diff --git a/ems/src/main/java/org/openremote/extension/ems/manager/gopacs/GOPACSAddressBookResource.java b/ems/src/main/java/org/openremote/extension/ems/manager/gopacs/GOPACSAddressBookResource.java
index 6ed4a0d..a119a08 100644
--- a/ems/src/main/java/org/openremote/extension/ems/manager/gopacs/GOPACSAddressBookResource.java
+++ b/ems/src/main/java/org/openremote/extension/ems/manager/gopacs/GOPACSAddressBookResource.java
@@ -19,17 +19,22 @@
  */
 package org.openremote.extension.ems.manager.gopacs;
 
-import jakarta.ws.rs.Consumes;
 import jakarta.ws.rs.GET;
+import jakarta.ws.rs.HeaderParam;
 import jakarta.ws.rs.Path;
-import jakarta.ws.rs.QueryParam;
+import jakarta.ws.rs.PathParam;
+import jakarta.ws.rs.Produces;
 import jakarta.ws.rs.core.Response;
 
-import static jakarta.ws.rs.core.MediaType.APPLICATION_XML;
+import static jakarta.ws.rs.core.MediaType.APPLICATION_JSON;
 
-@Path("v2/participants/DSO")
+@Path("uftp-participants/v3/participants")
 public interface GOPACSAddressBookResource {
     @GET
-    @Consumes(APPLICATION_XML)
-    Response fetchParticipants(@QueryParam("contractedEan") String contractedEan);
+    @Path("{uftpDomainName}")
+    @Produces(APPLICATION_JSON)
+    Response fetchParticipantByDomain(
+        @HeaderParam("Authorization") String authorization,
+        @PathParam("uftpDomainName") String uftpDomainName
+    );
 }
diff --git a/ems/src/main/java/org/openremote/extension/ems/manager/gopacs/GOPACSHandler.java b/ems/src/main/java/org/openremote/extension/ems/manager/gopacs/GOPACSHandler.java
index 5443d56..0871d66 100644
--- a/ems/src/main/java/org/openremote/extension/ems/manager/gopacs/GOPACSHandler.java
+++ b/ems/src/main/java/org/openremote/extension/ems/manager/gopacs/GOPACSHandler.java
@@ -25,7 +25,6 @@
 import com.fasterxml.jackson.databind.ObjectMapper;
 import jakarta.ws.rs.WebApplicationException;
 import jakarta.ws.rs.core.Application;
-import jakarta.ws.rs.core.GenericType;
 import jakarta.ws.rs.core.Response;
 import org.jboss.resteasy.client.jaxrs.ResteasyClient;
 import org.lfenergy.shapeshifter.api.*;
@@ -88,8 +87,10 @@ public class GOPACSHandler implements UftpPayloadHandler, UftpParticipantService
 
     private static final Logger LOG = SyslogCategory.getLogger(API, GOPACSHandler.class);
     public static final String GOPACS_PRIVATE_KEY_FILE = "GOPACS_PRIVATE_KEY_FILE";
+    public static final String GOPACS_BROKER_URL = "GOPACS_BROKER_URL";
+    public static final String DEFAULT_GOPACS_BROKER_URL = "https://clc-message-broker.gopacs-services.eu";
     public static final String GOPACS_PARTICIPANT_URL = "GOPACS_PARTICIPANT_URL";
-    public static final String DEFAULT_GOPACS_PARTICIPANT_URL = "https://clc-message-broker.gopacs-services.eu";
+    public static final String DEFAULT_GOPACS_PARTICIPANT_URL = "https://api.gopacs-services.eu";
     public static final String GOPACS_OAUTH2_URL = "GOPACS_OAUTH2_URL";
     public static final String DEFAULT_GOPACS_OAUTH2_URL = "https://auth.gopacs-services.eu/realms/gopacs/protocol/openid-connect/token";
     public static final String GOPACS_CLIENT_ID = "GOPACS_CLIENT_ID";
@@ -107,6 +108,7 @@ public class GOPACSHandler implements UftpPayloadHandler, UftpParticipantService
     protected final String contractedEAN;
     protected final String electricitySupplierAssetId;
     protected final String realm;
+    protected final String gopacsBrokerUrl;
     protected final Map<String, UftpParticipantInformation> participants;
 
     protected final AssetProcessingService assetProcessingService;
@@ -160,6 +162,8 @@ protected GOPACSHandler(String contractedEAN, String realm, String electricitySu
         this.timerService = container.getService(TimerService.class);
         this.webService = container.getService(WebService.class);
 
+        // Strip trailing slashes so the synthesised broker endpoint never contains a double slash
+        this.gopacsBrokerUrl = container.getConfig().getOrDefault(GOPACS_BROKER_URL, DEFAULT_GOPACS_BROKER_URL).replaceAll("/+$", "");
         this.responseDelaySeconds = Integer.parseInt(container.getConfig().getOrDefault(GOPACS_RESPONSE_DELAY_SECONDS, DEFAULT_GOPACS_RESPONSE_DELAY_SECONDS));
         this.flexOfferDelaySeconds = Integer.parseInt(container.getConfig().getOrDefault(GOPACS_FLEX_OFFER_DELAY_SECONDS, DEFAULT_GOPACS_FLEX_OFFER_DELAY_SECONDS));
 
@@ -293,8 +297,15 @@ public void notifyNewOutgoingMessage(OutgoingUftpMessage<? extends PayloadMessag
     @Override
     public String getAuthorizationHeader(UftpParticipant uftpParticipant) {
         LOG.fine("Getting authorization header for: " + uftpParticipant);
+        String authorization = fetchBearerToken();
+        if (authorization.isBlank()) {
+            LOG.warning("No OAuth2 bearer token available for authorization header of " + uftpParticipant);
+        }
+        return authorization;
+    }
+
+    protected String fetchBearerToken() {
         try {
-            // Perform OAuth2 client credentials flow
             try (Response response = gopacsAuthResource.getAccessToken(
                     "client_credentials",
                     this.clientId,
@@ -303,13 +314,10 @@ public String getAuthorizationHeader(UftpParticipant uftpParticipant) {
                 if (response.getStatus() == 200) {
                     String responseBody = response.readEntity(String.class);
                     OAuth2TokenResponse tokenResponse = objectMapper.readValue(responseBody, OAuth2TokenResponse.class);
-
-                    // Return Bearer token header
                     return "Bearer " + tokenResponse.getAccessToken();
-                } else {
-                    LOG.warning("OAuth2 token request failed with status: " + response.getStatus());
-                    return "";
                 }
+                LOG.warning("OAuth2 token request failed with status: " + response.getStatus());
+                return "";
             }
         } catch (Exception e) {
             LOG.log(Level.SEVERE, "Failed to obtain OAuth2 access token", e);
@@ -493,6 +501,20 @@ protected long calculatePower(FlexRequestISPType requestIsp) {
         return requestIsp.getMaxPower() == 0 ? requestIsp.getMinPower() : requestIsp.getMaxPower();
     }
 
+    /**
+     * Only act on flex messages whose congestion point matches this handler's contracted EAN. Returns
+     * false (and logs a warning) for out-of-scope messages so they are dropped before any asset mutation
+     * or outbound response. See issue #28 for full per-contract/role scoping via the V3 contracts endpoint.
+     */
+    protected boolean isWithinContractedScope(String messageType, String conversationId, String congestionPoint) {
+        if (contractedEAN.equals(congestionPoint)) {
+            return true;
+        }
+        LOG.warning("Rejecting " + messageType + " " + conversationId + " for out-of-scope congestion point "
+                + congestionPoint + " (contracted EAN " + contractedEAN + ")");
+        return false;
+    }
+
     protected void processRawMessage(String transportXml) {
         try {
             SignedMessage signedMessage = serializer.fromSignedXml(transportXml);
@@ -501,6 +523,19 @@ protected void processRawMessage(String transportXml) {
                 LOG.fine("Received message:" + payloadXml);
             }
             PayloadMessageType payloadMessage = serializer.fromPayloadXml(payloadXml);
+
+            // Re-assert EAN scoping that the V2 participant lookup enforced implicitly. The V3 lookup
+            // resolves any participant domain, so a validly signed flex message from a participant outside
+            // this handler's contracted EAN would otherwise be applied to the asset. Full per-contract/role
+            // scoping via the V3 contracts endpoint is tracked in issue #28.
+            // Out-of-scope messages are intentionally dropped here: the transport call still returns 200
+            // (signed envelope accepted) but no FlexRequestResponse/FlexOffer is sent in reply.
+            if (payloadMessage instanceof FlexMessageType flexMessage
+                    && !isWithinContractedScope(payloadMessage.getClass().getSimpleName(),
+                            payloadMessage.getConversationID(), flexMessage.getCongestionPoint())) {
+                return;
+            }
+
             var incomingUftpMessage = IncomingUftpMessage.create(new UftpParticipant(signedMessage), payloadMessage, transportXml, payloadXml);
             notifyNewIncomingMessage(incomingUftpMessage);
 
@@ -545,23 +580,29 @@ protected void processRawMessage(String transportXml) {
     public Optional<UftpParticipantInformation> getParticipantInformation(USEFRoleType role, String domain) {
         if (participants.containsKey(domain)) {
             return Optional.of(participants.get(domain));
-        } else {
-            try (Response response = gopacsAddressBookResource.fetchParticipants(contractedEAN)) {
-                if (response != null && response.getStatus() == 200) {
-                    List<UftpParticipantInformation> participants = response.readEntity(new GenericType<>() {
-                    });
-                    for (UftpParticipantInformation participant : participants) {
-                        this.participants.put(participant.domain(), new UftpParticipantInformation(participant.domain(), participant.publicKey(), participant.endpoint(), true));
-                    }
-                    return participants.stream().filter(p -> p.domain().equals(domain)).findFirst();
-                }
-            } catch (Exception e) {
-                if (e.getCause() != null && e.getCause() instanceof IOException) {
-                    LOG.log(Level.SEVERE, "Exception when requesting participant information", e.getCause());
-                } else {
-                    LOG.log(Level.SEVERE, "Exception when requesting participant information", e);
-                }
+        }
+
+        String authorization = fetchBearerToken();
+        if (authorization.isBlank()) {
+            LOG.warning("Skipping participant lookup for " + domain + ": no OAuth2 bearer token available");
+            return Optional.empty();
+        }
+        try (Response response = gopacsAddressBookResource.fetchParticipantByDomain(authorization, domain)) {
+            int status = response != null ? response.getStatus() : -1;
+            if (status == 200) {
+                ParticipantView view = response.readEntity(ParticipantView.class);
+                UftpParticipantInformation info = new UftpParticipantInformation(view.domain(), view.publicKey(), this.gopacsBrokerUrl + "/shapeshifter/api/v3/message", true);
+                participants.put(view.domain(), info);
+                return Optional.of(info);
             }
+            if (status == 404) {
+                LOG.fine("Participant not found in GOPACS address book: " + domain);
+            } else {
+                LOG.severe("Unexpected status " + status + " when requesting participant information for " + domain);
+            }
+        } catch (Exception e) {
+            Throwable cause = e.getCause() instanceof IOException ? e.getCause() : e;
+            LOG.log(Level.SEVERE, "Exception when requesting participant information for " + domain, cause);
         }
         return Optional.empty();
     }
diff --git a/ems/src/main/java/org/openremote/extension/ems/manager/gopacs/ParticipantView.java b/ems/src/main/java/org/openremote/extension/ems/manager/gopacs/ParticipantView.java
new file mode 100644
index 0000000..f956a3a
--- /dev/null
+++ b/ems/src/main/java/org/openremote/extension/ems/manager/gopacs/ParticipantView.java
@@ -0,0 +1,25 @@
+/*
+ * Copyright 2026, OpenRemote Inc.
+ *
+ * See the CONTRIBUTORS.txt file in the distribution for a
+ * full listing of individual contributors.
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU Affero General Public License as
+ * published by the Free Software Foundation, either version 3 of the
+ * License, or (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+ * GNU Affero General Public License for more details.
+ *
+ * You should have received a copy of the GNU Affero General Public License
+ * along with this program. If not, see <http://www.gnu.org/licenses/>.
+ */
+package org.openremote.extension.ems.manager.gopacs;
+
+import com.fasterxml.jackson.annotation.JsonIgnoreProperties;
+
+@JsonIgnoreProperties(ignoreUnknown = true)
+public record ParticipantView(String domain, String publicKey) {}
diff --git a/gradle.properties b/gradle.properties
index db4133a..80a6b61 100644
--- a/gradle.properties
+++ b/gradle.properties
@@ -18,5 +18,5 @@ openremoteVersion = 1.22.1
 bouncyCastleVersion = 1.81
 jacksonVersion = 2.21.2
 resteasyVersion = 6.2.15.Final
-shapeshifterVersion = 3.2.2
+shapeshifterVersion = 3.5.0
 testLoggerVersion = 4.0.0