diff --git a/hive-c0re/src/dashboard.rs b/hive-c0re/src/dashboard.rs
index 121e6be0..b19855cd 100644
--- a/hive-c0re/src/dashboard.rs
+++ b/hive-c0re/src/dashboard.rs
@@ -1131,6 +1131,23 @@ struct RequestSpawnForm {
 #[cfg(test)]
 mod tests {
     use super::*;
+
+    #[test]
+    fn problem_body_has_rfc9457_members() {
+        // about:blank type → title is the canonical status reason phrase,
+        // status is the numeric code, detail is the caller message.
+        let body = problem_body(StatusCode::BAD_REQUEST, "bad input");
+        assert_eq!(body["type"], "about:blank");
+        assert_eq!(body["title"], "Bad Request");
+        assert_eq!(body["status"], 400);
+        assert_eq!(body["detail"], "bad input");
+        // error_response (the 500 wrapper) carries the same shape with the
+        // internal-error status.
+        let five = problem_body(StatusCode::INTERNAL_SERVER_ERROR, "boom");
+        assert_eq!(five["status"], 500);
+        assert_eq!(five["title"], "Internal Server Error");
+    }
+
     #[test]
     fn walk_meta_inputs_keeps_nixpkgs_under_hyperhive_post_follows_refactor() {
         // Reproduce the shape where meta has
@@ -1638,8 +1655,44 @@ fn strip_container_prefix(name: &str) -> String {
         .to_owned()
 }
 
+/// The RFC 9457 problem-details media type.
+const PROBLEM_JSON_CONTENT_TYPE: &str = "application/problem+json";
+
+/// Build the RFC 9457 problem-details body for `status` + `detail`. The
+/// object carries the standard members: `type` ("about:blank", i.e. no
+/// problem-specific type), `title` (the HTTP status reason phrase),
+/// `status` (numeric code) and `detail` (the caller-supplied message).
+/// Split from [`problem_response`] so the member shape is unit-testable
+/// without axum response plumbing.
+fn problem_body(status: StatusCode, detail: &str) -> serde_json::Value {
+    serde_json::json!({
+        "type": "about:blank",
+        "title": status.canonical_reason().unwrap_or("Error"),
+        "status": status.as_u16(),
+        "detail": detail,
+    })
+}
+
+/// Build an RFC 9457 (`application/problem+json`) error response.
+/// Centralising this keeps every dashboard error on one machine-readable
+/// shape the frontend can parse (read `detail` for display) instead of
+/// guessing between plain text and JSON.
+fn problem_response(status: StatusCode, detail: &str) -> Response {
+    let body = serde_json::to_string(&problem_body(status, detail))
+        .expect("problem+json body is always serialisable");
+    (
+        status,
+        [(axum::http::header::CONTENT_TYPE, PROBLEM_JSON_CONTENT_TYPE)],
+        body,
+    )
+        .into_response()
+}
+
+/// Convenience wrapper for the common internal-error case: a 500
+/// problem-details response (see [`problem_response`]). Most dashboard
+/// handlers funnel their errors through here; handlers with a more
+/// specific failure (bad input, not found) call [`problem_response`]
+/// directly with the right status.
 fn error_response(message: &str) -> Response {
-    // Plain text — the JS app surfaces this in an alert(), so HTML
-    // wrapping would just clutter the message.
-    (StatusCode::INTERNAL_SERVER_ERROR, message.to_owned()).into_response()
+    problem_response(StatusCode::INTERNAL_SERVER_ERROR, message)
 }