diff --git a/build.gradle.kts b/build.gradle.kts
index 12b6083..2c0d0a7 100644
--- a/build.gradle.kts
+++ b/build.gradle.kts
@@ -92,6 +92,13 @@ tasks.wrapper {
 java {
     toolchain.languageVersion.set(JavaLanguageVersion.of(21))
 }
+sourceSets {
+    main {
+        // bundles the official OMG spec (resources/openapi.json) so the
+        // /meta/datatypes endpoints can serve its schema definitions
+        resources.srcDir("resources")
+    }
+}
 kotlin {
     jvmToolchain(21)
 }
diff --git a/src/main/kotlin/org/openmbee/flexo/sysmlv2/AppMain.kt b/src/main/kotlin/org/openmbee/flexo/sysmlv2/AppMain.kt
index 482f3c5..6f2eb09 100644
--- a/src/main/kotlin/org/openmbee/flexo/sysmlv2/AppMain.kt
+++ b/src/main/kotlin/org/openmbee/flexo/sysmlv2/AppMain.kt
@@ -20,6 +20,7 @@ import io.ktor.server.routing.*
 import kotlinx.serialization.json.Json
 import io.ktor.server.application.ApplicationStopped
 import org.openmbee.flexo.sysmlv2.apis.*
+import org.openmbee.flexo.sysmlv2.models.Error
 
 lateinit var GlobalFlexoConfig: FlexoConfig
 lateinit var FlexoHttpClient: HttpClient
@@ -105,17 +106,26 @@ fun Application.module() {
     //install(HSTS, ApplicationHstsConfiguration()) // see https://ktor.io/docs/hsts.html
     install(Resources)
     install(StatusPages) {
+        // failures respond with the spec's Error shape so clients can
+        // parse them uniformly
         exception<InvalidSysmlSerializationError> { call, cause ->
-            call.respondText(cause.message ?: "Bad Request", status = HttpStatusCode.BadRequest)
+            call.respond(HttpStatusCode.BadRequest,
+                Error(Error.AtType.Error, cause.message ?: "Bad Request"))
         }
         exception<BadRequestException> { call, cause ->
-            call.respondText(cause.message ?: "Bad Request", status = HttpStatusCode.BadRequest)
+            call.respond(HttpStatusCode.BadRequest,
+                Error(Error.AtType.Error, cause.message ?: "Bad Request"))
         }
         exception<NotImplementedError> { call, cause ->
-            call.respondText(cause.message ?: "Not Implemented", status = HttpStatusCode.NotImplemented)
+            call.respond(HttpStatusCode.NotImplemented,
+                Error(Error.AtType.Error, cause.message ?: "Not Implemented"))
         }
         exception<Throwable> { call, cause ->
-            call.respondText(cause.message ?: "Internal Server Error", status = HttpStatusCode.InternalServerError)
+            // do not leak internal details (exception messages may contain
+            // IRIs, paths, or upstream internals)
+            call.application.log.error("unhandled exception", cause)
+            call.respond(HttpStatusCode.InternalServerError,
+                Error(Error.AtType.Error, "Internal Server Error"))
         }
     }
     routing {
diff --git a/src/main/kotlin/org/openmbee/flexo/sysmlv2/Http.kt b/src/main/kotlin/org/openmbee/flexo/sysmlv2/Http.kt
index c45e08b..596ef9e 100644
--- a/src/main/kotlin/org/openmbee/flexo/sysmlv2/Http.kt
+++ b/src/main/kotlin/org/openmbee/flexo/sysmlv2/Http.kt
@@ -1,6 +1,8 @@
 package org.openmbee.flexo.sysmlv2
 
 import io.ktor.http.*
+import io.ktor.server.plugins.*
+import io.ktor.server.request.*
 import io.ktor.server.response.*
 import io.ktor.server.routing.*
 
@@ -8,3 +10,30 @@ import io.ktor.server.routing.*
 suspend fun RoutingContext.notImplemented() {
     call.respondText("Not implemented", status= HttpStatusCode.NotImplemented)
 }
+
+/**
+ * Respond with a cursor page of [items]: ordered by id, resuming after
+ * page[after], bounded by page[size], with a Link rel="next" header when
+ * more items remain. With neither param set, responds with the unpaged
+ * historical list untouched.
+ */
+suspend inline fun <reified T : Any> RoutingContext.respondPage(
+    items: List<T>,
+    pageSize: Int?,
+    pageAfter: String?,
+    crossinline idOf: (T) -> String
+) {
+    if (pageSize == null && pageAfter == null) {
+        return call.respond(items)
+    }
+    if (pageSize != null && pageSize < 1) {
+        throw BadRequestException("page[size] must be a positive integer")
+    }
+    val remaining = items.sortedBy(idOf).filter { pageAfter == null || idOf(it) > pageAfter }
+    val page = if (pageSize == null) remaining else remaining.take(pageSize)
+    if (pageSize != null && remaining.size > pageSize) {
+        val nextUrl = "${call.request.path()}?page%5Bsize%5D=$pageSize&page%5Bafter%5D=${idOf(page.last())}"
+        call.response.headers.append(HttpHeaders.Link, "<$nextUrl>; rel=\"next\"")
+    }
+    call.respond(page)
+}
diff --git a/src/main/kotlin/org/openmbee/flexo/sysmlv2/Paths.kt b/src/main/kotlin/org/openmbee/flexo/sysmlv2/Paths.kt
index 92ed710..0d316c5 100644
--- a/src/main/kotlin/org/openmbee/flexo/sysmlv2/Paths.kt
+++ b/src/main/kotlin/org/openmbee/flexo/sysmlv2/Paths.kt
@@ -35,7 +35,7 @@ object Paths {
      * @param pageBefore Page before (optional)
      * @param pageSize Page size (optional)
      */
-    @Serializable @Resource("/projects/{projectId}/branches") class getBranchesByProject(val projectId: kotlin.String, val pageAfter: kotlin.String? = null, val pageBefore: kotlin.String? = null, val pageSize: kotlin.Int? = null)
+    @Serializable @Resource("/projects/{projectId}/branches") class getBranchesByProject(val projectId: kotlin.String, @SerialName("page[after]") val pageAfter: kotlin.String? = null, @SerialName("page[before]") val pageBefore: kotlin.String? = null, @SerialName("page[size]") val pageSize: kotlin.Int? = null)
 
     /**
      * Get branch by project and ID
@@ -71,7 +71,7 @@ object Paths {
      * @param pageBefore Page before (optional)
      * @param pageSize Page size (optional)
      */
-    @Serializable @Resource("/projects/{projectId}/commits/{commitId}/changes") class getChangesByProjectCommit(val projectId: kotlin.String, val commitId: kotlin.String, val pageAfter: kotlin.String? = null, val pageBefore: kotlin.String? = null, val pageSize: kotlin.Int? = null)
+    @Serializable @Resource("/projects/{projectId}/commits/{commitId}/changes") class getChangesByProjectCommit(val projectId: kotlin.String, val commitId: kotlin.String, @SerialName("page[after]") val pageAfter: kotlin.String? = null, @SerialName("page[before]") val pageBefore: kotlin.String? = null, @SerialName("page[size]") val pageSize: kotlin.Int? = null)
 
     /**
      * Get commit by project and ID
@@ -89,7 +89,7 @@ object Paths {
      * @param pageBefore Page before (optional)
      * @param pageSize Page size (optional)
      */
-    @Serializable @Resource("/projects/{projectId}/commits") class getCommitsByProject(val projectId: kotlin.String, val pageAfter: kotlin.String? = null, val pageBefore: kotlin.String? = null, val pageSize: kotlin.Int? = null)
+    @Serializable @Resource("/projects/{projectId}/commits") class getCommitsByProject(val projectId: kotlin.String, @SerialName("page[after]") val pageAfter: kotlin.String? = null, @SerialName("page[before]") val pageBefore: kotlin.String? = null, @SerialName("page[size]") val pageSize: kotlin.Int? = null)
 
     /**
      * Create commit by project
@@ -110,7 +110,7 @@ object Paths {
      * @param pageBefore Page before (optional)
      * @param pageSize Page size (optional)
      */
-    @Serializable @Resource("/projects/{projectId}/commits/{compareCommitId}/diff") class diff(val projectId: kotlin.String, val baseCommitId: kotlin.String, val compareCommitId: kotlin.String, val pageAfter: kotlin.String? = null, val pageBefore: kotlin.String? = null, val pageSize: kotlin.Int? = null)
+    @Serializable @Resource("/projects/{projectId}/commits/{compareCommitId}/diff") class diff(val projectId: kotlin.String, val baseCommitId: kotlin.String, val compareCommitId: kotlin.String, @SerialName("page[after]") val pageAfter: kotlin.String? = null, @SerialName("page[before]") val pageBefore: kotlin.String? = null, @SerialName("page[size]") val pageSize: kotlin.Int? = null)
 
     /**
      * Merge source commit(s) into a target branch
@@ -124,7 +124,7 @@ object Paths {
      * @param pageSize Page size (optional)
      * @param &#x60;data&#x60;  (optional)
      */
-    @Serializable @Resource("/projects/{projectId}/branches/{targetBranchId}/merge") class merge(val projectId: kotlin.String, val sourceCommitId: kotlin.collections.List<kotlin.String>, val targetBranchId: kotlin.String, val description: kotlin.String? = null, val pageAfter: kotlin.String? = null, val pageBefore: kotlin.String? = null, val pageSize: kotlin.Int? = null, val `data`: kotlin.collections.List<JsonObject>? = null)
+    @Serializable @Resource("/projects/{projectId}/branches/{targetBranchId}/merge") class merge(val projectId: kotlin.String, val sourceCommitId: kotlin.collections.List<kotlin.String>, val targetBranchId: kotlin.String, val description: kotlin.String? = null, @SerialName("page[after]") val pageAfter: kotlin.String? = null, @SerialName("page[before]") val pageBefore: kotlin.String? = null, @SerialName("page[size]") val pageSize: kotlin.Int? = null, val `data`: kotlin.collections.List<JsonObject>? = null)
 
     /**
      * Get element by project, commit and ID
@@ -146,7 +146,7 @@ object Paths {
      * @param pageBefore Page before (optional)
      * @param pageSize Page size (optional)
      */
-    @Serializable @Resource("/projects/{projectId}/commits/{commitId}/elements") class getElementsByProjectCommit(val projectId: kotlin.String, val commitId: kotlin.String, val excludeUsed: kotlin.Boolean? = null, val pageAfter: kotlin.String? = null, val pageBefore: kotlin.String? = null, val pageSize: kotlin.Int? = null)
+    @Serializable @Resource("/projects/{projectId}/commits/{commitId}/elements") class getElementsByProjectCommit(val projectId: kotlin.String, val commitId: kotlin.String, val excludeUsed: kotlin.Boolean? = null, @SerialName("page[after]") val pageAfter: kotlin.String? = null, @SerialName("page[before]") val pageBefore: kotlin.String? = null, @SerialName("page[size]") val pageSize: kotlin.Int? = null)
 
     /**
      * Get ProjectUsage that originates the provided element
@@ -167,7 +167,7 @@ object Paths {
      * @param pageBefore Page before (optional)
      * @param pageSize Page size (optional)
      */
-    @Serializable @Resource("/projects/{projectId}/commits/{commitId}/roots") class getRootsByProjectCommit(val projectId: kotlin.String, val commitId: kotlin.String, val excludeUsed: kotlin.Boolean? = null, val pageAfter: kotlin.String? = null, val pageBefore: kotlin.String? = null, val pageSize: kotlin.Int? = null)
+    @Serializable @Resource("/projects/{projectId}/commits/{commitId}/roots") class getRootsByProjectCommit(val projectId: kotlin.String, val commitId: kotlin.String, val excludeUsed: kotlin.Boolean? = null, @SerialName("page[after]") val pageAfter: kotlin.String? = null, @SerialName("page[before]") val pageBefore: kotlin.String? = null, @SerialName("page[size]") val pageSize: kotlin.Int? = null)
 
     /**
      * Get datatype by ID
@@ -183,7 +183,7 @@ object Paths {
      * @param pageBefore Page before (optional)
      * @param pageSize Page size (optional)
      */
-    @Serializable @Resource("/meta/datatypes") class getDatatypes(val pageAfter: kotlin.String? = null, val pageBefore: kotlin.String? = null, val pageSize: kotlin.Int? = null)
+    @Serializable @Resource("/meta/datatypes") class getDatatypes(@SerialName("page[after]") val pageAfter: kotlin.String? = null, @SerialName("page[before]") val pageBefore: kotlin.String? = null, @SerialName("page[size]") val pageSize: kotlin.Int? = null)
 
     /**
      * Delete project by ID
@@ -206,7 +206,7 @@ object Paths {
      * @param pageBefore Page before (optional)
      * @param pageSize Page size (optional)
      */
-    @Serializable @Resource("/projects") class getProjects(val pageAfter: kotlin.String? = null, val pageBefore: kotlin.String? = null, val pageSize: kotlin.Int? = null)
+    @Serializable @Resource("/projects") class getProjects(@SerialName("page[after]") val pageAfter: kotlin.String? = null, @SerialName("page[before]") val pageBefore: kotlin.String? = null, @SerialName("page[size]") val pageSize: kotlin.Int? = null)
 
     /**
      * Create project
@@ -239,7 +239,7 @@ object Paths {
      * @param pageBefore Page before (optional)
      * @param pageSize Page size (optional)
      */
-    @Serializable @Resource("/projects/{projectId}/queries") class getQueriesByProject(val projectId: kotlin.String, val pageAfter: kotlin.String? = null, val pageBefore: kotlin.String? = null, val pageSize: kotlin.Int? = null)
+    @Serializable @Resource("/projects/{projectId}/queries") class getQueriesByProject(val projectId: kotlin.String, @SerialName("page[after]") val pageAfter: kotlin.String? = null, @SerialName("page[before]") val pageBefore: kotlin.String? = null, @SerialName("page[size]") val pageSize: kotlin.Int? = null)
 
     /**
      * Get query by project and ID
@@ -259,7 +259,7 @@ object Paths {
      * @param pageBefore Page before (optional)
      * @param pageSize Page size (optional)
      */
-    @Serializable @Resource("/projects/{projectId}/query-results") class getQueryResultsByProjectIdQuery(val projectId: kotlin.String, val queryRequest: QueryRequest, val commitId: kotlin.String? = null, val pageAfter: kotlin.String? = null, val pageBefore: kotlin.String? = null, val pageSize: kotlin.Int? = null)
+    @Serializable @Resource("/projects/{projectId}/query-results") class getQueryResultsByProjectIdQuery(val projectId: kotlin.String, val queryRequest: QueryRequest, val commitId: kotlin.String? = null, @SerialName("page[after]") val pageAfter: kotlin.String? = null, @SerialName("page[before]") val pageBefore: kotlin.String? = null, @SerialName("page[size]") val pageSize: kotlin.Int? = null)
 
     /**
      * Get query results by project and query
@@ -271,7 +271,7 @@ object Paths {
      * @param pageBefore Page before (optional)
      * @param pageSize Page size (optional)
      */
-    @Serializable @Resource("/projects/{projectId}/queries/{queryId}/results") class getQueryResultsByProjectIdQueryId(val projectId: kotlin.String, val queryId: kotlin.String, val commitId: kotlin.String? = null, val pageAfter: kotlin.String? = null, val pageBefore: kotlin.String? = null, val pageSize: kotlin.Int? = null)
+    @Serializable @Resource("/projects/{projectId}/queries/{queryId}/results") class getQueryResultsByProjectIdQueryId(val projectId: kotlin.String, val queryId: kotlin.String, val commitId: kotlin.String? = null, @SerialName("page[after]") val pageAfter: kotlin.String? = null, @SerialName("page[before]") val pageBefore: kotlin.String? = null, @SerialName("page[size]") val pageSize: kotlin.Int? = null)
 
     /**
      * Get query results by project and query definition via POST
@@ -283,7 +283,7 @@ object Paths {
      * @param pageBefore Page before (optional)
      * @param pageSize Page size (optional)
      */
-    @Serializable @Resource("/projects/{projectId}/query-results") class getQueryResultsByProjectIdQueryPost(val projectId: kotlin.String, val queryRequest: QueryRequest, val commitId: kotlin.String? = null, val pageAfter: kotlin.String? = null, val pageBefore: kotlin.String? = null, val pageSize: kotlin.Int? = null)
+    @Serializable @Resource("/projects/{projectId}/query-results") class getQueryResultsByProjectIdQueryPost(val projectId: kotlin.String, val queryRequest: QueryRequest, val commitId: kotlin.String? = null, @SerialName("page[after]") val pageAfter: kotlin.String? = null, @SerialName("page[before]") val pageBefore: kotlin.String? = null, @SerialName("page[size]") val pageSize: kotlin.Int? = null)
 
     /**
      * Create query by project
@@ -314,7 +314,7 @@ object Paths {
      * @param pageBefore Page before (optional)
      * @param pageSize Page size (optional)
      */
-    @Serializable @Resource("/projects/{projectId}/commits/{commitId}/elements/{relatedElementId}/relationships") class getRelationshipsByProjectCommitRelatedElement(val projectId: kotlin.String, val commitId: kotlin.String, val relatedElementId: kotlin.String, val direction: kotlin.String? = null, val excludeUsed: kotlin.Boolean? = null, val pageAfter: kotlin.String? = null, val pageBefore: kotlin.String? = null, val pageSize: kotlin.Int? = null)
+    @Serializable @Resource("/projects/{projectId}/commits/{commitId}/elements/{relatedElementId}/relationships") class getRelationshipsByProjectCommitRelatedElement(val projectId: kotlin.String, val commitId: kotlin.String, val relatedElementId: kotlin.String, val direction: kotlin.String? = null, val excludeUsed: kotlin.Boolean? = null, @SerialName("page[after]") val pageAfter: kotlin.String? = null, @SerialName("page[before]") val pageBefore: kotlin.String? = null, @SerialName("page[size]") val pageSize: kotlin.Int? = null)
 
     /**
      * Delete tag by project and ID
@@ -340,7 +340,7 @@ object Paths {
      * @param pageBefore Page before (optional)
      * @param pageSize Page size (optional)
      */
-    @Serializable @Resource("/projects/{projectId}/tags") class getTagsByProject(val projectId: kotlin.String, val pageAfter: kotlin.String? = null, val pageBefore: kotlin.String? = null, val pageSize: kotlin.Int? = null)
+    @Serializable @Resource("/projects/{projectId}/tags") class getTagsByProject(val projectId: kotlin.String, @SerialName("page[after]") val pageAfter: kotlin.String? = null, @SerialName("page[before]") val pageBefore: kotlin.String? = null, @SerialName("page[size]") val pageSize: kotlin.Int? = null)
 
     /**
      * Create tag by project
diff --git a/src/main/kotlin/org/openmbee/flexo/sysmlv2/apis/BranchApi.kt b/src/main/kotlin/org/openmbee/flexo/sysmlv2/apis/BranchApi.kt
index 9e49148..6957fba 100644
--- a/src/main/kotlin/org/openmbee/flexo/sysmlv2/apis/BranchApi.kt
+++ b/src/main/kotlin/org/openmbee/flexo/sysmlv2/apis/BranchApi.kt
@@ -89,13 +89,14 @@ fun Route.BranchApi() {
             return@get forward(flexoResponse)
         }
         // parse the response model, convert it to JSON, and reply to client
-        call.respond(flexoResponse.parseModel {
+        val branches = flexoResponse.parseModel {
             model.listSubjectsWithProperty(RDF.type, MMS.Branch).filterDrop {
                 (it.hasProperty(SYSMLV2.DELETED) || it.uri.uriSuffix == "master")
             }.mapWith {
                 branchFromResponse(it.outgoing(), path.projectId, it.uri.uriSuffix)
             }.toList().filterNotNull()
-        })
+        }
+        respondPage(branches, path.pageSize, path.pageAfter) { it.atId }
     }
 
     get<Paths.getBranchByProjectAndId> { path ->
diff --git a/src/main/kotlin/org/openmbee/flexo/sysmlv2/apis/CommitApi.kt b/src/main/kotlin/org/openmbee/flexo/sysmlv2/apis/CommitApi.kt
index cec5999..39af837 100644
--- a/src/main/kotlin/org/openmbee/flexo/sysmlv2/apis/CommitApi.kt
+++ b/src/main/kotlin/org/openmbee/flexo/sysmlv2/apis/CommitApi.kt
@@ -29,6 +29,8 @@ import io.ktor.http.*
 import org.openmbee.flexo.sysmlv2.*
 import org.openmbee.flexo.sysmlv2.models.Commit
 import org.openmbee.flexo.sysmlv2.models.CommitRequest
+import org.openmbee.flexo.sysmlv2.models.DataIdentity
+import org.openmbee.flexo.sysmlv2.models.DataVersion
 
 import org.openmbee.flexo.sysmlv2.infrastructure.generateId
 import org.openmbee.flexo.sysmlv2.infrastructure.requireValidId
@@ -42,6 +44,21 @@ fun FlexoModelHandler.commitFromModel(
     properties: Map<Property, Set<RDFNode>?>,
     projectId: String
 ): Commit {
+    // expose previousCommit only when the parent is itself a user-visible
+    // commit: flexo auto-creates a root commit (its own mms:parent is
+    // rdf:nil) that this API hides from the commit list, so linking to it
+    // would produce a dangling reference. When the model lacks the parent's
+    // triples (e.g. a freshly created commit response), the linkage is
+    // unknowable here and stays null — clients can re-GET the commit.
+    val previousCommit = properties[MMS.parent]?.resource()
+        ?.takeIf { it != MMS.nil }
+        ?.let { model.getResource(it.uri) }
+        ?.takeIf { parent ->
+            parent.hasProperty(MMS.parent) &&
+                    parent.getProperty(MMS.parent).`object` != MMS.nil
+        }
+        ?.let { Identified(atId = it.uri.uriSuffix) }
+
     // generate commit object
     return Commit(
         atId = commitIri.uriSuffix,
@@ -51,10 +68,7 @@ fun FlexoModelHandler.commitFromModel(
         // dct:description for forward/backward compatibility.
         description = properties[MMS.message]?.literal()?: properties[DCTerms.description]?.literal()?: "",
         owningProject = Identified(atId = projectId),
-        //previousCommit = properties[MMS.parent]?.map {
-        //    Identified(atId = UUID.fromString(it.asResource().uri.uriSuffix))
-        //}?: emptyList()
-        previousCommit = null
+        previousCommit = previousCommit
     )
 }
 
@@ -74,21 +88,105 @@ fun JsonPrimitive.toRdfLiteralNode(): Node {
     return NodeFactory.createLiteralDT(content, datatype)
 }
 
+// index every element in the model by IRI as its API JSON shape
+fun FlexoModelHandler.elementsByIri(): Map<String, JsonObject> =
+    model.listSubjects().toList()
+        .filter { !it.isAnon && it.uri.startsWith(SYSMLV2.ELEMENT) }
+        .associate { it.uri to extractModelElementToJson(it.uri) }
+
+// change ids must be stable across requests: derive them from the commit
+// and element ids
+fun changeId(commitId: String, elementId: String): String =
+    java.util.UUID.nameUUIDFromBytes("$commitId:$elementId".toByteArray()).toString()
+
+/**
+ * Compute a commit's changes by diffing its model graph against its
+ * parent's (layer1 stores the raw SPARQL patch, which cannot be mapped
+ * back to per-element DataVersions).
+ *
+ * Returns (failureToForward, changes): a non-null failure must be
+ * forwarded to the client; null changes means the commit does not exist.
+ */
+suspend fun RoutingContext.computeCommitChanges(
+    projectId: String, commitId: String
+): Pair<FlexoResponse?, List<DataVersion>?> {
+    // resolve the commit and its parent from the commit collection
+    val commitsResponse = flexoRequestGet {
+        orgPath("/repos/$projectId/commits")
+    }
+    if (commitsResponse.isFailure()) return Pair(commitsResponse, null)
+    var found = false
+    var parentId: String? = null
+    commitsResponse.parseModel {
+        model.listSubjectsWithProperty(RDF.type, MMS.Commit).toList()
+            .firstOrNull { it.uri.uriSuffix == commitId }
+            ?.let { commit ->
+                found = true
+                parentId = commit.outgoing()[MMS.parent]?.resource()
+                    ?.takeIf { it != MMS.nil }?.uri?.uriSuffix
+            }
+    }
+    if (!found) return Pair(null, null)
+
+    val after = flexoRequestGet {
+        orgPath("/repos/$projectId/locks/Commit.$commitId/graph")
+    }
+    if (after.isFailure()) return Pair(after, null)
+    val afterElements = after.parseModel { elementsByIri() }
+
+    val beforeElements = parentId?.let { pid ->
+        val before = flexoRequestGet {
+            orgPath("/repos/$projectId/locks/Commit.$pid/graph")
+        }
+        // a parent graph that cannot be materialized (e.g. the empty root
+        // commit) diffs against an empty baseline
+        if (before.isFailure()) emptyMap() else before.parseModel { elementsByIri() }
+    } ?: emptyMap()
+
+    val changes = (afterElements.keys + beforeElements.keys).sorted().mapNotNull { iri ->
+        val payload = afterElements[iri]
+        // unchanged elements are not part of the commit's changes
+        if (payload == beforeElements[iri]) return@mapNotNull null
+        val elementId = iri.substringAfterLast(':')
+        DataVersion(
+            atId = changeId(commitId, elementId),
+            atType = DataVersion.AtType.DataVersion,
+            identity = DataIdentity(elementId, DataIdentity.AtType.DataIdentity),
+            payload = payload
+        )
+    }
+    return Pair(null, changes)
+}
+
 fun Route.CommitApi() {
-    get<Paths.getChangeByProjectCommitId> {
-        throw NotImplementedError()
+    get<Paths.getChangeByProjectCommitId> { params ->
+        requireValidId(params.projectId, "projectId")
+        requireValidId(params.commitId, "commitId")
+        requireValidId(params.changeId, "changeId")
+        val (failure, changes) = computeCommitChanges(params.projectId, params.commitId)
+        failure?.let { return@get forward(it) }
+        val change = changes?.firstOrNull { it.atId == params.changeId }
+            ?: return@get call.respond(HttpStatusCode.NotFound)
+        call.respond(change)
     }
 
-    get<Paths.getChangesByProjectCommit> {
-        throw NotImplementedError()
+    get<Paths.getChangesByProjectCommit> { params ->
+        requireValidId(params.projectId, "projectId")
+        requireValidId(params.commitId, "commitId")
+        val (failure, changes) = computeCommitChanges(params.projectId, params.commitId)
+        failure?.let { return@get forward(it) }
+        changes ?: return@get call.respond(HttpStatusCode.NotFound)
+        respondPage(changes, params.pageSize, params.pageAfter) { it.atId }
     }
 
     get<Paths.getCommitByProjectAndId> { getCommit ->
         requireValidId(getCommit.projectId, "projectId")
         requireValidId(getCommit.commitId, "commitId")
-        // submit GET request to retrieve project metadata
+        // fetch the whole commit collection: layer1's single-commit response
+        // does not include the parent commit's triples, which are needed to
+        // resolve previousCommit (and to hide flexo's auto-created root)
         val flexoResponse = flexoRequestGet {
-            orgPath("/repos/${getCommit.projectId}/commits/${getCommit.commitId}")
+            orgPath("/repos/${getCommit.projectId}/commits")
         }
 
         // forward failures to client
@@ -97,9 +195,12 @@ fun Route.CommitApi() {
         }
         // parse the response model, convert it to JSON, and reply to client
         val commit = flexoResponse.parseModel {
-            model.listSubjectsWithProperty(RDF.type, MMS.Commit).mapWith {
-                commitFromModel(it.uri, it.outgoing(), getCommit.projectId)
-            }.toList().firstOrNull()
+            model.listSubjectsWithProperty(RDF.type, MMS.Commit)
+                .toList()
+                .firstOrNull { it.uri.uriSuffix == getCommit.commitId }
+                // the hidden root commit is not addressable through this API
+                ?.takeIf { it.outgoing()[MMS.parent]?.resource() != MMS.nil }
+                ?.let { commitFromModel(it.uri, it.outgoing(), getCommit.projectId) }
         } ?: return@get call.respond(HttpStatusCode.NotFound)
         call.respond(commit)
     }
@@ -126,7 +227,7 @@ fun Route.CommitApi() {
             }
         }
         commits.sortByDescending {it.created}
-        call.respond(commits)
+        respondPage(commits, getCommits.pageSize, getCommits.pageAfter) { it.atId }
     }
 
     post<CommitRequest>("/projects/{projectId}/commits") { commit ->
diff --git a/src/main/kotlin/org/openmbee/flexo/sysmlv2/apis/ElementApi.kt b/src/main/kotlin/org/openmbee/flexo/sysmlv2/apis/ElementApi.kt
index 70373cd..36349be 100644
--- a/src/main/kotlin/org/openmbee/flexo/sysmlv2/apis/ElementApi.kt
+++ b/src/main/kotlin/org/openmbee/flexo/sysmlv2/apis/ElementApi.kt
@@ -13,6 +13,8 @@ package org.openmbee.flexo.sysmlv2.apis
 
 import io.ktor.http.*
 import io.ktor.server.application.*
+import io.ktor.server.plugins.*
+import io.ktor.server.request.*
 import io.ktor.server.resources.*
 import io.ktor.server.response.*
 import io.ktor.server.routing.*
@@ -37,25 +39,24 @@ fun modelElementConstructQuery(elementTarget: String="?__element"): String {
     """.trimIndent()
 }
 
-fun listElementsConstructQuery(limit: Int?=null, offset: Int?=null): String {
-    if (limit == null && offset == null) return """
-        construct {
-            ?s ?p ?o .
-        } where {
-            ?s ?p ?o .
-        }
-    """.trimIndent()
+/**
+ * Cursor-paged element page: subjects are ordered by IRI (the element URN
+ * embeds the id, so this is id order), optionally resuming after a cursor.
+ */
+fun pagedElementsConstructQuery(afterElementIri: String?, limit: Int): String {
+    val afterFilter = afterElementIri?.let { """filter(str(?e) > "${escapeRdfDoubleQuotedLiteralContents(it)}")""" } ?: ""
     return """
-        prefix sysml: <https://www.omg.org/spec/SysML#>
         construct {
             ?e ?element_p ?element_o .
         }
         where {
             {
-                 select ?e
-                 where {
-                     ?e sysml:elementId ?id .
-                 } order by ?id limit $limit offset $offset
+                select distinct ?e
+                where {
+                    ?e ?any_p ?any_o .
+                    filter(isIRI(?e) && strstarts(str(?e), "${SYSMLV2.ELEMENT}"))
+                    $afterFilter
+                } order by ?e limit $limit
             }
             ?e ?element_p ?element_o .
         }
@@ -207,25 +208,66 @@ fun Route.ElementApi() {
     get<Paths.getElementsByProjectCommit> { getElements ->
         requireValidId(getElements.projectId, "projectId")
         requireValidId(getElements.commitId, "commitId")
-        // submit POST request to query model
-        val flexoResponse = flexoRequestGet {
-            orgPath("/repos/${getElements.projectId}/locks/Commit.${getElements.commitId}/graph")
+        val pageSize = getElements.pageSize
+        getElements.pageAfter?.let { requireValidId(it, "page[after]") }
+
+        // unpaged: dump the whole model graph (historical behavior)
+        if (pageSize == null) {
+            val flexoResponse = flexoRequestGet {
+                orgPath("/repos/${getElements.projectId}/locks/Commit.${getElements.commitId}/graph")
+            }
+
+            // forward failures to client
+            if(flexoResponse.isFailure()) {
+                return@get forward(flexoResponse)
+            }
+            // parse the response model, extract the elements to JSON, and reply to client
+            val result = buildJsonArray {
+                flexoResponse.parseModel {
+                    for(subject in model.listSubjects()) {
+                        if (subject.isAnon) continue
+                        add(extractModelElementToJson(subject.uri))
+                    }
+                }
+            }
+            return@get call.respond(result)
+        }
+
+        if (pageSize < 1) {
+            throw BadRequestException("page[size] must be a positive integer")
+        }
+
+        // paged: cursor over element IRIs at the SPARQL level, fetching one
+        // extra element to detect whether a next page exists
+        val afterIri = getElements.pageAfter?.let { SYSMLV2.element(it).uri }
+        val flexoResponse = flexoRequestPost {
+            orgPath("/repos/${getElements.projectId}/locks/Commit.${getElements.commitId}/query")
+            sparqlQuery {
+                pagedElementsConstructQuery(afterIri, pageSize + 1)
+            }
         }
 
         // forward failures to client
         if(flexoResponse.isFailure()) {
             return@get forward(flexoResponse)
         }
-        // parse the response model, extract the elements to JSON, and reply to client
-        val result = buildJsonArray {
-            flexoResponse.parseModel {
-                for(subject in model.listSubjects()) {
-                    if (subject.isAnon) continue
-                    add(extractModelElementToJson(subject.uri))
-                }
+
+        val (page, hasNext) = flexoResponse.parseModel {
+            val ordered = model.listSubjects().toList()
+                .filter { !it.isAnon }
+                .map { it.uri }
+                .sorted()
+            val page = ordered.take(pageSize).map { iri ->
+                iri.urnSuffix to extractModelElementToJson(iri)
             }
+            page to (ordered.size > pageSize)
         }
-        call.respond(result)
+
+        if (hasNext) {
+            val nextUrl = "${call.request.path()}?page%5Bsize%5D=$pageSize&page%5Bafter%5D=${page.last().first}"
+            call.response.headers.append(HttpHeaders.Link, "<$nextUrl>; rel=\"next\"")
+        }
+        call.respond(buildJsonArray { page.forEach { add(it.second) } })
     }
     //TODO this is wrong
     get<Paths.getProjectUsageByProjectCommitElement> {
@@ -299,15 +341,14 @@ fun Route.ElementApi() {
         }
 
         // parse the response model, extract the elements to JSON, and reply to client
-        val result = buildJsonArray {
-            flexoResponse.parseModel {
-                for(subject in model.listSubjects()) {
-                    if (subject.isAnon) continue
-                    add(extractModelElementToJson(subject.uri))
-                }
-            }
+        val roots = flexoResponse.parseModel {
+            model.listSubjects().toList()
+                .filter { !it.isAnon }
+                .map { extractModelElementToJson(it.uri) }
+        }
+        respondPage(roots, it.pageSize, it.pageAfter) { root ->
+            root["@id"]!!.jsonPrimitive.content
         }
-        call.respond(result)
     }
 
 }
diff --git a/src/main/kotlin/org/openmbee/flexo/sysmlv2/apis/MetaApi.kt b/src/main/kotlin/org/openmbee/flexo/sysmlv2/apis/MetaApi.kt
index 3eb949c..53a90ed 100644
--- a/src/main/kotlin/org/openmbee/flexo/sysmlv2/apis/MetaApi.kt
+++ b/src/main/kotlin/org/openmbee/flexo/sysmlv2/apis/MetaApi.kt
@@ -13,47 +13,41 @@ package org.openmbee.flexo.sysmlv2.apis
 
 import io.ktor.http.*
 import io.ktor.server.application.*
-import io.ktor.server.auth.*
-import io.ktor.server.response.*
-import org.openmbee.flexo.sysmlv2.Paths
-import io.ktor.server.resources.options
 import io.ktor.server.resources.get
-import io.ktor.server.resources.post
-import io.ktor.server.resources.put
-import io.ktor.server.resources.delete
-import io.ktor.server.resources.head
-import io.ktor.server.resources.patch
+import io.ktor.server.response.*
 import io.ktor.server.routing.*
+import kotlinx.serialization.json.*
+import org.openmbee.flexo.sysmlv2.Paths
 
-fun Route.MetaApi() {
+private const val JSON_SCHEMA_DIALECT = "https://json-schema.org/draft/2020-12/schema"
+
+// the API's datatype schemas, served from the bundled official OMG spec
+// (resources/openapi.json); parsed once on first use
+private val datatypeSchemas: Map<String, JsonObject> by lazy {
+    val stream = object {}.javaClass.classLoader.getResourceAsStream("openapi.json")
+        ?: return@lazy emptyMap()
+    val doc = stream.reader().use { Json.parseToJsonElement(it.readText()) }.jsonObject
+    doc["components"]?.jsonObject?.get("schemas")?.jsonObject
+        ?.mapValues { (_, schema) -> schema.jsonObject }
+        ?: emptyMap()
+}
 
-    get<Paths.getDatatypeById> {
-        val exampleContentString = """{
-          "${'$'}schema" : "${'$'}schema",
-          "${'$'}defs" : {
-            "key" : ""
-          },
-          "additionalProperties" : true,
-          "title" : "title",
-          "type" : "type",
-          "properties" : {
-            "key" : ""
-          },
-          "required" : [ "required", "required" ],
-          "${'$'}id" : "https://openapi-generator.tech"
-        }"""
+fun Route.MetaApi() {
 
-        call.respondText(exampleContentString, ContentType.Application.Json)
+    get<Paths.getDatatypeById> { params ->
+        val schema = datatypeSchemas[params.datatypeId.toString()]
+            ?: return@get call.respond(HttpStatusCode.NotFound)
+        call.respond(buildJsonObject {
+            put("\$schema", JSON_SCHEMA_DIALECT)
+            schema.forEach { (key, value) -> put(key, value) }
+        })
     }
 
     get<Paths.getDatatypes> {
-        val exampleContentString = """{
-          "${'$'}schema" : "${'$'}schema",
-          "${'$'}defs" : {
-            "key" : ""
-          }
-        }"""
-        call.respondText(exampleContentString, ContentType.Application.Json)
+        call.respond(buildJsonObject {
+            put("\$schema", JSON_SCHEMA_DIALECT)
+            put("\$defs", JsonObject(datatypeSchemas))
+        })
     }
 
 }
diff --git a/src/main/kotlin/org/openmbee/flexo/sysmlv2/apis/ProjectApi.kt b/src/main/kotlin/org/openmbee/flexo/sysmlv2/apis/ProjectApi.kt
index 88305c1..a213ee1 100644
--- a/src/main/kotlin/org/openmbee/flexo/sysmlv2/apis/ProjectApi.kt
+++ b/src/main/kotlin/org/openmbee/flexo/sysmlv2/apis/ProjectApi.kt
@@ -140,7 +140,7 @@ fun Route.ProjectApi() {
     }
 
     // get all projects
-    get<Paths.getProjects> {
+    get<Paths.getProjects> { params ->
         val flexoResponse = flexoRequestGet {
             orgPath("/repos")
         }
@@ -148,7 +148,7 @@ fun Route.ProjectApi() {
             return@get forward(flexoResponse)
         }
         // parse the response model, convert it to JSON, and reply to client
-        call.respond(flexoResponse.parseModel {
+        val projects = flexoResponse.parseModel {
             // find all repos and transform each one into a project by its outgoing triples
             // filter out deleted = true
             model.listSubjectsWithProperty(RDF.type, MMS.Repo).filterDrop {
@@ -156,7 +156,8 @@ fun Route.ProjectApi() {
             }.mapWith {
                 projectFromResponse(it.outgoing())
             }.toList().filterNotNull()
-        })
+        }
+        respondPage(projects, params.pageSize, params.pageAfter) { it.atId }
     }
 
     // create new project via POST
diff --git a/src/main/kotlin/org/openmbee/flexo/sysmlv2/apis/QueryApi.kt b/src/main/kotlin/org/openmbee/flexo/sysmlv2/apis/QueryApi.kt
index 36cbb2a..8cb6c6d 100644
--- a/src/main/kotlin/org/openmbee/flexo/sysmlv2/apis/QueryApi.kt
+++ b/src/main/kotlin/org/openmbee/flexo/sysmlv2/apis/QueryApi.kt
@@ -190,6 +190,12 @@ suspend fun RoutingContext.runQuery(
     }
 }
 
+// apply a query's select projection: keep only the requested properties
+fun JsonObject.projectTo(select: List<String>?): JsonObject {
+    if (select.isNullOrEmpty()) return this
+    return JsonObject(filterKeys { it in select })
+}
+
 suspend fun RoutingContext.getQuery(projectId: String, queryId: String): Pair<FlexoResponse, Query?> {
     val uri = SYSMLV2.query(queryId).uri
     val flexoResponse = flexoRequestPost {
@@ -263,11 +269,12 @@ fun Route.QueryApi() {
             return@get forward(flexoResponse)
         }
         // parse the response model, convert it to JSON, and reply to client
-        call.respond(flexoResponse.parseModel {
+        val queries = flexoResponse.parseModel {
             model.listSubjects().mapWith { it2 ->
                 queryFromResponse(it2.outgoing())
             }.toList()
-        })
+        }
+        respondPage(queries, it.pageSize, it.pageAfter) { query -> query.atId }
     }
 
     get<Paths.getQueryByProjectAndId> {
@@ -311,7 +318,7 @@ fun Route.QueryApi() {
             flexoResponse.parseModel {
                 for(subject in model.listSubjects()) {
                     if (subject.isAnon) continue
-                    add(extractModelElementToJson(subject.uri))
+                    add(extractModelElementToJson(subject.uri).projectTo(query.select))
                 }
             }
         })
@@ -332,7 +339,7 @@ fun Route.QueryApi() {
             flexoResponse.parseModel {
                 for(subject in model.listSubjects()) {
                     if (subject.isAnon) continue
-                    add(extractModelElementToJson(subject.uri))
+                    add(extractModelElementToJson(subject.uri).projectTo(it.select))
                 }
             }
         })
diff --git a/src/main/kotlin/org/openmbee/flexo/sysmlv2/apis/TagApi.kt b/src/main/kotlin/org/openmbee/flexo/sysmlv2/apis/TagApi.kt
index dfa6072..2b661f7 100644
--- a/src/main/kotlin/org/openmbee/flexo/sysmlv2/apis/TagApi.kt
+++ b/src/main/kotlin/org/openmbee/flexo/sysmlv2/apis/TagApi.kt
@@ -110,7 +110,7 @@ fun Route.TagApi() {
             return@get forward(flexoResponse)
         }
         // parse the response model, convert it to JSON, and reply to client
-        call.respond(flexoResponse.parseModel {
+        val tags = flexoResponse.parseModel {
             // find all locks and transform each one into a tag by its outgoing triples
             model.listSubjectsWithProperty(RDF.type, MMS.Lock).filterDrop {
                 // ignore flexo created locks and deleted
@@ -118,7 +118,8 @@ fun Route.TagApi() {
             }.mapWith {
                 tagFromResponse(it.outgoing(), path.projectId, it.uri.uriSuffix)
             }.toList().filterNotNull()
-        })
+        }
+        respondPage(tags, path.pageSize, path.pageAfter) { it.atId }
     }
 
     post<TagRequest>("/projects/{projectId}/tags") { request ->
diff --git a/src/main/kotlin/org/openmbee/flexo/sysmlv2/models/DataVersion.kt b/src/main/kotlin/org/openmbee/flexo/sysmlv2/models/DataVersion.kt
index eeaecb5..b984a4d 100644
--- a/src/main/kotlin/org/openmbee/flexo/sysmlv2/models/DataVersion.kt
+++ b/src/main/kotlin/org/openmbee/flexo/sysmlv2/models/DataVersion.kt
@@ -31,7 +31,8 @@ data class DataVersion(
     @SerialName("@type")
     val atType: DataVersion.AtType,
     val identity: DataIdentity,
-    val payload: JsonObject
+    // null payload denotes a deleted element
+    val payload: JsonObject?
 )
 {
     /**
diff --git a/src/test/kotlin/org/openmbee/flexo/sysmlv2/ChangesTest.kt b/src/test/kotlin/org/openmbee/flexo/sysmlv2/ChangesTest.kt
new file mode 100644
index 0000000..b219bcf
--- /dev/null
+++ b/src/test/kotlin/org/openmbee/flexo/sysmlv2/ChangesTest.kt
@@ -0,0 +1,99 @@
+package org.openmbee.flexo.sysmlv2
+
+import io.kotest.assertions.ktor.client.shouldHaveStatus
+import io.kotest.matchers.collections.shouldContainExactlyInAnyOrder
+import io.kotest.matchers.shouldBe
+import io.ktor.client.statement.*
+import io.ktor.http.*
+import kotlinx.serialization.json.JsonNull
+import kotlinx.serialization.json.Json
+import kotlinx.serialization.json.jsonArray
+import kotlinx.serialization.json.jsonObject
+import kotlinx.serialization.json.jsonPrimitive
+import org.openmbee.flexo.sysmlv2.util.*
+
+/**
+ * GET .../commits/{id}/changes reports the elements a commit created,
+ * modified, or deleted (previously a 501 stub).
+ */
+class ChangesTest : ProjectAny() {
+    val keepId = "aaaaaaa1-0000-4000-8000-000000000001"
+    val editId = "aaaaaaa1-0000-4000-8000-000000000002"
+    val dropId = "aaaaaaa1-0000-4000-8000-000000000003"
+
+    val firstChange = """
+        {
+            "change": [
+                {"@type": "DataVersion", "identity": {"@id": "$keepId"}, "payload": {"@type": "PartDefinition", "name": "keep"}},
+                {"@type": "DataVersion", "identity": {"@id": "$editId"}, "payload": {"@type": "PartDefinition", "name": "before"}},
+                {"@type": "DataVersion", "identity": {"@id": "$dropId"}, "payload": {"@type": "PartDefinition", "name": "drop"}}
+            ]
+        }
+    """.trimIndent()
+
+    val secondChange = """
+        {
+            "change": [
+                {"@type": "DataVersion", "identity": {"@id": "$editId"}, "payload": {"@type": "PartDefinition", "name": "after"}},
+                {"@type": "DataVersion", "identity": {"@id": "$dropId"}, "payload": null}
+            ]
+        }
+    """.trimIndent()
+
+    init {
+        "the first commit reports every created element" {
+            testApplication {
+                val commitId = commitChanges(demoProjectId, firstChange).atId()
+
+                val response = getChanges(demoProjectId, commitId)
+                response shouldHaveStatus HttpStatusCode.OK
+                val changes = Json.parseToJsonElement(response.bodyAsText()).jsonArray
+                val identities = changes.map { it.jsonObject.nestedAtId("identity") }
+                identities shouldContainExactlyInAnyOrder listOf(keepId, editId, dropId)
+                changes.forEach {
+                    it.jsonObject["@type"]!!.jsonPrimitive.content shouldBe "DataVersion"
+                }
+            }
+        }
+
+        "a later commit reports modification and deletion, not untouched elements" {
+            testApplication {
+                commitChanges(demoProjectId, firstChange).atId()
+                val second = commitChanges(demoProjectId, secondChange).atId()
+
+                val changes = Json.parseToJsonElement(
+                    getChanges(demoProjectId, second).bodyAsText()).jsonArray
+                val byIdentity = changes.associate {
+                    it.jsonObject.nestedAtId("identity") to it.jsonObject["payload"]!!
+                }
+                byIdentity.keys shouldContainExactlyInAnyOrder setOf(editId, dropId)
+                // modified element carries its new payload
+                byIdentity[editId]!!.jsonObject["name"]!!.jsonPrimitive.content shouldBe "after"
+                // deleted element carries a null payload
+                byIdentity[dropId] shouldBe JsonNull
+            }
+        }
+
+        "a single change is addressable by its id" {
+            testApplication {
+                val commitId = commitChanges(demoProjectId, firstChange).atId()
+
+                val changes = Json.parseToJsonElement(
+                    getChanges(demoProjectId, commitId).bodyAsText()).jsonArray
+                val changeId = changes.first().jsonObject["@id"]!!.jsonPrimitive.content
+
+                val response = httpGet(
+                    "/projects/$demoProjectId/commits/$commitId/changes/$changeId")
+                response shouldHaveStatus HttpStatusCode.OK
+                response.bodyAsJsonObject()["@id"]!!.jsonPrimitive.content shouldBe changeId
+            }
+        }
+
+        "changes for a nonexistent commit is a 404" {
+            testApplication {
+                getChanges(demoProjectId, "00000000-dead-4000-8000-000000000000")
+                    .shouldHaveStatus(HttpStatusCode.NotFound)
+            }
+        }
+    }
+}
diff --git a/src/test/kotlin/org/openmbee/flexo/sysmlv2/CollectionPaginationTest.kt b/src/test/kotlin/org/openmbee/flexo/sysmlv2/CollectionPaginationTest.kt
new file mode 100644
index 0000000..41460a2
--- /dev/null
+++ b/src/test/kotlin/org/openmbee/flexo/sysmlv2/CollectionPaginationTest.kt
@@ -0,0 +1,75 @@
+package org.openmbee.flexo.sysmlv2
+
+import io.kotest.assertions.ktor.client.shouldHaveStatus
+import io.kotest.matchers.collections.shouldContainExactly
+import io.kotest.matchers.nulls.shouldBeNull
+import io.kotest.matchers.nulls.shouldNotBeNull
+import io.kotest.matchers.shouldBe
+import io.ktor.client.statement.*
+import io.ktor.http.*
+import kotlinx.serialization.json.Json
+import kotlinx.serialization.json.jsonArray
+import kotlinx.serialization.json.jsonObject
+import kotlinx.serialization.json.jsonPrimitive
+import org.openmbee.flexo.sysmlv2.util.*
+
+/**
+ * Cursor pagination on the collection list endpoints (branches here as
+ * the representative; all go through the same respondPage helper).
+ */
+class CollectionPaginationTest : ProjectAny() {
+    val elementChange = """
+        {
+            "change": [
+                {
+                    "@type": "DataVersion",
+                    "identity": {"@id": "99999999-9999-4999-8999-999999999999"},
+                    "payload": {"@type": "PartDefinition", "name": "anything"}
+                }
+            ]
+        }
+    """.trimIndent()
+
+    init {
+        "branch list pages by id with a next link" {
+            testApplication {
+                val commitId = commitChanges(demoProjectId, elementChange).atId()
+                val defaultBranchId = getProject(demoProjectId).bodyAsJsonObject()
+                    .nestedAtId("defaultBranch")
+                // the list contains the default branch plus the three created here
+                val created = ((1..3).map { n ->
+                    createBranch(demoProjectId, commitId, "branch-$n").atId()
+                } + defaultBranchId).sorted()
+
+                val first = httpGet("/projects/$demoProjectId/branches?page%5Bsize%5D=2")
+                first shouldHaveStatus HttpStatusCode.OK
+                val firstIds = Json.parseToJsonElement(first.bodyAsText()).jsonArray
+                    .map { it.jsonObject["@id"]!!.jsonPrimitive.content }
+                firstIds.size shouldBe 2
+                first.headers[HttpHeaders.Link].shouldNotBeNull()
+
+                val second = httpGet(
+                    "/projects/$demoProjectId/branches?page%5Bsize%5D=2&page%5Bafter%5D=${firstIds.last()}")
+                val secondIds = Json.parseToJsonElement(second.bodyAsText()).jsonArray
+                    .map { it.jsonObject["@id"]!!.jsonPrimitive.content }
+                secondIds.size shouldBe 2
+                second.headers[HttpHeaders.Link].shouldBeNull()
+
+                (firstIds + secondIds) shouldContainExactly created
+            }
+        }
+
+        "unpaged branch list is unchanged" {
+            testApplication {
+                val commitId = commitChanges(demoProjectId, elementChange).atId()
+                createBranch(demoProjectId, commitId, "solo").atId()
+
+                val response = getBranches(demoProjectId)
+                response shouldHaveStatus HttpStatusCode.OK
+                // default branch + the one created here
+                Json.parseToJsonElement(response.bodyAsText()).jsonArray.size shouldBe 2
+                response.headers[HttpHeaders.Link].shouldBeNull()
+            }
+        }
+    }
+}
diff --git a/src/test/kotlin/org/openmbee/flexo/sysmlv2/ElementPaginationTest.kt b/src/test/kotlin/org/openmbee/flexo/sysmlv2/ElementPaginationTest.kt
new file mode 100644
index 0000000..6012b93
--- /dev/null
+++ b/src/test/kotlin/org/openmbee/flexo/sysmlv2/ElementPaginationTest.kt
@@ -0,0 +1,96 @@
+package org.openmbee.flexo.sysmlv2
+
+import io.kotest.assertions.ktor.client.shouldHaveStatus
+import io.kotest.matchers.collections.shouldContainExactly
+import io.kotest.matchers.nulls.shouldBeNull
+import io.kotest.matchers.nulls.shouldNotBeNull
+import io.kotest.matchers.shouldBe
+import io.kotest.matchers.string.shouldContain
+import io.ktor.client.statement.*
+import io.ktor.http.*
+import kotlinx.serialization.json.Json
+import kotlinx.serialization.json.jsonArray
+import kotlinx.serialization.json.jsonObject
+import kotlinx.serialization.json.jsonPrimitive
+import org.openmbee.flexo.sysmlv2.util.*
+
+/**
+ * Cursor pagination on GET .../elements: page[size] bounds each response,
+ * page[after] resumes after a cursor, and a Link rel="next" header is
+ * present exactly when more elements remain.
+ */
+class ElementPaginationTest : ProjectAny() {
+    // ids chosen so lexicographic order is the numeric order
+    val elementIds = (1..5).map { "88888888-8888-4888-8888-00000000000$it" }
+
+    val seedChanges = """
+        {
+            "change": [
+                ${elementIds.joinToString(",\n") {
+                    """{"@type": "DataVersion", "identity": {"@id": "$it"}, "payload": {"@type": "PartDefinition", "name": "part-$it"}}"""
+                }}
+            ]
+        }
+    """.trimIndent()
+
+    init {
+        "walking pages of 2 visits every element exactly once in order" {
+            testApplication {
+                val commitId = commitChanges(demoProjectId, seedChanges).atId()
+
+                val seen = mutableListOf<String>()
+                var after: String? = null
+                var pages = 0
+                while (true) {
+                    val cursor = after?.let { "&page%5Bafter%5D=$it" } ?: ""
+                    val response = httpGet(
+                        "/projects/$demoProjectId/commits/$commitId/elements?page%5Bsize%5D=2$cursor")
+                    response shouldHaveStatus HttpStatusCode.OK
+                    val ids = Json.parseToJsonElement(response.bodyAsText()).jsonArray
+                        .map { it.jsonObject["@id"]!!.jsonPrimitive.content }
+                    seen.addAll(ids)
+                    pages++
+
+                    val link = response.headers[HttpHeaders.Link]
+                    if (link == null) break
+                    link shouldContain "rel=\"next\""
+                    after = ids.last()
+                }
+
+                pages shouldBe 3
+                seen shouldContainExactly elementIds
+            }
+        }
+
+        "a page larger than the model has no next link" {
+            testApplication {
+                val commitId = commitChanges(demoProjectId, seedChanges).atId()
+
+                val response = httpGet(
+                    "/projects/$demoProjectId/commits/$commitId/elements?page%5Bsize%5D=50")
+                response shouldHaveStatus HttpStatusCode.OK
+                Json.parseToJsonElement(response.bodyAsText()).jsonArray.size shouldBe 5
+                response.headers[HttpHeaders.Link].shouldBeNull()
+            }
+        }
+
+        "the first page advertises a next link" {
+            testApplication {
+                val commitId = commitChanges(demoProjectId, seedChanges).atId()
+
+                val response = httpGet(
+                    "/projects/$demoProjectId/commits/$commitId/elements?page%5Bsize%5D=2")
+                response.headers[HttpHeaders.Link].shouldNotBeNull()
+            }
+        }
+
+        "page[size] of zero is a 400" {
+            testApplication {
+                val commitId = commitChanges(demoProjectId, seedChanges).atId()
+
+                httpGet("/projects/$demoProjectId/commits/$commitId/elements?page%5Bsize%5D=0")
+                    .shouldHaveStatus(HttpStatusCode.BadRequest)
+            }
+        }
+    }
+}
diff --git a/src/test/kotlin/org/openmbee/flexo/sysmlv2/ErrorShapeTest.kt b/src/test/kotlin/org/openmbee/flexo/sysmlv2/ErrorShapeTest.kt
new file mode 100644
index 0000000..ec797c7
--- /dev/null
+++ b/src/test/kotlin/org/openmbee/flexo/sysmlv2/ErrorShapeTest.kt
@@ -0,0 +1,37 @@
+package org.openmbee.flexo.sysmlv2
+
+import io.kotest.assertions.ktor.client.shouldHaveStatus
+import io.kotest.matchers.shouldBe
+import io.kotest.matchers.string.shouldNotBeBlank
+import io.ktor.http.*
+import kotlinx.serialization.json.jsonPrimitive
+import org.openmbee.flexo.sysmlv2.util.*
+
+/**
+ * Failures respond with the spec's Error object, not plain text, so
+ * official clients can parse them.
+ */
+class ErrorShapeTest : ProjectAny() {
+    init {
+        "a 400 carries a parseable Error object" {
+            testApplication {
+                val response = httpPost("/projects/$demoProjectId/queries") {
+                    setJsonBody("""{ "@type": "Query" }""")
+                }
+                response shouldHaveStatus HttpStatusCode.BadRequest
+                val error = response.bodyAsJsonObject()
+                error["@type"]!!.jsonPrimitive.content shouldBe "Error"
+                error["description"]!!.jsonPrimitive.content.shouldNotBeBlank()
+            }
+        }
+
+        "a 501 stub carries a parseable Error object" {
+            testApplication {
+                val response = httpGet(
+                    "/projects/$demoProjectId/commits/00000000-dead-4000-8000-000000000000/diff?baseCommitId=00000000-dead-4000-8000-000000000001")
+                response shouldHaveStatus HttpStatusCode.NotImplemented
+                response.bodyAsJsonObject()["@type"]!!.jsonPrimitive.content shouldBe "Error"
+            }
+        }
+    }
+}
diff --git a/src/test/kotlin/org/openmbee/flexo/sysmlv2/MetaTest.kt b/src/test/kotlin/org/openmbee/flexo/sysmlv2/MetaTest.kt
new file mode 100644
index 0000000..73eb193
--- /dev/null
+++ b/src/test/kotlin/org/openmbee/flexo/sysmlv2/MetaTest.kt
@@ -0,0 +1,46 @@
+package org.openmbee.flexo.sysmlv2
+
+import io.kotest.assertions.ktor.client.shouldHaveStatus
+import io.kotest.matchers.maps.shouldContainKey
+import io.kotest.matchers.shouldBe
+import io.ktor.http.*
+import kotlinx.serialization.json.jsonObject
+import kotlinx.serialization.json.jsonPrimitive
+import org.openmbee.flexo.sysmlv2.util.*
+
+/**
+ * /meta/datatypes serves the official spec's schema definitions
+ * (previously a hardcoded openapi-generator example payload).
+ */
+class MetaTest : CommonSpec() {
+    init {
+        "GET /meta/datatypes lists the spec schemas under \$defs" {
+            testApplication {
+                val response = httpGet("/meta/datatypes")
+                response shouldHaveStatus HttpStatusCode.OK
+                val body = response.bodyAsJsonObject()
+                body["\$schema"]!!.jsonPrimitive.content shouldBe
+                        "https://json-schema.org/draft/2020-12/schema"
+                val defs = body["\$defs"]!!.jsonObject
+                defs shouldContainKey "Project"
+                defs shouldContainKey "PartDefinition"
+            }
+        }
+
+        "GET /meta/datatypes/{id} returns the named schema" {
+            testApplication {
+                val response = httpGet("/meta/datatypes/PartDefinition")
+                response shouldHaveStatus HttpStatusCode.OK
+                val body = response.bodyAsJsonObject()
+                body["title"]!!.jsonPrimitive.content shouldBe "PartDefinition"
+            }
+        }
+
+        "GET an unknown datatype is a 404" {
+            testApplication {
+                httpGet("/meta/datatypes/NotARealDatatype")
+                    .shouldHaveStatus(HttpStatusCode.NotFound)
+            }
+        }
+    }
+}
diff --git a/src/test/kotlin/org/openmbee/flexo/sysmlv2/PreviousCommitTest.kt b/src/test/kotlin/org/openmbee/flexo/sysmlv2/PreviousCommitTest.kt
new file mode 100644
index 0000000..eafa7cf
--- /dev/null
+++ b/src/test/kotlin/org/openmbee/flexo/sysmlv2/PreviousCommitTest.kt
@@ -0,0 +1,60 @@
+package org.openmbee.flexo.sysmlv2
+
+import io.kotest.assertions.ktor.client.shouldHaveStatus
+import io.kotest.matchers.shouldBe
+import io.ktor.client.statement.*
+import io.ktor.http.*
+import kotlinx.serialization.json.Json
+import kotlinx.serialization.json.JsonNull
+import kotlinx.serialization.json.jsonArray
+import kotlinx.serialization.json.jsonObject
+import kotlinx.serialization.json.jsonPrimitive
+import org.openmbee.flexo.sysmlv2.util.*
+
+/**
+ * Commit lineage: previousCommit links each commit to its parent, except
+ * that flexo's auto-created (hidden) root commit is never referenced.
+ */
+class PreviousCommitTest : ProjectAny() {
+    fun change(elementId: String, name: String) = """
+        {
+            "change": [
+                {
+                    "@type": "DataVersion",
+                    "identity": {"@id": "$elementId"},
+                    "payload": {"@type": "PartDefinition", "name": "$name"}
+                }
+            ]
+        }
+    """.trimIndent()
+
+    init {
+        "single GET resolves previousCommit; first commit has none" {
+            testApplication {
+                val first = commitChanges(demoProjectId, change("55555555-5555-4555-8555-000000000001", "one")).atId()
+                val second = commitChanges(demoProjectId, change("55555555-5555-4555-8555-000000000002", "two")).atId()
+
+                val firstBody = getCommit(demoProjectId, first).bodyAsJsonObject()
+                (firstBody["previousCommit"] ?: JsonNull) shouldBe JsonNull
+
+                val secondBody = getCommit(demoProjectId, second).bodyAsJsonObject()
+                secondBody.nestedAtId("previousCommit") shouldBe first
+            }
+        }
+
+        "commit list carries the same lineage" {
+            testApplication {
+                val first = commitChanges(demoProjectId, change("55555555-5555-4555-8555-000000000003", "one")).atId()
+                val second = commitChanges(demoProjectId, change("55555555-5555-4555-8555-000000000004", "two")).atId()
+
+                val response = getCommits(demoProjectId)
+                response shouldHaveStatus HttpStatusCode.OK
+                val byId = Json.parseToJsonElement(response.bodyAsText()).jsonArray
+                    .associateBy { it.jsonObject["@id"]!!.jsonPrimitive.content }
+
+                (byId[first]!!.jsonObject["previousCommit"] ?: JsonNull) shouldBe JsonNull
+                byId[second]!!.nestedAtId("previousCommit") shouldBe first
+            }
+        }
+    }
+}
diff --git a/src/test/kotlin/org/openmbee/flexo/sysmlv2/QueryResultsTest.kt b/src/test/kotlin/org/openmbee/flexo/sysmlv2/QueryResultsTest.kt
index 58f73a5..ba2fd68 100644
--- a/src/test/kotlin/org/openmbee/flexo/sysmlv2/QueryResultsTest.kt
+++ b/src/test/kotlin/org/openmbee/flexo/sysmlv2/QueryResultsTest.kt
@@ -2,6 +2,7 @@ package org.openmbee.flexo.sysmlv2
 
 import io.kotest.assertions.ktor.client.shouldHaveStatus
 import io.kotest.matchers.collections.shouldContainExactlyInAnyOrder
+import io.kotest.matchers.shouldBe
 import io.ktor.client.statement.*
 import io.ktor.http.*
 import io.ktor.server.testing.ApplicationTestBuilder
@@ -162,6 +163,44 @@ class QueryResultsTest : ProjectAny() {
             }
         }
 
+        "POST query-results - select projects the result properties" {
+            testApplication {
+                commitChanges(demoProjectId, seedChanges).atId()
+                val response = httpPost("/projects/$demoProjectId/query-results") {
+                    setJsonBody(
+                        """
+                        {
+                            "@type": "Query",
+                            "select": ["@id", "name"],
+                            "where": {"@type": "PrimitiveConstraint", "operator": "=", "property": "name", "value": ["Alpha"]}
+                        }
+                        """.trimIndent()
+                    )
+                }
+                response shouldHaveStatus HttpStatusCode.OK
+                val results = Json.parseToJsonElement(response.bodyAsText()).jsonArray
+                results.size shouldBe 1
+                val element = results[0].jsonObject
+                element.keys shouldBe setOf("@id", "name")
+                element["name"]!!.jsonPrimitive.content shouldBe "Alpha"
+            }
+        }
+
+        "GET queries/{queryId}/results - applies the saved select projection" {
+            testApplication {
+                commitChanges(demoProjectId, seedChanges).atId()
+                // createQuery stores select = ["@id"]
+                val queryId = createQuery(demoProjectId, "name", "Beta").atId()
+
+                val response = httpGet("/projects/$demoProjectId/queries/$queryId/results")
+                response shouldHaveStatus HttpStatusCode.OK
+                val results = Json.parseToJsonElement(response.bodyAsText()).jsonArray
+                results.size shouldBe 1
+                results[0].jsonObject.keys shouldBe setOf("@id")
+                results[0].jsonObject["@id"]!!.jsonPrimitive.content shouldBe betaId
+            }
+        }
+
         "POST queries - missing select is a 400, not a server error" {
             testApplication {
                 val response = httpPost("/projects/$demoProjectId/queries") {