mcp-loader v2: streamable-HTTP transport for remote MCP servers (context7)
- New RemoteMcpClient implementing MCP streamable-HTTP per spec 2025-03-26:
POST JSON-RPC, parse application/json or text/event-stream responses,
round-trip optional Mcp-Session-Id, optional auth via 'headers' config.
- Refactor StdioMcpClient to share an IMcpClient interface with the remote
client; extension entry dispatches on cfg.type. Drops the v1 'remote
skipped with warning' code path.
- Bump MCP_PROTOCOL_VERSION to 2025-11-25 (single constant, both clients).
- 404 self-heal: when a remote returns 404 to a request carrying our
Mcp-Session-Id, drop the id, re-initialize, retry the request once
(per spec 2025-11-25 \u00a72.2). allowReinitOn404=false on the retry path
prevents recursion. Verified via mock-server smoke test.
- Sanitize pi-facing tool names to ^[A-Za-z][A-Za-z0-9_]{0,63}$. Anthropic
allows hyphens but Bedrock's Anthropic shim rejects them, causing entire
turns to 4xx silently when context7's hyphenated tools (resolve-library-id,
query-docs) were registered. Original MCP-side names are preserved in the
tool-execute closure, so sanitization is purely pi-facing.
- /mcp slash command: drop 'remote (skipped)' status label.
- Docs: README and AGENTS updated for transports, headers config, 404
self-heal, tool-name sanitization rationale, OAuth limitation.
End-to-end verified: context7 connects through pi, returns useful docs
(Bun streaming/SSE example fetched successfully).
This commit is contained in:
+287
-48
@@ -22,7 +22,8 @@
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* },
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* "context7": {
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* "type": "remote",
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* "url": "https://mcp.context7.com/mcp"
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* "url": "https://mcp.context7.com/mcp",
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* "headers": { "Authorization": "Bearer …" }
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* }
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* }
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* }
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@@ -32,11 +33,13 @@
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* command argv array — first element is the executable, rest are args.
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* For local servers only.
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* url remote MCP endpoint URL. For remote servers only.
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* headers optional object of HTTP headers to send with every request to
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* a remote server (e.g. Authorization, X-API-Key). Remote only.
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* enabled default true. Set false to disable a server without
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* removing it from settings.json.
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* env optional object of environment variables to inject into
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* the spawned subprocess. Inherits parent env first, then
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* overlays these keys.
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* overlays these keys. Local only.
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*
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* Tool naming
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*
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@@ -47,22 +50,35 @@
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*
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* Lifecycle
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*
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* • Servers are spawned at extension load time (pi startup or /reload).
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* • Servers are connected at extension load time (pi startup or /reload).
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* • Each server's `tools/list` is awaited before pi finishes registering.
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* • Subprocess stderr is silenced unless PI_MCP_LOADER_DEBUG=1.
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* • Subprocesses receive SIGTERM at session_shutdown.
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* • A server that fails to start (binary missing, init handshake error)
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* logs a single line to stderr and is skipped. Other servers continue.
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* Pi keeps working without that server's tools.
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* • Local subprocess stderr is silenced unless PI_MCP_LOADER_DEBUG=1.
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* • Local subprocesses receive SIGTERM at session_shutdown; remote
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* servers receive an MCP DELETE if a session id was issued.
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* • A server that fails to start (binary missing, init handshake error,
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* remote 4xx/5xx) logs a single line to stderr and is skipped. Other
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* servers continue. Pi keeps working without that server's tools.
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*
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* Limitations (v1)
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* Transports
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*
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* • Only local/stdio transport is implemented. Remote (streamable-http,
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* SSE) servers are detected and skipped with a warning. v2 will add
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* remote support — context7 is the prime motivator.
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* • No reconnect: if a subprocess dies mid-session, its tools become
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* unavailable until pi is reloaded. The mempalace.ts extension has
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* the same limitation today.
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* local — stdio JSON-RPC subprocess (mcp-searxng, gitea-mcp, …).
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* remote — streamable-HTTP per MCP spec 2025-03-26: POST JSON-RPC, server
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* replies either application/json or text/event-stream. Optional
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* Mcp-Session-Id header round-tripped if the server issues one.
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* No GET-stream subscription (we don't consume server-initiated
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* notifications).
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*
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* Limitations
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*
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* • No stdio reconnect: if a stdio subprocess dies mid-session, its tools
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* become unavailable until pi is reloaded. The mempalace.ts extension
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* has the same limitation today.
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* • Remote sessions self-heal on 404: if a streamable-HTTP server
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* forgets our session id (e.g. server restart), the next request gets
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* HTTP 404; the client transparently re-initializes and retries the
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* request once. Persistent 404s after refresh surface as errors.
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* • No OAuth flow: remote servers requiring OAuth must be accessed with
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* a pre-issued bearer token via `headers`.
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*
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* Coexistence with mempalace.ts
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*
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@@ -76,7 +92,7 @@
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* Slash command
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*
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* `/mcp` lists configured MCP servers with their runtime status
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* (running / failed / disabled / remote-skipped / invalid) and lets
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* (running / failed / disabled / invalid) and lets
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* you toggle the `enabled` flag in settings.json. Same UX as `/ext`:
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* space stages, enter applies + reloads, esc cancels.
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*/
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@@ -116,6 +132,7 @@ type LocalServerConfig = {
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type RemoteServerConfig = {
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type: "remote";
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url: string;
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headers?: Record<string, string>;
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enabled?: boolean;
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};
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@@ -124,6 +141,13 @@ type ServerConfig = LocalServerConfig | RemoteServerConfig;
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const SETTINGS_PATH = path.join(os.homedir(), ".pi", "agent", "settings.json");
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const DEBUG = process.env.PI_MCP_LOADER_DEBUG === "1";
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// MCP protocol version we advertise. Servers we've tested:
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// - searxng (mcp-searxng) accepts both 2024-11-05 and 2025-11-25
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// - context7 accepts both
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// - mempalace bridge: not touched by this loader
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// Bump as the spec evolves; both stdio and remote clients use this.
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const MCP_PROTOCOL_VERSION = "2025-11-25";
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function dlog(msg: string) {
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if (DEBUG) process.stderr.write(`[mcp-loader] ${msg}\n`);
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}
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@@ -162,21 +186,35 @@ function readMcpServers(): Record<string, ServerConfig> {
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// ── Stdio MCP client ─────────────────────────────────────────────────────────
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class StdioMcpClient {
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interface IMcpClient {
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readonly serverName: string;
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tools: McpTool[];
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start(): Promise<void>;
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callTool(name: string, args: Record<string, unknown>): Promise<any>;
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stop(): void | Promise<void>;
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}
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class StdioMcpClient implements IMcpClient {
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private proc: ChildProcessWithoutNullStreams | null = null;
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private nextId = 1;
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private pending = new Map<number, { resolve: (v: any) => void; reject: (e: Error) => void }>();
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private stdoutBuf = "";
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private serverName: string;
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public readonly serverName: string;
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private command: string;
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private args: string[];
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private env: Record<string, string> | undefined;
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public tools: McpTool[] = [];
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constructor(serverName: string) {
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constructor(serverName: string, command: string, args: string[], env?: Record<string, string>) {
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this.serverName = serverName;
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this.command = command;
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this.args = args;
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this.env = env;
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}
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async start(command: string, args: string[], env?: Record<string, string>): Promise<void> {
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const childEnv = { ...process.env, ...(env ?? {}) };
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this.proc = spawn(command, args, { stdio: ["pipe", "pipe", "pipe"], env: childEnv });
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async start(): Promise<void> {
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const childEnv = { ...process.env, ...(this.env ?? {}) };
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this.proc = spawn(this.command, this.args, { stdio: ["pipe", "pipe", "pipe"], env: childEnv });
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this.proc.on("error", (err) => this.failAll(err));
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this.proc.on("exit", (code, signal) => {
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@@ -197,9 +235,9 @@ class StdioMcpClient {
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// MCP initialize handshake
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await this.request("initialize", {
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protocolVersion: "2024-11-05",
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protocolVersion: MCP_PROTOCOL_VERSION,
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capabilities: {},
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clientInfo: { name: "pi-mcp-loader", version: "0.1.0" },
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clientInfo: { name: "pi-mcp-loader", version: "0.2.0" },
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});
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this.notify("notifications/initialized", {});
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@@ -266,6 +304,195 @@ class StdioMcpClient {
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}
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}
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// ── Streamable-HTTP MCP client (remote) ──────────────────────────────────────
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//
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// Per MCP spec 2025-03-26: a single endpoint URL accepts POST with a
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// JSON-RPC body. The server responds either with application/json (a
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// single response) or text/event-stream (an SSE stream containing the
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// response, possibly preceded by notifications, terminated by stream
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// close). We do NOT open a separate GET subscription stream — we don't
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// consume server-initiated notifications today.
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//
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// Session id: if the server returns an Mcp-Session-Id header on
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// initialize, we round-trip it on every subsequent request and DELETE
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// it on stop().
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class RemoteMcpClient implements IMcpClient {
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public readonly serverName: string;
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private url: string;
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private extraHeaders: Record<string, string>;
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private sessionId: string | null = null;
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private nextId = 1;
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public tools: McpTool[] = [];
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constructor(serverName: string, url: string, headers?: Record<string, string>) {
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this.serverName = serverName;
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this.url = url;
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this.extraHeaders = headers ?? {};
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}
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async start(): Promise<void> {
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await this.request("initialize", {
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protocolVersion: MCP_PROTOCOL_VERSION,
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capabilities: {},
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clientInfo: { name: "pi-mcp-loader", version: "0.2.0" },
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});
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await this.notify("notifications/initialized", {});
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const listed = await this.request("tools/list", {});
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this.tools = (listed?.tools as McpTool[]) ?? [];
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}
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async callTool(name: string, args: Record<string, unknown>): Promise<any> {
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return this.request("tools/call", { name, arguments: args });
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}
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async stop(): Promise<void> {
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if (!this.sessionId) return;
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try {
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await fetch(this.url, { method: "DELETE", headers: this.buildHeaders() });
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} catch {}
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}
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private buildHeaders(): Record<string, string> {
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const h: Record<string, string> = {
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"Content-Type": "application/json",
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Accept: "application/json, text/event-stream",
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"MCP-Protocol-Version": MCP_PROTOCOL_VERSION,
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...this.extraHeaders,
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};
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if (this.sessionId) h["Mcp-Session-Id"] = this.sessionId;
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return h;
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}
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/**
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* Re-establish a session after the server returned 404 to a request that
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* carried our `Mcp-Session-Id`. Per spec 2025-11-25, the client MUST start
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* a new session by sending a fresh InitializeRequest without a session id.
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*
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* Caller must clear `this.sessionId` BEFORE invoking this so the new
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* initialize POST goes out without the stale id.
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*/
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private async reinitialize(): Promise<void> {
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dlog(`${this.serverName}: session lost (404) — reinitializing`);
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await this.request("initialize", {
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protocolVersion: MCP_PROTOCOL_VERSION,
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capabilities: {},
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clientInfo: { name: "pi-mcp-loader", version: "0.2.0" },
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}, /*allowReinitOn404=*/ false);
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await this.notify("notifications/initialized", {});
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}
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private async request(method: string, params: unknown, allowReinitOn404 = true): Promise<any> {
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const id = this.nextId++;
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const body = JSON.stringify({ jsonrpc: "2.0", id, method, params });
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const sessionAtStart = this.sessionId;
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let res: Response;
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try {
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res = await fetch(this.url, { method: "POST", headers: this.buildHeaders(), body });
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} catch (err) {
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throw new Error(`${this.serverName}: fetch failed on ${method}: ${(err as Error).message}`);
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}
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const sid = res.headers.get("Mcp-Session-Id") ?? res.headers.get("mcp-session-id");
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if (sid) this.sessionId = sid;
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// 404 + we sent a session id → server forgot us. Drop our id, re-init,
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// retry once. `allowReinitOn404=false` is passed by the recovery path to
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// prevent infinite recursion if the server is permanently broken.
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if (res.status === 404 && sessionAtStart && allowReinitOn404) {
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try { await res.arrayBuffer(); } catch {}
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this.sessionId = null;
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await this.reinitialize();
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return this.request(method, params, /*allowReinitOn404=*/ false);
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}
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if (!res.ok) {
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let detail = "";
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try { detail = (await res.text()).slice(0, 200); } catch {}
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throw new Error(`${this.serverName}: HTTP ${res.status} on ${method}${detail ? `: ${detail}` : ""}`);
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}
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const ct = (res.headers.get("content-type") ?? "").toLowerCase();
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if (ct.includes("text/event-stream")) {
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return await this.readSseForResponse(res, id);
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}
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if (ct.includes("application/json")) {
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const json: any = await res.json();
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if (json.error) throw new Error(json.error.message ?? "MCP error");
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return json.result;
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}
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if (res.status === 202) return undefined;
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throw new Error(`${this.serverName}: unexpected content-type "${ct}" on ${method}`);
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}
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private async notify(method: string, params: unknown): Promise<void> {
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const body = JSON.stringify({ jsonrpc: "2.0", method, params });
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let res: Response;
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try {
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res = await fetch(this.url, { method: "POST", headers: this.buildHeaders(), body });
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} catch (err) {
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throw new Error(`${this.serverName}: fetch failed on notify ${method}: ${(err as Error).message}`);
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}
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try { await res.arrayBuffer(); } catch {}
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if (!res.ok && res.status !== 202) {
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throw new Error(`${this.serverName}: HTTP ${res.status} on notify ${method}`);
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}
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}
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private async readSseForResponse(res: Response, expectedId: number): Promise<any> {
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if (!res.body) throw new Error(`${this.serverName}: SSE response had no body`);
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const reader = res.body.getReader();
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const decoder = new TextDecoder();
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let buf = "";
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let dataLines: string[] = [];
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const tryDispatch = (): { matched: boolean; result?: any } => {
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if (dataLines.length === 0) return { matched: false };
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const data = dataLines.join("\n");
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dataLines = [];
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let msg: any;
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try {
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msg = JSON.parse(data);
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} catch {
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return { matched: false };
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}
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if (typeof msg.id === "number" && msg.id === expectedId) {
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if (msg.error) throw new Error(msg.error.message ?? "MCP error");
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return { matched: true, result: msg.result };
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}
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return { matched: false };
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};
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try {
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while (true) {
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const { done, value } = await reader.read();
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if (done) break;
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buf += decoder.decode(value, { stream: true });
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let nl: number;
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while ((nl = buf.indexOf("\n")) !== -1) {
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const rawLine = buf.slice(0, nl);
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buf = buf.slice(nl + 1);
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const line = rawLine.endsWith("\r") ? rawLine.slice(0, -1) : rawLine;
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if (line === "") {
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const out = tryDispatch();
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if (out.matched) return out.result;
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} else if (line.startsWith(":")) {
|
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// SSE comment — ignore (often a keepalive)
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} else if (line.startsWith("data:")) {
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dataLines.push(line.slice(5).replace(/^ /, ""));
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}
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// event:, id:, retry: — ignored; only `data` matters for our use
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}
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}
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const out = tryDispatch();
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if (out.matched) return out.result;
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} finally {
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try { reader.cancel(); } catch {}
|
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}
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throw new Error(`${this.serverName}: SSE stream closed without response for id=${expectedId}`);
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}
|
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}
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// ── MCP result → pi tool result helpers ──────────────────────────────────────
|
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|
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function extractText(mcpResult: any): string {
|
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@@ -284,8 +511,20 @@ function namespacedToolName(serverName: string, toolName: string): string {
|
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// Avoid double-prefix: if the MCP server already names its tools
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// `<serverName>_<...>`, leave them alone.
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const prefix = `${serverName}_`;
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if (toolName.startsWith(prefix)) return toolName;
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return `${prefix}${toolName}`;
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const raw = toolName.startsWith(prefix) ? toolName : `${prefix}${toolName}`;
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// Sanitize for the strictest tool-name regex we have to satisfy.
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// Anthropic's Messages API allows `^[a-zA-Z0-9_-]{1,64}$` (hyphens OK),
|
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// but AWS Bedrock's Anthropic shim rejects names containing `-` outright
|
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// — it accepts `^[a-zA-Z][a-zA-Z0-9_]{0,63}$`. Hyphenated MCP tool names
|
||||
// (context7's `resolve-library-id`, `query-docs`) caused entire turns to
|
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// 4xx silently on Bedrock, manifesting as no output at all once the
|
||||
// server was enabled. Replace any non-[A-Za-z0-9_] with `_`, then prefix
|
||||
// a leading underscore with `t` if the result starts with a non-letter.
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||||
// Truncate to 64 to also satisfy Anthropic's length cap.
|
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let sanitized = raw.replace(/[^A-Za-z0-9_]/g, "_");
|
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if (!/^[A-Za-z]/.test(sanitized)) sanitized = `t_${sanitized}`;
|
||||
if (sanitized.length > 64) sanitized = sanitized.slice(0, 64);
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return sanitized;
|
||||
}
|
||||
|
||||
// ── Settings.json writer ─────────────────────────────────────────────────────
|
||||
@@ -316,7 +555,6 @@ type ServerRuntimeState =
|
||||
| { kind: "running"; toolCount: number }
|
||||
| { kind: "failed"; message: string }
|
||||
| { kind: "disabled" }
|
||||
| { kind: "remote-skipped" }
|
||||
| { kind: "invalid"; message: string };
|
||||
|
||||
// ── Extension entry ──────────────────────────────────────────────────────────
|
||||
@@ -324,7 +562,7 @@ type ServerRuntimeState =
|
||||
export default async function mcpLoaderExtension(pi: ExtensionAPI) {
|
||||
const servers = readMcpServers();
|
||||
const runtime = new Map<string, ServerRuntimeState>();
|
||||
const clients: StdioMcpClient[] = [];
|
||||
const clients: IMcpClient[] = [];
|
||||
|
||||
// Always register /mcp — even when no servers are configured, so users
|
||||
// discover the surface. Handler reads settings.json fresh on each invoke.
|
||||
@@ -342,30 +580,33 @@ export default async function mcpLoaderExtension(pi: ExtensionAPI) {
|
||||
continue;
|
||||
}
|
||||
|
||||
let client: IMcpClient;
|
||||
if (cfg.type === "remote") {
|
||||
warn(
|
||||
`${name}: remote MCP transport not yet supported by mcp-loader (v1 is stdio-only); skipping`,
|
||||
);
|
||||
runtime.set(name, { kind: "remote-skipped" });
|
||||
continue;
|
||||
const remote = cfg as RemoteServerConfig;
|
||||
if (typeof remote.url !== "string" || remote.url.length === 0) {
|
||||
warn(`${name}: remote server missing \`url\`, skipping`);
|
||||
runtime.set(name, { kind: "invalid", message: "remote server missing url" });
|
||||
continue;
|
||||
}
|
||||
client = new RemoteMcpClient(name, remote.url, remote.headers);
|
||||
} else {
|
||||
const local = cfg as LocalServerConfig;
|
||||
if (!Array.isArray(local.command) || local.command.length === 0) {
|
||||
warn(`${name}: invalid \`command\` (must be non-empty array), skipping`);
|
||||
runtime.set(name, { kind: "invalid", message: "command must be non-empty array" });
|
||||
continue;
|
||||
}
|
||||
const [bin, ...args] = local.command;
|
||||
client = new StdioMcpClient(name, bin, args, local.env);
|
||||
}
|
||||
|
||||
const local = cfg as LocalServerConfig;
|
||||
if (!Array.isArray(local.command) || local.command.length === 0) {
|
||||
warn(`${name}: invalid \`command\` (must be non-empty array), skipping`);
|
||||
runtime.set(name, { kind: "invalid", message: "command must be non-empty array" });
|
||||
continue;
|
||||
}
|
||||
const [bin, ...args] = local.command;
|
||||
|
||||
const client = new StdioMcpClient(name);
|
||||
try {
|
||||
await client.start(bin, args, local.env);
|
||||
await client.start();
|
||||
} catch (err) {
|
||||
const message = (err as Error).message;
|
||||
warn(`${name}: failed to start — ${message}`);
|
||||
runtime.set(name, { kind: "failed", message });
|
||||
client.stop();
|
||||
try { await client.stop(); } catch {}
|
||||
continue;
|
||||
}
|
||||
|
||||
@@ -413,9 +654,9 @@ export default async function mcpLoaderExtension(pi: ExtensionAPI) {
|
||||
}
|
||||
}
|
||||
|
||||
// Tear down subprocesses on session shutdown so reloads don't leak procs.
|
||||
// Tear down clients (stdio subprocesses, remote sessions) on session shutdown.
|
||||
pi.on("session_shutdown", async () => {
|
||||
for (const c of clients) c.stop();
|
||||
await Promise.allSettled(clients.map((c) => Promise.resolve(c.stop())));
|
||||
});
|
||||
}
|
||||
|
||||
@@ -434,8 +675,6 @@ function statusLabel(state: ServerRuntimeState | undefined, configEnabled: boole
|
||||
return `failed: ${state.message}`;
|
||||
case "disabled":
|
||||
return "disabled in settings";
|
||||
case "remote-skipped":
|
||||
return "remote (skipped — v1 stdio only)";
|
||||
case "invalid":
|
||||
return `invalid: ${state.message}`;
|
||||
}
|
||||
|
||||
Reference in New Issue
Block a user