diff --git a/internal/gateway/fallback.go b/internal/gateway/fallback.go
index be1e6f5c..04069242 100644
--- a/internal/gateway/fallback.go
+++ b/internal/gateway/fallback.go
@@ -987,6 +987,10 @@ type holdWriter struct {
 	// reasoning keeps the agent from its idle timeout with SSE comments
 	// meanwhile (#751)
 	alive *keptAlive
+	// notes are the headers magpie says of the try (noteMember), set
+	// under mu: all keepAlive sends of the try's before it has its status,
+	// as until then the try is still writing header, with no lock
+	notes http.Header
 
 	first firstToken // when its first content and text came (#196)
 
@@ -1026,6 +1030,18 @@ func newHoldWriter(w http.ResponseWriter, hold bool) *holdWriter {
 
 func (h *holdWriter) Header() http.Header { return h.header }
 
+// note sets a header magpie says of the try, which keepAlive may send
+// from watch's goroutine before the try has its status.
+func (h *holdWriter) note(k, v string) {
+	h.mu.Lock()
+	defer h.mu.Unlock()
+	h.header.Set(k, v)
+	if h.notes == nil {
+		h.notes = http.Header{}
+	}
+	h.notes.Set(k, v)
+}
+
 func (h *holdWriter) WriteHeader(code int) {
 	h.mu.Lock()
 	defer h.mu.Unlock()
@@ -1244,10 +1260,17 @@ func (h *holdWriter) keepAlive() {
 		return
 	}
 	if !a.sent {
+		// the try's header is whole once it has its status (WriteHeader,
+		// under mu); before, the vendor's are still being copied into it
+		// from the try's goroutine, and only magpie's notes are read
+		src := h.header
+		if h.status == 0 {
+			src = h.notes
+		}
 		dst := h.w.Header()
-		for k, v := range h.header {
+		for k, v := range src {
 			if k != resetsHeader && k != refusedHeader && k != "Content-Length" {
-				dst[k] = v
+				dst[k] = slices.Clone(v)
 			}
 		}
 		dst.Set("Content-Type", "text/event-stream; charset=utf-8")
diff --git a/internal/gateway/member.go b/internal/gateway/member.go
index bd46f4ef..6b728d2b 100644
--- a/internal/gateway/member.go
+++ b/internal/gateway/member.go
@@ -43,9 +43,15 @@ type memberKey struct{}
 // and tells the request's memberWriter, if it has one.
 func noteMember(w http.ResponseWriter, r *http.Request, p provider.Provider, model string) {
 	id := p.ID + "/" + model
-	h := w.Header()
-	h.Set(providerHeader, p.ID)
-	h.Set(modelHeader, id)
+	if hw, ok := w.(*holdWriter); ok {
+		// watch's keepAlive may send them meanwhile
+		hw.note(providerHeader, p.ID)
+		hw.note(modelHeader, id)
+	} else {
+		h := w.Header()
+		h.Set(providerHeader, p.ID)
+		h.Set(modelHeader, id)
+	}
 	if mw, _ := r.Context().Value(memberKey{}).(*memberWriter); mw != nil {
 		mw.setMember(id)
 	}
diff --git a/internal/gateway/quiet_keepalive_test.go b/internal/gateway/quiet_keepalive_test.go
index 521f68f1..89117360 100644
--- a/internal/gateway/quiet_keepalive_test.go
+++ b/internal/gateway/quiet_keepalive_test.go
@@ -5,6 +5,7 @@ import (
 	"io"
 	"net/http"
 	"net/http/httptest"
+	"strconv"
 	"strings"
 	"sync"
 	"sync/atomic"
@@ -270,3 +271,41 @@ func TestQuietStreamsKeptAliveEveryProtocol(t *testing.T) {
 		}
 	}
 }
+
+// A vendor slow to answer at all gets its stream's agent kept alive from
+// watch's goroutine while the try, once the vendor answers, copies the
+// vendor's headers in: keepAlive reads only magpie's notes until the try
+// has its status, so the two never touch one map (a race, and in a build
+// without -race "concurrent map iteration and map write").
+func TestKeepAliveBeforeTheVendorsHeaders(t *testing.T) {
+	quietFast(t)
+	rec := httptest.NewRecorder()
+	h := newHoldWriter(rec, true)
+	h.streams, h.alive = true, &keptAlive{proto: provider.Responses}
+	r := httptest.NewRequest("POST", "/v1/responses", nil)
+	noteMember(h, r, provider.Provider{ID: "q"}, "m")
+	end := h.watch()
+	// the vendor's headers come in as passthrough copies them, the first
+	// comment going out meanwhile
+	deadline := time.Now().Add(5 * time.Second)
+	for i := 0; ; i++ {
+		h.Header().Set("X-Vendor", strconv.Itoa(i))
+		h.mu.Lock()
+		sent := h.alive.sent
+		h.mu.Unlock()
+		if sent && i > 100 || time.Now().After(deadline) {
+			break
+		}
+		time.Sleep(100 * time.Microsecond)
+	}
+	end()
+	if !h.alive.sent {
+		t.Fatal("no comment went out")
+	}
+	if got := rec.Header().Get(providerHeader); got != "q" {
+		t.Fatalf("the stream's headers before its reply lack magpie's notes: %q", got)
+	}
+	if !strings.Contains(rec.Body.String(), ": keepalive") {
+		t.Fatalf("body: %q", rec.Body.String())
+	}
+}