diff --git a/mobile_app/app/dev/contract-spike.tsx b/mobile_app/app/dev/contract-spike.tsx
new file mode 100644
index 0000000..ef76e11
--- /dev/null
+++ b/mobile_app/app/dev/contract-spike.tsx
@@ -0,0 +1,101 @@
+import React, { useCallback, useState } from 'react';
+import { ScrollView, StyleSheet, Text } from 'react-native';
+
+import { DepthButton } from '@/components/primitives';
+import { solanaConnection } from '@/src/infrastructure/network/connection';
+import {
+  SPIKE_FIXTURES,
+  SPIKE_PROGRAM_ID,
+  simulateCosignedTransfer,
+  type SpikeSimResult,
+} from '@/src/services/contractSpike';
+import { fontFamily, fontSize, spacing, useTheme } from '@/theme';
+
+/**
+ * SPIKE — proves the app can build a valid anonbeta1 execute_cosigned_transfer
+ * and the deployed devnet program accepts it (simulation, no signatures, no
+ * funds moved). Dev builds only; not reachable from any production surface.
+ */
+export default function ContractSpikeScreen() {
+  const { colors } = useTheme();
+  const [busy, setBusy] = useState(false);
+  const [result, setResult] = useState<SpikeSimResult | null>(null);
+  const [error, setError] = useState<string | null>(null);
+
+  const run = useCallback(async () => {
+    if (busy) return;
+    setBusy(true);
+    setError(null);
+    setResult(null);
+    try {
+      setResult(await simulateCosignedTransfer(solanaConnection));
+    } catch (e) {
+      setError(e instanceof Error ? e.message : String(e));
+    } finally {
+      setBusy(false);
+    }
+  }, [busy]);
+
+  const mono = { fontFamily: fontFamily.mono, fontSize: fontSize.xs };
+
+  return (
+    <ScrollView style={{ backgroundColor: colors.surface0 }} contentContainerStyle={S.content}>
+      <Text style={[S.title, { color: colors.textPrimary }]}>anonbeta1 settlement spike</Text>
+      <Text style={[S.body, { color: colors.textSecondary }]}>
+        Builds an execute_cosigned_transfer against the live devnet program and
+        simulates it (no signatures, nothing moves). The real path co-signs with
+        a beacon operator — proven end-to-end from the workstation harness.
+      </Text>
+
+      <Text style={[S.label, { color: colors.textTertiary }]}>PROGRAM</Text>
+      <Text style={[mono, { color: colors.textPrimary }]}>{SPIKE_PROGRAM_ID.toBase58()}</Text>
+      <Text style={[S.label, { color: colors.textTertiary }]}>BEACON OPERATOR (devnet fixture)</Text>
+      <Text style={[mono, { color: colors.textPrimary }]}>{SPIKE_FIXTURES.operator.toBase58()}</Text>
+
+      <DepthButton
+        label={busy ? 'Simulating…' : 'Build + simulate on devnet'}
+        onPress={run}
+        size="lg"
+        tone="cyan"
+        variant="primary"
+        style={S.button}
+      />
+
+      {error && (
+        <Text style={[S.body, { color: colors.error }]}>{error}</Text>
+      )}
+
+      {result && (
+        <>
+          <Text style={[S.label, { color: result.ok ? colors.primary : colors.error }]}>
+            {result.ok ? 'SIMULATION OK — program accepted the instruction' : `SIMULATION FAILED: ${result.err}`}
+          </Text>
+          <Text style={[S.label, { color: colors.textTertiary }]}>BEACON PDA</Text>
+          <Text style={[mono, { color: colors.textPrimary }]}>{result.beaconPda}</Text>
+          <Text style={[S.label, { color: colors.textTertiary }]}>SETTLEMENT PDA (fresh)</Text>
+          <Text style={[mono, { color: colors.textPrimary }]}>{result.settlementPda}</Text>
+          <Text style={[S.label, { color: colors.textTertiary }]}>PROGRAM LOGS</Text>
+          {result.logs.slice(-12).map((l, i) => (
+            <Text key={`${i}-${l.slice(0, 24)}`} style={[mono, { color: colors.textSecondary }]}>
+              {l}
+            </Text>
+          ))}
+        </>
+      )}
+    </ScrollView>
+  );
+}
+
+const S = StyleSheet.create({
+  content: { padding: spacing[6], gap: spacing[3], paddingBottom: spacing[9] },
+  title: { fontFamily: fontFamily.sansBold, fontSize: fontSize['2xl'] },
+  body: { fontFamily: fontFamily.sans, fontSize: fontSize.sm, lineHeight: 20 },
+  label: {
+    fontFamily: fontFamily.sansSb,
+    fontSize: fontSize.xs,
+    letterSpacing: 1.2,
+    marginTop: spacing[3],
+    textTransform: 'uppercase',
+  },
+  button: { marginTop: spacing[5] },
+});
diff --git a/mobile_app/app/dev/index.tsx b/mobile_app/app/dev/index.tsx
index ac963e9..e60ff2c 100644
--- a/mobile_app/app/dev/index.tsx
+++ b/mobile_app/app/dev/index.tsx
@@ -1,5 +1,5 @@
 import { Feather } from '@expo/vector-icons';
-import { router } from 'expo-router';
+import { type Href, router } from 'expo-router';
 import React, { useState } from 'react';
 import { Pressable, ScrollView, StyleSheet, Text } from 'react-native';
 
@@ -30,6 +30,7 @@ export default function DevIndexScreen() {
 
       <Text style={[S.section, { color: colors.textTertiary }]}>SCREENS</Text>
       <Row colors={colors} icon="image" label="Pigeon loader" onPress={() => router.push('/dev/pigeon-loader')} />
+      <Row colors={colors} icon="cpu" label="Contract spike (anonbeta1)" onPress={() => router.push('/dev/contract-spike' as Href)} />
 
       <Text style={[S.section, { color: colors.textTertiary }]}>FIRST-RUN STATE</Text>
       <Row colors={colors} icon="rotate-ccw" label="Reset tutorial flag" onPress={handleResetTutorial} />
diff --git a/mobile_app/src/services/contractSpike.ts b/mobile_app/src/services/contractSpike.ts
new file mode 100644
index 0000000..d98c9f9
--- /dev/null
+++ b/mobile_app/src/services/contractSpike.ts
@@ -0,0 +1,137 @@
+// SPIKE — anonbeta1 execute_cosigned_transfer from the app (devnet, __DEV__ only).
+//
+// The live program (anon7uu8…, devnet) settles a co-signed SPL transfer:
+// sender + beacon operator both sign; the beacon takes share_bps and the
+// settlement PDA records it. This module proves the APP can construct that
+// transaction correctly — verified against the real chain via simulation
+// (sigVerify off), since the co-sign needs the beacon operator's key, which
+// lives with the beacon, not in this app.
+//
+// Machine-side twin: contract/scripts/cosign-transfer-spike.mjs executed the
+// full co-signed path on devnet (see overnight3 report for the signature).
+// Fixtures below come from that run. NOT wired into the live send path.
+
+import { Buffer } from 'buffer';
+import {
+  Connection,
+  PublicKey,
+  SystemProgram,
+  Transaction,
+  TransactionInstruction,
+} from '@solana/web3.js';
+import { TOKEN_PROGRAM_ID, getAssociatedTokenAddressSync } from '@solana/spl-token';
+
+export const SPIKE_PROGRAM_ID = new PublicKey('anon7uu8UtVoFgS8GCSfw2RqyphJhkN3xEjgPwznYDe');
+
+// anchor discriminator for execute_cosigned_transfer (target/idl/anonbeta1.json)
+const DISC = Buffer.from([77, 199, 16, 225, 200, 193, 184, 138]);
+
+// Devnet fixtures established by cosign-transfer-spike.mjs on 2026-06-10:
+// a binding_verified beacon (Arcium roundtrip) + a throwaway 6-dec mint with
+// funded sender/beacon ATAs. Public keys only.
+export const SPIKE_FIXTURES = {
+  operator: new PublicKey('96pAGQK9Fa4dD17oDH9qDw6n38aNteLEEKKakNgsYUWw'),
+  mint: new PublicKey('2enp3pWDd6eGFn2a9KcnjAypMBdDfHkjaHccK2nymzDy'),
+  fundedSender: new PublicKey('CaQAKBcwf7G5vXeu2RNuNGJafnJ8724Uj4wv9ivfxfQA'),
+} as const;
+
+export function deriveBeaconPda(operator: PublicKey): PublicKey {
+  return PublicKey.findProgramAddressSync(
+    [Buffer.from('beacon'), operator.toBuffer()],
+    SPIKE_PROGRAM_ID,
+  )[0];
+}
+
+export function deriveSettlementPda(sender: PublicKey, settlementId: Uint8Array): PublicKey {
+  return PublicKey.findProgramAddressSync(
+    [Buffer.from('settlement'), sender.toBuffer(), settlementId],
+    SPIKE_PROGRAM_ID,
+  )[0];
+}
+
+export type CosignedTransferParams = Readonly<{
+  sender: PublicKey;
+  operator: PublicKey;
+  mint: PublicKey;
+  recipientTokenAccount: PublicKey;
+  settlementId: Uint8Array; // 32 bytes, non-zero
+  amount: bigint;
+  beaconShareBps: number;
+}>;
+
+export function buildExecuteCosignedTransferIx(p: CosignedTransferParams): TransactionInstruction {
+  if (p.settlementId.length !== 32) throw new Error('settlementId must be 32 bytes');
+  const beacon = deriveBeaconPda(p.operator);
+  const settlement = deriveSettlementPda(p.sender, p.settlementId);
+  const senderAta = getAssociatedTokenAddressSync(p.mint, p.sender);
+  const beaconAta = getAssociatedTokenAddressSync(p.mint, p.operator);
+
+  const amount = Buffer.alloc(8);
+  amount.writeBigUInt64LE(p.amount);
+  const share = Buffer.alloc(2);
+  share.writeUInt16LE(p.beaconShareBps);
+  const data = Buffer.concat([DISC, Buffer.from(p.settlementId), amount, share]);
+
+  return new TransactionInstruction({
+    programId: SPIKE_PROGRAM_ID,
+    data,
+    keys: [
+      { pubkey: p.sender, isSigner: true, isWritable: true },
+      { pubkey: p.operator, isSigner: true, isWritable: false },
+      { pubkey: beacon, isSigner: false, isWritable: true },
+      { pubkey: p.operator, isSigner: false, isWritable: false },
+      { pubkey: settlement, isSigner: false, isWritable: true },
+      { pubkey: p.mint, isSigner: false, isWritable: false },
+      { pubkey: senderAta, isSigner: false, isWritable: true },
+      { pubkey: p.recipientTokenAccount, isSigner: false, isWritable: true },
+      { pubkey: beaconAta, isSigner: false, isWritable: true },
+      { pubkey: TOKEN_PROGRAM_ID, isSigner: false, isWritable: false },
+      { pubkey: SystemProgram.programId, isSigner: false, isWritable: false },
+    ],
+  });
+}
+
+export type SpikeSimResult = Readonly<{
+  ok: boolean;
+  beaconPda: string;
+  settlementPda: string;
+  logs: readonly string[];
+  err: string | null;
+}>;
+
+// Builds a fresh self-pay settlement (sender ATA doubles as the recipient leg —
+// the program leaves recipient ATA ownership to the signed body) and simulates
+// it against devnet without signatures. Proves discriminator, arg encoding,
+// PDA derivation, and account ordering against the deployed program.
+export async function simulateCosignedTransfer(connection: Connection): Promise<SpikeSimResult> {
+  const { operator, mint, fundedSender } = SPIKE_FIXTURES;
+  const settlementId = new Uint8Array(32);
+  // Expo provides getRandomValues via expo-crypto polyfill at app start.
+  globalThis.crypto.getRandomValues(settlementId);
+
+  const ix = buildExecuteCosignedTransferIx({
+    sender: fundedSender,
+    operator,
+    mint,
+    recipientTokenAccount: getAssociatedTokenAddressSync(mint, fundedSender),
+    settlementId,
+    amount: 250_000n,
+    beaconShareBps: 200,
+  });
+
+  const tx = new Transaction().add(ix);
+  tx.feePayer = fundedSender;
+  const { blockhash } = await connection.getLatestBlockhash('confirmed');
+  tx.recentBlockhash = blockhash;
+
+  // Legacy-transaction simulate path runs with sigVerify=false — exactly what
+  // we need: validate the instruction against the live program, no keys.
+  const sim = await connection.simulateTransaction(tx);
+  return {
+    ok: sim.value.err === null,
+    beaconPda: deriveBeaconPda(operator).toBase58(),
+    settlementPda: deriveSettlementPda(fundedSender, settlementId).toBase58(),
+    logs: sim.value.logs ?? [],
+    err: sim.value.err === null ? null : JSON.stringify(sim.value.err),
+  };
+}