diff --git a/builtin/runtime/lsp.lua b/builtin/runtime/lsp.lua
index 02c53af..ce59fe4 100644
--- a/builtin/runtime/lsp.lua
+++ b/builtin/runtime/lsp.lua
@@ -47,6 +47,33 @@ pmacs.lsp.config.python = pmacs.lsp.config.python or {
   },
 }
 
+-- C / C++ via clangd. One clangd binary serves both; `config.c` and
+-- `config.cpp` are separate entries only so the `language_id` sent in
+-- `didOpen` is accurate (clangd respects it). clangd takes its
+-- project model from `compile_commands.json` / `compile_flags.txt`
+-- at the project root, not `workspace/configuration`, so no
+-- `settings` here; `--background-index` enables cross-file features.
+-- Users override from init.lua before a C/C++ file opens.
+pmacs.lsp.config.c = pmacs.lsp.config.c or {
+  command = "clangd",
+  args = { "--background-index" },
+}
+pmacs.lsp.config.cpp = pmacs.lsp.config.cpp or {
+  command = "clangd",
+  args = { "--background-index" },
+}
+
+-- Go via gopls. `gopls` with no args serves LSP over stdio. gopls
+-- pulls its configuration via `workspace/configuration` (now
+-- answered, #13) under the `gopls` section; an empty section means
+-- "use defaults" — present, not null, which gopls prefers. Users
+-- populate it (e.g. staticcheck, analyses) from init.lua.
+pmacs.lsp.config.go = pmacs.lsp.config.go or {
+  command = "gopls",
+  args = {},
+  settings = { gopls = {} },
+}
+
 -- LSP-side extension → language map, deliberately independent of the
 -- tree-sitter detection in `pmacs.parse` (which is grammar-gated:
 -- Python has an LSP server but no bundled grammar). Consulted only
@@ -56,6 +83,16 @@ pmacs.lsp.config.python = pmacs.lsp.config.python or {
 pmacs.lsp.filetypes = pmacs.lsp.filetypes or {}
 pmacs.lsp.filetypes.py = pmacs.lsp.filetypes.py or "python"
 pmacs.lsp.filetypes.pyi = pmacs.lsp.filetypes.pyi or "python"
+-- C. `.h` is ambiguous C/C++; default it to C (clangd copes either
+-- way, and users can remap `pmacs.lsp.filetypes.h = "cpp"`).
+pmacs.lsp.filetypes.c = pmacs.lsp.filetypes.c or "c"
+pmacs.lsp.filetypes.h = pmacs.lsp.filetypes.h or "c"
+-- C++.
+for _, ext in ipairs({ "cpp", "cc", "cxx", "hpp", "hh", "hxx", "ipp", "inl", "cppm" }) do
+  pmacs.lsp.filetypes[ext] = pmacs.lsp.filetypes[ext] or "cpp"
+end
+-- Go.
+pmacs.lsp.filetypes.go = pmacs.lsp.filetypes.go or "go"
 
 -- Per-buffer attachment record: { language, server, uri, version }.
 -- Keyed by `tostring(BufferIdLua)` because BufferIdLua hands out fresh
diff --git a/tests/m4_acceptance.rs b/tests/m4_acceptance.rs
index 476b7bc..85d24e8 100644
--- a/tests/m4_acceptance.rs
+++ b/tests/m4_acceptance.rs
@@ -1190,6 +1190,70 @@ fn m4_5_basedpyright_initializes_and_negotiates_encoding() {
     let _ = mgr.borrow_mut().stop(sid);
 }
 
+/// Shared body for the PATH-gated real-server smoke tests: spawn,
+/// reach `Initialized`, and assert the server negotiated a
+/// `positionEncoding` pmacs can actually encode (absent ⇒ pmacs
+/// defaults to UTF-16, also fine; a third encoding must never slip
+/// through). Mirrors the basedpyright test for clangd / gopls —
+/// strict-by-default servers that exercise the Option B path against
+/// real implementations, not just the fake.
+fn assert_lsp_initializes_and_negotiates(
+    label: &str,
+    language_id: &str,
+    command: &str,
+    args: &[&str],
+) {
+    let (sup, mgr) = make_lsp_test_manager();
+    let mut spec = LspServerSpec::new(label, language_id, command);
+    spec.args = args.iter().map(|s| (*s).to_string()).collect();
+    spec.restart = LspRestartPolicy::Never;
+    let sid = mgr.borrow_mut().spawn(spec).expect("spawn server");
+    let evs = drain_lsp_until(&sup, &mgr, sid, Duration::from_secs(20), |evs| {
+        evs.iter()
+            .any(|e| matches!(e.kind, LspEventKind::Initialized { .. }))
+    });
+    let caps = evs
+        .iter()
+        .find_map(|e| match &e.kind {
+            LspEventKind::Initialized { capabilities } => Some(capabilities.clone()),
+            _ => None,
+        })
+        .expect("must observe Initialized event");
+    assert!(caps.is_object(), "capabilities should be a JSON object");
+    let enc = caps.get("positionEncoding").and_then(|v| v.as_str());
+    assert!(
+        matches!(enc, None | Some("utf-8" | "utf-16")),
+        "{label} negotiated an encoding pmacs cannot handle: {enc:?}"
+    );
+    let state = mgr.borrow().state(sid).cloned();
+    assert!(matches!(state, Some(LspClientState::Initialized { .. })));
+    let _ = mgr.borrow_mut().stop(sid);
+}
+
+/// C/C++ via clangd (PATH-gated). clangd defaults UTF-16 and also
+/// supports its own `offsetEncoding` extension; either way the
+/// negotiated encoding must be one pmacs encodes.
+#[test]
+fn m4_5_clangd_initializes_and_negotiates_encoding() {
+    let Ok(_) = which_binary("clangd") else {
+        eprintln!("clangd not on PATH; skipping");
+        return;
+    };
+    assert_lsp_initializes_and_negotiates("clangd", "cpp", "clangd", &["--background-index"]);
+}
+
+/// Go via gopls (PATH-gated). gopls implements LSP 3.17
+/// position-encoding, defaults UTF-16, and pulls config via
+/// `workspace/configuration` — exercises the full stack end to end.
+#[test]
+fn m4_5_gopls_initializes_and_negotiates_encoding() {
+    let Ok(_) = which_binary("gopls") else {
+        eprintln!("gopls not on PATH; skipping");
+        return;
+    };
+    assert_lsp_initializes_and_negotiates("gopls", "go", "gopls", &[]);
+}
+
 /// Helper: scan PATH for a binary by name. Returns the absolute
 /// path if found.
 fn which_binary(name: &str) -> std::io::Result<std::path::PathBuf> {