feat: tracker-as-first-layer-node inference entry point (US-014)

- Tracker: add GET /v1/tracker-nodes/<model> returning nodes registered
  with tracker_mode=true whose shard_start matches the model's first layer
- Node: StubNodeServer and TorchNodeServer accept tracker_mode/tracker_url;
  when tracker_mode=True (or auto-detected via shard_start==0 for Torch),
  /v1/chat/completions is served alongside /forward
- TorchNodeServer: full pipeline implementation — encode_prompt → route
  selection via tracker → binary forward through remaining hops → decode
- Gateway: _handle_chat_completions checks _get_tracker_nodes() first and
  proxies round-robin to tracker-nodes; falls back to existing direct
  pipeline when none found (preserves all US-005 backward compat)
- CLI: --tracker-mode and --tracker-url flags added to meshnet-node start
- Test: two stub tracker-nodes + two mid-shard nodes for gpt2; 10 requests;
  round-robin 5/5 split verified; all OpenAI-format responses validated
- All 78 tests pass

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

packages/gateway/meshnet_gateway/server.py

@@ -56,6 +56,7 @@ class _GatewayHTTPServer(http.server.HTTPServer):
         self.minimum_stake = minimum_stake
         self.cost_per_layer_token_lamport = cost_per_layer_token_lamport
         self.last_binary_chunk_responses: list[_BinaryActivation] = []
+        self.request_count: int = 0
 
 
 class _GatewayHandler(http.server.BaseHTTPRequestHandler):
@@ -243,11 +244,28 @@ class _GatewayHandler(http.server.BaseHTTPRequestHandler):
 
     def _handle_chat_completions(self):
         server: _GatewayHTTPServer = self.server  # type: ignore[assignment]
-        body = self._read_json_body()
-        if body is None:
+        # Read raw bytes first so we can proxy them if tracker-nodes are available
+        length = int(self.headers.get("Content-Length", 0))
+        raw_body = self.rfile.read(length)
+        try:
+            body = json.loads(raw_body or b"{}")
+        except (json.JSONDecodeError, ValueError):
+            self._send_json_error(400, "invalid JSON body")
+            return
+        if not isinstance(body, dict):
+            self._send_json_error(400, "JSON body must be an object")
             return
-        streaming = bool(body.get("stream", False))
 
+        model = str(body.get("model", "stub-model"))
+        tracker_nodes = _get_tracker_nodes(server, model)
+        if tracker_nodes:
+            # Proxy to a tracker-node (round-robin by request count)
+            target = tracker_nodes[server.request_count % len(tracker_nodes)]
+            server.request_count += 1
+            return self._proxy_to_tracker_node(target, raw_body)
+
+        # Fallback: use existing direct pipeline (backward compat)
+        streaming = bool(body.get("stream", False))
         try:
             completion = self._build_completion(body)
         except _ModelUnavailable as exc:
@@ -266,6 +284,37 @@ class _GatewayHandler(http.server.BaseHTTPRequestHandler):
         else:
             self._send_json(200, completion)
 
+    def _proxy_to_tracker_node(self, url: str, body_bytes: bytes) -> None:
+        """Forward a raw request body to a tracker-node and stream the response back."""
+        target_url = f"{url}/v1/chat/completions"
+        req = urllib.request.Request(
+            target_url,
+            data=body_bytes,
+            headers={"Content-Type": "application/json"},
+            method="POST",
+        )
+        try:
+            with urllib.request.urlopen(req, timeout=30.0) as r:
+                content_type = r.headers.get("Content-Type", "application/json")
+                resp_body = r.read()
+                status = r.status
+        except urllib.error.HTTPError as exc:
+            resp_body = exc.read()
+            self.send_response(exc.code)
+            self.send_header("Content-Type", "application/json")
+            self.send_header("Content-Length", str(len(resp_body)))
+            self.end_headers()
+            self.wfile.write(resp_body)
+            return
+        except Exception as exc:
+            self._send_json_error(503, f"tracker-node unreachable: {exc}")
+            return
+        self.send_response(status)
+        self.send_header("Content-Type", content_type)
+        self.send_header("Content-Length", str(len(resp_body)))
+        self.end_headers()
+        self.wfile.write(resp_body)
+
     def _handle_meshnet_request(self) -> None:
         body = self._read_json_body()
         if body is None:
@@ -767,6 +816,17 @@ def _get_json(url: str, timeout: float = 5.0) -> dict:
         return json.loads(r.read())
 
 
+def _get_tracker_nodes(server: _GatewayHTTPServer, model: str) -> list[str]:
+    """Return tracker-node endpoint URLs for this model, or empty list on any failure."""
+    if server.tracker_url is None:
+        return []
+    try:
+        resp = _get_json(f"{server.tracker_url}/v1/tracker-nodes/{urllib.parse.quote(model)}")
+        return [n["endpoint"] for n in resp.get("tracker_nodes", [])]
+    except Exception:
+        return []
+
+
 class GatewayServer:
     """HTTP gateway that routes /v1/chat/completions through an ordered inference route.
 

packages/node/meshnet_node/cli.py

@@ -39,6 +39,16 @@ def main() -> None:
         "--advertise-host",
         help="Reachable host/IP to advertise to the tracker (defaults to FQDN when binding 0.0.0.0)",
     )
+    start_cmd.add_argument(
+        "--tracker-mode",
+        action="store_true",
+        help="Enable client-facing /v1/chat/completions (auto-enabled when shard-start=0)",
+    )
+    start_cmd.add_argument(
+        "--tracker-url",
+        default=None,
+        help="Tracker URL for route selection (used in tracker mode)",
+    )
 
     args = parser.parse_args()
 

packages/node/meshnet_node/server.py

@@ -2,6 +2,7 @@
 
 import http.server
 import json
+import time
 import threading
 import urllib.parse
 from pathlib import Path
@@ -93,6 +94,8 @@ class _StubHTTPServer(http.server.HTTPServer):
         response_prefix: str,
         model: str,
         shard_path: Path | None,
+        tracker_mode: bool = False,
+        tracker_url: str | None = None,
     ):
         super().__init__(addr, handler)
         self.shard_start = shard_start
@@ -103,6 +106,9 @@ class _StubHTTPServer(http.server.HTTPServer):
         self.shard_path = shard_path
         self.received_activations: bool = False
         self.forward_chunk_count: int = 0
+        self.tracker_mode: bool = tracker_mode
+        self.tracker_url: str | None = tracker_url
+        self.chat_completion_count: int = 0
 
 
 class _StubHandler(http.server.BaseHTTPRequestHandler):
@@ -110,10 +116,13 @@ class _StubHandler(http.server.BaseHTTPRequestHandler):
         pass
 
     def do_POST(self):
+        server: _StubHTTPServer = self.server  # type: ignore[assignment]
         if self.path == "/v1/infer":
             self._handle_infer()
         elif self.path == "/forward":
             self._handle_forward()
+        elif self.path == "/v1/chat/completions" and server.tracker_mode:
+            self._handle_chat_completions()
         else:
             self.send_response(404)
             self.end_headers()
@@ -126,6 +135,82 @@ class _StubHandler(http.server.BaseHTTPRequestHandler):
             self.send_response(404)
             self.end_headers()
 
+    def _send_json(self, status: int, data: dict) -> None:
+        payload = json.dumps(data).encode()
+        self.send_response(status)
+        self.send_header("Content-Type", "application/json")
+        self.send_header("Content-Length", str(len(payload)))
+        self.end_headers()
+        self.wfile.write(payload)
+
+    def _handle_chat_completions(self) -> None:
+        server: _StubHTTPServer = self.server  # type: ignore[assignment]
+        length = int(self.headers.get("Content-Length", 0))
+        try:
+            body = json.loads(self.rfile.read(length) or b"{}")
+        except (json.JSONDecodeError, ValueError):
+            self._send_json(400, {"error": "invalid JSON body"})
+            return
+        if not isinstance(body, dict):
+            self._send_json(400, {"error": "JSON body must be an object"})
+            return
+        server.chat_completion_count += 1
+        streaming = bool(body.get("stream", False))
+        model = str(body.get("model", server.model))
+        messages = body.get("messages", [])
+        last_content = ""
+        if isinstance(messages, list) and messages:
+            last = messages[-1]
+            if isinstance(last, dict):
+                last_content = str(last.get("content", ""))
+        text = f"{server.response_prefix} {last_content}"
+        if streaming:
+            self._send_sse_response(text, model)
+        else:
+            created = int(time.time())
+            self._send_json(200, {
+                "id": "chatcmpl-stub",
+                "object": "chat.completion",
+                "created": created,
+                "model": model,
+                "choices": [{
+                    "index": 0,
+                    "message": {"role": "assistant", "content": text},
+                    "finish_reason": "stop",
+                }],
+                "usage": {"prompt_tokens": 0, "completion_tokens": 0, "total_tokens": 0},
+            })
+
+    def _send_sse_response(self, text: str, model: str) -> None:
+        chunk_id = "chatcmpl-stub"
+        created = int(time.time())
+        self.send_response(200)
+        self.send_header("Content-Type", "text/event-stream; charset=utf-8")
+        self.send_header("Cache-Control", "no-cache")
+        self.end_headers()
+
+        def _emit(data: str) -> None:
+            self.wfile.write(f"data: {data}\n\n".encode())
+            self.wfile.flush()
+
+        _emit(json.dumps({
+            "id": chunk_id, "object": "chat.completion.chunk", "created": created,
+            "model": model,
+            "choices": [{"index": 0, "delta": {"role": "assistant", "content": ""}, "finish_reason": None}],
+        }))
+        _emit(json.dumps({
+            "id": chunk_id, "object": "chat.completion.chunk", "created": created,
+            "model": model,
+            "choices": [{"index": 0, "delta": {"content": text}, "finish_reason": None}],
+        }))
+        _emit(json.dumps({
+            "id": chunk_id, "object": "chat.completion.chunk", "created": created,
+            "model": model,
+            "choices": [{"index": 0, "delta": {}, "finish_reason": "stop"}],
+        }))
+        self.wfile.write(b"data: [DONE]\n\n")
+        self.wfile.flush()
+
     def _handle_shard_download(self, parsed: urllib.parse.ParseResult):
         server: _StubHTTPServer = self.server  # type: ignore[assignment]
         params = urllib.parse.parse_qs(parsed.query)
@@ -246,6 +331,7 @@ class StubNodeServer:
     shard_start / shard_end define which transformer layer range this node owns.
     is_last_shard controls whether the node returns a text response (True) or
     activation tensors (False) after processing its shard.
+    tracker_mode enables the /v1/chat/completions endpoint for head-shard nodes.
     """
 
     def __init__(
@@ -258,6 +344,8 @@ class StubNodeServer:
         response_prefix: str = "stub response to:",
         model: str = "stub-model",
         shard_path: Path | None = None,
+        tracker_mode: bool = False,
+        tracker_url: str | None = None,
     ):
         self._host = host
         self._requested_port = port
@@ -269,6 +357,8 @@ class StubNodeServer:
         self._response_prefix = response_prefix
         self._model = model
         self._shard_path = shard_path
+        self._tracker_mode = tracker_mode
+        self._tracker_url = tracker_url
         self._server: _StubHTTPServer | None = None
         self._thread: threading.Thread | None = None
         self.port: int | None = None
@@ -283,6 +373,11 @@ class StubNodeServer:
         """Number of binary /forward chunks handled since this node was started."""
         return self._server.forward_chunk_count if self._server is not None else 0
 
+    @property
+    def chat_completion_count(self) -> int:
+        """Number of /v1/chat/completions requests handled since this node was started."""
+        return self._server.chat_completion_count if self._server is not None else 0
+
     def start(self) -> int:
         if self._server is not None:
             raise RuntimeError("StubNodeServer is already running")
@@ -296,6 +391,8 @@ class StubNodeServer:
             self._response_prefix,
             self._model,
             self._shard_path,
+            self._tracker_mode,
+            self._tracker_url,
         )
         self.port = self._server.server_address[1]
         self._thread = threading.Thread(target=self._server.serve_forever, daemon=True)

packages/node/meshnet_node/torch_server.py

@@ -6,6 +6,11 @@ import http.server
 import json
 import sys
 import threading
+import time
+import urllib.error
+import urllib.parse
+import urllib.request
+import uuid
 
 from .model_backend import (
     InsufficientVRAMError,
@@ -24,11 +29,20 @@ from .server import (
 
 
 class _TorchHTTPServer(http.server.HTTPServer):
-    def __init__(self, addr, handler, backend: TorchModelShard):
+    def __init__(
+        self,
+        addr,
+        handler,
+        backend: TorchModelShard,
+        tracker_mode: bool = False,
+        tracker_url: str | None = None,
+    ):
         super().__init__(addr, handler)
         self.backend = backend
         self.received_activations = False
         self.forward_chunk_count = 0
+        self.tracker_mode = tracker_mode
+        self.tracker_url = tracker_url
 
 
 class _TorchHandler(http.server.BaseHTTPRequestHandler):
@@ -36,10 +50,13 @@ class _TorchHandler(http.server.BaseHTTPRequestHandler):
         pass
 
     def do_POST(self):
+        server: _TorchHTTPServer = self.server  # type: ignore[assignment]
         if self.path == "/forward":
             self._handle_forward()
         elif self.path == "/v1/infer":
             self._handle_infer()
+        elif self.path == "/v1/chat/completions" and server.tracker_mode:
+            self._handle_chat_completions()
         else:
             self.send_response(404)
             self.end_headers()
@@ -190,6 +207,152 @@ class _TorchHandler(http.server.BaseHTTPRequestHandler):
         self.end_headers()
         self.wfile.write(payload)
 
+    def _handle_chat_completions(self) -> None:
+        server: _TorchHTTPServer = self.server  # type: ignore[assignment]
+        body = self._read_json_body()
+        if body is None:
+            return
+        messages = body.get("messages", [])
+        stream = bool(body.get("stream", False))
+        model = str(body.get("model", ""))
+        prompt = " ".join(
+            str(m.get("content", ""))
+            for m in messages
+            if isinstance(m, dict) and m.get("role") == "user"
+        )
+        try:
+            payload = server.backend.encode_prompt(prompt)
+        except Exception as exc:
+            self._send_json(500, {"error": f"encode_prompt failed: {exc}"})
+            return
+        remaining_route = self._get_remaining_route(model)
+        result_text = self._run_downstream_pipeline(payload, remaining_route)
+        self._send_openai_response(result_text, model, stream)
+
+    def _get_remaining_route(self, model: str) -> list[str]:
+        server: _TorchHTTPServer = self.server  # type: ignore[assignment]
+        if server.tracker_url is None:
+            return []
+        try:
+            url = f"{server.tracker_url}/v1/route?model={urllib.parse.quote(model)}"
+            with urllib.request.urlopen(url, timeout=5.0) as r:
+                route_resp = json.loads(r.read())
+            route = route_resp.get("route", [])
+            # Skip the first node in the route (self) since we're already the head
+            return list(route[1:])
+        except Exception:
+            return []
+
+    def _run_downstream_pipeline(self, payload: object, route: list[str]) -> str:
+        server: _TorchHTTPServer = self.server  # type: ignore[assignment]
+        if not route:
+            # Single-node mode: decode tail locally if we're the tail
+            if server.backend.is_tail:
+                try:
+                    tensor = server.backend.torch.frombuffer(
+                        bytearray(payload.body),  # type: ignore[union-attr]
+                        dtype=server.backend.torch.bfloat16,
+                    ).reshape(payload.shape).to(server.backend.device)  # type: ignore[union-attr]
+                    return server.backend.decode_tail(tensor)
+                except Exception as exc:
+                    return f"decode error: {exc}"
+            return ""
+
+        session = str(uuid.uuid4())
+        shape = payload.shape  # type: ignore[union-attr]
+        attn_mask = payload.attention_mask_header  # type: ignore[union-attr]
+        pos_ids = payload.position_ids_header  # type: ignore[union-attr]
+        current_body = payload.body  # type: ignore[union-attr]
+        current_shape = shape
+        current_attn = attn_mask
+        current_pos = pos_ids
+
+        for hop_index, node_url in enumerate(route):
+            headers: dict[str, str] = {
+                "Content-Type": "application/octet-stream",
+                "X-Meshnet-Wire": _WIRE_VERSION,
+                "X-Meshnet-Shape": ",".join(str(d) for d in current_shape),
+                "X-Meshnet-Dtype": "bfloat16",
+                "X-Meshnet-Session": session,
+                "X-Meshnet-Chunk-Index": "0",
+                "X-Meshnet-Chunk-Total": "1",
+                "X-Meshnet-Hop-Index": str(hop_index),
+            }
+            if current_attn:
+                headers["X-Meshnet-Attn-Mask"] = current_attn
+            if current_pos:
+                headers["X-Meshnet-Position-Ids"] = current_pos
+            req = urllib.request.Request(
+                f"{node_url}/forward",
+                data=current_body,
+                headers=headers,
+                method="POST",
+            )
+            try:
+                with urllib.request.urlopen(req, timeout=10.0) as r:
+                    resp_body = r.read()
+                    resp_headers = {k.lower(): v for k, v in r.headers.items()}
+            except Exception as exc:
+                return f"pipeline error at {node_url}: {exc}"
+            content_type = resp_headers.get("content-type", "")
+            if "application/json" in content_type:
+                try:
+                    data = json.loads(resp_body)
+                    return str(data.get("text", ""))
+                except json.JSONDecodeError:
+                    return resp_body.decode("utf-8", errors="replace")
+            # Binary activation — update and forward to next node
+            shape_header = resp_headers.get("x-meshnet-shape", ",".join(str(d) for d in current_shape))
+            current_shape = _parse_shape(shape_header)
+            current_body = resp_body
+            current_attn = resp_headers.get("x-meshnet-attn-mask")
+            current_pos = resp_headers.get("x-meshnet-position-ids")
+        return ""
+
+    def _send_openai_response(self, text: str, model: str, stream: bool) -> None:
+        chunk_id = "chatcmpl-node"
+        created = int(time.time())
+        if not stream:
+            self._send_json(200, {
+                "id": chunk_id,
+                "object": "chat.completion",
+                "created": created,
+                "model": model,
+                "choices": [{
+                    "index": 0,
+                    "message": {"role": "assistant", "content": text},
+                    "finish_reason": "stop",
+                }],
+                "usage": {"prompt_tokens": 0, "completion_tokens": 0, "total_tokens": 0},
+            })
+            return
+        self.send_response(200)
+        self.send_header("Content-Type", "text/event-stream; charset=utf-8")
+        self.send_header("Cache-Control", "no-cache")
+        self.end_headers()
+
+        def _emit(data: str) -> None:
+            self.wfile.write(f"data: {data}\n\n".encode())
+            self.wfile.flush()
+
+        _emit(json.dumps({
+            "id": chunk_id, "object": "chat.completion.chunk", "created": created,
+            "model": model,
+            "choices": [{"index": 0, "delta": {"role": "assistant", "content": ""}, "finish_reason": None}],
+        }))
+        _emit(json.dumps({
+            "id": chunk_id, "object": "chat.completion.chunk", "created": created,
+            "model": model,
+            "choices": [{"index": 0, "delta": {"content": text}, "finish_reason": None}],
+        }))
+        _emit(json.dumps({
+            "id": chunk_id, "object": "chat.completion.chunk", "created": created,
+            "model": model,
+            "choices": [{"index": 0, "delta": {}, "finish_reason": "stop"}],
+        }))
+        self.wfile.write(b"data: [DONE]\n\n")
+        self.wfile.flush()
+
 
 class TorchNodeServer:
     """HTTP server backed by a HuggingFace causal language model shard."""
@@ -203,6 +366,8 @@ class TorchNodeServer:
         shard_end: int = 6,
         quantization: str = "bfloat16",
         backend: TorchModelShard | None = None,
+        tracker_mode: bool | None = None,
+        tracker_url: str | None = None,
     ) -> None:
         self._host = host
         self._requested_port = port
@@ -212,6 +377,9 @@ class TorchNodeServer:
             shard_end,
             quantization,
         )
+        # Auto-detect tracker mode: enabled when shard_start == 0 or explicitly set
+        self._tracker_mode = tracker_mode if tracker_mode is not None else (shard_start == 0)
+        self._tracker_url = tracker_url
         self._server: _TorchHTTPServer | None = None
         self._thread: threading.Thread | None = None
         self.port: int | None = None
@@ -235,6 +403,8 @@ class TorchNodeServer:
             (self._host, self._requested_port),
             _TorchHandler,
             self._backend,
+            self._tracker_mode,
+            self._tracker_url,
         )
         self.port = self._server.server_address[1]
         self._thread = threading.Thread(target=self._server.serve_forever, daemon=True)

packages/tracker/meshnet_tracker/server.py

@@ -55,7 +55,7 @@ class _NodeEntry:
         "model", "shard_checksum", "hardware_profile", "wallet_address",
         "score", "vram_bytes", "ram_bytes", "quantizations",
         "benchmark_tokens_per_sec", "quantization", "managed_assignment",
-        "pending_directives", "last_heartbeat",
+        "pending_directives", "last_heartbeat", "tracker_mode",
     )
 
     def __init__(
@@ -75,6 +75,7 @@ class _NodeEntry:
         benchmark_tokens_per_sec: float = DEFAULT_BENCHMARK_TOKENS_PER_SEC,
         quantization: str | None = None,
         managed_assignment: bool = False,
+        tracker_mode: bool = False,
     ) -> None:
         self.node_id = node_id
         self.endpoint = endpoint
@@ -91,6 +92,7 @@ class _NodeEntry:
         self.benchmark_tokens_per_sec = benchmark_tokens_per_sec
         self.quantization = quantization
         self.managed_assignment = managed_assignment
+        self.tracker_mode = tracker_mode
         self.pending_directives: list[dict] = []
         self.last_heartbeat: float = time.monotonic()
 
@@ -429,6 +431,9 @@ class _TrackerHandler(http.server.BaseHTTPRequestHandler):
         elif parsed.path.startswith("/v1/coverage/"):
             model = urllib.parse.unquote(parsed.path.removeprefix("/v1/coverage/"))
             self._handle_coverage(model)
+        elif parsed.path.startswith("/v1/tracker-nodes/"):
+            model = urllib.parse.unquote(parsed.path.removeprefix("/v1/tracker-nodes/"))
+            self._handle_tracker_nodes(model)
         else:
             self.send_response(404)
             self.end_headers()
@@ -482,6 +487,40 @@ class _TrackerHandler(http.server.BaseHTTPRequestHandler):
         coverage = _coverage_map(alive, required_start, required_end)
         self._send_json(200, {"model": model, "coverage": coverage})
 
+    def _handle_tracker_nodes(self, model: str):
+        """Return nodes registered with tracker_mode=True whose shard starts at layer 0."""
+        server: _TrackerHTTPServer = self.server  # type: ignore[assignment]
+        preset = server.model_presets.get(model)
+        if preset is None:
+            self._send_json(404, {"error": f"unknown model preset: {model!r}"})
+            return
+        required_start, _ = _preset_layer_bounds(preset)
+        with server.lock:
+            self._purge_expired_nodes()
+            alive = [node for node in server.registry.values() if node.model == model]
+        if server.contracts is not None:
+            alive = [
+                node for node in alive
+                if not node.wallet_address or not server.contracts.registry.get_wallet(node.wallet_address).banned
+            ]
+        tracker_nodes = [
+            node for node in alive
+            if node.shard_start is not None
+            and node.shard_start == required_start
+            and node.tracker_mode
+        ]
+        self._send_json(200, {
+            "model": model,
+            "tracker_nodes": [
+                {
+                    "node_id": node.node_id,
+                    "endpoint": node.endpoint,
+                    "benchmark_tokens_per_sec": node.benchmark_tokens_per_sec,
+                }
+                for node in tracker_nodes
+            ],
+        })
+
     def _handle_register(self):
         server: _TrackerHTTPServer = self.server  # type: ignore[assignment]
         body = self._read_json_body()
@@ -567,6 +606,8 @@ class _TrackerHandler(http.server.BaseHTTPRequestHandler):
             self._send_json(403, {"error": ban_error})
             return
 
+        tracker_mode = bool(body.get("tracker_mode", False))
+
         node_id = str(uuid.uuid4())
         entry = _NodeEntry(
             node_id=node_id,
@@ -584,6 +625,7 @@ class _TrackerHandler(http.server.BaseHTTPRequestHandler):
             benchmark_tokens_per_sec=benchmark_tokens_per_sec,
             quantization=quantization,
             managed_assignment=not explicit_shard,
+            tracker_mode=tracker_mode,
         )
         with server.lock:
             self._purge_expired_nodes()