diff --git a/platform/internal/handlers/restart_context.go b/platform/internal/handlers/restart_context.go
new file mode 100644
index 00000000..3ae2f41d
--- /dev/null
+++ b/platform/internal/handlers/restart_context.go
@@ -0,0 +1,240 @@
+// Package handlers — restart_context.go implements Layer 1 of issue #19:
+// after a workspace is restarted and comes back online, the platform
+// generates a state snapshot (timestamp, previous session end, env-var
+// keys now available) and delivers it as a synthetic A2A message/send
+// so the agent sees what changed across the restart boundary.
+//
+// Layer 2 (user-defined restart_prompt via config.yaml / org.yaml) is
+// out of scope for this file — tracked as a separate follow-up issue.
+package handlers
+
+import (
+	"context"
+	"database/sql"
+	"encoding/json"
+	"fmt"
+	"log"
+	"sort"
+	"time"
+
+	"github.com/Molecule-AI/molecule-monorepo/platform/internal/db"
+	"github.com/google/uuid"
+)
+
+// restartContextOnlineTimeout bounds how long we wait for a workspace
+// to re-register after restart before dropping the context message.
+// The Restart HTTP handler has already returned 200 by the time this
+// waiter runs, so a timeout here is purely a best-effort skip.
+const restartContextOnlineTimeout = 30 * time.Second
+
+// restartContextOnlinePollInterval is the poll cadence while waiting
+// for WORKSPACE_ONLINE. 500ms keeps the typical-case latency low
+// without hammering Postgres.
+const restartContextOnlinePollInterval = 500 * time.Millisecond
+
+// restartContextData captures the platform-computed snapshot that will
+// be rendered into a human-readable message. Keeping it as a struct
+// (rather than building the string inline) makes the builder
+// unit-testable without stubbing time/DB calls.
+type restartContextData struct {
+	RestartAt     time.Time
+	PrevSessionAt time.Time // zero value = no prior session recorded
+	EnvKeys       []string  // sorted list of env-var keys (no values)
+}
+
+// buildRestartContextMessage renders the restart context into the
+// exact format proposed in issue #19. Fields that have no data (e.g.
+// first-ever session) are rendered with a neutral placeholder so the
+// agent always sees a consistent shape.
+func buildRestartContextMessage(d restartContextData) string {
+	msg := "=== WORKSPACE RESTART CONTEXT ===\n"
+	msg += fmt.Sprintf("Restart at: %s\n", d.RestartAt.UTC().Format(time.RFC3339))
+
+	if d.PrevSessionAt.IsZero() {
+		msg += "Previous session ended: (no prior session on record)\n"
+	} else {
+		delta := d.RestartAt.Sub(d.PrevSessionAt)
+		msg += fmt.Sprintf("Previous session ended: %s (%s ago)\n",
+			d.PrevSessionAt.UTC().Format(time.RFC3339),
+			humanDuration(delta))
+	}
+
+	if len(d.EnvKeys) == 0 {
+		msg += "Env vars now available: (none)\n"
+	} else {
+		msg += fmt.Sprintf("Env vars now available: %s\n", joinStrings(d.EnvKeys, ", "))
+	}
+
+	msg += "=== END RESTART CONTEXT ===\n"
+	return msg
+}
+
+// humanDuration formats a duration for display in the restart context.
+// Keeps the output terse ("2h14m", "38s") without pulling in a
+// humanize library. Negative/zero deltas render as "0s".
+func humanDuration(d time.Duration) string {
+	if d <= 0 {
+		return "0s"
+	}
+	d = d.Round(time.Second)
+	h := int(d / time.Hour)
+	m := int((d % time.Hour) / time.Minute)
+	s := int((d % time.Minute) / time.Second)
+	switch {
+	case h > 0:
+		return fmt.Sprintf("%dh%dm", h, m)
+	case m > 0:
+		return fmt.Sprintf("%dm%ds", m, s)
+	default:
+		return fmt.Sprintf("%ds", s)
+	}
+}
+
+// joinStrings is strings.Join — inlined to avoid an import cycle
+// concern in a file that already carries a handful of stdlib deps.
+func joinStrings(parts []string, sep string) string {
+	switch len(parts) {
+	case 0:
+		return ""
+	case 1:
+		return parts[0]
+	}
+	n := len(sep) * (len(parts) - 1)
+	for i := 0; i < len(parts); i++ {
+		n += len(parts[i])
+	}
+	b := make([]byte, 0, n)
+	b = append(b, parts[0]...)
+	for _, p := range parts[1:] {
+		b = append(b, sep...)
+		b = append(b, p...)
+	}
+	return string(b)
+}
+
+// loadRestartContextData gathers the snapshot inputs from the DB.
+// Called *before* the restart mutates workspace state so the "previous
+// session ended" timestamp reflects the pre-restart heartbeat, not the
+// newly-provisioning row.
+func loadRestartContextData(ctx context.Context, workspaceID string) restartContextData {
+	d := restartContextData{RestartAt: time.Now()}
+
+	var lastHB sql.NullTime
+	if err := db.DB.QueryRowContext(ctx,
+		`SELECT last_heartbeat_at FROM workspaces WHERE id = $1`, workspaceID,
+	).Scan(&lastHB); err == nil && lastHB.Valid {
+		d.PrevSessionAt = lastHB.Time
+	}
+
+	// Env-var keys: union of global secrets + workspace-specific
+	// secrets. Values are NEVER included — only keys — so the agent
+	// can reason about "did my missing credential arrive?" without
+	// the platform ever echoing secret material back into the
+	// message bus.
+	keySet := map[string]struct{}{}
+	if rows, err := db.DB.QueryContext(ctx, `SELECT key FROM global_secrets`); err == nil {
+		for rows.Next() {
+			var k string
+			if rows.Scan(&k) == nil {
+				keySet[k] = struct{}{}
+			}
+		}
+		rows.Close()
+	}
+	if rows, err := db.DB.QueryContext(ctx,
+		`SELECT key FROM workspace_secrets WHERE workspace_id = $1`, workspaceID,
+	); err == nil {
+		for rows.Next() {
+			var k string
+			if rows.Scan(&k) == nil {
+				keySet[k] = struct{}{}
+			}
+		}
+		rows.Close()
+	}
+	for k := range keySet {
+		d.EnvKeys = append(d.EnvKeys, k)
+	}
+	sort.Strings(d.EnvKeys)
+	return d
+}
+
+// waitForWorkspaceOnline polls the workspaces table until the target
+// workspace's status flips to 'online' or the deadline expires.
+// Returns true on success; callers log+drop on false.
+func waitForWorkspaceOnline(ctx context.Context, workspaceID string, timeout time.Duration) bool {
+	deadline := time.Now().Add(timeout)
+	for time.Now().Before(deadline) {
+		var status string
+		if err := db.DB.QueryRowContext(ctx,
+			`SELECT status FROM workspaces WHERE id = $1`, workspaceID,
+		).Scan(&status); err == nil && status == "online" {
+			return true
+		}
+		select {
+		case <-ctx.Done():
+			return false
+		case <-time.After(restartContextOnlinePollInterval):
+		}
+	}
+	return false
+}
+
+// buildRestartA2APayload wraps the rendered context string in the
+// JSON-RPC 2.0 / A2A message/send shape that the proxy already knows
+// how to normalize. Returns the marshalled body ready for ProxyA2ARequest.
+func buildRestartA2APayload(text string) ([]byte, error) {
+	payload := map[string]any{
+		"jsonrpc": "2.0",
+		"id":      uuid.New().String(),
+		"method":  "message/send",
+		"params": map[string]any{
+			"message": map[string]any{
+				"messageId": uuid.New().String(),
+				"role":      "user",
+				"parts":     []any{map[string]any{"kind": "text", "text": text}},
+				"metadata": map[string]any{
+					"source":          "platform",
+					"kind":            "restart_context",
+					"layer":           1,
+					"restart_context": true,
+				},
+			},
+		},
+	}
+	return json.Marshal(payload)
+}
+
+// sendRestartContext is called by the Restart handler in a background
+// goroutine. It waits for the workspace to come online, then delivers
+// the snapshot via the existing A2A proxy. Failures are logged and
+// dropped — the restart itself is already considered successful at
+// this point.
+func (h *WorkspaceHandler) sendRestartContext(workspaceID string, data restartContextData) {
+	// Detach from any request context — this runs after the HTTP
+	// response is flushed.
+	ctx, cancel := context.WithTimeout(context.Background(), restartContextOnlineTimeout+30*time.Second)
+	defer cancel()
+
+	if !waitForWorkspaceOnline(ctx, workspaceID, restartContextOnlineTimeout) {
+		log.Printf("restart-context: workspace %s did not come online within %s — dropping context message", workspaceID, restartContextOnlineTimeout)
+		return
+	}
+
+	text := buildRestartContextMessage(data)
+	body, err := buildRestartA2APayload(text)
+	if err != nil {
+		log.Printf("restart-context: failed to marshal payload for %s: %v", workspaceID, err)
+		return
+	}
+
+	// "system:restart-context" prefix flags this as a trusted
+	// non-workspace caller — bypasses CanCommunicate and the
+	// caller-token check in a2a_proxy.go.
+	status, _, proxyErr := h.ProxyA2ARequest(ctx, workspaceID, body, "system:restart-context", false)
+	if proxyErr != nil {
+		log.Printf("restart-context: ProxyA2ARequest failed for %s (status=%d): %v", workspaceID, status, proxyErr)
+		return
+	}
+	log.Printf("restart-context: delivered to %s (status=%d, keys=%d)", workspaceID, status, len(data.EnvKeys))
+}
diff --git a/platform/internal/handlers/restart_context_test.go b/platform/internal/handlers/restart_context_test.go
new file mode 100644
index 00000000..75c5e7e8
--- /dev/null
+++ b/platform/internal/handlers/restart_context_test.go
@@ -0,0 +1,120 @@
+package handlers
+
+import (
+	"encoding/json"
+	"strings"
+	"testing"
+	"time"
+)
+
+func TestHumanDuration(t *testing.T) {
+	cases := []struct {
+		in   time.Duration
+		want string
+	}{
+		{0, "0s"},
+		{-5 * time.Second, "0s"},
+		{45 * time.Second, "45s"},
+		{3 * time.Minute, "3m0s"},
+		{2*time.Hour + 14*time.Minute, "2h14m"},
+		{25 * time.Hour, "25h0m"},
+	}
+	for _, c := range cases {
+		got := humanDuration(c.in)
+		if got != c.want {
+			t.Errorf("humanDuration(%v) = %q, want %q", c.in, got, c.want)
+		}
+	}
+}
+
+func TestBuildRestartContextMessage_NoPriorSession(t *testing.T) {
+	d := restartContextData{
+		RestartAt:     time.Date(2026, 4, 13, 12, 0, 0, 0, time.UTC),
+		PrevSessionAt: time.Time{},
+		EnvKeys:       nil,
+	}
+	msg := buildRestartContextMessage(d)
+
+	mustContain(t, msg, "=== WORKSPACE RESTART CONTEXT ===")
+	mustContain(t, msg, "Restart at: 2026-04-13T12:00:00Z")
+	mustContain(t, msg, "Previous session ended: (no prior session on record)")
+	mustContain(t, msg, "Env vars now available: (none)")
+	mustContain(t, msg, "=== END RESTART CONTEXT ===")
+}
+
+func TestBuildRestartContextMessage_WithDataRendersKeysAndDelta(t *testing.T) {
+	prev := time.Date(2026, 4, 13, 10, 0, 0, 0, time.UTC)
+	d := restartContextData{
+		RestartAt:     prev.Add(2*time.Hour + 14*time.Minute),
+		PrevSessionAt: prev,
+		EnvKeys:       []string{"ANTHROPIC_API_KEY", "OPENAI_API_KEY"},
+	}
+	msg := buildRestartContextMessage(d)
+
+	mustContain(t, msg, "Previous session ended: 2026-04-13T10:00:00Z (2h14m ago)")
+	mustContain(t, msg, "Env vars now available: ANTHROPIC_API_KEY, OPENAI_API_KEY")
+	if strings.Contains(msg, "sk-") || strings.Contains(msg, "secret-value") {
+		t.Errorf("rendered message leaked secret-shaped content: %q", msg)
+	}
+}
+
+func TestBuildRestartA2APayload_ShapeIsJSONRPCMessageSend(t *testing.T) {
+	body, err := buildRestartA2APayload("hello world")
+	if err != nil {
+		t.Fatalf("buildRestartA2APayload: %v", err)
+	}
+
+	var parsed map[string]any
+	if err := json.Unmarshal(body, &parsed); err != nil {
+		t.Fatalf("payload is not valid JSON: %v", err)
+	}
+
+	if parsed["jsonrpc"] != "2.0" {
+		t.Errorf("jsonrpc = %v, want 2.0", parsed["jsonrpc"])
+	}
+	if parsed["method"] != "message/send" {
+		t.Errorf("method = %v, want message/send", parsed["method"])
+	}
+
+	params, ok := parsed["params"].(map[string]any)
+	if !ok {
+		t.Fatalf("params missing or wrong type: %v", parsed["params"])
+	}
+	message, ok := params["message"].(map[string]any)
+	if !ok {
+		t.Fatalf("params.message missing: %v", params)
+	}
+	if message["role"] != "user" {
+		t.Errorf("role = %v, want user", message["role"])
+	}
+	if message["messageId"] == nil || message["messageId"] == "" {
+		t.Errorf("messageId missing")
+	}
+
+	meta, ok := message["metadata"].(map[string]any)
+	if !ok {
+		t.Fatalf("metadata missing: %v", message)
+	}
+	if meta["kind"] != "restart_context" {
+		t.Errorf("metadata.kind = %v, want restart_context", meta["kind"])
+	}
+	if meta["source"] != "platform" {
+		t.Errorf("metadata.source = %v, want platform", meta["source"])
+	}
+
+	parts, ok := message["parts"].([]any)
+	if !ok || len(parts) != 1 {
+		t.Fatalf("parts wrong shape: %v", message["parts"])
+	}
+	firstPart, _ := parts[0].(map[string]any)
+	if firstPart["text"] != "hello world" {
+		t.Errorf("parts[0].text = %v, want hello world", firstPart["text"])
+	}
+}
+
+func mustContain(t *testing.T, haystack, needle string) {
+	t.Helper()
+	if !strings.Contains(haystack, needle) {
+		t.Errorf("expected output to contain %q, got:\n%s", needle, haystack)
+	}
+}
diff --git a/platform/internal/handlers/workspace_restart.go b/platform/internal/handlers/workspace_restart.go
index e40f9670..503c6d68 100644
--- a/platform/internal/handlers/workspace_restart.go
+++ b/platform/internal/handlers/workspace_restart.go
@@ -159,7 +159,12 @@ func (h *WorkspaceHandler) Restart(c *gin.Context) {
 		log.Printf("Restart: reset=true — will discard claude-sessions volume for %s (%s)", wsName, id)
 	}
 
+	// Capture restart-context data BEFORE provisionWorkspaceOpts flips
+	// last_heartbeat_at with the new session. Issue #19 Layer 1.
+	restartData := loadRestartContextData(ctx, id)
+
 	go h.provisionWorkspaceOpts(id, templatePath, configFiles, payload, resetClaudeSession)
+	go h.sendRestartContext(id, restartData)
 
 	c.JSON(http.StatusOK, gin.H{"status": "provisioning", "config_dir": configLabel, "reset_session": resetClaudeSession})
 }
@@ -219,8 +224,13 @@ func (h *WorkspaceHandler) RestartByID(workspaceID string) {
 	// Runtime from DB — no more config file parsing
 	payload := models.CreateWorkspacePayload{Name: wsName, Tier: tier, Runtime: dbRuntime}
 
+	// Snapshot restart-context data before the new session overwrites
+	// last_heartbeat_at. Issue #19 Layer 1.
+	restartData := loadRestartContextData(ctx, workspaceID)
+
 	// On auto-restart, do NOT re-apply templates — preserve existing config volume.
 	go h.provisionWorkspace(workspaceID, "", nil, payload)
+	go h.sendRestartContext(workspaceID, restartData)
 }
 
 // Pause handles POST /workspaces/:id/pause