diff --git a/tests/lsp_features/completion.zig b/tests/lsp_features/completion.zig
new file mode 100644
index 0000000..ca3959d
--- /dev/null
+++ b/tests/lsp_features/completion.zig
@@ -0,0 +1,535 @@
+const std = @import("std");
+const zls = @import("zls");
+const builtin = @import("builtin");
+
+const helper = @import("../helper.zig");
+const Context = @import("../context.zig").Context;
+const ErrorBuilder = @import("../ErrorBuilder.zig");
+
+const types = zls.types;
+const offsets = zls.offsets;
+const requests = zls.requests;
+
+const allocator: std.mem.Allocator = std.testing.allocator;
+
+const Completion = struct {
+    label: []const u8,
+    kind: types.CompletionItem.Kind,
+    detail: ?[]const u8 = null,
+};
+
+const CompletionSet = std.StringArrayHashMapUnmanaged(Completion);
+
+test "completion - root scope" {
+    try testCompletion(
+        \\const foo = 5;
+        \\const bar = <cursor>;
+    , &.{
+        .{ .label = "foo", .kind = .Constant },
+    });
+
+    try testCompletion(
+        \\const foo = 5;
+        \\const bar = <cursor>
+    , &.{
+        .{ .label = "foo", .kind = .Constant },
+    });
+
+    try testCompletion(
+        \\const foo = 5;
+        \\const bar = <cursor>;
+        \\const baz = 5;
+    , &.{
+        .{ .label = "foo", .kind = .Constant },
+        .{ .label = "baz", .kind = .Constant },
+    });
+}
+
+test "completion - local scope" {
+    if (true) return error.SkipZigTest;
+    try testCompletion(
+        \\const foo = {
+        \\    var bar = 5;
+        \\    const alpha = <cursor>;
+        \\    const baz = 3;
+        \\};
+    , &.{
+        .{ .label = "foo", .kind = .Constant }, // should foo be referencable?
+        .{ .label = "bar", .kind = .Variable },
+    });
+}
+
+test "completion - function" {
+    try testCompletion(
+        \\fn foo(alpha: u32, beta: []const u8) void {
+        \\    <cursor>
+        \\}
+    , &.{
+        // TODO detail should be 'fn(alpha: u32, beta: []const u8) void' or 'foo: fn(alpha: u32, beta: []const u8) void'
+        .{ .label = "foo", .kind = .Function, .detail = "fn foo(alpha: u32, beta: []const u8) void" },
+        .{ .label = "alpha", .kind = .Constant, .detail = "alpha: u32" },
+        .{ .label = "beta", .kind = .Constant, .detail = "beta: []const u8" },
+    });
+
+    try testCompletion(
+        \\const S = struct { alpha: u32 };
+        \\fn foo() S { return undefined; }
+        \\const bar = foo().<cursor>;
+    , &.{
+        .{ .label = "alpha", .kind = .Field, .detail = "alpha: u32" },
+    });
+}
+
+test "completion - generic function" {
+    // TODO doesn't work for std.ArrayList
+
+    try testCompletion(
+        \\const S = struct { alpha: u32 };
+        \\fn ArrayList(comptime T: type) type {
+        \\    return struct { items: []const T };
+        \\}
+        \\const array_list: ArrayList(S);
+        \\const foo = array_list.items[0].<cursor>
+    , &.{
+        .{ .label = "alpha", .kind = .Field, .detail = "alpha: u32" },
+    });
+}
+
+test "completion - optional" {
+    try testCompletion(
+        \\const foo: ?u32 = undefined;
+        \\const bar = foo.<cursor>
+    , &.{
+        // TODO detail should be 'u32'
+        .{ .label = "?", .kind = .Operator },
+    });
+
+    try testCompletion(
+        \\const S = struct { alpha: u32 };
+        \\const foo: ?S = undefined;
+        \\const bar = foo.?.<cursor>
+    , &.{
+        .{ .label = "alpha", .kind = .Field, .detail = "alpha: u32" },
+    });
+}
+
+test "completion - pointer" {
+    try testCompletion(
+        \\const foo: *u32 = undefined;
+        \\const bar = foo.<cursor>
+    , &.{
+        // TODO detail should be 'u32'
+        .{ .label = "*", .kind = .Operator },
+    });
+
+    try testCompletion(
+        \\const S = struct { alpha: u32 };
+        \\const foo: *S = undefined;
+        \\const bar = foo.*.<cursor>
+    , &.{
+        .{ .label = "alpha", .kind = .Field, .detail = "alpha: u32" },
+    });
+
+    try testCompletion(
+        \\const foo: []const u8 = undefined;
+        \\const bar = foo.<cursor>
+    , &.{
+        // TODO detail should be 'u32'
+        .{ .label = "len", .kind = .Field, .detail = "const len: usize" },
+        // TODO detail should be 'const ptr: [*]const u8'
+        .{ .label = "ptr", .kind = .Field },
+    });
+
+    try testCompletion(
+        \\const S = struct { alpha: u32 };
+        \\const foo: []S = undefined;
+        \\const bar = foo[0].<cursor>
+    , &.{
+        .{ .label = "alpha", .kind = .Field, .detail = "alpha: u32" },
+    });
+
+    try testCompletion(
+        \\const S = struct { alpha: u32 };
+        \\const foo: *S = undefined;
+        \\const bar = foo.<cursor>
+    , &.{
+        // TODO detail should be 'S'
+        .{ .label = "*", .kind = .Operator },
+        .{ .label = "alpha", .kind = .Field, .detail = "alpha: u32" },
+    });
+
+    // try testCompletion(
+    //     \\const S = struct {
+    //     \\    alpha: u32,
+    //     \\};
+    //     \\const foo: []S = undefined;
+    //     \\const bar = foo.ptr[0].<cursor>
+    // , &.{
+    //     .{ .label = "alpha", .kind = .Field, .detail = "alpha: u32" },
+    // });
+}
+
+test "completion - captures" {
+    try testCompletion(
+        \\const S = struct { alpha: u32 };
+        \\fn foo(bar: ?S) void {
+        \\    if(bar) |baz| {
+        \\        baz.<cursor>
+        \\    }
+        \\}
+    , &.{
+        .{ .label = "alpha", .kind = .Field, .detail = "alpha: u32" },
+    });
+
+    try testCompletion(
+        \\const S = struct { alpha: u32 };
+        \\fn foo(items: []S) void {
+        \\    for (items) |bar, i| {
+        \\        bar.<cursor>
+        \\    }
+        \\}
+    , &.{
+        .{ .label = "alpha", .kind = .Field, .detail = "alpha: u32" },
+    });
+
+    try testCompletion(
+        \\const S = struct { alpha: u32 };
+        \\fn foo(bar: ?S) void {
+        \\    while (bar) |baz| {
+        \\        baz.<cursor>
+        \\    }
+        \\}
+    , &.{
+        .{ .label = "alpha", .kind = .Field, .detail = "alpha: u32" },
+    });
+
+    // TODO fix value capture without block scope
+    // try testCompletion(
+    //     \\const S = struct { alpha: u32 };
+    //     \\const foo: ?S = undefined;
+    //     \\const bar = if(foo) |baz| baz.<cursor>
+    // , &.{
+    //     .{ .label = "alpha", .kind = .Field, .detail = "alpha: u32" },
+    // });
+}
+
+test "completion - struct" {
+    try testCompletion(
+        \\const S = struct {
+        \\    alpha: u32,
+        \\    beta: []const u8,
+        \\};
+        \\const foo: S = undefined;
+        \\const bar = foo.<cursor>
+    , &.{
+        .{ .label = "alpha", .kind = .Field, .detail = "alpha: u32" },
+        .{ .label = "beta", .kind = .Field, .detail = "beta: []const u8" },
+    });
+
+    try testCompletion(
+        \\const S = struct {
+        \\    alpha: u32,
+        \\    beta: []const u8,
+        \\};
+        \\const foo = S{ .alpha = 0, .beta = "" };
+        \\const bar = foo.<cursor>
+    , &.{
+        .{ .label = "alpha", .kind = .Field, .detail = "alpha: u32" },
+        .{ .label = "beta", .kind = .Field, .detail = "beta: []const u8" },
+    });
+}
+
+test "completion - union" {
+    try testCompletion(
+        \\const U = union {
+        \\    alpha: u32,
+        \\    beta: []const u8,
+        \\};
+        \\const foo: U = undefined;
+        \\const bar = foo.<cursor>
+    , &.{
+        .{ .label = "alpha", .kind = .Field, .detail = "alpha: u32" },
+        .{ .label = "beta", .kind = .Field, .detail = "beta: []const u8" },
+    });
+
+    try testCompletion(
+        \\const U = union {
+        \\    alpha: ?u32,
+        \\};
+        \\fn foo(bar: U) void {
+        \\    switch (bar) {
+        \\        .alpha => |a| {
+        \\            a.<cursor>
+        \\        }
+        \\    }
+        \\}
+    , &.{
+        .{ .label = "?", .kind = .Operator },
+    });
+}
+
+test "completion - enum" {
+    try testCompletion(
+        \\const E = enum {
+        \\    alpha,
+        \\    beta,
+        \\};
+        \\const foo = E.<cursor>
+    , &.{
+        // TODO kind should be Enum
+        .{ .label = "alpha", .kind = .Field },
+        .{ .label = "beta", .kind = .Field },
+    });
+}
+
+test "completion - error union" {
+    try testCompletion(
+        \\const E = error {
+        \\    Foo,
+        \\    Bar,
+        \\};
+        \\const baz = error.<cursor>
+    , &.{
+        .{ .label = "Foo", .kind = .Constant },
+        .{ .label = "Bar", .kind = .Constant },
+    });
+
+    // TODO implement completion for error unions
+    // try testCompletion(
+    //     \\const E = error {
+    //     \\    foo,
+    //     \\    bar,
+    //     \\};
+    //     \\const baz = E.<cursor>
+    // , &.{
+    //     .{ .label = "foo", .kind = .Constant },
+    //     .{ .label = "bar", .kind = .Constant },
+    // });
+
+    try testCompletion(
+        \\const S = struct { alpha: u32 };
+        \\fn foo() error{Foo}!S {}
+        \\fn bar() error{Foo}!void {
+        \\    const baz = try foo();
+        \\    baz.<cursor>
+        \\}
+    , &.{
+        .{ .label = "alpha", .kind = .Field, .detail = "alpha: u32" },
+    });
+
+    // try testCompletion(
+    //     \\const S = struct { alpha: u32 };
+    //     \\fn foo() error{Foo}!S {}
+    //     \\fn bar() error{Foo}!void {
+    //     \\    (try foo()).<cursor>
+    //     \\}
+    // , &.{
+    //     .{ .label = "alpha", .kind = .Field, .detail = "alpha: u32" },
+    // });
+
+    try testCompletion(
+        \\const S = struct { alpha: u32 };
+        \\fn foo() error{Foo}!S {}
+        \\fn bar() error{Foo}!void {
+        \\    const baz = foo() catch return;
+        \\    baz.<cursor>
+        \\}
+    , &.{
+        .{ .label = "alpha", .kind = .Field, .detail = "alpha: u32" },
+    });
+}
+
+test "completion - declarations" {
+    try testCompletion(
+        \\const S = struct {
+        \\    pub fn public() S {}
+        \\    fn private() !void {}
+        \\};
+        \\const foo: S = undefined;
+        \\const bar = foo.<cursor>
+    , &.{
+        .{ .label = "public", .kind = .Function, .detail = "fn public() S" },
+        // TODO private should not be visible
+        .{ .label = "private", .kind = .Function, .detail = "fn private() !void" },
+    });
+
+    try testCompletion(
+        \\const S = struct {
+        \\    pub fn public() S {}
+        \\    fn private() !void {}
+        \\};
+        \\const foo = S.<cursor>
+    , &.{
+        .{ .label = "public", .kind = .Function, .detail = "fn public() S" },
+        // TODO private should not be visible
+        .{ .label = "private", .kind = .Function, .detail = "fn private() !void" },
+    });
+}
+
+test "completion - usingnamespace" {
+    try testCompletion(
+        \\const S1 = struct {
+        \\    member: u32,
+        \\    pub fn public() S1 {}
+        \\    fn private() !void {}
+        \\};
+        \\const S2 = struct {
+        \\    usingnamespace S1;
+        \\};
+        \\const foo = S2.<cursor>
+    , &.{
+        .{ .label = "public", .kind = .Function, .detail = "fn public() S1" },
+        // TODO private should not be visible
+        .{ .label = "private", .kind = .Function, .detail = "fn private() !void" },
+    });
+}
+
+test "completion - block" {
+    if (true) return error.SkipZigTest;
+    try testCompletion(
+        \\const foo = blk: {
+        \\    break :<cursor>
+        \\};
+    , &.{
+        .{ .label = "blk", .kind = .Text }, // idk what kind this should be
+    });
+}
+
+fn testCompletion(source: []const u8, expected_completions: []const Completion) !void {
+    const cursor_idx = std.mem.indexOf(u8, source, "<cursor>").?;
+    const text = try std.mem.concat(allocator, u8, &.{ source[0..cursor_idx], source[cursor_idx + "<cursor>".len ..] });
+    defer allocator.free(text);
+
+    var ctx = try Context.init();
+    defer ctx.deinit();
+
+    const test_uri: []const u8 = switch (builtin.os.tag) {
+        .windows => "file:///C:\\test.zig",
+        else => "file:///test.zig",
+    };
+
+    try ctx.requestDidOpen(test_uri, text);
+
+    const request = requests.Completion{
+        .params = .{
+            .textDocument = .{ .uri = test_uri },
+            .position = offsets.indexToPosition(source, cursor_idx, ctx.server.offset_encoding),
+        },
+    };
+
+    @setEvalBranchQuota(2000);
+    const response = try ctx.requestGetResponse(?types.CompletionList, "textDocument/completion", request);
+    defer response.deinit();
+
+    const completion_list: types.CompletionList = response.result orelse {
+        std.debug.print("Server returned `null` as the result\n", .{});
+        return error.InvalidResponse;
+    };
+
+    var actual = try extractCompletionLabels(completion_list.items);
+    defer actual.deinit(allocator);
+
+    var expected = try extractCompletionLabels(expected_completions);
+    defer expected.deinit(allocator);
+
+    var found = try set_intersection(actual, expected);
+    defer found.deinit(allocator);
+
+    var missing = try set_difference(expected, actual);
+    defer missing.deinit(allocator);
+
+    var unexpected = try set_difference(actual, expected);
+    defer unexpected.deinit(allocator);
+
+    var error_builder = ErrorBuilder.init(allocator, text);
+    defer error_builder.deinit();
+    errdefer error_builder.writeDebug();
+
+    for (found.keys()) |label| {
+        const actual_completion: types.CompletionItem = blk: {
+            for (completion_list.items) |item| {
+                if (std.mem.eql(u8, label, item.label)) break :blk item;
+            }
+            unreachable;
+        };
+
+        const expected_completion: Completion = blk: {
+            for (expected_completions) |item| {
+                if (std.mem.eql(u8, label, item.label)) break :blk item;
+            }
+            unreachable;
+        };
+
+        if (expected_completion.kind != actual_completion.kind) {
+            try error_builder.msgAtIndex("label '{s}' should be of kind '{s}' but was '{s}'!", cursor_idx, .err, .{
+                label,
+                @tagName(expected_completion.kind),
+                @tagName(actual_completion.kind),
+            });
+            return error.InvalidCompletionKind;
+        }
+
+        if (expected_completion.detail == null) continue;
+        if (actual_completion.detail != null and std.mem.eql(u8, expected_completion.detail.?, actual_completion.detail.?)) continue;
+
+        try error_builder.msgAtIndex("label '{s}' should have detail '{?s}' but was '{?s}'!", cursor_idx, .err, .{
+            label,
+            expected_completion.detail,
+            actual_completion.detail,
+        });
+        return error.InvalidCompletionDetail;
+    }
+
+    if (missing.count() != 0 or unexpected.count() != 0) {
+        var buffer = std.ArrayListUnmanaged(u8){};
+        defer buffer.deinit(allocator);
+        var out = buffer.writer(allocator);
+
+        try printLabels(out, found, "found");
+        try printLabels(out, missing, "missing");
+        try printLabels(out, unexpected, "unexpected");
+        try error_builder.msgAtIndex("invalid completions\n{s}", cursor_idx, .err, .{buffer.items});
+        return error.InvalidCompletions;
+    }
+}
+
+fn extractCompletionLabels(items: anytype) error{ DuplicateCompletionLabel, OutOfMemory }!std.StringArrayHashMapUnmanaged(void) {
+    var set = std.StringArrayHashMapUnmanaged(void){};
+    errdefer set.deinit(allocator);
+    try set.ensureTotalCapacity(allocator, items.len);
+    for (items) |item| {
+        switch (item.kind) {
+            .Keyword, .Snippet => continue,
+            else => {},
+        }
+        if (set.fetchPutAssumeCapacity(item.label, {}) != null) return error.DuplicateCompletionLabel;
+    }
+    return set;
+}
+
+fn set_intersection(a: std.StringArrayHashMapUnmanaged(void), b: std.StringArrayHashMapUnmanaged(void)) error{OutOfMemory}!std.StringArrayHashMapUnmanaged(void) {
+    var result = std.StringArrayHashMapUnmanaged(void){};
+    errdefer result.deinit(allocator);
+    for (a.keys()) |key| {
+        if (b.contains(key)) try result.putNoClobber(allocator, key, {});
+    }
+    return result;
+}
+
+fn set_difference(a: std.StringArrayHashMapUnmanaged(void), b: std.StringArrayHashMapUnmanaged(void)) error{OutOfMemory}!std.StringArrayHashMapUnmanaged(void) {
+    var result = std.StringArrayHashMapUnmanaged(void){};
+    errdefer result.deinit(allocator);
+    for (a.keys()) |key| {
+        if (!b.contains(key)) try result.putNoClobber(allocator, key, {});
+    }
+    return result;
+}
+
+fn printLabels(writer: anytype, labels: std.StringArrayHashMapUnmanaged(void), name: []const u8) @TypeOf(writer).Error!void {
+    if (labels.count() != 0) {
+        try writer.print("{s}:\n", .{name});
+        for (labels.keys()) |label| {
+            try writer.print("  - {s}\n", .{label});
+        }
+    }
+}
diff --git a/tests/tests.zig b/tests/tests.zig
index 3f85a87..c1772cb 100644
--- a/tests/tests.zig
+++ b/tests/tests.zig
@@ -13,6 +13,7 @@ comptime {
     _ = @import("lsp_features/semantic_tokens.zig");
     _ = @import("lsp_features/inlay_hints.zig");
     _ = @import("lsp_features/references.zig");
+    _ = @import("lsp_features/completion.zig");
 
     // Language features
     _ = @import("language_features/cimport.zig");