From 9cc633dcb04d401b8884612b00cc9a1bd075cf56 Mon Sep 17 00:00:00 2001
From: Oscar Dowson <o.dowson@gmail.com>
Date: Wed, 29 Jul 2026 13:56:28 +1200
Subject: [PATCH 1/4] Improve error messages of various errors

---
 src/Bridges/lazy_bridge_optimizer.jl | 20 +++++++++-
 src/attributes.jl                    | 48 ++++++++++++++++------
 src/constraints.jl                   | 60 +++++++++++++++++++---------
 src/indextypes.jl                    |  7 +++-
 test/General/test_attributes.jl      |  9 ++++-
 test/General/test_errors.jl          | 47 +++++++++++++---------
 6 files changed, 137 insertions(+), 54 deletions(-)

diff --git a/src/Bridges/lazy_bridge_optimizer.jl b/src/Bridges/lazy_bridge_optimizer.jl
index 7d0d97f050..8823451864 100644
--- a/src/Bridges/lazy_bridge_optimizer.jl
+++ b/src/Bridges/lazy_bridge_optimizer.jl
@@ -544,6 +544,22 @@ function is_variable_bridged(b::LazyBridgeOptimizer, S::Type{<:MOI.AbstractSet})
     return is_bridged(b, S) && is_variable_edge_best(b.graph, node(b, S))
 end
 
+function MOI.UnsupportedConstraint{F,S}(
+    model::LazyBridgeOptimizer,
+) where {F<:MOI.AbstractFunction,S<:MOI.AbstractSet}
+    msg = """
+    ## More information
+
+    Bridges are enabled, but there are none that can rewrite the \
+    constraint into a form supported by the solver.
+
+    The model that we were unable to add the constraint to is:
+
+    $(sprint(show, model))
+    """
+    return MOI.UnsupportedConstraint{F,S}(msg)
+end
+
 function bridge_type(b::LazyBridgeOptimizer, S::Type{<:MOI.AbstractSet})
     bt = get(b.cached_bridge_type, (S,), nothing)
     if bt !== nothing
@@ -552,7 +568,7 @@ function bridge_type(b::LazyBridgeOptimizer, S::Type{<:MOI.AbstractSet})
     index = bridge_index(b, S)
     if iszero(index)
         F = MOI.Utilities.variable_function_type(S)
-        throw(MOI.UnsupportedConstraint{F,S}())
+        throw(MOI.UnsupportedConstraint{F,S}(b))
     end
     new_bt = Variable.concrete_bridge_type(b.variable_bridge_types[index], S)
     b.cached_bridge_type[(S,)] = new_bt
@@ -570,7 +586,7 @@ function bridge_type(
     end
     index = bridge_index(b, F, S)
     if iszero(index)
-        throw(MOI.UnsupportedConstraint{F,S}())
+        throw(MOI.UnsupportedConstraint{F,S}(b))
     end
     new_bt =
         Constraint.concrete_bridge_type(b.constraint_bridge_types[index], F, S)
diff --git a/src/attributes.jl b/src/attributes.jl
index f671e3d86f..efaeca0c8b 100644
--- a/src/attributes.jl
+++ b/src/attributes.jl
@@ -243,8 +243,11 @@ end
 function Base.showerror(io::IO, err::ResultIndexBoundsError)
     return print(
         io,
-        "Result index of attribute $(err.attr) out of bounds. There are " *
-        "currently $(err.result_count) solution(s) in the model.",
+        """
+        Result index of attribute $(err.attr) is out of bounds.
+
+        There are currently $(err.result_count) solution(s) in the model.
+        """,
     )
 end
 
@@ -285,8 +288,11 @@ end
 function Base.showerror(io::IO, err::ConflictIndexBoundsError)
     return print(
         io,
-        "Conflict index of attribute $(err.attr) out of bounds. There are " *
-        "currently $(err.conflict_count) conflict(s) in the model.",
+        """
+        Conflict index of attribute $(err.attr) is out of bounds.
+
+        There are currently $(err.conflict_count) conflict(s) in the model.
+        """,
     )
 end
 
@@ -1261,9 +1267,13 @@ end
 function Base.showerror(io::IO, err::OptimizeInProgress)
     return print(
         io,
-        typeof(err),
-        ": Cannot get result as the `MOI.optimize!` has not",
-        " finished.",
+        """
+        Unable to get the attribute $(err.attr) because `MOI.optimize!` in progress.
+
+        This error occurs when you try to query an unsupported attribute from \
+        inside a callback. Consult the solver's documentation to learn which \
+        attributes may be queried inside a callback.
+        """
     )
 end
 
@@ -2633,8 +2643,15 @@ struct FunctionTypeMismatch{F1,F2} <: Exception end
 function Base.showerror(io::IO, err::FunctionTypeMismatch{F1,F2}) where {F1,F2}
     return print(
         io,
-        """$(typeof(err)): Cannot modify functions of different types.
-  Constraint type is $F1 while the replacement function is of type $F2.""",
+        """
+        Cannot modify functions of different types.
+
+        The original function is a $F1
+
+        The replacement function is a $F2
+
+        When modifying the `ConstraintFunction` the types must be the same.
+        """,
     )
 end
 
@@ -2714,9 +2731,16 @@ struct SetTypeMismatch{S1,S2} <: Exception end
 function Base.showerror(io::IO, err::SetTypeMismatch{S1,S2}) where {S1,S2}
     return print(
         io,
-        """$(typeof(err)): Cannot modify sets of different types. Constraint
-  type is $S1 while the replacement set is of type $S2. Use `transform`
-  instead.""",
+        """
+        Cannot modify sets of different types.
+
+        The original set is a $S1
+
+        The replacement set is a $S2
+
+        When modifying the `ConstraintSet` the types must be the same.
+        Alternatively, you may have meant to call `MOI.transform` instead.
+        """,
     )
 end
 
diff --git a/src/constraints.jl b/src/constraints.jl
index bba3a5a01a..dfc17a6218 100644
--- a/src/constraints.jl
+++ b/src/constraints.jl
@@ -35,35 +35,45 @@ the model, that is, that [`supports_constraint`](@ref) returns `false`.
 
 ```jldoctest
 julia> showerror(stdout, MOI.UnsupportedConstraint{MOI.VariableIndex,MOI.ZeroOne}())
-UnsupportedConstraint: `MathOptInterface.VariableIndex`-in-`MathOptInterface.ZeroOne` constraints are not supported by the
-solver you have chosen, and we could not reformulate your model into a
-form that is supported.
+UnsupportedConstraint{
+    MathOptInterface.VariableIndex,
+    MathOptInterface.ZeroOne,
+}
 
-To fix this error you must choose a different solver.
+This constraint type is not supported by the solver.
 
+To fix this error you must choose a different solver.
 ```
 """
 struct UnsupportedConstraint{F<:AbstractFunction,S<:AbstractSet} <:
        UnsupportedError
     # Human-friendly explanation why the attribute cannot be set
     message::String
-end
 
-UnsupportedConstraint{F,S}() where {F,S} = UnsupportedConstraint{F,S}("")
+    function UnsupportedConstraint{F,S}(
+        msg::String = "",
+    ) where {F<:AbstractFunction,S<:AbstractSet}
+        return new{F,S}(msg)
+    end
+end
 
 function Base.showerror(io::IO, err::UnsupportedConstraint{F,S}) where {F,S}
     print(
         io,
         """
-        UnsupportedConstraint: `$F`-in-`$S` constraints are not supported by the
-        solver you have chosen, and we could not reformulate your model into a
-        form that is supported.
+        UnsupportedConstraint{
+            $F,
+            $S,
+        }
 
-        To fix this error you must choose a different solver.
+        This constraint type is not supported by the solver.
 
-        $(err.message)
+        To fix this error you must choose a different solver.
         """,
     )
+    if !isempty(err.message)
+        print(io, "\n", err.message)
+    end
     return
 end
 
@@ -289,10 +299,16 @@ end
 function Base.showerror(io::IO, err::LowerBoundAlreadySet{S1,S2}) where {S1,S2}
     return print(
         io,
-        typeof(err),
-        ": Cannot add `VariableIndex`-in-`$(S2)` constraint for variable ",
-        "$(err.vi) as a `VariableIndex`-in-`$(S1)` constraint was already ",
-        "set for this variable and both constraints set a lower bound.",
+        """
+        LowerBoundAlreadySet{
+            $S1,
+            $S2,
+        }
+
+        You cannot add a `VariableIndex`-in-`$(S2)` constraint for variable \
+        "$(err.vi) because a `VariableIndex`-in-`$(S1)` constraint was already \
+        set for this variable and both constraints define a lower bound.
+        """,
     )
 end
 
@@ -311,10 +327,16 @@ end
 function Base.showerror(io::IO, err::UpperBoundAlreadySet{S1,S2}) where {S1,S2}
     return print(
         io,
-        typeof(err),
-        ": Cannot add `VariableIndex`-in-`$(S2)` constraint for variable ",
-        "$(err.vi) as a `VariableIndex`-in-`$(S1)` constraint was already ",
-        "set for this variable and both constraints set an upper bound.",
+        """
+        UpperBoundAlreadySet{
+            $S1,
+            $S2,
+        }
+
+        You cannot add a `VariableIndex`-in-`$(S2)` constraint for variable \
+        "$(err.vi) because a `VariableIndex`-in-`$(S1)` constraint was already \
+        set for this variable and both constraints define an upper bound.
+        """,
     )
 end
 
diff --git a/src/indextypes.jl b/src/indextypes.jl
index 4fa471dbab..6568c91772 100644
--- a/src/indextypes.jl
+++ b/src/indextypes.jl
@@ -94,7 +94,12 @@ end
 function Base.showerror(io::IO, err::InvalidIndex)
     return print(
         io,
-        "The index $(err.index) is invalid. Note that an index becomes invalid after it has been deleted.",
+        """
+        The index $(err.index) is invalid.
+
+        An index becomes invalid after it has been deleted. Alternatively, this \
+        error might mean that you have re-used the index from a different model.
+        """,
     )
 end
 
diff --git a/test/General/test_attributes.jl b/test/General/test_attributes.jl
index 68e6685df7..b7420ca723 100644
--- a/test/General/test_attributes.jl
+++ b/test/General/test_attributes.jl
@@ -393,7 +393,14 @@ end
 function test_showerror_OptimizeInProgress()
     err = MOI.OptimizeInProgress(MOI.VariablePrimal())
     @test sprint(showerror, err) ==
-          "$(typeof(err)): Cannot get result as the `MOI.optimize!` has not finished."
+          """
+          Unable to get the attribute MathOptInterface.VariablePrimal(1) \
+          because `MOI.optimize!` in progress.
+
+          This error occurs when you try to query an unsupported attribute from \
+          inside a callback. Consult the solver's documentation to learn which \
+          attributes may be queried inside a callback.
+          """
     return
 end
 
diff --git a/test/General/test_errors.jl b/test/General/test_errors.jl
index 2f3da89a08..e7a599c6cf 100644
--- a/test/General/test_errors.jl
+++ b/test/General/test_errors.jl
@@ -67,13 +67,14 @@ function test_errors_UnsupportedConstraint()
         MOI.add_constraint(model, vi, MOI.EqualTo(0)),
     )
     msg = """
-    UnsupportedConstraint: `$(MOI.VariableIndex)`-in-`$(MOI.EqualTo{Int})` constraints are not supported by the
-    solver you have chosen, and we could not reformulate your model into a
-    form that is supported.
-
-    To fix this error you must choose a different solver.
+    UnsupportedConstraint{
+        $(MOI.VariableIndex),
+        $(MOI.EqualTo{Int}),
+    }
 
+    This constraint type is not supported by the solver.
 
+    To fix this error you must choose a different solver.
     """
     try
         MOI.add_constraint(model, vi, MOI.EqualTo(0))
@@ -297,22 +298,25 @@ function test_errors_show_SetAttributeNotAllowed()
 end
 
 function test_errors_ResultIndexBoundsError()
-    @test sprint(
-        showerror,
-        MOI.ResultIndexBoundsError(MOI.VariablePrimal(1), 0),
-    ) ==
-          "Result index of attribute MathOptInterface.VariablePrimal(1) out of" *
-          " bounds. There are currently 0 solution(s) in the model."
+    err = MOI.ResultIndexBoundsError(MOI.VariablePrimal(1), 0)
+    @test sprint(showerror, err) ==
+          """
+          Result index of attribute MathOptInterface.VariablePrimal(1) is out of bounds.
+
+          There are currently 0 solution(s) in the model.
+          """
+    return
 end
 
 function test_errors_ConflictIndexBoundsError()
-    @test sprint(
-        showerror,
-        MOI.ConflictIndexBoundsError(MOI.ConstraintConflictStatus(1), 0),
-    ) ==
-          "Conflict index of attribute " *
-          "MathOptInterface.ConstraintConflictStatus(1) out of bounds. " *
-          "There are currently 0 conflict(s) in the model."
+    err = MOI.ConflictIndexBoundsError(MOI.ConstraintConflictStatus(1), 0)
+    @test sprint(showerror, err) ==
+          """
+          Conflict index of attribute MathOptInterface.ConstraintConflictStatus(1) is out of bounds.
+
+          There are currently 0 conflict(s) in the model.
+          """
+    return
 end
 
 function test_errors_InvalidCalbackUsage()
@@ -393,7 +397,12 @@ end
 function test_showerror_InvalidIndex()
     x = MOI.VariableIndex(1)
     @test sprint(showerror, MOI.InvalidIndex(x)) ==
-          "The index $x is invalid. Note that an index becomes invalid after it has been deleted."
+          """
+          The index $x is invalid.
+
+          An index becomes invalid after it has been deleted. Alternatively, this \
+          error might mean that you have re-used the index from a different model.
+          """
     return
 end
 

From 42dab1fb1f378bb180e2516b569bf0a51eac7e71 Mon Sep 17 00:00:00 2001
From: Oscar Dowson <o.dowson@gmail.com>
Date: Wed, 29 Jul 2026 14:07:43 +1200
Subject: [PATCH 2/4] Update

---
 src/constraints.jl               |  4 ++--
 test/General/test_constraints.jl | 26 ++++++++++++++++++++------
 2 files changed, 22 insertions(+), 8 deletions(-)

diff --git a/src/constraints.jl b/src/constraints.jl
index dfc17a6218..6f32342995 100644
--- a/src/constraints.jl
+++ b/src/constraints.jl
@@ -306,7 +306,7 @@ function Base.showerror(io::IO, err::LowerBoundAlreadySet{S1,S2}) where {S1,S2}
         }
 
         You cannot add a `VariableIndex`-in-`$(S2)` constraint for variable \
-        "$(err.vi) because a `VariableIndex`-in-`$(S1)` constraint was already \
+        $(err.vi) because a `VariableIndex`-in-`$(S1)` constraint was already \
         set for this variable and both constraints define a lower bound.
         """,
     )
@@ -334,7 +334,7 @@ function Base.showerror(io::IO, err::UpperBoundAlreadySet{S1,S2}) where {S1,S2}
         }
 
         You cannot add a `VariableIndex`-in-`$(S2)` constraint for variable \
-        "$(err.vi) because a `VariableIndex`-in-`$(S1)` constraint was already \
+        $(err.vi) because a `VariableIndex`-in-`$(S1)` constraint was already \
         set for this variable and both constraints define an upper bound.
         """,
     )
diff --git a/test/General/test_constraints.jl b/test/General/test_constraints.jl
index 50baa5e5d8..686a8e9b39 100644
--- a/test/General/test_constraints.jl
+++ b/test/General/test_constraints.jl
@@ -47,9 +47,16 @@ function test_LowerBoundAlreadySet_error()
     err = MOI.LowerBoundAlreadySet{S1,S2}(x)
     @test err isa Exception
     @test sprint(showerror, err) ==
-          "$(typeof(err)): Cannot add `VariableIndex`-in-`$(S2)` constraint " *
-          "for variable $(x) as a `VariableIndex`-in-`$(S1)` constraint was " *
-          "already set for this variable and both constraints set a lower bound."
+          """
+          LowerBoundAlreadySet{
+              $S1,
+              $S2,
+          }
+
+          You cannot add a `VariableIndex`-in-`$(S2)` constraint for variable \
+          $(x) because a `VariableIndex`-in-`$(S1)` constraint was already set \
+          for this variable and both constraints define a lower bound.
+          """
     return
 end
 
@@ -60,9 +67,16 @@ function test_UpperBoundAlreadySet_error()
     err = MOI.UpperBoundAlreadySet{S1,S2}(x)
     @test err isa Exception
     @test sprint(showerror, err) ==
-          "$(typeof(err)): Cannot add `VariableIndex`-in-`$(S2)` constraint " *
-          "for variable $(x) as a `VariableIndex`-in-`$(S1)` constraint was " *
-          "already set for this variable and both constraints set an upper bound."
+          """
+          UpperBoundAlreadySet{
+              $S1,
+              $S2,
+          }
+
+          You cannot add a `VariableIndex`-in-`$(S2)` constraint for variable \
+          $(x) because a `VariableIndex`-in-`$(S1)` constraint was already set \
+          for this variable and both constraints define an upper bound.
+          """
     return
 end
 

From 9a9e8742d806ad7c0b1b848dc03357cf7a5e982a Mon Sep 17 00:00:00 2001
From: Oscar Dowson <o.dowson@gmail.com>
Date: Wed, 29 Jul 2026 14:32:01 +1200
Subject: [PATCH 3/4] Update

---
 src/attributes.jl                             |  2 +-
 .../General/test_lazy_bridge_optimizer.jl     | 29 +++++++-------
 test/General/test_attributes.jl               | 18 ++++-----
 test/General/test_constraints.jl              | 40 +++++++++----------
 test/General/test_errors.jl                   | 36 ++++++++---------
 5 files changed, 63 insertions(+), 62 deletions(-)

diff --git a/src/attributes.jl b/src/attributes.jl
index efaeca0c8b..b88d6258dc 100644
--- a/src/attributes.jl
+++ b/src/attributes.jl
@@ -1273,7 +1273,7 @@ function Base.showerror(io::IO, err::OptimizeInProgress)
         This error occurs when you try to query an unsupported attribute from \
         inside a callback. Consult the solver's documentation to learn which \
         attributes may be queried inside a callback.
-        """
+        """,
     )
 end
 
diff --git a/test/Bridges/General/test_lazy_bridge_optimizer.jl b/test/Bridges/General/test_lazy_bridge_optimizer.jl
index bae95797b9..fcd65517b5 100644
--- a/test/Bridges/General/test_lazy_bridge_optimizer.jl
+++ b/test/Bridges/General/test_lazy_bridge_optimizer.jl
@@ -1772,17 +1772,18 @@ function test_UnsupportedConstraint_when_it_cannot_be_bridged()
     mock = MOI.Utilities.MockOptimizer(NoRSOCModel{Float64}())
     bridged_mock = MOI.Bridges.LazyBridgeOptimizer(mock)
     x = MOI.add_variables(bridged_mock, 4)
-    err = MOI.UnsupportedConstraint{
-        MOI.VectorOfVariables,
-        MOI.RotatedSecondOrderCone,
-    }()
-    @test_throws err begin
+    @test_throws(
+        MOI.UnsupportedConstraint{
+            MOI.VectorOfVariables,
+            MOI.RotatedSecondOrderCone,
+        },
         MOI.add_constraint(
             bridged_mock,
             MOI.VectorOfVariables(x),
             MOI.RotatedSecondOrderCone(4),
-        )
-    end
+        ),
+    )
+    return
 end
 
 function test_MOI_runtests_No_RSOCModel()
@@ -1839,17 +1840,17 @@ function test_bridge_selection()
         MOI.LogDetConeTriangle,
     ))
     x = MOI.add_variables(bridged_mock, 3)
-    err = MOI.UnsupportedConstraint{
-        MOI.VectorAffineFunction{Float64},
-        MOI.LogDetConeTriangle,
-    }()
-    @test_throws err begin
+    @test_throws(
+        MOI.UnsupportedConstraint{
+            MOI.VectorAffineFunction{Float64},
+            MOI.LogDetConeTriangle,
+        },
         MOI.Bridges.bridge_type(
             bridged_mock,
             MOI.VectorAffineFunction{Float64},
             MOI.LogDetConeTriangle,
-        )
-    end
+        ),
+    )
     c = MOI.add_constraint(
         bridged_mock,
         MOI.VectorOfVariables(x),
diff --git a/test/General/test_attributes.jl b/test/General/test_attributes.jl
index b7420ca723..6c6cdc740b 100644
--- a/test/General/test_attributes.jl
+++ b/test/General/test_attributes.jl
@@ -392,15 +392,15 @@ end
 
 function test_showerror_OptimizeInProgress()
     err = MOI.OptimizeInProgress(MOI.VariablePrimal())
-    @test sprint(showerror, err) ==
-          """
-          Unable to get the attribute MathOptInterface.VariablePrimal(1) \
-          because `MOI.optimize!` in progress.
-
-          This error occurs when you try to query an unsupported attribute from \
-          inside a callback. Consult the solver's documentation to learn which \
-          attributes may be queried inside a callback.
-          """
+    ret = """
+    Unable to get the attribute MathOptInterface.VariablePrimal(1) \
+    because `MOI.optimize!` in progress.
+
+    This error occurs when you try to query an unsupported attribute from \
+    inside a callback. Consult the solver's documentation to learn which \
+    attributes may be queried inside a callback.
+    """
+    @test sprint(showerror, err) == ret
     return
 end
 
diff --git a/test/General/test_constraints.jl b/test/General/test_constraints.jl
index 686a8e9b39..ac0e72743a 100644
--- a/test/General/test_constraints.jl
+++ b/test/General/test_constraints.jl
@@ -46,17 +46,17 @@ function test_LowerBoundAlreadySet_error()
     S2 = MOI.Interval{Int}
     err = MOI.LowerBoundAlreadySet{S1,S2}(x)
     @test err isa Exception
-    @test sprint(showerror, err) ==
-          """
-          LowerBoundAlreadySet{
-              $S1,
-              $S2,
-          }
+    ret = """
+    LowerBoundAlreadySet{
+        $S1,
+        $S2,
+    }
 
-          You cannot add a `VariableIndex`-in-`$(S2)` constraint for variable \
-          $(x) because a `VariableIndex`-in-`$(S1)` constraint was already set \
-          for this variable and both constraints define a lower bound.
-          """
+    You cannot add a `VariableIndex`-in-`$(S2)` constraint for variable \
+    $(x) because a `VariableIndex`-in-`$(S1)` constraint was already set \
+    for this variable and both constraints define a lower bound.
+    """
+    @test sprint(showerror, err) == ret
     return
 end
 
@@ -66,17 +66,17 @@ function test_UpperBoundAlreadySet_error()
     S2 = MOI.Interval{Int}
     err = MOI.UpperBoundAlreadySet{S1,S2}(x)
     @test err isa Exception
-    @test sprint(showerror, err) ==
-          """
-          UpperBoundAlreadySet{
-              $S1,
-              $S2,
-          }
+    ret = """
+    UpperBoundAlreadySet{
+        $S1,
+        $S2,
+    }
 
-          You cannot add a `VariableIndex`-in-`$(S2)` constraint for variable \
-          $(x) because a `VariableIndex`-in-`$(S1)` constraint was already set \
-          for this variable and both constraints define an upper bound.
-          """
+    You cannot add a `VariableIndex`-in-`$(S2)` constraint for variable \
+    $(x) because a `VariableIndex`-in-`$(S1)` constraint was already set \
+    for this variable and both constraints define an upper bound.
+    """
+    @test sprint(showerror, err) == ret
     return
 end
 
diff --git a/test/General/test_errors.jl b/test/General/test_errors.jl
index e7a599c6cf..0f518eef9b 100644
--- a/test/General/test_errors.jl
+++ b/test/General/test_errors.jl
@@ -66,7 +66,7 @@ function test_errors_UnsupportedConstraint()
         MOI.UnsupportedConstraint,
         MOI.add_constraint(model, vi, MOI.EqualTo(0)),
     )
-    msg = """
+    ret = """
     UnsupportedConstraint{
         $(MOI.VariableIndex),
         $(MOI.EqualTo{Int}),
@@ -79,7 +79,7 @@ function test_errors_UnsupportedConstraint()
     try
         MOI.add_constraint(model, vi, MOI.EqualTo(0))
     catch err
-        @test sprint(showerror, err) == msg
+        @test sprint(showerror, err) == ret
     end
     return
 end
@@ -299,23 +299,23 @@ end
 
 function test_errors_ResultIndexBoundsError()
     err = MOI.ResultIndexBoundsError(MOI.VariablePrimal(1), 0)
-    @test sprint(showerror, err) ==
-          """
-          Result index of attribute MathOptInterface.VariablePrimal(1) is out of bounds.
+    ret = """
+    Result index of attribute MathOptInterface.VariablePrimal(1) is out of bounds.
 
-          There are currently 0 solution(s) in the model.
-          """
+    There are currently 0 solution(s) in the model.
+    """
+    @test sprint(showerror, err) == ret
     return
 end
 
 function test_errors_ConflictIndexBoundsError()
     err = MOI.ConflictIndexBoundsError(MOI.ConstraintConflictStatus(1), 0)
-    @test sprint(showerror, err) ==
-          """
-          Conflict index of attribute MathOptInterface.ConstraintConflictStatus(1) is out of bounds.
+    ret = """
+    Conflict index of attribute MathOptInterface.ConstraintConflictStatus(1) is out of bounds.
 
-          There are currently 0 conflict(s) in the model.
-          """
+    There are currently 0 conflict(s) in the model.
+    """
+    @test sprint(showerror, err) == ret
     return
 end
 
@@ -396,13 +396,13 @@ end
 
 function test_showerror_InvalidIndex()
     x = MOI.VariableIndex(1)
-    @test sprint(showerror, MOI.InvalidIndex(x)) ==
-          """
-          The index $x is invalid.
+    err = """
+    The index $x is invalid.
 
-          An index becomes invalid after it has been deleted. Alternatively, this \
-          error might mean that you have re-used the index from a different model.
-          """
+    An index becomes invalid after it has been deleted. Alternatively, this \
+    error might mean that you have re-used the index from a different model.
+    """
+    @test sprint(showerror, MOI.InvalidIndex(x)) == err
     return
 end
 

From 323cac5a926ccce71081fd4598065cce0c35a987 Mon Sep 17 00:00:00 2001
From: Oscar Dowson <o.dowson@gmail.com>
Date: Wed, 29 Jul 2026 15:27:46 +1200
Subject: [PATCH 4/4] Update

---
 .../General/test_lazy_bridge_optimizer.jl     | 41 +++++++++++++++++++
 1 file changed, 41 insertions(+)

diff --git a/test/Bridges/General/test_lazy_bridge_optimizer.jl b/test/Bridges/General/test_lazy_bridge_optimizer.jl
index fcd65517b5..2e73b72c6f 100644
--- a/test/Bridges/General/test_lazy_bridge_optimizer.jl
+++ b/test/Bridges/General/test_lazy_bridge_optimizer.jl
@@ -2704,6 +2704,47 @@ function test_custom_cost_model_bridge_selection()
     return
 end
 
+struct _UnsupportedSet <: MOI.AbstractScalarSet end
+
+function test_bridge_unsupported_constraint()
+    model = MOI.instantiate(
+        MOI.Utilities.Model{Float64};
+        with_bridge_type = Float64,
+    )
+    x = MOI.add_variable(model)
+    err = MOI.UnsupportedConstraint{MOI.VariableIndex,_UnsupportedSet}(model)
+    @test_throws err MOI.add_constraint(model, x, _UnsupportedSet())
+    @test_throws err MOI.add_constrained_variable(model, _UnsupportedSet())
+    ret = """
+    UnsupportedConstraint{
+        MathOptInterface.VariableIndex,
+        $_UnsupportedSet,
+    }
+
+    This constraint type is not supported by the solver.
+
+    To fix this error you must choose a different solver.
+
+    ## More information
+
+    Bridges are enabled, but there are none that can rewrite the constraint into a form supported by the solver.
+
+    The model that we were unable to add the constraint to is:
+
+    MOIB.LazyBridgeOptimizer{MOIU.Model{Float64}}
+    ├ Variable bridges: none
+    ├ Constraint bridges: none
+    ├ Objective bridges: none
+    └ model: MOIU.Model{Float64}
+      ├ ObjectiveSense: FEASIBILITY_SENSE
+      ├ ObjectiveFunctionType: MOI.ScalarAffineFunction{Float64}
+      ├ NumberOfVariables: 1
+      └ NumberOfConstraints: 0
+    """
+    @test sprint(showerror, err) == ret
+    return
+end
+
 end  # module
 
 TestBridgesLazyBridgeOptimizer.runtests()