diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index d10db54..ee962a4 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -54,6 +54,9 @@ jobs:
       - name: Smoke test CLI
         run: node packages/cli/dist/index.js --version
 
+      - name: Smoke test MCP server
+        run: node packages/cli/test/mcp-smoke.mjs
+
       - name: Build web
         run: pnpm build:web
 
diff --git a/.github/workflows/publish.yml b/.github/workflows/publish.yml
index 950bef8..c105279 100644
--- a/.github/workflows/publish.yml
+++ b/.github/workflows/publish.yml
@@ -44,6 +44,9 @@ jobs:
       - name: Smoke test CLI
         run: node packages/cli/dist/index.js --version
 
+      - name: Smoke test MCP server
+        run: node packages/cli/test/mcp-smoke.mjs
+
       - name: Upgrade npm for OIDC trusted publishing
         run: npm install -g npm@latest
 
diff --git a/packages/cli/package.json b/packages/cli/package.json
index 3f3483d..715a1bc 100644
--- a/packages/cli/package.json
+++ b/packages/cli/package.json
@@ -1,5 +1,6 @@
 {
   "name": "@flaid/devmap",
+  "mcpName": "io.github.itsflaid/devmap",
   "version": "0.8.4",
   "description": "CLI that maps your codebase into a reusable context layer for developers and AI agents: entry points, critical files, features and routes for JS/TS projects.",
   "bin": {
diff --git a/packages/cli/src/commands/explain.ts b/packages/cli/src/commands/explain.ts
index cbb332b..34d643f 100644
--- a/packages/cli/src/commands/explain.ts
+++ b/packages/cli/src/commands/explain.ts
@@ -9,7 +9,7 @@ import {
 import { buildExplainMessages } from "../ai/prompts.js";
 import type { AiClient } from "../ai/types.js";
 import type { FileIndexEntry, ProjectMap } from "../analyzers/pipeline/index.js";
-import { readSnapshotOrThrow } from "../cache/snapshot.js";
+import { readSnapshotOrThrow, type LoadedSnapshot } from "../cache/snapshot.js";
 import { resolveEffectiveConfig, type DevmapConfig } from "../utils/config.js";
 import { DevmapError } from "../utils/errors.js";
 import { output, withJsonOutput } from "../utils/output.js";
@@ -20,6 +20,11 @@ export type ExplainOptions = {
   json?: boolean;
   write?: boolean;
   projectRoot?: string;
+  /**
+   * A snapshot the caller already loaded (long-lived front-ends cache it).
+   * When set, the snapshot is not re-read from disk.
+   */
+  loaded?: LoadedSnapshot;
 };
 
 export type ExplainDependencies = {
@@ -74,7 +79,7 @@ export async function runExplain(
   dependencies: ExplainDependencies
 ): Promise<ExplainResult> {
   const projectRoot = resolve(options.projectRoot ?? ".");
-  const snapshot = await readSnapshotOrThrow(projectRoot);
+  const snapshot = options.loaded?.snapshot ?? await readSnapshotOrThrow(projectRoot);
 
   const loadConfig = dependencies.loadConfig
     ?? (() => resolveEffectiveConfig(projectRoot));
diff --git a/packages/cli/src/mcp/server.ts b/packages/cli/src/mcp/server.ts
index 9fc7b78..025fdef 100644
--- a/packages/cli/src/mcp/server.ts
+++ b/packages/cli/src/mcp/server.ts
@@ -3,6 +3,7 @@ import { join } from "node:path";
 import { McpServer, ResourceTemplate } from "@modelcontextprotocol/sdk/server/mcp.js";
 import type { CallToolResult } from "@modelcontextprotocol/sdk/types.js";
 import { z } from "zod";
+import { runExplain, type ExplainDependencies } from "../commands/explain.js";
 import { runFlow } from "../commands/flow.js";
 import { runMap } from "../commands/map.js";
 import { DevmapError } from "../utils/errors.js";
@@ -33,6 +34,9 @@ const SERVER_INSTRUCTIONS = [
   "Prompts devmap_onboard and devmap_plan_change describe ready-made workflows."
 ].join(" ");
 
+const EXPLAIN_NOTE =
+  "devmap_explain sends excerpts of the project's source files to the user's configured AI provider, so it only exists when the server was started with --allow-ai.";
+
 const DEFAULT_ANALYZE_TIMEOUT_SECONDS = 120;
 const DEFAULT_HEARTBEAT_MS = 5_000;
 
@@ -55,6 +59,8 @@ export type DevmapServerOptions = {
   refresher?: Refresher;
   /** Inject the analysis function (tests). */
   analyze?: AnalyzeFn;
+  /** Inject the AI config/client used by devmap_explain (tests). */
+  explainDependencies?: ExplainDependencies;
   /** Interval of progress notifications while devmap_analyze runs. */
   heartbeatMs?: number;
 };
@@ -376,6 +382,49 @@ export function createDevmapServer(options: DevmapServerOptions): DevmapServer {
     }
   );
 
+  if (options.allowAi) {
+    server.registerTool(
+      "devmap_explain",
+      {
+        title: "Explain code with AI",
+        description:
+          "Ask the user's configured AI provider to explain what a file, feature or top-level function does, "
+          + "using DevMap's index to pick the relevant code. Prefer devmap_map, devmap_flow and devmap_impact for structure; "
+          + "use this for \"what does this do and why\". "
+          + EXPLAIN_NOTE
+          + " Requires a provider configured with `devmap init`. Spends the user's AI quota.",
+        inputSchema: {
+          target: z.string().min(1).describe("A project-relative file path, a feature name, or a top-level function name.")
+        },
+        annotations: {
+          readOnlyHint: true,
+          destructiveHint: false,
+          idempotentHint: false,
+          openWorldHint: true
+        }
+      },
+      ({ target }) => respond(async () => {
+        const loaded = await provider.get();
+        const result = await withJsonOutput(() => runExplain(
+          target,
+          { projectRoot, json: true, write: false, loaded },
+          options.explainDependencies ?? {}
+        ));
+        const answer = truncateText(result.answer);
+        return {
+          mode: result.mode,
+          target: result.target,
+          ...(result.resolvedFile ? { resolvedFile: result.resolvedFile } : {}),
+          answer: answer.text,
+          truncated: answer.truncated,
+          model: result.model,
+          contextFiles: result.contextFiles,
+          snapshot: snapshotMeta(loaded)
+        };
+      })
+    );
+  }
+
   // ── resources ────────────────────────────────────────────────────────────
 
   server.registerResource(
diff --git a/packages/cli/test/mcp-explain.test.ts b/packages/cli/test/mcp-explain.test.ts
new file mode 100644
index 0000000..5569c7e
--- /dev/null
+++ b/packages/cli/test/mcp-explain.test.ts
@@ -0,0 +1,265 @@
+import assert from "node:assert/strict";
+import { cp, mkdtemp, readdir, readFile, rm, writeFile } from "node:fs/promises";
+import { tmpdir } from "node:os";
+import { dirname, join, resolve } from "node:path";
+import test from "node:test";
+import { fileURLToPath } from "node:url";
+import { Client } from "@modelcontextprotocol/sdk/client/index.js";
+import { InMemoryTransport } from "@modelcontextprotocol/sdk/inMemory.js";
+import type {
+  AiClient,
+  AiCompletionRequest,
+  AiCompletionResult
+} from "../src/ai/types.js";
+import { createProjectMap } from "../src/analyzers/pipeline/projectMap.js";
+import { saveSnapshot } from "../src/cache/snapshot.js";
+import type { ExplainDependencies } from "../src/commands/explain.js";
+import { createDevmapServer, MAX_TEXT_CHARS } from "../src/mcp/server.js";
+
+const testDirectory = dirname(fileURLToPath(import.meta.url));
+const packageRoot = resolve(testDirectory, "..");
+const nextFixture = join(testDirectory, "fixtures", "nextjs-project");
+
+async function copyFixture(): Promise<string> {
+  const projectRoot = await mkdtemp(join(tmpdir(), "devmap-mcp-explain-"));
+  await cp(nextFixture, projectRoot, { recursive: true });
+  await saveSnapshot(projectRoot, await createProjectMap(projectRoot));
+  return projectRoot;
+}
+
+function fakeAi(content = "It validates the session and returns the current user."): {
+  dependencies: ExplainDependencies;
+  requests: AiCompletionRequest[];
+} {
+  const requests: AiCompletionRequest[] = [];
+  const client: AiClient = {
+    async complete(request): Promise<AiCompletionResult> {
+      requests.push(request);
+      return {
+        content,
+        model: request.model,
+        usage: { promptTokens: 10, completionTokens: 5, totalTokens: 15 }
+      };
+    }
+  };
+  return {
+    requests,
+    dependencies: {
+      loadConfig: async () => ({ provider: "groq", apiKey: "gsk_fixture", model: "auto" }),
+      createAiClient: () => client
+    }
+  };
+}
+
+async function connect(
+  projectRoot: string,
+  options: { allowAi?: boolean; explainDependencies?: ExplainDependencies } = {}
+): Promise<{ client: Client; close: () => Promise<void> }> {
+  const { server } = createDevmapServer({ projectRoot, ...options });
+  const client = new Client({ name: "test-client", version: "0.0.0" });
+  const [clientTransport, serverTransport] = InMemoryTransport.createLinkedPair();
+  await Promise.all([server.connect(serverTransport), client.connect(clientTransport)]);
+  return {
+    client,
+    close: async () => {
+      await client.close();
+      await server.close();
+    }
+  };
+}
+
+async function callTool(
+  client: Client,
+  name: string,
+  args: Record<string, unknown> = {}
+): Promise<{ isError: boolean; data: any }> {
+  const result = (await client.callTool({ name, arguments: args })) as {
+    isError?: boolean;
+    content: Array<{ text: string }>;
+  };
+  return { isError: result.isError === true, data: JSON.parse(result.content[0]!.text) };
+}
+
+test("devmap_explain does not exist unless the server was started with allowAi", async () => {
+  const projectRoot = await copyFixture();
+  const { client, close } = await connect(projectRoot, { explainDependencies: fakeAi().dependencies });
+
+  try {
+    const { tools } = await client.listTools();
+    assert.equal(tools.some((tool) => tool.name === "devmap_explain"), false);
+    assert.equal(tools.length, 7);
+
+    // Calling a tool that was never registered must fail loudly, not run anything.
+    const attempt = await callTool(client, "devmap_explain", { target: "x" }).catch(() => undefined);
+    assert.equal(attempt, undefined, "the response is a plain-text protocol error, not DevMap JSON");
+    const raw = (await client.callTool({ name: "devmap_explain", arguments: { target: "x" } })) as {
+      isError?: boolean;
+      content: Array<{ text: string }>;
+    };
+    assert.equal(raw.isError, true);
+    assert.match(raw.content[0]!.text, /Tool devmap_explain not found/);
+  } finally {
+    await close();
+    await rm(projectRoot, { recursive: true, force: true });
+  }
+});
+
+test("with allowAi, devmap_explain is listed and honest about the AI provider", async () => {
+  const projectRoot = await copyFixture();
+  const { client, close } = await connect(projectRoot, {
+    allowAi: true,
+    explainDependencies: fakeAi().dependencies
+  });
+
+  try {
+    const { tools } = await client.listTools();
+    const explain = tools.find((tool) => tool.name === "devmap_explain");
+    assert.ok(explain, "expected devmap_explain");
+    assert.equal(explain.annotations?.openWorldHint, true);
+    assert.equal(explain.annotations?.destructiveHint, false);
+    assert.equal(explain.annotations?.idempotentHint, false);
+    assert.match(explain.description ?? "", /AI provider/);
+    assert.match(explain.description ?? "", /--allow-ai/);
+    assert.equal(tools.length, 8);
+  } finally {
+    await close();
+    await rm(projectRoot, { recursive: true, force: true });
+  }
+});
+
+test("devmap_explain answers through the configured AI client without writing anything", async () => {
+  const projectRoot = await copyFixture();
+  const { dependencies, requests } = fakeAi();
+  const { client, close } = await connect(projectRoot, { allowAi: true, explainDependencies: dependencies });
+  const printed: string[] = [];
+  const originalLog = console.log;
+  console.log = (...values: unknown[]) => { printed.push(values.join(" ")); };
+
+  try {
+    const snapshotFiles = await readdir(join(projectRoot, ".devmap"));
+    const result = await callTool(client, "devmap_explain", { target: "lib/auth.ts" });
+
+    assert.equal(result.isError, false);
+    assert.equal(result.data.mode, "file");
+    assert.equal(result.data.target, "lib/auth.ts");
+    assert.equal(result.data.answer, "It validates the session and returns the current user.");
+    assert.equal(result.data.truncated, false);
+    assert.ok(result.data.contextFiles.includes("lib/auth.ts"));
+    assert.equal(typeof result.data.model, "string");
+    assert.equal(result.data.snapshot.stale, false);
+
+    assert.equal(requests.length, 1);
+    assert.deepEqual(printed, [], "no terminal output may leak into the protocol stream");
+    assert.deepEqual(await readdir(join(projectRoot, ".devmap")), snapshotFiles, "explain must not write .devmap/explain");
+  } finally {
+    console.log = originalLog;
+    await close();
+    await rm(projectRoot, { recursive: true, force: true });
+  }
+});
+
+test("devmap_explain truncates very long answers and says so", async () => {
+  const projectRoot = await copyFixture();
+  const { dependencies } = fakeAi("x".repeat(MAX_TEXT_CHARS + 500));
+  const { client, close } = await connect(projectRoot, { allowAi: true, explainDependencies: dependencies });
+
+  try {
+    const result = await callTool(client, "devmap_explain", { target: "lib/auth.ts" });
+    assert.equal(result.data.truncated, true);
+    assert.match(result.data.answer, /truncated 500 characters/);
+  } finally {
+    await close();
+    await rm(projectRoot, { recursive: true, force: true });
+  }
+});
+
+test("devmap_explain returns an actionable error when no AI provider is configured", async () => {
+  const projectRoot = await copyFixture();
+  const { client, close } = await connect(projectRoot, {
+    allowAi: true,
+    explainDependencies: { loadConfig: async () => null }
+  });
+
+  try {
+    const result = await callTool(client, "devmap_explain", { target: "lib/auth.ts" });
+    assert.equal(result.isError, true);
+    assert.match(result.data.error, /requires an AI provider/);
+    assert.match(result.data.hint, /devmap init/);
+  } finally {
+    await close();
+    await rm(projectRoot, { recursive: true, force: true });
+  }
+});
+
+test("devmap_explain reports unknown targets with the known features, without calling the AI", async () => {
+  const projectRoot = await copyFixture();
+  const { dependencies, requests } = fakeAi();
+  const { client, close } = await connect(projectRoot, { allowAi: true, explainDependencies: dependencies });
+
+  try {
+    const result = await callTool(client, "devmap_explain", { target: "definitelyNotAThing" });
+    assert.equal(result.isError, true);
+    assert.match(result.data.error, /isn't a known file, feature, or function/);
+    assert.match(result.data.hint, /Known features/);
+    assert.equal(requests.length, 0);
+  } finally {
+    await close();
+    await rm(projectRoot, { recursive: true, force: true });
+  }
+});
+
+test("devmap_explain flags a stale snapshot instead of hiding it", async () => {
+  const projectRoot = await copyFixture();
+  const { dependencies } = fakeAi();
+  const { client, close } = await connect(projectRoot, { allowAi: true, explainDependencies: dependencies });
+
+  try {
+    await writeFile(join(projectRoot, "lib", "brand-new.ts"), "export const fresh = 1;\n");
+    const result = await callTool(client, "devmap_explain", { target: "lib/auth.ts" });
+    assert.equal(result.isError, false);
+    assert.equal(result.data.snapshot.stale, true);
+    assert.match(result.data.snapshot.note, /devmap_analyze/);
+  } finally {
+    await close();
+    await rm(projectRoot, { recursive: true, force: true });
+  }
+});
+
+test("registry metadata: mcpName is set, and server.json (when present) matches the package", async () => {
+  const pkg = JSON.parse(await readFile(join(packageRoot, "package.json"), "utf8")) as {
+    name: string;
+    version: string;
+    mcpName?: string;
+  };
+
+  // The MCP Registry verifies ownership through this field; with GitHub login
+  // it must start with io.github.<github-username>/.
+  assert.match(pkg.mcpName ?? "", /^io\.github\.itsflaid\/[a-z0-9-]+$/);
+
+  const serverJsonPath = join(packageRoot, "..", "..", "server.json");
+  const raw = await readFile(serverJsonPath, "utf8").catch(() => null);
+  if (raw === null) return; // prepared together with the release
+
+  const serverJson = JSON.parse(raw) as {
+    name: string;
+    version: string;
+    packages: Array<{
+      registryType: string;
+      identifier: string;
+      version: string;
+      transport: { type: string };
+      packageArguments?: Array<{ type: string; value?: string }>;
+    }>;
+  };
+  assert.equal(serverJson.name, pkg.mcpName);
+  assert.equal(serverJson.version, pkg.version);
+  const npmPackage = serverJson.packages.find((entry) => entry.registryType === "npm");
+  assert.ok(npmPackage, "server.json must describe the npm package");
+  assert.equal(npmPackage.identifier, pkg.name);
+  assert.equal(npmPackage.version, pkg.version);
+  assert.equal(npmPackage.transport.type, "stdio");
+  assert.ok(
+    npmPackage.packageArguments?.some((argument) => argument.type === "positional" && argument.value === "mcp"),
+    "the server is started with `devmap mcp`"
+  );
+});
diff --git a/packages/cli/test/mcp-smoke.mjs b/packages/cli/test/mcp-smoke.mjs
new file mode 100644
index 0000000..35fbab1
--- /dev/null
+++ b/packages/cli/test/mcp-smoke.mjs
@@ -0,0 +1,90 @@
+// Smoke test for the compiled CLI: starts `devmap mcp` from dist/ as a real stdio
+// server, builds the first snapshot through the protocol, and queries it.
+//
+// Run after `pnpm build:cli`:  node packages/cli/test/mcp-smoke.mjs
+// Used by CI and by the publish workflow so a broken MCP server never ships.
+import assert from "node:assert/strict";
+import { mkdir, mkdtemp, rm, writeFile } from "node:fs/promises";
+import { tmpdir } from "node:os";
+import { dirname, join, resolve } from "node:path";
+import { fileURLToPath } from "node:url";
+import { Client } from "@modelcontextprotocol/sdk/client/index.js";
+import { StdioClientTransport } from "@modelcontextprotocol/sdk/client/stdio.js";
+
+const packageRoot = resolve(dirname(fileURLToPath(import.meta.url)), "..");
+const entry = join(packageRoot, "dist", "index.js");
+const TIMEOUT_MS = 90_000;
+
+const projectRoot = await mkdtemp(join(tmpdir(), "devmap-mcp-smoke-"));
+const watchdog = setTimeout(() => {
+  console.error(`MCP smoke test timed out after ${TIMEOUT_MS / 1000}s`);
+  process.exit(1);
+}, TIMEOUT_MS);
+watchdog.unref();
+
+let client;
+try {
+  await mkdir(join(projectRoot, "src"), { recursive: true });
+  await writeFile(
+    join(projectRoot, "package.json"),
+    JSON.stringify({ name: "mcp-smoke-app", version: "1.0.0" })
+  );
+  await writeFile(join(projectRoot, "src", "index.ts"), 'import { greet } from "./greet";\nconsole.log(greet());\n');
+  await writeFile(join(projectRoot, "src", "greet.ts"), 'export const greet = () => "hello";\n');
+
+  const transport = new StdioClientTransport({
+    command: process.execPath,
+    args: [entry, "mcp", "--root", projectRoot],
+    stderr: "pipe"
+  });
+  const protocolErrors = [];
+  transport.onerror = (error) => protocolErrors.push(error);
+  client = new Client({ name: "mcp-smoke", version: "0.0.0" });
+  await client.connect(transport);
+
+  const { tools } = await client.listTools();
+  const names = tools.map((tool) => tool.name);
+  for (const expected of [
+    "devmap_overview",
+    "devmap_map",
+    "devmap_flow",
+    "devmap_routes",
+    "devmap_impact",
+    "devmap_find",
+    "devmap_analyze"
+  ]) {
+    assert.ok(names.includes(expected), `missing tool ${expected}`);
+  }
+  assert.ok(!names.includes("devmap_explain"), "devmap_explain must require --allow-ai");
+
+  const call = async (name, args = {}) => {
+    const result = await client.callTool({ name, arguments: args });
+    return { isError: result.isError === true, data: JSON.parse(result.content[0].text) };
+  };
+
+  // No snapshot yet: a structured error that tells the agent what to do.
+  const missing = await call("devmap_overview");
+  assert.equal(missing.isError, true);
+  assert.match(missing.data.hint, /devmap analyze/);
+
+  const analyzed = await call("devmap_analyze");
+  assert.equal(analyzed.isError, false);
+  assert.equal(analyzed.data.status, "refreshed");
+  assert.equal(analyzed.data.mode, "static");
+
+  const overview = await call("devmap_overview");
+  assert.equal(overview.isError, false);
+  assert.ok(overview.data.stats.totalFiles >= 2);
+  assert.equal(overview.data.snapshot.stale, false);
+
+  const impact = await call("devmap_impact", { file: "src/greet.ts", depth: 2 });
+  assert.equal(impact.isError, false);
+  assert.ok(impact.data.transitive.files.some((entryFile) => entryFile.file === "src/index.ts"));
+
+  assert.deepEqual(protocolErrors, [], "stdout must carry JSON-RPC only");
+  console.log(`MCP smoke test passed (${names.length} tools).`);
+} finally {
+  clearTimeout(watchdog);
+  await client?.close().catch(() => undefined);
+  await rm(projectRoot, { recursive: true, force: true });
+}