diff --git a/CHANGELOG b/CHANGELOG
index d8155323d..36b5bb76e 100644
--- a/CHANGELOG
+++ b/CHANGELOG
@@ -14,6 +14,7 @@ This project _loosely_ adheres to [Semantic Versioning](https://semver.org/spec/
 - Show an error when the code is wrong. [PR#502](https://github.com/coasys/ad4m/pull/502)
 - Error handling in connect when hosting is not working [PR#502](https://github.com/coasys/ad4m/pull/502)
 - Make new Rust create_subject() implementation wait for Prolog engine to be updated, reflecting the new instance to avoid race conditions [PR#520](https://github.com/coasys/ad4m/pull/520)
+- Fix problems with Subject Classes created by other non-progenitor agents, and make isSubjectInstance() more stable agains race conditions [PR#521](https://github.com/coasys/ad4m/pull/521)
 
 ### Added
 - Prolog predicates needed in new Flux mention notification trigger:
diff --git a/core/src/perspectives/PerspectiveProxy.ts b/core/src/perspectives/PerspectiveProxy.ts
index 26bdb0171..b47b24cef 100644
--- a/core/src/perspectives/PerspectiveProxy.ts
+++ b/core/src/perspectives/PerspectiveProxy.ts
@@ -365,7 +365,16 @@ export class PerspectiveProxy {
     */
     async isSubjectInstance<T>(expression: string, subjectClass: T): Promise<boolean> {
         let className = await this.stringOrTemplateObjectToSubjectClass(subjectClass)
-        return await this.infer(`subject_class("${className}", C), instance(C, "${expression}")`)
+        let isInstance = false;
+        const maxAttempts = 5;
+        let attempts = 0;
+
+        while (attempts < maxAttempts && !isInstance) {
+            isInstance = await this.infer(`subject_class("${className}", C), instance(C, "${expression}")`);
+            attempts++;
+          }
+
+        return isInstance
     }
 
 
diff --git a/rust-executor/src/perspectives/perspective_instance.rs b/rust-executor/src/perspectives/perspective_instance.rs
index 8f6aa7207..8afaa1165 100644
--- a/rust-executor/src/perspectives/perspective_instance.rs
+++ b/rust-executor/src/perspectives/perspective_instance.rs
@@ -3,7 +3,7 @@ use super::update_perspective;
 use super::utils::{
     prolog_get_all_string_bindings, prolog_get_first_string_binding, prolog_resolution_to_string,
 };
-use crate::agent::create_signed_expression;
+use crate::agent::{self, create_signed_expression};
 use crate::graphql::graphql_types::{
     DecoratedPerspectiveDiff, ExpressionRendered, JsResultType, LinkMutations, LinkQuery,
     LinkStatus, NeighbourhoodSignalFilter, OnlineAgent, PerspectiveExpression, PerspectiveHandle,
@@ -889,8 +889,15 @@ impl PerspectiveInstance {
             })
             .await?;
 
+        let author = agent::did();
+
         let mut sdna_links: Vec<Link> = Vec::new();
 
+        let links = links
+            .into_iter()
+            .filter(|l| l.author == author)
+            .collect::<Vec<DecoratedLinkExpression>>();
+
         if (Literal::from_url(sdna_code.clone())).is_err() {
             sdna_code = Literal::from_string(sdna_code)
                 .to_url()
@@ -905,7 +912,7 @@ impl PerspectiveInstance {
             });
 
             sdna_links.push(Link {
-                source: literal_name,
+                source: literal_name.clone(),
                 predicate: Some("ad4m://sdna".to_string()),
                 target: sdna_code,
             });