diff --git a/.changeset/standard-schema-spec-dependency.md b/.changeset/standard-schema-spec-dependency.md
new file mode 100644
index 000000000..a248214d6
--- /dev/null
+++ b/.changeset/standard-schema-spec-dependency.md
@@ -0,0 +1,7 @@
+---
+'@tanstack/ai': patch
+---
+
+Move `@standard-schema/spec` from `devDependencies` to `dependencies`. Closes #602.
+
+The package's published `.d.ts` files (`types.d.ts`, `activities/chat/tools/tool-definition.d.ts`, `activities/chat/tools/schema-converter.d.ts`) import types from `@standard-schema/spec`, so consumers need it installed for type resolution to succeed. With `skipLibCheck: true`, `tsc` silently ignored the unresolved module, but type-aware tools like `@typescript-eslint` (with `recommendedTypeChecked` / `projectService: true`) failed to resolve return types — surfacing as `Unsafe assignment of an error typed value` on `useChat()` destructuring and cascading errors through downstream usages.
diff --git a/packages/typescript/ai/package.json b/packages/typescript/ai/package.json
index 48c68c650..8d598ef9c 100644
--- a/packages/typescript/ai/package.json
+++ b/packages/typescript/ai/package.json
@@ -66,6 +66,7 @@
   ],
   "dependencies": {
     "@ag-ui/core": "^0.0.52",
+    "@standard-schema/spec": "^1.1.0",
     "@tanstack/ai-event-client": "workspace:*",
     "partial-json": "^0.1.7"
   },
@@ -79,7 +80,6 @@
   },
   "devDependencies": {
     "@opentelemetry/api": "^1.9.0",
-    "@standard-schema/spec": "^1.1.0",
     "@vitest/coverage-v8": "4.0.14",
     "zod": "^4.2.0"
   }
diff --git a/pnpm-lock.yaml b/pnpm-lock.yaml
index c86e7cd66..f4e548b8b 100644
--- a/pnpm-lock.yaml
+++ b/pnpm-lock.yaml
@@ -953,6 +953,9 @@ importers:
       '@ag-ui/core':
         specifier: ^0.0.52
         version: 0.0.52
+      '@standard-schema/spec':
+        specifier: ^1.1.0
+        version: 1.1.0
       '@tanstack/ai-event-client':
         specifier: workspace:*
         version: link:../ai-event-client
@@ -963,9 +966,6 @@ importers:
       '@opentelemetry/api':
         specifier: ^1.9.0
         version: 1.9.1
-      '@standard-schema/spec':
-        specifier: ^1.1.0
-        version: 1.1.0
       '@vitest/coverage-v8':
         specifier: 4.0.14
         version: 4.0.14(vitest@4.0.14(@opentelemetry/api@1.9.1)(@types/node@24.10.3)(happy-dom@20.0.11)(jiti@2.6.1)(jsdom@27.3.0(postcss@8.5.9))(lightningcss@1.30.2)(terser@5.44.1)(tsx@4.21.0)(yaml@2.8.2))