diff --git a/include/mp/flat/constr_algebraic.h b/include/mp/flat/constr_algebraic.h
index 34aa3564c..96f7339b2 100644
--- a/include/mp/flat/constr_algebraic.h
+++ b/include/mp/flat/constr_algebraic.h
@@ -145,10 +145,18 @@ template <int kind_>
 class AlgConRhs {
   static constexpr const char* kind_str_[] =
   { "LT", "LE", "EQ", "GE", "GT" };
+  static constexpr const char* kind_cmp_[] =
+  { "<", "<=", "==", ">=", ">" };
 public:
   /// name
   static std::string GetTypeName()
   { return std::string("Rhs") + kind_str_[kind_+2]; }
+  /// Comparison name as string
+  static const char* GetCmpName()
+  { return kind_str_[kind_+2]; }
+  /// Comparison operator as string
+  static const char* GetCmpStr()
+  { return kind_cmp_[kind_+2]; }
   /// Constructor
   AlgConRhs(double r) : rhs_(r) { }
   /// Kind
@@ -227,6 +235,28 @@ using QuadConGE = QuadConRhs< 1>;
 /// Quadratic constraint c'x+x`Qx >  d
 using QuadConGT = QuadConRhs< 2>;
 
+
+/// Write an algebraic constraint
+template <class Body, class RhsOrRange>
+inline void WriteJSON(JSONW jw,
+                      const AlgebraicConstraint<Body, RhsOrRange>& algc) {
+  WriteJSON(jw["body"], algc.GetBody());
+  WriteJSON(jw["rhs_or_range"], algc.GetRhsOrRange());
+}
+
+/// Write alg con range
+inline void WriteJSON(JSONW jw,
+                      const AlgConRange& acr) {
+  jw << acr.lb() << acr.ub();
+}
+
+/// Write alg con rhs
+template <int kind>
+inline void WriteJSON(JSONW jw,
+                      const AlgConRhs<kind>& acrhs) {
+  jw << acrhs.GetCmpName() << acrhs.rhs();
+}
+
 } // namespace mp
 
 #endif // CONSTRAINTS_ALGEBRAIC_H
diff --git a/include/mp/flat/constr_base.h b/include/mp/flat/constr_base.h
index af94db1ce..a5e73cfb2 100644
--- a/include/mp/flat/constr_base.h
+++ b/include/mp/flat/constr_base.h
@@ -182,6 +182,16 @@ class CustomFunctionalConstraint :
 };
 
 
+/// Write a CustomFunctionalConstraint<>
+template <class JW, class A, class P, class N, class I>
+inline void WriteJSON(JW jw,
+                      const CustomFunctionalConstraint<A,P,N,I>& cfc) {
+  jw["res_var"] = cfc.GetResultVar();
+  WriteJSON(jw["args"], cfc.GetArguments());
+  WriteJSON(jw["params"], cfc.GetParameters());
+}
+
+
 /// Dummy template: compute result of functional constraint.
 /// Should be implemented for proper functional specializations,
 /// but has no sense for conic constraints, for example.
@@ -336,6 +346,15 @@ class ConditionalConstraint :
   }
 };
 
+/// Write a CondCon
+template <class JW, class Con>
+inline void WriteJSON(JW jw,
+                      const ConditionalConstraint<Con>& condc) {
+  jw["res_var"] = condc.GetResultVar();
+  WriteJSON(jw["con"], condc.GetConstraint());
+}
+
+
 ////////////////////////////////////////////////////////////////////////
 /// Args is the argument type, e.g., array of variables, or an expression
 /// Params is the parameter type, e.g., array of numbers. Can be empty
diff --git a/include/mp/flat/constr_functional.h b/include/mp/flat/constr_functional.h
index 1790f6568..e24c62b00 100644
--- a/include/mp/flat/constr_functional.h
+++ b/include/mp/flat/constr_functional.h
@@ -215,6 +215,15 @@ class LinearFunctionalConstraint :
   }
 };
 
+
+/// Write a LinFuncCon
+inline void WriteJSON(JSONW jw,
+                      const LinearFunctionalConstraint& lfc) {
+  jw["res_var"] = lfc.GetResultVar();
+  WriteJSON(jw["expr"], lfc.GetAffineExpr());
+}
+
+
 ////////////////////////////////////////////////////////////////////////
 /// Quadratic Functional Constraint: r = quad_expr
 class QuadraticFunctionalConstraint :
@@ -267,6 +276,14 @@ class QuadraticFunctionalConstraint :
 };
 
 
+/// Write a QuadrFuncCon
+inline void WriteJSON(JSONW jw,
+                      const QuadraticFunctionalConstraint& qfc) {
+  jw["res_var"] = qfc.GetResultVar();
+  WriteJSON(jw["expr"], qfc.GetQuadExpr());
+}
+
+
 ////////////////////////////////////////////////////////////////////////
 /// AMPL represents PWL by a list of slopes
 /// and breakpoints between them, assuming (X0,Y0) is on the line
@@ -375,6 +392,16 @@ class PLConParams {
 DEF_NUMERIC_FUNC_CONSTR_WITH_PRM( PLConstraint,
                   VarArray1, PLConParams, "r = piecewise_linear(x)");
 
+
+/// Write PLConParams
+inline void WriteJSON(JSONW jw,
+                      const PLConParams& plp) {
+  const auto& p = plp.GetPLPoints();
+  jw["pl_x"] = p.x_;
+  jw["pl_y"] = p.y_;
+}
+
+
 } // namespace mp
 
 #endif // CONSTRAINTS_FUNCTIONAL_H
diff --git a/include/mp/flat/constr_general.h b/include/mp/flat/constr_general.h
index 4c5a86ff4..e3ce1d9cf 100644
--- a/include/mp/flat/constr_general.h
+++ b/include/mp/flat/constr_general.h
@@ -79,6 +79,16 @@ using IndicatorConstraintQuadEQ = IndicatorConstraint<QuadConEQ>;
 using IndicatorConstraintQuadGE = IndicatorConstraint<QuadConGE>;
 
 
+/// Write an indicator constraint
+template <class Con>
+inline void WriteJSON(JSONW jw,
+                      const IndicatorConstraint<Con>& ic) {
+  jw["bin_var"] = ic.get_binary_var();
+  jw["bin_val"] = ic.get_binary_value();
+  WriteJSON(jw["con"], ic.get_constraint());
+}
+
+
 /// Unary encoding.
 /// Currently a dummy constraint just to build
 /// the reformulation graph.
@@ -222,18 +232,13 @@ using SOS2Constraint = SOS_1or2_Constraint<2>;
 
 
 /// Write a SOS1, SOS2 constraint
-template <class Writer, int type>
-inline void WriteJSON(Writer& wrt, const SOS_1or2_Constraint<type>& sos) {
-  wrt.write("{} ", '{');
-  wrt.write("\"sos_type\": {}, ", sos.get_sos_type());
-  wrt.write("\"vars\": ");
-  WriteJSONVec(wrt, sos.get_vars());
-  wrt.write(", \"weights\": ");
-  WriteJSONVec(wrt, sos.get_weights());
+template <int type>
+inline void WriteJSON(JSONW jw, const SOS_1or2_Constraint<type>& sos) {
+  jw["SOS_type"] = sos.get_sos_type();
+  jw["vars"] = sos.get_vars();
+  jw["weights"] = sos.get_weights();
   auto bnds = sos.get_sum_of_vars_range();
-  wrt.write(", \"sum_of_vars_range\": [{}, {}]",
-            bnds.lb_, bnds.ub_);
-  wrt.write(" {}", '}');
+  jw["sum_of_vars_range"] << bnds.lb_ << bnds.ub_;
 }
 
 ////////////////////////////////////////////////////////////////////////
@@ -291,6 +296,16 @@ using ComplementarityLinear = ComplementarityConstraint<AffineExpr>;
 /// Typedef ComplementarityQuadRange
 using ComplementarityQuadratic = ComplementarityConstraint<QuadraticExpr>;
 
+
+/// Write a ComplementarityCon
+template <class Expr>
+inline void WriteJSON(JSONW jw,
+                      const ComplementarityConstraint<Expr>& cc) {
+  WriteJSON(jw["expr"], cc.GetExpression());
+  jw["compl_var"] = cc.GetVariable();
+}
+
+
 /// Quadratic cone
 DEF_STATIC_CONSTR_WITH_PRM( QuadraticConeConstraint, VarArray, DblParamArray,
                             "Quadratic cone p1*x1 >= sqrt((p2*x2)^2 + ...)),"
diff --git a/include/mp/flat/constr_keeper.h b/include/mp/flat/constr_keeper.h
index 00f6b3337..a92becea7 100644
--- a/include/mp/flat/constr_keeper.h
+++ b/include/mp/flat/constr_keeper.h
@@ -884,8 +884,7 @@ class ConstraintKeeper final
         jw["CON_TYPE"] = GetShortTypeName();
         jw["index"] = i_con;
         jw["depth"] = cnt.GetDepth();
-        //      wrt.write("\"data\": ");
-        //      WriteJSON(wrt, cnt.con_);
+        WriteJSON(jw["data"], cnt.con_);
       }
       wrt.write("\n");                     // EOL
       GetLogger()->Append(wrt);
diff --git a/include/mp/flat/converter_model.h b/include/mp/flat/converter_model.h
index b7046f8ef..4c08eadca 100644
--- a/include/mp/flat/converter_model.h
+++ b/include/mp/flat/converter_model.h
@@ -74,7 +74,7 @@ class FlatModel
       fmt::MemoryWriter wrt;
       {
         MiniJSONWriter jw(wrt);
-        jw["VAR_INDEX"] = i+i_start;
+        jw["VAR_index"] = i+i_start;
         jw["bounds"] << lbs[i] << ubs[i];
         jw["type"] = (int)types[i];
         jw["is_from_nl"] = (int)is_var_original(i);
@@ -238,13 +238,10 @@ class FlatModel
       fmt::MemoryWriter wrt;
       {
         MiniJSONWriter jw(wrt);
-        jw["OBJ_INDEX"] = i_obj;
+        jw["OBJECTIVE_index"] = i_obj;
         jw["sense"] = (int)obj.obj_sense();
-        wrt.write("\"qp_terms\": ");
-        WriteJSON(wrt, obj.GetQPTerms());
-        wrt.write("\"lin_terms\": ");
-        WriteJSON(wrt, obj.GetLinTerms());
-        wrt.write(" {}\n", '}');                      // with EOL
+        WriteJSON(jw["qp_terms"], obj.GetQPTerms());
+        WriteJSON(jw["lin_terms"], obj.GetLinTerms());
       }
       wrt.write("\n");                     // EOL
       GetFileAppender().Append(wrt);
diff --git a/include/mp/flat/expr_affine.h b/include/mp/flat/expr_affine.h
index dbbad883b..262d7db2f 100644
--- a/include/mp/flat/expr_affine.h
+++ b/include/mp/flat/expr_affine.h
@@ -155,6 +155,10 @@ class LinTerms {
   std::vector<int> vars_;
 };
 
+/// Specialize
+template <>
+void WriteJSON(JSONW jw, const LinTerms& qt);
+
 
 /// Typedef AffineExpr
 using AffineExpr = AlgebraicExpression<LinTerms>;
diff --git a/include/mp/flat/expr_algebraic.h b/include/mp/flat/expr_algebraic.h
index 4ce007984..5f147ac13 100644
--- a/include/mp/flat/expr_algebraic.h
+++ b/include/mp/flat/expr_algebraic.h
@@ -9,6 +9,7 @@
 #include <string>
 #include <cmath>
 
+#include "mp/util-json-write.hpp"
 #include "mp/arrayref.h"
 
 namespace mp {
@@ -112,13 +113,23 @@ class AlgebraicExpression : public Body {
 };
 
 /// Very general template to write
-/// a JSON representation of anything
+/// a JSON-like representation of anything
 template <class Writer, class Obj>
-void WriteJSON(Writer& , const Obj& );
+void WriteJSON(Writer w, const Obj& o) {
+  w = o;       // default: assume assignable
+}              // unless WriteJSON is specialized for Obj
+
+/// Specialize our JSON writer
+using JSONW = MiniJSONWriter<fmt::MemoryWriter>;
+
+/// Write an AlgebraicExpression
+template <class Terms>
+inline void WriteJSON(JSONW jw,
+                      const AlgebraicExpression<Terms>& ale) {
+  WriteJSON(jw["body"], ale.GetBody());
+  jw["const_term"] = ale.constant_term();
+}
 
-/// Write JSON string of a vector
-template <class Writer, class Vec>
-void WriteJSONVec(Writer& wrt, const Vec& vec);
 
 } // namespace mp
 
diff --git a/include/mp/flat/expr_quadratic.h b/include/mp/flat/expr_quadratic.h
index 014fc0718..2e9be94a4 100644
--- a/include/mp/flat/expr_quadratic.h
+++ b/include/mp/flat/expr_quadratic.h
@@ -131,6 +131,9 @@ class QuadTerms {
   std::vector<int> vars2_;
 };
 
+/// Specialize
+template <>
+void WriteJSON(JSONW jw, const QuadTerms& qt);
 
 ////////////////////////////////////////////////////////////////////////
 /// Quadratic and linear terms.
@@ -229,6 +232,10 @@ class QuadAndLinTerms :
   bool operator==(const QuadAndLinTerms& qlc) const { return equals(qlc); }
 };
 
+/// Specialize
+template <>
+void WriteJSON(JSONW jw, const QuadAndLinTerms& qt);
+
 
 /// Typedef QuadraticExpr
 using QuadraticExpr = AlgebraicExpression<QuadAndLinTerms>;
diff --git a/include/mp/util-json-write.h b/include/mp/util-json-write.h
index 10ce0660d..b8f2a8020 100644
--- a/include/mp/util-json-write.h
+++ b/include/mp/util-json-write.h
@@ -65,24 +65,34 @@ class MiniJSONWriter {
   /// operator<<: make/ensure *this an array
   /// and write \a val a new element.
   ///
+  /// Equivalent: `++(*this) = val; return *this;`
+  ///
   /// @param val: scalar, tuple or container.
-  ///   For non-supported types, define global method
-  ///   Serialize(MiniJSONWriter& , const YourType& ).
+  ///   For non-supported types,
+  ///   write elements manually into
+  ///     `auto jw_child = ++jw;`,
+  ///   or define, e.g.,
+  ///     `Serialize(MiniJSONWriter , const YourType& )`,
+  ///   to be used as follows:
+  ///     `Serialize(++jw, obj[5]);`
+  ///     (instead of `jw << obj[5]`.)
   ///
   /// @return *this.
   template <class Value>
-  Node& operator<<(const Value& val) {
-    EnsureArray();
-    InsertElementSeparator();
-    Write(val);
-    return *this;
-  }
+  Node& operator<<(const Value& val)
+  { ++(*this) = val; return *this; }
 
   /// operator=: write \a val as a whole.
   ///
   /// @param val: scalar, tuple or container.
-  ///   For non-supported types, define global method
-  ///   Serialize(MiniJSONWriter& , const YourType& ).
+  ///   For non-supported types,
+  ///   write elements manually into
+  ///     `auto jw_child = jw["data"];`,
+  ///   or define, e.g.,
+  ///     `Serialize(MiniJSONWriter , const YourType& )`,
+  ///   to be used as follows:
+  ///     `Serialize(jw["data"], obj_data);`
+  ///     (instead of `jw["data"] = obj_data`.)
   ///
   /// @note Can be called only once on a single node.
   template <class Value>
@@ -155,6 +165,7 @@ class MiniJSONWriter {
                              int> = 0> // C++17
   void DoWrite(Arithmetic v) { DoWriteScalar(v); }
 
+  /// Write a container
   template <typename C,
       typename T = std::decay_t<
           decltype(*begin(std::declval<C>()))> >
diff --git a/include/mp/valcvt.h b/include/mp/valcvt.h
index 3d60b868f..21fe98b8e 100644
--- a/include/mp/valcvt.h
+++ b/include/mp/valcvt.h
@@ -206,7 +206,7 @@ class ValuePresolverImpl : public BasicValuePresolver {
   void WriteNodes(JSON jw, const std::vector<NodeRange>& nodes) {
     for (size_t i=0; i<nodes.size(); ++i) {
       const auto& nd = nodes[i];
-      auto jelement = ++jw;       // We write an array
+      auto jelement = ++jw;       // jw is an array of dict's
       if (nd.IsSingleIndex())
         jelement[nd.GetValueNode()->GetName()] = (int)nd;
       else
diff --git a/src/std_constr.cc b/src/std_constr.cc
index fbf79f987..b223aa348 100644
--- a/src/std_constr.cc
+++ b/src/std_constr.cc
@@ -46,37 +46,23 @@ void QuadTerms::sort_terms()  {
   }
 }
 
-template <class Writer, class Vec>
-void WriteJSONVec(Writer& wrt, const Vec& vec) {
-  wrt.write("{} ", '[');
-  for (size_t i=0; i<vec.size(); ++i) {
-    if (i)
-      wrt.write(", ");
-    wrt.write("{}", vec[i]);
-  }
-  wrt.write(" {}", ']');
+template <>
+void WriteJSON(JSONW jw, const QuadTerms& qt) {
+  jw["coefs"] = qt.coefs();
+  jw["vars1"] = qt.vars1();
+  jw["vars2"] = qt.vars2();
 }
 
 template <>
-void WriteJSON(fmt::MemoryWriter& wrt, const QuadTerms& qt) {
-  wrt.write("{} ", '{');
-  wrt.write("\"coefs\": ");
-  WriteJSONVec(wrt, qt.coefs());
-  wrt.write(", \"vars1\": ");
-  WriteJSONVec(wrt, qt.vars1());
-  wrt.write(", \"vars2\": ");
-  WriteJSONVec(wrt, qt.vars2());
-  wrt.write(" {}", '}');
+void WriteJSON(JSONW jw, const LinTerms& qt) {
+  jw["coefs"] = qt.coefs();
+  jw["vars"] = qt.vars();
 }
 
 template <>
-void WriteJSON(fmt::MemoryWriter& wrt, const LinTerms& qt) {
-  wrt.write("{} ", '{');
-  wrt.write("\"coefs\": ");
-  WriteJSONVec(wrt, qt.coefs());
-  wrt.write(", \"vars\": ");
-  WriteJSONVec(wrt, qt.vars());
-  wrt.write(" {}", '}');
+void WriteJSON(JSONW jw, const QuadAndLinTerms& qlt) {
+  WriteJSON(jw["qp_terms"], qlt.GetQPTerms());
+  WriteJSON(jw["lin_terms"], qlt.GetLinTerms());
 }
 
 } // namespace mp