diff --git a/src/Firely.Fhir.Validation/Impl/BindingValidator.cs b/src/Firely.Fhir.Validation/Impl/BindingValidator.cs
index 63cfefb6..e6aa02f2 100644
--- a/src/Firely.Fhir.Validation/Impl/BindingValidator.cs
+++ b/src/Firely.Fhir.Validation/Impl/BindingValidator.cs
@@ -146,11 +146,11 @@ private ResultReport verifyContentRequirements(PocoNode source, DataType bindabl
                 case Coding cd when string.IsNullOrEmpty(cd.Code) && Strength == BindingStrength.Required:
                 case CodeableConcept cc when !codeableConceptHasCode(cc) && Strength == BindingStrength.Required:
                     return new IssueAssertion(Issue.TERMINOLOGY_INCOMPLETE_CODE_ERROR,
-                        $"No code found in {source.Poco.TypeName} with a required binding to valueset '{ValueSetUri}'.").AsResult(s, source, nameof(BindingValidator));
+                        $"No code found in {source.Poco.TypeName} with a required binding to valueset '{ValueSetUri}'.").AsResult(s, source, nameof(BindingValidator), this);
                 case CodeableConcept cc when !codeableConceptHasCode(cc) && string.IsNullOrEmpty(cc.Text) &&
                                 Strength == BindingStrength.Extensible:
                     return new IssueAssertion(Issue.TERMINOLOGY_INCOMPLETE_CODE_WARNING,
-                        $"Extensible binding to valueset '{ValueSetUri}' requires code or text.").AsResult(s, source, nameof(BindingValidator));
+                        $"Extensible binding to valueset '{ValueSetUri}' requires code or text.").AsResult(s, source, nameof(BindingValidator), this);
                 default:
                     return ResultReport.SUCCESS;      // nothing wrong then
             }
@@ -196,7 +196,8 @@ ValidateCodeParameters buildParams()
             return result switch
             {
                 (null, _) => ResultReport.SUCCESS,
-                ({ } issue, var message) => new IssueAssertion(issue, (issue.Severity == OperationOutcome.IssueSeverity.Error ? message! + ", but the binding is of strength 'required'" : message!)).AsResult(s, input, nameof(BindingValidator))
+                ({ } issue, var message) => new IssueAssertion(issue, (issue.Severity == OperationOutcome.IssueSeverity.Error ? message! + ", but the binding is of strength 'required'" : message!))
+                    .AsResult(s, input, nameof(BindingValidator), this)
             };
         }
 
diff --git a/src/Firely.Fhir.Validation/Impl/CanonicalValidator.cs b/src/Firely.Fhir.Validation/Impl/CanonicalValidator.cs
index 03856ca9..1246f28a 100644
--- a/src/Firely.Fhir.Validation/Impl/CanonicalValidator.cs
+++ b/src/Firely.Fhir.Validation/Impl/CanonicalValidator.cs
@@ -39,10 +39,10 @@ ResultReport IValidatable.Validate(PocoNode input, ValidationSettings vc, Valida
                 return canonical.HasAnchor || canonical.IsAbsolute
                     ? ResultReport.SUCCESS
                     : new IssueAssertion(Issue.CONTENT_ELEMENT_INVALID_PRIMITIVE_VALUE,
-                        $"Canonical URLs must be absolute URLs if they are not fragment references").AsResult(state, input, nameof(CanonicalValidator));
+                        $"Canonical URLs must be absolute URLs if they are not fragment references").AsResult(state, input, nameof(CanonicalValidator), this);
 
             return new IssueAssertion(Issue.CONTENT_ELEMENT_INVALID_PRIMITIVE_VALUE,
-                $"Primitive does not have the correct type ({input.Poco.TypeName})").AsResult(state, input, nameof(CanonicalValidator));
+                $"Primitive does not have the correct type ({input.Poco.TypeName})").AsResult(state, input, nameof(CanonicalValidator), this);
         }
     }
 }
\ No newline at end of file
diff --git a/src/Firely.Fhir.Validation/Impl/CardinalityValidator.cs b/src/Firely.Fhir.Validation/Impl/CardinalityValidator.cs
index f2fe9f61..5105fb9d 100644
--- a/src/Firely.Fhir.Validation/Impl/CardinalityValidator.cs
+++ b/src/Firely.Fhir.Validation/Impl/CardinalityValidator.cs
@@ -101,7 +101,7 @@ ResultReport IGroupValidatable.Validate(IEnumerable<PocoNode> input, ValidationS
 
         private ResultReport buildResult(PocoNode parent, string elemName, int count, ValidationState s) => !inRange(count) ?
                         new IssueAssertion(Issue.CONTENT_INCORRECT_OCCURRENCE,
-                        $"Instance count at element '{elemName}' is {count}, which is not within the specified cardinality of {CardinalityDisplay}").AsResult(s, parent, nameof(CardinalityValidator))
+                        $"Instance count at element '{elemName}' is {count}, which is not within the specified cardinality of {CardinalityDisplay}").AsResult(s, parent, nameof(CardinalityValidator), this)
                         : ResultReport.SUCCESS;
 
         /// <inheritdoc />
diff --git a/src/Firely.Fhir.Validation/Impl/ChildrenValidator.cs b/src/Firely.Fhir.Validation/Impl/ChildrenValidator.cs
index b45cefa3..1c8040c7 100644
--- a/src/Firely.Fhir.Validation/Impl/ChildrenValidator.cs
+++ b/src/Firely.Fhir.Validation/Impl/ChildrenValidator.cs
@@ -100,7 +100,7 @@ ResultReport IValidatable.Validate(PocoNode input, ValidationSettings vc, Valida
             {
                 var elementList = string.Join(",", matchResult.UnmatchedInstanceElements.Select(e => $"'{e.Name}'"));
                 evidence.Add(new IssueAssertion(Issue.CONTENT_ELEMENT_HAS_UNKNOWN_CHILDREN, $"Encountered unknown child elements {elementList}")
-                    .AsResult(state, input, nameof(ChildrenValidator)));
+                    .AsResult(state, input, nameof(ChildrenValidator), this));
             }
 
             evidence.AddRange(
diff --git a/src/Firely.Fhir.Validation/Impl/ExtensionContextValidator.cs b/src/Firely.Fhir.Validation/Impl/ExtensionContextValidator.cs
index 8a029c1c..cd53ccdc 100644
--- a/src/Firely.Fhir.Validation/Impl/ExtensionContextValidator.cs
+++ b/src/Firely.Fhir.Validation/Impl/ExtensionContextValidator.cs
@@ -27,6 +27,7 @@ namespace Firely.Fhir.Validation;
 #endif
 public class ExtensionContextValidator : IValidatable
 {
+    private const string CONTEXT_INVARIANT_KEY = "ctx-inv";
     /// <summary>
     /// Creates a new ExtensionContextValidator with the given allowed contexts and invariants.
     /// </summary>
@@ -69,7 +70,7 @@ public ResultReport Validate(PocoNode input, ValidationSettings vc, ValidationSt
         {
             return new IssueAssertion(Issue.CONTENT_INCORRECT_OCCURRENCE,
                     $"Extension used outside of appropriate contexts. Expected context to be one of: {RenderExpectedContexts}")
-                .AsResult(state, input, nameof(ExtensionContextValidator));
+                .AsResult(state, input, nameof(ExtensionContextValidator), this);
         }
 
         var invariantResults = Invariants
@@ -86,9 +87,8 @@ public ResultReport Validate(PocoNode input, ValidationSettings vc, ValidationSt
                 {
                     // If eval to false, throw an error
                     (false, null) =>
-                        new IssueAssertion(
-                            Issue.CONTENT_ELEMENT_FAILS_ERROR_CONSTRAINT,
-                            $"Extension context failed invariant constraint {res.Invariant}").AsResult(state, input, nameof(ExtensionContextValidator)),
+                        new IssueAssertion(Issue.CONTENT_ELEMENT_FAILS_ERROR_CONSTRAINT, $"Extension context failed invariant constraint {res.Invariant}")
+                            .AsResult(state, input, nameof(ExtensionContextValidator), new FhirPathValidator(res.Invariant ?? CONTEXT_INVARIANT_KEY, "")),
                     // If evalutation threw an exception, return that exception
                     (_, { } report) => report,
                     // Otherwise return success
@@ -287,7 +287,7 @@ private static InvariantValidator.InvariantResult runContextInvariant(PocoNode i
     {
         // our invariant is defined with %extension, but the FhirPathValidator expects %%extension because that is our syntax for environment variables
         // TODO investigate changing this in the SDK
-        var fhirPathValidator = new FhirPathValidator("ctx-inv", invariant.Replace("%extension", "%%extension"));
+        var fhirPathValidator = new FhirPathValidator(CONTEXT_INVARIANT_KEY, invariant.Replace("%extension", "%%extension"));
         return fhirPathValidator.RunInvariant(input.ToPocoNode().Parent!, vc, state, ("extension", [input.ToPocoNode()]));
     }
 
diff --git a/src/Firely.Fhir.Validation/Impl/ExtensionSchema.cs b/src/Firely.Fhir.Validation/Impl/ExtensionSchema.cs
index 8213f5c8..73b27360 100644
--- a/src/Firely.Fhir.Validation/Impl/ExtensionSchema.cs
+++ b/src/Firely.Fhir.Validation/Impl/ExtensionSchema.cs
@@ -98,7 +98,7 @@ internal override ResultReport ValidateInternal(IEnumerable<PocoNode> input, Val
 
                             evidence.Add(new ResultReport(vr,
                                 new IssueAssertion(issue, $"Unable to resolve reference to extension '{group.Key}'.")
-                                    .AsResult(state, group.First(), nameof(ExtensionSchema)).Evidence));
+                                    .AsResult(state, group.First(), nameof(ExtensionSchema), this).Evidence));
 
                             // No url available - validate the Extension schema itself.
                             evidence.Add(ValidateExtensionSchema(group, vc, state));
diff --git a/src/Firely.Fhir.Validation/Impl/FhirPathValidator.cs b/src/Firely.Fhir.Validation/Impl/FhirPathValidator.cs
index 3611b76a..f3367858 100644
--- a/src/Firely.Fhir.Validation/Impl/FhirPathValidator.cs
+++ b/src/Firely.Fhir.Validation/Impl/FhirPathValidator.cs
@@ -129,7 +129,7 @@ private InvariantResult runInvariantInternal(PocoNode input, ValidationSettings
             {
                 return new(false, new IssueAssertion(Issue.PROFILE_ELEMENTDEF_INVALID_FHIRPATH_EXPRESSION,
                         $"Evaluation of FhirPath for constraint '{Key}' failed: {e.Message}")
-                    .AsResult(s, input, nameof(FhirPathValidator)));
+                    .AsResult(s, input, nameof(FhirPathValidator), this));
             }
         }
 
diff --git a/src/Firely.Fhir.Validation/Impl/FhirStringValidator.cs b/src/Firely.Fhir.Validation/Impl/FhirStringValidator.cs
index 81ffcb3f..437733cb 100644
--- a/src/Firely.Fhir.Validation/Impl/FhirStringValidator.cs
+++ b/src/Firely.Fhir.Validation/Impl/FhirStringValidator.cs
@@ -28,10 +28,10 @@ ResultReport IValidatable.Validate(PocoNode input, ValidationSettings vc, Valida
         {
             if (input is not PrimitiveNode { Primitive: FhirString str })
                 return new IssueAssertion(Issue.CONTENT_ELEMENT_INVALID_PRIMITIVE_VALUE,
-                    $"Primitive does not have the correct type ({input.Poco.TypeName})").AsResult(state, input, nameof(FhirStringValidator));
+                    $"Primitive does not have the correct type ({input.Poco.TypeName})").AsResult(state, input, nameof(FhirStringValidator), this);
             if (!str.HasValidValue())
                 return new IssueAssertion(Issue.CONTENT_ELEMENT_INVALID_PRIMITIVE_VALUE,
-                    $"String values cannot be empty").AsResult(state, input, nameof(FhirStringValidator));
+                    $"String values cannot be empty").AsResult(state, input, nameof(FhirStringValidator), this);
             return ResultReport.SUCCESS;
         }
     }
diff --git a/src/Firely.Fhir.Validation/Impl/FhirTxt1Validator.cs b/src/Firely.Fhir.Validation/Impl/FhirTxt1Validator.cs
index 669d2615..f242a585 100644
--- a/src/Firely.Fhir.Validation/Impl/FhirTxt1Validator.cs
+++ b/src/Firely.Fhir.Validation/Impl/FhirTxt1Validator.cs
@@ -50,7 +50,7 @@ internal override InvariantResult RunInvariant(PocoNode input, ValidationSetting
             if (primitive is not {Poco: XHtml xhtml})
                 return new(false, 
                     new IssueAssertion(Issue.CONTENT_ELEMENT_INVALID_PRIMITIVE_VALUE,
-                        $"Narrative should be of type string, but is of type ({primitive.Poco.GetType()})").AsResult(state, input));
+                        $"Narrative should be of type string, but is of type ({primitive.Poco.GetType()})").AsResult(state, input, nameof(FhirTxt1Validator), this));
         
             // Check if the narrative contains only the basic HTML formatting elements and attributesvar result = XHtml.IsValidNarrativeXhtml(input.Value.ToString()!, out var malformedError, out var narrativeIssues);
 
@@ -63,8 +63,8 @@ internal override InvariantResult RunInvariant(PocoNode input, ValidationSetting
             else
             {
                 var issueReports = malformedError is null 
-                    ? narrativeIssues.Select(e => new IssueAssertion(Issue.XSD_VALIDATION_ERROR, e).AsResult(state, input)).ToArray()
-                    : [new IssueAssertion(Issue.XSD_VALIDATION_ERROR, malformedError).AsResult(state, input)];
+                    ? narrativeIssues.Select(e => new IssueAssertion(Issue.XSD_VALIDATION_ERROR, e).AsResult(state, input, nameof(FhirTxt1Validator), this)).ToArray()
+                    : [new IssueAssertion(Issue.XSD_VALIDATION_ERROR, malformedError).AsResult(state, input, nameof(FhirTxt1Validator), this)];
                 return new(false, ResultReport.Combine(issueReports));
             }
         }
diff --git a/src/Firely.Fhir.Validation/Impl/FhirTypeLabelValidator.cs b/src/Firely.Fhir.Validation/Impl/FhirTypeLabelValidator.cs
index 3e30fdde..ec0b4a70 100644
--- a/src/Firely.Fhir.Validation/Impl/FhirTypeLabelValidator.cs
+++ b/src/Firely.Fhir.Validation/Impl/FhirTypeLabelValidator.cs
@@ -54,7 +54,7 @@ internal override ResultReport BasicValidate(PocoNode input, ValidationSettings
                 ResultReport.SUCCESS :
                 new IssueAssertion(Issue.CONTENT_ELEMENT_HAS_INCORRECT_TYPE,
                     $"The declared type of the element ({Label}) is incompatible with that of the instance ({input.Poco.TypeName}).")
-                    .AsResult(s, input, nameof(FhirTypeLabelValidator));
+                    .AsResult(s, input, nameof(FhirTypeLabelValidator), this);
             //
             return result;
         }
diff --git a/src/Firely.Fhir.Validation/Impl/FhirUriValidator.cs b/src/Firely.Fhir.Validation/Impl/FhirUriValidator.cs
index bb320466..491195bd 100644
--- a/src/Firely.Fhir.Validation/Impl/FhirUriValidator.cs
+++ b/src/Firely.Fhir.Validation/Impl/FhirUriValidator.cs
@@ -21,7 +21,7 @@ public class FhirUriValidator : BasicValidator
 {
     internal override ResultReport BasicValidate(PocoNode input, ValidationSettings vc, ValidationState state) => 
         (input is PrimitiveNode {Poco: FhirUri uri}) 
-            ? uri.HasValidValue() && !string.IsNullOrWhiteSpace(uri.Value) ? ResultReport.SUCCESS : new IssueAssertion(Issue.CONTENT_ELEMENT_INVALID_PRIMITIVE_VALUE, $"Value '{uri}' is not a valid URI").AsResult(state, input, nameof(FhirUriValidator))
+            ? uri.HasValidValue() && !string.IsNullOrWhiteSpace(uri.Value) ? ResultReport.SUCCESS : new IssueAssertion(Issue.CONTENT_ELEMENT_INVALID_PRIMITIVE_VALUE, $"Value '{uri}' is not a valid URI").AsResult(state, input, nameof(FhirUriValidator), this)
             : ResultReport.SUCCESS; // TODO remove this check when the SDK is updated
 
     /// <inheritdoc />
diff --git a/src/Firely.Fhir.Validation/Impl/FixedValidator.cs b/src/Firely.Fhir.Validation/Impl/FixedValidator.cs
index ee0e768c..d1ae645c 100644
--- a/src/Firely.Fhir.Validation/Impl/FixedValidator.cs
+++ b/src/Firely.Fhir.Validation/Impl/FixedValidator.cs
@@ -51,7 +51,7 @@ ResultReport IValidatable.Validate(PocoNode input, ValidationSettings _, Validat
             {
                 return new IssueAssertion(Issue.CONTENT_DOES_NOT_MATCH_FIXED_VALUE,
                         $"Value '{displayValue(input)}' is not exactly equal to fixed value '{displayValue(FixedValue)}'")
-                        .AsResult(s, input, nameof(FixedValidator));
+                        .AsResult(s, input, nameof(FixedValidator), this);
             }
 
             return ResultReport.SUCCESS;
diff --git a/src/Firely.Fhir.Validation/Impl/InvariantValidator.cs b/src/Firely.Fhir.Validation/Impl/InvariantValidator.cs
index d1ebb016..0f780211 100644
--- a/src/Firely.Fhir.Validation/Impl/InvariantValidator.cs
+++ b/src/Firely.Fhir.Validation/Impl/InvariantValidator.cs
@@ -87,7 +87,8 @@ ResultReport IValidatable.Validate(PocoNode input, ValidationSettings vc, Valida
                 return new IssueAssertion(sev == IssueSeverity.Error ?
                         Issue.CONTENT_ELEMENT_FAILS_ERROR_CONSTRAINT :
                         Issue.CONTENT_ELEMENT_FAILS_WARNING_CONSTRAINT,
-                        $"Instance failed constraint {getDescription()}").AsResult(s, input, nameof(InvariantValidator));
+                        $"Instance failed constraint {getDescription()}").AsResult(s, input, nameof(InvariantValidator),
+                        this);
             }
             else
                 return ResultReport.SUCCESS;
diff --git a/src/Firely.Fhir.Validation/Impl/IssueAssertion.cs b/src/Firely.Fhir.Validation/Impl/IssueAssertion.cs
index 57851e5e..251f5c56 100644
--- a/src/Firely.Fhir.Validation/Impl/IssueAssertion.cs
+++ b/src/Firely.Fhir.Validation/Impl/IssueAssertion.cs
@@ -6,7 +6,6 @@
  * available at https://github.com/FirelyTeam/firely-validator-api/blob/main/LICENSE
  */
 
-using Hl7.Fhir.ElementModel;
 using Hl7.Fhir.Model;
 using Hl7.Fhir.Serialization;
 using Hl7.Fhir.Support;
@@ -14,7 +13,6 @@
 using Newtonsoft.Json.Linq;
 using System;
 using System.Collections.Generic;
-using System.Runtime.CompilerServices;
 using System.Runtime.Serialization;
 using static Hl7.Fhir.Model.OperationOutcome;
 
@@ -88,6 +86,11 @@ public class IssueAssertion : IFixedResult, IValidatable, IEquatable<IssueAssert
     /// </summary>
     internal string? IssueSource { get; private set; }
 
+    /// <summary>
+    /// An assertion that resulted in the raised issue.
+    /// </summary>
+    internal IAssertion? Assertion { get; private set; }
+
     /// <summary>
     /// Interprets the <see cref="IssueSeverity" /> of the assertion as a <see cref="ValidationResult" />
     /// to be used by the validator for deriving the result of the validation.
@@ -125,7 +128,7 @@ public IssueAssertion(int issueNumber, string message, IssueSeverity severity, I
     {
     }
 
-    private IssueAssertion(int issueNumber, string? location, DefinitionPath? definitionPath, string message, IssueSeverity severity, IssueType? type = null, IPositionInfo? positionInfo = null, string? issueSource = null)
+    private IssueAssertion(int issueNumber, string? location, DefinitionPath? definitionPath, string message, IssueSeverity severity, IssueType? type = null, IPositionInfo? positionInfo = null, string? issueSource = null, IAssertion? assertion = null)
     {
         IssueNumber = issueNumber;
         Location = location;
@@ -135,6 +138,7 @@ private IssueAssertion(int issueNumber, string? location, DefinitionPath? defini
         Type = type;
         PositionInfo = positionInfo;
         IssueSource = issueSource;
+        Assertion = assertion;
     }
 
     /// <inheritdoc />
@@ -190,7 +194,7 @@ ResultReport IValidatable.Validate(PocoNode input, ValidationSettings _, Validat
             // Also, we replace some "magic" tags in the message with common runtime data
             var message = Message.Replace(Pattern.INSTANCETYPE, input.Poco.TypeName).Replace(Pattern.RESOURCEURL, state.Instance.ResourceUrl);
 
-        return new IssueAssertion(IssueNumber, message, Severity, Type).AsResult(state, input, IssueSource);
+        return new IssueAssertion(IssueNumber, message, Severity, Type).AsResult(state, input, IssueSource, this);
     }
 
     /// <summary>
@@ -200,7 +204,8 @@ ResultReport IValidatable.Validate(PocoNode input, ValidationSettings _, Validat
     /// <param name="input"></param>
     /// <returns></returns>
 #pragma warning disable CS0618 // Type or member is obsolete
-    public ResultReport AsResult(ValidationState state, PocoNode input) => asResult(input.GetLocation(), state.Location.DefinitionPath);
+    public ResultReport AsResult(ValidationState state, PocoNode input) 
+        => AsResult(state, input, issueSource: null, assertion: null);
 
     /// <summary>
     /// Package this <see cref="IssueAssertion"/> as a <see cref="ResultReport"/>, adding information from the current state of <paramref name="instance"/>.
@@ -209,17 +214,33 @@ ResultReport IValidatable.Validate(PocoNode input, ValidationSettings _, Validat
     /// <param name="instance"></param>
     /// <param name="issueSource"></param>
     /// <returns></returns>
-    public ResultReport AsResult(ValidationState state, PocoNode instance, string? issueSource)
+    public ResultReport AsResult(ValidationState state, PocoNode instance, string? issueSource) 
+        => AsResult(state, instance, issueSource, assertion: null);
+
+    /// <summary>
+    /// Package this <see cref="IssueAssertion"/> as a <see cref="ResultReport"/>, adding information from the current
+    /// state of <paramref name="instance"/>, and registering a callback that will be invoked with the
+    /// <see cref="IssueComponent"/> generated from this assertion when it is turned into an
+    /// <see cref="OperationOutcome"/> (e.g. to add extensions), without coupling this class to that concern.
+    /// </summary>
+    /// <param name="state"></param>
+    /// <param name="instance"></param>
+    /// <param name="issueSource"></param>
+    /// <param name="assertion">A callback invoked with the generated <see cref="IssueComponent"/>
+    /// when this assertion is turned into an <see cref="OperationOutcome"/>.</param>
+    /// <returns></returns>
+    public ResultReport AsResult(ValidationState state, PocoNode instance, string? issueSource, IAssertion? assertion)
     {
         this.PositionInfo ??= ((IAnnotated)instance).Annotation<JsonSerializationDetails>();
         this.PositionInfo ??= ((IAnnotated)instance).Annotation<XmlSerializationDetails>();
         this.PositionInfo ??= ((IAnnotated)instance).Annotation<PositionInfo>();
 
         this.IssueSource = issueSource;
-        
+        this.Assertion = assertion;
+
         return asResult(instance.GetLocation(), state.Location.DefinitionPath);
     }
-    
+
 #pragma warning restore CS0618 // Type or member is obsolete
 
     /// <summary>
@@ -231,7 +252,7 @@ public ResultReport AsResult(ValidationState state, PocoNode instance, string? i
     /// Package this <see cref="IssueAssertion"/> as a <see cref="ResultReport"/>
     /// </summary>
     internal ResultReport asResult(string location, DefinitionPath? definitionPath) =>
-        new(Result, new IssueAssertion(IssueNumber, location, definitionPath, Message, Severity, Type, PositionInfo, IssueSource));
+        new(Result, new IssueAssertion(IssueNumber, location, definitionPath, Message, Severity, Type, PositionInfo, IssueSource, Assertion));
 
     /// <inheritdoc/>
     public override bool Equals(object? obj) => Equals(obj as IssueAssertion);
diff --git a/src/Firely.Fhir.Validation/Impl/MaxLengthValidator.cs b/src/Firely.Fhir.Validation/Impl/MaxLengthValidator.cs
index 5558f800..b10a34c8 100644
--- a/src/Firely.Fhir.Validation/Impl/MaxLengthValidator.cs
+++ b/src/Firely.Fhir.Validation/Impl/MaxLengthValidator.cs
@@ -61,7 +61,7 @@ internal override ResultReport BasicValidate(PocoNode input, ValidationSettings
             {
                 return str.Length > MaximumLength
                     ? new IssueAssertion(Issue.CONTENT_ELEMENT_VALUE_TOO_LONG,
-                        $"Value '{str}' is too long (maximum length is {MaximumLength})").AsResult(s, input, nameof(MaxLengthValidator))
+                        $"Value '{str}' is too long (maximum length is {MaximumLength})").AsResult(s, input, nameof(MaxLengthValidator), this)
                     : ResultReport.SUCCESS;
             }
             else
diff --git a/src/Firely.Fhir.Validation/Impl/MinMaxValueValidator.cs b/src/Firely.Fhir.Validation/Impl/MinMaxValueValidator.cs
index 425de438..40902d50 100644
--- a/src/Firely.Fhir.Validation/Impl/MinMaxValueValidator.cs
+++ b/src/Firely.Fhir.Validation/Impl/MinMaxValueValidator.cs
@@ -118,7 +118,7 @@ ResultReport IValidatable.Validate(PocoNode input, ValidationSettings _, Validat
             else if (!Any.TryConvert(input.GetValue(), out instanceValue!))
             {
                 return new IssueAssertion(Issue.CONTENT_ELEMENT_PRIMITIVE_VALUE_NOT_COMPARABLE,
-                            $"Value '{input.GetValue()}' cannot be compared with {_minMaxAnyValue}.").AsResult(s, input, nameof(MinMaxValueValidator));
+                            $"Value '{input.GetValue()}' cannot be compared with {_minMaxAnyValue}.").AsResult(s, input, nameof(MinMaxValueValidator), this);
             }
             try
             {
@@ -133,19 +133,19 @@ ResultReport IValidatable.Validate(PocoNode input, ValidationSettings _, Validat
                 if (intResult == _comparisonOutcome)
                 {
                     return new IssueAssertion(_comparisonIssue, $"Value '{instanceValue}' is {_comparisonLabel} {_minMaxAnyValue}.")
-                        .AsResult(s, input, nameof(MinMaxValueValidator));
+                        .AsResult(s, input, nameof(MinMaxValueValidator), this);
                 }
             }
             catch (ArgumentException){
                 return new IssueAssertion(Issue.CONTENT_ELEMENT_PRIMITIVE_VALUE_NOT_COMPARABLE,
                         $"Value '{instanceValue}' cannot be compared with {_minMaxAnyValue})")
-                    .AsResult(s, input);
+                    .AsResult(s, input, nameof(MinMaxValueValidator), this);
             }
             catch (InvalidOperationException)
             {
                 return new IssueAssertion(Issue.CONTENT_ELEMENT_PRIMITIVE_VALUE_NOT_COMPARABLE,
                         $"Value '{instanceValue}' cannot be compared with {_minMaxAnyValue}.")
-                    .AsResult(s, input, nameof(MinMaxValueValidator));
+                    .AsResult(s, input, nameof(MinMaxValueValidator), this);
             }
 
             return ResultReport.SUCCESS;
diff --git a/src/Firely.Fhir.Validation/Impl/PatternValidator.cs b/src/Firely.Fhir.Validation/Impl/PatternValidator.cs
index 57b53f23..3cf24c3d 100644
--- a/src/Firely.Fhir.Validation/Impl/PatternValidator.cs
+++ b/src/Firely.Fhir.Validation/Impl/PatternValidator.cs
@@ -59,7 +59,7 @@ ResultReport IValidatable.Validate(PocoNode input, ValidationSettings _, Validat
                 return ResultReport.SUCCESS;
                     
             return new IssueAssertion(Issue.CONTENT_DOES_NOT_MATCH_PATTERN_VALUE.Code, $"Value '{displayValue(input)}' does not match pattern '{displayValue(PatternValue)}'", OperationOutcome.IssueSeverity.Error, OperationOutcome.IssueType.Invalid)
-                  .AsResult(s, input, nameof(PatternValidator));
+                  .AsResult(s, input, nameof(PatternValidator), this);
 
             static string displayValue(ITypedElement te) =>
               te.Children().Any() ? te.ToJson() : te.Value?.ToString()!;
diff --git a/src/Firely.Fhir.Validation/Impl/ReferencedInstanceValidator.cs b/src/Firely.Fhir.Validation/Impl/ReferencedInstanceValidator.cs
index e93c1693..f8ae7a8f 100644
--- a/src/Firely.Fhir.Validation/Impl/ReferencedInstanceValidator.cs
+++ b/src/Firely.Fhir.Validation/Impl/ReferencedInstanceValidator.cs
@@ -77,7 +77,7 @@ ResultReport IValidatable.Validate(PocoNode input, ValidationSettings vc, Valida
             if (!IsSupportedReferenceType(input.Poco.TypeName))
                 return new IssueAssertion(Issue.CONTENT_REFERENCE_OF_INVALID_KIND,
                     $"Expected a reference type here (reference or canonical) not a {input.Poco.TypeName}.")
-                    .AsResult(state, input, nameof(ReferencedInstanceValidator));
+                    .AsResult(state, input, nameof(ReferencedInstanceValidator), this);
 
             // Get the actual reference from the instance by the pre-configured name.
             // The name is usually "reference" in case we are dealing with a FHIR reference type,
@@ -107,7 +107,7 @@ ResultReport IValidatable.Validate(PocoNode input, ValidationSettings vc, Valida
                     null when vc.ResolveExternalReference is null => ResultReport.SUCCESS,
                     null => new IssueAssertion(
                         Issue.UNAVAILABLE_REFERENCED_RESOURCE,
-                        $"Cannot resolve reference {reference}").AsResult(state, input, nameof(ReferencedInstanceValidator)),
+                        $"Cannot resolve reference {reference}").AsResult(state, input, nameof(ReferencedInstanceValidator), this),
                     _ => validateReferencedResource(reference, vc, resolution, state)
                 };
 
@@ -139,7 +139,7 @@ private record ResolutionResult(PocoNode? ReferencedResource, AggregationMode? R
                 var allowed = string.Join(", ", AggregationRules);
                 evidence.Add(new IssueAssertion(Issue.CONTENT_REFERENCE_OF_INVALID_KIND,
                     $"Encountered a reference ({reference}) of kind '{resolution.ReferenceKind}', which is not one of the allowed kinds ({allowed}).")
-                    .AsResult(s, input, nameof(ReferencedInstanceValidator)));
+                    .AsResult(s, input, nameof(ReferencedInstanceValidator), this));
             }
 
             if (VersioningRules is not null && VersioningRules != ReferenceVersionRules.Either)
@@ -147,7 +147,7 @@ private record ResolutionResult(PocoNode? ReferencedResource, AggregationMode? R
                 if (VersioningRules != resolution.VersioningKind)
                     evidence.Add(new IssueAssertion(Issue.CONTENT_REFERENCE_OF_INVALID_KIND,
                         $"Expected a {VersioningRules} versioned reference but found {resolution.VersioningKind}.")
-                        .AsResult(s, input, nameof(ReferencedInstanceValidator)));
+                        .AsResult(s, input, nameof(ReferencedInstanceValidator), this));
             }
 
             if (resolution.ReferenceKind == AggregationMode.Referenced)
@@ -169,7 +169,7 @@ private record ResolutionResult(PocoNode? ReferencedResource, AggregationMode? R
                         evidence.Add(new IssueAssertion(
                             Issue.UNAVAILABLE_REFERENCED_RESOURCE,
                             $"Resolution of external reference {reference} failed. Message: {e.Message}")
-                            .AsResult(s, input, nameof(ReferencedInstanceValidator)));
+                            .AsResult(s, input, nameof(ReferencedInstanceValidator), this));
                     }
                 }
             }
@@ -180,7 +180,7 @@ private record ResolutionResult(PocoNode? ReferencedResource, AggregationMode? R
         /// <summary>
         /// Try to fetch the resource within this instance (e.g. a contained or bundled resource).
         /// </summary>
-        private static ResultReport resolveLocally(PocoNode instance, string reference, ValidationState s, out ResolutionResult resolution)
+        private ResultReport resolveLocally(PocoNode instance, string reference, ValidationState s, out ResolutionResult resolution)
         {
             resolution = new ResolutionResult(null, null, null);
             var identity = new ResourceIdentity(reference);
@@ -193,7 +193,7 @@ private static ResultReport resolveLocally(PocoNode instance, string reference,
                 if (!Uri.IsWellFormedUriString(Uri.EscapeDataString(reference), UriKind.RelativeOrAbsolute))
                 {
                     return new IssueAssertion(Issue.CONTENT_UNPARSEABLE_REFERENCE,
-                        $"Encountered an unparseable reference ({reference}").AsResult(s, instance.ToPocoNode(), nameof(ReferencedInstanceValidator));
+                        $"Encountered an unparseable reference ({reference}").AsResult(s, instance.ToPocoNode(), nameof(ReferencedInstanceValidator), this);
                 }
             }
 
@@ -206,7 +206,7 @@ private static ResultReport resolveLocally(PocoNode instance, string reference,
             catch (Exception e)
             {
                 return new IssueAssertion(Issue.CONTENT_REFERENCE_NOT_RESOLVABLE,
-                    $"Encountered an issue during reference resolution. Message: {e.Message}").AsResult(s, instance.ToPocoNode(), nameof(ReferencedInstanceValidator));
+                    $"Encountered an issue during reference resolution. Message: {e.Message}").AsResult(s, instance.ToPocoNode(), nameof(ReferencedInstanceValidator), this);
             }
             
 
diff --git a/src/Firely.Fhir.Validation/Impl/RegExValidator.cs b/src/Firely.Fhir.Validation/Impl/RegExValidator.cs
index 8836d49e..4a710e3e 100644
--- a/src/Firely.Fhir.Validation/Impl/RegExValidator.cs
+++ b/src/Firely.Fhir.Validation/Impl/RegExValidator.cs
@@ -65,7 +65,7 @@ internal override ResultReport BasicValidate(PocoNode input, ValidationSettings
                 return ResultReport.SUCCESS;
 
             return new IssueAssertion(Issue.CONTENT_ELEMENT_INVALID_PRIMITIVE_VALUE.Code, $"Value '{value}' does not match regex '{Pattern}'", OperationOutcome.IssueSeverity.Error, OperationOutcome.IssueType.Invalid)
-                .AsResult(s, input, nameof(RegExValidator));
+                .AsResult(s, input, nameof(RegExValidator), this);
         }
 
         private static string? toStringRepresentation(PocoNode vp)
diff --git a/src/Firely.Fhir.Validation/Impl/RenderingXhtmlValidator.cs b/src/Firely.Fhir.Validation/Impl/RenderingXhtmlValidator.cs
index 6099a16e..0e773a34 100644
--- a/src/Firely.Fhir.Validation/Impl/RenderingXhtmlValidator.cs
+++ b/src/Firely.Fhir.Validation/Impl/RenderingXhtmlValidator.cs
@@ -55,8 +55,8 @@ ResultReport IValidatable.Validate(PocoNode input, ValidationSettings vc, Valida
             if (isValid) return ResultReport.SUCCESS;
 
             var issueReports = malformedError is null
-                ? narrativeIssues.Select(e => new IssueAssertion(Issue.XSD_VALIDATION_ERROR, e).AsResult(state, valueNode)).ToArray()
-                : [new IssueAssertion(Issue.XSD_VALIDATION_ERROR, malformedError).AsResult(state, valueNode)];
+                ? narrativeIssues.Select(e => new IssueAssertion(Issue.XSD_VALIDATION_ERROR, e).AsResult(state, valueNode, nameof(RenderingXhtmlValidator), this)).ToArray()
+                : [new IssueAssertion(Issue.XSD_VALIDATION_ERROR, malformedError).AsResult(state, valueNode, nameof(RenderingXhtmlValidator), this)];
 
             return ResultReport.Combine(issueReports);
         }
diff --git a/src/Firely.Fhir.Validation/Impl/RequiredValidator.cs b/src/Firely.Fhir.Validation/Impl/RequiredValidator.cs
index ec09e2ad..17eeaee0 100644
--- a/src/Firely.Fhir.Validation/Impl/RequiredValidator.cs
+++ b/src/Firely.Fhir.Validation/Impl/RequiredValidator.cs
@@ -56,7 +56,7 @@ ResultReport IValidatable.Validate(PocoNode input, ValidationSettings vc, Valida
                 _requiredMembers
                     .Where(memberName => input.Child(memberName) is null)
                     .Select(memberName => new IssueAssertion(Issue.CONTENT_INCORRECT_OCCURRENCE, $"Missing required member: '{memberName}'")
-                        .AsResult(state, input, nameof(RequiredValidator))
+                        .AsResult(state, input, nameof(RequiredValidator), this)
                     ).ToList();
             
             return ResultReport.Combine(evidence);
diff --git a/src/Firely.Fhir.Validation/Impl/SliceValidator.cs b/src/Firely.Fhir.Validation/Impl/SliceValidator.cs
index dd29bccc..5ca3ce08 100644
--- a/src/Firely.Fhir.Validation/Impl/SliceValidator.cs
+++ b/src/Firely.Fhir.Validation/Impl/SliceValidator.cs
@@ -172,7 +172,7 @@ ResultReport IGroupValidatable.Validate(IEnumerable<PocoNode> input, ValidationS
                         if (sliceNumber < lastMatchingSlice && Ordered)
                             evidence.Add(new IssueAssertion(Issue.CONTENT_ELEMENT_SLICING_OUT_OF_ORDER,
                                     $"Element matches slice {sliceLocation!}:{sliceName}', but this is out of order for group {sliceLocation!}, since a previous element already matched slice '{sliceLocation!}:{Slices[lastMatchingSlice].Name}'")
-                                .AsResult(state, candidate, nameof(SliceValidator)));
+                                .AsResult(state, candidate, nameof(SliceValidator), this));
                         else
                             lastMatchingSlice = sliceNumber;
 
@@ -181,7 +181,7 @@ ResultReport IGroupValidatable.Validate(IEnumerable<PocoNode> input, ValidationS
                             // We found a match while we already added a non-match to a "open at end" slicegroup, that's not allowed
                             evidence.Add(new IssueAssertion(Issue.CONTENT_ELEMENT_FAILS_SLICING_RULE,
                                     $"Element matched slice '{sliceLocation!}:{sliceName}', but it appears after a non-match, which is not allowed for an open-at-end group")
-                                .AsResult(state, candidate, nameof(SliceValidator)));
+                                .AsResult(state, candidate, nameof(SliceValidator), this));
                         }
 
                         hasSucceeded = true;
@@ -212,7 +212,7 @@ ResultReport IGroupValidatable.Validate(IEnumerable<PocoNode> input, ValidationS
             evidence.AddRange(buckets
                 .Where(slice => slice.Value is null && slice.Key.Required)
                 .Select(slice => new IssueAssertion(Issue.CONTENT_INCORRECT_OCCURRENCE, $"No elements matched required slice: '{pn.Name}:{slice.Key.Name}'")
-                    .AsResult(state, pn.Parent!, nameof (SliceValidator))));
+                    .AsResult(state, pn.Parent!, nameof (SliceValidator), this)));
             evidence.AddRange(buckets.Validate(vc, state));
 
             return ResultReport.Combine(evidence);
diff --git a/src/Firely.Fhir.Validation/PublicAPI.Unshipped.txt b/src/Firely.Fhir.Validation/PublicAPI.Unshipped.txt
index c9dc4f6f..b88b276c 100644
--- a/src/Firely.Fhir.Validation/PublicAPI.Unshipped.txt
+++ b/src/Firely.Fhir.Validation/PublicAPI.Unshipped.txt
@@ -175,6 +175,7 @@ Firely.Fhir.Validation.InvariantValidator.InvariantValidator() -> void
 Firely.Fhir.Validation.IssueAssertion
 Firely.Fhir.Validation.IssueAssertion.AsResult(Firely.Fhir.Validation.ValidationState! state, Hl7.Fhir.Model.PocoNode! input) -> Firely.Fhir.Validation.ResultReport!
 Firely.Fhir.Validation.IssueAssertion.AsResult(Firely.Fhir.Validation.ValidationState! state, Hl7.Fhir.Model.PocoNode! instance, string? issueSource) -> Firely.Fhir.Validation.ResultReport!
+Firely.Fhir.Validation.IssueAssertion.AsResult(Firely.Fhir.Validation.ValidationState! state, Hl7.Fhir.Model.PocoNode! instance, string? issueSource, Firely.Fhir.Validation.IAssertion? assertion) -> Firely.Fhir.Validation.ResultReport!
 Firely.Fhir.Validation.IssueAssertion.AsResult(string! location) -> Firely.Fhir.Validation.ResultReport!
 Firely.Fhir.Validation.IssueAssertion.Equals(Firely.Fhir.Validation.IssueAssertion? other) -> bool
 Firely.Fhir.Validation.IssueAssertion.IssueAssertion(Hl7.Fhir.Support.Issue! issue, string! message) -> void
diff --git a/src/Firely.Fhir.Validation/Schema/AssertionToOperationOutcomeExtensions.cs b/src/Firely.Fhir.Validation/Schema/AssertionToOperationOutcomeExtensions.cs
index 0ed0dae4..6bd4003b 100644
--- a/src/Firely.Fhir.Validation/Schema/AssertionToOperationOutcomeExtensions.cs
+++ b/src/Firely.Fhir.Validation/Schema/AssertionToOperationOutcomeExtensions.cs
@@ -65,6 +65,13 @@ public static OperationOutcome ToOperationOutcome(this ResultReport result)
                 {
                     newIssueComponent.AddExtension("http://hl7.org/fhir/StructureDefinition/operationoutcome-issue-source", new FhirString(item.IssueSource));
                 }
+
+ #pragma warning disable CS0618 // Type or member is obsolete
+                if (item.Assertion is InvariantValidator invariant)
+                {
+ #pragma warning restore CS0618 // Type or member is obsolete
+                    newIssueComponent.AddExtension("http://hl7.org/fhir/StructureDefinition/operationoutcome-message-id", new FhirString(invariant.Key));
+                }
             }
 
             return outcome;
diff --git a/src/Firely.Fhir.Validation/Support/OperationOutcomeDefinitionExtensions.cs b/src/Firely.Fhir.Validation/Support/OperationOutcomeDefinitionExtensions.cs
index eb914019..39c0c943 100644
--- a/src/Firely.Fhir.Validation/Support/OperationOutcomeDefinitionExtensions.cs
+++ b/src/Firely.Fhir.Validation/Support/OperationOutcomeDefinitionExtensions.cs
@@ -24,4 +24,6 @@ public static void SetStructureDefinitionPath(this OperationOutcome.IssueCompone
         public static string? GetStructureDefinitionPath(this OperationOutcome.IssueComponent ic) =>
             ic.Details?.Coding.FirstOrDefault(c => c.System == OPERATION_OUTCOME_SDREF)?.Code;
     }
+
+
 }
\ No newline at end of file
diff --git a/test/Firely.Fhir.Validation.Tests/Impl/FhirPathValidatorTests.cs b/test/Firely.Fhir.Validation.Tests/Impl/FhirPathValidatorTests.cs
index 08475062..dbc77d6b 100644
--- a/test/Firely.Fhir.Validation.Tests/Impl/FhirPathValidatorTests.cs
+++ b/test/Firely.Fhir.Validation.Tests/Impl/FhirPathValidatorTests.cs
@@ -61,6 +61,25 @@ public void ValidateSuccess()
             Assert.IsTrue(result.IsSuccessful, "the FhirPath Expression must be valid for this input");
         }
 
+        [TestMethod]
+        public void ValidateFailureExposesInvariantIdOnOperationOutcome()
+        {
+            var validatable = new FhirPathValidator("ele-1", "$this = 'other'", "human description", IssueSeverity.Error, false);
+
+            var input = PocoNode.ForPrimitive<FhirString>("test");
+
+            var minimalContextWithFp = ValidationSettings.BuildMinimalContext(fpCompiler: _fpCompiler);
+            var result = validatable.Validate(input, minimalContextWithFp);
+
+            Assert.IsFalse(result.IsSuccessful);
+
+            var outcome = result.ToOperationOutcome();
+            var extension = outcome.Issue.Single().GetExtension("http://hl7.org/fhir/StructureDefinition/operationoutcome-message-id");
+
+            Assert.IsNotNull(extension);
+            Assert.AreEqual("ele-1", (extension.Value as FhirString)?.Value);
+        }
+
         [TestMethod]
         public void ValidateIncorrectFhirPath()
         {