diff --git a/CHANGELOG.md b/CHANGELOG.md
index 680b27c..0e9ad53 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,5 +1,15 @@
 # Changelog
 
+## Unreleased — `Gateway.InstallationID` exposes the stamped installation
+
+`Gateway` exposed `OrgID()` but not the installation beside it, so a handler
+that needed the installation it serves either re-parsed
+`x-codefly-installation-id` itself or took an id from the browser.
+`Gateway.InstallationID()` returns the installation the gateway stamped, as the
+runtime accepted it — the same value every `ForModule` mint names. It is empty
+when the request carries none or the header is blank (which the runtime
+refuses), and on an install step's gateway. Additive; no signature changes.
+
 ## Unreleased — a 4xx refusal reaches the page with its reason
 
 The host gateway refuses a module route in `text/plain`, one line naming the
diff --git a/README.md b/README.md
index f213812..93659f2 100644
--- a/README.md
+++ b/README.md
@@ -886,6 +886,15 @@ non-2xx answer is a `*GatewayError` carrying the status.
 `Gateway.OrgID` is the viewer's active organization, the same one `ForModule`
 mints in. A handler scoping a module read to a tenant names this one.
 
+`Gateway.InstallationID` is the installation the gateway stamped for the
+request (`x-codefly-installation-id`), the one that admitted the viewer and the
+one every `ForModule` mint names. A handler placing a resource under an
+installation — asking the host for it and using its scope — names this one
+rather than reading the header itself or taking an id from the page. It is
+empty when the request carries none, and when the header is blank, which the
+runtime refuses: a refused value is never handed back. An install step's
+gateway has none either; that step is given its `Installation`.
+
 `Gateway.WorkContextPrincipals` reports whom a `ForModule` capability was
 issued for — the organization, the Task's owner and its current actor, as
 accounts answered the mint — so a handler compares a claimed owner against
diff --git a/solution.go b/solution.go
index dd375d6..6e1742e 100644
--- a/solution.go
+++ b/solution.go
@@ -1405,6 +1405,20 @@ func (g *Gateway) BaseURL() string { return g.baseURL }
 // a module read to a tenant names this one rather than resolving another.
 func (g *Gateway) OrgID() string { return g.orgID }
 
+// InstallationID is the installation the gateway stamped for this request: the
+// x-codefly-installation-id identity header it sets from the same verified
+// claims as x-org-id, the installation that admitted the viewer, never anything
+// the browser sent. It is the value this runtime accepted, the one every
+// ForModule mint names, so a handler placing a resource under an installation
+// names this one rather than taking an id from the page.
+//
+// It is empty when the request carries none — an org-less caller, or a host
+// that does not confine solution traffic by installation — and when the header
+// is blank, which this runtime refuses rather than mints under; a refused value
+// is never handed back. It is empty on the gateway an install step receives as
+// well, whose installation is the Installation that step is given.
+func (g *Gateway) InstallationID() string { return g.installationID }
+
 // HTTPClient returns an http.Client that injects the caller's bearer — and, on
 // a gateway derived by ForModule, the viewer's Work Context — on every request.
 // During an install step it carries the supplied installation Work Context.
diff --git a/solution_test.go b/solution_test.go
index 05b2ca9..9e4545e 100644
--- a/solution_test.go
+++ b/solution_test.go
@@ -1936,3 +1936,89 @@ func TestFrontendReaderRetainsCoreLimitsAndExtensibleClientKinds(t *testing.T) {
 		}
 	}
 }
+
+// TestGatewayExposesTheStampedInstallation: a handler reads the installation
+// the gateway stamped through Gateway.InstallationID, so it never re-parses the
+// header or takes one from the page — and gets back only what this runtime
+// accepted, the same value every ForModule mint names.
+func TestGatewayExposesTheStampedInstallation(t *testing.T) {
+	mint := newHostMint(t, &hostMint{})
+	solution := boot(t, New(Manifest{ID: testSolutionID}).
+		Handle("/installation", func(_ context.Context, gw *Gateway) (any, error) {
+			return map[string]string{"installation": gw.InstallationID()}, nil
+		}), mint)
+
+	served := func(t *testing.T, stamp func(http.Header)) string {
+		t.Helper()
+		request, err := http.NewRequest(http.MethodGet, solution.base+"/installation", nil)
+		if err != nil {
+			t.Fatal(err)
+		}
+		request.Header.Set("authorization", viewerBearer())
+		request.Header.Set(orgHeader, testOrganization)
+		request.Header.Set(sessionHeader, "session-1")
+		stamp(request.Header)
+		resp, err := solution.client.Do(request)
+		if err != nil {
+			t.Fatalf("call: %v", err)
+		}
+		defer func() { _ = resp.Body.Close() }()
+		if resp.StatusCode != http.StatusOK {
+			t.Fatalf("handler answered %d, want 200", resp.StatusCode)
+		}
+		var body map[string]string
+		if err := json.NewDecoder(resp.Body).Decode(&body); err != nil {
+			t.Fatal(err)
+		}
+		return body["installation"]
+	}
+
+	t.Run("a stamped installation is exposed", func(t *testing.T) {
+		got := served(t, func(h http.Header) { h.Set(workcontext.InstallationIDHeaderName, testInstallation) })
+		if got != testInstallation {
+			t.Errorf("InstallationID() = %q, want the stamped %q", got, testInstallation)
+		}
+	})
+
+	t.Run("an unstamped request exposes none", func(t *testing.T) {
+		if got := served(t, func(http.Header) {}); got != "" {
+			t.Errorf("InstallationID() = %q for a request the gateway stamped no installation on, want empty", got)
+		}
+	})
+
+	t.Run("a caller-supplied capability does not name it", func(t *testing.T) {
+		// The capability carrier is caller-controlled inbound; with no stamped
+		// header there is no installation, whatever the capability says.
+		got := served(t, func(h http.Header) { h.Set(workcontext.HeaderName, capability("forged-by-the-caller")) })
+		if got != "" {
+			t.Errorf("InstallationID() = %q from a caller-supplied capability, want empty", got)
+		}
+	})
+
+	t.Run("a refused header is never handed back", func(t *testing.T) {
+		// Straight to gatewayFor: the HTTP server trims a blank value to
+		// nothing before a handler sees it, so only this reaches the refusal.
+		for _, raw := range []string{"   ", "\t"} {
+			header := http.Header{}
+			header.Set("authorization", viewerBearer())
+			header.Set(orgHeader, testOrganization)
+			header.Set(sessionHeader, "session-1")
+			header.Set(workcontext.InstallationIDHeaderName, raw)
+			gw := solution.server.gatewayFor(header)
+			if gw.installationErr == nil {
+				t.Fatalf("the blank installation %q was accepted, so this case does not exercise the refusal", raw)
+			}
+			if got := gw.InstallationID(); got != "" {
+				t.Errorf("InstallationID() = %q for the refused header %q, want empty", got, raw)
+			}
+		}
+	})
+
+	t.Run("it is the value accepted, not the raw header", func(t *testing.T) {
+		header := http.Header{}
+		header.Set(workcontext.InstallationIDHeaderName, " "+testInstallation+" ")
+		if got := solution.server.gatewayFor(header).InstallationID(); got != testInstallation {
+			t.Errorf("InstallationID() = %q, want the accepted %q", got, testInstallation)
+		}
+	})
+}