diff --git a/CHANGELOG.md b/CHANGELOG.md
index 18ca52d..1c07210 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,5 +1,12 @@
 # Changelog
 
+## Pending
+
+### Fixed
+
+- `scout traces list` and `scout traces show` no longer fail with `cannot unmarshal number ... into Go struct field ... mem_delta of type int64` — the API reports `mem_delta` as a float in megabytes, so it is now decoded as such and displayed as MB (#19)
+- `scout traces list` Duration column and the legacy-trace header in `scout traces show` treated `total_call_time` as seconds; the API reports it in milliseconds, so an 83-second request no longer shows as `83483.9s` (#19)
+
 ## [0.4.0] - 2026-06-04
 
 ### Added
diff --git a/cmd/traces.go b/cmd/traces.go
index 9be244c..26a968a 100644
--- a/cmd/traces.go
+++ b/cmd/traces.go
@@ -71,8 +71,8 @@ func runTracesList(cmd *cobra.Command, args []string) {
 		rows[i] = []string{
 			strconv.Itoa(t.ID),
 			output.FormatRelativeTime(t.Time),
-			output.FormatSeconds(t.TotalCallTime),
-			output.FormatBytes(t.MemDelta),
+			output.FormatMs(t.TotalCallTime),
+			output.FormatMB(t.MemDelta),
 			t.MetricName,
 			t.URI,
 		}
diff --git a/internal/api/traces_test.go b/internal/api/traces_test.go
new file mode 100644
index 0000000..6c3db68
--- /dev/null
+++ b/internal/api/traces_test.go
@@ -0,0 +1,63 @@
+package api
+
+import (
+	"net/http"
+	"net/http/httptest"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+// The API reports mem_delta as a float (megabytes), e.g. 0.0 or 2.9296875.
+// Regression test for https://github.com/scoutapp/scout-cli/issues/19.
+func TestListTracesFloatMemDelta(t *testing.T) {
+	server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		assert.Equal(t, "/api/v0/apps/6/endpoints/YXBpL21ldHJpY3Mvc2hvdw==/traces", r.URL.Path)
+		_, _ = w.Write([]byte(`{
+			"header": {"status": {"code": 200, "message": "OK"}, "apiVersion": "0.1"},
+			"results": {"traces": [
+				{"id": 1, "time": "2026-01-01T00:05:00Z", "total_call_time": 83.48, "mem_delta": 2.9296875, "metric_name": "Controller/users/index", "uri": "/users/42", "context": {}},
+				{"id": 2, "time": "2026-01-01T00:06:00Z", "total_call_time": 1.5, "mem_delta": 0.0, "metric_name": "Controller/users/index", "uri": null, "context": {}},
+				{"id": 3, "time": "2026-01-01T00:07:00Z", "total_call_time": 1.5, "mem_delta": null, "metric_name": "Controller/users/index", "uri": "/x", "context": {}},
+				{"id": 4, "time": "2026-01-01T00:08:00Z", "total_call_time": 1.5, "mem_delta": 12, "metric_name": "Controller/users/index", "uri": "/x", "context": {}}
+			]}
+		}`))
+	}))
+	defer server.Close()
+
+	client := NewClient(server.URL, "test-key")
+	traces, err := client.ListTraces(6, "YXBpL21ldHJpY3Mvc2hvdw==", "2026-09-03T00:00:00Z", "2026-09-09T00:00:00Z")
+	require.NoError(t, err)
+	require.Len(t, traces, 4)
+	assert.InDelta(t, 2.9296875, traces[0].MemDelta, 1e-9)
+	assert.Equal(t, 0.0, traces[1].MemDelta)
+	assert.Equal(t, 0.0, traces[2].MemDelta, "null mem_delta should decode as zero")
+	assert.Equal(t, 12.0, traces[3].MemDelta, "integer mem_delta should still decode")
+}
+
+func TestGetTraceFloatMemDelta(t *testing.T) {
+	server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		assert.Equal(t, "/api/v0/apps/6/traces/12345", r.URL.Path)
+		_, _ = w.Write([]byte(`{
+			"header": {"status": {"code": 200, "message": "OK"}, "apiVersion": "0.1"},
+			"results": {"trace": {
+				"id": 12345, "time": "2026-01-01T00:05:00Z", "total_call_time": 83483.85,
+				"mem_delta": 2.9296875, "metric_name": "Controller/users/index", "uri": "/x", "context": {},
+				"transaction_id": "abc", "hostname": "web-1", "git_sha": "deadbeef",
+				"duration_in_seconds": 83.483853, "allocations_count": 15467, "limited": false,
+				"spans": [{"id": "span-1", "parent_id": null, "operation": "Middleware/Summary", "type": "Middleware",
+				           "description": null, "duration_ms": 83.48, "exclusive_duration_ms": 0.0038, "allocations": 15467, "children": []}]
+			}}
+		}`))
+	}))
+	defer server.Close()
+
+	client := NewClient(server.URL, "test-key")
+	trace, err := client.GetTrace(6, 12345)
+	require.NoError(t, err)
+	assert.InDelta(t, 2.9296875, trace.MemDelta, 1e-9)
+	assert.Equal(t, int64(15467), trace.AllocationsCount)
+	require.Len(t, trace.Spans, 1)
+	assert.Equal(t, int64(15467), trace.Spans[0].Allocations)
+}
diff --git a/internal/api/types.go b/internal/api/types.go
index 7660d03..c3a337c 100644
--- a/internal/api/types.go
+++ b/internal/api/types.go
@@ -86,7 +86,7 @@ type TraceEntry struct {
 	ID            int                    `json:"id"`
 	Time          string                 `json:"time"`
 	TotalCallTime float64                `json:"total_call_time"`
-	MemDelta      int64                  `json:"mem_delta"`
+	MemDelta      float64                `json:"mem_delta"` // memory increase in MB
 	MetricName    string                 `json:"metric_name"`
 	URI           string                 `json:"uri"`
 	Context       map[string]interface{} `json:"context"`
@@ -115,7 +115,7 @@ type TraceDetail struct {
 	ID                int         `json:"id"`
 	Time              string      `json:"time"`
 	TotalCallTime     float64     `json:"total_call_time"`
-	MemDelta          int64       `json:"mem_delta"`
+	MemDelta          float64     `json:"mem_delta"` // memory increase in MB
 	MetricName        string      `json:"metric_name"`
 	URI               string      `json:"uri"`
 	TransactionID     string      `json:"transaction_id"`
diff --git a/internal/output/format_mb_test.go b/internal/output/format_mb_test.go
new file mode 100644
index 0000000..908fff2
--- /dev/null
+++ b/internal/output/format_mb_test.go
@@ -0,0 +1,63 @@
+package output
+
+import (
+	"testing"
+
+	"github.com/scoutapm/scout/internal/api"
+	"github.com/stretchr/testify/assert"
+)
+
+func TestFormatMB(t *testing.T) {
+	tests := []struct {
+		input    float64
+		expected string
+	}{
+		{0, "0 MB"},
+		{0.35546875, "0.4 MB"},
+		{2.9296875, "2.9 MB"},
+		{7.29296875, "7.3 MB"},
+		{12, "12 MB"},
+		{256.4, "256 MB"},
+		{1024, "1.0 GB"},
+		{1536, "1.5 GB"},
+		{-2.5, "-2.5 MB"},
+	}
+	for _, tt := range tests {
+		t.Run(tt.expected, func(t *testing.T) {
+			assert.Equal(t, tt.expected, FormatMB(tt.input))
+		})
+	}
+}
+
+func TestRenderSpanTreeMemoryFooter(t *testing.T) {
+	trace := api.TraceDetail{
+		ID:                1,
+		MetricName:        "Controller/users/index",
+		DurationInSeconds: 0.5,
+		MemDelta:          2.9296875,
+		Spans: []api.TraceSpan{
+			{ID: "a", Operation: "Controller/users/index", DurationMs: 500},
+		},
+	}
+	out := RenderSpanTree(trace)
+	assert.Contains(t, out, "Memory: +2.9 MB")
+
+	trace.MemDelta = -0.5
+	out = RenderSpanTree(trace)
+	assert.Contains(t, out, "Memory: -0.5 MB")
+
+	trace.MemDelta = 0
+	out = RenderSpanTree(trace)
+	assert.NotContains(t, out, "Memory:")
+}
+
+func TestRenderSpanTreeLegacyDurationIsMilliseconds(t *testing.T) {
+	trace := api.TraceDetail{
+		ID:            2,
+		MetricName:    "Controller/home/index",
+		TotalCallTime: 1500, // ms
+		LegacyFormat:  true,
+	}
+	out := RenderSpanTree(trace)
+	assert.Contains(t, out, "Trace #2 — Controller/home/index — 1.5s")
+}
diff --git a/internal/output/style.go b/internal/output/style.go
index 507e599..84e6448 100644
--- a/internal/output/style.go
+++ b/internal/output/style.go
@@ -95,6 +95,20 @@ func FormatBytes(bytes int64) string {
 	return fmt.Sprintf("%d B", bytes)
 }
 
+// FormatMB formats a memory size given in megabytes (the unit the Scout API
+// uses for trace mem_delta values).
+func FormatMB(mb float64) string {
+	abs := math.Abs(mb)
+	switch {
+	case abs >= 1000:
+		return fmt.Sprintf("%.1f GB", mb/1000)
+	case abs >= 10, abs == 0:
+		return fmt.Sprintf("%.0f MB", mb)
+	default:
+		return fmt.Sprintf("%.1f MB", mb)
+	}
+}
+
 func FormatRelativeTime(iso string) string {
 	t, err := time.Parse(time.RFC3339, iso)
 	if err != nil {
diff --git a/internal/output/tree.go b/internal/output/tree.go
index 37bd1b0..aeac902 100644
--- a/internal/output/tree.go
+++ b/internal/output/tree.go
@@ -2,6 +2,7 @@ package output
 
 import (
 	"fmt"
+	"math"
 	"strings"
 
 	"github.com/charmbracelet/lipgloss"
@@ -13,7 +14,8 @@ func RenderSpanTree(trace api.TraceDetail) string {
 
 	totalMs := trace.DurationInSeconds * 1000
 	if totalMs == 0 {
-		totalMs = trace.TotalCallTime * 1000
+		// total_call_time is reported in milliseconds by the API.
+		totalMs = trace.TotalCallTime
 	}
 
 	// Header
@@ -41,7 +43,7 @@ func RenderSpanTree(trace api.TraceDetail) string {
 	var meta []string
 	if trace.MemDelta != 0 {
 		meta = append(meta, fmt.Sprintf("Memory: %s%s",
-			sign(trace.MemDelta), FormatBytes(abs(trace.MemDelta))))
+			sign(trace.MemDelta), FormatMB(math.Abs(trace.MemDelta))))
 	}
 	if trace.AllocationsCount > 0 {
 		meta = append(meta, fmt.Sprintf("Allocations: %s", FormatNumber(trace.AllocationsCount)))
@@ -118,16 +120,9 @@ func renderSpan(sb *strings.Builder, span api.TraceSpan, totalMs float64, prefix
 	}
 }
 
-func sign(n int64) string {
+func sign(n float64) string {
 	if n >= 0 {
 		return "+"
 	}
 	return "-"
 }
-
-func abs(n int64) int64 {
-	if n < 0 {
-		return -n
-	}
-	return n
-}