diff --git a/editor/CMakeLists.txt b/editor/CMakeLists.txt
index 96cf8e4..63b1bd3 100644
--- a/editor/CMakeLists.txt
+++ b/editor/CMakeLists.txt
@@ -4738,4 +4738,13 @@ target_link_libraries(step650_test PRIVATE
     tree_sitter_javascript tree_sitter_typescript
     tree_sitter_java tree_sitter_rust tree_sitter_go)
 
+add_executable(step651_test tests/step651_test.cpp)
+target_include_directories(step651_test PRIVATE src)
+target_link_libraries(step651_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/LanguageGeneratorPluginManifest.h b/editor/src/LanguageGeneratorPluginManifest.h
new file mode 100644
index 0000000..7167a62
--- /dev/null
+++ b/editor/src/LanguageGeneratorPluginManifest.h
@@ -0,0 +1,30 @@
+#pragma once
+// Step 651: Language generator plugin manifest
+
+#include <string>
+#include <vector>
+
+struct PluginManifest {
+    std::string pluginName;
+    std::string sharedObject;
+    std::string generatorClass;
+    std::vector<std::string> languages;
+};
+
+class LanguageGeneratorPluginManifest {
+public:
+    static bool isValid(const PluginManifest& m) {
+        return !m.pluginName.empty() && !m.sharedObject.empty() &&
+               !m.generatorClass.empty() && !m.languages.empty();
+    }
+
+    static bool isPluginFile(const std::string& fileName) {
+        return fileName.rfind("whetstone-plugin-", 0) == 0 &&
+               fileName.size() > 3 &&
+               fileName.substr(fileName.size() - 3) == ".so";
+    }
+
+    static std::string symbolName(const PluginManifest& m) {
+        return "create_" + m.generatorClass;
+    }
+};
diff --git a/editor/tests/step651_test.cpp b/editor/tests/step651_test.cpp
new file mode 100644
index 0000000..3035f88
--- /dev/null
+++ b/editor/tests/step651_test.cpp
@@ -0,0 +1,26 @@
+// Step 651: Language generator plugin manifest (12 tests)
+
+#include "LanguageGeneratorPluginManifest.h"
+#include <iostream>
+static int p=0,f=0;
+#define T(n) { std::cout<<"  "<<#n<<"... "; }
+#define P() { std::cout<<"PASS\n"; ++p; }
+#define F(m) { std::cout<<"FAIL: "<<m<<"\n"; ++f; }
+#define C(c,m) if(!(c)){F(m);return;}
+
+static PluginManifest good(){return {"RustGen","whetstone-plugin-rust.so","RustGenerator",{"rust"}};}
+
+void t1(){T(valid_manifest_passes);C(LanguageGeneratorPluginManifest::isValid(good()),"valid expected");P();}
+void t2(){T(invalid_missing_plugin_name);auto m=good();m.pluginName="";C(!LanguageGeneratorPluginManifest::isValid(m),"invalid expected");P();}
+void t3(){T(invalid_missing_shared_object);auto m=good();m.sharedObject="";C(!LanguageGeneratorPluginManifest::isValid(m),"invalid expected");P();}
+void t4(){T(invalid_missing_generator_class);auto m=good();m.generatorClass="";C(!LanguageGeneratorPluginManifest::isValid(m),"invalid expected");P();}
+void t5(){T(invalid_missing_languages);auto m=good();m.languages.clear();C(!LanguageGeneratorPluginManifest::isValid(m),"invalid expected");P();}
+void t6(){T(plugin_file_pattern_accepts_valid_name);C(LanguageGeneratorPluginManifest::isPluginFile("whetstone-plugin-rust.so"),"should pass");P();}
+void t7(){T(plugin_file_pattern_rejects_wrong_prefix);C(!LanguageGeneratorPluginManifest::isPluginFile("plugin-rust.so"),"should fail");P();}
+void t8(){T(plugin_file_pattern_rejects_wrong_suffix);C(!LanguageGeneratorPluginManifest::isPluginFile("whetstone-plugin-rust.dll"),"should fail");P();}
+void t9(){T(symbol_name_uses_generator_class);C(LanguageGeneratorPluginManifest::symbolName(good())=="create_RustGenerator","symbol mismatch");P();}
+void t10(){T(symbol_name_allows_custom_class);auto m=good();m.generatorClass="GoGenerator";C(LanguageGeneratorPluginManifest::symbolName(m)=="create_GoGenerator","symbol mismatch");P();}
+void t11(){T(plugin_file_detection_handles_short_names);C(!LanguageGeneratorPluginManifest::isPluginFile(".so"),"short should fail");P();}
+void t12(){T(plugin_file_detection_handles_empty_name);C(!LanguageGeneratorPluginManifest::isPluginFile(""),"empty should fail");P();}
+
+int main(){std::cout<<"Step 651: language generator plugin manifest\n";t1();t2();t3();t4();t5();t6();t7();t8();t9();t10();t11();t12();std::cout<<"\nResults: "<<p<<"/"<<(p+f)<<" passed\n";return f?1:0;}
diff --git a/progress.md b/progress.md
index 59d889d..93567ad 100644
--- a/progress.md
+++ b/progress.md
@@ -13349,3 +13349,27 @@ validation, hash verification, binary replace gating, and restart scheduling.
 - `editor/src/HiveMindAutoUpdateModel.h` (`41` <= `600`)
 - `editor/tests/step650_test.cpp` within test-file size guidance (`26` lines)
 - Header-only architecture and naming conventions remain aligned with `ARCHITECTURE.md`
+
+### Step 651: Language generator plugin manifest
+**Status:** PASS (12/12 tests)
+
+Adds plugin-manifest validation and discovery helpers for shared-library
+language generator extensions (`whetstone-plugin-*.so`) and factory symbol naming.
+
+**Files added:**
+- `editor/src/LanguageGeneratorPluginManifest.h` - plugin manifest model
+- `editor/tests/step651_test.cpp` - 12 tests for validation/discovery behavior
+
+**Files modified:**
+- `editor/CMakeLists.txt` - `step651_test` target
+
+**Verification run:**
+- `cmake -S editor -B editor/build-native` - PASS
+- `cmake --build editor/build-native --target step651_test step650_test` - PASS
+- `./editor/build-native/step651_test` - PASS (12/12)
+- `./editor/build-native/step650_test` - PASS (12/12) regression coverage
+
+**Architecture gate check:**
+- `editor/src/LanguageGeneratorPluginManifest.h` (`30` <= `600`)
+- `editor/tests/step651_test.cpp` within test-file size guidance (`26` lines)
+- Header-only architecture and naming conventions remain aligned with `ARCHITECTURE.md`