diff --git a/classical/unstable.v b/classical/unstable.v
index e331d2836..8a82f1539 100644
--- a/classical/unstable.v
+++ b/classical/unstable.v
@@ -47,6 +47,20 @@ Unset Printing Implicit Defensive.
 Import Order.TTheory GRing.Theory Num.Theory.
 Local Open Scope ring_scope.
 
+(* Backward compatibility with math-comp < 2.6, which has no [conjFieldType]
+ (the common ancestor of [rcfType] and [numClosedFieldType]).  When it is
+ absent we alias it to [rcfType] via a global parsing-only abbreviation, so
+ that MCA files endowing [conjFieldType] with topological/normed  structures
+ still compile: those instances then become redundant copies of  the [rcfType]
+ ones (their [HB.no-new-instance] is silenced at each site). *)
+Elpi Command analysis_declare_conjFieldType_compat.
+Elpi Accumulate lp:{{
+  main [] :- coq.locate-all "Num.ConjField.type" L, L = [_|_], !.
+  main [] :- @global! => coq.notation.add-abbreviation "conjFieldType" 0
+               {{ Num.RealClosedField.type }} tt _.
+}}.
+Elpi analysis_declare_conjFieldType_compat.
+
 Section IntervalNumDomain.
 Variable R : numDomainType.
 Implicit Types x : R.
diff --git a/theories/normedtype_theory/normed_module.v b/theories/normedtype_theory/normed_module.v
index 30e02866e..d0d20b781 100644
--- a/theories/normedtype_theory/normed_module.v
+++ b/theories/normedtype_theory/normed_module.v
@@ -221,6 +221,16 @@ HB.instance Definition _ := NormedModule.copy R R^o.
 HB.instance Definition _ := Num.ClosedField.on R.
 End numClosedFieldType.
 
+Section conjFieldType.
+Variable R : conjFieldType.
+#[export, non_forgetful_inheritance, warnings="-HB.no-new-instance"]
+HB.instance Definition _ := GRing.ComAlgebra.copy R R^o.
+#[export, non_forgetful_inheritance, warnings="-HB.no-new-instance"]
+HB.instance Definition _ := Vector.copy R R^o.
+#[export, non_forgetful_inheritance, warnings="-HB.no-new-instance"]
+HB.instance Definition _ := NormedModule.copy R R^o.
+End conjFieldType.
+
 Section numFieldType.
 Variable (R : numFieldType).
 #[export, non_forgetful_inheritance]
diff --git a/theories/topology_theory/num_topology.v b/theories/topology_theory/num_topology.v
index 9c90ac510..12985b65c 100644
--- a/theories/topology_theory/num_topology.v
+++ b/theories/topology_theory/num_topology.v
@@ -1,6 +1,8 @@
 (* mathcomp analysis (c) 2026 Inria and AIST. License: CeCILL-C.              *)
 From HB Require Import structures.
 From mathcomp Require Import all_ssreflect_compat all_algebra all_classical.
+#[warning="-warn-library-file-internal-analysis"]
+From mathcomp Require Import unstable.
 From mathcomp Require Import interval_inference reals topology_structure.
 From mathcomp Require Import uniform_structure pseudometric_structure.
 From mathcomp Require Import order_topology matrix_topology.
@@ -130,6 +132,10 @@ HB.instance Definition _ (R : realFieldType) := PseudoPointedMetric.copy R R^o.
 HB.instance Definition _ (R : numClosedFieldType) :=
   PseudoPointedMetric.copy R R^o.
 
+#[export, non_forgetful_inheritance, warnings="-HB.no-new-instance"]
+HB.instance Definition _ (R : conjFieldType) :=
+  PseudoPointedMetric.copy R R^o.
+
 #[export, non_forgetful_inheritance]
 HB.instance Definition _ (R : numFieldType) := PseudoPointedMetric.copy R R^o.