diff --git a/.github/workflows/root-ci.yml b/.github/workflows/root-ci.yml
index 86b81cf657292..4f452d63fd26e 100644
--- a/.github/workflows/root-ci.yml
+++ b/.github/workflows/root-ci.yml
@@ -369,7 +369,7 @@ jobs:
             is_special: true
           - image: alma8
           - image: alma9
-            overrides: ["CMAKE_BUILD_TYPE=Debug", "CMAKE_CXX_FLAGS=-Wsuggest-override"]
+            overrides: ["CMAKE_BUILD_TYPE=Debug"]
           - image: alma10
           - image: ubuntu22
             overrides: ["imt=Off", "CMAKE_BUILD_TYPE=Debug"]
@@ -378,7 +378,7 @@ jobs:
           - image: ubuntu2504
             overrides: ["CMAKE_CXX_STANDARD=23"]
           - image: debian125
-            overrides: ["CMAKE_CXX_STANDARD=20", "dev=ON"]
+            overrides: ["CMAKE_CXX_STANDARD=20", "dev=ON", "CMAKE_CXX_FLAGS=-Wsuggest-override"]
           # Special builds
           - image: alma9
             is_special: true
diff --git a/core/multiproc/inc/TMPWorkerExecutor.h b/core/multiproc/inc/TMPWorkerExecutor.h
index 26ed8c3ecb9eb..fec9ddf5fb5a8 100644
--- a/core/multiproc/inc/TMPWorkerExecutor.h
+++ b/core/multiproc/inc/TMPWorkerExecutor.h
@@ -201,7 +201,7 @@ class TMPWorkerExecutor<F, void, void> : public TMPWorker {
 public:
    explicit TMPWorkerExecutor(F func) : TMPWorker(), fFunc(func) {}
    ~TMPWorkerExecutor() {}
-   void HandleInput(MPCodeBufPair &msg)
+   void HandleInput(MPCodeBufPair &msg) override
    {
       unsigned code = msg.first;
       TSocket *s = GetSocket();
diff --git a/roottest/root/io/abstractclass/DataBlock1.h b/roottest/root/io/abstractclass/DataBlock1.h
index 38359c94bc68d..907b5e00ee220 100644
--- a/roottest/root/io/abstractclass/DataBlock1.h
+++ b/roottest/root/io/abstractclass/DataBlock1.h
@@ -11,12 +11,12 @@ class DataBlock1 : public DataBlockBase, public PureAbstractInterface {
   DataBlock1();
   virtual ~DataBlock1();
  
-  virtual Short_t      GetXyzzy() const;
-  virtual Short_t      GetAbc()   const;
+  Short_t      GetXyzzy() const override;
+  Short_t      GetAbc()   const override;
 
  private:
 
-  ClassDef(DataBlock1,1)
+  ClassDefOverride(DataBlock1,1)
 };
 
 inline Short_t DataBlock1::GetXyzzy() const { return fRawBlock[0]; }
diff --git a/roottest/root/io/abstractclass/DataBlock2.h b/roottest/root/io/abstractclass/DataBlock2.h
index 8218efef91244..cdfe92e5f8fcd 100644
--- a/roottest/root/io/abstractclass/DataBlock2.h
+++ b/roottest/root/io/abstractclass/DataBlock2.h
@@ -11,12 +11,12 @@ class DataBlock2 : public DataBlockBase, public PureAbstractInterface {
   DataBlock2();
   virtual ~DataBlock2();
  
-  virtual Short_t      GetXyzzy() const;
-  virtual Short_t      GetAbc()   const;
+  Short_t      GetXyzzy() const override;
+  Short_t      GetAbc()   const override;
 
  private:
 
-  ClassDef(DataBlock2,1)
+  ClassDefOverride(DataBlock2,1)
 };
 
 inline Short_t DataBlock2::GetXyzzy() const { return fRawBlock[0]; }
diff --git a/roottest/root/io/abstractclass/DataBlockBase.h b/roottest/root/io/abstractclass/DataBlockBase.h
index 66e28b92c39d7..7311c6374a0cb 100644
--- a/roottest/root/io/abstractclass/DataBlockBase.h
+++ b/roottest/root/io/abstractclass/DataBlockBase.h
@@ -13,7 +13,7 @@ class DataBlockBase : public TObject {
   inline const Int_t*  GetData() const { return fRawBlock; }
   inline Int_t         GetSize() const { return fSize; }
 
-  virtual void         Print(Option_t *option="") const;
+  void         Print(Option_t *option="") const override;
 
  protected:   // allow derived classes direct access to the data
 
@@ -22,7 +22,7 @@ class DataBlockBase : public TObject {
 
  private:
 
-  ClassDef(DataBlockBase,1)
+  ClassDefOverride(DataBlockBase,1)
 };
 
 #endif
diff --git a/roottest/root/io/arrayobject/foo.h b/roottest/root/io/arrayobject/foo.h
index 748173a27bccb..117ba25dcb9a1 100644
--- a/roottest/root/io/arrayobject/foo.h
+++ b/roottest/root/io/arrayobject/foo.h
@@ -13,7 +13,7 @@ class foobj : public TObject {
   Int_t i;
   Float_t f;
   
-  ClassDef(foobj,1)
+  ClassDefOverride(foobj,1)
 };
 
 class foo {
diff --git a/roottest/root/io/clones/RtObj.h b/roottest/root/io/clones/RtObj.h
index 869bf264bc39f..7f53442b6e15c 100644
--- a/roottest/root/io/clones/RtObj.h
+++ b/roottest/root/io/clones/RtObj.h
@@ -6,10 +6,5 @@ class RtObj : public T
   public:
     RtObj() {}
     RtObj( const T & val ) : T(val) {}
-    ClassDefT(RtObj,1)
+    ClassDefOverride(RtObj,1)
  } ;
-
-ClassDefT2(RtObj,T)
-
-ClassImpT(RtObj,T)
-
diff --git a/roottest/root/io/compression/Event.h b/roottest/root/io/compression/Event.h
index dbff9179d881e..600baa1758554 100644
--- a/roottest/root/io/compression/Event.h
+++ b/roottest/root/io/compression/Event.h
@@ -148,7 +148,7 @@ class Event : public TObject {
                  Event();
                  Event(Int_t i);
    virtual      ~Event();
-   void          Clear(Option_t *option ="");
+   void          Clear(Option_t *option ="") override;
    TDatime      &GetDatime() {return fDatime;}
    static void   Reset(Option_t *option ="");
    void          ResetHistogramPointer() {fH=0;}
@@ -179,7 +179,7 @@ class Event : public TObject {
 
    UShortVector* GetUshort() { return &fUshort; }
 
-   ClassDef(Event,1)  //Event structure
+   ClassDefOverride(Event,1)  //Event structure
 };
 
 
@@ -238,7 +238,7 @@ class Track : public TObject {
    Short_t       GetValid()  const { return fValid; }
    virtual void  SetValid(Int_t valid=1) { fValid = valid; }
 
-   ClassDef(Track,1)  //A track segment
+   ClassDefOverride(Track,1)  //A track segment
 };
 
 class BigTrack : public Track {
@@ -252,7 +252,7 @@ class BigTrack : public Track {
    BigTrack(Float_t random, Int_t special);
    virtual ~BigTrack() { }
 
-   ClassDef(BigTrack,1)  //A Big track
+   ClassDefOverride(BigTrack,1)  //A Big track
 };
 
 
diff --git a/roottest/root/io/evolution/equivalent/ioReadRuleEquivalentCode.h b/roottest/root/io/evolution/equivalent/ioReadRuleEquivalentCode.h
index f01d1023b1ac9..7e38bbca4f6d6 100644
--- a/roottest/root/io/evolution/equivalent/ioReadRuleEquivalentCode.h
+++ b/roottest/root/io/evolution/equivalent/ioReadRuleEquivalentCode.h
@@ -4,7 +4,7 @@ class A:public TObject{
 
 public:
    A(){}
-   ClassDef(A,1)
+   ClassDefOverride(A,1)
 
 };
 
@@ -13,7 +13,7 @@ class B:public TObject{
 public:
    B(){}
    void initializeTransients(){transient_=2;}
-   ClassDef(B,1)
+   ClassDefOverride(B,1)
 private:
    double transient_; //!
 
diff --git a/roottest/root/io/evolution/pragma_read/v1/AliAODForwardMult.h b/roottest/root/io/evolution/pragma_read/v1/AliAODForwardMult.h
index 4ac86ad3beeb6..4dd7868c9453d 100644
--- a/roottest/root/io/evolution/pragma_read/v1/AliAODForwardMult.h
+++ b/roottest/root/io/evolution/pragma_read/v1/AliAODForwardMult.h
@@ -277,19 +277,19 @@ class AliAODForwardMult : public TObject
    * 
    * @param option  Passed on to TH2::Reset verbatim
    */
-  void Clear(Option_t* option="");
+  void Clear(Option_t* option="") override;
   /** 
    * browse this object 
    * 
    * @param b Browser 
    */
-  void Browse(TBrowser* b);
+  void Browse(TBrowser* b) override;
   /** 
    * This is a folder 
    * 
    * @return Always true
    */
-  Bool_t IsFolder() const { return kTRUE; } // Always true 
+  Bool_t IsFolder() const override { return kTRUE; } // Always true 
 
   Bool_t IsSecondaryCorrected() const { return TestBit(kSecondary); }
   Bool_t IsVertexBiasCorrected() const { return TestBit(kVertexBias); }
@@ -303,7 +303,7 @@ class AliAODForwardMult : public TObject
    * 
    * @param option Passed verbatim to TH2::Print 
    */
-  void Print(Option_t* option="") const;
+  void Print(Option_t* option="") const override;
   /** 
    * Set the z coordinate of the interaction point
    * 
@@ -401,7 +401,7 @@ class AliAODForwardMult : public TObject
    * 
    * @return Name of object 
    */
-  const Char_t* GetName() const { return (fIsMC ? "ForwardMC" : "Forward"); }
+  const Char_t* GetName() const override { return (fIsMC ? "ForwardMC" : "Forward"); }
   /** 
    * Check if event meets the passses requirements.   
    *
@@ -497,7 +497,7 @@ class AliAODForwardMult : public TObject
   UShort_t fNClusters;  // Number of SPD clusters in |eta|<1
   /** Invalid value for interaction point @f$z@f$ coordiante */
   static const Float_t fgkInvalidIpZ; // Invalid IpZ value 
-  ClassDef(AliAODForwardMult,5); // AOD forward multiplicity 
+  ClassDefOverride(AliAODForwardMult,5); // AOD forward multiplicity 
 };
 
 //____________________________________________________________________
diff --git a/roottest/root/io/evolution/pragma_read/v2/AliAODForwardHeader.h b/roottest/root/io/evolution/pragma_read/v2/AliAODForwardHeader.h
index 9439ad8524891..8a40e7773dd96 100644
--- a/roottest/root/io/evolution/pragma_read/v2/AliAODForwardHeader.h
+++ b/roottest/root/io/evolution/pragma_read/v2/AliAODForwardHeader.h
@@ -108,7 +108,7 @@ class AliAODForwardHeader : public TObject
    * 
    * @param option  Not used
    */
-  void Clear(Option_t* option="");
+  void Clear(Option_t* option="") override;
   /** 
    * Set the trigger mask 
    * 
@@ -271,25 +271,25 @@ class AliAODForwardHeader : public TObject
    * 
    * @param option Passed verbatim to TH2::Print 
    */
-  void Print(Option_t* option="") const;
+  void Print(Option_t* option="") const override;
   /** 
    * Get the name of the object 
    * 
    * @return Name of object 
    */
-  const Char_t* GetName() const { return "ForwardHeader"; }
+  const Char_t* GetName() const override { return "ForwardHeader"; }
   /** 
    * browse this object 
    * 
    * @param b Browser 
    */
-  void Browse(TBrowser* b);
+  void Browse(TBrowser* b) override;
   /** 
    * This is a folder 
    * 
    * @return Always true
    */
-  Bool_t IsFolder() const { return kTRUE; } // Always true 
+  Bool_t IsFolder() const override { return kTRUE; } // Always true 
   /* @} */
   /** 
    * @{
@@ -354,7 +354,7 @@ class AliAODForwardHeader : public TObject
   UShort_t fNClusters;  // Number of SPD clusters in |eta|<1
   /** Invalid value for interaction point @f$z@f$ coordiante */
   static const Float_t fgkInvalidIpZ; // Invalid IpZ value 
-  ClassDef(AliAODForwardHeader,1); // AOD forward header 
+  ClassDefOverride(AliAODForwardHeader,1); // AOD forward header 
 };
 
 //____________________________________________________________________
diff --git a/roottest/root/io/evolution/pragma_read/v2/AliAODForwardMult.h b/roottest/root/io/evolution/pragma_read/v2/AliAODForwardMult.h
index 14cceae20d29e..ba60619435af9 100644
--- a/roottest/root/io/evolution/pragma_read/v2/AliAODForwardMult.h
+++ b/roottest/root/io/evolution/pragma_read/v2/AliAODForwardMult.h
@@ -175,19 +175,19 @@ class AliAODForwardMult : public TObject
    * 
    * @param option  Passed on to TH2::Reset verbatim
    */
-  void Clear(Option_t* option="");
+  void Clear(Option_t* option="") override;
   /** 
    * browse this object 
    * 
    * @param b Browser 
    */
-  void Browse(TBrowser* b);
+  void Browse(TBrowser* b) override;
   /** 
    * This is a folder 
    * 
    * @return Always true
    */
-  Bool_t IsFolder() const { return kTRUE; } // Always true 
+  Bool_t IsFolder() const override { return kTRUE; } // Always true 
   /** 
    * @return @c true if secondary corrected
    */
@@ -237,13 +237,13 @@ class AliAODForwardMult : public TObject
    * 
    * @param option Passed verbatim to TH2::Print 
    */
-  void Print(Option_t* option="") const;
+  void Print(Option_t* option="") const override;
   /** 
    * Get the name of the object 
    * 
    * @return Name of object 
    */
-  const Char_t* GetName() const { return (fIsMC ? "ForwardMC" : "Forward"); }
+  const Char_t* GetName() const override { return (fIsMC ? "ForwardMC" : "Forward"); }
   /** 
    * Create a backward compatiblity header 
    * 
@@ -263,7 +263,7 @@ class AliAODForwardMult : public TObject
   TH2D     fHist;       // Histogram of d^2N_{ch}/(deta dphi) for this event
   AliAODForwardHeader* fHeader; //! Cached header 
 
-  ClassDef(AliAODForwardMult,6); // AOD forward multiplicity 
+  ClassDefOverride(AliAODForwardMult,6); // AOD forward multiplicity 
 };
 
 #endif
diff --git a/roottest/root/io/json/test_classes.h b/roottest/root/io/json/test_classes.h
index 7c13b453a3c10..aca359adde3cc 100644
--- a/roottest/root/io/json/test_classes.h
+++ b/roottest/root/io/json/test_classes.h
@@ -305,7 +305,7 @@ class TJsonEx4 : public TJsonEx1 {
          strcpy((char*)fStr4, "--- normal string value ---");
       }
 
-      void Print()
+      void Print() override
       {
           TJsonEx1::Print();
           std::cout << "   fStr1 = " << fStr1 << std::endl;
diff --git a/roottest/root/io/rootcint/foo.hh b/roottest/root/io/rootcint/foo.hh
index 75609662b7801..d97a4ad3415fa 100644
--- a/roottest/root/io/rootcint/foo.hh
+++ b/roottest/root/io/rootcint/foo.hh
@@ -9,7 +9,7 @@ class foo
 {
 public:
   void a();
-  ClassDef (foo, 1);
+  ClassDefOverride (foo, 1);
 };
 
 }
diff --git a/roottest/root/io/treeproblem/Foo.h b/roottest/root/io/treeproblem/Foo.h
index 3fcf4a81e46cc..64d91e15be267 100644
--- a/roottest/root/io/treeproblem/Foo.h
+++ b/roottest/root/io/treeproblem/Foo.h
@@ -22,9 +22,9 @@ class Foo : public TObject
 
   void  SetFoo(Int_t foo=0) { fFoo = foo; }
   Int_t GetFoo() const { return fFoo; }
-  void  Print(Option_t * = "") const;
+  void  Print(Option_t * = "") const override;
 
-  ClassDef(Foo,1) // DOCUMENT ME
+  ClassDefOverride(Foo,1) // DOCUMENT ME
 };
 
 #endif
diff --git a/roottest/root/io/xml/test_classes.h b/roottest/root/io/xml/test_classes.h
index 1a2e852871f2e..a367ba0a2aa65 100644
--- a/roottest/root/io/xml/test_classes.h
+++ b/roottest/root/io/xml/test_classes.h
@@ -204,7 +204,7 @@ class TXmlEx4 : public TXmlEx1 {
          delete[] fStr3;
       }
 
-      void Print()
+      void Print() override
       {
           TXmlEx1::Print();
           std::cout << "   fStr1 = " << fStr1 << std::endl;
diff --git a/roottest/root/meta/iotypeioname/classes.h b/roottest/root/meta/iotypeioname/classes.h
index 2441b11c60e53..81e3671e96231 100644
--- a/roottest/root/meta/iotypeioname/classes.h
+++ b/roottest/root/meta/iotypeioname/classes.h
@@ -48,7 +48,7 @@ class Container: public TObject {
    SuperCluster m_sc;
    Particle m_part;
 
-ClassDef(Container,1);
+ClassDefOverride(Container,1);
 };
 
 #endif
diff --git a/roottest/root/ntuple/streamerfield/Event_v2.hxx b/roottest/root/ntuple/streamerfield/Event_v2.hxx
index a83bb08a14427..a18b8cb897c5c 100644
--- a/roottest/root/ntuple/streamerfield/Event_v2.hxx
+++ b/roottest/root/ntuple/streamerfield/Event_v2.hxx
@@ -17,7 +17,7 @@ struct StreamerDerived : public StreamerBase {
    int fSecond = 2;
    virtual ~StreamerDerived() = default;
 
-   ClassDef(StreamerDerived, 2)
+   ClassDefOverride(StreamerDerived, 2)
 };
 
 struct StreamerContainer {
diff --git a/roottest/root/ntuple/streamerfield/Event_v3.hxx b/roottest/root/ntuple/streamerfield/Event_v3.hxx
index 67730d351b728..59f4ec5cb1e49 100644
--- a/roottest/root/ntuple/streamerfield/Event_v3.hxx
+++ b/roottest/root/ntuple/streamerfield/Event_v3.hxx
@@ -17,7 +17,7 @@ struct StreamerDerived : public StreamerBase {
    int fFirst = 1;
    virtual ~StreamerDerived() = default;
 
-   ClassDef(StreamerDerived, 3)
+   ClassDefOverride(StreamerDerived, 3)
 };
 
 struct StreamerContainer {