diff --git a/crates/assistant/src/slash_command/active_command.rs b/crates/assistant/src/slash_command/active_command.rs
index 235a38118a24a661d60b76c4e2679d057c7cb7a2..6ab8684362ea28a8a68c908ee0c93441aaa9de05 100644
--- a/crates/assistant/src/slash_command/active_command.rs
+++ b/crates/assistant/src/slash_command/active_command.rs
@@ -6,6 +6,7 @@ use anyhow::{anyhow, Result};
 use editor::Editor;
 use gpui::{AppContext, Task, WeakView};
 use language::LspAdapterDelegate;
+use std::sync::atomic::AtomicBool;
 use std::sync::Arc;
 use ui::WindowContext;
 use workspace::Workspace;
@@ -26,9 +27,9 @@ impl SlashCommand for ActiveSlashCommand {
     }
 
     fn complete_argument(
-        &self,
+        self: Arc<Self>,
         _query: String,
-        _cancel: std::sync::Arc<std::sync::atomic::AtomicBool>,
+        _cancel: Arc<AtomicBool>,
         _workspace: Option<WeakView<Workspace>>,
         _cx: &mut AppContext,
     ) -> Task<Result<Vec<String>>> {
diff --git a/crates/assistant/src/slash_command/default_command.rs b/crates/assistant/src/slash_command/default_command.rs
index d1f657269222758941a0c533f4f1c83a896f8762..44ffdfcd11f1d4322d1f68df81cc5efa2ec21d6f 100644
--- a/crates/assistant/src/slash_command/default_command.rs
+++ b/crates/assistant/src/slash_command/default_command.rs
@@ -31,7 +31,7 @@ impl SlashCommand for DefaultSlashCommand {
     }
 
     fn complete_argument(
-        &self,
+        self: Arc<Self>,
         _query: String,
         _cancellation_flag: Arc<AtomicBool>,
         _workspace: Option<WeakView<Workspace>>,
diff --git a/crates/assistant/src/slash_command/diagnostics_command.rs b/crates/assistant/src/slash_command/diagnostics_command.rs
index 67afdae2dc7c167e744bfd6f22f7cd38822e37fb..c7e89a3fb8ed4be09e51b566a9144d4953db37ab 100644
--- a/crates/assistant/src/slash_command/diagnostics_command.rs
+++ b/crates/assistant/src/slash_command/diagnostics_command.rs
@@ -98,7 +98,7 @@ impl SlashCommand for DiagnosticsCommand {
     }
 
     fn complete_argument(
-        &self,
+        self: Arc<Self>,
         query: String,
         cancellation_flag: Arc<AtomicBool>,
         workspace: Option<WeakView<Workspace>>,
diff --git a/crates/assistant/src/slash_command/fetch_command.rs b/crates/assistant/src/slash_command/fetch_command.rs
index 37be2928009133b767f2f882574fe07fea130ee5..cc675ecebe4c0e36bf8ddc83c6bdd41a3a5bf381 100644
--- a/crates/assistant/src/slash_command/fetch_command.rs
+++ b/crates/assistant/src/slash_command/fetch_command.rs
@@ -113,7 +113,7 @@ impl SlashCommand for FetchSlashCommand {
     }
 
     fn complete_argument(
-        &self,
+        self: Arc<Self>,
         _query: String,
         _cancel: Arc<AtomicBool>,
         _workspace: Option<WeakView<Workspace>>,
diff --git a/crates/assistant/src/slash_command/file_command.rs b/crates/assistant/src/slash_command/file_command.rs
index 4fd7cd6a9aa311f181ee2f975824116f40901ae6..31792292d596de4dae32bd45492deb0e27dc331b 100644
--- a/crates/assistant/src/slash_command/file_command.rs
+++ b/crates/assistant/src/slash_command/file_command.rs
@@ -101,7 +101,7 @@ impl SlashCommand for FileSlashCommand {
     }
 
     fn complete_argument(
-        &self,
+        self: Arc<Self>,
         query: String,
         cancellation_flag: Arc<AtomicBool>,
         workspace: Option<WeakView<Workspace>>,
diff --git a/crates/assistant/src/slash_command/now_command.rs b/crates/assistant/src/slash_command/now_command.rs
index 6db984469bc976ec5023f141d058428bf5c75ac2..43e5daa05b5788570604b51c4057a54565471941 100644
--- a/crates/assistant/src/slash_command/now_command.rs
+++ b/crates/assistant/src/slash_command/now_command.rs
@@ -29,7 +29,7 @@ impl SlashCommand for NowSlashCommand {
     }
 
     fn complete_argument(
-        &self,
+        self: Arc<Self>,
         _query: String,
         _cancel: Arc<AtomicBool>,
         _workspace: Option<WeakView<Workspace>>,
diff --git a/crates/assistant/src/slash_command/project_command.rs b/crates/assistant/src/slash_command/project_command.rs
index d5f4d74404f891bbc63ad8e4ec75f600bb141ccc..b51c051e1180c1b2d9f2534d16744b424d7563b5 100644
--- a/crates/assistant/src/slash_command/project_command.rs
+++ b/crates/assistant/src/slash_command/project_command.rs
@@ -102,7 +102,7 @@ impl SlashCommand for ProjectSlashCommand {
     }
 
     fn complete_argument(
-        &self,
+        self: Arc<Self>,
         _query: String,
         _cancel: Arc<AtomicBool>,
         _workspace: Option<WeakView<Workspace>>,
diff --git a/crates/assistant/src/slash_command/prompt_command.rs b/crates/assistant/src/slash_command/prompt_command.rs
index 37610cc0177c1ddb71e97dccf6cb684a10e6fe22..ac4d77789e438ee500a9863c85eedb422a42d608 100644
--- a/crates/assistant/src/slash_command/prompt_command.rs
+++ b/crates/assistant/src/slash_command/prompt_command.rs
@@ -28,7 +28,7 @@ impl SlashCommand for PromptSlashCommand {
     }
 
     fn complete_argument(
-        &self,
+        self: Arc<Self>,
         query: String,
         _cancellation_flag: Arc<AtomicBool>,
         _workspace: Option<WeakView<Workspace>>,
diff --git a/crates/assistant/src/slash_command/rustdoc_command.rs b/crates/assistant/src/slash_command/rustdoc_command.rs
index 3104189eadf7d7b1c184096e7616a1d129544099..1658852e3cbddfe5b00a10f15ed366508caf79c4 100644
--- a/crates/assistant/src/slash_command/rustdoc_command.rs
+++ b/crates/assistant/src/slash_command/rustdoc_command.rs
@@ -107,7 +107,7 @@ impl SlashCommand for RustdocSlashCommand {
     }
 
     fn complete_argument(
-        &self,
+        self: Arc<Self>,
         query: String,
         _cancel: Arc<AtomicBool>,
         workspace: Option<WeakView<Workspace>>,
diff --git a/crates/assistant/src/slash_command/search_command.rs b/crates/assistant/src/slash_command/search_command.rs
index ca2328ca6b92946a27b31cec20f587fb6f281e18..ad4beef5b339473f6b2030a6333ba5b37c38273d 100644
--- a/crates/assistant/src/slash_command/search_command.rs
+++ b/crates/assistant/src/slash_command/search_command.rs
@@ -47,7 +47,7 @@ impl SlashCommand for SearchSlashCommand {
     }
 
     fn complete_argument(
-        &self,
+        self: Arc<Self>,
         _query: String,
         _cancel: Arc<AtomicBool>,
         _workspace: Option<WeakView<Workspace>>,
diff --git a/crates/assistant/src/slash_command/tabs_command.rs b/crates/assistant/src/slash_command/tabs_command.rs
index 6c4daf0afa7f26cf758f6087e30f9f78ab75679f..45220f8f4e38eb327b392c0a4d036285f5d66b17 100644
--- a/crates/assistant/src/slash_command/tabs_command.rs
+++ b/crates/assistant/src/slash_command/tabs_command.rs
@@ -31,7 +31,7 @@ impl SlashCommand for TabsSlashCommand {
     }
 
     fn complete_argument(
-        &self,
+        self: Arc<Self>,
         _query: String,
         _cancel: Arc<std::sync::atomic::AtomicBool>,
         _workspace: Option<WeakView<Workspace>>,
diff --git a/crates/assistant_slash_command/src/assistant_slash_command.rs b/crates/assistant_slash_command/src/assistant_slash_command.rs
index 691816ed7c6a8e31f41740bfb1c038ff957cba36..374cb817df8ec8652ed465935e2ba583e06c7331 100644
--- a/crates/assistant_slash_command/src/assistant_slash_command.rs
+++ b/crates/assistant_slash_command/src/assistant_slash_command.rs
@@ -23,7 +23,7 @@ pub trait SlashCommand: 'static + Send + Sync {
     fn description(&self) -> String;
     fn menu_text(&self) -> String;
     fn complete_argument(
-        &self,
+        self: Arc<Self>,
         query: String,
         cancel: Arc<AtomicBool>,
         workspace: Option<WeakView<Workspace>>,
diff --git a/crates/extension/src/extension_slash_command.rs b/crates/extension/src/extension_slash_command.rs
index 23de82174c56b91ef987b5d073dacd0a95d60daa..e10007bfc2eec67c3c039bcdcb8be1a0cc994850 100644
--- a/crates/extension/src/extension_slash_command.rs
+++ b/crates/extension/src/extension_slash_command.rs
@@ -36,13 +36,34 @@ impl SlashCommand for ExtensionSlashCommand {
     }
 
     fn complete_argument(
-        &self,
-        _query: String,
+        self: Arc<Self>,
+        query: String,
         _cancel: Arc<AtomicBool>,
         _workspace: Option<WeakView<Workspace>>,
-        _cx: &mut AppContext,
+        cx: &mut AppContext,
     ) -> Task<Result<Vec<String>>> {
-        Task::ready(Ok(Vec::new()))
+        cx.background_executor().spawn(async move {
+            self.extension
+                .call({
+                    let this = self.clone();
+                    move |extension, store| {
+                        async move {
+                            let completions = extension
+                                .call_complete_slash_command_argument(
+                                    store,
+                                    &this.command,
+                                    query.as_ref(),
+                                )
+                                .await?
+                                .map_err(|e| anyhow!("{}", e))?;
+
+                            anyhow::Ok(completions)
+                        }
+                        .boxed()
+                    }
+                })
+                .await
+        })
     }
 
     fn run(
diff --git a/crates/extension/src/wasm_host/wit.rs b/crates/extension/src/wasm_host/wit.rs
index 9963840b14423c9eb91a3283c0d3d18e32b4ba23..399f0a9119bb7b66c81f96dba5b42eb8bd3a01dd 100644
--- a/crates/extension/src/wasm_host/wit.rs
+++ b/crates/extension/src/wasm_host/wit.rs
@@ -257,6 +257,21 @@ impl Extension {
         }
     }
 
+    pub async fn call_complete_slash_command_argument(
+        &self,
+        store: &mut Store<WasmState>,
+        command: &SlashCommand,
+        query: &str,
+    ) -> Result<Result<Vec<String>, String>> {
+        match self {
+            Extension::V007(ext) => {
+                ext.call_complete_slash_command_argument(store, command, query)
+                    .await
+            }
+            Extension::V001(_) | Extension::V004(_) | Extension::V006(_) => Ok(Ok(Vec::new())),
+        }
+    }
+
     pub async fn call_run_slash_command(
         &self,
         store: &mut Store<WasmState>,
diff --git a/crates/extension_api/src/extension_api.rs b/crates/extension_api/src/extension_api.rs
index a15826f2c18bd2aca29cb7c865685e55d0bd9878..5e9f3ad12f428ca90acf542a95a15e7f88c46b2a 100644
--- a/crates/extension_api/src/extension_api.rs
+++ b/crates/extension_api/src/extension_api.rs
@@ -108,7 +108,16 @@ pub trait Extension: Send + Sync {
         None
     }
 
-    /// Runs the given slash command.
+    /// Returns the completions that should be shown when completing the provided slash command with the given query.
+    fn complete_slash_command_argument(
+        &self,
+        _command: SlashCommand,
+        _query: String,
+    ) -> Result<Vec<String>, String> {
+        Ok(Vec::new())
+    }
+
+    /// Returns the output from running the provided slash command.
     fn run_slash_command(
         &self,
         _command: SlashCommand,
@@ -225,6 +234,13 @@ impl wit::Guest for Component {
         Ok(labels)
     }
 
+    fn complete_slash_command_argument(
+        command: SlashCommand,
+        query: String,
+    ) -> Result<Vec<String>, String> {
+        extension().complete_slash_command_argument(command, query)
+    }
+
     fn run_slash_command(
         command: SlashCommand,
         argument: Option<String>,
diff --git a/crates/extension_api/wit/since_v0.0.7/extension.wit b/crates/extension_api/wit/since_v0.0.7/extension.wit
index 253011194fc02d778844aa56cb0dcb98a820f7c9..522df4ccb3d7da0590baff1675666c8bf7d09f3f 100644
--- a/crates/extension_api/wit/since_v0.0.7/extension.wit
+++ b/crates/extension_api/wit/since_v0.0.7/extension.wit
@@ -122,6 +122,9 @@ world extension {
     export labels-for-completions: func(language-server-id: string, completions: list<completion>) -> result<list<option<code-label>>, string>;
     export labels-for-symbols: func(language-server-id: string, symbols: list<symbol>) -> result<list<option<code-label>>, string>;
 
-    /// Runs the provided slash command.
+    /// Returns the completions that should be shown when completing the provided slash command with the given query.
+    export complete-slash-command-argument: func(command: slash-command, query: string) -> result<list<string>, string>;
+
+    /// Returns the output from running the provided slash command.
     export run-slash-command: func(command: slash-command, argument: option<string>, worktree: borrow<worktree>) -> result<slash-command-output, string>;
 }
diff --git a/extensions/gleam/src/gleam.rs b/extensions/gleam/src/gleam.rs
index 1671fdf400b0a2537c063d3047c1cd6e0ed33e39..fd3ddf64ec7b60bfc5a7a0a4e1e4f5b440f0e796 100644
--- a/extensions/gleam/src/gleam.rs
+++ b/extensions/gleam/src/gleam.rs
@@ -146,6 +146,21 @@ impl zed::Extension for GleamExtension {
         })
     }
 
+    fn complete_slash_command_argument(
+        &self,
+        command: SlashCommand,
+        _query: String,
+    ) -> Result<Vec<String>, String> {
+        match command.name.as_str() {
+            "gleam-project" => Ok(vec![
+                "apple".to_string(),
+                "banana".to_string(),
+                "cherry".to_string(),
+            ]),
+            _ => Ok(Vec::new()),
+        }
+    }
+
     fn run_slash_command(
         &self,
         command: SlashCommand,