diff --git a/docs/hardening-backlog.md b/docs/hardening-backlog.md
index c3cf7a9..7cd26ef 100644
--- a/docs/hardening-backlog.md
+++ b/docs/hardening-backlog.md
@@ -50,7 +50,7 @@ All items below survived an independent skeptic re-reading the cited code.
 | 20 | 🟡 medium | trivial | resource | `cas-unbounded-readall-error-body` | ✅ done (#87) |
 | 21 | 🟡 medium | small | security | `ban-reset-on-disconnect` | ✅ done (#93) |
 | 22 | ⚪ low | trivial | security | `keystore-open-no-perm-check` | ✅ done (#97) |
-| 23 | ⚪ low | trivial | operational | `buildpsbt-missing-mainnet-gate` | open |
+| 23 | ⚪ low | trivial | operational | `buildpsbt-missing-mainnet-gate` | ✅ done (#109) |
 | 24 | ⚪ low | small | correctness | `scan-cid-shape-only-no-multihash-validation` | open |
 | 25 | ⚪ low | trivial | operational | `config-command-hides-trust-anchors` | ✅ done (#107) |
 | 26 | ⚪ low | trivial | correctness | `scan-zero-and-overcount-consume-block-cap-slot` | open |
diff --git a/internal/publish/psbt.go b/internal/publish/psbt.go
index d8bd298..6f2a15c 100644
--- a/internal/publish/psbt.go
+++ b/internal/publish/psbt.go
@@ -64,6 +64,12 @@ func (w *Writer) BuildPSBT(ctx context.Context, ops []anchor.Op, utxos []btctx.U
 	if w.cfg.Params == nil {
 		return nil, fmt.Errorf("publish: nil chain params")
 	}
+	// Same mainnet gate as Submit: building a PSBT pins the DID files to public IPFS
+	// and funds a real-BTC spend (the out-of-band signer then completes it), so it
+	// must not proceed on mainnet without AllowMainnet. Checked BEFORE the IPFS pin.
+	if w.cfg.Params.Name == chaincfg.MainNetParams.Name && !w.cfg.AllowMainnet {
+		return nil, fmt.Errorf("publish: refusing to build a mainnet anchoring PSBT without AllowMainnet (pins to public IPFS and funds a real-BTC spend)")
+	}
 	if err := btctx.ValidateFeeRate(w.cfg.FeeRate); err != nil {
 		return nil, err
 	}
diff --git a/internal/publish/psbt_test.go b/internal/publish/psbt_test.go
index 090efef..1335bfd 100644
--- a/internal/publish/psbt_test.go
+++ b/internal/publish/psbt_test.go
@@ -275,3 +275,21 @@ func TestSignPSBTRefusesFeeOverCap(t *testing.T) {
 		t.Fatalf("SignPSBT under a generous cap should succeed: %v", err)
 	}
 }
+
+// TestBuildPSBTMainnetGuard guards #23: BuildPSBT pins to public IPFS and funds a
+// real-BTC spend, so it must enforce the same AllowMainnet gate as Submit.
+func TestBuildPSBTMainnetGuard(t *testing.T) {
+	key := mkFundingKey(t)
+	w := New(&fakeCAS{}, &fakeBroadcaster{}, key, Config{Prefix: "ion:", FeeRate: 2, Params: &chaincfg.MainNetParams}, nil)
+	ops := []anchor.Op{mkCreateOp(t, "psbtmain")}
+	utxos := []btctx.UTXO{fundingUTXO(key, 100_000)}
+
+	if _, err := w.BuildPSBT(context.Background(), ops, utxos, key.PkScript()); err == nil || !strings.Contains(err.Error(), "AllowMainnet") {
+		t.Errorf("BuildPSBT must refuse mainnet without AllowMainnet, got %v", err)
+	}
+	// With the gate open, the mainnet guard no longer blocks (the PSBT builds).
+	w.cfg.AllowMainnet = true
+	if _, err := w.BuildPSBT(context.Background(), ops, utxos, key.PkScript()); err != nil {
+		t.Errorf("with AllowMainnet the gate should not block: %v", err)
+	}
+}