diff --git a/apps/journal/app/routes/api.v1.activities._index.ts b/apps/journal/app/routes/api.v1.activities._index.ts
index d9545b1..a323e88 100644
--- a/apps/journal/app/routes/api.v1.activities._index.ts
+++ b/apps/journal/app/routes/api.v1.activities._index.ts
@@ -5,6 +5,7 @@ import {
   PaginationQuerySchema,
   CreateActivityRequestSchema,
   ERROR_CODES,
+  zodIssuesToFieldErrors,
 } from "@trails-cool/api";
 
 /** GET /api/v1/activities — paginated activity list */
@@ -59,7 +60,7 @@ export async function action({ request }: Route.ActionArgs) {
   const parsed = CreateActivityRequestSchema.safeParse(body);
   if (!parsed.success) {
     return apiError(400, ERROR_CODES.VALIDATION_ERROR, "Validation failed",
-      parsed.error.issues.map((i) => ({ field: i.path.join("."), message: i.message })));
+      zodIssuesToFieldErrors(parsed.error));
   }
 
   const id = await createActivity(user.id, {
diff --git a/apps/journal/app/routes/api.v1.routes.$id.ts b/apps/journal/app/routes/api.v1.routes.$id.ts
index 8458312..86e3690 100644
--- a/apps/journal/app/routes/api.v1.routes.$id.ts
+++ b/apps/journal/app/routes/api.v1.routes.$id.ts
@@ -1,7 +1,7 @@
 import type { Route } from "./+types/api.v1.routes.$id";
 import { requireApiUser, apiError } from "~/lib/api-guard.server";
 import { getRouteWithVersions, updateRoute, deleteRoute } from "~/lib/routes.server";
-import { UpdateRouteRequestSchema, ERROR_CODES } from "@trails-cool/api";
+import { UpdateRouteRequestSchema, ERROR_CODES, zodIssuesToFieldErrors } from "@trails-cool/api";
 
 /** GET /api/v1/routes/:id — full route detail */
 export async function loader({ request, params }: Route.LoaderArgs) {
@@ -45,7 +45,7 @@ export async function action({ request, params }: Route.ActionArgs) {
     const parsed = UpdateRouteRequestSchema.safeParse(body);
     if (!parsed.success) {
       return apiError(400, ERROR_CODES.VALIDATION_ERROR, "Validation failed",
-        parsed.error.issues.map((i) => ({ field: i.path.join("."), message: i.message })));
+        zodIssuesToFieldErrors(parsed.error));
     }
 
     await updateRoute(params.id, user.id, parsed.data);
diff --git a/apps/journal/app/routes/api.v1.routes._index.ts b/apps/journal/app/routes/api.v1.routes._index.ts
index 8ce9821..5f72767 100644
--- a/apps/journal/app/routes/api.v1.routes._index.ts
+++ b/apps/journal/app/routes/api.v1.routes._index.ts
@@ -5,6 +5,7 @@ import {
   PaginationQuerySchema,
   CreateRouteRequestSchema,
   ERROR_CODES,
+  zodIssuesToFieldErrors,
 } from "@trails-cool/api";
 
 /** GET /api/v1/routes — paginated route list */
@@ -58,7 +59,7 @@ export async function action({ request }: Route.ActionArgs) {
   const parsed = CreateRouteRequestSchema.safeParse(body);
   if (!parsed.success) {
     return apiError(400, ERROR_CODES.VALIDATION_ERROR, "Validation failed",
-      parsed.error.issues.map((i) => ({ field: i.path.join("."), message: i.message })));
+      zodIssuesToFieldErrors(parsed.error));
   }
 
   const id = await createRoute(user.id, parsed.data);
diff --git a/apps/journal/app/routes/api.v1.routes.compute.ts b/apps/journal/app/routes/api.v1.routes.compute.ts
index 077d733..cd111ff 100644
--- a/apps/journal/app/routes/api.v1.routes.compute.ts
+++ b/apps/journal/app/routes/api.v1.routes.compute.ts
@@ -1,6 +1,6 @@
 import type { Route } from "./+types/api.v1.routes.compute";
 import { requireApiUser, apiError } from "~/lib/api-guard.server";
-import { ComputeRouteRequestSchema, ERROR_CODES } from "@trails-cool/api";
+import { ComputeRouteRequestSchema, ERROR_CODES, zodIssuesToFieldErrors } from "@trails-cool/api";
 
 const PLANNER_URL = process.env.PLANNER_URL ?? "http://localhost:3001";
 
@@ -13,7 +13,7 @@ export async function action({ request }: Route.ActionArgs) {
   const parsed = ComputeRouteRequestSchema.safeParse(body);
   if (!parsed.success) {
     return apiError(400, ERROR_CODES.VALIDATION_ERROR, "Validation failed",
-      parsed.error.issues.map((i) => ({ field: i.path.join("."), message: i.message })));
+      zodIssuesToFieldErrors(parsed.error));
   }
 
   try {
diff --git a/apps/journal/app/routes/api.v1.uploads.ts b/apps/journal/app/routes/api.v1.uploads.ts
index 83a18da..4998bdc 100644
--- a/apps/journal/app/routes/api.v1.uploads.ts
+++ b/apps/journal/app/routes/api.v1.uploads.ts
@@ -1,6 +1,6 @@
 import type { Route } from "./+types/api.v1.uploads";
 import { requireApiUser, apiError } from "~/lib/api-guard.server";
-import { PresignedUploadRequestSchema, ERROR_CODES } from "@trails-cool/api";
+import { PresignedUploadRequestSchema, ERROR_CODES, zodIssuesToFieldErrors } from "@trails-cool/api";
 import { randomUUID } from "node:crypto";
 
 const S3_ENDPOINT = process.env.S3_ENDPOINT ?? "http://localhost:3902";
@@ -16,7 +16,7 @@ export async function action({ request }: Route.ActionArgs) {
   const parsed = PresignedUploadRequestSchema.safeParse(body);
   if (!parsed.success) {
     return apiError(400, ERROR_CODES.VALIDATION_ERROR, "Validation failed",
-      parsed.error.issues.map((i) => ({ field: i.path.join("."), message: i.message })));
+      zodIssuesToFieldErrors(parsed.error));
   }
 
   const { filename, resourceType, resourceId } = parsed.data;
diff --git a/packages/api/src/errors.ts b/packages/api/src/errors.ts
index 7d7fb88..381dd95 100644
--- a/packages/api/src/errors.ts
+++ b/packages/api/src/errors.ts
@@ -14,6 +14,18 @@ export const ApiErrorResponseSchema = z.object({
 export type FieldError = z.infer<typeof FieldErrorSchema>;
 export type ApiErrorResponse = z.infer<typeof ApiErrorResponseSchema>;
 
+/**
+ * Map a ZodError's issues to the FieldError[] shape used in API
+ * validation responses. Keeps the path-flattening rule in one place so
+ * every route returns validation errors in the same shape.
+ */
+export function zodIssuesToFieldErrors(error: z.ZodError): FieldError[] {
+  return error.issues.map((i) => ({
+    field: i.path.join("."),
+    message: i.message,
+  }));
+}
+
 /** Standard error codes */
 export const ERROR_CODES = {
   VALIDATION_ERROR: "VALIDATION_ERROR",
diff --git a/packages/api/src/index.ts b/packages/api/src/index.ts
index 8f7e100..a6e14a6 100644
--- a/packages/api/src/index.ts
+++ b/packages/api/src/index.ts
@@ -4,6 +4,7 @@ export { ENDPOINTS } from "./endpoints.ts";
 // Error schemas
 export {
   ApiErrorResponseSchema, FieldErrorSchema, ERROR_CODES,
+  zodIssuesToFieldErrors,
   type ApiErrorResponse, type FieldError,
 } from "./errors.ts";