diff --git a/crates/capsem-api/src/lifecycle.rs b/crates/capsem-api/src/lifecycle.rs
index e7c2a9a9b..d80ab6e0e 100644
--- a/crates/capsem-api/src/lifecycle.rs
+++ b/crates/capsem-api/src/lifecycle.rs
@@ -15,6 +15,27 @@ pub struct StopResponse {
     pub persistent: bool,
 }
 
+/// Validate a VM name or identifier (`1..=64` ASCII `[A-Za-z0-9_-]` characters,
+/// starting with an ASCII letter or digit) so service paths and gateway routes
+/// enforce one rule.
+pub fn validate_name(kind: &str, value: &str) -> Result<(), String> {
+    let Some(first) = value.chars().next() else {
+        return Err(format!("{kind} cannot be empty"));
+    };
+    if value.len() > 64 {
+        return Err(format!("{kind} too long (max 64 characters)"));
+    }
+    if !first.is_ascii_alphanumeric() {
+        return Err(format!("{kind} must start with a letter or digit"));
+    }
+    if !value.chars().all(|c| c.is_ascii_alphanumeric() || c == '-' || c == '_') {
+        return Err(format!(
+            "{kind} must contain only letters, digits, hyphens, and underscores"
+        ));
+    }
+    Ok(())
+}
+
 /// Unknown fields are refused rather than ignored.
 #[derive(Serialize, Deserialize, Debug, Clone, Default, ToSchema)]
 #[serde(deny_unknown_fields)]
diff --git a/crates/capsem-api/src/tests.rs b/crates/capsem-api/src/tests.rs
index 4a876ff35..cbccfa57c 100644
--- a/crates/capsem-api/src/tests.rs
+++ b/crates/capsem-api/src/tests.rs
@@ -422,3 +422,42 @@ fn exec_target_defaults_to_the_workload_only_where_there_is_one() {
     // An absent target stays absent on the wire, so older services decode it.
     assert_eq!(serde_json::to_value(&absent).unwrap(), json!({"command": "id -u"}));
 }
+
+#[test]
+fn validate_name_enforces_shared_vm_identifier_rule() {
+    for valid in [
+        "a",
+        "9lives",
+        "my-vm",
+        "project_alpha",
+        "vm-1712678400",
+        &"a".repeat(64),
+    ] {
+        assert_eq!(validate_name("VM name", valid), Ok(()), "{valid}");
+        assert_eq!(validate_name("VM id", valid), Ok(()), "{valid}");
+    }
+
+    assert_eq!(validate_name("VM name", ""), Err("VM name cannot be empty".to_string()));
+    assert_eq!(validate_name("VM id", ""), Err("VM id cannot be empty".to_string()));
+    assert_eq!(
+        validate_name("VM name", &"a".repeat(65)),
+        Err("VM name too long (max 64 characters)".to_string())
+    );
+    for bad_start in ["-bad", "_bad", ".bad"] {
+        assert_eq!(
+            validate_name("VM id", bad_start),
+            Err("VM id must start with a letter or digit".to_string()),
+            "{bad_start}"
+        );
+    }
+    for bad_body in ["my/vm", "../escape", "my vm", "my.vm", "vm\0id", "nai\u{00ef}ve"] {
+        assert!(
+            validate_name("VM name", bad_body).is_err(),
+            "{bad_body} should be rejected"
+        );
+    }
+    assert_eq!(
+        validate_name("VM name", "my.vm"),
+        Err("VM name must contain only letters, digits, hyphens, and underscores".to_string())
+    );
+}
diff --git a/crates/capsem-gateway/src/stream.rs b/crates/capsem-gateway/src/stream.rs
index 76c6eacfc..40de0c1ed 100644
--- a/crates/capsem-gateway/src/stream.rs
+++ b/crates/capsem-gateway/src/stream.rs
@@ -22,21 +22,9 @@ const REQUEST_HEADERS: [http::HeaderName; 5] = [
 const RESPONSE_HEADERS: [http::HeaderName; 4] = [CONNECTION, UPGRADE, SEC_WEBSOCKET_ACCEPT, SEC_WEBSOCKET_PROTOCOL];
 
 /// Validate VM ID: alphanumeric, hyphens, underscores. Must start with
-/// alphanumeric, length 1-64. Matches capsem-service's `validate_vm_name`.
-pub(crate) fn validate_vm_id(id: &str) -> Result<(), &'static str> {
-    if id.is_empty() {
-        return Err("VM id cannot be empty");
-    }
-    if id.len() > 64 {
-        return Err("VM id too long (max 64 characters)");
-    }
-    if !id.chars().next().unwrap().is_ascii_alphanumeric() {
-        return Err("VM id must start with a letter or digit");
-    }
-    if !id.chars().all(|c| c.is_ascii_alphanumeric() || c == '-' || c == '_') {
-        return Err("VM id must contain only letters, digits, hyphens, and underscores");
-    }
-    Ok(())
+/// alphanumeric, length 1-64.
+pub(crate) fn validate_vm_id(id: &str) -> Result<(), String> {
+    capsem_api::validate_name("VM id", id)
 }
 
 fn refuse(status: http::StatusCode, message: &str) -> Response {
@@ -49,7 +37,7 @@ pub async fn handle_stream_tunnel(
     mut request: Request,
 ) -> Response {
     if let Err(message) = validate_vm_id(&id) {
-        return refuse(http::StatusCode::BAD_REQUEST, message);
+        return refuse(http::StatusCode::BAD_REQUEST, &message);
     }
     let is_upgrade = request
         .headers()
diff --git a/crates/capsem-service/src/naming.rs b/crates/capsem-service/src/naming.rs
index 62873ba86..47f682a3e 100644
--- a/crates/capsem-service/src/naming.rs
+++ b/crates/capsem-service/src/naming.rs
@@ -1,6 +1,6 @@
 //! VM-name helpers: generated session names and persistent-name validation.
 
-use anyhow::{anyhow, Result};
+use anyhow::Result;
 use rand::Rng;
 
 /// The first free `vm-N` among `existing` names.
@@ -31,21 +31,7 @@ where
 /// - starts with an ASCII letter or digit (no leading hyphen/underscore)
 /// - consists only of ASCII alphanumerics, `-`, or `_`
 pub fn validate_vm_name(name: &str) -> Result<()> {
-    if name.is_empty() {
-        return Err(anyhow!("VM name cannot be empty"));
-    }
-    if name.len() > 64 {
-        return Err(anyhow!("VM name too long (max 64 characters)"));
-    }
-    if !name.chars().next().unwrap().is_ascii_alphanumeric() {
-        return Err(anyhow!("VM name must start with a letter or digit"));
-    }
-    if !name.chars().all(|c| c.is_ascii_alphanumeric() || c == '-' || c == '_') {
-        return Err(anyhow!(
-            "VM name must contain only letters, digits, hyphens, and underscores"
-        ));
-    }
-    Ok(())
+    capsem_api::validate_name("VM name", name).map_err(anyhow::Error::msg)
 }
 
 #[cfg(test)]