diff --git a/docs/releases/unreleased.md b/docs/releases/unreleased.md
index b75bbe1..011ae7c 100644
--- a/docs/releases/unreleased.md
+++ b/docs/releases/unreleased.md
@@ -5,3 +5,7 @@
 - All successfully read license states exit 0. Configuration, request, decoding, and output failures exit nonzero. No token or database header is sent to `/health`, and no authentication verification is performed. Normal connection selection, timeout, TLS verification, and identity opt-out settings apply.
 
 License installation and key/file configuration remain server responsibilities. Existing `ping -o json` license pass-through is unchanged. Closes [arcli#34](https://github.com/Basekick-Labs/arcli/issues/34).
+
+### Fixed
+
+- Accept large integers that can be represented exactly as float64 in mixed numeric JSON write columns. Lossy conversions remain rejected, and integer-only columns retain int64 encoding.
diff --git a/internal/client/msgpack_numeric_columns_test.go b/internal/client/msgpack_numeric_columns_test.go
new file mode 100644
index 0000000..c3fe7a3
--- /dev/null
+++ b/internal/client/msgpack_numeric_columns_test.go
@@ -0,0 +1,76 @@
+package client
+
+import (
+	"bytes"
+	"encoding/hex"
+	"encoding/json"
+	"fmt"
+	"math"
+	"strconv"
+	"strings"
+	"testing"
+)
+
+func TestJSONToMsgPack_MixedNumericColumnExactIntegers(t *testing.T) {
+	for _, tc := range []struct {
+		value int64
+		exact bool
+	}{
+		{1 << 53, true},
+		{(1 << 53) + 2, true},
+		{-(1 << 53) - 2, true},
+		{1 << 54, true},
+		{math.MaxInt64 - 1023, true},
+		{math.MinInt64, true},
+		{(1 << 53) + 1, false},
+		{-(1 << 53) - 1, false},
+		{math.MaxInt64, false},
+		{math.MinInt64 + 1, false},
+	} {
+		for _, floatFirst := range []bool{false, true} {
+			t.Run(fmt.Sprintf("%d/float-first=%t", tc.value, floatFirst), func(t *testing.T) {
+				values := []any{nil, json.Number(strconv.FormatInt(tc.value, 10)), json.Number("1.5")}
+				wantValues := []any{nil, float64(tc.value), float64(1.5)}
+				if floatFirst {
+					values[1], values[2] = values[2], values[1]
+					wantValues[1], wantValues[2] = wantValues[2], wantValues[1]
+				}
+				in, err := json.Marshal(map[string]any{"m": "cpu", "columns": map[string]any{"value": values}})
+				if err != nil {
+					t.Fatal(err)
+				}
+				out, sum, err := JSONToMsgPack(bytes.NewReader(in))
+				if !tc.exact {
+					if err == nil || !strings.Contains(err.Error(), "cannot be represented exactly") || out != nil {
+						t.Errorf("inexact integer: output=%x error=%v", out, err)
+					}
+					return
+				}
+				if err != nil {
+					t.Fatal(err)
+				}
+				want := encodeHex(t, map[string]any{"m": "cpu", "columns": map[string]any{"value": wantValues}})
+				if got := hex.EncodeToString(out); got != want {
+					t.Errorf("MessagePack = %s, want %s", got, want)
+				}
+				if sum.Items != 1 || sum.Columnar != 1 || sum.Rows != 3 {
+					t.Errorf("summary = %+v", sum)
+				}
+			})
+		}
+	}
+}
+
+func TestJSONToMsgPack_IntegerColumnRetainsInt64(t *testing.T) {
+	const in = `{"m":"cpu","columns":{"value":[9007199254740993,9223372036854775807,-9223372036854775808]}}`
+	out, _, err := JSONToMsgPack(strings.NewReader(in))
+	if err != nil {
+		t.Fatal(err)
+	}
+	want := encodeHex(t, map[string]any{
+		"m": "cpu", "columns": map[string]any{"value": []any{int64(9007199254740993), int64(math.MaxInt64), int64(math.MinInt64)}},
+	})
+	if got := hex.EncodeToString(out); got != want {
+		t.Errorf("integer column = %s, want %s", got, want)
+	}
+}
diff --git a/internal/client/msgpackenc.go b/internal/client/msgpackenc.go
index 7595fd3..a78ed71 100644
--- a/internal/client/msgpackenc.go
+++ b/internal/client/msgpackenc.go
@@ -395,10 +395,12 @@ func convertColumn(col string, arr []any, where string) ([]any, error) {
 	if !isTime && hasInt && hasFloat {
 		for i, v := range out {
 			if iv, ok := v.(int64); ok {
-				if iv > 1<<53 || iv < -(1<<53) {
+				f := float64(iv)
+				// MaxInt64 rounds to 2^63; reject that before converting back.
+				if f >= 1<<63 || int64(f) != iv {
 					return nil, fmt.Errorf("%s: column %q mixes integers and floats and %d cannot be represented exactly as a float", where, col, iv)
 				}
-				out[i] = float64(iv)
+				out[i] = f
 			}
 		}
 	}
diff --git a/internal/commands/write_numeric_columns_test.go b/internal/commands/write_numeric_columns_test.go
new file mode 100644
index 0000000..70b1059
--- /dev/null
+++ b/internal/commands/write_numeric_columns_test.go
@@ -0,0 +1,43 @@
+package commands
+
+import (
+	"bytes"
+	"encoding/hex"
+	"io"
+	"net/http"
+	"net/http/httptest"
+	"strings"
+	"testing"
+)
+
+func TestWriteJSON_ExactLargeIntegerInMixedColumn(t *testing.T) {
+	bodies := make(chan []byte, 1)
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		if r.URL.Path != "/api/v1/write/msgpack" {
+			t.Errorf("path = %q", r.URL.Path)
+		}
+		body, _ := io.ReadAll(r.Body)
+		bodies <- body
+		w.WriteHeader(http.StatusNoContent)
+	}))
+	defer srv.Close()
+	writeTestConfig(t, srv.URL, "tok")
+	cmd := newWriteCmd()
+	cmd.SetIn(strings.NewReader(`{"m":"cpu","columns":{"value":[18014398509481984,1.5]}}`))
+	out, _, err := execCmd(t, cmd, "--format", "json")
+	if err != nil {
+		t.Fatal(err)
+	}
+	body := <-bodies
+	// Both values use float64 tags; 2^54 is exactly representable.
+	want, err := hex.DecodeString("82a7636f6c756d6e7381a576616c756592cb4350000000000000cb3ff8000000000000a16da3637075")
+	if err != nil {
+		t.Fatal(err)
+	}
+	if !bytes.Equal(body, want) {
+		t.Errorf("uploaded MessagePack = %x, want %x", body, want)
+	}
+	if !strings.Contains(out, "OK: 1 measurement(s), 2 row(s)") {
+		t.Errorf("stdout = %q", out)
+	}
+}