Add test for handler exception to JSON-RPC error conversion

Validates that when typed handlers throw exceptions, the SDK properly
converts them to JSON-RPC error responses with code -32603 (Internal
Error). This confirms the correct error handling pattern for SDK users.

Author: Mark Pollack

acp-core/src/test/java/com/agentclientprotocol/sdk/client/HandlerExceptionTest.java

@@ -0,0 +1,100 @@
+/*
+ * Copyright 2025-2025 the original author or authors.
+ */
+package com.agentclientprotocol.sdk.client;
+
+import java.util.Map;
+import java.util.function.Function;
+
+import com.agentclientprotocol.sdk.MockAcpClientTransport;
+import com.agentclientprotocol.sdk.spec.AcpSchema;
+import org.junit.jupiter.api.Test;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+/**
+ * Tests that handler exceptions are properly converted to JSON-RPC error responses.
+ * This validates the correct error handling pattern for SDK users.
+ */
+class HandlerExceptionTest {
+
+    /**
+     * Verifies that when a typed handler throws an exception, the SDK converts it
+     * to a proper JSON-RPC error response with code -32603 (Internal Error).
+     */
+    @Test
+    void handlerExceptionConvertedToJsonRpcError() throws InterruptedException {
+        MockAcpClientTransport transport = new MockAcpClientTransport();
+        String errorMessage = "File not found: /nonexistent.txt";
+
+        // Create a handler that throws an exception
+        Function<AcpSchema.ReadTextFileRequest, AcpSchema.ReadTextFileResponse> handler = req -> {
+            throw new RuntimeException(errorMessage);
+        };
+
+        AcpSyncClient client = AcpClient.sync(transport)
+            .readTextFileHandler(handler)
+            .build();
+
+        // Simulate incoming request
+        AcpSchema.JSONRPCRequest request = new AcpSchema.JSONRPCRequest(
+                AcpSchema.JSONRPC_VERSION,
+                "test-id",
+                AcpSchema.METHOD_FS_READ_TEXT_FILE,
+                Map.of("sessionId", "session-123", "path", "/nonexistent.txt")
+        );
+        transport.simulateIncomingMessage(request);
+
+        // Wait for async processing
+        Thread.sleep(500);
+
+        // Verify it's a JSON-RPC error response
+        assertThat(transport.getSentMessages()).hasSize(1);
+        assertThat(transport.getSentMessages().get(0)).isInstanceOf(AcpSchema.JSONRPCResponse.class);
+        AcpSchema.JSONRPCResponse response = (AcpSchema.JSONRPCResponse) transport.getSentMessages().get(0);
+
+        assertThat(response.id()).isEqualTo("test-id");
+        assertThat(response.result()).isNull();
+        assertThat(response.error()).isNotNull();
+        assertThat(response.error().code()).isEqualTo(-32603); // Internal Error
+        assertThat(response.error().message()).contains(errorMessage);
+
+        client.close();
+    }
+
+    /**
+     * Verifies that IOException from file operations is also converted to JSON-RPC error.
+     */
+    @Test
+    void ioExceptionConvertedToJsonRpcError() throws InterruptedException {
+        MockAcpClientTransport transport = new MockAcpClientTransport();
+
+        // Create a handler that throws IOException (wrapped in RuntimeException per Java patterns)
+        Function<AcpSchema.WriteTextFileRequest, AcpSchema.WriteTextFileResponse> handler = req -> {
+            throw new RuntimeException(new java.io.IOException("Permission denied: " + req.path()));
+        };
+
+        AcpSyncClient client = AcpClient.sync(transport)
+            .writeTextFileHandler(handler)
+            .build();
+
+        AcpSchema.JSONRPCRequest request = new AcpSchema.JSONRPCRequest(
+                AcpSchema.JSONRPC_VERSION,
+                "test-id-2",
+                AcpSchema.METHOD_FS_WRITE_TEXT_FILE,
+                Map.of("sessionId", "session-123", "path", "/readonly/file.txt", "content", "test")
+        );
+        transport.simulateIncomingMessage(request);
+
+        // Wait for async processing
+        Thread.sleep(500);
+
+        assertThat(transport.getSentMessages()).hasSize(1);
+        AcpSchema.JSONRPCResponse response = (AcpSchema.JSONRPCResponse) transport.getSentMessages().get(0);
+        assertThat(response.error()).isNotNull();
+        assertThat(response.error().code()).isEqualTo(-32603);
+        assertThat(response.error().message()).contains("Permission denied");
+
+        client.close();
+    }
+}