diff --git a/wire/test/run-bigsize.c b/common/test/run-bigsize.c
similarity index 89%
rename from wire/test/run-bigsize.c
rename to common/test/run-bigsize.c
index 22645b1a1..387f08fef 100644
--- a/wire/test/run-bigsize.c
+++ b/common/test/run-bigsize.c
@@ -4,14 +4,17 @@
 #include <ccan/tal/str/str.h>
 #include <common/json.c>
 #include <common/utils.h>
+#include <wire/wire.h>
 
 static const char *reason;
 #define SUPERVERBOSE(r) do { if (!reason) reason = (r); } while(0)
 
-#include <wire/fromwire.c>
-#include <wire/towire.c>
+#include <common/bigsize.c>
 
 /* AUTOGENERATED MOCKS START */
+/* Generated stub for fromwire_fail */
+const void *fromwire_fail(const u8 **cursor UNNEEDED, size_t *max UNNEEDED)
+{ fprintf(stderr, "fromwire_fail called!\n"); abort(); }
 /* AUTOGENERATED MOCKS END */
 
 /* BOLT-EXPERIMENTAL #1:
@@ -132,7 +135,7 @@ static void test_decode(const char *json, const jsmntok_t toks[])
 		const jsmntok_t *bytes = json_get_member(json, t, "bytes");
 		u64 num, expect;
 		const u8 *b;
-		size_t max;
+		size_t len;
 
 		if (!json_to_u64(json, json_get_member(json, t, "value"),
 				 &expect))
@@ -140,25 +143,14 @@ static void test_decode(const char *json, const jsmntok_t toks[])
 		b = tal_hexdata(tmpctx, json + bytes->start,
 				bytes->end - bytes->start);
 
-		max = tal_bytelen(b);
 		reason = NULL;
-		num = fromwire_bigsize(&b, &max);
+		len = bigsize_get(b, tal_bytelen(b), &num);
 		if (err) {
-			assert(b == NULL);
-			assert(max == 0);
-			/* Map our errors to the test strings. */
-			if (json_tok_streq(json, err,
-					   "decoded varint is not canonical"))
-				assert(streq(reason, "not minimal encoded"));
-			else if (json_tok_streq(json, err, "unexpected EOF")
-				 || json_tok_streq(json, err, "EOF"))
-				assert(streq(reason, "less than encoding length"));
-			else
-				abort();
+			assert(len == 0);
+			assert(json_tok_streq(json, err, reason));
 		} else {
-			assert(b != NULL);
+			assert(len == tal_bytelen(b));
 			assert(num == expect);
-			assert(max == 0);
 		}
 	}
 }
@@ -215,12 +207,13 @@ static void test_encode(const char *json, const jsmntok_t toks[])
 {
 	size_t i;
 	const jsmntok_t *t;
+	u8 buf[BIGSIZE_MAX_LEN];
 
 	json_for_each_arr(i, t, toks) {
 		const jsmntok_t *bytes = json_get_member(json, t, "bytes");
 		u64 num;
 		const u8 *expect;
-		u8 *b;
+		size_t len;
 
 		if (!json_to_u64(json, json_get_member(json, t, "value"),
 				 &num))
@@ -228,9 +221,8 @@ static void test_encode(const char *json, const jsmntok_t toks[])
 		expect = tal_hexdata(tmpctx, json + bytes->start,
 				     bytes->end - bytes->start);
 
-		b = tal_arr(tmpctx, u8, 0);
-		towire_bigsize(&b, num);
-		assert(memeq(b, tal_bytelen(b), expect, tal_bytelen(expect)));
+		len = bigsize_put(buf, num);
+		assert(memeq(buf, len, expect, tal_bytelen(expect)));
 	}
 }