diff --git a/Cargo.lock b/Cargo.lock
index b18613c3a8..b742f00b96 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -3763,6 +3763,7 @@ dependencies = [
  "shellexpand",
  "smallvec",
  "tempfile",
+ "test-case",
  "thiserror 2.0.20",
  "toml 1.1.4+spec-1.1.0",
  "tracing",
diff --git a/agents/references/language-server-threat-model.md b/agents/references/language-server-threat-model.md
index 65a65e9430..3d1bfb5b63 100644
--- a/agents/references/language-server-threat-model.md
+++ b/agents/references/language-server-threat-model.md
@@ -14,6 +14,7 @@ when deciding which inputs are trusted. Browser integrations have a separate
 - **Trusted local input:** the editor, its extensions and their bundled files, the local machine and
     its file system, and the user's configuration and trust decisions.
 
+Ruff treats workspaces as trusted unless the server starts with `--untrusted-workspace`.
 ty treats workspaces as trusted unless `untrustedWorkspace` is true at initialization. Trust extends
 to everything in the workspace, including source code, configuration, notebooks, and the targets of
 symlinks that point outside it.
diff --git a/crates/ruff/src/args.rs b/crates/ruff/src/args.rs
index 4b3cfa131d..de5d497a68 100644
--- a/crates/ruff/src/args.rs
+++ b/crates/ruff/src/args.rs
@@ -627,6 +627,10 @@ pub struct FormatCommand {
 
 #[derive(Copy, Clone, Debug, clap::Parser)]
 pub struct ServerCommand {
+    /// Treat all workspaces as untrusted, disabling the uv formatter backend.
+    #[arg(long)]
+    untrusted_workspace: bool,
+
     /// Enable preview mode. Use `--no-preview` to disable.
     ///
     /// This enables unstable server features and turns on the preview mode for the linter
@@ -638,6 +642,14 @@ pub struct ServerCommand {
 }
 
 impl ServerCommand {
+    pub(crate) fn resolve_workspace_trust(self) -> ruff_server::WorkspaceTrust {
+        if self.untrusted_workspace {
+            ruff_server::WorkspaceTrust::Untrusted
+        } else {
+            ruff_server::WorkspaceTrust::Trusted
+        }
+    }
+
     pub(crate) fn resolve_preview(self) -> Option<bool> {
         resolve_bool_arg(self.preview, self.no_preview)
     }
diff --git a/crates/ruff/src/commands/server.rs b/crates/ruff/src/commands/server.rs
index 837662e7d3..eba983ccb8 100644
--- a/crates/ruff/src/commands/server.rs
+++ b/crates/ruff/src/commands/server.rs
@@ -1,7 +1,11 @@
 use crate::ExitStatus;
 use anyhow::Result;
+use ruff_server::WorkspaceTrust;
 
-pub(crate) fn run_server(preview: Option<bool>) -> Result<ExitStatus> {
-    ruff_server::run(preview)?;
+pub(crate) fn run_server(
+    preview: Option<bool>,
+    workspace_trust: WorkspaceTrust,
+) -> Result<ExitStatus> {
+    ruff_server::run(preview, workspace_trust)?;
     Ok(ExitStatus::Success)
 }
diff --git a/crates/ruff/src/lib.rs b/crates/ruff/src/lib.rs
index f64359e567..b0cbd385b2 100644
--- a/crates/ruff/src/lib.rs
+++ b/crates/ruff/src/lib.rs
@@ -231,7 +231,7 @@ fn analyze_graph(
 }
 
 fn server(args: ServerCommand) -> Result<ExitStatus> {
-    commands::server::run_server(args.resolve_preview())
+    commands::server::run_server(args.resolve_preview(), args.resolve_workspace_trust())
 }
 
 pub fn check(args: CheckCommand, global_options: GlobalConfigArgs) -> Result<ExitStatus> {
diff --git a/crates/ruff_server/Cargo.toml b/crates/ruff_server/Cargo.toml
index 7140d72de8..82e1d093d3 100644
--- a/crates/ruff_server/Cargo.toml
+++ b/crates/ruff_server/Cargo.toml
@@ -54,6 +54,7 @@ dunce = { workspace = true }
 regex = { workspace = true }
 smallvec = { workspace = true }
 tempfile = { workspace = true }
+test-case = { workspace = true }
 
 [features]
 test-uv = []
diff --git a/crates/ruff_server/src/lib.rs b/crates/ruff_server/src/lib.rs
index 2df7a8bdfd..366d5efd24 100644
--- a/crates/ruff_server/src/lib.rs
+++ b/crates/ruff_server/src/lib.rs
@@ -36,11 +36,18 @@ const NOTEBOOK_SOURCE_ORGANIZE_IMPORTS_RUFF: CodeActionKind =
 /// result type is needed.
 pub(crate) type Result<T> = anyhow::Result<T>;
 
+/// Whether the server may execute code from the workspace.
+#[derive(Copy, Clone, Debug, Eq, PartialEq)]
+pub enum WorkspaceTrust {
+    Trusted,
+    Untrusted,
+}
+
 fn version() -> &'static str {
     ruff_linter::VERSION
 }
 
-pub fn run(preview: Option<bool>) -> Result<()> {
+pub fn run(preview: Option<bool>, workspace_trust: WorkspaceTrust) -> Result<()> {
     let four = NonZeroUsize::new(4).unwrap();
 
     // by default, we set the number of worker threads to `num_cpus`, with a maximum of 4.
@@ -50,7 +57,7 @@ pub fn run(preview: Option<bool>) -> Result<()> {
 
     let (connection, io_threads) = ConnectionInitializer::stdio();
 
-    let server_result = Server::new(worker_threads, connection, preview, false)
+    let server_result = Server::new(worker_threads, connection, preview, workspace_trust, false)
         .context("Failed to start server")?
         .run();
 
diff --git a/crates/ruff_server/src/server.rs b/crates/ruff_server/src/server.rs
index c046266d44..2cbb599438 100644
--- a/crates/ruff_server/src/server.rs
+++ b/crates/ruff_server/src/server.rs
@@ -24,11 +24,11 @@ use types::WorkspaceFoldersServerCapabilities;
 pub use self::connection::ConnectionInitializer;
 pub(crate) use self::connection::ConnectionSender;
 use self::schedule::spawn_main_loop;
-use crate::PositionEncoding;
 pub(crate) use crate::server::main_loop::MainLoopSender;
 pub(crate) use crate::server::main_loop::{Event, MainLoopReceiver};
 use crate::session::{AllOptions, Client, Session};
 use crate::workspace::Workspaces;
+use crate::{PositionEncoding, WorkspaceTrust};
 pub(crate) use api::Error;
 
 mod api;
@@ -52,6 +52,7 @@ impl Server {
         worker_threads: NonZeroUsize,
         connection: ConnectionInitializer,
         preview: Option<bool>,
+        workspace_trust: WorkspaceTrust,
         is_test: bool,
     ) -> crate::Result<Self> {
         let (id, init_params) = connection.initialize_start()?;
@@ -121,6 +122,7 @@ impl Server {
                 global,
                 &workspaces,
                 &client,
+                workspace_trust,
             )?,
             client_capabilities,
         })
diff --git a/crates/ruff_server/src/server/api/requests/format.rs b/crates/ruff_server/src/server/api/requests/format.rs
index 36948e5266..4b201613b1 100644
--- a/crates/ruff_server/src/server/api/requests/format.rs
+++ b/crates/ruff_server/src/server/api/requests/format.rs
@@ -45,10 +45,7 @@ impl super::BackgroundDocumentRequestHandler for Format {
 pub(super) fn format_full_document(snapshot: &DocumentSnapshot) -> Result<Fixes> {
     let mut fixes = Fixes::default();
     let query = snapshot.query();
-    let backend = snapshot
-        .client_settings()
-        .editor_settings()
-        .format_backend();
+    let backend = snapshot.format_backend();
 
     match snapshot.query() {
         DocumentQuery::Notebook { notebook, .. } => {
@@ -84,10 +81,7 @@ fn format_document(snapshot: &DocumentSnapshot) -> Result<super::FormatResponse>
         .context("Failed to get text document for the format request")
         .unwrap();
     let query = snapshot.query();
-    let backend = snapshot
-        .client_settings()
-        .editor_settings()
-        .format_backend();
+    let backend = snapshot.format_backend();
     format_text_document(
         text_document,
         query,
diff --git a/crates/ruff_server/src/server/api/requests/format_range.rs b/crates/ruff_server/src/server/api/requests/format_range.rs
index 3329a3358d..1e3d6f0dc7 100644
--- a/crates/ruff_server/src/server/api/requests/format_range.rs
+++ b/crates/ruff_server/src/server/api/requests/format_range.rs
@@ -47,10 +47,7 @@ fn format_document_range(
         .context("Failed to get text document for the format range request")
         .unwrap();
     let query = snapshot.query();
-    let backend = snapshot
-        .client_settings()
-        .editor_settings()
-        .format_backend();
+    let backend = snapshot.format_backend();
     format_text_document_range(text_document, range, query, snapshot.encoding(), backend)
 }
 
diff --git a/crates/ruff_server/src/session.rs b/crates/ruff_server/src/session.rs
index a6dda25a22..0b3a1d35a1 100644
--- a/crates/ruff_server/src/session.rs
+++ b/crates/ruff_server/src/session.rs
@@ -7,10 +7,11 @@ use lsp_types::{ClientCapabilities, FileEvent, NotebookDocumentCellChanges, Uri}
 use settings::ClientSettings;
 
 use crate::edit::{DocumentKey, DocumentVersion, NotebookDocument};
+use crate::format::FormatBackend;
 use crate::session::request_queue::RequestQueue;
 use crate::session::settings::GlobalClientSettings;
 use crate::workspace::Workspaces;
-use crate::{PositionEncoding, TextDocument};
+use crate::{PositionEncoding, TextDocument, WorkspaceTrust};
 
 pub(crate) use self::capabilities::ResolvedClientCapabilities;
 pub(crate) use self::index::DocumentQuery;
@@ -34,6 +35,9 @@ pub(crate) struct Session {
     /// Global settings provided by the client.
     global_settings: GlobalClientSettings,
 
+    /// Set at startup so client settings cannot enable external formatting in an untrusted workspace.
+    workspace_trust: WorkspaceTrust,
+
     /// Tracks what LSP features the client supports and doesn't support.
     resolved_client_capabilities: Arc<ResolvedClientCapabilities>,
 
@@ -51,6 +55,7 @@ pub(crate) struct DocumentSnapshot {
     client_settings: Arc<settings::ClientSettings>,
     document_ref: index::DocumentQuery,
     position_encoding: PositionEncoding,
+    workspace_trust: WorkspaceTrust,
 }
 
 impl Session {
@@ -60,11 +65,13 @@ impl Session {
         global: GlobalClientSettings,
         workspaces: &Workspaces,
         client: &Client,
+        workspace_trust: WorkspaceTrust,
     ) -> crate::Result<Self> {
         Ok(Self {
             position_encoding,
             index: index::Index::new(workspaces, &global, client)?,
             global_settings: global,
+            workspace_trust,
             resolved_client_capabilities: Arc::new(ResolvedClientCapabilities::new(
                 client_capabilities,
             )),
@@ -104,6 +111,7 @@ impl Session {
                 .unwrap_or_else(|| self.global_settings.to_settings_arc()),
             document_ref: self.index.make_document_ref(key, &self.global_settings)?,
             position_encoding: self.position_encoding,
+            workspace_trust: self.workspace_trust,
         })
     }
 
@@ -215,6 +223,21 @@ impl Session {
 }
 
 impl DocumentSnapshot {
+    pub(crate) fn format_backend(&self) -> FormatBackend {
+        let backend = self.client_settings.editor_settings().format_backend();
+        match self.workspace_trust {
+            WorkspaceTrust::Trusted => backend,
+            WorkspaceTrust::Untrusted => {
+                if backend == FormatBackend::Uv {
+                    tracing::info!(
+                        "Using the internal formatter because the workspace is untrusted; the uv backend is disabled"
+                    );
+                }
+                FormatBackend::Internal
+            }
+        }
+    }
+
     pub(crate) fn resolved_client_capabilities(&self) -> &ResolvedClientCapabilities {
         &self.resolved_client_capabilities
     }
@@ -242,3 +265,49 @@ impl DocumentSnapshot {
         )
     }
 }
+
+#[cfg(test)]
+mod tests {
+    use std::sync::Arc;
+
+    use anyhow::{Context, Result};
+    use lsp_types::Uri;
+    use serde_json::json;
+    use test_case::test_case;
+
+    use super::index::Index;
+    use super::options::GlobalOptions;
+    use super::{Client, Session};
+    use crate::format::FormatBackend;
+    use crate::session::request_queue::RequestQueue;
+    use crate::{PositionEncoding, TextDocument, WorkspaceTrust};
+
+    #[test_case(WorkspaceTrust::Untrusted, FormatBackend::Internal)]
+    #[test_case(WorkspaceTrust::Trusted, FormatBackend::Uv)]
+    fn format_backend(workspace_trust: WorkspaceTrust, expected: FormatBackend) -> Result<()> {
+        let (main_loop_sender, _) = crossbeam::channel::unbounded();
+        let (client_sender, _) = crossbeam::channel::unbounded();
+        let client = Client::new(main_loop_sender, client_sender);
+        let options: GlobalOptions = serde_json::from_value(json!({
+            "format": {"backend": "uv"},
+        }))?;
+        let mut session = Session {
+            index: Index::default(),
+            position_encoding: PositionEncoding::default(),
+            global_settings: options.into_settings(client),
+            workspace_trust,
+            resolved_client_capabilities: Arc::default(),
+            request_queue: RequestQueue::new(),
+            shutdown_requested: false,
+        };
+        let uri: Uri = "untitled:test.py".parse()?;
+        session.open_text_document(uri.clone(), TextDocument::new(String::new(), 1));
+        let snapshot = session
+            .take_snapshot(uri)
+            .context("missing document snapshot")?;
+
+        assert_eq!(snapshot.format_backend(), expected);
+
+        Ok(())
+    }
+}
diff --git a/crates/ruff_server/tests/e2e/main.rs b/crates/ruff_server/tests/e2e/main.rs
index d8e53ee40d..1cd945105f 100644
--- a/crates/ruff_server/tests/e2e/main.rs
+++ b/crates/ruff_server/tests/e2e/main.rs
@@ -66,7 +66,7 @@ use lsp_types::{
     DidChangeWorkspaceFoldersNotification, DidCloseTextDocumentNotification,
     DidOpenTextDocumentNotification, ExitNotification, InitializedNotification, Notification,
 };
-use ruff_server::{ConnectionInitializer, LogLevel, Server, init_logging};
+use ruff_server::{ConnectionInitializer, LogLevel, Server, WorkspaceTrust, init_logging};
 use rustc_hash::FxHashMap;
 use tempfile::TempDir;
 
@@ -207,7 +207,13 @@ impl TestServer {
             // TODO: This should probably be configurable to test concurrency issues
             let worker_threads = NonZeroUsize::new(1).unwrap();
 
-            match Server::new(worker_threads, server_connection, None, true) {
+            match Server::new(
+                worker_threads,
+                server_connection,
+                None,
+                WorkspaceTrust::Trusted,
+                true,
+            ) {
                 Ok(server) => {
                     if let Err(err) = server.run() {
                         panic!("Server stopped with error: {err:?}");
diff --git a/docs/editors/settings.md b/docs/editors/settings.md
index 5d5949a7fd..ab9a9c5b83 100644
--- a/docs/editors/settings.md
+++ b/docs/editors/settings.md
@@ -981,6 +981,10 @@ The backend to use for formatting files. Following options are available:
 For `internal`, the formatter version will match the selected Ruff version while for `uv`, the
 formatter version may differ.
 
+Starting the server with `ruff server --untrusted-workspace` disables the uv backend. With this flag,
+the server always uses the internal formatter, regardless of this setting. This prevents workspace
+configuration from causing uv to execute untrusted code.
+
 **Default value**: `"internal"`
 
 **Type**: `"internal" | "uv"`