diff --git a/extensions/memory-core/src/memory/index.test.ts b/extensions/memory-core/src/memory/index.test.ts
index a9c959abf0cb..31ad28e49902 100644
--- a/extensions/memory-core/src/memory/index.test.ts
+++ b/extensions/memory-core/src/memory/index.test.ts
@@ -880,7 +880,7 @@ describe("memory index", () => {
 
       expect(providerRuntimeBatchCalls).toHaveLength(3);
       expect(providerRuntimeBatchCalls.every((call) => call.length === 1)).toBe(true);
-      expect(providerRuntimeBatchCalls.map((call) => call[0]).toSorted()).toEqual(
+      expect(providerRuntimeBatchCalls.map((call) => call[0] ?? "").toSorted()).toEqual(
         [
           "# Log\nAlpha memory line.\nZebra memory line.",
           "# Log\nBeta memory line.",
diff --git a/test/tsconfig/tsconfig.test.json b/test/tsconfig/tsconfig.test.json
index 82cbb3af6d7b..344a18aaeb45 100644
--- a/test/tsconfig/tsconfig.test.json
+++ b/test/tsconfig/tsconfig.test.json
@@ -1,6 +1,9 @@
 {
   "extends": "../../tsconfig.json",
   "compilerOptions": {
+    // Test lanes opt out of noUncheckedIndexedAccess for now (~4,400 fixture
+    // indexing sites); tracked as a lane-by-lane follow-up to #104600.
+    "noUncheckedIndexedAccess": false,
     "rootDir": "../.."
   },
   "include": [
diff --git a/tsconfig.core.json b/tsconfig.core.json
index 8e6b0b4eaa86..19c5c8b9892d 100644
--- a/tsconfig.core.json
+++ b/tsconfig.core.json
@@ -4,7 +4,6 @@
     // Node-side production code must not see browser globals; ui/ owns DOM via tsconfig.ui.json.
     "lib": ["ES2023"],
     // Production indexed reads must account for missing entries.
-    "noUncheckedIndexedAccess": true,
     "noUnusedLocals": true,
     "noUnusedParameters": true,
     "tsBuildInfoFile": ".artifacts/tsgo-cache/core.tsbuildinfo"
diff --git a/tsconfig.extensions.json b/tsconfig.extensions.json
index 6b756eaf84bc..c1ee3c571f16 100644
--- a/tsconfig.extensions.json
+++ b/tsconfig.extensions.json
@@ -2,7 +2,6 @@
   "extends": "./tsconfig.json",
   "compilerOptions": {
     // Plugin production indexed reads must account for missing entries.
-    "noUncheckedIndexedAccess": true,
     "noUnusedLocals": true,
     "noUnusedParameters": true,
     "tsBuildInfoFile": ".artifacts/tsgo-cache/extensions.tsbuildinfo"
diff --git a/tsconfig.json b/tsconfig.json
index ac6429e4a452..9d70f3406a6f 100644
--- a/tsconfig.json
+++ b/tsconfig.json
@@ -12,6 +12,7 @@
     "moduleResolution": "NodeNext",
     "noImplicitOverride": true,
     "noImplicitReturns": true,
+    "noUncheckedIndexedAccess": true,
     "noEmit": true,
     "noEmitOnError": true,
     "noUncheckedSideEffectImports": true,
diff --git a/tsconfig.scripts.json b/tsconfig.scripts.json
index 2639e7947027..c253fa9a762f 100644
--- a/tsconfig.scripts.json
+++ b/tsconfig.scripts.json
@@ -1,7 +1,6 @@
 {
   "extends": "./tsconfig.json",
   "compilerOptions": {
-    "noUncheckedIndexedAccess": true,
     "noUnusedLocals": true,
     "noUnusedParameters": true,
     "tsBuildInfoFile": ".artifacts/tsgo-cache/scripts.tsbuildinfo"
diff --git a/tsconfig.ui.json b/tsconfig.ui.json
index 02539fb38332..0d9682fa90c0 100644
--- a/tsconfig.ui.json
+++ b/tsconfig.ui.json
@@ -2,7 +2,6 @@
   "extends": "./tsconfig.json",
   "compilerOptions": {
     // Control UI indexed reads must account for missing entries.
-    "noUncheckedIndexedAccess": true,
     "noUnusedLocals": true,
     "noUnusedParameters": true,
     "tsBuildInfoFile": ".artifacts/tsgo-cache/ui.tsbuildinfo"