Index: test/clangd/compile-commands-path-in-initialize.test
===================================================================
--- /dev/null
+++ test/clangd/compile-commands-path-in-initialize.test
@@ -0,0 +1,30 @@
+# Test that we can set choose a configuration/build directly in the initialize
+# request.
+
+# RUN: rm -rf %t.dir/* && mkdir -p %t.dir
+# RUN: mkdir %t.dir/build-1
+# RUN: mkdir %t.dir/build-2
+# RUN: echo '[{"directory": "%t.dir", "command": "c++ the-file.cpp", "file": "the-file.cpp"}]' > %t.dir/compile_commands.json
+# RUN: echo '[{"directory": "%t.dir/build-1", "command": "c++ -DMACRO=1 the-file.cpp", "file": "../the-file.cpp"}]' > %t.dir/build-1/compile_commands.json
+# RUN: echo '[{"directory": "%t.dir/build-2", "command": "c++ -DMACRO=2 the-file.cpp", "file": "../the-file.cpp"}]' > %t.dir/build-2/compile_commands.json
+# RUN: sed -e "s|INPUT_DIR|%t.dir|g" %s > %t.test
+
+# RUN: clangd -lit-test < %t.test | FileCheck -strict-whitespace %t.test
+
+{"jsonrpc":"2.0","id":0,"method":"initialize","params":{"initializationOptions":{"compilationDatabasePath":"INPUT_DIR/build-1"}}}
+---
+{"jsonrpc":"2.0","method":"textDocument/didOpen","params":{"textDocument":{"uri":"file://INPUT_DIR/the-file.cpp","languageId":"cpp","version":1,"text":"#if !defined(MACRO)\n#pragma message (\"MACRO is not defined\")\n#elif MACRO == 1\n#pragma message (\"MACRO is one\")\n#elif MACRO == 2\n#pragma message (\"MACRO is two\")\n#else\n#pragma message (\"woops\")\n#endif\nint main() {}\n"}}}
+# CHECK:   "method": "textDocument/publishDiagnostics",
+# CHECK-NEXT:   "params": {
+# CHECK-NEXT:     "diagnostics": [
+# CHECK-NEXT:       {
+# CHECK-NEXT:         "message": "MACRO is one",
+---
+{"jsonrpc":"2.0","id":0,"method":"workspace/didChangeConfiguration","params":{"settings":{"compilationDatabasePath":"INPUT_DIR/build-2"}}}
+# CHECK:   "method": "textDocument/publishDiagnostics",
+# CHECK-NEXT:   "params": {
+# CHECK-NEXT:     "diagnostics": [
+# CHECK-NEXT:       {
+# CHECK-NEXT:         "message": "MACRO is two",
+---
+{"jsonrpc":"2.0","id":10000,"method":"shutdown"}
Index: test/clangd/compile-commands-path.test
===================================================================
--- /dev/null
+++ test/clangd/compile-commands-path.test
@@ -0,0 +1,37 @@
+# Test that we can switch between configuration/build using the
+# workspace/didChangeConfiguration notification.
+
+# RUN: rm -rf %t.dir/* && mkdir -p %t.dir
+# RUN: mkdir %t.dir/build-1
+# RUN: mkdir %t.dir/build-2
+# RUN: echo '[{"directory": "%t.dir", "command": "c++ the-file.cpp", "file": "the-file.cpp"}]' > %t.dir/compile_commands.json
+# RUN: echo '[{"directory": "%t.dir/build-1", "command": "c++ -DMACRO=1 the-file.cpp", "file": "../the-file.cpp"}]' > %t.dir/build-1/compile_commands.json
+# RUN: echo '[{"directory": "%t.dir/build-2", "command": "c++ -DMACRO=2 the-file.cpp", "file": "../the-file.cpp"}]' > %t.dir/build-2/compile_commands.json
+# RUN: sed -e "s|INPUT_DIR|%t.dir|g" %s > %t.test
+
+# RUN: clangd -lit-test < %t.test | FileCheck -strict-whitespace %t.test
+
+{"jsonrpc":"2.0","id":0,"method":"initialize","params":{}}
+---
+{"jsonrpc":"2.0","method":"textDocument/didOpen","params":{"textDocument":{"uri":"file://INPUT_DIR/the-file.cpp","languageId":"cpp","version":1,"text":"#if !defined(MACRO)\n#pragma message (\"MACRO is not defined\")\n#elif MACRO == 1\n#pragma message (\"MACRO is one\")\n#elif MACRO == 2\n#pragma message (\"MACRO is two\")\n#else\n#pragma message (\"woops\")\n#endif\nint main() {}\n"}}}
+# CHECK:   "method": "textDocument/publishDiagnostics",
+# CHECK-NEXT:   "params": {
+# CHECK-NEXT:     "diagnostics": [
+# CHECK-NEXT:       {
+# CHECK-NEXT:         "message": "MACRO is not defined",
+---
+{"jsonrpc":"2.0","id":0,"method":"workspace/didChangeConfiguration","params":{"settings":{"compilationDatabasePath":"INPUT_DIR/build-1"}}}
+# CHECK:   "method": "textDocument/publishDiagnostics",
+# CHECK-NEXT:   "params": {
+# CHECK-NEXT:     "diagnostics": [
+# CHECK-NEXT:       {
+# CHECK-NEXT:         "message": "MACRO is one",
+---
+{"jsonrpc":"2.0","id":0,"method":"workspace/didChangeConfiguration","params":{"settings":{"compilationDatabasePath":"INPUT_DIR/build-2"}}}
+# CHECK:   "method": "textDocument/publishDiagnostics",
+# CHECK-NEXT:   "params": {
+# CHECK-NEXT:     "diagnostics": [
+# CHECK-NEXT:       {
+# CHECK-NEXT:         "message": "MACRO is two",
+---
+{"jsonrpc":"2.0","id":10000,"method":"shutdown"}