diff --git a/crates/extension/src/extension_slash_command.rs b/crates/extension/src/extension_slash_command.rs
index 3c8a9659846ffb6f750fefac04ba769d9c29b5d3..086ddafbb019cf3b2a95cf382a6b7ff21640b751 100644
--- a/crates/extension/src/extension_slash_command.rs
+++ b/crates/extension/src/extension_slash_command.rs
@@ -63,9 +63,9 @@ impl SlashCommand for ExtensionSlashCommand {
                                 completions
                                     .into_iter()
                                     .map(|completion| ArgumentCompletion {
-                                        label: completion.clone(),
-                                        new_text: completion,
-                                        run_command: true,
+                                        label: completion.label,
+                                        new_text: completion.new_text,
+                                        run_command: completion.run_command,
                                     })
                                     .collect(),
                             )
diff --git a/crates/extension/src/wasm_host/wit.rs b/crates/extension/src/wasm_host/wit.rs
index 374a337063bc45d815910b6dda391a9c321d459e..25e928c97249cc3383d91426280a25b68b1a5e24 100644
--- a/crates/extension/src/wasm_host/wit.rs
+++ b/crates/extension/src/wasm_host/wit.rs
@@ -20,7 +20,7 @@ use wasmtime::{
 pub use latest::CodeLabelSpanLiteral;
 pub use latest::{
     zed::extension::lsp::{Completion, CompletionKind, InsertTextFormat, Symbol, SymbolKind},
-    zed::extension::slash_command::SlashCommandOutput,
+    zed::extension::slash_command::{SlashCommandArgumentCompletion, SlashCommandOutput},
     CodeLabel, CodeLabelSpan, Command, Range, SlashCommand,
 };
 pub use since_v0_0_4::LanguageServerConfig;
@@ -263,7 +263,7 @@ impl Extension {
         store: &mut Store<WasmState>,
         command: &SlashCommand,
         query: &str,
-    ) -> Result<Result<Vec<String>, String>> {
+    ) -> Result<Result<Vec<SlashCommandArgumentCompletion>, String>> {
         match self {
             Extension::V007(ext) => {
                 ext.call_complete_slash_command_argument(store, command, query)
diff --git a/crates/extension_api/src/extension_api.rs b/crates/extension_api/src/extension_api.rs
index 88c27bc05226efe62b1cd3a5aa077a4a38187799..e0adf1b84bfa57ef0f57aab3a0a497181e044b53 100644
--- a/crates/extension_api/src/extension_api.rs
+++ b/crates/extension_api/src/extension_api.rs
@@ -25,7 +25,9 @@ pub use wit::{
         npm_package_latest_version,
     },
     zed::extension::platform::{current_platform, Architecture, Os},
-    zed::extension::slash_command::{SlashCommand, SlashCommandOutput, SlashCommandOutputSection},
+    zed::extension::slash_command::{
+        SlashCommand, SlashCommandArgumentCompletion, SlashCommandOutput, SlashCommandOutputSection,
+    },
     CodeLabel, CodeLabelSpan, CodeLabelSpanLiteral, Command, DownloadedFileType, EnvVars,
     KeyValueStore, LanguageServerInstallationStatus, Range, Worktree,
 };
@@ -114,7 +116,7 @@ pub trait Extension: Send + Sync {
         &self,
         _command: SlashCommand,
         _query: String,
-    ) -> Result<Vec<String>, String> {
+    ) -> Result<Vec<SlashCommandArgumentCompletion>, String> {
         Ok(Vec::new())
     }
 
@@ -247,7 +249,7 @@ impl wit::Guest for Component {
     fn complete_slash_command_argument(
         command: SlashCommand,
         query: String,
-    ) -> Result<Vec<String>, String> {
+    ) -> Result<Vec<SlashCommandArgumentCompletion>, String> {
         extension().complete_slash_command_argument(command, query)
     }
 
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 f1d26dabc8af9eb011be643f8822eeecde5af4e0..90f7c9978ace19b26b11ec46189167acb0bafedd 100644
--- a/crates/extension_api/wit/since_v0.0.7/extension.wit
+++ b/crates/extension_api/wit/since_v0.0.7/extension.wit
@@ -8,7 +8,7 @@ world extension {
 
     use common.{range};
     use lsp.{completion, symbol};
-    use slash-command.{slash-command, slash-command-output};
+    use slash-command.{slash-command, slash-command-argument-completion, slash-command-output};
 
     /// Initializes the extension.
     export init-extension: func();
@@ -130,7 +130,7 @@ world extension {
     export labels-for-symbols: func(language-server-id: string, symbols: list<symbol>) -> result<list<option<code-label>>, string>;
 
     /// 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>;
+    export complete-slash-command-argument: func(command: slash-command, query: string) -> result<list<slash-command-argument-completion>, 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/crates/extension_api/wit/since_v0.0.7/slash-command.wit b/crates/extension_api/wit/since_v0.0.7/slash-command.wit
index 17d4a615527f7438635ef3468d805e6fb2ec0a83..f52561c2ef412be071820f3a71621c3c4f3f9da3 100644
--- a/crates/extension_api/wit/since_v0.0.7/slash-command.wit
+++ b/crates/extension_api/wit/since_v0.0.7/slash-command.wit
@@ -28,4 +28,14 @@ interface slash-command {
         /// The label to display in the placeholder for this section.
         label: string,
     }
+
+    /// A completion for a slash command argument.
+    record slash-command-argument-completion {
+        /// The label to display for this completion.
+        label: string,
+        /// The new text that should be inserted into the command when this completion is accepted.
+        new-text: string,
+        /// Whether the command should be run when accepting this completion.
+        run-command: bool,
+    }
 }
diff --git a/extensions/gleam/src/gleam.rs b/extensions/gleam/src/gleam.rs
index 27478a2915f0a7e9a8bdf189108f8e23d152d50a..1ed1c536fa1fa4d1b2e0e0c9c20d8030874aae00 100644
--- a/extensions/gleam/src/gleam.rs
+++ b/extensions/gleam/src/gleam.rs
@@ -4,7 +4,7 @@ use std::fs;
 use zed::lsp::CompletionKind;
 use zed::{
     CodeLabel, CodeLabelSpan, HttpRequest, KeyValueStore, LanguageServerId, SlashCommand,
-    SlashCommandOutput, SlashCommandOutputSection,
+    SlashCommandArgumentCompletion, SlashCommandOutput, SlashCommandOutputSection,
 };
 use zed_extension_api::{self as zed, Result};
 
@@ -154,12 +154,24 @@ impl zed::Extension for GleamExtension {
         &self,
         command: SlashCommand,
         _query: String,
-    ) -> Result<Vec<String>, String> {
+    ) -> Result<Vec<SlashCommandArgumentCompletion>, String> {
         match command.name.as_str() {
             "gleam-project" => Ok(vec![
-                "apple".to_string(),
-                "banana".to_string(),
-                "cherry".to_string(),
+                SlashCommandArgumentCompletion {
+                    label: "apple".to_string(),
+                    new_text: "Apple".to_string(),
+                    run_command: false,
+                },
+                SlashCommandArgumentCompletion {
+                    label: "banana".to_string(),
+                    new_text: "Banana".to_string(),
+                    run_command: false,
+                },
+                SlashCommandArgumentCompletion {
+                    label: "cherry".to_string(),
+                    new_text: "Cherry".to_string(),
+                    run_command: true,
+                },
             ]),
             _ => Ok(Vec::new()),
         }