diff --git a/devtools/Makefile b/devtools/Makefile
index 577b0dd2b..8cfa41663 100644
--- a/devtools/Makefile
+++ b/devtools/Makefile
@@ -1,6 +1,6 @@
 DEVTOOLS_SRC := devtools/gen_print_wire.c devtools/gen_print_onion_wire.c devtools/print_wire.c
 DEVTOOLS_OBJS := $(DEVTOOLS_SRC:.c=.o)
-DEVTOOLS := devtools/bolt11-cli devtools/decodemsg devtools/onion devtools/dump-gossipstore devtools/gossipwith devtools/create-gossipstore devtools/mkcommit devtools/mkfunding
+DEVTOOLS := devtools/bolt11-cli devtools/decodemsg devtools/onion devtools/dump-gossipstore devtools/gossipwith devtools/create-gossipstore devtools/mkcommit devtools/mkfunding devtools/mkclose
 DEVTOOLS_TOOL_SRC := $(DEVTOOLS:=.c)
 DEVTOOLS_TOOL_OBJS := $(DEVTOOLS_TOOL_SRC:.c=.o)
 
@@ -66,6 +66,8 @@ devtools/mkcommit: $(DEVTOOLS_COMMON_OBJS) $(CCAN_OBJS) $(BITCOIN_OBJS) common/d
 
 devtools/mkfunding: $(DEVTOOLS_COMMON_OBJS) $(CCAN_OBJS) $(BITCOIN_OBJS) wire/fromwire.o wire/towire.o wire/tlvstream.o common/funding_tx.o common/utxo.o common/permute_tx.o common/key_derive.o devtools/mkfunding.o
 
+devtools/mkclose: $(DEVTOOLS_COMMON_OBJS) $(CCAN_OBJS) $(BITCOIN_OBJS) wire/fromwire.o wire/towire.o wire/tlvstream.o devtools/mkclose.o
+
 # Make sure these depend on everything.
 ALL_PROGRAMS += $(DEVTOOLS)
 ALL_OBJS += $(DEVTOOLS_OBJS) $(DEVTOOLS_TOOL_OBJS)
diff --git a/devtools/mkclose.c b/devtools/mkclose.c
new file mode 100644
index 000000000..a9f9abf37
--- /dev/null
+++ b/devtools/mkclose.c
@@ -0,0 +1,188 @@
+/* Code to make a mutual close tx, useful for generating test cases.
+ *
+ * For example, in the spec tests we use the following:
+ *
+ * lightning/devtools/mkclose 189c40b0728f382fe91c87270926584e48e0af3a6789f37454afee6c7560311d 0 0.00999877btc 253 0.00999877btc 0000000000000000000000000000000000000000000000000000000000000010 0000000000000000000000000000000000000000000000000000000000000020 026957e53b46df017bd6460681d068e1d23a7b027de398272d0b15f59b78d060a9 03a9f795ff2e4c27091f40e8f8277301824d1c3dfa6b0204aa92347314e41b1033
+ */
+#include <bitcoin/script.h>
+#include <bitcoin/tx.h>
+#include <ccan/err/err.h>
+#include <ccan/str/hex/hex.h>
+#include <channeld/full_channel.h>
+#include <common/amount.h>
+#include <common/derive_basepoints.h>
+#include <common/htlc_wire.h>
+#include <common/initial_commit_tx.h>
+#include <common/key_derive.h>
+#include <common/keyset.h>
+#include <common/status.h>
+#include <common/type_to_string.h>
+#include <inttypes.h>
+#include <stdarg.h>
+#include <stdio.h>
+
+static bool verbose = false;
+
+void status_fmt(enum log_level level, const char *fmt, ...)
+{
+	if (verbose) {
+		va_list ap;
+
+		va_start(ap, fmt);
+		printf("#TRACE: ");
+		vprintf(fmt, ap);
+		printf("\n");
+		va_end(ap);
+	}
+}
+
+void status_failed(enum status_failreason reason, const char *fmt, ...)
+{
+	abort();
+}
+
+static char *sig_as_hex(const struct bitcoin_signature *sig)
+{
+	u8 compact_sig[64];
+
+	secp256k1_ecdsa_signature_serialize_compact(secp256k1_ctx,
+						    compact_sig,
+						    &sig->s);
+	return tal_hexstr(NULL, compact_sig, sizeof(compact_sig));
+}
+
+int main(int argc, char *argv[])
+{
+	struct pubkey funding_pubkey[NUM_SIDES], outkey[NUM_SIDES];
+	struct privkey funding_privkey[NUM_SIDES];
+	struct amount_sat funding_amount;
+	struct bitcoin_txid funding_txid;
+	unsigned int funding_outnum;
+	u32 feerate_per_kw;
+	struct amount_sat fee;
+	struct bitcoin_signature local_sig, remote_sig;
+	struct amount_msat local_msat, remote_msat;
+	int argnum, num_outputs;
+	struct bitcoin_tx *tx;
+	u8 **witness;
+	const u8 *funding_wscript;
+	const struct chainparams *chainparams = chainparams_for_network("bitcoin");
+	const struct amount_sat dust_limit = AMOUNT_SAT(546);
+
+	setup_locale();
+
+	secp256k1_ctx = secp256k1_context_create(SECP256K1_CONTEXT_VERIFY |
+						 SECP256K1_CONTEXT_SIGN);
+
+	if (argv[1] && streq(argv[1], "-v")) {
+		verbose = true;
+		argv++;
+		argc--;
+	}
+
+	if (argc != 10)
+		errx(1, "Usage: mkclose [-v] <funding-txid> <funding-txout> <funding-amount> <feerate-per-kw> <local-msat> <local-funding-privkey> <remote-funding-privkey> <local-close-pubkey> <remote-close-pubkey>\n"
+			"(Fee is deducted from local-msat)");
+
+	argnum = 1;
+	if (!bitcoin_txid_from_hex(argv[argnum],
+				   strlen(argv[argnum]), &funding_txid))
+		errx(1, "Bad funding-txid");
+	argnum++;
+	funding_outnum = atoi(argv[argnum++]);
+	if (!parse_amount_sat(&funding_amount, argv[argnum], strlen(argv[argnum])))
+		errx(1, "Bad funding-amount");
+	argnum++;
+	feerate_per_kw = atoi(argv[argnum++]);
+	if (!parse_amount_msat(&local_msat,
+			       argv[argnum], strlen(argv[argnum])))
+		errx(1, "Bad local-msat");
+	argnum++;
+	if (!hex_decode(argv[argnum], strlen(argv[argnum]),
+			&funding_privkey[LOCAL], sizeof(funding_privkey[LOCAL])))
+		errx(1, "Parsing local-funding_privkey");
+	argnum++;
+	if (!hex_decode(argv[argnum], strlen(argv[argnum]),
+			&funding_privkey[REMOTE], sizeof(funding_privkey[REMOTE])))
+		errx(1, "Parsing remote-funding_privkey");
+	argnum++;
+	if (!pubkey_from_hexstr(argv[argnum], strlen(argv[argnum]),
+				&outkey[LOCAL]))
+		errx(1, "Parsing local-close-pubkey");
+	argnum++;
+	if (!pubkey_from_hexstr(argv[argnum], strlen(argv[argnum]),
+				&outkey[REMOTE]))
+		errx(1, "Parsing remote-close-pubkey");
+	argnum++;
+
+	fee = commit_tx_base_fee(feerate_per_kw, 0);
+	if (!amount_msat_sub_sat(&local_msat, local_msat, fee))
+		errx(1, "Can't afford fee %s",
+		     type_to_string(NULL, struct amount_sat, &fee));
+	if (!amount_sat_sub_msat(&remote_msat, funding_amount, local_msat))
+		errx(1, "Can't afford local_msat");
+
+	if (!pubkey_from_privkey(&funding_privkey[LOCAL], &funding_pubkey[LOCAL])
+	    || !pubkey_from_privkey(&funding_privkey[REMOTE], &funding_pubkey[REMOTE]))
+		errx(1, "Bad deriving funding pubkeys");
+
+	tx = bitcoin_tx(NULL, chainparams, 1, 2);
+
+	/* Our input spends the anchor tx output. */
+	bitcoin_tx_add_input(tx, &funding_txid, funding_outnum,
+			     BITCOIN_TX_DEFAULT_SEQUENCE, funding_amount, NULL);
+
+	num_outputs = 0;
+	if (amount_msat_greater_eq_sat(local_msat, dust_limit)) {
+		u8 *script = scriptpubkey_p2wpkh(NULL, &outkey[LOCAL]);
+		printf("# local witness script: %s\n", tal_hex(NULL, script));
+		/* One output is to us. */
+		bitcoin_tx_add_output(tx, script,
+				      amount_msat_to_sat_round_down(local_msat));
+		num_outputs++;
+	} else
+		printf("# local output trimmed\n");
+
+	if (amount_msat_greater_eq_sat(remote_msat, dust_limit)) {
+		u8 *script = scriptpubkey_p2wpkh(NULL, &outkey[REMOTE]);
+		printf("# remote witness script: %s\n", tal_hex(NULL, script));
+		/* Other output is to them. */
+		bitcoin_tx_add_output(tx, script,
+				      amount_msat_to_sat_round_down(remote_msat));
+		num_outputs++;
+	} else
+		printf("# remote output trimmed\n");
+
+	/* Can't have no outputs at all! */
+	if (num_outputs == 0)
+		errx(1, "Can't afford any output!");
+
+	funding_wscript = bitcoin_redeem_2of2(NULL,
+					      &funding_pubkey[LOCAL],
+					      &funding_pubkey[REMOTE]);
+	printf("# funding witness script = %s\n",
+	       tal_hex(NULL, funding_wscript));
+
+	/* Need input amount for signing */
+	tx->input_amounts[0] = tal_dup(tx, struct amount_sat, &funding_amount);
+	sign_tx_input(tx, 0, NULL, funding_wscript,
+		      &funding_privkey[LOCAL],
+		      &funding_pubkey[LOCAL],
+		      SIGHASH_ALL, &local_sig);
+	printf("localsig: %s\n", sig_as_hex(&local_sig));
+	sign_tx_input(tx, 0, NULL, funding_wscript,
+		      &funding_privkey[REMOTE],
+		      &funding_pubkey[REMOTE],
+		      SIGHASH_ALL, &remote_sig);
+	printf("remotesig: %s\n", sig_as_hex(&remote_sig));
+
+	witness =
+		bitcoin_witness_2of2(NULL, &local_sig, &remote_sig,
+				     &funding_pubkey[LOCAL],
+				     &funding_pubkey[REMOTE]);
+	bitcoin_tx_input_set_witness(tx, 0, witness);
+	printf("# signed close transaction: %s\n",
+	       tal_hex(NULL, linearize_tx(NULL, tx)));
+
+	return 0;
+}