diff --git a/guards/github-guard/rust-guard/src/labels/mod.rs b/guards/github-guard/rust-guard/src/labels/mod.rs
index 804dcc13..61ee4ee6 100644
--- a/guards/github-guard/rust-guard/src/labels/mod.rs
+++ b/guards/github-guard/rust-guard/src/labels/mod.rs
@@ -4770,6 +4770,40 @@ mod tests {
         }
     }
 
+    #[test]
+    fn test_apply_tool_labels_set_issue_fields_writer_integrity() {
+        let ctx = default_ctx();
+        let repo_id = "github/copilot";
+        let tool_args = json!({
+            "owner": "github",
+            "repo": "copilot",
+            "issue_number": 1,
+            "fields": [
+                {
+                    "field_id": "PVTSSF_example",
+                    "text_value": "In progress"
+                }
+            ]
+        });
+
+        let (secrecy, integrity, _desc) = apply_tool_labels(
+            "set_issue_fields",
+            &tool_args,
+            repo_id,
+            vec![],
+            vec![],
+            String::new(),
+            &ctx,
+        );
+
+        assert_eq!(secrecy, vec![] as Vec<String>, "set_issue_fields secrecy mismatch");
+        assert_eq!(
+            integrity,
+            writer_integrity(repo_id, &ctx),
+            "set_issue_fields should have writer integrity"
+        );
+    }
+
     #[test]
     fn test_apply_tool_labels_granular_sub_issue_tools_writer_integrity() {
         let ctx = default_ctx();
diff --git a/guards/github-guard/rust-guard/src/labels/tool_rules.rs b/guards/github-guard/rust-guard/src/labels/tool_rules.rs
index 6ba98578..d0948e5b 100644
--- a/guards/github-guard/rust-guard/src/labels/tool_rules.rs
+++ b/guards/github-guard/rust-guard/src/labels/tool_rules.rs
@@ -533,6 +533,15 @@ pub fn apply_tool_labels(
             integrity = writer_integrity(repo_id, ctx);
         }
 
+        // === Issue custom fields mutation (repo-scoped write) ===
+        "set_issue_fields" => {
+            // Field definitions are organization-level, but the mutation targets a specific
+            // issue in owner/repo and returns issue-scoped metadata.
+            // S = S(repo); I = writer
+            secrecy = apply_repo_visibility_secrecy(&owner, &repo, repo_id, secrecy, ctx);
+            integrity = writer_integrity(repo_id, ctx);
+        }
+
         // === Granular repo-scoped write operations ===
         // Covers granular issue PATCH tools, sub-issue management, granular PR PATCH tools,
         // and PR review tools. All follow: S = S(repo), I = writer.
diff --git a/guards/github-guard/rust-guard/src/tools.rs b/guards/github-guard/rust-guard/src/tools.rs
index 375a6673..2bd68f54 100644
--- a/guards/github-guard/rust-guard/src/tools.rs
+++ b/guards/github-guard/rust-guard/src/tools.rs
@@ -100,6 +100,9 @@ pub const READ_WRITE_OPERATIONS: &[&str] = &[
     "update_issue_title",     // PATCH — modifies issue title
     "update_issue_type",      // PATCH — modifies issue type
 
+    // Issue custom field mutation (field definitions are org-level; target issue is repo-scoped)
+    "set_issue_fields", // GraphQL — sets custom field values on a specific repository issue
+
     // Sub-issue management tools (alongside sub_issue_write composite)
     "add_sub_issue",          // POST  /repos/.../issues/{number}/sub_issues
     "remove_sub_issue",       // DELETE/POST — remove sub-issue link
@@ -362,6 +365,21 @@ mod tests {
         }
     }
 
+    #[test]
+    fn test_set_issue_fields_is_read_write_operation() {
+        let op = "set_issue_fields";
+        assert!(
+            is_read_write_operation(op),
+            "{} must be classified as a read-write operation",
+            op
+        );
+        assert!(
+            !is_write_operation(op),
+            "{} should not be in WRITE_OPERATIONS (it is in READ_WRITE_OPERATIONS)",
+            op
+        );
+    }
+
     #[test]
     fn test_sub_issue_management_tools_are_read_write_operations() {
         for op in &["add_sub_issue", "remove_sub_issue", "reprioritize_sub_issue"] {