diff --git a/src/app/api/medications/[id]/phase-config/__tests__/route.test.ts b/src/app/api/medications/[id]/phase-config/__tests__/route.test.ts
new file mode 100644
index 000000000..92010a61b
--- /dev/null
+++ b/src/app/api/medications/[id]/phase-config/__tests__/route.test.ts
@@ -0,0 +1,96 @@
+/**
+ * v1.5.5 F-1 H-3 — phase-config PUT now returns the multi-issue 422
+ * envelope rather than the flat `400 Invalid input`.
+ */
+
+import { describe, it, expect, vi, beforeEach } from "vitest";
+import { NextRequest } from "next/server";
+
+vi.mock("@/lib/db", () => ({
+  prisma: {
+    reminderPhaseConfig: { upsert: vi.fn() },
+  },
+}));
+
+vi.mock("@/lib/medications/route-guards", () => ({
+  assertMedicationOwnership: vi.fn().mockResolvedValue(null),
+}));
+
+vi.mock("@/lib/auth/session", () => ({ getSession: vi.fn() }));
+vi.mock("@/lib/logging/transports", () => ({ emitIfSampled: vi.fn() }));
+vi.mock("@/lib/db-compat", () => ({
+  ensureDbCompatibility: vi.fn().mockResolvedValue(undefined),
+}));
+vi.mock("next/headers", () => ({
+  headers: vi.fn(async () => ({ get: () => null })),
+  cookies: vi.fn(async () => ({
+    get: () => undefined,
+    set: () => {},
+    delete: () => {},
+  })),
+}));
+
+import { PUT } from "../route";
+import { prisma } from "@/lib/db";
+import { getSession } from "@/lib/auth/session";
+import { assertMedicationOwnership } from "@/lib/medications/route-guards";
+
+const SESSION_OK = {
+  session: { id: "sess-1", expiresAt: new Date(Date.now() + 3_600_000) },
+  user: { id: "user-1", username: "tester", role: "USER" as const },
+};
+
+const ROUTE_PARAMS = { params: Promise.resolve({ id: "med-1" }) };
+
+function putReq(body: unknown): NextRequest {
+  return new NextRequest(
+    "http://localhost/api/medications/med-1/phase-config",
+    {
+      method: "PUT",
+      headers: { "Content-Type": "application/json" },
+      body: JSON.stringify(body),
+    },
+  );
+}
+
+beforeEach(() => {
+  vi.resetAllMocks();
+  vi.mocked(getSession).mockResolvedValue(SESSION_OK as never);
+  vi.mocked(assertMedicationOwnership).mockResolvedValue(null);
+  vi.mocked(prisma.reminderPhaseConfig.upsert).mockResolvedValue({
+    medicationId: "med-1",
+  } as never);
+});
+
+describe("PUT /api/medications/[id]/phase-config — F-1 H-3 multi-issue 422", () => {
+  it("returns the multi-issue envelope on a malformed body", async () => {
+    // Three issues: greenValue not int, greenMode not enum, redValue overflow.
+    const res = await PUT(
+      putReq({
+        greenValue: "not-a-number",
+        greenMode: "BOGUS",
+        yellowValue: 30,
+        yellowMode: "MINUTES",
+        orangeValue: 0,
+        orangeMode: "MINUTES",
+        redValue: 10_000,
+        redMode: "MINUTES",
+      }),
+      ROUTE_PARAMS,
+    );
+    expect(res.status).toBe(422);
+    const body = (await res.json()) as {
+      data: null;
+      error: string;
+      details: {
+        issues: Array<{ path: string; code: string; message: string }>;
+      };
+    };
+    expect(body.data).toBeNull();
+    expect(body.error).toBe("Validation failed");
+    expect(body.details.issues.length).toBeGreaterThanOrEqual(2);
+    for (const issue of body.details.issues) {
+      expect(Object.keys(issue).sort()).toEqual(["code", "message", "path"]);
+    }
+  });
+});
diff --git a/src/app/api/medications/[id]/phase-config/route.ts b/src/app/api/medications/[id]/phase-config/route.ts
index 65214be30..e64e87861 100644
--- a/src/app/api/medications/[id]/phase-config/route.ts
+++ b/src/app/api/medications/[id]/phase-config/route.ts
@@ -1,7 +1,11 @@
 import { prisma } from "@/lib/db";
 import { apiHandler, requireAuth } from "@/lib/api-handler";
 import { annotate } from "@/lib/logging/context";
-import { apiSuccess, apiError, safeJson } from "@/lib/api-response";
+import {
+  apiSuccess,
+  returnAllZodIssues,
+  safeJson,
+} from "@/lib/api-response";
 import { phaseConfigSchema } from "@/lib/validations/phase-config";
 import { assertMedicationOwnership } from "@/lib/medications/route-guards";
 import { NextRequest } from "next/server";
@@ -59,7 +63,11 @@ export const PUT = apiHandler(
     if (jsonError) return jsonError;
     const parsed = phaseConfigSchema.safeParse(body);
     if (!parsed.success) {
-      return apiError("Invalid input", 400);
+      // v1.5.5 F-1 H-3 — every v1.5.5 route returns the multi-issue
+      // 422 envelope so iOS sees per-field feedback the same way the
+      // bulk-delete and intake routes already do. The old `400 Invalid
+      // input` flat response was the last hold-out in the surface.
+      return returnAllZodIssues(parsed.error, 422);
     }
 
     const config = await prisma.reminderPhaseConfig.upsert({