feat: Add MCP Resources and Prompts support

src/main/java/io/naftiko/engine/exposes/McpPromptHandler.java

@@ -0,0 +1,162 @@
+/**
+ * Copyright 2025-2026 Naftiko
+ * 
+ * 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
+ * 
+ * http://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.
+ */
+package io.naftiko.engine.exposes;
+
+import java.io.IOException;
+import java.net.URI;
+import java.nio.charset.StandardCharsets;
+import java.nio.file.Files;
+import java.nio.file.Path;
+import java.nio.file.Paths;
+import java.util.ArrayList;
+import java.util.List;
+import java.util.Map;
+import java.util.concurrent.ConcurrentHashMap;
+import java.util.regex.Matcher;
+import java.util.regex.Pattern;
+import io.naftiko.spec.exposes.McpPromptMessageSpec;
+import io.naftiko.spec.exposes.McpServerPromptSpec;
+
+/**
+ * Handles MCP {@code prompts/get} requests by rendering prompt templates.
+ *
+ * <p><b>Inline prompts</b> substitute {@code {{arg}}} placeholders in the declared message list.
+ * Argument values are treated as literal text — nested {@code {{...}}} in values are NOT
+ * re-interpolated (prevents prompt injection).</p>
+ *
+ * <p><b>File-based prompts</b> load the file at the {@code location} URI, substitute arguments,
+ * and return the content as a single {@code user} role message.</p>
+ */
+public class McpPromptHandler {
+
+    /** Matches {{argName}} placeholders — arg names are alphanumeric + underscore. */
+    private static final Pattern PLACEHOLDER = Pattern.compile("\\{\\{([a-zA-Z0-9_]+)\\}\\}");
+
+    private final Map<String, McpServerPromptSpec> promptSpecs;
+
+    public McpPromptHandler(List<McpServerPromptSpec> prompts) {
+        this.promptSpecs = new ConcurrentHashMap<>();
+        for (McpServerPromptSpec prompt : prompts) {
+            promptSpecs.put(prompt.getName(), prompt);
+        }
+    }
+
+    /**
+     * A rendered prompt message returned by {@link #render(String, Map)}.
+     */
+    public static class RenderedMessage {
+        public final String role;
+        public final String text;
+
+        public RenderedMessage(String role, String text) {
+            this.role = role;
+            this.text = text;
+        }
+    }
+
+    /**
+     * Render a prompt by name with the provided arguments.
+     *
+     * @param name      the prompt name
+     * @param arguments key-value argument map (values are always treated as strings)
+     * @return ordered list of rendered messages
+     * @throws IllegalArgumentException when the prompt is unknown
+     * @throws IOException              when a file-based prompt cannot be read
+     */
+    public List<RenderedMessage> render(String name, Map<String, Object> arguments)
+            throws IOException {
+
+        McpServerPromptSpec spec = promptSpecs.get(name);
+        if (spec == null) {
+            throw new IllegalArgumentException("Unknown prompt: " + name);
+        }
+
+        Map<String, String> args = toStringMap(arguments);
+
+        if (spec.isFileBased()) {
+            return renderFileBased(spec, args);
+        } else {
+            return renderInline(spec, args);
+        }
+    }
+
+    /**
+     * Return all prompt specs (for {@code prompts/list}).
+     */
+    public List<McpServerPromptSpec> listAll() {
+        return new ArrayList<>(promptSpecs.values());
+    }
+
+    // ── Inline rendering ─────────────────────────────────────────────────────────────────────────
+
+    private List<RenderedMessage> renderInline(McpServerPromptSpec spec,
+            Map<String, String> args) {
+        List<RenderedMessage> messages = new ArrayList<>();
+        for (McpPromptMessageSpec msg : spec.getTemplate()) {
+            String rendered = substitute(msg.getContent(), args);
+            messages.add(new RenderedMessage(msg.getRole(), rendered));
+        }
+        return messages;
+    }
+
+    // ── File-based rendering ─────────────────────────────────────────────────────────────────────
+
+    private List<RenderedMessage> renderFileBased(McpServerPromptSpec spec,
+            Map<String, String> args) throws IOException {
+        Path file = Paths.get(URI.create(spec.getLocation()));
+        if (!Files.isRegularFile(file)) {
+            throw new IOException(
+                    "Prompt file not found for '" + spec.getName() + "': " + spec.getLocation());
+        }
+        String content = Files.readString(file, StandardCharsets.UTF_8);
+        String rendered = substitute(content, args);
+        return List.of(new RenderedMessage("user", rendered));
+    }
+
+    // ── Argument substitution ────────────────────────────────────────────────────────────────────
+
+    /**
+     * Replace all {@code {{argName}}} occurrences in {@code template} with the corresponding value
+     * from {@code args}. Unrecognised placeholders are left as-is. Argument values are escaped so
+     * that any {@code {{...}}} sequences they contain are NOT treated as additional placeholders
+     * (prevents prompt injection).
+     */
+    public static String substitute(String template, Map<String, String> args) {
+        if (template == null) {
+            return "";
+        }
+        Matcher m = PLACEHOLDER.matcher(template);
+        StringBuilder sb = new StringBuilder();
+        while (m.find()) {
+            String argName = m.group(1);
+            String value = args.getOrDefault(argName, m.group(0)); // leave placeholder if missing
+            // Escape literal backslashes and dollar signs for Matcher.appendReplacement
+            m.appendReplacement(sb, Matcher.quoteReplacement(value));
+        }
+        m.appendTail(sb);
+        return sb.toString();
+    }
+
+    private static Map<String, String> toStringMap(Map<String, Object> arguments) {
+        Map<String, String> result = new ConcurrentHashMap<>();
+        if (arguments != null) {
+            for (Map.Entry<String, Object> entry : arguments.entrySet()) {
+                if (entry.getValue() != null) {
+                    result.put(entry.getKey(), String.valueOf(entry.getValue()));
+                }
+            }
+        }
+        return result;
+    }
+}