diff --git a/src/__tests__/portalPets.test.ts b/src/__tests__/portalPets.test.ts
index a95cb23..c23bca0 100644
--- a/src/__tests__/portalPets.test.ts
+++ b/src/__tests__/portalPets.test.ts
@@ -177,7 +177,10 @@ describe("PATCH /portal/pets/:petId", () => {
     expect(persisted.weightKg).toBe("18.25");
     expect(persisted.groomingNotes).toBe("old grooming notes");
     expect(persisted.healthAlerts).toBe("Allergic to oatmeal shampoo");
-    expect(persisted.photoKey).toBe("pets/rex.jpg");
+    // photoKey is NOT writable via portal PATCH (GRO-2187 S3 key-hijack fix):
+    // the web form round-trips the GET-shaped photoUrl, but the server must not
+    // persist it. Photo changes go through the key-validated upload flow.
+    expect(persisted.photoKey).toBeUndefined();
     expect(persisted.coatType).toBe("double");
     expect(persisted.petSizeCategory).toBe("extra_large");
     expect(persisted.preferredCuts).toEqual(["teddy bear", "puppy cut"]);
@@ -187,6 +190,55 @@ describe("PATCH /portal/pets/:petId", () => {
     expect(persisted.updatedAt).toBeInstanceOf(Date);
   });
 
+  // GRO-2187 security regression: a portal customer must not be able to set the
+  // S3 object key. photoKey is consumed server-side by getPresignedGetUrl /
+  // deleteObject; the upload path guards keys with a pets/{petId}/ prefix, and the
+  // portal PATCH must not offer a bypass. A foreign/arbitrary photoUrl is accepted
+  // (Zod strips the unknown key) but must leave photoKey untouched.
+  it("does not mutate photoKey when a foreign photoUrl is supplied (200)", async () => {
+    selectSessionRow = ACTIVE_SESSION;
+    const ownKey = `pets/${PET_ID}/original.jpg`;
+    selectPetRow = { ...PET, photoKey: ownKey };
+
+    const res = await jsonPatch(
+      `/portal/pets/${PET_ID}`,
+      {
+        name: "Rex",
+        // attacker-chosen key pointing at another tenant's object
+        photoUrl: "pets/00000000-0000-0000-0000-0000000000ff/victim-secret.jpg",
+      },
+      { "X-Impersonation-Session-Id": SESSION_ID }
+    );
+
+    expect(res.status).toBe(200);
+    const persisted = updatedValues[0]!;
+    // The attacker-supplied key never reaches the update payload.
+    expect(persisted.photoKey).toBeUndefined();
+    // And the stored key is unchanged from the pet's own value.
+    const body = await res.json();
+    expect(body.photoUrl).toBe(ownKey);
+  });
+
+  // The length/array caps live in the Zod schema, so violations are rejected by
+  // zValidator with 400 (in-handler enum checks are what return 422).
+  it("returns 400 when a medicalAlert description exceeds the length cap", async () => {
+    selectSessionRow = ACTIVE_SESSION;
+    selectPetRow = PET;
+
+    const res = await jsonPatch(
+      `/portal/pets/${PET_ID}`,
+      {
+        medicalAlerts: [
+          { type: "allergy", description: "x".repeat(2001), severity: "low" },
+        ],
+      },
+      { "X-Impersonation-Session-Id": SESSION_ID }
+    );
+
+    expect(res.status).toBe(400);
+    expect(updatedValues).toHaveLength(0);
+  });
+
   it("falls back to the weight key when weightKg is absent", async () => {
     selectSessionRow = ACTIVE_SESSION;
     selectPetRow = PET;
diff --git a/src/routes/portal.ts b/src/routes/portal.ts
index 3f15745..0b106fb 100644
--- a/src/routes/portal.ts
+++ b/src/routes/portal.ts
@@ -261,8 +261,8 @@ const PORTAL_PET_SIZE_ALIASES: Record<string, string> = { xlarge: "extra_large"
 
 const portalMedicalAlertSchema = z.object({
   id: z.string().optional(),
-  type: z.string(),
-  description: z.string(),
+  type: z.string().max(2000),
+  description: z.string().max(2000),
   severity: z.enum(["low", "medium", "high"]),
 });
 
@@ -275,12 +275,16 @@ const portalPetUpdateSchema = z.object({
   birthDate: z.string().nullable().optional(),
   notes: z.string().max(2000).nullable().optional(),
   healthAlerts: z.string().max(2000).nullable().optional(),
-  photoUrl: z.string().nullable().optional(),
+  // photoUrl/photoKey are intentionally NOT writable here: photoKey is a trusted
+  // S3 object key consumed server-side (getPresignedGetUrl / deleteObject), and the
+  // upload path (pets.ts) already enforces a pets/{petId}/ prefix guard against key
+  // hijacking. Photo changes go through the dedicated upload + /photo/confirm flow.
+  // The web form round-trips the GET-shaped photoUrl; Zod strips it as an unknown key.
   // coatType / petSizeCategory validated in-handler so bad values return 422.
   coatType: z.string().nullable().optional(),
   petSizeCategory: z.string().nullable().optional(),
-  preferredCuts: z.array(z.string()).nullable().optional(),
-  medicalAlerts: z.array(portalMedicalAlertSchema).nullable().optional(),
+  preferredCuts: z.array(z.string().max(2000)).max(50).nullable().optional(),
+  medicalAlerts: z.array(portalMedicalAlertSchema).max(50).nullable().optional(),
 });
 
 portalRouter.patch(
@@ -322,7 +326,8 @@ portalRouter.patch(
 
     if (body.notes !== undefined) updateData.groomingNotes = body.notes;
     if (body.healthAlerts !== undefined) updateData.healthAlerts = body.healthAlerts;
-    if (body.photoUrl !== undefined) updateData.photoKey = body.photoUrl;
+    // photoKey is intentionally not writable here — see portalPetUpdateSchema note.
+    // Photo changes go through the key-validated upload + /photo/confirm flow.
 
     if (body.coatType !== undefined) {
       if (body.coatType !== null && !PORTAL_COAT_TYPES.includes(body.coatType)) {