oratis · oratis · May 28, 2026 · May 28, 2026
diff --git a/apps/cli/src/commands.ts b/apps/cli/src/commands.ts
@@ -1,7 +1,12 @@
 // CLI slash commands — pure functions over a session context.
 // Spec: docs/DEVELOPMENT_PLAN.md §3.6 (30+ commands; M2 ships a core subset)
 
-import type { DeepCodeSettings, SessionManager, SessionMeta } from '@deepcode/core';
+import type {
+  DeepCodeSettings,
+  McpClientHandle,
+  SessionManager,
+  SessionMeta,
+} from '@deepcode/core';
 import { redact, type Credentials } from '@deepcode/core';
 
 export interface SessionContext {
@@ -18,6 +23,10 @@ export interface SessionContext {
   exitRequested?: boolean;
   /** Replace history entirely (used by /clear, /resume). */
   clearHistory?: boolean;
+  /** Connected MCP server handles (M3c). */
+  mcpServers?: McpClientHandle[];
+  /** MCP servers that failed to connect on startup (M3c). */
+  mcpErrors?: Array<{ serverName: string; error: string }>;
 }
 
 export interface SlashCommand {
@@ -215,6 +224,37 @@ export const InitCommand: SlashCommand = {
   },
 };
 
+export const McpCommand: SlashCommand = {
+  name: '/mcp',
+  description: 'List connected MCP servers and their tools.',
+  async run(_args, ctx) {
+    const servers = ctx.mcpServers ?? [];
+    if (servers.length === 0) {
+      return [
+        'No MCP servers connected.',
+        '',
+        'Add servers in settings.json under "mcpServers". Example:',
+        '  { "mcpServers": { "filesystem": { "command": "npx",',
+        '      "args": ["@modelcontextprotocol/server-filesystem", "/tmp"] } } }',
+      ];
+    }
+    const lines = [`Connected MCP servers (${servers.length}):`];
+    for (const s of servers) {
+      lines.push(`  ● ${s.serverName}  ·  ${s.tools.length} tools`);
+      for (const t of s.tools.slice(0, 6)) {
+        lines.push(`     - ${t.name}`);
+      }
+      if (s.tools.length > 6) lines.push(`     … and ${s.tools.length - 6} more`);
+    }
+    if ((ctx.mcpErrors ?? []).length > 0) {
+      lines.push('');
+      lines.push('Servers that failed to connect:');
+      for (const e of ctx.mcpErrors!) lines.push(`  ✕ ${e.serverName}  ${e.error}`);
+    }
+    return lines;
+  },
+};
+
 export const TodosCommand: SlashCommand = {
   name: '/todos',
   description: 'Show active TODO list (M3 wires TodoWrite tool).',
@@ -237,6 +277,7 @@ export const BUILTIN_COMMANDS: SlashCommand[] = [
   AddDirCommand,
   ResumeCommand,
   InitCommand,
+  McpCommand,
   TodosCommand,
 ];
 

diff --git a/apps/cli/src/repl.ts b/apps/cli/src/repl.ts
@@ -10,6 +10,8 @@ import {
   ToolRegistry,
   applyStyle,
   buildSkillsDescriptionBlock,
+  closeAllMcpServers,
+  connectAllMcpServers,
   findStyle,
   loadMemory,
   loadOutputStyles,
@@ -20,6 +22,7 @@ import {
   runAgent,
   type DeepCodeSettings,
   type Effort,
+  type McpClientHandle,
   type Mode,
   type AgentEvent,
   type StoredMessage,
@@ -98,6 +101,32 @@ export async function startRepl(opts: ReplOpts): Promise<number> {
     tools.register(makeSkillTool(skills));
   }
 
+  // M3c: connect MCP servers (best-effort; individual failures don't abort)
+  let mcpServers: McpClientHandle[] = [];
+  let mcpErrors: Array<{ serverName: string; error: string }> = [];
+  if (settings.mcpServers && Object.keys(settings.mcpServers).length > 0) {
+    const enabled = settings.enabledMcpjsonServers;
+    const disabled = settings.disabledMcpjsonServers ?? [];
+    const result = await connectAllMcpServers(settings.mcpServers, {
+      enabledOnly: enabled,
+      disabled,
+    });
+    mcpServers = result.handles;
+    mcpErrors = result.errors;
+    // Register every MCP-tool handler into the live registry
+    for (const handle of mcpServers) {
+      for (const tool of handle.tools) tools.register(tool);
+    }
+    if (mcpServers.length > 0) {
+      output.write(
+        `  ⊞ MCP: ${mcpServers.length} server(s) connected (${mcpServers.reduce((n, h) => n + h.tools.length, 0)} tools)\n`,
+      );
+    }
+    if (mcpErrors.length > 0) {
+      output.write(`  ⊞ MCP: ${mcpErrors.length} server(s) failed (see /mcp)\n`);
+    }
+  }
+
   // Build the composite system prompt
   let systemPrompt = DEFAULT_SYSTEM_PROMPT;
   if (memory.text) systemPrompt += '\n\n' + memory.text;
@@ -122,6 +151,8 @@ export async function startRepl(opts: ReplOpts): Promise<number> {
     sessionId: session.id,
     sessions,
     usage: { inputTokens: 0, outputTokens: 0, reasoningTokens: 0 },
+    mcpServers,
+    mcpErrors,
   };
 
   output.write(`\n  ▎ DeepCode  ·  ${ctx.model}  ·  mode: ${ctx.mode}  ·  effort: ${ctx.effort}\n`);
@@ -202,6 +233,10 @@ export async function startRepl(opts: ReplOpts): Promise<number> {
   }
 
   rl.close();
+  // Clean up MCP server connections
+  if (mcpServers.length > 0) {
+    await closeAllMcpServers(mcpServers);
+  }
   return 0;
 }
 

diff --git a/docs/m1-validation.md b/docs/m1-validation.md
@@ -14,11 +14,11 @@
 
 ## End-to-end runs
 
-| Scenario | Result |
-|---|---|
-| Agent reads a file via Read tool | ✓ 2 turns, 2523 in / 137 out tokens, ended `end_turn`, correct answer |
+| Scenario                            | Result                                                                                |
+| ----------------------------------- | ------------------------------------------------------------------------------------- |
+| Agent reads a file via Read tool    | ✓ 2 turns, 2523 in / 137 out tokens, ended `end_turn`, correct answer                 |
 | Reasoner solves a math word problem | ✓ 1 turn, 1188 in / 500 out / 427 reasoning, both `thinking` + `text` blocks streamed |
-| `/v1/models` + alias mapping | ✓ documented in §3.1 update |
+| `/v1/models` + alias mapping        | ✓ documented in §3.1 update                                                           |
 
 ## Changes in this PR
 

diff --git a/docs/milestones/M3c-mcp.md b/docs/milestones/M3c-mcp.md
@@ -0,0 +1,44 @@
+# M3c · MCP Client (stdio)
+
+> **Status**: ✅ stdio transport shipped + wired into REPL  
+> **Branch**: `feat/m3c-mcp-client`
+
+## Shipped
+
+- `packages/core/src/mcp/client.ts` — `connectMcpServer(name, config)` + `connectAllMcpServers(servers)` + `closeAllMcpServers(handles)`.
+- Uses official `@modelcontextprotocol/sdk@^1.29.0` for protocol details.
+- Tools are exposed with **`mcp__<server>__<tool>` qualified name**, registered into the same `ToolRegistry` that powers the 6 P0 tools. The agent loop sees them identically.
+- Tool results unwrap MCP's `content[].text` array into a single string for the `tool_result` block.
+- Individual server failures are isolated — one bad config doesn't kill others.
+- `/mcp` slash command lists connected servers + tool count + errors.
+- CLI REPL now auto-connects MCP servers from `settings.mcpServers` on startup, honoring `enabledMcpjsonServers` / `disabledMcpjsonServers`.
+- Graceful shutdown closes all connections.
+
+## Tests (6 new)
+
+`packages/core/src/mcp/client.test.ts` spawns tiny in-disk MCP server scripts that import the SDK via absolute path (so /tmp doesn't need node_modules):
+
+1. lists tools and qualifies names
+2. calls a remote tool, returns its text output
+3. `connectAllMcpServers` continues on individual failures
+4. respects `disabled` list
+5. respects `enabledOnly` list  
+6. rejects server config missing `command`
+
+## NOT in this PR (M3c-ext, next)
+
+- HTTP transport
+- SSE transport
+- OAuth (2.0 client credentials + dynamic flows)
+- `headersHelper` (dynamic auth via shell)
+- Elicitation hooks (server-requested user input)
+- `deepcode mcp serve` (reverse-expose DeepCode as MCP server)
+- `_meta["anthropic/maxResultSizeChars"]` per-tool output caps
+- MCP resource references in composer (`@server:proto://path`)
+- `mcp__server__prompt` slash commands
+
+## Verified
+
+- `pnpm typecheck` → green
+- `pnpm test` → 264 passed / 0 failed / 7 skipped (was 258)
+- CLI builds: `node apps/cli/dist/cli.js --help` still works
diff --git a/packages/core/package.json b/packages/core/package.json
@@ -26,6 +26,7 @@
     "clean": "rm -rf dist *.tsbuildinfo"
   },
   "dependencies": {
+    "@modelcontextprotocol/sdk": "^1.29.0",
     "openai": "^6.0.0"
   },
   "devDependencies": {

diff --git a/packages/core/src/index.ts b/packages/core/src/index.ts
@@ -136,6 +136,16 @@ export {
   type Frontmatter,
 } from './skills/index.js';
 
+// MCP client (M3c — stdio transport; http/sse/OAuth/serve → M3c-ext)
+export {
+  connectMcpServer,
+  connectAllMcpServers,
+  closeAllMcpServers,
+  type McpClientHandle,
+  type McpToolMeta,
+  type ConnectAllResult,
+} from './mcp/index.js';
+
 // Plugins (M5 — manifest + hash pinning + local install + discovery)
 export {
   installLocal,