diff --git a/ts/server/package.json b/ts/server/package.json
index c8205cd..c017c60 100644
--- a/ts/server/package.json
+++ b/ts/server/package.json
@@ -24,7 +24,7 @@
         "@nestjs/core": "^10.0.0",
         "@nestjs/platform-express": "^10.0.0",
         "@types/fhir": "^0.0.37",
-        "fhirpath": "^3.6.1",
+        "fhirpath": "^3.13.2",
         "reflect-metadata": "^0.1.13",
         "rxjs": "^7.8.1"
     },
diff --git a/ts/server/src/utils/extract.ts b/ts/server/src/utils/extract.ts
index 4571fb9..ace8824 100644
--- a/ts/server/src/utils/extract.ts
+++ b/ts/server/src/utils/extract.ts
@@ -2,20 +2,8 @@ import * as fhirpath from 'fhirpath';
 
 type Resource = Record<string, any>;
 
-
-type UserInvocationTable = {
-    [name: string]: {
-      fn: (...args: any) => any,
-      arity: {
-        [numberOfParams: number]: Array<'Expr' | 'AnyAtRoot' | 'Identifier' | 'TypeSpecifier' | 'Any' | 'Integer' | 'Boolean' | 'Number' | 'String'>
-      },
-      nullable?: boolean,
-      internalStructures?: boolean
-    }
-  };
-
 interface FPOptions {
-    // userInvocationTable?: UserInvocationTable;
+    userInvocationTable?: UserInvocationTable;
 }
 
 interface Embedded {
@@ -62,7 +50,7 @@ function resolveTemplateRecur(
     model?: Model,
     fpOptions?: FPOptions,
 ): any {
-    return iterateObject(template, initialContext?? {}, (node, context) => {
+    return iterateObject(template, initialContext ?? {}, (node, context) => {
         if (isPlainObject(node)) {
             const { node: newNode, context: newContext } = processAssignBlock(
                 resource,
@@ -94,7 +82,7 @@ function resolveTemplateRecur(
                     fhirpath.evaluate(
                         resource,
                         embedded.expression,
-                        context,
+                        { ...context }, // fhirpath mutates context!
                         model,
                         fpOptions,
                     )[0] ?? null;
@@ -175,7 +163,13 @@ function processForBlock(
         const itemKey = hasIndexKey ? matches[2] : matches[1];
         const expr = hasIndexKey ? matches[3] : matches[2];
 
-        const answers = fhirpath.evaluate(resource, expr, context, model, fpOptions);
+        const answers = fhirpath.evaluate(
+            resource,
+            expr,
+            { ...context }, // fhirpath mutates context!
+            model,
+            fpOptions,
+        );
         return {
             node: answers.map((answer, index) =>
                 resolveTemplate(
@@ -212,7 +206,13 @@ function processContextBlock(
         const matches = contextKey.match(contextRegExp);
 
         const expr = matches[1];
-        const answers = fhirpath.evaluate(resource, expr, context, model, fpOptions);
+        const answers = fhirpath.evaluate(
+            resource,
+            expr,
+            { ...context }, // fhirpath mutates context!
+            model,
+            fpOptions,
+        );
         const result: any[] = answers.map((answer) =>
             resolveTemplate(answer, node[contextKey], context, model, fpOptions),
         );
@@ -280,7 +280,7 @@ function processIfBlock(
         const answer = fhirpath.evaluate(
             resource,
             `iif(${expr}, true, false)`,
-            context,
+            { ...context }, // fhirpath mutates context!
             model,
             fpOptions,
         )[0];
diff --git a/ts/server/yarn.lock b/ts/server/yarn.lock
index 81d8baf..cd9206c 100644
--- a/ts/server/yarn.lock
+++ b/ts/server/yarn.lock
@@ -677,10 +677,10 @@
     "@jridgewell/resolve-uri" "^3.1.0"
     "@jridgewell/sourcemap-codec" "^1.4.14"
 
-"@lhncbc/ucum-lhc@^4.1.3":
-  version "4.2.0"
-  resolved "https://registry.yarnpkg.com/@lhncbc/ucum-lhc/-/ucum-lhc-4.2.0.tgz#522cc16fe37739d7da16c27c8b9ad039ee39c2d7"
-  integrity sha512-OEiWX7IHFHLTFs7+w5EvGtI5dhXhhL0341LqZ9WEBWErtoY0/9xl/vn+wwT9vnBHnjQ7Ux0o7iEUXvN8uVn4xg==
+"@lhncbc/ucum-lhc@^5.0.0":
+  version "5.0.4"
+  resolved "https://registry.yarnpkg.com/@lhncbc/ucum-lhc/-/ucum-lhc-5.0.4.tgz#1357a039954e03154f89ce51f2d90c93b9c1a1f3"
+  integrity sha512-khuV9GV51DF80b0wJmhZTR5Bf23fhS6SSIWnyGT9X+Uvn0FsHFl2LKViQ2TTOuvwagUOUSq8/0SyoE2ZDGwrAA==
   dependencies:
     coffeescript "^2.7.0"
     csv-parse "^4.4.6"
@@ -2455,12 +2455,12 @@ fb-watchman@^2.0.0:
   dependencies:
     bser "2.1.1"
 
-fhirpath@^3.6.1:
-  version "3.6.1"
-  resolved "https://registry.yarnpkg.com/fhirpath/-/fhirpath-3.6.1.tgz#05843eacacb4b9d94d18154771a019c0e4d8122d"
-  integrity sha512-Sgneaohzc2IYun17OS4107GQXeXd6umVT9P5rn+9YD6v0JDJOvoDixqWeBEY3MokSk7Hpco5NQbTa61TWF4gtg==
+fhirpath@^3.13.2:
+  version "3.13.2"
+  resolved "https://registry.yarnpkg.com/fhirpath/-/fhirpath-3.13.2.tgz#4c9e3a77267a1c080fc8d295f5aa6aabd26d1fb9"
+  integrity sha512-pPlMHEj1TlES+QaUdLlGmYgvSYiwecXTHbPjX4WKHj7nR1ZZr0YIj/jBo7k9ADrSKQizqvURGYCqufN7sHnnQA==
   dependencies:
-    "@lhncbc/ucum-lhc" "^4.1.3"
+    "@lhncbc/ucum-lhc" "^5.0.0"
     antlr4 "~4.9.3"
     commander "^2.18.0"
     date-fns "^1.30.1"