fix(mcp): validate remote URLs up-front with a clear error (#27105)

Port from anomalyco/opencode#25019 ("fix: handle invalid mcp urls").

Previously: a typo in `config.yaml` (missing scheme, wrong scheme,
empty string, non-string value) slipped past `_is_http()` and hit
`httpx.URL(url)` or `streamablehttp_client(url, ...)` deep in the
transport layer. That raised a generic exception which went through
the reconnect-backoff loop, so a bad URL caused _MAX_INITIAL_CONNECT_RETRIES
attempts with doubling backoff — about a minute of pointless retries
plus an opaque error — before the server was marked failed.

Now: we validate the URL once, at the top of `run()`, before
entering the retry loop. A malformed URL raises `InvalidMcpUrlError`
(a `ValueError` subclass) with a message that names the offending
server and explains exactly what was wrong. `_ready` is set and
`_error` is populated, so `start()` re-raises and the server shows
up as failed in `hermes mcp list` without any backoff burn.

Validation rules:
- Must be a string (rejects None, dict, int)
- Must be non-empty (rejects '' and whitespace-only)
- Scheme must be http or https (rejects file://, ws://, stdio://)
- Must have a non-empty host (rejects http:///, http://:8080)

Tests (21 new cases in tests/tools/test_mcp_invalid_url.py):
- TestValidUrlsAccepted: http, https, IPv6, ports, paths, query strings
- TestInvalidUrlsRejected: every rejection path above + clear error text
- TestErrorIsValueError: downstream code catching ValueError still works

E2E verified: a misconfigured server with `url: not-a-valid-url`
now fails in <0.001s with the clear error, instead of minutes of retries.

Doesn't touch stdio servers (they use `command`, not `url`) — the
validator only fires when `_is_http()` returns True.

tests/tools/test_mcp_invalid_url.py

@@ -0,0 +1,125 @@
+"""Tests for the MCP remote-URL validator.
+
+Ported from anomalyco/opencode#25019 (``fix: handle invalid mcp urls``).
+
+Previously, a typo in ``config.yaml`` (missing scheme, wrong scheme, empty
+string, dict where a URL was expected) caused the MCP server startup code
+to enter httpx's URL-parsing path and crash inside the transport layer.
+The reconnect-backoff loop would then retry
+``_MAX_INITIAL_CONNECT_RETRIES`` times with doubling backoff — a minute or
+more of pointless retries plus a confusing opaque error message — before
+eventually giving up.
+
+The fix validates the URL once, up front, and fails fast with a specific
+error message identifying the offending server.
+"""
+
+from __future__ import annotations
+
+import pytest
+
+from tools.mcp_tool import (
+    InvalidMcpUrlError,
+    _validate_remote_mcp_url,
+)
+
+
+class TestValidUrlsAccepted:
+    """Every valid http(s) URL must pass through untouched (stripped of whitespace)."""
+
+    @pytest.mark.parametrize(
+        "url",
+        [
+            "http://localhost:3000/mcp",
+            "https://example.com/mcp",
+            "https://context7.liam.com/mcp",
+            "http://127.0.0.1:8080",
+            "https://api.example.com:443/v1/mcp?session=abc",
+            "http://[::1]:9000/mcp",  # IPv6
+            "https://host.example.com",  # no port, no path
+        ],
+    )
+    def test_accepts_valid_http_url(self, url):
+        assert _validate_remote_mcp_url("test", url) == url
+
+    def test_strips_surrounding_whitespace(self):
+        assert (
+            _validate_remote_mcp_url("test", "  https://example.com/mcp  ")
+            == "https://example.com/mcp"
+        )
+
+
+class TestInvalidUrlsRejected:
+    """Every broken shape must raise ``InvalidMcpUrlError`` with a clear message."""
+
+    def test_none_rejected(self):
+        with pytest.raises(InvalidMcpUrlError, match="context7.*expected a string"):
+            _validate_remote_mcp_url("context7", None)
+
+    def test_dict_rejected(self):
+        with pytest.raises(InvalidMcpUrlError, match="expected a string, got dict"):
+            _validate_remote_mcp_url("ctx", {"url": "nested"})
+
+    def test_int_rejected(self):
+        with pytest.raises(InvalidMcpUrlError, match="expected a string, got int"):
+            _validate_remote_mcp_url("ctx", 8080)
+
+    def test_empty_string_rejected(self):
+        with pytest.raises(InvalidMcpUrlError, match="empty url"):
+            _validate_remote_mcp_url("ctx", "")
+
+    def test_whitespace_only_rejected(self):
+        with pytest.raises(InvalidMcpUrlError, match="empty url"):
+            _validate_remote_mcp_url("ctx", "   \t\n")
+
+    def test_missing_scheme_rejected(self):
+        # The most common typo — users copy a host from a web page.
+        with pytest.raises(
+            InvalidMcpUrlError, match="scheme must be http or https"
+        ):
+            _validate_remote_mcp_url("ctx", "example.com/mcp")
+
+    def test_file_scheme_rejected(self):
+        with pytest.raises(
+            InvalidMcpUrlError, match="scheme must be http or https"
+        ):
+            _validate_remote_mcp_url("ctx", "file:///etc/passwd")
+
+    def test_ws_scheme_rejected(self):
+        # WebSocket is not MCP's remote transport.
+        with pytest.raises(
+            InvalidMcpUrlError, match="scheme must be http or https"
+        ):
+            _validate_remote_mcp_url("ctx", "ws://example.com/mcp")
+
+    def test_stdio_scheme_rejected(self):
+        # stdio servers use the ``command`` key, not ``url``.
+        with pytest.raises(
+            InvalidMcpUrlError, match="scheme must be http or https"
+        ):
+            _validate_remote_mcp_url("ctx", "stdio:///node server.js")
+
+    def test_empty_host_rejected(self):
+        with pytest.raises(InvalidMcpUrlError, match="missing host"):
+            _validate_remote_mcp_url("ctx", "http:///")
+
+    def test_empty_host_with_path_rejected(self):
+        with pytest.raises(InvalidMcpUrlError, match="missing host"):
+            _validate_remote_mcp_url("ctx", "https:///path/only")
+
+    def test_error_mentions_server_name(self):
+        # So users can find the bad entry when there are multiple configured.
+        with pytest.raises(InvalidMcpUrlError, match="my-weird-server"):
+            _validate_remote_mcp_url("my-weird-server", "not a url at all")
+
+
+class TestErrorIsValueError:
+    """InvalidMcpUrlError must be a ValueError for broad downstream catch blocks."""
+
+    def test_is_value_error(self):
+        try:
+            _validate_remote_mcp_url("ctx", "garbage")
+        except ValueError:
+            pass  # expected
+        else:
+            pytest.fail("expected ValueError")

tools/mcp_tool.py

@@ -91,6 +91,7 @@ import threading
 import time
 from datetime import datetime
 from typing import Any, Dict, List, Optional
+from urllib.parse import urlparse
 
 logger = logging.getLogger(__name__)
 
@@ -492,6 +493,72 @@ def _cache_mcp_image_block(block) -> str:
     return f"MEDIA:{image_path}"
 
 
+# ---------------------------------------------------------------------------
+# Remote MCP URL validation
+# ---------------------------------------------------------------------------
+
+
+class InvalidMcpUrlError(ValueError):
+    """Raised when a remote MCP server's ``url`` cannot be parsed as http(s)://.
+
+    Validated once at startup so we fail fast with a clear message instead of
+    burning through the reconnect-backoff loop on every attempt.  (Ported from
+    anomalyco/opencode#25019.)
+    """
+
+
+def _validate_remote_mcp_url(server_name: str, url: Any) -> str:
+    """Return the URL as a string if it's a valid http(s) remote MCP URL.
+
+    Raises :class:`InvalidMcpUrlError` otherwise with a message naming the
+    offending server, so users can spot the bad entry in their config.
+
+    Accepts:
+    - ``http://host`` / ``https://host`` with optional port, path, query
+    - IPv4, IPv6 (bracketed), DNS hostnames
+
+    Rejects:
+    - Non-string values (``None``, dicts, ints)
+    - Missing scheme (``example.com/mcp``)
+    - Non-http(s) schemes (``file://``, ``ws://``, ``stdio:`` — stdio servers
+      use the ``command`` key, not ``url``)
+    - Empty host (``http://``, ``https:///path``)
+    """
+    if not isinstance(url, str):
+        raise InvalidMcpUrlError(
+            f"Invalid MCP URL for '{server_name}': expected a string, got "
+            f"{type(url).__name__}"
+        )
+    stripped = url.strip()
+    if not stripped:
+        raise InvalidMcpUrlError(
+            f"Invalid MCP URL for '{server_name}': empty url"
+        )
+    try:
+        parsed = urlparse(stripped)
+    except Exception as exc:  # urlparse is very permissive — belt and braces
+        raise InvalidMcpUrlError(
+            f"Invalid MCP URL for '{server_name}': {stripped!r} ({exc})"
+        ) from exc
+    if parsed.scheme.lower() not in ("http", "https"):
+        raise InvalidMcpUrlError(
+            f"Invalid MCP URL for '{server_name}': scheme must be http or "
+            f"https, got {parsed.scheme!r} ({stripped!r})"
+        )
+    if not parsed.netloc:
+        raise InvalidMcpUrlError(
+            f"Invalid MCP URL for '{server_name}': missing host ({stripped!r})"
+        )
+    # ``urlparse`` accepts ``http://:8080`` (empty host, explicit port).
+    # Reject that — we need a real host.
+    if not parsed.hostname:
+        raise InvalidMcpUrlError(
+            f"Invalid MCP URL for '{server_name}': missing hostname "
+            f"({stripped!r})"
+        )
+    return stripped
+
+
 def _format_connect_error(exc: BaseException) -> str:
     """Render nested MCP connection errors into an actionable short message."""
 
@@ -1458,6 +1525,21 @@ class MCPServerTask:
                 "this warning.",
                 self.name,
             )
+
+        # Validate remote URL once, up front.  Raising here (rather than
+        # letting it blow up inside the SDK's httpx layer on every retry)
+        # means a typo in config.yaml fails fast with a clear error — and
+        # critically, no reconnect-backoff burn.  (Ported from
+        # anomalyco/opencode#25019.)
+        if self._is_http():
+            try:
+                _validate_remote_mcp_url(self.name, config.get("url"))
+            except InvalidMcpUrlError as exc:
+                logger.warning("%s", exc)
+                self._error = exc
+                self._ready.set()
+                return
+
         retries = 0
         initial_retries = 0
         backoff = 1.0