diff --git a/docs/development/contributing.md b/docs/development/contributing.md
index 1bb04e730..27367b946 100644
--- a/docs/development/contributing.md
+++ b/docs/development/contributing.md
@@ -72,6 +72,7 @@ Make sure your code adheres to our Rust and TypeScript code style by running:
 
  - `cargo +nightly fmt` (with the nightly toolchain installed)
  - `npm run format` in the `frontend` directory
+ - `make fmt` in the `policies` directory (if changed)
 
 When updating SQL queries in the `crates/storage-pg/` crate, you may need to update the `sqlx` introspection data. To do this, make sure to install `cargo-sqlx` (`cargo install sqlx-cli`) and:
 
@@ -86,6 +87,7 @@ While you're developing and before submitting a patch, you'll want to test your
 
 - Run `cargo clippy --workspace` to lint the Rust code.
 - Run `npm run lint` in the `frontend` directory to lint the frontend code.
+- Run `make fmt` and `make lint` in the `policies` directory to format and lint the included policy.
 
 ### Run the tests
 
@@ -93,6 +95,10 @@ If you haven't already, install [Cargo-Nextest](https://nexte.st/docs/installati
 
 - Run the tests to the backend by running `cargo nextest run --workspace`. This requires a connection to a PostgreSQL database, set via the `DATABASE_URL` environment variable.
 - Run the tests to the frontend by running `npm run test` in the `frontend` directory.
+- To run the tests for the included policy, change to the `policies` directory and run one of:
+  - `make test` (needs Open Policy Agent installed)
+  - `make PODMAN=1 test` (runs inside a container; needs Podman installed)
+  - `make DOCKER=1 test` (runs inside a container; needs Docker installed)
 
 ## 8. Submit a pull request
 
diff --git a/policies/Makefile b/policies/Makefile
index 421990d07..0d515b904 100644
--- a/policies/Makefile
+++ b/policies/Makefile
@@ -24,9 +24,9 @@ ifeq ($(DOCKER), 1)
 	REGAL := docker run -i -v $(shell pwd):/policies:ro -w /policies --rm $(REGAL_DOCKER_IMAGE)
 else ifeq ($(PODMAN), 1)
 	# When running rootless, the volume directory may need to be given global write permissions on the host
-	OPA := podman run -i -v $(shell pwd):/policies:ro:Z -w /policies --rm $(OPA_DOCKER_IMAGE)
+	OPA := podman run -i -v $(shell pwd):/policies:ro,Z -w /policies --rm $(OPA_DOCKER_IMAGE)
 	OPA_RW := podman run -i -v $(shell pwd):/policies:Z -w /policies --rm $(OPA_DOCKER_IMAGE)
-	REGAL := podman run -i -v $(shell pwd):/policies:ro:Z -w /policies --rm $(REGAL_DOCKER_IMAGE)
+	REGAL := podman run -i -v $(shell pwd):/policies:ro,Z -w /policies --rm $(REGAL_DOCKER_IMAGE)
 else
 	OPA := opa
 	OPA_RW := opa
diff --git a/policies/authorization_grant/authorization_grant.rego b/policies/authorization_grant/authorization_grant.rego
index b409cc889..79f737af1 100644
--- a/policies/authorization_grant/authorization_grant.rego
+++ b/policies/authorization_grant/authorization_grant.rego
@@ -98,6 +98,26 @@ uses_stable_scopes if {
 	count({scope | some scope in scope_list; startswith(scope, "urn:matrix:client:")}) > 0
 }
 
+has_device_scope if {
+	scope_list := split(input.scope, " ")
+	count({scope | some scope in scope_list; startswith(scope, "urn:matrix:client:device:")}) > 0
+}
+
+has_device_scope if {
+	scope_list := split(input.scope, " ")
+	count({scope | some scope in scope_list; startswith(scope, "urn:matrix:org.matrix.msc2967.client:device:")}) > 0
+}
+
+has_cs_api_scope if {
+	scope_list := split(input.scope, " ")
+	count({scope | some scope in scope_list; startswith(scope, "urn:matrix:client:api:")}) > 0
+}
+
+has_cs_api_scope if {
+	scope_list := split(input.scope, " ")
+	count({scope | some scope in scope_list; startswith(scope, "urn:matrix:org.matrix.msc2967.client:api:")}) > 0
+}
+
 # METADATA
 # entrypoint: true
 violation contains {"msg": msg} if {
@@ -116,6 +136,12 @@ violation contains {"msg": "only one device scope is allowed at a time"} if {
 	count({scope | some scope in scope_list; startswith(scope, "urn:matrix:client:device:")}) > 1
 }
 
+# Prevent the creation of C-S API devices for sessions that don't have C-S API access.
+violation contains {"msg": "device scopes are only allowed when the client-server API scope is requested"} if {
+	has_device_scope
+	not has_cs_api_scope
+}
+
 violation contains {"msg": "request cannot mix unstable and stable scopes"} if {
 	uses_stable_scopes
 	uses_unstable_scopes
diff --git a/policies/authorization_grant/authorization_grant_test.rego b/policies/authorization_grant/authorization_grant_test.rego
index fdde534d0..6634eacb9 100644
--- a/policies/authorization_grant/authorization_grant_test.rego
+++ b/policies/authorization_grant/authorization_grant_test.rego
@@ -78,70 +78,84 @@ test_unstable_device_scopes if {
 	authorization_grant.allow with input.user as user
 		with input.client as client
 		with input.grant_type as "authorization_code"
-		with input.scope as "urn:matrix:org.matrix.msc2967.client:device:AAbbCCdd01"
+		with input.scope as "urn:matrix:org.matrix.msc2967.client:api:* urn:matrix:org.matrix.msc2967.client:device:AAbbCCdd01"
 
 	authorization_grant.allow with input.user as user
 		with input.client as client
 		with input.grant_type as "authorization_code"
-		with input.scope as "urn:matrix:org.matrix.msc2967.client:device:AAbbCCdd01-asdasdsa1-2313"
+		with input.scope as "urn:matrix:org.matrix.msc2967.client:api:* urn:matrix:org.matrix.msc2967.client:device:AAbbCCdd01-asdasdsa1-2313"
 
 	# Too short
 	not authorization_grant.allow with input.user as user
 		with input.client as client
 		with input.grant_type as "authorization_code"
-		with input.scope as "urn:matrix:org.matrix.msc2967.client:device:abcd"
+		with input.scope as "urn:matrix:org.matrix.msc2967.client:api:* urn:matrix:org.matrix.msc2967.client:device:abcd"
 
 	# Multiple device scope
 	not authorization_grant.allow with input.user as user
 		with input.client as client
 		with input.grant_type as "authorization_code"
-		with input.scope as "urn:matrix:org.matrix.msc2967.client:device:AAbbCCdd01 urn:matrix:org.matrix.msc2967.client:device:AAbbCCdd02"
+		with input.scope as "urn:matrix:org.matrix.msc2967.client:api:* urn:matrix:org.matrix.msc2967.client:device:AAbbCCdd01 urn:matrix:org.matrix.msc2967.client:device:AAbbCCdd02"
 
 	# Allowed with the device code grant
 	authorization_grant.allow with input.user as user
 		with input.client as client
 		with input.grant_type as "urn:ietf:params:oauth:grant-type:device_code"
-		with input.scope as "urn:matrix:org.matrix.msc2967.client:device:AAbbCCdd01"
+		with input.scope as "urn:matrix:org.matrix.msc2967.client:api:* urn:matrix:org.matrix.msc2967.client:device:AAbbCCdd01"
 
 	# Not authorization_grant.allowed for the client credentials grant
 	not authorization_grant.allow with input.client as client
 		with input.grant_type as "client_credentials"
-		with input.scope as "urn:matrix:org.matrix.msc2967.client:device:AAbbCCdd01"
+		with input.scope as "urn:matrix:org.matrix.msc2967.client:api:* urn:matrix:org.matrix.msc2967.client:device:AAbbCCdd01"
 }
 
 test_stable_device_scopes if {
 	authorization_grant.allow with input.user as user
 		with input.client as client
 		with input.grant_type as "authorization_code"
-		with input.scope as "urn:matrix:client:device:AAbbCCdd01"
+		with input.scope as "urn:matrix:client:api:* urn:matrix:client:device:AAbbCCdd01"
 
 	authorization_grant.allow with input.user as user
 		with input.client as client
 		with input.grant_type as "authorization_code"
-		with input.scope as "urn:matrix:client:device:AAbbCCdd01-asdasdsa1-2313"
+		with input.scope as "urn:matrix:client:api:* urn:matrix:client:device:AAbbCCdd01-asdasdsa1-2313"
 
 	# Too short
 	not authorization_grant.allow with input.user as user
 		with input.client as client
 		with input.grant_type as "authorization_code"
-		with input.scope as "urn:matrix:client:device:abcd"
+		with input.scope as "urn:matrix:client:api:* urn:matrix:client:device:abcd"
 
 	# Multiple device scope
 	not authorization_grant.allow with input.user as user
 		with input.client as client
 		with input.grant_type as "authorization_code"
-		with input.scope as "urn:matrix:client:device:AAbbCCdd01 urn:matrix:client:device:AAbbCCdd02"
+		with input.scope as "urn:matrix:client:api:* urn:matrix:client:device:AAbbCCdd01 urn:matrix:client:device:AAbbCCdd02"
 
 	# Allowed with the device code grant
 	authorization_grant.allow with input.user as user
 		with input.client as client
 		with input.grant_type as "urn:ietf:params:oauth:grant-type:device_code"
-		with input.scope as "urn:matrix:client:device:AAbbCCdd01"
+		with input.scope as "urn:matrix:client:api:* urn:matrix:client:device:AAbbCCdd01"
 
 	# Not authorization_grant.allowed for the client credentials grant
 	not authorization_grant.allow with input.client as client
 		with input.grant_type as "client_credentials"
-		with input.scope as "urn:matrix:client:device:AAbbCCdd01"
+		with input.scope as "urn:matrix:client:api:* urn:matrix:client:device:AAbbCCdd01"
+}
+
+test_device_scope_only_with_cs_api_scope if {
+	not authorization_grant.allow with input.user as user
+		with input.client as client
+		with input.grant_type as "authorization_code"
+		# Requested a device scope but no C-S API scope:
+with 		input.scope as "urn:matrix:client:device:AAbbCCdd01"
+
+	not authorization_grant.allow with input.user as user
+		with input.client as client
+		with input.grant_type as "authorization_code"
+		# Requested a device scope but no C-S API scope:
+with 		input.scope as "urn:matrix:org.matrix.msc2967.client:device:AAbbCCdd01"
 }
 
 test_mix_stable_and_unstable_scopes if {