diff --git a/editor/CMakeLists.txt b/editor/CMakeLists.txt
index 6a08831..7a70eb9 100644
--- a/editor/CMakeLists.txt
+++ b/editor/CMakeLists.txt
@@ -4459,4 +4459,13 @@ target_link_libraries(step619_test PRIVATE
     tree_sitter_javascript tree_sitter_typescript
     tree_sitter_java tree_sitter_rust tree_sitter_go)
 
+add_executable(step620_test tests/step620_test.cpp)
+target_include_directories(step620_test PRIVATE src)
+target_link_libraries(step620_test PRIVATE
+    nlohmann_json::nlohmann_json
+    unofficial::tree-sitter::tree-sitter
+    tree_sitter_python tree_sitter_cpp tree_sitter_elisp
+    tree_sitter_javascript tree_sitter_typescript
+    tree_sitter_java tree_sitter_rust tree_sitter_go)
+
 # Step 12: Dear ImGui shell scaffolding created (main.cpp exists but not built due to dependencies)
diff --git a/editor/src/MCPProjectConfig.h b/editor/src/MCPProjectConfig.h
index ca13b92..5a69de6 100644
--- a/editor/src/MCPProjectConfig.h
+++ b/editor/src/MCPProjectConfig.h
@@ -18,6 +18,7 @@ struct MCPProjectConfigData {
 
 struct MCPProjectConfigLoadResult {
     bool found = false;
+    std::string sourceWorkspaceRoot;
     MCPProjectConfigData config;
     std::string error;
 };
@@ -69,6 +70,7 @@ public:
             out.error = "workspace_missing";
             return out;
         }
+        out.sourceWorkspaceRoot = workspaceRoot;
 
         std::filesystem::path path = std::filesystem::path(workspaceRoot) / ".whetstone.json";
         if (!std::filesystem::exists(path)) return out;
@@ -86,6 +88,30 @@ public:
         return out;
     }
 
+    static MCPProjectConfigLoadResult discoverFromCwd(const std::string& startDir = "") {
+        MCPProjectConfigLoadResult out;
+        std::filesystem::path current = startDir.empty()
+            ? std::filesystem::current_path()
+            : std::filesystem::path(startDir);
+        if (!std::filesystem::exists(current)) {
+            out.error = "start_dir_not_found";
+            return out;
+        }
+
+        current = std::filesystem::weakly_canonical(current);
+        while (true) {
+            std::filesystem::path candidate = current / ".whetstone.json";
+            if (std::filesystem::exists(candidate)) {
+                return loadFromWorkspace(current.string());
+            }
+            if (current == current.root_path()) break;
+            std::filesystem::path parent = current.parent_path();
+            if (parent == current) break;
+            current = parent;
+        }
+        return out;
+    }
+
 private:
     static bool readOptionalString(const json& parsed,
                                    const char* key,
diff --git a/editor/src/mcp_main.cpp b/editor/src/mcp_main.cpp
index d5d93ee..6dccba1 100644
--- a/editor/src/mcp_main.cpp
+++ b/editor/src/mcp_main.cpp
@@ -124,12 +124,19 @@ int main(int argc, char** argv) {
         if (!configLoad.error.empty()) {
             std::cerr << "[whetstone-mcp] Config load warning: " << configLoad.error << "\n";
         }
+    } else {
+        configLoad = MCPProjectConfig::discoverFromCwd();
+        if (!configLoad.error.empty()) {
+            std::cerr << "[whetstone-mcp] Config discovery warning: " << configLoad.error << "\n";
+        }
     }
 
     if (configLoad.found && !configLoad.config.workspace.empty()) {
         state.workspaceRoot = configLoad.config.workspace;
     } else if (!workspace.empty()) {
         state.workspaceRoot = workspace;
+    } else if (configLoad.found) {
+        state.workspaceRoot = configLoad.sourceWorkspaceRoot;
     }
 
     if (!language.empty()) {
diff --git a/editor/tests/step620_test.cpp b/editor/tests/step620_test.cpp
new file mode 100644
index 0000000..d722e58
--- /dev/null
+++ b/editor/tests/step620_test.cpp
@@ -0,0 +1,191 @@
+// Step 620: Config auto-discovery (walk up from CWD) (12 tests)
+
+#include "MCPProjectConfig.h"
+
+#include <filesystem>
+#include <fstream>
+#include <iostream>
+
+static int passed = 0, failed = 0;
+#define TEST(name) { std::cout << "  " << #name << "... "; }
+#define PASS() { std::cout << "PASS\n"; ++passed; }
+#define FAIL(msg) { std::cout << "FAIL: " << msg << "\n"; ++failed; }
+#define CHECK(cond, msg) if (!(cond)) { FAIL(msg); return; } else {}
+
+static std::filesystem::path tempRoot() {
+    std::filesystem::path root = std::filesystem::temp_directory_path() / "whetstone_step620";
+    std::filesystem::remove_all(root);
+    std::filesystem::create_directories(root);
+    return root;
+}
+
+static void writeFile(const std::filesystem::path& path, const std::string& content) {
+    std::ofstream out(path);
+    out << content;
+}
+
+void test_discover_from_exact_directory() {
+    TEST(discover_from_exact_directory);
+    auto root = tempRoot() / "exact";
+    std::filesystem::create_directories(root);
+    writeFile(root / ".whetstone.json", R"({"defaultLanguage":"go"})");
+    auto result = MCPProjectConfig::discoverFromCwd(root.string());
+    CHECK(result.found, "config should be found");
+    CHECK(result.sourceWorkspaceRoot == root.string(), "source workspace mismatch");
+    CHECK(result.config.defaultLanguage == "go", "language mismatch");
+    PASS();
+}
+
+void test_discover_walks_to_parent() {
+    TEST(discover_walks_to_parent);
+    auto root = tempRoot() / "parent";
+    auto child = root / "src" / "feature";
+    std::filesystem::create_directories(child);
+    writeFile(root / ".whetstone.json", R"({"defaultLanguage":"rust"})");
+    auto result = MCPProjectConfig::discoverFromCwd(child.string());
+    CHECK(result.found, "config should be found");
+    CHECK(result.sourceWorkspaceRoot == root.string(), "should resolve parent workspace");
+    PASS();
+}
+
+void test_discover_prefers_nearest_config() {
+    TEST(discover_prefers_nearest_config);
+    auto root = tempRoot() / "nearest";
+    auto mid = root / "apps";
+    auto leaf = mid / "api";
+    std::filesystem::create_directories(leaf);
+    writeFile(root / ".whetstone.json", R"({"mcpWorkspaceAlias":"root"})");
+    writeFile(mid / ".whetstone.json", R"({"mcpWorkspaceAlias":"mid"})");
+    auto result = MCPProjectConfig::discoverFromCwd(leaf.string());
+    CHECK(result.found, "config should be found");
+    CHECK(result.sourceWorkspaceRoot == mid.string(), "should choose nearest parent");
+    CHECK(result.config.mcpWorkspaceAlias == "mid", "alias should come from nearest file");
+    PASS();
+}
+
+void test_discover_returns_not_found_when_missing() {
+    TEST(discover_returns_not_found_when_missing);
+    auto root = tempRoot() / "missing";
+    auto child = root / "sub";
+    std::filesystem::create_directories(child);
+    auto result = MCPProjectConfig::discoverFromCwd(child.string());
+    CHECK(!result.found, "should not find config");
+    CHECK(result.error.empty(), "error should be empty");
+    PASS();
+}
+
+void test_discover_reports_invalid_start_directory() {
+    TEST(discover_reports_invalid_start_directory);
+    auto root = tempRoot() / "nope" / "does-not-exist";
+    auto result = MCPProjectConfig::discoverFromCwd(root.string());
+    CHECK(!result.found, "should not find config");
+    CHECK(result.error == "start_dir_not_found", "wrong error");
+    PASS();
+}
+
+void test_discover_reports_parse_error_from_nearest_file() {
+    TEST(discover_reports_parse_error_from_nearest_file);
+    auto root = tempRoot() / "bad";
+    auto child = root / "src";
+    std::filesystem::create_directories(child);
+    writeFile(root / ".whetstone.json", "{bad");
+    auto result = MCPProjectConfig::discoverFromCwd(child.string());
+    CHECK(result.found, "file exists so found should be true");
+    CHECK(result.error == "config_json_parse_failed", "wrong error");
+    PASS();
+}
+
+void test_discover_parses_workspace_override() {
+    TEST(discover_parses_workspace_override);
+    auto root = tempRoot() / "workspace-override";
+    std::filesystem::create_directories(root);
+    writeFile(root / ".whetstone.json", R"({"workspace":"/opt/project"})");
+    auto result = MCPProjectConfig::discoverFromCwd(root.string());
+    CHECK(result.found, "should find config");
+    CHECK(result.config.workspace == "/opt/project", "workspace override mismatch");
+    PASS();
+}
+
+void test_discover_parses_agent_role() {
+    TEST(discover_parses_agent_role);
+    auto root = tempRoot() / "role";
+    std::filesystem::create_directories(root);
+    writeFile(root / ".whetstone.json", R"({"agentRole":"generator"})");
+    auto result = MCPProjectConfig::discoverFromCwd(root.string());
+    CHECK(result.found, "should find config");
+    CHECK(result.config.agentRole == "generator", "agent role mismatch");
+    PASS();
+}
+
+void test_discover_parses_workspace_alias() {
+    TEST(discover_parses_workspace_alias);
+    auto root = tempRoot() / "alias";
+    std::filesystem::create_directories(root);
+    writeFile(root / ".whetstone.json", R"({"mcpWorkspaceAlias":"payments"})");
+    auto result = MCPProjectConfig::discoverFromCwd(root.string());
+    CHECK(result.found, "should find config");
+    CHECK(result.config.mcpWorkspaceAlias == "payments", "alias mismatch");
+    PASS();
+}
+
+void test_discover_with_no_argument_uses_current_path() {
+    TEST(discover_with_no_argument_uses_current_path);
+    auto root = tempRoot() / "cwd-default";
+    std::filesystem::create_directories(root);
+    writeFile(root / ".whetstone.json", R"({"defaultLanguage":"cpp"})");
+
+    auto previous = std::filesystem::current_path();
+    std::filesystem::current_path(root);
+    auto result = MCPProjectConfig::discoverFromCwd();
+    std::filesystem::current_path(previous);
+
+    CHECK(result.found, "should find config from cwd");
+    CHECK(result.config.defaultLanguage == "cpp", "language mismatch");
+    PASS();
+}
+
+void test_discover_ignores_grandparent_when_parent_has_invalid_file() {
+    TEST(discover_ignores_grandparent_when_parent_has_invalid_file);
+    auto root = tempRoot() / "invalid-nearest";
+    auto parent = root / "service";
+    auto leaf = parent / "api";
+    std::filesystem::create_directories(leaf);
+    writeFile(root / ".whetstone.json", R"({"defaultLanguage":"python"})");
+    writeFile(parent / ".whetstone.json", "{bad");
+    auto result = MCPProjectConfig::discoverFromCwd(leaf.string());
+    CHECK(result.found, "nearest file exists");
+    CHECK(result.error == "config_json_parse_failed", "should fail on nearest invalid file");
+    PASS();
+}
+
+void test_discover_preserves_source_workspace_root_for_nested_start() {
+    TEST(discover_preserves_source_workspace_root_for_nested_start);
+    auto root = tempRoot() / "source-root";
+    auto nested = root / "a" / "b" / "c";
+    std::filesystem::create_directories(nested);
+    writeFile(root / ".whetstone.json", R"({"defaultLanguage":"typescript"})");
+    auto result = MCPProjectConfig::discoverFromCwd(nested.string());
+    CHECK(result.found, "should find config");
+    CHECK(result.sourceWorkspaceRoot == root.string(), "source root mismatch");
+    PASS();
+}
+
+int main() {
+    std::cout << "Step 620: Config auto-discovery walk-up\n";
+
+    test_discover_from_exact_directory();                         // 1
+    test_discover_walks_to_parent();                              // 2
+    test_discover_prefers_nearest_config();                       // 3
+    test_discover_returns_not_found_when_missing();               // 4
+    test_discover_reports_invalid_start_directory();              // 5
+    test_discover_reports_parse_error_from_nearest_file();        // 6
+    test_discover_parses_workspace_override();                    // 7
+    test_discover_parses_agent_role();                            // 8
+    test_discover_parses_workspace_alias();                       // 9
+    test_discover_with_no_argument_uses_current_path();           // 10
+    test_discover_ignores_grandparent_when_parent_has_invalid_file();// 11
+    test_discover_preserves_source_workspace_root_for_nested_start();// 12
+
+    std::cout << "\nResults: " << passed << "/" << (passed + failed) << " passed\n";
+    return failed == 0 ? 0 : 1;
+}
diff --git a/progress.md b/progress.md
index 59a2b32..608a986 100644
--- a/progress.md
+++ b/progress.md
@@ -12252,3 +12252,38 @@ at startup when present.
 - `editor/src/mcp_main.cpp` within main-size limit (`177` <= `1500`)
 - `editor/tests/step619_test.cpp` within test-file size guidance (`169` lines)
 - Header-only architecture and naming conventions remain aligned with `ARCHITECTURE.md`
+
+### Step 620: Config auto-discovery (walk up from CWD)
+**Status:** PASS (12/12 tests)
+
+Adds upward-walk config discovery so `whetstone_mcp` can boot without an
+explicit `--workspace` flag by finding the nearest `.whetstone.json` from CWD.
+
+**Files modified:**
+- `editor/src/MCPProjectConfig.h` - added:
+  - `discoverFromCwd()` walk-up discovery
+  - `sourceWorkspaceRoot` capture for discovered config location
+- `editor/src/mcp_main.cpp` - startup now:
+  - uses discovery path when `--workspace` is omitted
+  - applies discovered workspace root when config file omits `workspace`
+  - emits discovery warnings on invalid start/config parse errors
+- `editor/CMakeLists.txt` - `step620_test` target
+
+**Files added:**
+- `editor/tests/step620_test.cpp` - 12 tests covering:
+  - exact/parent/nearest config discovery behavior
+  - no-config and invalid-start directory cases
+  - parse-error propagation from nearest config file
+  - field parsing during discovery and source-root tracking
+
+**Verification run:**
+- `cmake -S editor -B editor/build-native` - PASS
+- `cmake --build editor/build-native --target step620_test step619_test` - PASS
+- `./editor/build-native/step620_test` - PASS (12/12)
+- `./editor/build-native/step619_test` - PASS (12/12) regression coverage
+
+**Architecture gate check:**
+- `editor/src/MCPProjectConfig.h` within header-size limit (`128` <= `600`)
+- `editor/src/mcp_main.cpp` within main-size limit (`184` <= `1500`)
+- `editor/tests/step620_test.cpp` within test-file size guidance (`191` lines)
+- Header-only architecture and naming conventions remain aligned with `ARCHITECTURE.md`