diff --git a/src/common/sstream.c b/src/common/sstream.c
index aabdd6ab..3cd98d93 100644
--- a/src/common/sstream.c
+++ b/src/common/sstream.c
@@ -7,6 +7,8 @@
 #include <stdarg.h>
 #include <assert.h>
 #include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
 #include <ctype.h>
 
 void c11_sbuf__ctor(c11_sbuf* self) {
@@ -53,21 +55,26 @@ void c11_sbuf__write_f64(c11_sbuf* self, double val, int precision) {
     char b[32];
     int size;
     if(precision < 0) {
-        int prec = 17 - 1;  // std::numeric_limits<double>::max_digits10 == 17
-        size = snprintf(b, sizeof(b), "%.*g", prec, val);
+        // Pick the shortest decimal width that round-trips to the same binary double.
+        for(int prec = 15; prec <= 17; prec++) {
+            size = snprintf(b, sizeof(b), "%.*g", prec, val);
+            double parsed = strtod(b, NULL);
+            if(memcmp(&parsed, &val, sizeof(double)) == 0) break;
+        }
+        bool all_is_digit = true;
+        for(int i = 1; i < size; i++) {
+            if(!isdigit(b[i])) {
+                all_is_digit = false;
+                break;
+            }
+        }
+        c11_sbuf__write_cstr(self, b);
+        if(all_is_digit) c11_sbuf__write_cstr(self, ".0");
     } else {
         int prec = precision;
         size = snprintf(b, sizeof(b), "%.*f", prec, val);
+        c11_sbuf__write_cstr(self, b);
     }
-    c11_sbuf__write_cstr(self, b);
-    bool all_is_digit = true;
-    for(int i = 1; i < size; i++) {
-        if(!isdigit(b[i])) {
-            all_is_digit = false;
-            break;
-        }
-    }
-    if(all_is_digit) c11_sbuf__write_cstr(self, ".0");
 }
 
 void c11_sbuf__write_sv(c11_sbuf* self, c11_sv sv) {
diff --git a/src/modules/json.c b/src/modules/json.c
index 181d4aff..fca273f6 100644
--- a/src/modules/json.c
+++ b/src/modules/json.c
@@ -102,7 +102,7 @@ static bool json__write_namedict_kv(py_Name k, py_Ref v, void* ctx_) {
 static bool json__write_object(c11_sbuf* buf, py_TValue* obj, int indent, int depth) {
     switch(obj->type) {
         case tp_NoneType: c11_sbuf__write_cstr(buf, "null"); return true;
-        case tp_int: c11_sbuf__write_int(buf, obj->_i64); return true;
+        case tp_int: c11_sbuf__write_i64(buf, obj->_i64); return true;
         case tp_float: {
             if(dmath_isnan(obj->_f64)) {
                 c11_sbuf__write_cstr(buf, "NaN");
diff --git a/tests/250_rfstring.py b/tests/250_rfstring.py
index 5b687bd6..d2f93ed8 100644
--- a/tests/250_rfstring.py
+++ b/tests/250_rfstring.py
@@ -64,6 +64,7 @@ assert f'{a:010f}' == '010.000000'
 assert f'{a:010.2f}' == '0000010.00'
 assert f'{a:.2f}' == '10.00'
 assert f'{a:.5f}' == '10.00000'
+assert f'{2.5:.0f}' == '2'
 
 b = '123'
 assert f'{b:10}' == '123       '
diff --git a/tests/721_json.py b/tests/721_json.py
index 29d168bc..8509d02b 100644
--- a/tests/721_json.py
+++ b/tests/721_json.py
@@ -48,6 +48,9 @@ _j = json.dumps(c)
 _c = json.loads(_j)
 assert c == _c
 
+assert json.dumps(9223372036854775807) == '9223372036854775807'
+assert json.loads(json.dumps(0.1 + 0.2)) == 0.1 + 0.2
+
 d = True
 _j = json.dumps(d)
 _d = json.loads(_j)