diff --git a/test/usage.test.js b/test/usage.test.js
index 4b2e4c9..c11b48a 100644
--- a/test/usage.test.js
+++ b/test/usage.test.js
@@ -95,3 +95,32 @@ test('usage log: writes are batched (one read and one write for several calls),
   assert.equal(file.slice(old.length).trim().split('\n').length, 2);
   assert.deepEqual(calls, [`GET /repos/Fizzl13/usage-log/contents/${path}`, `PUT /repos/Fizzl13/usage-log/contents/${path}`]);
 });
+
+test('usage log: the next write to the same day file skips the read; a file changed elsewhere is read again', async () => {
+  const crypto = require('node:crypto');
+  const { createUsageLog } = require('../usage-log');
+  const shaOf = (text) => { const b = Buffer.from(text); return crypto.createHash('sha1').update(`blob ${b.length}\0`).update(b).digest('hex'); };
+  let file = null;
+  const calls = [];
+  // Like GitHub: a write must name the sha of the file it replaces.
+  const fetchFn = async (url, opts = {}) => {
+    const method = opts.method || 'GET';
+    calls.push(method);
+    if (method === 'GET') return file === null ? new Response('{}', { status: 404 }) : new Response(file);
+    const body = JSON.parse(opts.body);
+    if (file !== null && body.sha !== shaOf(file)) return new Response('{}', { status: 409 });
+    file = Buffer.from(body.content, 'base64').toString('utf8');
+    return Response.json({}, { status: 201 });
+  };
+  const log = createUsageLog({ service: 'plaintext', env: { USAGE_LOG_TOKEN: 't' }, fetchFn, now: () => new Date('2026-10-14T10:00:00Z'), log: { warn() {}, error() {} } });
+  log.record({ route: 'check-wallet', status: 402 });
+  await log.flush();
+  log.record({ route: 'explain', status: 200 });
+  await log.flush();
+  assert.deepEqual(calls, ['GET', 'PUT', 'PUT'], 'one read, then writes only');
+  file += '{"t":"2026-10-14T10:00:01.000Z","service":"plaintext","route":"other"}\n'; // another writer appended
+  log.record({ route: 'explain', status: 402 });
+  await log.flush();
+  assert.deepEqual(calls.slice(3), ['PUT', 'GET', 'PUT']);
+  assert.deepEqual(file.trim().split('\n').map((l) => JSON.parse(l).route), ['check-wallet', 'explain', 'other', 'explain'], 'no line lost');
+});
diff --git a/usage-log.js b/usage-log.js
index 0e6ed33..a7e56b8 100644
--- a/usage-log.js
+++ b/usage-log.js
@@ -170,29 +170,43 @@ function createUsageLog({ service, env = process.env, fetchFn = globalThis.fetch
     'user-agent': `usage-log/${service}`,
   };
 
+  // The day file as this service last wrote it. Only this service writes its own files, so the next append to
+  // the same file skips the read (half the bandwidth of a write); if the file did change (a deploy's old and new
+  // instance both writing), GitHub refuses the stale sha with a 409 and the file is read again.
+  let last = null; // { path, sha, text }
+
   // Appends lines to one file: read (for the sha), write, retry on a race.
   async function append(path, lines) {
     for (let attempt = 0; attempt < 4; attempt++) {
       const url = `${API}/repos/${repo}/contents/${path}`;
-      // The raw file: the JSON form leaves out the content of files over 1 MB, which would then be overwritten
-      // with only the new lines. The sha GitHub wants back is the git blob sha of exactly these bytes.
-      const current = await fetchFn(url, { headers: { ...headers, accept: 'application/vnd.github.raw+json' } });
       let sha;
       let text = '';
-      if (current.status === 200) {
-        const raw = Buffer.from(await current.arrayBuffer());
-        sha = blobSha(raw);
-        text = raw.toString('utf8');
-      } else if (current.status !== 404) {
-        throw new Error(`read ${path}: HTTP ${current.status}`);
+      if (attempt === 0 && last && last.path === path) {
+        ({ sha, text } = last);
+      } else {
+        // The raw file: the JSON form leaves out the content of files over 1 MB, which would then be overwritten
+        // with only the new lines. The sha GitHub wants back is the git blob sha of exactly these bytes.
+        const current = await fetchFn(url, { headers: { ...headers, accept: 'application/vnd.github.raw+json' } });
+        if (current.status === 200) {
+          const raw = Buffer.from(await current.arrayBuffer());
+          sha = blobSha(raw);
+          text = raw.toString('utf8');
+        } else if (current.status !== 404) {
+          throw new Error(`read ${path}: HTTP ${current.status}`);
+        }
       }
+      const content = Buffer.from(text + lines.join('\n') + '\n');
       const body = {
         message: `${service}: ${lines.length} call${lines.length === 1 ? '' : 's'}`,
-        content: Buffer.from(text + lines.join('\n') + '\n').toString('base64'),
+        content: content.toString('base64'),
         ...(sha ? { sha } : {}),
       };
       const put = await fetchFn(url, { method: 'PUT', headers: { ...headers, 'content-type': 'application/json' }, body: JSON.stringify(body) });
-      if (put.status === 200 || put.status === 201) return;
+      if (put.status === 200 || put.status === 201) {
+        last = { path, sha: blobSha(content), text: content.toString('utf8') };
+        return;
+      }
+      last = null;
       if (put.status !== 409 && put.status !== 422) throw new Error(`write ${path}: HTTP ${put.status}`);
       await new Promise((r) => setTimeout(r, 250 * (attempt + 1)));
     }