CPP validation library

a2a_agents/cpp/.gitignore

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+build/

a2a_agents/cpp/CMakeLists.txt

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+cmake_minimum_required(VERSION 3.14)
+project(A2UI_CPP_Agent)
+
+set(CMAKE_CXX_STANDARD 17)
+set(CMAKE_CXX_STANDARD_REQUIRED True)
+
+include(FetchContent)
+
+# nlohmann/json
+FetchContent_Declare(
+    nlohmann_json
+    GIT_REPOSITORY https://github.com/nlohmann/json.git
+    GIT_TAG        v3.11.3
+)
+FetchContent_MakeAvailable(nlohmann_json)
+
+# pboettch/json-schema-validator
+FetchContent_Declare(
+    nlohmann_json_schema_validator
+    GIT_REPOSITORY https://github.com/pboettch/json-schema-validator.git
+    GIT_TAG        2.3.0
+)
+FetchContent_MakeAvailable(nlohmann_json_schema_validator)
+
+# GoogleTest
+FetchContent_Declare(
+    googletest
+    GIT_REPOSITORY https://github.com/google/googletest.git
+    GIT_TAG        v1.14.0
+)
+# For Windows: Prevent overriding the parent project's compiler/linker settings
+set(gtest_force_shared_crt ON CACHE BOOL "" FORCE)
+FetchContent_MakeAvailable(googletest)
+
+# library: a2ui_validation
+add_library(a2ui_validation src/validation.cpp)
+target_include_directories(a2ui_validation PUBLIC include)
+target_link_libraries(a2ui_validation PUBLIC nlohmann_json::nlohmann_json nlohmann_json_schema_validator)
+
+# executable: test_validation
+enable_testing()
+add_executable(test_validation tests/test_validation.cpp)
+target_link_libraries(test_validation PRIVATE a2ui_validation GTest::gtest_main GTest::gmock)
+
+include(GoogleTest)
+gtest_discover_tests(test_validation)

a2a_agents/cpp/README.md

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+# A2UI C++ Agent implementation
+
+The `a2a_agents/cpp/` directory contains the C++ implementation of the A2UI agent library.
+
+## Components
+
+The library provides validation logic for A2UI protocol messages:
+
+*   **`a2ui_validation`**: A library for validating A2UI JSON messages against the schema and semantic rules.
+    *   Header: `include/a2ui/validation.hpp`
+    *   Source: `src/validation.cpp`
+
+## Running tests
+
+The project uses CMake and GoogleTest.
+
+1.  Navigate to the C++ agent directory:
+    ```bash
+    cd a2a_agents/cpp
+    ```
+
+2.  Create a build directory and configure:
+    ```bash
+    mkdir -p build
+    cd build
+    cmake ..
+    ```
+
+3.  Build and run the tests:
+    ```bash
+    make
+    ./test_validation
+    ```
+
+## Dependencies
+
+The following dependencies are automatically fetched via CMake `FetchContent`:
+*   [nlohmann/json](https://github.com/nlohmann/json): JSON for Modern C++
+*   [pboettch/json-schema-validator](https://github.com/pboettch/json-schema-validator): JSON Schema Validator for JSON for Modern C++
+*   [GoogleTest](https://github.com/google/googletest): Google Testing and Mocking Framework

a2a_agents/cpp/include/a2ui/validation.hpp

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+/*
+ * Copyright 2025 Google LLC
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#pragma once
+
+#include <nlohmann/json.hpp>
+
+namespace a2ui {
+
+/**
+ * Validates the A2UI JSON payload against the provided schema and checks for integrity.
+ *
+ * The payload can be a single message object or an array of message objects.
+ *
+ * Note: Does not support streaming or partial messages. 
+ * TODO: Support streaming.
+ *
+ * Checks performed:
+ * 1.  **JSON Schema Validation**: Ensures payload adheres to the A2UI schema.
+ * 2.  **Component Integrity**:
+ *     -   All component IDs are unique.
+ *     -   A 'root' component exists.
+ *     -   All unique component references point to valid IDs.
+ * 3.  **Topology**:
+ *     -   No circular references (including self-references).
+ *     -   No orphaned components (all components must be reachable from 'root').
+ * 4.  **Recursion Limits**:
+ *     -   Global recursion depth limit (50).
+ *     -   FunctionCall recursion depth limit (5).
+ * 5.  **Path Syntax**:
+ *     -   Validates JSON Pointer syntax for data paths.
+ *
+ * @param a2ui_json The JSON payload to validate.
+ * @param a2ui_schema The schema to validate against.
+ *
+ * @throws std::invalid_argument If integrity, topology, or recursion checks fail, or if payload does not match schema.
+ */
+void validate_a2ui_json(const nlohmann::json& a2ui_json, const nlohmann::json& a2ui_schema);
+
+} // namespace a2ui