diff --git a/.github/workflows/ci-c-e2e-mqttv5-adr-namespace.yml b/.github/workflows/ci-c-e2e-mqttv5-adr-namespace.yml
new file mode 100644
index 000000000..b32f9ba79
--- /dev/null
+++ b/.github/workflows/ci-c-e2e-mqttv5-adr-namespace.yml
@@ -0,0 +1,80 @@
+# Copyright (c) Microsoft. All rights reserved.
+# Licensed under the MIT license. See LICENSE file in the project root for full license information.
+
+# Keeps the shared mqttv5 e2e environment's ADR namespace present and linked.
+#
+# Workaround: the namespace can disappear from ARM (taking its role assignments
+# with it), after which DPS registrations fail. c/eng/e2e-mqttv5-adr-namespace.ps1
+# rebuilds and re-links it; when it is healthy the run is one ARM read.
+#
+# Runs hourly, on demand, and as the first job of the mqttv5 e2e workflow
+# (workflow_call), so tests start against a linked namespace.
+#
+#   vars:    E2E_MQTTV5_SHARED_SUBSCRIPTION_ID, E2E_MQTTV5_SHARED_RESOURCE_GROUP,
+#            E2E_MQTTV5_SHARED_HUB_NAME, E2E_MQTTV5_SHARED_DPS_NAME,
+#            E2E_MQTTV5_SHARED_ADR_NAMESPACE,
+#            E2E_MQTTV5_SHARED_ADR_API_VERSION (optional; default 2026-11-02-preview)
+#   secrets: AZURE_CLIENT_ID, AZURE_TENANT_ID (OIDC). On the resource group the
+#            identity needs Contributor, and role assignment write for
+#            Contributor, IoT Hub Data Contributor and Azure Device Registry
+#            Contributor.
+
+name: ci-c-e2e-mqttv5-adr-namespace
+
+on:
+  workflow_dispatch:
+  workflow_call:
+    secrets:
+      AZURE_CLIENT_ID:
+        required: true
+      AZURE_TENANT_ID:
+        required: true
+  schedule:
+    - cron: '17 * * * *'
+
+permissions:
+  contents: read
+  id-token: write   # azure/login OIDC
+
+jobs:
+  ensure:
+    name: ensure ADR namespace
+    if: ${{ vars.E2E_MQTTV5_SHARED_ADR_NAMESPACE != '' }}
+    runs-on: ubuntu-24.04
+    # One repair at a time across schedule, dispatch and callers. GitHub keeps
+    # one pending run per group; a third queued run cancels the pending one.
+    concurrency:
+      group: mqttv5-adr-namespace
+      cancel-in-progress: false
+    # Above the script's worst case: delete wait 15 + namespace 10 + role
+    # grants 14 + 4 link attempts x 13 + 3 settle waits x 5 + reconcile 5 +
+    # DPS 5 = 116 min, plus ARM latency.
+    timeout-minutes: 135
+    steps:
+      - uses: actions/checkout@11d5960a326750d5838078e36cf38b85af677262 # v4.4.0
+        with:
+          persist-credentials: false
+          sparse-checkout: c/eng
+
+      - name: Azure Login via OIDC
+        uses: azure/login@7184910d9eb2b1c5e48f7073824a90609bb9b6d6 # v2.3.1
+        with:
+          client-id: ${{ secrets.AZURE_CLIENT_ID }}
+          tenant-id: ${{ secrets.AZURE_TENANT_ID }}
+          subscription-id: ${{ vars.E2E_MQTTV5_SHARED_SUBSCRIPTION_ID }}
+
+      - name: Ensure ADR namespace
+        shell: pwsh
+        env:
+          SUBSCRIPTION_ID: ${{ vars.E2E_MQTTV5_SHARED_SUBSCRIPTION_ID }}
+          RESOURCE_GROUP: ${{ vars.E2E_MQTTV5_SHARED_RESOURCE_GROUP }}
+          HUB_NAME: ${{ vars.E2E_MQTTV5_SHARED_HUB_NAME }}
+          DPS_NAME: ${{ vars.E2E_MQTTV5_SHARED_DPS_NAME }}
+          ADR_NAMESPACE: ${{ vars.E2E_MQTTV5_SHARED_ADR_NAMESPACE }}
+          ADR_API_VERSION: ${{ vars.E2E_MQTTV5_SHARED_ADR_API_VERSION }}
+        run: |
+          $extra = @{}
+          if ($env:ADR_API_VERSION) { $extra.AdrApiVersion = $env:ADR_API_VERSION }
+          ./c/eng/e2e-mqttv5-adr-namespace.ps1 -SubscriptionId $env:SUBSCRIPTION_ID `
+            -ResourceGroup $env:RESOURCE_GROUP -HubName $env:HUB_NAME `
+            -DpsName $env:DPS_NAME -NamespaceName $env:ADR_NAMESPACE @extra
diff --git a/c/docs/eng/end-to-end-tests.md b/c/docs/eng/end-to-end-tests.md
index aeddc0a3d..01311f467 100644
--- a/c/docs/eng/end-to-end-tests.md
+++ b/c/docs/eng/end-to-end-tests.md
@@ -235,6 +235,18 @@ CA certificate and key, then its root). `E2E_CSR_SHARED_DPS_HOST` optionally set
 device endpoint. `E2E_CSR_SHARED_SAS_GROUP_KEY` is the key of a symmetric-key group linked
 to the same certificate policy.
 
+The shared mqttv5 environment (ADR-linked mqttv5 hub and DPS) is kept usable by
+[`ci-c-e2e-mqttv5-adr-namespace.yml`](../../../.github/workflows/ci-c-e2e-mqttv5-adr-namespace.yml)
+(hourly, manual, or `workflow_call` as the first job of a mqttv5 e2e workflow). The ADR namespace
+can disappear from ARM, removing its role assignments; DPS registrations then fail.
+[`c/eng/e2e-mqttv5-adr-namespace.ps1`](../../eng/e2e-mqttv5-adr-namespace.ps1) re-creates it
+(same name, hub region), re-grants the 8 role assignments, re-links hub and DPS, and pushes the
+namespace to DPS; when the namespace is healthy it makes one ARM read. Inputs are repository
+variables `E2E_MQTTV5_SHARED_SUBSCRIPTION_ID`, `_RESOURCE_GROUP`, `_HUB_NAME`, `_DPS_NAME`
+and `_ADR_NAMESPACE` (unset `_ADR_NAMESPACE` skips the job); optional `_ADR_API_VERSION`
+(default `2026-11-02-preview`) must match the version the namespace was linked with. The OIDC identity needs Contributor
+and role assignment write on the resource group.
+
 > **Software updates e2e** runs in its own workflow
 > ([`ci-c-e2e-adu.yml`](../../../.github/workflows/ci-c-e2e-adu.yml), Linux, manual dispatch). See [Software updates e2e](#software-updates-e2e).
 
diff --git a/c/eng/e2e-mqttv5-adr-namespace.ps1 b/c/eng/e2e-mqttv5-adr-namespace.ps1
new file mode 100644
index 000000000..16f4aa110
--- /dev/null
+++ b/c/eng/e2e-mqttv5-adr-namespace.ps1
@@ -0,0 +1,260 @@
+# Copyright (c) Microsoft. All rights reserved.
+# Licensed under the MIT license. See LICENSE file in the project root for full license information.
+
+<#
+.SYNOPSIS
+Ensures the ADR namespace of the shared mqttv5 e2e environment exists and links its hub and DPS.
+
+.DESCRIPTION
+Workaround: in some regions the ADR namespace can disappear from ARM while the ADR service still
+holds it; its role assignments go with it, and DPS registrations then fail. When the namespace
+is missing, or an endpoint is not linked, this script:
+  1. re-creates it (same name/region; if ADR still holds it, re-asserts its endpoints),
+  2. re-grants the 8 role assignments the link needs (idempotent),
+  3. links the hub (messaging/hub-1) and DPS (provisioning/dps-1) and waits for Succeeded,
+  4. pushes the namespace to the DPS data plane (tags-only DPS update).
+If the namespace is present with both endpoints Succeeded it makes one ARM read and exits.
+
+Requires `az` logged in with, on the resource group: Contributor, and
+Microsoft.Authorization/roleAssignments/write (e.g. Role Based Access Control Administrator)
+for Contributor (b24988ac-...), IoT Hub Data Contributor (4fc6c259-...) and
+Azure Device Registry Contributor (a5c3590a-...).
+Hub and DPS must have system-assigned identities.
+#>
+[CmdletBinding(PositionalBinding = $false)]
+param(
+    [Parameter(Mandatory)][string]$SubscriptionId,
+    [Parameter(Mandatory)][string]$ResourceGroup,
+    [Parameter(Mandatory)][string]$HubName,
+    [Parameter(Mandatory)][string]$DpsName,
+    [Parameter(Mandatory)][string]$NamespaceName,
+    # Use the version the namespace was linked with: the namespace principalId differs per
+    # api-version, so grants and link must share one. 2026-11-01-preview where 11-02 is not enabled.
+    [string]$AdrApiVersion = '2026-11-02-preview',
+    [string]$HubApiVersion = '2026-10-01-preview',
+    # Empty: the first of 2026-11-02-preview / 2026-03-01-preview that ARM serves, else its newest.
+    [string]$DpsApiVersion = '',
+    [int]$LinkTimeoutMinutes = 12,
+    [int]$LinkAttempts = 4
+)
+
+$ErrorActionPreference = 'Stop'
+
+$Arm = 'https://management.azure.com'
+$Rg = "/subscriptions/$SubscriptionId/resourceGroups/$ResourceGroup"
+$HubId = "$Rg/providers/Microsoft.Devices/IotHubs/$HubName"
+$DpsId = "$Rg/providers/Microsoft.Devices/provisioningServices/$DpsName"
+$NsId = "$Rg/providers/Microsoft.DeviceRegistry/namespaces/$NamespaceName"
+$HubUrl = "$Arm${HubId}?api-version=$HubApiVersion"
+$NsUrl = "$Arm${NsId}?api-version=$AdrApiVersion"
+$RoleContributor = 'b24988ac-6180-42a0-ab88-20f7382dd24c'
+$RoleHubData = '4fc6c259-987e-4a07-842e-c321cc9d413f'
+$RoleAdr = 'a5c3590a-3a1a-4cd4-9648-ea0a32b15137'
+# Link failures that clear once new role assignments replicate.
+$PropagationCodes = 'AdrMiNotAuthorized|LinkableResourceNotReady|AuthorizationFailed|LinkInitiateFailed|NamespaceMiTokenAcquisitionFailed|OutboundIdentityUnavailable'
+# Link submissions that may be rejected outright yet succeed on a later attempt.
+$RetryableSubmit = "$PropagationCodes|ResourceProvisioningInProgress|Service ?Unavailable|Gateway ?Timeout|Bad ?Gateway|Too ?Many ?Requests|Internal ?Server ?Error|ServerTimeout|ServerBusy"
+# ADR feature flag: without it the link takes another path and reports the linked resource as unreadable.
+$FeatureTags = @{ useMiSdk = 'true' }
+
+# Returns $null on 404. Response bodies are not logged: DPS reads can carry keys.
+function Invoke-Arm([string]$Method, [string]$Url, $Body) {
+    $a = @('rest', '--method', $Method, '--url', $Url, '-o', 'json')
+    $tmp = $null
+    if ($null -ne $Body) {
+        $tmp = New-TemporaryFile
+        $Body | ConvertTo-Json -Depth 20 -Compress | Set-Content -LiteralPath $tmp -Encoding utf8NoBOM
+        $a += @('--headers', 'Content-Type=application/json', '--body', "@$tmp")
+    }
+    try {
+        $out = ((& az @a 2>&1) | Out-String).Trim()
+        if ($LASTEXITCODE -ne 0) {
+            if ($Method -eq 'get' -and $out -match 'ResourceNotFound|\(NotFound\)|Not Found') { return $null }
+            $msg = $out -replace '(?i)("(?:primaryKey|secondaryKey|accessToken)"\s*:\s*")[^"]*', '$1<redacted>'
+            throw "ARM $($Method.ToUpper()) $($Url -replace '\?.*') failed: $msg"
+        }
+        if ($out) { $out | ConvertFrom-Json -Depth 50 }
+    }
+    finally { if ($tmp) { Remove-Item -LiteralPath $tmp -Force -ErrorAction SilentlyContinue } }
+}
+
+function Get-EndpointStates($Ns) {
+    foreach ($g in 'messaging', 'provisioning') {
+        $eps = $Ns.properties.$g.endpoints
+        if (-not $eps) { continue }
+        foreach ($p in $eps.PSObject.Properties) {
+            [pscustomobject]@{ Name = "$g/$($p.Name)"; State = [string]$p.Value.linkingState; Code = [string]$p.Value.linkingError.code; Message = [string]$p.Value.linkingError.message }
+        }
+    }
+}
+
+# hub-1 and dps-1 Succeeded and pointing at this hub and DPS.
+function Test-EndpointsLinked($Ns) {
+    if (-not $Ns) { return $false }
+    foreach ($e in @(@($Ns.properties.messaging.endpoints.'hub-1', $HubId), @($Ns.properties.provisioning.endpoints.'dps-1', $DpsId))) {
+        if (-not $e[0] -or $e[0].linkingState -ne 'Succeeded') { return $false }
+        if ($e[0].resourceId -and $e[0].resourceId -ne $e[1]) { return $false }
+    }
+    $true
+}
+
+function Test-Linked($Ns) { $Ns -and $Ns.properties.provisioningState -eq 'Succeeded' -and (Test-EndpointsLinked $Ns) }
+
+function Get-NsTags($Ns) {
+    $t = @{}
+    if ($Ns -and $Ns.tags) { $Ns.tags.PSObject.Properties | ForEach-Object { $t[$_.Name] = $_.Value } }
+    $FeatureTags.GetEnumerator() | ForEach-Object { $t[$_.Key] = $_.Value }
+    $t
+}
+
+function Wait-For([string]$What, [int]$Minutes, [scriptblock]$Probe) {
+    $deadline = (Get-Date).AddMinutes($Minutes)
+    while ($true) {
+        $r = & $Probe
+        if ($r.Done) { return $r }
+        if ($r.ContainsKey('Failed') -and $r.Failed) { throw "$What failed: $($r.Detail)" }
+        if ((Get-Date) -ge $deadline) { throw "$What did not finish within $Minutes min (last: $($r.Detail))." }
+        Write-Host "  $What : $($r.Detail)"
+        Start-Sleep -Seconds 15
+    }
+}
+
+function Grant-Role([string]$Scope, [string]$PrincipalId, [string]$RoleId, [string]$What) {
+    for ($i = 1; $i -le 8; $i++) {
+        $out = ((& az role assignment create --assignee-object-id $PrincipalId --assignee-principal-type ServicePrincipal --role $RoleId --scope $Scope -o none 2>&1) | Out-String).Trim()
+        if ($LASTEXITCODE -eq 0 -or $out -match 'RoleAssignmentExists|already exists') { Write-Host "  role: $What"; return }
+        if ($i -eq 8) { throw "Role assignment '$What' failed: $out" }
+        Start-Sleep -Seconds 15
+    }
+}
+
+[void](& az account set --subscription $SubscriptionId)
+if ($LASTEXITCODE -ne 0) { throw "az account set --subscription $SubscriptionId failed." }
+
+$ns = Invoke-Arm get $NsUrl
+if (Test-Linked $ns) { Write-Host "ADR namespace $NamespaceName present and linked."; return }
+Write-Host "::warning::ADR namespace $NamespaceName $(if ($ns) { 'not linked' } else { 'missing from ARM' }); rebuilding it."
+
+if (-not $DpsApiVersion) {
+    $served = @(((& az provider show --namespace Microsoft.Devices -o json 2>$null) | Out-String | ConvertFrom-Json).resourceTypes |
+        Where-Object resourceType -eq 'provisioningServices' | ForEach-Object apiVersions)
+    if (-not $served) { throw 'Could not read the Microsoft.Devices/provisioningServices api-versions.' }
+    $DpsApiVersion = @(@('2026-11-02-preview', '2026-03-01-preview') | Where-Object { $_ -in $served }) + @($served | Sort-Object -Descending) | Select-Object -First 1
+}
+Write-Host "  DPS api-version: $DpsApiVersion"
+$DpsUrl = "$Arm${DpsId}?api-version=$DpsApiVersion"
+$hub = Invoke-Arm get $HubUrl
+$dps = Invoke-Arm get $DpsUrl
+if (-not $hub -or -not $dps) { throw 'Hub or DPS not found.' }
+$hubPid = $hub.identity.principalId; $dpsPid = $dps.identity.principalId
+if (-not $hubPid -or -not $dpsPid) { throw 'Hub and DPS need system-assigned identities.' }
+# ADR requires the namespace in the hub's region.
+$location = $hub.location
+
+# Full PUTs replace tags: keep existing ones, re-assert the feature flag.
+$tags = Get-NsTags $ns
+$tags['restoredBy'] = 'e2e-mqttv5-adr-namespace'
+$linkBody = @{
+    location = $location; identity = @{ type = 'SystemAssigned' }; tags = $tags
+    properties = @{
+        messaging    = @{ endpoints = @{ 'hub-1' = @{ endpointType = 'Microsoft.Devices/IotHubs'; resourceId = $HubId
+                        inboundCallerIdentity = @{ type = 'SystemAssigned' }; provisioning = @{ availability = 'Available'; allocationWeight = 1 } } } }
+        provisioning = @{ endpoints = @{ 'dps-1' = @{ endpointType = 'Microsoft.Devices/provisioningServices'; resourceId = $DpsId
+                        inboundCallerIdentity = @{ type = 'SystemAssigned' } } } }
+    }
+}
+
+# A namespace being deleted is waited out, then created again.
+if ($ns -and $ns.properties.provisioningState -eq 'Deleting') {
+    [void](Wait-For 'namespace delete' 15 {
+            $n = Invoke-Arm get $NsUrl
+            @{ Done = (-not $n); Detail = if ($n) { $n.properties.provisioningState } else { 'absent' } } })
+    $ns = $null
+}
+
+# 1. Namespace. A bare create fails with EndpointUsedByNestedDevices when ADR still holds it; then re-assert its endpoints.
+if (-not $ns) {
+    try { [void](Invoke-Arm put $NsUrl @{ location = $location; identity = @{ type = 'SystemAssigned' }; tags = $tags }) }
+    catch {
+        if ($_.Exception.Message -notmatch 'EndpointUsedByNestedDevices') { throw }
+        Write-Host '  ADR still holds the namespace; re-asserting its endpoints.'
+        [void](Invoke-Arm put $NsUrl $linkBody)
+    }
+}
+$ns = (Wait-For 'namespace' 10 {
+        $n = Invoke-Arm get $NsUrl
+        $ps = if ($n) { $n.properties.provisioningState } else { 'absent' }
+        # Any terminal state with an identity can go on: the link PUT below re-drives the namespace.
+        $terminal = $ps -in 'Succeeded', 'Failed', 'Canceled'
+        @{ Done = ($terminal -and $n.identity.principalId); Failed = ($terminal -and -not $n.identity.principalId); Detail = $ps; Ns = $n } }).Ns
+$nsPid = $ns.identity.principalId
+
+# 2. Role assignments (removed with the namespace; the namespace identity may be new).
+Grant-Role $HubId $nsPid $RoleContributor 'namespace -> hub (Contributor)'
+Grant-Role $NsId $hubPid $RoleContributor 'hub -> namespace (Contributor)'
+Grant-Role $DpsId $nsPid $RoleContributor 'namespace -> DPS (Contributor)'
+Grant-Role $NsId $dpsPid $RoleContributor 'DPS -> namespace (Contributor)'
+Grant-Role $NsId $dpsPid $RoleAdr 'DPS -> namespace (Azure Device Registry Contributor)'
+Grant-Role $HubId $dpsPid $RoleHubData 'DPS -> hub (IoT Hub Data Contributor)'
+Grant-Role $NsId $nsPid $RoleContributor 'namespace -> namespace (Contributor)'
+Grant-Role $HubId $nsPid $RoleHubData 'namespace -> hub (IoT Hub Data Contributor)'
+
+# 3. Link hub + DPS in one write (ADR refuses a messaging endpoint alone).
+for ($attempt = 1; ; $attempt++) {
+    Start-Sleep -Seconds 60   # role assignment replication
+    if ($attempt -gt 1) {
+        # ARM rejects writes (ResourceProvisioningInProgress) until a failed link settles.
+        try { [void](Wait-For 'namespace settle' 5 {
+                    $ps = (Invoke-Arm get $NsUrl).properties.provisioningState
+                    @{ Done = (-not $ps -or $ps -in 'Succeeded', 'Failed', 'Canceled'); Detail = $ps } }) }
+        catch { Write-Host "  $($_.Exception.Message)" }
+    }
+    Write-Host "  link attempt $attempt/$LinkAttempts"
+    try { [void](Invoke-Arm put $NsUrl $linkBody) }
+    catch {
+        $msg = $_.Exception.Message
+        if ($msg -notmatch $RetryableSubmit -or $attempt -ge $LinkAttempts) { throw }
+        Write-Host "  link rejected, retrying: $msg"
+        continue
+    }
+    $r = Wait-For 'link' $LinkTimeoutMinutes {
+        $n = Invoke-Arm get $NsUrl
+        $s = @(if ($n) { Get-EndpointStates $n })
+        $failed = @($s | Where-Object State -eq 'Failed')
+        $done = (Test-Linked $n) -or $failed.Count -gt 0 -or ($n -and $n.properties.provisioningState -in 'Failed', 'Canceled')
+        @{ Done = $done; Linked = (Test-Linked $n); Failed = $false; States = $failed; Ns = $n
+           Detail = if ($s) { ($s | ForEach-Object { "$($_.Name)=$($_.State)" }) -join ', ' } else { '<no endpoints>' } } }
+    if ($r.Linked) { break }
+    # Endpoints linked but namespace Failed: a tags-only update re-runs reconciliation
+    # (re-sending endpoints is rejected as immutable). Failed can persist briefly after it is accepted.
+    if ((Test-EndpointsLinked $r.Ns) -and $r.Ns.properties.provisioningState -eq 'Failed') {
+        Write-Host '  endpoints linked, namespace Failed; reconciling (tags-only update)'
+        $t = Get-NsTags $r.Ns
+        $t['adrReconcile'] = [guid]::NewGuid().ToString('N')
+        [void](Invoke-Arm patch $NsUrl @{ tags = $t })
+        [void](Wait-For 'namespace reconcile' 5 {
+                $ps = (Invoke-Arm get $NsUrl).properties.provisioningState
+                @{ Done = ($ps -eq 'Succeeded'); Detail = $ps } })
+        break
+    }
+    $why = ($r.States | ForEach-Object { "$($_.Name): $($_.Code) $($_.Message)" }) -join '; '
+    $retryable = $r.States.Count -gt 0 -and -not ($r.States | Where-Object { $_.Code -notmatch "^($PropagationCodes)$" })
+    if (-not $retryable -or $attempt -ge $LinkAttempts) { throw "ADR link failed: $(if ($why) { $why } else { 'namespace Failed' })" }
+    Write-Host "  role replication pending ($why); retrying"
+}
+
+# 4. Push the namespace to the DPS data plane (tags-only update).
+# Fresh read: recovery can take over an hour and the PATCH replaces the whole tag map.
+$dps = Invoke-Arm get $DpsUrl
+$dpsTags = @{}
+if ($dps.tags) { $dps.tags.PSObject.Properties | ForEach-Object { $dpsTags[$_.Name] = $_.Value } }
+$marker = [guid]::NewGuid().ToString('N')
+$dpsTags['adrNamespaceRestored'] = $marker
+[void](Invoke-Arm patch $DpsUrl @{ tags = $dpsTags })
+[void](Wait-For 'DPS update' 5 {
+        $d = Invoke-Arm get $DpsUrl
+        # Succeeded/Active is also the pre-update state; require the new marker.
+        @{ Done = ($d.tags.adrNamespaceRestored -eq $marker -and $d.properties.provisioningState -eq 'Succeeded' -and $d.properties.state -eq 'Active'); Detail = "$($d.properties.provisioningState) / $($d.properties.state)" } })
+
+Write-Host "ADR namespace $NamespaceName rebuilt and linked."
+if ($env:GITHUB_STEP_SUMMARY) { "ADR namespace ``$NamespaceName`` was rebuilt and re-linked by this run." | Add-Content -LiteralPath $env:GITHUB_STEP_SUMMARY }