diff --git a/.github/workflows/test.yml b/.github/workflows/test.yml
index 5cc8ae7..e552fac 100644
--- a/.github/workflows/test.yml
+++ b/.github/workflows/test.yml
@@ -417,7 +417,14 @@ jobs:
           node-version: '22'
 
       - name: Install codegen dependency
-        run: npm install --no-save --no-package-lock json-schema-to-typescript@^15
+        # Pin BOTH json-schema-to-typescript and prettier to exact versions.
+        # jstt formats its output with prettier (dep range "^3.2.5"), so an
+        # unpinned install silently adopts new prettier releases and reformats
+        # the generated types — drift with no schema change. prettier 3.9.0
+        # (2026-06-27) collapsed short union types onto one line and broke this
+        # gate on main. The committed tests/types/eep-schemas.d.ts is generated
+        # against exactly these versions; bump them here and regenerate together.
+        run: npm install --no-save --no-package-lock json-schema-to-typescript@15.0.4 prettier@3.9.0
 
       - name: Check generated schema types are up to date
         # The script's --check mode generates the output to memory and
diff --git a/scripts/codegen-schema-types.mjs b/scripts/codegen-schema-types.mjs
index 564b593..0cc7342 100644
--- a/scripts/codegen-schema-types.mjs
+++ b/scripts/codegen-schema-types.mjs
@@ -14,10 +14,16 @@
  *   node scripts/codegen-schema-types.mjs            # generate
  *   node scripts/codegen-schema-types.mjs --check    # generate to a temp dir and compare
  *
- * Dependencies (devDependencies of the repo root):
- *   - json-schema-to-typescript
+ * Dependencies (pin these EXACTLY — json-schema-to-typescript formats its
+ * output with prettier, so an unpinned prettier silently reformats the
+ * generated types and breaks the drift gate with no schema change):
+ *   - json-schema-to-typescript@15.0.4
+ *   - prettier@3.9.0
  *   - datamodel-code-generator (Python, run via pipx in CI)
  *
+ * Install locally before regenerating:
+ *   npm install --no-save --no-package-lock json-schema-to-typescript@15.0.4 prettier@3.9.0
+ *
  * This script is intentionally modest: it does not aim to replace the
  * hand-maintained TypeScript surfaces in @eep-dev/* packages. Its job
  * is to be a *drift sentinel*: if a schema changes, the generated file
diff --git a/tests/types/eep-schemas.d.ts b/tests/types/eep-schemas.d.ts
index 45daf50..0601263 100644
--- a/tests/types/eep-schemas.d.ts
+++ b/tests/types/eep-schemas.d.ts
@@ -224,14 +224,7 @@ export interface AuditEntry {
    * For gate events: the type of gate requirement (credential, payment, agreement, data_request, identity, combined).
    */
   gate_type?:
-    | 'credential'
-    | 'payment'
-    | 'agreement'
-    | 'data_request'
-    | 'identity'
-    | 'allowlist'
-    | 'reciprocal'
-    | 'combined';
+    'credential' | 'payment' | 'agreement' | 'data_request' | 'identity' | 'allowlist' | 'reciprocal' | 'combined';
   /**
    * Short machine-readable reason code for failures. Not surfaced to requesting agents (logged internally only per EEP §10.8).
    */
@@ -2122,14 +2115,7 @@ export interface RegistryEntry {
    * Gate requirement types supported by this publisher. Agents filter by gate type when selecting interaction partners (e.g., ?gate=payment).
    */
   gate_types?: (
-    | 'credential'
-    | 'identity'
-    | 'agreement'
-    | 'data_request'
-    | 'payment'
-    | 'combined'
-    | 'proof_of_intent'
-    | 'public'
+    'credential' | 'identity' | 'agreement' | 'data_request' | 'payment' | 'combined' | 'proof_of_intent' | 'public'
   )[];
   /**
    * Protocol layers supported. Agents filter by layer capability (e.g., ?supports=sse).