diff --git a/src/domain/analysisView/environmentStateWriteApply.test.ts b/src/domain/analysisView/environmentStateWriteApply.test.ts
index a4fc8133f1..33f0937519 100644
--- a/src/domain/analysisView/environmentStateWriteApply.test.ts
+++ b/src/domain/analysisView/environmentStateWriteApply.test.ts
@@ -142,6 +142,8 @@ it.each<[string, JsonValue | undefined]>([
   ["/x~1y/m~0n", true],
   ["/", "blank"],
   ["/a/2/b", undefined],
+  ["/a/01/b", undefined],
+  ["/a/999999999999999999999999/b", undefined],
   ["/a/first", undefined],
   ["/a/0/b/c", undefined],
   ["/missing", undefined],
diff --git a/src/domain/analysisView/environmentStateWriteApply.ts b/src/domain/analysisView/environmentStateWriteApply.ts
index 255dd25d89..3386f1a3d0 100644
--- a/src/domain/analysisView/environmentStateWriteApply.ts
+++ b/src/domain/analysisView/environmentStateWriteApply.ts
@@ -180,8 +180,15 @@ export const valueAt = (
 ): JsonValue | undefined => {
   let value: JsonValue | undefined = body;
   for (const token of pointerTokens(pointer)) {
-    if (Array.isArray(value)) value = value[Number(token)];
-    else if (isObject(value) && Object.hasOwn(value, token))
+    if (Array.isArray(value)) {
+      // JSON Pointer array indexes are canonical decimal tokens; coercing
+      // arbitrary text would make /01 alias /1 and can turn huge indexes into
+      // Infinity.
+      if (!/^(?:0|[1-9]\d*)$/u.test(token)) return undefined;
+      const index = Number(token);
+      if (!Number.isSafeInteger(index)) return undefined;
+      value = value[index];
+    } else if (isObject(value) && Object.hasOwn(value, token))
       value = value[token];
     else return undefined;
   }