diff --git a/fixtures/block-identity-rpc.cjs b/fixtures/block-identity-rpc.cjs
new file mode 100644
index 0000000..0b9a02e
--- /dev/null
+++ b/fixtures/block-identity-rpc.cjs
@@ -0,0 +1,20 @@
+// SPDX-FileCopyrightText: 2026 1932819331hy-debug
+// SPDX-License-Identifier: MIT
+
+const assert = require('node:assert/strict');
+
+// Preloaded only by CLI tests. Replace fetch completely, without opening a
+// socket or allowing any request to fall through to a real RPC provider.
+globalThis.fetch = async (url, init) => {
+  assert.equal(url, 'https://block-identity.invalid');
+  assert.equal(init.method, 'POST');
+  const request = JSON.parse(init.body);
+  assert.equal(request.method, 'eth_getBlockByNumber');
+  assert.equal(request.params[1], false);
+  const tag = request.params[0];
+  let number = tag === 'latest' ? 1000 : Number(BigInt(tag));
+  if (process.env.BLOCKCLOCK_WRONG_END === '1' && number === 300) number = 301;
+  return new Response(JSON.stringify({ jsonrpc: '2.0', id: request.id, result: {
+    number: '0x' + number.toString(16), timestamp: '0x' + (10000 + number).toString(16),
+  } }));
+};
diff --git a/src/rpc.js b/src/rpc.js
index 5bb7b49..790dbf1 100644
--- a/src/rpc.js
+++ b/src/rpc.js
@@ -121,8 +121,15 @@ export async function getBlock(url, blockNumber, opts) {
   if (block === null || block === undefined) {
     throw new BlockNotFoundError(`block ${blockNumber} not found`, { url, method });
   }
+  const number = quantity(block.number, { url, method, what: `block ${blockNumber} number` });
+  // A concrete block request must not silently use another header's timestamp.
+  // Named tags such as 'latest' have no predetermined height to compare.
+  if (typeof tag === 'string' && /^0x[0-9a-fA-F]+$/.test(tag)
+      && BigInt(tag) !== BigInt(number)) {
+    throw new RpcShapeError(`block ${blockNumber}: node returned block ${number}`, { url, method });
+  }
   return {
-    number: quantity(block.number, { url, method, what: `block ${blockNumber} number` }),
+    number,
     timestamp: quantity(block.timestamp, { url, method, what: `block ${blockNumber} timestamp` }),
   };
 }
diff --git a/test/block-identity.test.js b/test/block-identity.test.js
new file mode 100644
index 0000000..b44e0b5
--- /dev/null
+++ b/test/block-identity.test.js
@@ -0,0 +1,107 @@
+// SPDX-FileCopyrightText: 2026 1932819331hy-debug
+// SPDX-License-Identifier: MIT
+
+import test from 'node:test';
+import assert from 'node:assert/strict';
+import { getBlock, measureBlockTime, checkSchedule, RpcShapeError, BlockNotFoundError } from '../src/index.js';
+
+const url = 'https://block-identity.invalid';
+const header = (number, timestamp) => ({
+  number: '0x' + number.toString(16), timestamp: '0x' + timestamp.toString(16),
+});
+
+// Only the HTTP transport is synthetic; the real RPC client decodes the JSON
+// and the public functions parse and use the returned header.
+function nodeFor(resultFor) {
+  let calls = 0;
+  return {
+    get calls() { return calls; },
+    rpc: { retries: 1, fetchImpl: async (requestUrl, init) => {
+      assert.equal(requestUrl, url);
+      assert.equal(init.method, 'POST');
+      const request = JSON.parse(init.body);
+      assert.equal(request.method, 'eth_getBlockByNumber');
+      assert.equal(request.params[1], false);
+      calls++;
+      return new Response(JSON.stringify({
+        jsonrpc: '2.0', id: request.id, result: resultFor(request.params[0]),
+      }));
+    } },
+  };
+}
+
+function isShapeError(error) {
+  assert.ok(error instanceof RpcShapeError, `${error.name}: ${error.message}`);
+  assert.equal(error.url, url);
+  assert.equal(error.method, 'eth_getBlockByNumber');
+  return true;
+}
+
+for (const [requested, returned] of [[100, 101], ['0x64', 101], [0, 1]]) {
+  test(`getBlock rejects block ${returned} when ${requested} was requested`, async () => {
+    const node = nodeFor(() => header(returned, 10000));
+    await assert.rejects(() => getBlock(url, requested, node.rpc), isShapeError);
+    assert.equal(node.calls, 1, 'a mismatched header must not be retried');
+  });
+}
+
+for (const [requested, returned] of [
+  [0, 0], ['0x0', 0], [100, 100], ['0xAB', 171],
+  ['earliest', 0], ['latest', 171], ['safe', 171], ['finalized', 171],
+  [Number.MAX_SAFE_INTEGER, Number.MAX_SAFE_INTEGER],
+]) {
+  test(`getBlock preserves a valid ${requested} response`, async () => {
+    const node = nodeFor(() => header(returned, 10000));
+    assert.deepEqual(await getBlock(url, requested, node.rpc), { number: returned, timestamp: 10000 });
+    assert.equal(node.calls, 1);
+  });
+}
+
+test('a missing numbered block remains BlockNotFoundError', async () => {
+  const node = nodeFor(() => null);
+  await assert.rejects(() => getBlock(url, 100, node.rpc), BlockNotFoundError);
+  assert.equal(node.calls, 1);
+});
+
+// A correct fixture has one-second blocks and head 1000. Wrong replies remain
+// monotonic, so monotonicity alone cannot catch selecting the wrong header.
+function chain(wrongTag) {
+  return nodeFor((tag) => {
+    const n = tag === 'latest' ? 1000 : Number(BigInt(tag));
+    const returned = tag === wrongTag ? n + 1 : n;
+    return header(returned, 10000 + returned);
+  });
+}
+
+test('measureBlockTime rejects a different block at the sample boundary', async () => {
+  const node = chain('0x384'); // asked for 900, received 901
+  await assert.rejects(() => measureBlockTime(url, {
+    sampleBlocks: 100, rpc: node.rpc,
+  }), isShapeError);
+});
+
+const schedule = {
+  rpcUrl: url, startBlock: 100, periodBlocks: 100, periods: 2,
+  designSecondsPerBlock: 1, sampleBlocks: 100,
+};
+
+for (const [role, tag] of [['start', '0x64'], ['end', '0x12c']]) {
+  test(`checkSchedule rejects a different ${role} block instead of reporting its timestamp`, async () => {
+    const node = chain(tag);
+    await assert.rejects(() => checkSchedule({ ...schedule, rpc: node.rpc }), isShapeError);
+  });
+}
+
+test('checkSchedule still reports the actual finish for matching headers', async () => {
+  const node = chain();
+  const report = await checkSchedule({ ...schedule, rpc: node.rpc });
+  assert.equal(report.startTimestamp, 10100);
+  assert.equal(report.currentBlock, 1000);
+  assert.equal(report.observedSecondsPerBlock, 1);
+  assert.equal(report.sample.fromBlock, 900);
+  assert.equal(report.sample.toBlock, 1000);
+  assert.equal(report.projectedEnd, 10300);
+  assert.equal(report.complete, true);
+  assert.equal(report.projectedEndIsEstimate, false);
+  assert.equal(report.driftPercent, 0);
+});
diff --git a/test/cli-block-identity.test.js b/test/cli-block-identity.test.js
new file mode 100644
index 0000000..2f51f11
--- /dev/null
+++ b/test/cli-block-identity.test.js
@@ -0,0 +1,53 @@
+// SPDX-FileCopyrightText: 2026 1932819331hy-debug
+// SPDX-License-Identifier: MIT
+
+import test from 'node:test';
+import assert from 'node:assert/strict';
+import { spawnSync } from 'node:child_process';
+import { fileURLToPath } from 'node:url';
+
+const cli = fileURLToPath(new URL('../bin/blockclock.js', import.meta.url));
+const fixture = fileURLToPath(new URL('../fixtures/block-identity-rpc.cjs', import.meta.url));
+
+function run({ wrongEnd, json }) {
+  const result = spawnSync(process.execPath, ['--require', fixture, cli,
+    '--rpc', 'https://block-identity.invalid', '--start-block', '100',
+    '--period-blocks', '100', '--periods', '2', '--design-block-time', '1',
+    '--sample-blocks', '100', ...(json ? ['--json'] : [])], {
+    encoding: 'utf8', timeout: 10000,
+    // Keep the subprocess's only preload deterministic. The fixture itself
+    // supplies all responses and never delegates to the network.
+    env: { ...process.env, NODE_OPTIONS: '', BLOCKCLOCK_WRONG_END: wrongEnd ? '1' : '0' },
+  });
+  assert.ifError(result.error);
+  assert.equal(result.signal, null);
+  return result;
+}
+
+for (const json of [true, false]) {
+  const mode = json ? 'JSON' : 'text';
+  test(`${mode} CLI rejects the wrong end header without printing a success report`, () => {
+    const result = run({ wrongEnd: true, json });
+    assert.equal(result.status, 2, result.stderr);
+    assert.equal(result.stdout, '');
+    assert.match(result.stderr, /block 300: node returned block 301/);
+  });
+
+  test(`${mode} CLI still reports success for matching block headers`, () => {
+    const result = run({ wrongEnd: false, json });
+    assert.equal(result.status, 0, result.stderr);
+    assert.equal(result.stderr, '');
+    if (json) {
+      const report = JSON.parse(result.stdout);
+      assert.equal(report.endBlock, 300);
+      assert.equal(report.projectedEnd, 10300);
+      assert.equal(report.projectedEndIsEstimate, false);
+      assert.equal(report.withinTolerance, true);
+    } else {
+      assert.match(result.stdout, /Ends\s+block 300\s+\(already passed\)/);
+      assert.match(result.stdout, /actual\s+1970-01-01/);
+      assert.match(result.stdout, /OK\s+drift of 0\.0%/);
+      assert.doesNotMatch(result.stdout, /estimated/);
+    }
+  });
+}