diff --git a/packages/cli/CHANGELOG.md b/packages/cli/CHANGELOG.md
index e1dfe84..56faba9 100644
--- a/packages/cli/CHANGELOG.md
+++ b/packages/cli/CHANGELOG.md
@@ -7,6 +7,7 @@ All notable changes to the `nirium-cli` package are documented here.
 ### Added
 
 - `nirium anchor` ([#133](https://github.com/nirium-protocol/nirium/issues/133)): anchors a sha-256 hash (`--hash`), a file's sha-256 (`--file`, the file itself is never uploaded), or a small JSON record (`--record`, max 8KB, enforced client-side) to IPFS via an Audit Trail node's `POST /api/audit/log`. Optional `--tx`/`--tag`; no `--network` flag by design - the agent box decides its own network. Prints the CID, content hash, gateway URL, and a `nirium verify` suggestion.
+- `nirium anchor --sign` ([#134](https://github.com/nirium-protocol/nirium/issues/134)): attaches an ed25519 attestation of which key produced the anchor. Signs `nirium-audit-v1:<content_sha256>` locally with a key from `--key-env <VAR>` or the config store's `secretKey` - the secret key itself never leaves this process. A rejected signature gets a `400` and anchors nothing; `nirium verify` reports `SIGNATURE: VALID` on the round trip.
 
 ## 3.0.1 - 2026-10-24
 
diff --git a/packages/cli/README.md b/packages/cli/README.md
index 58aa02a..a30c416 100644
--- a/packages/cli/README.md
+++ b/packages/cli/README.md
@@ -235,6 +235,8 @@ client-supplied network field is rejected server-side anyway.
 | `--record <json>` | anchor this small JSON object verbatim | none |
 | `--tx <hash>` | associate a transaction hash | none |
 | `--tag <label>` | free-form label | none |
+| `--sign` | attach an ed25519 attestation of which key produced this anchor | off |
+| `--key-env <VAR>` | env var holding the secret key to sign with (`--sign` only) | config store's `secretKey` |
 | `-u, --url <url>` | agent URL | `http://localhost:3002` |
 | `--json` | machine-readable result | off |
 
@@ -242,6 +244,23 @@ On success, prints the CID, the content hash, the gateway URL, and a
 `nirium verify <cid>` suggestion so the anchor can be independently
 re-checked right away.
 
+### Attesting who produced an anchor
+
+```bash
+nirium config set secretKey          # hidden prompt, stored in ~/.niriumrc.json
+nirium anchor --hash <64-hex> --sign
+# or, without touching the config store:
+NIRIUM_KEY=S... nirium anchor --hash <64-hex> --sign --key-env NIRIUM_KEY
+```
+
+The CID alone proves the anchored fact wasn't altered; `--sign` proves
+*who* declared it. The secret key signs `nirium-audit-v1:<content_sha256>`
+locally with ed25519 - only the resulting signature and the derived
+public key (`G...`) ever leave this process, never the secret itself. If
+the node rejects the signature, nothing is anchored (`400`, exit non-zero).
+`nirium verify <cid>` independently re-checks it afterward and reports
+`SIGNATURE: VALID` against the same public key.
+
 ## Verify an audit record
 
 ```bash
diff --git a/packages/cli/bin/nirium.js b/packages/cli/bin/nirium.js
index 49d255f..47c829c 100755
--- a/packages/cli/bin/nirium.js
+++ b/packages/cli/bin/nirium.js
@@ -197,6 +197,8 @@ program
     .option('--record <json>', 'Small JSON object (max 8KB) to anchor verbatim, e.g. \'{"order":"A-12"}\'')
     .option('--tx <hash>', 'Transaction hash to associate with this anchor')
     .option('--tag <tag>', 'Free-form label for this anchor')
+    .option('--sign', 'Attach an ed25519 attestation of which key produced this anchor')
+    .option('--key-env <VAR>', 'Environment variable holding the Stellar secret key to sign with (--sign only; default: the config store\'s secretKey)')
     .option('-u, --url <url>', 'Agent URL', 'http://localhost:3002')
     .option('--json', 'Output result as JSON')
     .action(async (options) => {
@@ -1784,10 +1786,49 @@ async function executeAnchorCommand(options) {
 
 // Returns the `agent` object for the request body, `undefined` when
 // `--sign` was not passed, or the string 'failed' when fail() already
-// ran (caller must return without doing anything else — placeholder for
-// #134, which adds `--sign`/`--key-env` on top of this command).
-async function maybeBuildAgentAttestation(_options, _body, _fail) {
-    return undefined;
+// ran (caller must return without doing anything else).
+//
+// Signs locally and never sends the secret key anywhere - only the
+// resulting signature and the derived public key leave this process.
+// The statement signed, `nirium-audit-v1:<content_sha256>`, must match
+// exactly what the server itself computes as contentSha before it
+// verifies: the hash as-is for `--hash`/`--file` (already normalized to
+// lowercase hex at this point), or sha-256 of the record's JSON text for
+// `--record` - never the file's own content, which the server never sees.
+async function maybeBuildAgentAttestation(options, body, fail) {
+    if (!options.sign) return undefined;
+
+    let secretKey;
+    if (options.keyEnv !== undefined) {
+        secretKey = process.env[options.keyEnv];
+        if (!secretKey) {
+            fail(`--key-env ${options.keyEnv} is not set (or is empty) in the environment.`);
+            return 'failed';
+        }
+    } else {
+        secretKey = loadConfig().secretKey;
+        if (!secretKey) {
+            fail('--sign needs a key: run `nirium config set secretKey`, or pass --key-env <VAR> '
+                + 'with the secret key in that environment variable.');
+            return 'failed';
+        }
+    }
+
+    let keypair;
+    try {
+        keypair = Keypair.fromSecret(secretKey);
+    } catch (err) {
+        fail(`--sign: invalid Stellar secret key - ${err?.message || String(err)}`);
+        return 'failed';
+    }
+
+    const contentSha = body.hash
+        ? body.hash
+        : crypto.createHash('sha256').update(JSON.stringify(body.record)).digest('hex');
+    const statement = `nirium-audit-v1:${contentSha}`;
+    const signature = Buffer.from(keypair.sign(Buffer.from(statement, 'utf8'))).toString('base64');
+
+    return { key: keypair.publicKey(), signature };
 }
 
 program.parse();
diff --git a/packages/cli/package.json b/packages/cli/package.json
index 8282a9f..50ceaae 100644
--- a/packages/cli/package.json
+++ b/packages/cli/package.json
@@ -9,7 +9,7 @@
   },
   "scripts": {
     "prepublishOnly": "node --check bin/nirium.js",
-    "test": "node --test test/doctor-seller.test.js test/config-secret.test.js test/serve-api-key.test.js test/verify-hash-only.test.js test/pay-agent-construction.test.js test/pay-status-and-cap.test.js test/status.test.js test/anchor.test.js"
+    "test": "node --test test/doctor-seller.test.js test/config-secret.test.js test/serve-api-key.test.js test/verify-hash-only.test.js test/pay-agent-construction.test.js test/pay-status-and-cap.test.js test/status.test.js test/anchor.test.js test/anchor-sign.test.js"
   },
   "keywords": [
     "stellar",
diff --git a/packages/cli/test/anchor-sign.test.js b/packages/cli/test/anchor-sign.test.js
new file mode 100644
index 0000000..915e2eb
--- /dev/null
+++ b/packages/cli/test/anchor-sign.test.js
@@ -0,0 +1,215 @@
+/**
+ * `nirium anchor --sign` (#134): attaches an agent attestation signed
+ * locally with a throwaway Keypair.random(). The fake "agent" server
+ * here verifies the signature exactly like the real node does (ed25519
+ * over `nirium-audit-v1:<content_sha256>`), so a genuinely bad
+ * signature gets a real 400 - not a stub that always accepts.
+ */
+import assert from 'node:assert/strict';
+import crypto from 'node:crypto';
+import path from 'node:path';
+import { spawn } from 'node:child_process';
+import http from 'node:http';
+import { once } from 'node:events';
+import { fileURLToPath } from 'node:url';
+import test from 'node:test';
+import { Keypair, StrKey } from '@stellar/stellar-sdk';
+
+const cliRoot = path.resolve(path.dirname(fileURLToPath(import.meta.url)), '..');
+const cli = path.join(cliRoot, 'bin/nirium.js');
+
+function verifyAgentSignature(gAddress, statement, signatureB64) {
+    try {
+        const rawPubKey = Buffer.from(StrKey.decodeEd25519PublicKey(gAddress));
+        const spkiHeader = Buffer.from('302a300506032b6570032100', 'hex');
+        const keyObject = crypto.createPublicKey({
+            key: Buffer.concat([spkiHeader, rawPubKey]),
+            format: 'der',
+            type: 'spki',
+        });
+        const sigBuf = Buffer.from(signatureB64, 'base64');
+        if (sigBuf.length !== 64) return false;
+        return crypto.verify(null, Buffer.from(statement, 'utf8'), keyObject, sigBuf);
+    } catch {
+        return false;
+    }
+}
+
+let lastRequest = null;
+let docsByCid = {};
+let cidCounter = 0;
+let forceSignatureReject = false;
+
+const agentServer = http.createServer((req, res) => {
+    let chunks = [];
+    req.on('data', (d) => chunks.push(d));
+    req.on('end', () => {
+        let body;
+        try { body = JSON.parse(Buffer.concat(chunks).toString('utf8') || '{}'); } catch { body = null; }
+        lastRequest = body;
+
+        const contentSha = body?.hash
+            ? String(body.hash).toLowerCase().replace(/^sha-?256:/, '')
+            : crypto.createHash('sha256').update(JSON.stringify(body?.record ?? {})).digest('hex');
+
+        if (body?.agent) {
+            const statement = `nirium-audit-v1:${contentSha}`;
+            const valid = !forceSignatureReject && verifyAgentSignature(body.agent.key, statement, body.agent.signature);
+            if (!valid) {
+                res.writeHead(400, { 'content-type': 'application/json' });
+                res.end(JSON.stringify({ error: 'agent attestation signature is invalid' }));
+                return;
+            }
+        }
+
+        cidCounter += 1;
+        const cid = `QmFakeSignTest${cidCounter}`;
+        const doc = {
+            content_sha256: contentSha,
+            anchoredAt: '2026-11-08T00:00:00.000Z',
+            ...(body?.record !== undefined ? { record: body.record } : {}),
+            ...(body?.agent ? { agent: { key: body.agent.key, signature: body.agent.signature } } : {}),
+        };
+        docsByCid[cid] = doc;
+
+        res.writeHead(201, { 'content-type': 'application/json' });
+        res.end(JSON.stringify({
+            cid,
+            gatewayUrl: `${gatewayBase}/ipfs/${cid}`,
+            content_sha256: contentSha,
+            ...(body?.agent ? { attestedBy: body.agent.key } : {}),
+            anchoredAt: doc.anchoredAt,
+        }));
+    });
+});
+
+const gatewayServer = http.createServer((req, res) => {
+    const cid = decodeURIComponent(req.url.replace(/^\/ipfs\//, ''));
+    const doc = docsByCid[cid];
+    if (!doc) { res.writeHead(404).end(); return; }
+    res.writeHead(200, { 'content-type': 'application/json' }).end(JSON.stringify(doc));
+});
+
+let agentBase;
+let gatewayBase;
+
+test.before(async () => {
+    agentServer.listen(0, '127.0.0.1');
+    await once(agentServer, 'listening');
+    agentBase = `http://127.0.0.1:${agentServer.address().port}`;
+
+    gatewayServer.listen(0, '127.0.0.1');
+    await once(gatewayServer, 'listening');
+    gatewayBase = `http://127.0.0.1:${gatewayServer.address().port}`;
+});
+
+test.after(() => Promise.all([
+    new Promise((resolve, reject) => agentServer.close((err) => (err ? reject(err) : resolve()))),
+    new Promise((resolve, reject) => gatewayServer.close((err) => (err ? reject(err) : resolve()))),
+]));
+
+test.beforeEach(() => { lastRequest = null; docsByCid = {}; forceSignatureReject = false; });
+
+function run(args, env = {}) {
+    return new Promise((resolve, reject) => {
+        const child = spawn(process.execPath, [cli, ...args], {
+            env: { ...process.env, ...env },
+            stdio: ['ignore', 'pipe', 'pipe'],
+        });
+        let stdout = '';
+        let stderr = '';
+        const timer = setTimeout(() => { child.kill('SIGKILL'); reject(new Error('timed out')); }, 10_000);
+        child.stdout.on('data', (d) => { stdout += d; });
+        child.stderr.on('data', (d) => { stderr += d; });
+        child.on('close', (code) => { clearTimeout(timer); resolve({ code, stdout, stderr }); });
+        child.on('error', reject);
+    });
+}
+
+const HASH = 'a'.repeat(64);
+
+test('round trip: --sign with --key-env, then `verify` reports SIGNATURE: VALID', async () => {
+    const kp = Keypair.random();
+    const anchorResult = await run(
+        ['anchor', '--url', agentBase, '--hash', HASH, '--sign', '--key-env', 'TEST_SECRET', '--json'],
+        { TEST_SECRET: kp.secret() },
+    );
+    assert.equal(anchorResult.code, 0, anchorResult.stdout + anchorResult.stderr);
+    const anchored = JSON.parse(anchorResult.stdout);
+    assert.equal(anchored.attestedBy, kp.publicKey());
+    assert.equal(lastRequest.agent.key, kp.publicKey());
+
+    const verifyResult = await run(['verify', anchored.cid, '--gateway', gatewayBase, '--json']);
+    assert.equal(verifyResult.code, 0, verifyResult.stdout + verifyResult.stderr);
+    const verified = JSON.parse(verifyResult.stdout);
+    assert.equal(verified.signatureStatus, 'valid');
+    assert.equal(verified.signerKey, kp.publicKey());
+    assert.equal(verified.ok, true);
+});
+
+test('round trip works with --record too (statement hashes the record JSON, not the hash field)', async () => {
+    const kp = Keypair.random();
+    const anchorResult = await run(
+        ['anchor', '--url', agentBase, '--record', '{"order":"A-12"}', '--sign', '--key-env', 'TEST_SECRET', '--json'],
+        { TEST_SECRET: kp.secret() },
+    );
+    assert.equal(anchorResult.code, 0, anchorResult.stdout + anchorResult.stderr);
+    const anchored = JSON.parse(anchorResult.stdout);
+
+    const verifyResult = await run(['verify', anchored.cid, '--gateway', gatewayBase, '--json']);
+    assert.equal(verifyResult.code, 0, verifyResult.stdout + verifyResult.stderr);
+    const verified = JSON.parse(verifyResult.stdout);
+    assert.equal(verified.signatureStatus, 'valid');
+    assert.equal(verified.mode, 'record');
+    assert.equal(verified.hashMatch, true);
+});
+
+test('a real bad signature (server rejects it) gets 400, nothing anchored', async () => {
+    forceSignatureReject = true;
+    const kp = Keypair.random();
+    const r = await run(
+        ['anchor', '--url', agentBase, '--hash', HASH, '--sign', '--key-env', 'TEST_SECRET', '--json'],
+        { TEST_SECRET: kp.secret() },
+    );
+    assert.equal(r.code, 1);
+    const json = JSON.parse(r.stdout);
+    assert.equal(json.ok, false);
+    assert.match(json.error, /invalid/);
+    assert.equal(Object.keys(docsByCid).length, 0);
+});
+
+test('--sign with a malformed secret key fails locally, never reaches the network', async () => {
+    const r = await run(
+        ['anchor', '--url', agentBase, '--hash', HASH, '--sign', '--key-env', 'TEST_SECRET', '--json'],
+        { TEST_SECRET: 'not-a-real-secret-key' },
+    );
+    assert.equal(r.code, 1);
+    const json = JSON.parse(r.stdout);
+    assert.equal(json.ok, false);
+    assert.match(json.error, /invalid Stellar secret key/i);
+    assert.equal(lastRequest, null);
+});
+
+test('--sign with --key-env pointing at an unset variable fails locally with a clear message', async () => {
+    const r = await run(['anchor', '--url', agentBase, '--hash', HASH, '--sign', '--key-env', 'NIRIUM_TEST_DOES_NOT_EXIST', '--json']);
+    assert.equal(r.code, 1);
+    const json = JSON.parse(r.stdout);
+    assert.equal(json.ok, false);
+    assert.match(json.error, /NIRIUM_TEST_DOES_NOT_EXIST/);
+    assert.equal(lastRequest, null);
+});
+
+test('--sign without --key-env and no secretKey in the config store fails with guidance, never reaches the network', async () => {
+    const r = await run(['anchor', '--url', agentBase, '--hash', HASH, '--sign', '--json'], { HOME: '/nonexistent-home-dir-for-test' });
+    assert.equal(r.code, 1);
+    const json = JSON.parse(r.stdout);
+    assert.equal(json.ok, false);
+    assert.match(json.error, /config set secretKey/);
+    assert.equal(lastRequest, null);
+});
+
+test('without --sign, the request body has no `agent` field at all (no behavior change)', async () => {
+    const r = await run(['anchor', '--url', agentBase, '--hash', HASH, '--json']);
+    assert.equal(r.code, 0, r.stdout + r.stderr);
+    assert.equal(lastRequest.agent, undefined);
+});