @@
+use std::path::Path;
+
+use color_eyre::{eyre::Context, Result};
+use serde::Deserialize;
+use tokio::fs;
+
+pub mod app;
+pub mod game;
+pub mod worker;
+
+#[tracing::instrument]
+async fn read_sjson_file<P, T>(path: P) -> Result<T>
+where
+    T: for<'a> Deserialize<'a>,
+    P: AsRef<Path> + std::fmt::Debug,
+{
+    let path = path.as_ref();
+    let buf = fs::read(path)
+        .await
+        .wrap_err_with(|| format!("failed to read file '{}'", path.display()))?;
+    let data = String::from_utf8(buf).wrap_err("invalid UTF8")?;
+    serde_sjson::from_str(&data).wrap_err("failed to deserialize SJSON")
+}
diff --git a/crates/dtmm/src/main.rs b/crates/dtmm/src/main.rs
index 667fc07..21b9f37 100644
--- a/crates/dtmm/src/main.rs
+++ b/crates/dtmm/src/main.rs
@@ -17,11 +17,7 @@ use crate::controller::app::load_mods;
 use crate::controller::worker::work_thread;
 use crate::state::{Delegate, State};
 
-mod controller {
-    pub mod app;
-    pub mod game;
-    pub mod worker;
-}
+mod controller;
 mod state;
 mod util {
     pub mod config;
@@ -66,6 +62,8 @@ fn main() -> Result<()> {
 
     let game_info = dtmt_shared::collect_game_info()?;
 
+    tracing::debug!(?config, ?game_info);
+
     let initial_state = {
         let mut state = State::new(
             config.path,
diff --git a/lib/dtmt-shared/src/lib.rs b/lib/dtmt-shared/src/lib.rs
index 46e7950..fa8c407 100644
--- a/lib/dtmt-shared/src/lib.rs
+++ b/lib/dtmt-shared/src/lib.rs
@@ -34,6 +34,7 @@ pub struct ModConfig {
 
 pub const STEAMAPP_ID: u32 = 1361210;
 
+#[derive(Debug)]
 pub struct GameInfo {
     pub path: PathBuf,
     pub last_updated: OffsetDateTime,