diff --git a/TODO.md b/TODO.md
index af47eb4..f3b30bb 100644
--- a/TODO.md
+++ b/TODO.md
@@ -863,5 +863,14 @@ not obvious from any one of them.
 - [ ] **Report the MegaMek defects in `crates/helm-bv/UPSTREAM.md`.** One
       unit file so far, found by conforming against the bridge. Worth
       sending upstream rather than only carrying.
-- [ ] **`helm-mcp` has no place to report a stale database.** If the `.sqlite`
-      was built from a different MegaMek than the one on disk, nothing says so.
+- [x] **`helm-mcp` has no place to report a stale database.** It has one now:
+      pass the install beside the database - `helm-mcp helm.sqlite <install>`,
+      or `HELM_MEGAMEK` - and a mismatch is warned about on startup and carried
+      in `list_facets` as `stale`, where an agent reading the library will see
+      it. Without the install it says nothing rather than guessing.
+
+      A database built from one MegaMek and queried about another answers every
+      question and answers them about a library that is not there: designs
+      since added are missing and figures since corrected are the old ones. The
+      version was already recorded in both places; the comparison just had
+      nowhere to be made.
diff --git a/crates/helm-mcp/src/library.rs b/crates/helm-mcp/src/library.rs
index 668dec6..70f06d9 100644
--- a/crates/helm-mcp/src/library.rs
+++ b/crates/helm-mcp/src/library.rs
@@ -45,9 +45,34 @@ pub struct Library {
     pub quirk_counts: HashMap<String, usize>,
     pub megamek_version: String,
     pub stats_producer: String,
+    /// Set when the database was built from a different MegaMek than the one
+    /// on disk beside it. Everything still answers; the answers are just about
+    /// a library that is no longer there.
+    pub stale: Option<String>,
 }
 
 impl Library {
+    /// Say whether this database was built from the MegaMek install it is
+    /// being served beside.
+    ///
+    /// A database built from one MegaMek and queried about another answers
+    /// every question and answers them about the wrong library: designs that
+    /// have since been added are missing, battle values that have since been
+    /// corrected are the old ones, and nothing anywhere says so. The version
+    /// is in both places, so the check is a string comparison - it just had
+    /// nowhere to be made.
+    pub fn check_against(&mut self, install: &std::path::Path) {
+        let Some(on_disk) = megamek_version(install) else {
+            return;
+        };
+        if on_disk != self.megamek_version {
+            self.stale = Some(format!(
+                "built from MegaMek {} and serving beside {on_disk}: rebuild with `helm build`",
+                self.megamek_version
+            ));
+        }
+    }
+
     pub fn open(path: &str) -> Result<Self, String> {
         let db = Connection::open(path).map_err(|e| format!("opening {path}: {e}"))?;
 
@@ -186,6 +211,7 @@ impl Library {
             quirk_counts,
             megamek_version,
             stats_producer,
+            stale: None,
         })
     }
 
@@ -347,3 +373,15 @@ impl Library {
         })
     }
 }
+
+/// The MegaMek version an install declares, from the first release line of its
+/// own changelog. The same read `helm build` does, kept here because helm-mcp
+/// deliberately does not depend on the unit file readers.
+fn megamek_version(install: &std::path::Path) -> Option<String> {
+    let text = std::fs::read_to_string(install.join("docs/history.txt")).ok()?;
+    text.lines()
+        .map(str::trim)
+        .find(|line| line.starts_with(|c: char| c.is_ascii_digit()))
+        .and_then(|line| line.split_whitespace().next())
+        .map(str::to_string)
+}
diff --git a/crates/helm-mcp/src/main.rs b/crates/helm-mcp/src/main.rs
index 0433def..0e1b0c7 100644
--- a/crates/helm-mcp/src/main.rs
+++ b/crates/helm-mcp/src/main.rs
@@ -1,6 +1,9 @@
 //! An MCP server over helm's unit database.
 //!
-//!     helm-mcp <path-to-helm.sqlite>      (or set HELM_DB)
+//!     helm-mcp <path-to-helm.sqlite> [megamek-install]
+//!
+//! Or set `HELM_DB` and `HELM_MEGAMEK`. The install is optional and is only
+//! used to say whether the database was built from it.
 //!
 //! Scoped to BattleMeks. The tools answer from `helm-facet`'s predicate rather
 //! than from SQL, so what they return is what MegaMek's own filtering would
@@ -209,7 +212,15 @@ async fn main() -> Result<(), Box<dyn std::error::Error>> {
         .or_else(|| std::env::var("HELM_DB").ok())
         .ok_or("usage: helm-mcp <helm.sqlite>   (or set HELM_DB)")?;
 
-    let lib = Library::open(&path)?;
+    let mut lib = Library::open(&path)?;
+    // Where the install is, if the caller says: a database built from another
+    // MegaMek answers every question about a library that is not there.
+    if let Some(install) = std::env::args()
+        .nth(2)
+        .or_else(|| std::env::var("HELM_MEGAMEK").ok())
+    {
+        lib.check_against(std::path::Path::new(&install));
+    }
     // stdout is the protocol channel, so progress goes to stderr or nowhere.
     eprintln!(
         "helm-mcp: {} Meks from MegaMek {} ({})",
@@ -218,6 +229,10 @@ async fn main() -> Result<(), Box<dyn std::error::Error>> {
         path
     );
 
+    if let Some(stale) = &lib.stale {
+        eprintln!("helm-mcp: WARNING - {stale}");
+    }
+
     let service = Helm::new(Arc::new(lib)).serve(stdio()).await?;
     service.waiting().await?;
     Ok(())
diff --git a/crates/helm-mcp/src/tools.rs b/crates/helm-mcp/src/tools.rs
index 32b71f4..888d243 100644
--- a/crates/helm-mcp/src/tools.rs
+++ b/crates/helm-mcp/src/tools.rs
@@ -374,6 +374,7 @@ pub fn list_facets(lib: &Library) -> Value {
         "scope": "BattleMeks only, canon and valid",
         "unit_count": lib.len(),
         "megamek_version": lib.megamek_version,
+        "stale": lib.stale,
         "computed_by": lib.stats_producer,
         "tech_base": facet_values(lib, |f| f.tech_base.clone()),
         "role": facet_values(lib, |f| f.role.clone()),
diff --git a/crates/helm-mcp/tests/tools.rs b/crates/helm-mcp/tests/tools.rs
index 967e3a6..1bf26a4 100644
--- a/crates/helm-mcp/tests/tools.rs
+++ b/crates/helm-mcp/tests/tools.rs
@@ -672,3 +672,33 @@ fn a_book_selects_the_designs_that_are_in_it() {
         .collect();
     assert!(names.contains(&"TR:SW"), "{names:?}");
 }
+
+// A database built from one MegaMek and served beside another answers every
+// question about a library that is not there. The check is a string
+// comparison; it just had nowhere to be made.
+#[test]
+#[ignore = "needs a built database; set HELM_DB"]
+fn a_database_says_when_it_was_built_from_another_megamek() {
+    let mut lib = library();
+    assert!(lib.stale.is_none(), "a fresh library is not stale");
+    assert_eq!(tools::list_facets(&lib)["stale"], serde_json::Value::Null);
+
+    // An install that is not there says nothing, rather than guessing.
+    lib.check_against(std::path::Path::new("/nonexistent"));
+    assert!(lib.stale.is_none());
+
+    // One that declares a different version says so, and says it where an
+    // agent reading list_facets will see it.
+    let older = std::env::temp_dir().join("helm-stale-check/docs");
+    std::fs::create_dir_all(&older).unwrap();
+    std::fs::write(older.join("history.txt"), "0.49.19 (2019-01-01)\n").unwrap();
+    lib.check_against(older.parent().unwrap());
+    let stale = lib.stale.clone().expect("a version mismatch is stale");
+    assert!(stale.contains("0.49.19"), "{stale}");
+    assert!(
+        tools::list_facets(&lib)["stale"]
+            .as_str()
+            .is_some_and(|s| s.contains("rebuild")),
+        "the warning has to reach the tool output"
+    );
+}