diff --git a/README.md b/README.md
index 98513ee..f8447b3 100644
--- a/README.md
+++ b/README.md
@@ -62,7 +62,7 @@ No resources.
 |------|-------------|------|---------|:--------:|
 | <a name="input_name"></a> [name](#input\_name) | Deployment name. | `string` | n/a | yes |
 | <a name="input_cloud_provider"></a> [cloud\_provider](#input\_cloud\_provider) | Compatibility selector for the root GCP entrypoint. Use providers/do or providers/linode for other clouds. | `string` | `"gcp"` | no |
-| <a name="input_gcp"></a> [gcp](#input\_gcp) | Google Cloud infrastructure settings. | <pre>object({<br/>    project_id     = optional(string, "")<br/>    project_number = optional(string, "")<br/>    region         = optional(string, "us-east5")<br/>    zone           = optional(string, "us-east5-b")<br/><br/>    identity = optional(object({<br/>      vm_service_account_email  = optional(string, "")<br/>      app_service_account_email = optional(string, "")<br/>      app_credentials_enabled   = optional(bool, false)<br/>    }), {})<br/><br/>    instance = optional(object({<br/>      machine_type = optional(string, "n4-standard-2")<br/>      os           = optional(string, "cos-125-19216-220-185")<br/>      production   = optional(bool, false)<br/>    }), {})<br/><br/>    disks = optional(object({<br/>      type                   = optional(string, "hyperdisk-balanced")<br/>      docker_volumes_size_gb = optional(number, 50)<br/>    }), {})<br/><br/>    network = optional(object({<br/>      create                   = optional(bool, true)<br/>      project_id               = optional(string, "")<br/>      name                     = optional(string, "")<br/>      subnetwork               = optional(string, "")<br/>      ip_cidr_range            = optional(string, "10.42.0.0/24")<br/>      mtu                      = optional(number, 1460)<br/>      power_button_allowed_ips = optional(list(string), [])<br/>      power_button_ip_depth    = optional(number)<br/>      ssh_ipv4                 = optional(list(string), [])<br/>      ssh_ipv6                 = optional(list(string), [])<br/>    }), {})<br/><br/>    snapshots = optional(object({<br/>      enabled = optional(bool, false)<br/>    }), {})<br/><br/>    overlay = optional(object({<br/>      source_instance = optional(string, "")<br/>      volume_names    = optional(list(string), [])<br/>    }), {})<br/><br/>    cloud_init = optional(object({<br/>      initcmd = optional(list(string), [])<br/>      runcmd  = optional(list(string), [])<br/>    }), {})<br/><br/>    artifact_registry = optional(object({<br/>      repository = optional(string, "")<br/>      location   = optional(string, "us")<br/>    }), {})<br/><br/>    power_management = optional(object({<br/>      enabled      = optional(bool, false)<br/>      start_role   = optional(string, "")<br/>      suspend_role = optional(string, "")<br/>      frontend = optional(object({<br/>        image  = string<br/>        port   = optional(number, 8080)<br/>        cpu    = optional(string, "1000m")<br/>        memory = optional(string, "1Gi")<br/>      }), null)<br/>    }), {})<br/><br/>    rollout = optional(object({<br/>      enabled        = optional(bool, false)<br/>      release_url    = optional(string, "")<br/>      release_sha256 = optional(string, "")<br/>      port           = optional(number, 8081)<br/>      jwks_uri       = optional(string, "")<br/>      jwt_audience   = optional(string, "")<br/>      custom_claims  = optional(string, "")<br/>      allowed_ipv4   = optional(list(string), ["10.0.0.0/8"])<br/>    }), {})<br/>  })</pre> | `{}` | no |
+| <a name="input_gcp"></a> [gcp](#input\_gcp) | Google Cloud infrastructure settings. | <pre>object({<br/>    project_id     = optional(string, "")<br/>    project_number = optional(string, "")<br/>    region         = optional(string, "us-east5")<br/>    zone           = optional(string, "us-east5-b")<br/><br/>    identity = optional(object({<br/>      vm_service_account_email  = optional(string, "")<br/>      app_service_account_email = optional(string, "")<br/>      app_credentials_enabled   = optional(bool, false)<br/>    }), {})<br/><br/>    instance = optional(object({<br/>      machine_type = optional(string, "n4-standard-2")<br/>      os           = optional(string, "cos-125-19216-220-185")<br/>      production   = optional(bool, false)<br/>    }), {})<br/><br/>    disks = optional(object({<br/>      type                   = optional(string, "hyperdisk-balanced")<br/>      docker_volumes_size_gb = optional(number, 50)<br/>      attachments = optional(map(object({<br/>        source = string<br/>        mode   = optional(string, "READ_WRITE")<br/>      })), {})<br/>    }), {})<br/><br/>    network = optional(object({<br/>      create                   = optional(bool, true)<br/>      project_id               = optional(string, "")<br/>      name                     = optional(string, "")<br/>      subnetwork               = optional(string, "")<br/>      ip_cidr_range            = optional(string, "10.42.0.0/24")<br/>      mtu                      = optional(number, 1460)<br/>      power_button_allowed_ips = optional(list(string), [])<br/>      power_button_ip_depth    = optional(number)<br/>      ssh_ipv4                 = optional(list(string), [])<br/>      ssh_ipv6                 = optional(list(string), [])<br/>    }), {})<br/><br/>    snapshots = optional(object({<br/>      enabled = optional(bool, false)<br/>    }), {})<br/><br/>    overlay = optional(object({<br/>      source_instance = optional(string, "")<br/>      volume_names    = optional(list(string), [])<br/>    }), {})<br/><br/>    cloud_init = optional(object({<br/>      initcmd = optional(list(string), [])<br/>      runcmd  = optional(list(string), [])<br/>    }), {})<br/><br/>    artifact_registry = optional(object({<br/>      repository = optional(string, "")<br/>      location   = optional(string, "us")<br/>    }), {})<br/><br/>    power_management = optional(object({<br/>      enabled      = optional(bool, false)<br/>      start_role   = optional(string, "")<br/>      suspend_role = optional(string, "")<br/>      frontend = optional(object({<br/>        image  = string<br/>        port   = optional(number, 8080)<br/>        cpu    = optional(string, "1000m")<br/>        memory = optional(string, "1Gi")<br/>      }), null)<br/>    }), {})<br/><br/>    rollout = optional(object({<br/>      enabled        = optional(bool, false)<br/>      release_url    = optional(string, "")<br/>      release_sha256 = optional(string, "")<br/>      port           = optional(number, 8081)<br/>      jwks_uri       = optional(string, "")<br/>      jwt_audience   = optional(string, "")<br/>      custom_claims  = optional(string, "")<br/>      allowed_ipv4   = optional(list(string), ["10.0.0.0/8"])<br/>    }), {})<br/>  })</pre> | `{}` | no |
 | <a name="input_runtime"></a> [runtime](#input\_runtime) | Provider-neutral compose/runtime settings. | <pre>object({<br/>    rootfs                = optional(string, "")<br/>    rootfs_archive_url    = optional(string, "")<br/>    rootfs_archive_sha256 = optional(string, "")<br/>    users                 = optional(map(list(string)), {})<br/><br/>    compose = optional(object({<br/>      primary      = optional(string, "")<br/>      ingress_port = optional(number, 80)<br/>      ingress = optional(object({<br/>        letsencrypt     = optional(bool, false)<br/>        bot_mitigation  = optional(bool, false)<br/>        mode            = optional(string, "")<br/>        domain          = optional(string, "")<br/>        acme_email      = optional(string, "")<br/>        trusted_ips     = optional(list(string), [])<br/>        max_upload_size = optional(string, "")<br/>        upload_timeout  = optional(string, "")<br/>      }), {})<br/>      repo   = optional(string, "")<br/>      branch = optional(string, "")<br/>      projects = optional(map(object({<br/>        docker_compose_repo   = string<br/>        docker_compose_branch = optional(string)<br/>        project_dir           = optional(string)<br/>        compose_project_name  = optional(string)<br/>        ingress_port          = optional(number)<br/>        ingress = optional(object({<br/>          letsencrypt     = optional(bool)<br/>          bot_mitigation  = optional(bool)<br/>          mode            = optional(string)<br/>          domain          = optional(string)<br/>          acme_email      = optional(string)<br/>          trusted_ips     = optional(list(string))<br/>          max_upload_size = optional(string)<br/>          upload_timeout  = optional(string)<br/>        }), {})<br/>        sitectl_context_name   = optional(string)<br/>        sitectl_plugin         = optional(string)<br/>        sitectl_environment    = optional(string)<br/>        sitectl_packages       = optional(list(string))<br/>        sitectl_verify_args    = optional(list(string))<br/>        docker_compose_init    = optional(list(string))<br/>        docker_compose_up      = optional(list(string))<br/>        docker_compose_down    = optional(list(string))<br/>        docker_compose_rollout = optional(list(string))<br/>      })), {})<br/>      init    = optional(list(string))<br/>      up      = optional(list(string))<br/>      down    = optional(list(string))<br/>      rollout = optional(list(string))<br/>    }), {})<br/><br/>    sitectl = optional(object({<br/>      packages         = optional(list(string))<br/>      version          = optional(string, "latest")<br/>      package_versions = optional(map(string), {})<br/>      context_name     = optional(string, "")<br/>      plugin           = optional(string, "core")<br/>      environment      = optional(string, "production")<br/>      verify_args      = optional(list(string), [])<br/>    }), {})<br/><br/>    docker = optional(object({<br/>      # renovate: datasource=github-releases depName=docker-compose packageName=docker/compose versioning=semver<br/>      compose_version = optional(string, "v5.3.1")<br/>      # renovate: datasource=github-releases depName=docker-buildx packageName=docker/buildx versioning=semver<br/>      buildx_version = optional(string, "v0.35.0")<br/>    }), {})<br/><br/>    managed_runtime = optional(object({<br/>      enabled                       = optional(bool, true)<br/>      internal_services_enabled     = optional(bool, false)<br/>      internal_services_auto_update = optional(bool, false)<br/>      artifacts = optional(list(object({<br/>        name    = string<br/>        url     = string<br/>        sha256  = string<br/>        path    = string<br/>        mode    = optional(string, "0755")<br/>        owner   = optional(string, "root")<br/>        group   = optional(string, "root")<br/>        restart = optional(string, "")<br/>      })), [])<br/>    }), {})<br/><br/>    vault = optional(object({<br/>      addr                    = optional(string, "")<br/>      namespace               = optional(string, "")<br/>      role                    = optional(string, "")<br/>      agent_enabled           = optional(bool, false)<br/>      auth_method             = optional(string, "auto")<br/>      gcp_auth_mount_path     = optional(string, "auth/gcp")<br/>      agent_token_path        = optional(string, "/mnt/disks/data/vault/token")<br/>      agent_additional_config = optional(string, "")<br/>      agent_templates = optional(list(object({<br/>        destination = string<br/>        contents    = string<br/>        perms       = optional(string, "0640")<br/>        command     = optional(string, "")<br/>      })), [])<br/>    }), {})<br/><br/>    extra_env = optional(map(string), {})<br/>  })</pre> | `{}` | no |
 | <a name="input_template"></a> [template](#input\_template) | Optional compose template preset. Supported values are archivesspace, ojs, isle, drupal, wp, omeka-s, and omeka-classic. Explicit runtime settings override preset defaults. | `string` | `""` | no |
 
diff --git a/docs/runtime-contracts.md b/docs/runtime-contracts.md
index 9ad177c..beac237 100644
--- a/docs/runtime-contracts.md
+++ b/docs/runtime-contracts.md
@@ -226,6 +226,23 @@ Dependency installation dispatches by OS family:
 Provider modules should mount persistent data and Docker-volume disks before the
 runtime starts. Destroying/recreating the VM must not destroy these volumes.
 
+GCP callers can add persistent disks through `gcp.disks.attachments`. The map
+key is the GCE device name, `source` is the caller-owned disk self-link, and
+`mode` defaults to `READ_WRITE` but can be set to `READ_ONLY`. Cloud-compose
+owns the complete inline attachment set on the instance so every configured
+disk is present when the VM starts; the caller continues to own each disk's
+lifecycle. Device names must be valid GCE names and cannot collide with the
+module-owned `boot`, `data`, `docker-volumes`, or `prod-volumes` names.
+
+Do not manage the same VM/disk pair with a standalone
+`google_compute_attached_disk`; that creates two Terraform owners for one
+attachment. When migrating an existing standalone attachment, abandon the old
+resource from state without destroying it (for example, a `removed` block with
+`destroy = false` on supported Terraform versions), add the inline attachment,
+and inspect the transition plan to confirm it contains no detach. A normal
+destroy of the old standalone resource can detach the disk before the inline
+owner takes over.
+
 Every Compose `project_dir` must be a normalized descendant of the fixed
 `/mnt/disks/data` ownership boundary. Terraform rejects paths such as `/`,
 `/etc`, repeated separators, and dot-segment traversal during planning. The
diff --git a/main.tf b/main.tf
index dab7d7c..038452a 100644
--- a/main.tf
+++ b/main.tf
@@ -114,8 +114,9 @@ module "gcp" {
   os           = local.gcp_instance.os
   production   = local.gcp_instance.production
 
-  disk_type    = local.gcp_disks.type
-  disk_size_gb = local.gcp_disks.docker_volumes_size_gb
+  disk_type        = local.gcp_disks.type
+  disk_size_gb     = local.gcp_disks.docker_volumes_size_gb
+  disk_attachments = local.gcp_disks.attachments
 
   ingress_port            = local.compose.ingress_port
   primary_compose_project = local.compose.primary
diff --git a/modules/gcp/main.tf b/modules/gcp/main.tf
index 09012d9..fd19032 100644
--- a/modules/gcp/main.tf
+++ b/modules/gcp/main.tf
@@ -799,6 +799,16 @@ resource "google_compute_instance" "cloud-compose" {
     }
   }
 
+  dynamic "attached_disk" {
+    for_each = var.disk_attachments
+    iterator = caller_disk
+    content {
+      device_name = caller_disk.key
+      source      = trimspace(caller_disk.value.source)
+      mode        = caller_disk.value.mode
+    }
+  }
+
   metadata = {
     google-logging-enabled       = "true"
     google-logging-use-fluentbit = "true"
diff --git a/modules/gcp/outputs.tf b/modules/gcp/outputs.tf
index 8d62e29..49d7a6b 100644
--- a/modules/gcp/outputs.tf
+++ b/modules/gcp/outputs.tf
@@ -29,13 +29,19 @@ output "instance" {
       name      = google_compute_disk.docker-volumes.name
       self_link = google_compute_disk.docker-volumes.self_link
     }
+    attached_disks = {
+      for disk in google_compute_instance.cloud-compose.attached_disk : disk.device_name => {
+        source = disk.source
+        mode   = disk.mode
+      } if contains(keys(var.disk_attachments), disk.device_name)
+    }
     gsa : {
       email : local.vm_service_account_email,
       id : local.vm_service_account_id,
       name : local.vm_service_account_name,
     }
   }
-  description = "The GCP instance identity, network placement, replaceable boot disk, persistent disks, and VM service account. The legacy disk field remains the Docker-volume disk name."
+  description = "The GCP instance identity, network placement, replaceable boot disk, persistent disks, caller-owned attachments, and VM service account. The legacy disk field remains the Docker-volume disk name."
 }
 
 output "network" {
diff --git a/modules/gcp/runtime_contracts.tftest.hcl b/modules/gcp/runtime_contracts.tftest.hcl
index 5d3e1d2..3d9c901 100644
--- a/modules/gcp/runtime_contracts.tftest.hcl
+++ b/modules/gcp/runtime_contracts.tftest.hcl
@@ -88,6 +88,137 @@ run "disables_privileged_services_by_default" {
   }
 }
 
+run "attaches_caller_owned_disks_inline" {
+  command = plan
+
+  variables {
+    name                = "gcp-contract"
+    project_id          = "test-project"
+    docker_compose_repo = "https://github.com/libops/wp.git"
+    disk_attachments = {
+      mariadb-backups = {
+        source = "https://www.googleapis.com/compute/v1/projects/test-project/zones/us-east5-b/disks/mariadb-backups"
+      }
+      reference-data = {
+        source = "https://www.googleapis.com/compute/v1/projects/test-project/zones/us-east5-b/disks/reference-data"
+        mode   = "READ_ONLY"
+      }
+    }
+  }
+
+  assert {
+    condition = (
+      length(google_compute_instance.cloud-compose.attached_disk) == 4 &&
+      one([
+        for disk in google_compute_instance.cloud-compose.attached_disk : disk
+        if disk.device_name == "mariadb-backups"
+      ]).source == "https://www.googleapis.com/compute/v1/projects/test-project/zones/us-east5-b/disks/mariadb-backups" &&
+      one([
+        for disk in google_compute_instance.cloud-compose.attached_disk : disk
+        if disk.device_name == "mariadb-backups"
+      ]).mode == "READ_WRITE" &&
+      one([
+        for disk in google_compute_instance.cloud-compose.attached_disk : disk
+        if disk.device_name == "reference-data"
+      ]).source == "https://www.googleapis.com/compute/v1/projects/test-project/zones/us-east5-b/disks/reference-data" &&
+      one([
+        for disk in google_compute_instance.cloud-compose.attached_disk : disk
+        if disk.device_name == "reference-data"
+      ]).mode == "READ_ONLY"
+    )
+    error_message = "Caller-owned disks must be part of the VM's authoritative inline attachment set with explicit modes."
+  }
+}
+
+run "rejects_reserved_disk_attachment_name" {
+  command = plan
+
+  variables {
+    name                = "gcp-contract"
+    project_id          = "test-project"
+    docker_compose_repo = "https://github.com/libops/wp.git"
+    disk_attachments = {
+      data = {
+        source = "https://www.googleapis.com/compute/v1/projects/test-project/zones/us-east5-b/disks/extra-data"
+      }
+    }
+  }
+
+  expect_failures = [var.disk_attachments]
+}
+
+run "rejects_invalid_disk_attachment_name" {
+  command = plan
+
+  variables {
+    name                = "gcp-contract"
+    project_id          = "test-project"
+    docker_compose_repo = "https://github.com/libops/wp.git"
+    disk_attachments = {
+      "Invalid_Name" = {
+        source = "https://www.googleapis.com/compute/v1/projects/test-project/zones/us-east5-b/disks/invalid-name"
+      }
+    }
+  }
+
+  expect_failures = [var.disk_attachments]
+}
+
+run "rejects_blank_disk_attachment_source" {
+  command = plan
+
+  variables {
+    name                = "gcp-contract"
+    project_id          = "test-project"
+    docker_compose_repo = "https://github.com/libops/wp.git"
+    disk_attachments = {
+      backups = {
+        source = "   "
+      }
+    }
+  }
+
+  expect_failures = [var.disk_attachments]
+}
+
+run "rejects_invalid_disk_attachment_mode" {
+  command = plan
+
+  variables {
+    name                = "gcp-contract"
+    project_id          = "test-project"
+    docker_compose_repo = "https://github.com/libops/wp.git"
+    disk_attachments = {
+      backups = {
+        source = "https://www.googleapis.com/compute/v1/projects/test-project/zones/us-east5-b/disks/backups"
+        mode   = "READ_MOSTLY"
+      }
+    }
+  }
+
+  expect_failures = [var.disk_attachments]
+}
+
+run "rejects_duplicate_disk_attachment_source" {
+  command = plan
+
+  variables {
+    name                = "gcp-contract"
+    project_id          = "test-project"
+    docker_compose_repo = "https://github.com/libops/wp.git"
+    disk_attachments = {
+      backups-a = {
+        source = "https://www.googleapis.com/compute/v1/projects/test-project/zones/us-east5-b/disks/backups"
+      }
+      backups-b = {
+        source = "https://www.googleapis.com/compute/v1/projects/test-project/zones/us-east5-b/disks/backups"
+      }
+    }
+  }
+
+  expect_failures = [var.disk_attachments]
+}
+
 run "creates_app_key_management_only_when_explicitly_enabled" {
   command = plan
 
diff --git a/modules/gcp/upgrade_dependency.tftest.hcl b/modules/gcp/upgrade_dependency.tftest.hcl
index a45c1dc..42e568c 100644
--- a/modules/gcp/upgrade_dependency.tftest.hcl
+++ b/modules/gcp/upgrade_dependency.tftest.hcl
@@ -32,6 +32,11 @@ variables {
   vault_auth_method          = "gcp-iam"
   vault_addr                 = "https://vault.example"
   vault_role                 = "wordpress"
+  disk_attachments = {
+    mariadb-backups = {
+      source = "https://www.googleapis.com/compute/v1/projects/test-project/zones/us-east5-b/disks/mariadb-backups"
+    }
+  }
 }
 
 run "apply_power_managed_vault_baseline" {
@@ -54,8 +59,16 @@ run "plan_vm_replacement_while_retiring_vault_signer" {
       length(google_compute_instance_iam_member.gce-start) == 1 &&
       length(google_compute_instance_iam_member.gce-suspend) == 1 &&
       google_compute_instance_iam_member.gce-start[0].instance_name == google_compute_instance.cloud-compose.name &&
-      google_compute_instance_iam_member.gce-suspend[0].instance_name == google_compute_instance.cloud-compose.name
+      google_compute_instance_iam_member.gce-suspend[0].instance_name == google_compute_instance.cloud-compose.name &&
+      one([
+        for disk in google_compute_instance.cloud-compose.attached_disk : disk
+        if disk.device_name == "mariadb-backups"
+      ]).source == "https://www.googleapis.com/compute/v1/projects/test-project/zones/us-east5-b/disks/mariadb-backups" &&
+      one([
+        for disk in google_compute_instance.cloud-compose.attached_disk : disk
+        if disk.device_name == "mariadb-backups"
+      ]).mode == "READ_WRITE"
     )
-    error_message = "A VM replacement must retain both instance-scoped power bindings without introducing an upgrade dependency cycle."
+    error_message = "A VM replacement must retain instance-scoped power bindings and caller-owned disks without introducing an upgrade dependency cycle."
   }
 }
diff --git a/modules/gcp/variables.tf b/modules/gcp/variables.tf
index eb1e3f9..b50e437 100644
--- a/modules/gcp/variables.tf
+++ b/modules/gcp/variables.tf
@@ -179,6 +179,48 @@ variable "disk_size_gb" {
   description = "Data disk size in GB"
 }
 
+variable "disk_attachments" {
+  type = map(object({
+    source = string
+    mode   = optional(string, "READ_WRITE")
+  }))
+  default     = {}
+  description = "Caller-owned persistent disks to attach inline to the VM, keyed by GCE device name. The caller retains disk lifecycle ownership."
+
+  validation {
+    condition = alltrue([
+      for device_name in keys(var.disk_attachments) :
+      can(regex("^[a-z]([a-z0-9-]{0,61}[a-z0-9])?$", device_name)) &&
+      !contains(["boot", "data", "docker-volumes", "prod-volumes"], device_name)
+    ])
+    error_message = "disk_attachments keys must be valid GCE device names and must not use boot, data, docker-volumes, or prod-volumes."
+  }
+
+  validation {
+    condition = alltrue([
+      for attachment in values(var.disk_attachments) :
+      trimspace(attachment.source) != ""
+    ])
+    error_message = "disk_attachments sources must not be empty."
+  }
+
+  validation {
+    condition = alltrue([
+      for attachment in values(var.disk_attachments) :
+      contains(["READ_ONLY", "READ_WRITE"], attachment.mode)
+    ])
+    error_message = "disk_attachments modes must be READ_ONLY or READ_WRITE."
+  }
+
+  validation {
+    condition = length(distinct([
+      for attachment in values(var.disk_attachments) :
+      trimspace(attachment.source)
+    ])) == length(var.disk_attachments)
+    error_message = "disk_attachments must not attach the same source more than once."
+  }
+}
+
 variable "os" {
   type        = string
   default     = "cos-125-19216-220-185"
diff --git a/providers/gcp/main.tf b/providers/gcp/main.tf
index 308a9d3..d869006 100644
--- a/providers/gcp/main.tf
+++ b/providers/gcp/main.tf
@@ -98,8 +98,9 @@ module "gcp" {
   os           = local.gcp_instance.os
   production   = local.gcp_instance.production
 
-  disk_type    = local.gcp_disks.type
-  disk_size_gb = local.gcp_disks.docker_volumes_size_gb
+  disk_type        = local.gcp_disks.type
+  disk_size_gb     = local.gcp_disks.docker_volumes_size_gb
+  disk_attachments = local.gcp_disks.attachments
 
   ingress_port            = local.compose.ingress_port
   primary_compose_project = local.compose.primary
diff --git a/providers/gcp/template_versions.tftest.hcl b/providers/gcp/template_versions.tftest.hcl
index dd10162..1b5c5fb 100644
--- a/providers/gcp/template_versions.tftest.hcl
+++ b/providers/gcp/template_versions.tftest.hcl
@@ -73,6 +73,35 @@ run "explicit_core_only_package_set_disables_template_plugins" {
   }
 }
 
+run "gcp_entrypoint_forwards_caller_owned_disks" {
+  command = plan
+
+  variables {
+    name     = "template-versions"
+    template = "wp"
+    gcp = {
+      project_id = "test-project"
+      disks = {
+        attachments = {
+          mariadb-backups = {
+            source = "https://www.googleapis.com/compute/v1/projects/test-project/zones/us-east5-b/disks/mariadb-backups"
+          }
+        }
+      }
+    }
+  }
+
+  assert {
+    condition = output.instance.attached_disks == {
+      mariadb-backups = {
+        source = "https://www.googleapis.com/compute/v1/projects/test-project/zones/us-east5-b/disks/mariadb-backups"
+        mode   = "READ_WRITE"
+      }
+    }
+    error_message = "The GCP entrypoint must forward caller-owned disks into the VM's inline attachment set."
+  }
+}
+
 run "accepts_direct_cloud_run_proxy_depth" {
   command = plan
 
diff --git a/providers/gcp/variables.tf b/providers/gcp/variables.tf
index 0cbf042..4522e61 100644
--- a/providers/gcp/variables.tf
+++ b/providers/gcp/variables.tf
@@ -42,6 +42,10 @@ variable "gcp" {
     disks = optional(object({
       type                   = optional(string, "hyperdisk-balanced")
       docker_volumes_size_gb = optional(number, 50)
+      attachments = optional(map(object({
+        source = string
+        mode   = optional(string, "READ_WRITE")
+      })), {})
     }), {})
 
     network = optional(object({
@@ -110,6 +114,39 @@ variable "gcp" {
     error_message = "gcp.disks.type must be hyperdisk-balanced, pd-ssd, or pd-standard."
   }
 
+  validation {
+    condition = alltrue([
+      for device_name in keys(var.gcp.disks.attachments) :
+      can(regex("^[a-z]([a-z0-9-]{0,61}[a-z0-9])?$", device_name)) &&
+      !contains(["boot", "data", "docker-volumes", "prod-volumes"], device_name)
+    ])
+    error_message = "gcp.disks.attachments keys must be valid GCE device names and must not use boot, data, docker-volumes, or prod-volumes."
+  }
+
+  validation {
+    condition = alltrue([
+      for attachment in values(var.gcp.disks.attachments) :
+      trimspace(attachment.source) != ""
+    ])
+    error_message = "gcp.disks.attachments sources must not be empty."
+  }
+
+  validation {
+    condition = alltrue([
+      for attachment in values(var.gcp.disks.attachments) :
+      contains(["READ_ONLY", "READ_WRITE"], attachment.mode)
+    ])
+    error_message = "gcp.disks.attachments modes must be READ_ONLY or READ_WRITE."
+  }
+
+  validation {
+    condition = length(distinct([
+      for attachment in values(var.gcp.disks.attachments) :
+      trimspace(attachment.source)
+    ])) == length(var.gcp.disks.attachments)
+    error_message = "gcp.disks.attachments must not attach the same source more than once."
+  }
+
   validation {
     condition = contains([
       "e2-micro",
diff --git a/runtime_contracts.tftest.hcl b/runtime_contracts.tftest.hcl
index e149839..cfbafff 100644
--- a/runtime_contracts.tftest.hcl
+++ b/runtime_contracts.tftest.hcl
@@ -74,6 +74,36 @@ run "public_entrypoint_exposes_sitectl_package_versions" {
   }
 }
 
+run "public_entrypoint_forwards_caller_owned_disks" {
+  command = plan
+
+  variables {
+    name           = "root-contract"
+    cloud_provider = "gcp"
+    template       = "wp"
+    gcp = {
+      project_id = "test-project"
+      disks = {
+        attachments = {
+          mariadb-backups = {
+            source = "https://www.googleapis.com/compute/v1/projects/test-project/zones/us-east5-b/disks/mariadb-backups"
+          }
+        }
+      }
+    }
+  }
+
+  assert {
+    condition = output.instance.attached_disks == {
+      mariadb-backups = {
+        source = "https://www.googleapis.com/compute/v1/projects/test-project/zones/us-east5-b/disks/mariadb-backups"
+        mode   = "READ_WRITE"
+      }
+    }
+    error_message = "The root entrypoint must forward caller-owned disks into the VM's inline attachment set."
+  }
+}
+
 run "public_entrypoint_rejects_reserved_extra_environment" {
   command = plan
 
diff --git a/variables.tf b/variables.tf
index 9905ee6..e8fa435 100644
--- a/variables.tf
+++ b/variables.tf
@@ -48,6 +48,10 @@ variable "gcp" {
     disks = optional(object({
       type                   = optional(string, "hyperdisk-balanced")
       docker_volumes_size_gb = optional(number, 50)
+      attachments = optional(map(object({
+        source = string
+        mode   = optional(string, "READ_WRITE")
+      })), {})
     }), {})
 
     network = optional(object({
@@ -116,6 +120,39 @@ variable "gcp" {
     error_message = "gcp.disks.type must be hyperdisk-balanced, pd-ssd, or pd-standard."
   }
 
+  validation {
+    condition = alltrue([
+      for device_name in keys(var.gcp.disks.attachments) :
+      can(regex("^[a-z]([a-z0-9-]{0,61}[a-z0-9])?$", device_name)) &&
+      !contains(["boot", "data", "docker-volumes", "prod-volumes"], device_name)
+    ])
+    error_message = "gcp.disks.attachments keys must be valid GCE device names and must not use boot, data, docker-volumes, or prod-volumes."
+  }
+
+  validation {
+    condition = alltrue([
+      for attachment in values(var.gcp.disks.attachments) :
+      trimspace(attachment.source) != ""
+    ])
+    error_message = "gcp.disks.attachments sources must not be empty."
+  }
+
+  validation {
+    condition = alltrue([
+      for attachment in values(var.gcp.disks.attachments) :
+      contains(["READ_ONLY", "READ_WRITE"], attachment.mode)
+    ])
+    error_message = "gcp.disks.attachments modes must be READ_ONLY or READ_WRITE."
+  }
+
+  validation {
+    condition = length(distinct([
+      for attachment in values(var.gcp.disks.attachments) :
+      trimspace(attachment.source)
+    ])) == length(var.gcp.disks.attachments)
+    error_message = "gcp.disks.attachments must not attach the same source more than once."
+  }
+
   validation {
     condition = contains([
       "e2-micro",