diff --git a/web/src/components/app/contacts/importShared.test.ts b/web/src/components/app/contacts/importShared.test.ts
new file mode 100644
index 00000000..3abb510a
--- /dev/null
+++ b/web/src/components/app/contacts/importShared.test.ts
@@ -0,0 +1,70 @@
+// Issue #207: a 1,000-row import failed every row with "invalid custom field
+// key: Company Mobile". The client now enforces the same rule the API does, so
+// a name it cannot use is caught on the mapping screen. These cases mirror
+// internal/utils/json.go — if that rule moves, this is where it shows up.
+
+import { describe, it, expect } from "vitest";
+import {
+    isValidCustomKey,
+    mappingProblem,
+    normalizeCustomKey,
+    suggestCustomKey,
+} from "./importShared";
+import type { ImportColumnMapping } from "@/lib/api/client/app/contacts/importContacts";
+
+describe("custom field names", () => {
+    it("accepts what the template engine can resolve", () => {
+        for (const key of ["role", "Job_Title", "Company Mobile", "first-name", "plan tier 2"]) {
+            expect(isValidCustomKey(key)).toBe(true);
+        }
+    });
+
+    it("rejects names nothing could merge into an email", () => {
+        for (const key of ["", "   ", "Company/Mobile", "Revenue ($)", "a.b", "тест"]) {
+            expect(isValidCustomKey(key)).toBe(false);
+        }
+    });
+
+    it("collapses stray whitespace so one column is one field", () => {
+        expect(normalizeCustomKey("  Company   Mobile ")).toBe("Company Mobile");
+        expect(isValidCustomKey("  Company   Mobile ")).toBe(true);
+    });
+
+    it("turns a spreadsheet header into a usable name", () => {
+        expect(suggestCustomKey("Company Mobile")).toBe("Company Mobile");
+        expect(suggestCustomKey("Revenue ($)")).toBe("Revenue");
+        expect(suggestCustomKey("Annual Revenue (USD)")).toBe("Annual Revenue USD");
+        expect(suggestCustomKey("###")).toBe("");
+    });
+});
+
+describe("mappingProblem", () => {
+    const email: ImportColumnMapping = { index: 0, target: "email" };
+
+    it("passes a mapping the API would accept", () => {
+        expect(
+            mappingProblem([email, { index: 5, target: "custom", custom_key: "Company Mobile" }]),
+        ).toBeNull();
+    });
+
+    it("names the column when a custom field has no name", () => {
+        expect(mappingProblem([email, { index: 5, target: "custom", custom_key: "  " }])).toBe(
+            "Column 6 needs a custom field name.",
+        );
+    });
+
+    it("explains an unusable name instead of letting the import fail row by row", () => {
+        const msg = mappingProblem([email, { index: 5, target: "custom", custom_key: "Company/Mobile" }]);
+        expect(msg).toContain("Company/Mobile");
+        expect(msg).toContain("letters");
+    });
+
+    it("still requires an email column", () => {
+        expect(mappingProblem([{ index: 1, target: "first_name" }])).toBe("Map a column to Email.");
+    });
+
+    it("understands the legacy custom:<key> spelling", () => {
+        expect(mappingProblem([email, { index: 2, target: "custom:plan_tier" }])).toBeNull();
+        expect(mappingProblem([email, { index: 2, target: "custom:plan/tier" }])).toContain("plan/tier");
+    });
+});
diff --git a/web/src/components/app/contacts/importShared.ts b/web/src/components/app/contacts/importShared.ts
index c48a8341..6b6dc43c 100644
--- a/web/src/components/app/contacts/importShared.ts
+++ b/web/src/components/app/contacts/importShared.ts
@@ -6,6 +6,7 @@
 
 import toast from "react-hot-toast";
 import type {
+    ImportColumnMapping,
     ImportDedupStrategy,
     ImportResult,
 } from "@/lib/api/client/app/contacts/importContacts";
@@ -61,3 +62,57 @@ export function announceResult(res: ImportResult) {
         toast(`Synced with ${res.failed} errors`, { icon: "⚠️" });
     }
 }
+
+// ----- Custom-field names -----------------------------------------
+//
+// Mirrors internal/utils.IsValidJSONKey. A custom field is addressable in
+// campaign copy either as {{.Role}} or, for a spaced/dashed name, through the
+// server-side rewrite to (index . "Company Mobile"). Anything else would make
+// a field the user can store but never merge into an email, so the API rejects
+// it — we catch it here so a mistyped name never costs a whole import.
+const CUSTOM_KEY_RE = /^[A-Za-z0-9_]+(?:[ -]+[A-Za-z0-9_]+)*$/;
+
+export const CUSTOM_KEY_RULES = "Use letters, numbers, underscores, spaces or dashes.";
+
+export function normalizeCustomKey(key: string): string {
+    return key.trim().split(/\s+/).filter(Boolean).join(" ");
+}
+
+export function isValidCustomKey(key: string): boolean {
+    const k = normalizeCustomKey(key);
+    return k.length > 0 && k.length <= 255 && CUSTOM_KEY_RE.test(k);
+}
+
+// suggestCustomKey turns a raw spreadsheet header into a name the API accepts,
+// so picking "Use as custom field" on a "Company Mobile" column just works.
+// Returns "" when nothing usable survives and the user has to type a name.
+export function suggestCustomKey(header: string): string {
+    const cleaned = normalizeCustomKey(header.replace(/[^A-Za-z0-9_ -]+/g, " "))
+        .replace(/^[-\s]+/, "")
+        .replace(/[-\s]+$/, "");
+    return isValidCustomKey(cleaned) ? cleaned : "";
+}
+
+export function isCustomTarget(target: string): boolean {
+    return target === "custom" || target.startsWith("custom:");
+}
+
+// mappingProblem returns the first reason the mapping cannot be committed, or
+// null when it is good to go. Same order of checks as the server so the two
+// never disagree about which column is at fault.
+export function mappingProblem(mapping: ImportColumnMapping[]): string | null {
+    for (const m of mapping) {
+        if (!isCustomTarget(m.target)) continue;
+        const key = m.custom_key ?? (m.target.startsWith("custom:") ? m.target.slice(7) : "");
+        if (normalizeCustomKey(key) === "") {
+            return `Column ${m.index + 1} needs a custom field name.`;
+        }
+        if (!isValidCustomKey(key)) {
+            return `"${key.trim()}" is not a valid field name. ${CUSTOM_KEY_RULES}`;
+        }
+    }
+    if (!mapping.some((m) => m.target === "email")) {
+        return "Map a column to Email.";
+    }
+    return null;
+}