diff --git a/next.config.ts b/next.config.ts
index 1767b99..bb33b73 100644
--- a/next.config.ts
+++ b/next.config.ts
@@ -1,5 +1,6 @@
 import type { NextConfig } from "next";
 import withBundleAnalyzer from "@next/bundle-analyzer";
+import { validateEnv } from "./src/config/validateEnv";
 
 const raw = process.env.BASE_PATH?.trim() ?? "";
 const basePath = raw.startsWith("/") ? raw : raw ? `/${raw}` : "";
@@ -13,6 +14,19 @@ const basePath = raw.startsWith("/") ? raw : raw ? `/${raw}` : "";
  * the Stellar Horizon API directly from the browser.
  */
 const isStaticExport = process.env.NEXT_EXPORT === "true";
+
+// Fail the build immediately on missing/malformed env vars instead of
+// silently falling back to demo mode or a runtime 500 (issue #199).
+//
+// Netlify sets CONTEXT to "production" | "deploy-preview" | "branch-deploy"
+// (unset locally and in most other CI). Preview/branch-deploy builds don't
+// necessarily carry production Soroban config, and previously fell into
+// silent demo mode the same as any other missing-config build — so those
+// contexts warn instead of failing the build. Local dev/build and the
+// production context stay strict.
+const netlifyContext = process.env.CONTEXT;
+const isNetlifyPreview = netlifyContext === "deploy-preview" || netlifyContext === "branch-deploy";
+validateEnv({ isStaticExport, strict: !isNetlifyPreview });
 const backendApiOrigin = (() => {
   try {
     return new URL(
diff --git a/src/config/validateEnv.test.ts b/src/config/validateEnv.test.ts
new file mode 100644
index 0000000..ccb10f6
--- /dev/null
+++ b/src/config/validateEnv.test.ts
@@ -0,0 +1,96 @@
+import { afterEach, describe, expect, it, vi } from 'vitest';
+import { StrKey } from '@stellar/stellar-sdk';
+import { validateEnv } from './validateEnv';
+
+// Built via StrKey.encode*, not Keypair.random()/fromSecret(): the latter
+// route through @noble/ed25519 signing, which hits a pre-existing
+// Vitest-only Buffer-polyfill bug (see feeBumpGuard.test.ts's note on
+// Transaction.hash()) — unrelated to this change.
+const VALID_CONTRACT_ID = StrKey.encodeContract(new Uint8Array(32).fill(1));
+const VALID_SPONSOR_SECRET = StrKey.encodeEd25519SecretSeed(new Uint8Array(32).fill(2));
+
+const ENV_KEYS = [
+  'NEXT_PUBLIC_FACTORY_CONTRACT_ID',
+  'NEXT_PUBLIC_SOROBAN_RPC_URL',
+  'STELLAR_FEE_SPONSOR_SECRET',
+] as const;
+
+function setValidEnv() {
+  process.env.NEXT_PUBLIC_FACTORY_CONTRACT_ID = VALID_CONTRACT_ID;
+  process.env.NEXT_PUBLIC_SOROBAN_RPC_URL = 'https://soroban-testnet.stellar.org';
+  process.env.STELLAR_FEE_SPONSOR_SECRET = VALID_SPONSOR_SECRET;
+}
+
+describe('validateEnv', () => {
+  afterEach(() => {
+    for (const key of ENV_KEYS) delete process.env[key];
+  });
+
+  it('passes with a fully valid server-mode env', () => {
+    setValidEnv();
+    expect(() => validateEnv({ isStaticExport: false })).not.toThrow();
+  });
+
+  it('throws when NEXT_PUBLIC_FACTORY_CONTRACT_ID is missing', () => {
+    setValidEnv();
+    delete process.env.NEXT_PUBLIC_FACTORY_CONTRACT_ID;
+    expect(() => validateEnv({ isStaticExport: false })).toThrow(
+      /NEXT_PUBLIC_FACTORY_CONTRACT_ID is not set/,
+    );
+  });
+
+  it('throws when NEXT_PUBLIC_FACTORY_CONTRACT_ID is not a valid contract id', () => {
+    setValidEnv();
+    process.env.NEXT_PUBLIC_FACTORY_CONTRACT_ID = 'not-a-contract-id';
+    expect(() => validateEnv({ isStaticExport: false })).toThrow(
+      /not a valid Soroban contract id/,
+    );
+  });
+
+  it('throws when NEXT_PUBLIC_SOROBAN_RPC_URL is set but not a valid URL', () => {
+    setValidEnv();
+    process.env.NEXT_PUBLIC_SOROBAN_RPC_URL = 'not a url';
+    expect(() => validateEnv({ isStaticExport: false })).toThrow(/not a valid URL/);
+  });
+
+  it('does not require STELLAR_FEE_SPONSOR_SECRET for static export', () => {
+    setValidEnv();
+    delete process.env.STELLAR_FEE_SPONSOR_SECRET;
+    expect(() => validateEnv({ isStaticExport: true })).not.toThrow();
+  });
+
+  it('requires STELLAR_FEE_SPONSOR_SECRET in server mode', () => {
+    setValidEnv();
+    delete process.env.STELLAR_FEE_SPONSOR_SECRET;
+    expect(() => validateEnv({ isStaticExport: false })).toThrow(
+      /STELLAR_FEE_SPONSOR_SECRET is not set/,
+    );
+  });
+
+  it('throws when STELLAR_FEE_SPONSOR_SECRET is not a valid secret key', () => {
+    setValidEnv();
+    process.env.STELLAR_FEE_SPONSOR_SECRET = 'not-a-secret-key';
+    expect(() => validateEnv({ isStaticExport: false })).toThrow(
+      /not a valid Stellar secret key/,
+    );
+  });
+
+  it('warns instead of throwing when strict is false', () => {
+    delete process.env.NEXT_PUBLIC_FACTORY_CONTRACT_ID;
+    delete process.env.STELLAR_FEE_SPONSOR_SECRET;
+    const warnSpy = vi.spyOn(console, 'warn').mockImplementation(() => {});
+    expect(() => validateEnv({ isStaticExport: false, strict: false })).not.toThrow();
+    expect(warnSpy).toHaveBeenCalledWith(
+      expect.stringContaining('NEXT_PUBLIC_FACTORY_CONTRACT_ID is not set'),
+    );
+    warnSpy.mockRestore();
+  });
+
+  it('defaults to strict when the option is omitted', () => {
+    delete process.env.NEXT_PUBLIC_FACTORY_CONTRACT_ID;
+    delete process.env.STELLAR_FEE_SPONSOR_SECRET;
+    expect(() => validateEnv({ isStaticExport: false })).toThrow(
+      /NEXT_PUBLIC_FACTORY_CONTRACT_ID is not set/,
+    );
+  });
+});
diff --git a/src/config/validateEnv.ts b/src/config/validateEnv.ts
new file mode 100644
index 0000000..349e7bc
--- /dev/null
+++ b/src/config/validateEnv.ts
@@ -0,0 +1,61 @@
+import { StrKey } from '@stellar/stellar-sdk';
+
+/**
+ * Fails fast with a clear message instead of letting a missing or malformed
+ * env var fall through to demo-mode data or a runtime 500 (issue #199).
+ * Called from next.config.ts, which Next.js loads before both `next build`
+ * and `next dev`/`next start`.
+ *
+ * `strict: false` downgrades a failure to a console warning instead of
+ * throwing. next.config.ts uses this for Netlify's `deploy-preview` /
+ * `branch-deploy` contexts (Netlify sets `CONTEXT`), which don't necessarily
+ * have production Soroban config — those builds fell into silent demo mode
+ * before this change too, so warn-and-continue there is not a regression,
+ * just a louder version of the previous behavior. Local dev/build and
+ * Netlify's `production` context stay strict.
+ */
+export function validateEnv(options: { isStaticExport: boolean; strict?: boolean }): void {
+  const { strict = true } = options;
+  const problems: string[] = [];
+
+  const factoryContractId = process.env.NEXT_PUBLIC_FACTORY_CONTRACT_ID;
+  if (!factoryContractId) {
+    problems.push('NEXT_PUBLIC_FACTORY_CONTRACT_ID is not set.');
+  } else if (!StrKey.isValidContract(factoryContractId)) {
+    problems.push(
+      `NEXT_PUBLIC_FACTORY_CONTRACT_ID ("${factoryContractId}") is not a valid Soroban contract id.`,
+    );
+  }
+
+  const sorobanRpcUrl = process.env.NEXT_PUBLIC_SOROBAN_RPC_URL;
+  if (sorobanRpcUrl) {
+    try {
+      new URL(sorobanRpcUrl);
+    } catch {
+      problems.push(`NEXT_PUBLIC_SOROBAN_RPC_URL ("${sorobanRpcUrl}") is not a valid URL.`);
+    }
+  }
+
+  // STELLAR_FEE_SPONSOR_SECRET only backs the /api/sign-fee-bump route, which
+  // doesn't exist in static-export builds (see next.config.ts).
+  if (!options.isStaticExport) {
+    const sponsorSecret = process.env.STELLAR_FEE_SPONSOR_SECRET;
+    if (!sponsorSecret) {
+      problems.push(
+        'STELLAR_FEE_SPONSOR_SECRET is not set (required in server mode for /api/sign-fee-bump).',
+      );
+    } else if (!StrKey.isValidEd25519SecretSeed(sponsorSecret)) {
+      problems.push('STELLAR_FEE_SPONSOR_SECRET is set but is not a valid Stellar secret key.');
+    }
+  }
+
+  if (problems.length > 0) {
+    const message =
+      `Invalid environment configuration:\n  - ${problems.join('\n  - ')}\n` +
+      'Set the required variables in .env.local before building or starting the app.';
+    if (strict) {
+      throw new Error(message);
+    }
+    console.warn(`\n⚠️  ${message}\n`);
+  }
+}
diff --git a/src/lib/soroban.ts b/src/lib/soroban.ts
index 0b75857..f0e8378 100644
--- a/src/lib/soroban.ts
+++ b/src/lib/soroban.ts
@@ -4,6 +4,7 @@
  */
 
 import {
+  Account,
   Contract,
   TransactionBuilder,
   BASE_FEE,
@@ -1484,7 +1485,7 @@ export class SorobanService {
   async getPoolHistory(
     poolId: string,
     days: number = 7,
-  ): Promise<{ date: string; tvl: string; truncated?: boolean }> {
+  ): Promise<{ date: string; tvl: string; truncated?: boolean }[]> {
     try {
       const latest = await this.rpcServer.getLatestLedger();
       // ~5 s per ledger; days * 86400 / 5