diff --git a/platform/internal/middleware/tenant_guard.go b/platform/internal/middleware/tenant_guard.go
index a48c0bbc..d59b37af 100644
--- a/platform/internal/middleware/tenant_guard.go
+++ b/platform/internal/middleware/tenant_guard.go
@@ -7,6 +7,16 @@ import (
 	"github.com/gin-gonic/gin"
 )
 
+// flyReplaySrcHeader is the header Fly injects on requests it replays via
+// the `fly-replay: ...;state=...` mechanism. Format is a semicolon-
+// separated list of k=v pairs, e.g.
+//   instance=91854...;region=ord;t=1700000000000;state=org-id=<uuid>
+// We care only about the `state=` segment; the control plane encodes
+// the org id as `state=org-id=<uuid>` so we can treat it equivalently
+// to the X-Molecule-Org-Id header.
+const flyReplaySrcHeader = "Fly-Replay-Src"
+const flyReplayStatePrefix = "org-id="
+
 // Tenant-mode guard — public repo's only SaaS hook.
 //
 // The SaaS control plane (private `molecule-controlplane` repo) provisions one
@@ -58,12 +68,45 @@ func TenantGuardWithOrgID(configuredOrgID string) gin.HandlerFunc {
 			c.Next()
 			return
 		}
-		if c.GetHeader(tenantOrgIDHeader) != configuredOrgID {
-			// 404 not 403 — existence of this tenant must not be inferable by
-			// probing other orgs' machines.
-			c.AbortWithStatus(404)
+		// Primary: explicit X-Molecule-Org-Id header (direct access path,
+		// e.g. from molecli or internal tooling that sets it directly).
+		if c.GetHeader(tenantOrgIDHeader) == configuredOrgID {
+			c.Next()
 			return
 		}
-		c.Next()
+		// Secondary: org id encoded in Fly-Replay-Src state by the control
+		// plane. This is the path every production request takes, because
+		// response headers set by the cp don't travel to the replayed
+		// tenant — only the state= param does.
+		if orgIDFromReplaySrc(c.GetHeader(flyReplaySrcHeader)) == configuredOrgID {
+			c.Next()
+			return
+		}
+		// 404 not 403 — existence of this tenant must not be inferable by
+		// probing other orgs' machines.
+		c.AbortWithStatus(404)
 	}
 }
+
+// orgIDFromReplaySrc extracts the org id the control plane encoded via
+// `state=org-id=<uuid>` in the fly-replay response header. Returns "" if
+// the header is missing, malformed, or the state segment isn't ours.
+// Separated from TenantGuardWithOrgID so tests can round-trip header →
+// id without spinning a full Gin context.
+func orgIDFromReplaySrc(header string) string {
+	if header == "" {
+		return ""
+	}
+	for _, seg := range strings.Split(header, ";") {
+		seg = strings.TrimSpace(seg)
+		const statePrefix = "state="
+		if !strings.HasPrefix(seg, statePrefix) {
+			continue
+		}
+		value := seg[len(statePrefix):]
+		if strings.HasPrefix(value, flyReplayStatePrefix) {
+			return value[len(flyReplayStatePrefix):]
+		}
+	}
+	return ""
+}
diff --git a/platform/internal/middleware/tenant_guard_test.go b/platform/internal/middleware/tenant_guard_test.go
index 97c0679c..034e4dda 100644
--- a/platform/internal/middleware/tenant_guard_test.go
+++ b/platform/internal/middleware/tenant_guard_test.go
@@ -82,6 +82,58 @@ func TestTenantGuard_AllowlistBypassesCheck(t *testing.T) {
 	}
 }
 
+// Fly-Replay-Src state path: the production path. Control plane sends the
+// org id as `state=org-id=<uuid>` via fly-replay; Fly injects that into
+// the replayed request as a segment of the Fly-Replay-Src header.
+func TestTenantGuard_AcceptsFlyReplaySrcState(t *testing.T) {
+	gin.SetMode(gin.TestMode)
+	r := gin.New()
+	r.Use(TenantGuardWithOrgID("org-abc"))
+	r.GET("/workspaces", func(c *gin.Context) { c.String(200, "ok") })
+
+	req := httptest.NewRequest("GET", "/workspaces", nil)
+	req.Header.Set("Fly-Replay-Src", "instance=src-123;region=ord;t=1700000000000;state=org-id=org-abc")
+	w := httptest.NewRecorder()
+	r.ServeHTTP(w, req)
+
+	if w.Code != 200 {
+		t.Errorf("Fly-Replay-Src state match: expected 200, got %d", w.Code)
+	}
+}
+
+func TestTenantGuard_RejectsFlyReplaySrcMismatch(t *testing.T) {
+	gin.SetMode(gin.TestMode)
+	r := gin.New()
+	r.Use(TenantGuardWithOrgID("org-abc"))
+	r.GET("/workspaces", func(c *gin.Context) { c.String(200, "ok") })
+
+	req := httptest.NewRequest("GET", "/workspaces", nil)
+	req.Header.Set("Fly-Replay-Src", "state=org-id=org-xyz")
+	w := httptest.NewRecorder()
+	r.ServeHTTP(w, req)
+
+	if w.Code != 404 {
+		t.Errorf("mismatched Fly-Replay-Src state: expected 404, got %d", w.Code)
+	}
+}
+
+func TestOrgIDFromReplaySrc(t *testing.T) {
+	cases := map[string]string{
+		"instance=x;region=ord;state=org-id=abc-123":             "abc-123",
+		"state=org-id=abc-123;instance=x":                        "abc-123",
+		"   state=org-id=abc-123  ":                              "abc-123",
+		"state=other=foo;instance=x":                             "",  // wrong state key
+		"instance=x;region=ord":                                  "",  // no state
+		"":                                                       "",  // empty header
+		"garbage":                                                "",  // unparseable
+	}
+	for in, want := range cases {
+		if got := orgIDFromReplaySrc(in); got != want {
+			t.Errorf("orgIDFromReplaySrc(%q) = %q, want %q", in, got, want)
+		}
+	}
+}
+
 // The allowlist is exact-match, not prefix. "/health/debug" must NOT bypass.
 func TestTenantGuard_AllowlistIsExactMatch(t *testing.T) {
 	gin.SetMode(gin.TestMode)