diff --git a/lsp/lsp-harness/src/lib.rs b/lsp/lsp-harness/src/lib.rs
index 061888bf..2829ee2b 100644
--- a/lsp/lsp-harness/src/lib.rs
+++ b/lsp/lsp-harness/src/lib.rs
@@ -4,8 +4,8 @@ mod output;
 pub use jsonrpc::Server;
 use log::error;
 use lsp_types::{
-    GotoDefinitionParams, Location, Position, Range, TextDocumentIdentifier,
-    TextDocumentPositionParams, Url,
+    CompletionContext, CompletionParams, GotoDefinitionParams, Location, Position, Range,
+    TextDocumentIdentifier, TextDocumentPositionParams, Url,
 };
 pub use output::LspDebug;
 
@@ -34,6 +34,7 @@ pub struct TestFile {
 /// A subset of LSP requests that our harness supports.
 pub enum Request {
     GotoDefinition(GotoDefinitionParams),
+    Completion(CompletionParams),
 }
 
 /// A private FromStr, with simpler error handling and (because it's
@@ -92,6 +93,28 @@ impl Parse for Request {
                 work_done_progress_params: Default::default(),
                 partial_result_params: Default::default(),
             })),
+            // The format of a Completion request is the file:line:char location,
+            // followed by an optional trigger character.
+            "Completion" => {
+                let (pos, context) = if let Some((pos, trigger)) = params.split_once(' ') {
+                    (
+                        pos,
+                        Some(CompletionContext {
+                            trigger_kind: lsp_types::CompletionTriggerKind::TriggerCharacter,
+                            trigger_character: Some(trigger.to_owned()),
+                        }),
+                    )
+                } else {
+                    (params, None)
+                };
+                Some(Request::Completion(CompletionParams {
+                    text_document_position: TextDocumentPositionParams::parse(pos)?,
+                    work_done_progress_params: Default::default(),
+                    partial_result_params: Default::default(),
+                    context,
+                }))
+            }
+
             _ => None,
         }
     }
diff --git a/lsp/lsp-harness/src/output.rs b/lsp/lsp-harness/src/output.rs
index fe1007bd..4d69b0a7 100644
--- a/lsp/lsp-harness/src/output.rs
+++ b/lsp/lsp-harness/src/output.rs
@@ -85,3 +85,27 @@ impl LspDebug for lsp_types::GotoDefinitionResponse {
         }
     }
 }
+
+impl LspDebug for lsp_types::CompletionItem {
+    fn debug(&self, mut w: impl Write) -> std::io::Result<()> {
+        write!(w, "{}", self.label)
+    }
+}
+
+impl LspDebug for lsp_types::CompletionResponse {
+    fn debug(&self, w: impl Write) -> std::io::Result<()> {
+        match self {
+            lsp_types::CompletionResponse::Array(items) => {
+                // The order of completions is non-deterministic, so sort them.
+                let mut items = items.clone();
+                items.sort_by_key(|i| i.label.clone());
+                Iter(items.iter()).debug(w)
+            }
+            lsp_types::CompletionResponse::List(list) => {
+                let mut items = list.items.clone();
+                items.sort_by_key(|i| i.label.clone());
+                Iter(list.items.iter()).debug(w)
+            }
+        }
+    }
+}
diff --git a/lsp/nls/tests/inputs/completion-basic.ncl b/lsp/nls/tests/inputs/completion-basic.ncl
new file mode 100644
index 00000000..bd1b8ade
--- /dev/null
+++ b/lsp/nls/tests/inputs/completion-basic.ncl
@@ -0,0 +1,17 @@
+### /completion-basic.ncl
+let config = {
+  version = "1.2",
+  verified = { really = true },
+  foo = "bar",
+}
+in
+{
+    a = config.version,
+    b = config.verified.really,
+}
+### Completion /completion-basic.ncl:7:12
+### Completion /completion-basic.ncl:7:15 .
+### Completion /completion-basic.ncl:8:15 .
+### Completion /completion-basic.ncl:8:23
+### Completion /completion-basic.ncl:8:24 .
+### Completion /completion-basic.ncl:8:27
diff --git a/lsp/nls/tests/main.rs b/lsp/nls/tests/main.rs
index 6427bf96..0eb42b86 100644
--- a/lsp/nls/tests/main.rs
+++ b/lsp/nls/tests/main.rs
@@ -1,5 +1,5 @@
 use assert_cmd::cargo::CommandCargoExt;
-use lsp_types::request::{GotoDefinition, Request as LspRequest};
+use lsp_types::request::{Completion, GotoDefinition, Request as LspRequest};
 use nickel_lang_utils::project_root::project_root;
 use test_generator::test_resources;
 
@@ -32,6 +32,7 @@ impl TestHarness {
     fn request_dyn(&mut self, req: Request) {
         match req {
             Request::GotoDefinition(d) => self.request::<GotoDefinition>(d),
+            Request::Completion(c) => self.request::<Completion>(c),
         }
     }
 
diff --git a/lsp/nls/tests/snapshots/main__lsp__nls__tests__inputs__completion-basic.ncl.snap b/lsp/nls/tests/snapshots/main__lsp__nls__tests__inputs__completion-basic.ncl.snap
new file mode 100644
index 00000000..135a2612
--- /dev/null
+++ b/lsp/nls/tests/snapshots/main__lsp__nls__tests__inputs__completion-basic.ncl.snap
@@ -0,0 +1,11 @@
+---
+source: lsp/nls/tests/main.rs
+expression: output
+---
+[a, b, config]
+[foo, verified, version]
+[foo, verified, version]
+[foo, verified, version]
+[really]
+[really]
+