diff --git a/README.md b/README.md
index 15e844c..789ea20 100644
--- a/README.md
+++ b/README.md
@@ -113,6 +113,15 @@ To add an environment-specific page, add one `SURFACES` entry (middleware and th
 llms generator pick it up automatically) and gate its nav entry / layout / API
 guard on `surfaceEnabled(...)`. `deploy.config.test.mjs` covers the matrix logic.
 
+`NEXT_PUBLIC_DEPLOY_TARGET` is a **build-time** switch, so it can only separate
+deployments that are built separately. The internal target builds one image and
+promotes it between environments, so anything that must differ *within* the
+internal target has to be runtime config instead. `TIPS_CHAINS` is the current
+example: a comma-separated allowlist (`mainnet,sepolia`) of the chains the TIPS
+chain switcher offers and its API will serve, read per request in
+`app/tips/enabledChains.ts`. Unset — the default, including local dev — means
+every known chain.
+
 ## Deployment
 
 Deployed on Vercel (external target). Push to the default branch to ship; pull
diff --git a/app/api/tips/block/[hash]/route.ts b/app/api/tips/block/[hash]/route.ts
index fd808f5..720f7b1 100644
--- a/app/api/tips/block/[hash]/route.ts
+++ b/app/api/tips/block/[hash]/route.ts
@@ -10,7 +10,7 @@ import {
   transactionMetadataFromAuditEvents,
 } from '../../audit-events';
 import { getAuditRpcUrl, getRpcUrl } from '../../config';
-import { tipsDisabledResponse } from '../../guard';
+import { tipsChainDisabledResponse, tipsDisabledResponse } from '../../guard';
 import { getTransactionReceiptSummaries } from '../../receipts';
 import {
   cacheBlockData,
@@ -305,6 +305,8 @@ export async function GET(request: Request, { params }: { params: Promise<{ hash
   const disabled = tipsDisabledResponse();
   if (disabled) return disabled;
   const chain = resolveTipsChain(new URL(request.url).searchParams.get('chain'));
+  const chainDisabled = tipsChainDisabledResponse(chain);
+  if (chainDisabled) return chainDisabled;
   const rpcUrl = getRpcUrl(chain);
 
   try {
diff --git a/app/api/tips/blocks/route.ts b/app/api/tips/blocks/route.ts
index ed746b5..4d9ec88 100644
--- a/app/api/tips/blocks/route.ts
+++ b/app/api/tips/blocks/route.ts
@@ -6,7 +6,7 @@ import {
   parseBlockListQuery,
 } from '../block-list';
 import { getRpcUrl } from '../config';
-import { tipsDisabledResponse } from '../guard';
+import { tipsChainDisabledResponse, tipsDisabledResponse } from '../guard';
 
 export const runtime = 'nodejs';
 
@@ -18,6 +18,8 @@ export async function GET(request: Request) {
   const disabled = tipsDisabledResponse();
   if (disabled) return disabled;
   const chain = resolveTipsChain(new URL(request.url).searchParams.get('chain'));
+  const chainDisabled = tipsChainDisabledResponse(chain);
+  if (chainDisabled) return chainDisabled;
 
   try {
     const query = parseBlockListQuery(new URL(request.url).searchParams);
diff --git a/app/api/tips/bundle/[hash]/route.ts b/app/api/tips/bundle/[hash]/route.ts
index 65ea320..5833f3a 100644
--- a/app/api/tips/bundle/[hash]/route.ts
+++ b/app/api/tips/bundle/[hash]/route.ts
@@ -6,7 +6,7 @@ import {
   getJoinedAuditEventsByBundle,
 } from '../../audit-events';
 import { getAuditRpcUrl, getRpcUrl } from '../../config';
-import { tipsDisabledResponse } from '../../guard';
+import { tipsChainDisabledResponse, tipsDisabledResponse } from '../../guard';
 import { getBundleHistory } from '../../s3';
 import type { BundleEvent, BundleHistory, BundleTransaction } from '../../transaction-data';
 import { publicClientFor, type TipsPublicClient } from '../../viem';
@@ -96,6 +96,8 @@ export async function GET(request: Request, { params }: { params: Promise<{ hash
   const disabled = tipsDisabledResponse();
   if (disabled) return disabled;
   const chain = resolveTipsChain(new URL(request.url).searchParams.get('chain'));
+  const chainDisabled = tipsChainDisabledResponse(chain);
+  if (chainDisabled) return chainDisabled;
 
   try {
     const { hash } = await params;
diff --git a/app/api/tips/guard.ts b/app/api/tips/guard.ts
index 7a1197f..36fd29a 100644
--- a/app/api/tips/guard.ts
+++ b/app/api/tips/guard.ts
@@ -1,3 +1,5 @@
+import type { TipsChain } from '../../tips/chains';
+import { isTipsChainEnabled } from '../../tips/enabledChains';
 import { TIPS_ENABLED } from '../../tips/flag';
 
 // Returns a 404 Response when TIPS is disabled (the public/Vercel build), else
@@ -7,3 +9,15 @@ import { TIPS_ENABLED } from '../../tips/flag';
 export function tipsDisabledResponse(): Response | null {
   return TIPS_ENABLED ? null : Response.json({ error: 'Not found' }, { status: 404 });
 }
+
+// Returns a 404 Response when this deployment does not serve `chain`, else
+// null. The UI never asks for a disabled chain — useTipsChain clamps `?chain=`
+// to the enabled list — so this catches hand-edited URLs and stale links. 404
+// rather than a silent fall back to the default chain, which would return one
+// chain's data under another chain's name; and 404 rather than letting the
+// request through to per-chain config that is unset in this environment, where
+// it would read the default bucket and 500 or, worse, succeed against the
+// wrong source.
+export function tipsChainDisabledResponse(chain: TipsChain): Response | null {
+  return isTipsChainEnabled(chain) ? null : Response.json({ error: 'Not found' }, { status: 404 });
+}
diff --git a/app/api/tips/rejected/route.ts b/app/api/tips/rejected/route.ts
index caf2d08..fbb9cc3 100644
--- a/app/api/tips/rejected/route.ts
+++ b/app/api/tips/rejected/route.ts
@@ -4,7 +4,7 @@ import {
   rejectedTransactionFromAuditEvent,
 } from '../audit-events';
 import { getAuditRpcUrl } from '../config';
-import { tipsDisabledResponse } from '../guard';
+import { tipsChainDisabledResponse, tipsDisabledResponse } from '../guard';
 import { getRejectedTransaction, listRejectedTransactions } from '../s3';
 import type { RejectedTransaction } from '../transaction-data';
 
@@ -18,6 +18,8 @@ export async function GET(request: Request) {
   const disabled = tipsDisabledResponse();
   if (disabled) return disabled;
   const chain = resolveTipsChain(new URL(request.url).searchParams.get('chain'));
+  const chainDisabled = tipsChainDisabledResponse(chain);
+  if (chainDisabled) return chainDisabled;
 
   try {
     // Audit-first, S3 fallback: use the S3 archive only when audit is not
diff --git a/app/api/tips/txn/[hash]/route.ts b/app/api/tips/txn/[hash]/route.ts
index 3d0f2a1..fa7307b 100644
--- a/app/api/tips/txn/[hash]/route.ts
+++ b/app/api/tips/txn/[hash]/route.ts
@@ -1,5 +1,5 @@
 import { resolveTipsChain } from '../../../../tips/chains';
-import { tipsDisabledResponse } from '../../guard';
+import { tipsChainDisabledResponse, tipsDisabledResponse } from '../../guard';
 import {
   InvalidTransactionHashError,
   lookupTransaction,
@@ -17,6 +17,8 @@ export async function GET(request: Request, { params }: { params: Promise<{ hash
   const disabled = tipsDisabledResponse();
   if (disabled) return disabled;
   const chain = resolveTipsChain(new URL(request.url).searchParams.get('chain'));
+  const chainDisabled = tipsChainDisabledResponse(chain);
+  if (chainDisabled) return chainDisabled;
 
   try {
     const { hash } = await params;
diff --git a/app/tips/chains.test.ts b/app/tips/chains.test.ts
new file mode 100644
index 0000000..23b058a
--- /dev/null
+++ b/app/tips/chains.test.ts
@@ -0,0 +1,87 @@
+import { afterEach, describe, expect, it } from 'vitest';
+
+import { ALL_TIPS_CHAINS, parseTipsChains, resolveTipsChain } from './chains';
+import { enabledTipsChains, isTipsChainEnabled } from './enabledChains';
+
+describe('parseTipsChains', () => {
+  it('treats unset and empty as every known chain', () => {
+    expect(parseTipsChains(undefined)).toEqual([...ALL_TIPS_CHAINS]);
+    expect(parseTipsChains(null)).toEqual([...ALL_TIPS_CHAINS]);
+    expect(parseTipsChains('')).toEqual([...ALL_TIPS_CHAINS]);
+  });
+
+  it('parses an allowlist, tolerating whitespace and case', () => {
+    expect(parseTipsChains('mainnet,sepolia')).toEqual(['mainnet', 'sepolia']);
+    expect(parseTipsChains(' MAINNET , Sepolia ')).toEqual(['mainnet', 'sepolia']);
+  });
+
+  it('keeps catalogue order and drops duplicates', () => {
+    expect(parseTipsChains('zeronet,mainnet,mainnet')).toEqual(['mainnet', 'zeronet']);
+  });
+
+  it('drops unknown names but keeps the recognized ones', () => {
+    expect(parseTipsChains('mainnet,nope')).toEqual(['mainnet']);
+  });
+
+  it('falls back to every chain when nothing recognizable is named', () => {
+    // A typo should not empty the section; unset semantics are the safer default.
+    expect(parseTipsChains('nope,alsonope')).toEqual([...ALL_TIPS_CHAINS]);
+  });
+});
+
+describe('resolveTipsChain', () => {
+  it('defaults to mainnet for missing or unknown values', () => {
+    expect(resolveTipsChain(null)).toBe('mainnet');
+    expect(resolveTipsChain('nope')).toBe('mainnet');
+  });
+
+  it('returns the requested chain when it is enabled', () => {
+    expect(resolveTipsChain('zeronet')).toBe('zeronet');
+    expect(resolveTipsChain('sepolia', ['mainnet', 'sepolia'])).toBe('sepolia');
+  });
+
+  it('falls back to the default when the requested chain is not served here', () => {
+    // The production case: a zeronet link opened against the prod deployment.
+    expect(resolveTipsChain('zeronet', ['mainnet', 'sepolia'])).toBe('mainnet');
+  });
+
+  it('falls back to the first enabled chain when the default is not served', () => {
+    expect(resolveTipsChain('mainnet', ['sepolia'])).toBe('sepolia');
+    expect(resolveTipsChain('zeronet', ['sepolia', 'zeronet'])).toBe('zeronet');
+  });
+});
+
+describe('enabledTipsChains', () => {
+  const original = process.env.TIPS_CHAINS;
+  afterEach(() => {
+    if (original === undefined) delete process.env.TIPS_CHAINS;
+    else process.env.TIPS_CHAINS = original;
+  });
+
+  it('serves every chain when TIPS_CHAINS is unset (local dev)', () => {
+    delete process.env.TIPS_CHAINS;
+    expect(enabledTipsChains()).toEqual([...ALL_TIPS_CHAINS]);
+    expect(isTipsChainEnabled('zeronet')).toBe(true);
+  });
+
+  it('honours the production allowlist', () => {
+    process.env.TIPS_CHAINS = 'mainnet,sepolia';
+    expect(enabledTipsChains()).toEqual(['mainnet', 'sepolia']);
+    expect(isTipsChainEnabled('mainnet')).toBe(true);
+    expect(isTipsChainEnabled('sepolia')).toBe(true);
+    expect(isTipsChainEnabled('zeronet')).toBe(false);
+  });
+
+  it('honours the development allowlist', () => {
+    process.env.TIPS_CHAINS = 'mainnet,sepolia,zeronet';
+    expect(enabledTipsChains()).toEqual(['mainnet', 'sepolia', 'zeronet']);
+    expect(isTipsChainEnabled('zeronet')).toBe(true);
+  });
+
+  it('is read per call, not cached at module load', () => {
+    process.env.TIPS_CHAINS = 'mainnet';
+    expect(enabledTipsChains()).toEqual(['mainnet']);
+    process.env.TIPS_CHAINS = 'mainnet,zeronet';
+    expect(enabledTipsChains()).toEqual(['mainnet', 'zeronet']);
+  });
+});
diff --git a/app/tips/chains.ts b/app/tips/chains.ts
index e454e67..f3d37d0 100644
--- a/app/tips/chains.ts
+++ b/app/tips/chains.ts
@@ -24,9 +24,10 @@ function explorerFor(chain: TipsChain): string {
   return configured && configured.length > 0 ? configured : DEFAULT_EXPLORERS[chain];
 }
 
-export const TIPS_CHAINS: readonly TipsChainInfo[] = (
-  ['mainnet', 'sepolia', 'zeronet'] as const
-).map((id) => ({
+/** Every chain the TIPS surface knows how to render, in display order. */
+export const ALL_TIPS_CHAINS = ['mainnet', 'sepolia', 'zeronet'] as const;
+
+export const TIPS_CHAINS: readonly TipsChainInfo[] = ALL_TIPS_CHAINS.map((id) => ({
   id,
   label: id === 'mainnet' ? 'Base Mainnet' : id === 'sepolia' ? 'Base Sepolia' : 'Zeronet',
   explorerUrl: explorerFor(id),
@@ -38,9 +39,42 @@ export function isTipsChain(value: string | null | undefined): value is TipsChai
   return value === 'mainnet' || value === 'sepolia' || value === 'zeronet';
 }
 
-/** Normalize an unknown ?chain= value to a valid chain (falls back to default). */
-export function resolveTipsChain(value: string | null | undefined): TipsChain {
-  return isTipsChain(value) ? value : DEFAULT_TIPS_CHAIN;
+/**
+ * Normalize an unknown ?chain= value to a chain that is actually available.
+ *
+ * `enabled` defaults to every known chain, so callers with no deployment
+ * context behave as before. When the requested chain is absent from `enabled`
+ * — a stale link, or a URL hand-edited to a chain this deployment does not
+ * serve — this falls back to the default chain if it is enabled, else to the
+ * first enabled one, so the caller always gets a chain it can serve.
+ */
+export function resolveTipsChain(
+  value: string | null | undefined,
+  enabled: readonly TipsChain[] = ALL_TIPS_CHAINS,
+): TipsChain {
+  if (enabled.length === 0) return DEFAULT_TIPS_CHAIN;
+  if (isTipsChain(value) && enabled.includes(value)) return value;
+  return enabled.includes(DEFAULT_TIPS_CHAIN) ? DEFAULT_TIPS_CHAIN : enabled[0];
+}
+
+/**
+ * Parse a `TIPS_CHAINS` allowlist ("mainnet,sepolia") into chain ids.
+ *
+ * Unset or empty means every known chain — the local-dev and pre-configuration
+ * default, which keeps behaviour unchanged for deployments that do not set it.
+ * Unknown names are dropped rather than failing the request: the env var is
+ * operator-supplied, and a typo should not take the whole section down. A value
+ * naming only unknown chains is treated as unset for the same reason.
+ */
+export function parseTipsChains(raw: string | null | undefined): readonly TipsChain[] {
+  if (!raw) return ALL_TIPS_CHAINS;
+  const named = raw
+    .split(',')
+    .map((part) => part.trim().toLowerCase())
+    .filter(isTipsChain);
+  if (named.length === 0) return ALL_TIPS_CHAINS;
+  // Keep the catalogue's display order and drop duplicates.
+  return ALL_TIPS_CHAINS.filter((id) => named.includes(id));
 }
 
 export function tipsChainInfo(chain: TipsChain): TipsChainInfo {
diff --git a/app/tips/components/ChainToggle.tsx b/app/tips/components/ChainToggle.tsx
index e99986d..ae2d2df 100644
--- a/app/tips/components/ChainToggle.tsx
+++ b/app/tips/components/ChainToggle.tsx
@@ -2,21 +2,26 @@
 
 import { Tabs } from '../../components/ui/Tabs';
 import { trackTipsChainSelect } from '../../analytics/events';
-import { TIPS_CHAINS, type TipsChain } from '../chains';
+import { tipsChainInfo, type TipsChain } from '../chains';
 import { useTipsChain } from '../library/useTipsChain';
+import { useEnabledTipsChains } from './TipsChainsProvider';
 
-// Segmented control over the TIPS chains (Base Mainnet / Base Sepolia /
-// Zeronet). Rewrites `?chain=` via useTipsChain's setter so the selection
-// persists across navigation, and reports the choice to analytics.
+// Segmented control over the chains this deployment serves. Rewrites `?chain=`
+// via useTipsChain's setter so the selection persists across navigation, and
+// reports the choice to analytics. Hidden when there is nothing to choose
+// between — a one-chain deployment gets a label-less single tab otherwise.
 export function ChainToggle() {
   const { chain, setChain } = useTipsChain();
+  const enabled = useEnabledTipsChains();
+
+  if (enabled.length < 2) return null;
 
   return (
     <Tabs
       ariaLabel="Select chain"
       size="sm"
       value={chain}
-      items={TIPS_CHAINS.map((c) => ({ value: c.id, label: c.label }))}
+      items={enabled.map((id) => ({ value: id, label: tipsChainInfo(id).label }))}
       onChange={(value) => {
         const next = value as TipsChain;
         setChain(next);
diff --git a/app/tips/components/TipsChainsProvider.tsx b/app/tips/components/TipsChainsProvider.tsx
new file mode 100644
index 0000000..80c1a6b
--- /dev/null
+++ b/app/tips/components/TipsChainsProvider.tsx
@@ -0,0 +1,32 @@
+'use client';
+
+import { createContext, useContext, type ReactNode } from 'react';
+
+import { ALL_TIPS_CHAINS, type TipsChain } from '../chains';
+
+// Carries the deployment's chain allowlist from the server (TIPS_CHAINS, read
+// in app/tips/enabledChains.ts) down to the client components that render and
+// resolve the chain. The list is runtime config the client cannot read itself,
+// so the section layout resolves it once and provides it here.
+const TipsChainsContext = createContext<readonly TipsChain[]>(ALL_TIPS_CHAINS);
+
+export function TipsChainsProvider({
+  chains,
+  children,
+}: {
+  chains: readonly TipsChain[];
+  children: ReactNode;
+}) {
+  return <TipsChainsContext.Provider value={chains}>{children}</TipsChainsContext.Provider>;
+}
+
+/**
+ * The chains this deployment serves, in display order.
+ *
+ * Defaults to every known chain when no provider is present, matching the
+ * unset-TIPS_CHAINS behaviour so a component rendered outside the section
+ * (or in a test) still works.
+ */
+export function useEnabledTipsChains(): readonly TipsChain[] {
+  return useContext(TipsChainsContext);
+}
diff --git a/app/tips/enabledChains.ts b/app/tips/enabledChains.ts
new file mode 100644
index 0000000..c2efbcc
--- /dev/null
+++ b/app/tips/enabledChains.ts
@@ -0,0 +1,20 @@
+// Which TIPS chains this deployment serves. Server-only: reads TIPS_CHAINS,
+// a plain (non-NEXT_PUBLIC_) env var, so it must never be imported from a
+// client component — the client gets the list through TipsChainsProvider.
+//
+// This is deliberately runtime rather than build-time config. The internal
+// deployment builds one image (protocols/ui Dockerfile.ui) and promotes that
+// same image from development to production, so a NEXT_PUBLIC_* flag — inlined
+// into the bundle at build time — cannot differ between the two environments.
+// The Helm chart already varies per-chain TIPS_* env this way; TIPS_CHAINS
+// joins it. Unset means every known chain, which is what local dev sees.
+import { parseTipsChains, type TipsChain } from './chains';
+
+export function enabledTipsChains(): readonly TipsChain[] {
+  return parseTipsChains(process.env.TIPS_CHAINS);
+}
+
+/** Is this chain served by this deployment? */
+export function isTipsChainEnabled(chain: TipsChain): boolean {
+  return enabledTipsChains().includes(chain);
+}
diff --git a/app/tips/layout.tsx b/app/tips/layout.tsx
index 1567134..d8a1060 100644
--- a/app/tips/layout.tsx
+++ b/app/tips/layout.tsx
@@ -2,6 +2,8 @@ import type { Metadata } from 'next';
 import { notFound } from 'next/navigation';
 import type { ReactNode } from 'react';
 
+import { TipsChainsProvider } from './components/TipsChainsProvider';
+import { enabledTipsChains } from './enabledChains';
 import { TIPS_ENABLED } from './flag';
 
 // Metadata for the TIPS section. The app-wide chrome (sidebar, header) comes
@@ -18,5 +20,11 @@ export default function TipsLayout({ children }: { children: ReactNode }) {
   // disabled. With the flag off this branch is a compile-time constant, so the
   // section is unreachable in the public build.
   if (!TIPS_ENABLED) notFound();
-  return <div className="mx-auto flex w-full max-w-5xl flex-1 flex-col">{children}</div>;
+  // Resolved here, once per request, because TIPS_CHAINS is server-only runtime
+  // config: the client components below cannot read it themselves.
+  return (
+    <TipsChainsProvider chains={enabledTipsChains()}>
+      <div className="mx-auto flex w-full max-w-5xl flex-1 flex-col">{children}</div>
+    </TipsChainsProvider>
+  );
 }
diff --git a/app/tips/library/useTipsChain.ts b/app/tips/library/useTipsChain.ts
index fe9ec1b..e785da2 100644
--- a/app/tips/library/useTipsChain.ts
+++ b/app/tips/library/useTipsChain.ts
@@ -4,9 +4,10 @@ import { useCallback } from 'react';
 import { usePathname, useRouter, useSearchParams } from 'next/navigation';
 
 import { resolveTipsChain, type TipsChain } from '../chains';
+import { useEnabledTipsChains } from '../components/TipsChainsProvider';
 
 type UseTipsChain = {
-  /** The chain currently selected in the URL (defaults via resolveTipsChain). */
+  /** The chain selected in the URL, clamped to the chains this deployment serves. */
   chain: TipsChain;
   /** Update `?chain=` in place, preserving the path and other query params. */
   setChain: (next: TipsChain) => void;
@@ -20,7 +21,10 @@ export function useTipsChain(): UseTipsChain {
   const pathname = usePathname();
   const searchParams = useSearchParams();
 
-  const chain = resolveTipsChain(searchParams.get('chain'));
+  // Clamped to what this deployment serves, so a `?chain=` naming a chain this
+  // environment has no data for reads as the default rather than erroring.
+  const enabled = useEnabledTipsChains();
+  const chain = resolveTipsChain(searchParams.get('chain'), enabled);
 
   const setChain = useCallback(
     (next: TipsChain) => {