diff --git a/e2e/agentnetwork/chat_test.go b/e2e/agentnetwork/chat_test.go
index 6cd23c34e..5e3a79273 100644
--- a/e2e/agentnetwork/chat_test.go
+++ b/e2e/agentnetwork/chat_test.go
@@ -212,6 +212,10 @@ func TestProvidersMatrix(t *testing.T) {
 		t.Run(pc.name, func(t *testing.T) {
 			before, _ := srv.ListAccessLogs(ctx)
 
+			// Unique per provider so we can find this provider's row by its
+			// session id and confirm the marker propagated end-to-end.
+			sessionID := "e2e-session-" + pc.name
+
 			// Retry briefly to absorb tunnel/DNS jitter on the first call.
 			var code int
 			var body string
@@ -221,9 +225,9 @@ func TestProvidersMatrix(t *testing.T) {
 				var b string
 				var cerr error
 				if pc.kind == harness.WireVertex {
-					c, b, cerr = cl.Vertex(ctx, settings.Endpoint, proxyIP, pc.project, pc.region, pc.model, "Reply with exactly: pong")
+					c, b, cerr = cl.Vertex(ctx, settings.Endpoint, proxyIP, pc.project, pc.region, pc.model, "Reply with exactly: pong", sessionID)
 				} else {
-					c, b, cerr = cl.Chat(ctx, settings.Endpoint, proxyIP, pc.kind, pc.model, "Reply with exactly: pong")
+					c, b, cerr = cl.Chat(ctx, settings.Endpoint, proxyIP, pc.kind, pc.model, "Reply with exactly: pong", sessionID)
 				}
 				if cerr == nil {
 					code, body = c, b
@@ -239,6 +243,21 @@ func TestProvidersMatrix(t *testing.T) {
 				logs, lerr := srv.ListAccessLogs(ctx)
 				return lerr == nil && logs.TotalRecords > before.TotalRecords
 			}, 30*time.Second, 2*time.Second, "an access-log row should be ingested for %s", pc.name)
+
+			// The session id sent as x-session-id must round-trip into the
+			// access-log row for this provider.
+			require.Eventually(t, func() bool {
+				logs, lerr := srv.ListAccessLogs(ctx)
+				if lerr != nil {
+					return false
+				}
+				for _, r := range logs.Data {
+					if r.SessionId != nil && *r.SessionId == sessionID {
+						return true
+					}
+				}
+				return false
+			}, 30*time.Second, 2*time.Second, "session id %q must be recorded in an access-log row for %s", sessionID, pc.name)
 		})
 	}
 
diff --git a/e2e/harness/client.go b/e2e/harness/client.go
index 1552ad3d4..cf7ef8945 100644
--- a/e2e/harness/client.go
+++ b/e2e/harness/client.go
@@ -158,8 +158,9 @@ const (
 
 // Chat issues a chat-completion POST to the agent-network endpoint over the
 // client's tunnel, returning the HTTP status and response body. kind selects
-// the wire shape: WireChat (OpenAI) or WireMessages (Anthropic).
-func (cl *Client) Chat(ctx context.Context, endpoint, proxyIP, kind, model, prompt string) (int, string, error) {
+// the wire shape: WireChat (OpenAI) or WireMessages (Anthropic). A non-empty
+// sessionID is sent as the universal x-session-id header the proxy records.
+func (cl *Client) Chat(ctx context.Context, endpoint, proxyIP, kind, model, prompt, sessionID string) (int, string, error) {
 	var path, body string
 	var headers []string
 	switch kind {
@@ -171,17 +172,26 @@ func (cl *Client) Chat(ctx context.Context, endpoint, proxyIP, kind, model, prom
 		path = "/v1/chat/completions"
 		body = fmt.Sprintf(`{"model":%q,"messages":[{"role":"user","content":%q}]}`, model, prompt)
 	}
-	return cl.post(ctx, endpoint, proxyIP, path, body, headers)
+	return cl.post(ctx, endpoint, proxyIP, path, body, withSessionID(headers, sessionID))
 }
 
 // Vertex issues an Anthropic-on-Vertex rawPredict POST over the tunnel. Unlike
 // Chat, the model is carried in the request path (project/region/model), so the
 // proxy routes by path and mints the service-account OAuth token; the body uses
-// the Vertex anthropic_version rather than a model field.
-func (cl *Client) Vertex(ctx context.Context, endpoint, proxyIP, project, region, model, prompt string) (int, string, error) {
+// the Vertex anthropic_version rather than a model field. A non-empty sessionID
+// is sent as the universal x-session-id header the proxy records.
+func (cl *Client) Vertex(ctx context.Context, endpoint, proxyIP, project, region, model, prompt, sessionID string) (int, string, error) {
 	path := fmt.Sprintf("/v1/projects/%s/locations/%s/publishers/anthropic/models/%s:rawPredict", project, region, model)
 	body := fmt.Sprintf(`{"anthropic_version":"vertex-2023-10-16","max_tokens":64,"messages":[{"role":"user","content":%q}]}`, prompt)
-	return cl.post(ctx, endpoint, proxyIP, path, body, nil)
+	return cl.post(ctx, endpoint, proxyIP, path, body, withSessionID(nil, sessionID))
+}
+
+// withSessionID appends the x-session-id header when sessionID is non-empty.
+func withSessionID(headers []string, sessionID string) []string {
+	if sessionID == "" {
+		return headers
+	}
+	return append(headers, "x-session-id: "+sessionID)
 }
 
 // post runs curl in a throwaway container sharing the client's network