diff --git a/README.md b/README.md
index 818ca85..78c3c83 100644
--- a/README.md
+++ b/README.md
@@ -69,6 +69,7 @@ The current source includes:
 - [`docs/release-evidence-index.md`](docs/release-evidence-index.md) — claim-to-evidence map for reviewers, contributors, auditors, partners, and funders
 - [`docs/grant-reviewer-guide.md`](docs/grant-reviewer-guide.md) — concise reviewer routes, safety boundaries, and known limitations
 - [`docs/reviewer-publication-runbook.md`](docs/reviewer-publication-runbook.md) — exact-source manifest and owner-gated publication procedure
+- [`docs/base-builder-attribution.md`](docs/base-builder-attribution.md) — owner-gated Base.dev registration, ERC-8021 configuration and verification
 - [`GOVERNANCE.md`](GOVERNANCE.md) — current decision process, roles, conflicts, funding independence, and succession direction
 - [`MAINTAINERS.md`](MAINTAINERS.md) — current maintainers and the path to shared stewardship
 - [`SECURITY.md`](SECURITY.md) — private vulnerability reporting and safe research boundaries
diff --git a/docs/base-builder-attribution.md b/docs/base-builder-attribution.md
new file mode 100644
index 0000000..de4188a
--- /dev/null
+++ b/docs/base-builder-attribution.md
@@ -0,0 +1,55 @@
+# Base Builder Code attribution
+
+OpenEscrow can append an ERC-8021 attribution suffix to user-initiated Base transactions after the
+owner registers and verifies the app on Base.dev. Attribution is disabled by default and no Builder
+Code is hardcoded in source.
+
+## Configuration
+
+1. Register and verify `https://openescrow.io` on <https://base.dev>.
+2. Review Base's current terms, select the real payout address and copy the Builder Code from
+   **Settings → Builder Code**.
+3. Set the public build-time environment variable:
+
+   ```dotenv
+   VITE_BASE_BUILDER_CODE=bc_owner_issued_value
+   ```
+
+4. Build and deploy through the normal reviewed release process.
+
+The value is a public attribution identifier, not a private key or secret. It must still come from
+the owner-controlled Base.dev project and must not be guessed or fabricated.
+
+## Transaction coverage
+
+- Direct Privy transactions receive the suffix through Privy's `dataSuffix` plugin.
+- Wagmi contract writes receive the same suffix through viem's per-transaction `dataSuffix`
+  parameter. This explicit path is required because Privy's plugin is not currently applied to
+  transactions sent through the `@privy-io/wagmi` adapter.
+- When the environment variable is blank, both paths behave exactly as before and append nothing.
+
+No contract upgrade or redeployment is required. ERC-8021 data is appended to calldata and ignored
+by the called contract while Base's offchain indexer records the app attribution.
+
+## Verification
+
+After deployment, submit one authentic Base Sepolia lifecycle transaction—never synthetic volume
+for rewards—and preserve its transaction hash. Verify it in both places:
+
+1. Base's Builder Code validation tool confirms the expected code and ERC-8021 marker.
+2. Base.dev shows the attributed transaction in OpenEscrow's onchain analytics after indexing.
+
+The testnet/mainnet distinction remains explicit. Attribution evidence is not proof of production
+adoption, a grant, or a reward, and no reward should be recorded until an attributable payment is
+received.
+
+## Local checks
+
+```powershell
+cd frontend
+npm run test:client-logic
+npm run build
+```
+
+The client-logic suite verifies that an unset or whitespace-only value disables attribution and
+that an owner-supplied code is trimmed and encoded with the official `ox/erc8021` utility.
diff --git a/frontend/.env.example b/frontend/.env.example
index b2ef68c..283b984 100644
--- a/frontend/.env.example
+++ b/frontend/.env.example
@@ -2,6 +2,11 @@
 # Leave unset to retain OpenEscrow's direct injected-wallet fallback.
 VITE_PRIVY_APP_ID=
 
+# Optional Base.dev Builder Code (for example, bc_...). When set, OpenEscrow
+# appends an ERC-8021 attribution suffix to user-initiated Base transactions.
+# Obtain the real code by verifying the app on base.dev; never invent one.
+VITE_BASE_BUILDER_CODE=
+
 # Real card/bank funding must target a supported mainnet stablecoin. Keep this
 # disabled for Base Sepolia, where the free faucet remains the funding method.
 VITE_FIAT_ONRAMP_ENABLED=false
diff --git a/frontend/package-lock.json b/frontend/package-lock.json
index d2197b1..e256278 100644
--- a/frontend/package-lock.json
+++ b/frontend/package-lock.json
@@ -11,6 +11,7 @@
         "@privy-io/react-auth": "^3.35.2",
         "@privy-io/wagmi": "^4.0.15",
         "@tanstack/react-query": "^5.101.4",
+        "ox": "1.6.2",
         "react": "^19.2.7",
         "react-dom": "^19.2.7",
         "viem": "2.55.5",
@@ -61,6 +62,47 @@
         "zustand": "5.0.3"
       }
     },
+    "node_modules/@base-org/account/node_modules/ox": {
+      "version": "0.6.9",
+      "resolved": "https://registry.npmjs.org/ox/-/ox-0.6.9.tgz",
+      "integrity": "sha512-wi5ShvzE4eOcTwQVsIPdFr+8ycyX+5le/96iAJutaZAvCes1J0+RvpEPg5QDPDiaR0XQQAvZVl7AwqQcINuUug==",
+      "funding": [
+        {
+          "type": "github",
+          "url": "https://github.com/sponsors/wevm"
+        }
+      ],
+      "license": "MIT",
+      "dependencies": {
+        "@adraffy/ens-normalize": "^1.10.1",
+        "@noble/curves": "^1.6.0",
+        "@noble/hashes": "^1.5.0",
+        "@scure/bip32": "^1.5.0",
+        "@scure/bip39": "^1.4.0",
+        "abitype": "^1.0.6",
+        "eventemitter3": "5.0.1"
+      },
+      "peerDependencies": {
+        "typescript": ">=5.4.0"
+      },
+      "peerDependenciesMeta": {
+        "typescript": {
+          "optional": true
+        }
+      }
+    },
+    "node_modules/@base-org/account/node_modules/ox/node_modules/@noble/hashes": {
+      "version": "1.8.0",
+      "resolved": "https://registry.npmjs.org/@noble/hashes/-/hashes-1.8.0.tgz",
+      "integrity": "sha512-jCs9ldd7NwzpgXDIf6P3+NrHh9/sD6CQdxHyjQI+h/6rDNo88ypBxxz45UDuZHz9r3tNz7N/VInSVoVdtXEI4A==",
+      "license": "MIT",
+      "engines": {
+        "node": "^14.21.3 || >=16"
+      },
+      "funding": {
+        "url": "https://paulmillr.com/funding/"
+      }
+    },
     "node_modules/@base-org/account/node_modules/zustand": {
       "version": "5.0.3",
       "resolved": "https://registry.npmjs.org/zustand/-/zustand-5.0.3.tgz",
@@ -2183,6 +2225,62 @@
         "url": "https://paulmillr.com/funding/"
       }
     },
+    "node_modules/@noble/post-quantum": {
+      "version": "0.6.1",
+      "resolved": "https://registry.npmjs.org/@noble/post-quantum/-/post-quantum-0.6.1.tgz",
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+      "license": "MIT",
+      "dependencies": {
+        "@noble/ciphers": "~2.2.0",
+        "@noble/curves": "~2.2.0",
+        "@noble/hashes": "~2.2.0"
+      },
+      "engines": {
+        "node": ">= 20.19.0"
+      },
+      "funding": {
+        "url": "https://paulmillr.com/funding/"
+      }
+    },
+    "node_modules/@noble/post-quantum/node_modules/@noble/ciphers": {
+      "version": "2.2.0",
+      "resolved": "https://registry.npmjs.org/@noble/ciphers/-/ciphers-2.2.0.tgz",
+      "integrity": "sha512-Z6pjIZ/8IJcCGzb2S/0Px5J81yij85xASuk1teLNeg75bfT07MV3a/O2Mtn1I2se43k3lkVEcFaR10N4cgQcZA==",
+      "license": "MIT",
+      "engines": {
+        "node": ">= 20.19.0"
+      },
+      "funding": {
+        "url": "https://paulmillr.com/funding/"
+      }
+    },
+    "node_modules/@noble/post-quantum/node_modules/@noble/curves": {
+      "version": "2.2.0",
+      "resolved": "https://registry.npmjs.org/@noble/curves/-/curves-2.2.0.tgz",
+      "integrity": "sha512-T/BoHgFXirb0ENSPBquzX0rcjXeM6Lo892a2jlYJkqk83LqZx0l1Of7DzlKJ6jkpvMrkHSnAcgb5JegL8SeIkQ==",
+      "license": "MIT",
+      "dependencies": {
+        "@noble/hashes": "2.2.0"
+      },
+      "engines": {
+        "node": ">= 20.19.0"
+      },
+      "funding": {
+        "url": "https://paulmillr.com/funding/"
+      }
+    },
+    "node_modules/@noble/post-quantum/node_modules/@noble/hashes": {
+      "version": "2.2.0",
+      "resolved": "https://registry.npmjs.org/@noble/hashes/-/hashes-2.2.0.tgz",
+      "integrity": "sha512-IYqDGiTXab6FniAgnSdZwgWbomxpy9FtYvLKs7wCUs2a8RkITG+DFGO1DM9cr+E3/RgADRpFjrKVaJ1z6sjtEg==",
+      "license": "MIT",
+      "engines": {
+        "node": ">= 20.19.0"
+      },
+      "funding": {
+        "url": "https://paulmillr.com/funding/"
+      }
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     "node_modules/@oxc-project/types": {
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       "license": "MIT"
     },
+    "node_modules/@wagmi/connectors/node_modules/ox": {
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+      "resolved": "https://registry.npmjs.org/ox/-/ox-0.6.9.tgz",
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+      "funding": [
+        {
+          "type": "github",
+          "url": "https://github.com/sponsors/wevm"
+        }
+      ],
+      "license": "MIT",
+      "dependencies": {
+        "@adraffy/ens-normalize": "^1.10.1",
+        "@noble/curves": "^1.6.0",
+        "@noble/hashes": "^1.5.0",
+        "@scure/bip32": "^1.5.0",
+        "@scure/bip39": "^1.4.0",
+        "abitype": "^1.0.6",
+        "eventemitter3": "5.0.1"
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+      "peerDependencies": {
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+      "license": "MIT",
+      "engines": {
+        "node": "^14.21.3 || >=16"
+      },
+      "funding": {
+        "url": "https://paulmillr.com/funding/"
+      }
+    },
     "node_modules/@wagmi/connectors/node_modules/pino": {
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@@ -9674,13 +9802,16 @@
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-        "@scure/bip39": "^1.4.0",
-        "abitype": "^1.0.6",
-        "eventemitter3": "5.0.1"
+        "@adraffy/ens-normalize": "^1.11.0",
+        "@noble/ciphers": "^2.2.0",
+        "@noble/curves": "^2.2.0",
+        "@noble/hashes": "^2.2.0",
+        "@noble/post-quantum": "^0.6.1",
+        "@scure/bip32": "^2.0.0",
+        "@scure/bip39": "^2.0.0",
+        "abitype": "^1.3.0",
+        "eventemitter3": "5.0.1",
+        "zod": "^4.4.3"
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@@ -9691,18 +9822,89 @@
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+      "engines": {
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+      "funding": {
+        "url": "https://paulmillr.com/funding/"
+      }
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+      "funding": {
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+      "funding": {
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-        "eventemitter3": "^5.0.1",
-        "preact": "^10.24.2",
-        "viem": "^2.27.2"
-      }
-    },
     "node_modules/wagmi/node_modules/@wagmi/connectors": {
       "version": "8.0.24",
       "resolved": "https://registry.npmjs.org/@wagmi/connectors/-/connectors-8.0.24.tgz",
diff --git a/frontend/package.json b/frontend/package.json
index 372a075..4251337 100644
--- a/frontend/package.json
+++ b/frontend/package.json
@@ -77,6 +77,7 @@
     "@privy-io/react-auth": "^3.35.2",
     "@privy-io/wagmi": "^4.0.15",
     "@tanstack/react-query": "^5.101.4",
+    "ox": "1.6.2",
     "react": "^19.2.7",
     "react-dom": "^19.2.7",
     "viem": "2.55.5",
diff --git a/frontend/src/AppProviders.tsx b/frontend/src/AppProviders.tsx
index 0c587c3..21e05a1 100644
--- a/frontend/src/AppProviders.tsx
+++ b/frontend/src/AppProviders.tsx
@@ -1,7 +1,8 @@
 import type { ReactNode } from "react";
-import { PrivyProvider } from "@privy-io/react-auth";
+import { PrivyProvider, dataSuffix } from "@privy-io/react-auth";
 import { baseSepolia } from "wagmi/chains";
 import { PRIVY_APP_ID } from "./lib/accountConfig";
+import { BASE_BUILDER_DATA_SUFFIX } from "./lib/baseBuilderConfig";
 
 export function AppProviders({ children }: { children: ReactNode }) {
   return (
@@ -30,6 +31,9 @@ export function AppProviders({ children }: { children: ReactNode }) {
         },
         defaultChain: baseSepolia,
         supportedChains: [baseSepolia],
+        plugins: BASE_BUILDER_DATA_SUFFIX
+          ? [dataSuffix(BASE_BUILDER_DATA_SUFFIX)]
+          : [],
       }}
     >
       {children}
diff --git a/frontend/src/components/CreateAgreementForm.tsx b/frontend/src/components/CreateAgreementForm.tsx
index a0c67a5..14e967d 100644
--- a/frontend/src/components/CreateAgreementForm.tsx
+++ b/frontend/src/components/CreateAgreementForm.tsx
@@ -25,6 +25,7 @@ import {
   YIELD_USDC_ADDRESS,
   chain,
 } from "../contracts/config";
+import { BASE_BUILDER_DATA_SUFFIX } from "../lib/baseBuilderConfig";
 import { formatUSDC, parseUSDC } from "../lib/format";
 import {
   CALIFORNIA_POLICY,
@@ -1842,6 +1843,7 @@ function AgreementForm({
       functionName: "createMultiTenantAgreementWithToken",
       account: address,
       chain,
+      dataSuffix: BASE_BUILDER_DATA_SUFFIX,
       args: [
         tenantWallets,
         draft.tenants.map((tenant) => tenant.depositShareBps),
diff --git a/frontend/src/components/TxButton.tsx b/frontend/src/components/TxButton.tsx
index 64608ba..591e1f9 100644
--- a/frontend/src/components/TxButton.tsx
+++ b/frontend/src/components/TxButton.tsx
@@ -2,6 +2,7 @@ import { useEffect, useMemo, useRef, useState } from "react";
 import { useAccount, useWaitForTransactionReceipt, useWriteContract } from "wagmi";
 import type { Abi } from "viem";
 import { chain } from "../contracts/config";
+import { BASE_BUILDER_DATA_SUFFIX } from "../lib/baseBuilderConfig";
 import { createSubmittedCallbackSlot } from "../lib/submittedCallback";
 import { transactionTerminalState } from "../lib/transactionTerminalState";
 
@@ -91,7 +92,15 @@ export function TxButton({
             onSubmit?.();
             setSubmitted(true);
             reset();
-            writeContract({ address, abi, functionName, args, account, chain });
+            writeContract({
+              address,
+              abi,
+              functionName,
+              args,
+              account,
+              chain,
+              dataSuffix: BASE_BUILDER_DATA_SUFFIX,
+            });
           } catch (cause) {
             submittedSuccessCallback.clear();
             setSubmitted(false);
diff --git a/frontend/src/env.d.ts b/frontend/src/env.d.ts
index 40a3a90..b8234b3 100644
--- a/frontend/src/env.d.ts
+++ b/frontend/src/env.d.ts
@@ -1,6 +1,7 @@
 /// <reference types="vite/client" />
 
 interface ImportMetaEnv {
+  readonly VITE_BASE_BUILDER_CODE?: string;
   readonly VITE_PRIVY_APP_ID?: string;
   readonly VITE_FIAT_ONRAMP_ENABLED?: string;
   readonly VITE_FIAT_ONRAMP_ENVIRONMENT?: "sandbox" | "production";
diff --git a/frontend/src/lib/baseBuilderAttribution.test.ts b/frontend/src/lib/baseBuilderAttribution.test.ts
new file mode 100644
index 0000000..51079a0
--- /dev/null
+++ b/frontend/src/lib/baseBuilderAttribution.test.ts
@@ -0,0 +1,21 @@
+import assert from "node:assert/strict";
+import test from "node:test";
+import { Attribution } from "ox/erc8021";
+import { createBaseBuilderDataSuffix } from "./baseBuilderAttribution.ts";
+
+test("builder attribution stays disabled without an owner-issued code", () => {
+  assert.equal(createBaseBuilderDataSuffix(), undefined);
+  assert.equal(createBaseBuilderDataSuffix("   "), undefined);
+});
+
+test("builder attribution trims and encodes the supplied Base.dev code", () => {
+  const expected = Attribution.toDataSuffix({ codes: ["bc_openescrow"] });
+  assert.equal(createBaseBuilderDataSuffix("  bc_openescrow  "), expected);
+});
+
+test("builder attribution fails closed when the code cannot fit the schema", () => {
+  assert.throws(
+    () => createBaseBuilderDataSuffix("x".repeat(256)),
+    /could not be encoded as an ERC-8021 Builder Code/,
+  );
+});
diff --git a/frontend/src/lib/baseBuilderAttribution.ts b/frontend/src/lib/baseBuilderAttribution.ts
new file mode 100644
index 0000000..2786e67
--- /dev/null
+++ b/frontend/src/lib/baseBuilderAttribution.ts
@@ -0,0 +1,19 @@
+import { Attribution } from "ox/erc8021";
+import type { Hex } from "viem";
+
+/** Build the ERC-8021 suffix for one owner-issued Base.dev Builder Code. */
+export function createBaseBuilderDataSuffix(
+  rawBuilderCode?: string,
+): Hex | undefined {
+  const builderCode = rawBuilderCode?.trim();
+  if (!builderCode) return undefined;
+
+  try {
+    return Attribution.toDataSuffix({ codes: [builderCode] });
+  } catch (cause) {
+    throw new Error(
+      "VITE_BASE_BUILDER_CODE could not be encoded as an ERC-8021 Builder Code.",
+      { cause },
+    );
+  }
+}
diff --git a/frontend/src/lib/baseBuilderConfig.ts b/frontend/src/lib/baseBuilderConfig.ts
new file mode 100644
index 0000000..28b8601
--- /dev/null
+++ b/frontend/src/lib/baseBuilderConfig.ts
@@ -0,0 +1,8 @@
+import { createBaseBuilderDataSuffix } from "./baseBuilderAttribution";
+
+export const BASE_BUILDER_CODE =
+  import.meta.env.VITE_BASE_BUILDER_CODE?.trim() ?? "";
+
+export const BASE_BUILDER_DATA_SUFFIX = createBaseBuilderDataSuffix(
+  BASE_BUILDER_CODE,
+);
diff --git a/frontend/src/testing/privyReactAuthMock.tsx b/frontend/src/testing/privyReactAuthMock.tsx
index 9abc2e5..e0a42a0 100644
--- a/frontend/src/testing/privyReactAuthMock.tsx
+++ b/frontend/src/testing/privyReactAuthMock.tsx
@@ -242,3 +242,7 @@ export function useCreateWallet() {
   const { createWallet } = useMockPrivyContext();
   return { createWallet };
 }
+
+export function dataSuffix(value: `0x${string}`) {
+  return { name: "dataSuffix", value };
+}