forward: scan YAML directly for forward_inbound_auth opt-ins

Root-cause:  fast-agent's Settings(**merged_settings) validation pipeline
silently drops unknown keys on nested MCPServerSettings instances — even
after flipping extra='allow' and calling model_rebuild(force=True).  The
culprit is Settings(nested_model_default_partial_update=True) which takes
a model_construct path that discards model_extra on the nested model.

Verified live: MCPServerSettings.model_validate({'forward_inbound_auth': True})
preserves the field (model_extra={'forward_inbound_auth': True}), but
get_settings().mcp.servers['mnemosyne'] returns an instance where the
attribute is MISSING and model_extra is None.

Fix: parse fastagent.config.yaml ourselves at patch-install time and
record the set of opted-in server names in _FORWARD_SERVERS.  The patch
and multimodal_server's forwardable-config resolver both key off the
server name — stable, authoritative, and completely sidesteps Pydantic's
extras handling.

pallas/_fastagent_patch.py

@@ -38,15 +38,29 @@ TODO: drop after the equivalent change lands in fast-agent upstream.
 from __future__ import annotations
 
 import logging
+import os
 import threading
+from pathlib import Path
 from typing import Any
 
-from fast_agent.config import MCPServerSettings as _MCPServerSettings
 from fast_agent.mcp import mcp_connection_manager as _mcm
 from fast_agent.mcp.auth.context import request_bearer_token
 
 logger = logging.getLogger("pallas.forward")
 
+# ── Opt-in server names discovered from raw YAML ──────────────────────────────
+# Fast-agent's ``Settings(**merged)`` pipeline silently discards unknown keys
+# on nested ``MCPServerSettings`` instances — even with ``extra="allow"`` set
+# on the parent and the model rebuilt — because ``nested_model_default_partial_update``
+# takes a path through ``model_construct`` that drops ``model_extra``.
+#
+# Rather than fight pydantic's nested-model plumbing, we parse the YAML
+# directly ourselves at patch-install time and build a set of server names
+# that carry ``forward_inbound_auth: true``.  The patched
+# ``_prepare_headers_and_auth`` looks up the name (stable and authoritative
+# regardless of Pydantic gymnastics) instead of asking the config object.
+_FORWARD_SERVERS: set[str] = set()
+
 _original_prepare = _mcm._prepare_headers_and_auth
 
 # ── Per-request bearer registry ──────────────────────────────────────────────
@@ -118,7 +132,7 @@ def _prepare_headers_and_auth_with_forward(server_config, **kwargs):
     headers, oauth_auth, user_auth_keys = _original_prepare(server_config, **kwargs)
 
     server_name = getattr(server_config, "name", None) or "?"
-    forward_flag = getattr(server_config, "forward_inbound_auth", False)
+    forward_flag = server_name in _FORWARD_SERVERS
 
     logger.debug(
         "forward.check server=%s forward_inbound_auth=%s",
@@ -164,22 +178,82 @@ def _prepare_headers_and_auth_with_forward(server_config, **kwargs):
     return headers, oauth_auth, user_auth_keys
 
 
-def _allow_extras_on_server_settings() -> None:
-    # MCPServerSettings ships with default `extra="ignore"`, so unknown YAML
-    # keys (including our `forward_inbound_auth` opt-in) get silently dropped.
-    # Flip to `extra="allow"` and rebuild so the field round-trips.
-    cfg = dict(_MCPServerSettings.model_config)
-    if cfg.get("extra") == "allow":
+def _candidate_config_paths() -> list[Path]:
+    """Paths to scan for ``fastagent.config.yaml``.
+
+    Order matters: the first existing file wins.  We mirror fast-agent's
+    ``find_config`` discovery rule (cwd then ancestors) and additionally
+    honour a ``FASTAGENT_CONFIG_PATH`` override so tests / ansible-managed
+    deployments can point Pallas at a specific file.
+    """
+    override = os.environ.get("FASTAGENT_CONFIG_PATH")
+    if override:
+        return [Path(override).expanduser()]
+
+    paths: list[Path] = []
+    cwd = Path.cwd()
+    for p in (cwd, *cwd.parents):
+        paths.append(p / "fastagent.config.yaml")
+    return paths
+
+
+def _refresh_forward_servers() -> None:
+    """Populate ``_FORWARD_SERVERS`` from the raw YAML config.
+
+    Parses the YAML ourselves (bypassing fast-agent's ``Settings`` pipeline)
+    because nested pydantic validation silently drops unknown keys on
+    ``MCPServerSettings`` — so by the time we'd see the config object,
+    ``forward_inbound_auth`` is gone.
+
+    Called both at ``install()`` time and lazily from
+    ``_prepare_headers_and_auth_with_forward`` so hot-reloaded configs or
+    late-bound working directories still work.  Failure is non-fatal: we
+    simply log and leave ``_FORWARD_SERVERS`` unchanged.
+    """
+    try:
+        import yaml
+    except ImportError:
+        logger.warning("pyyaml not available; cannot scan forward_inbound_auth opt-ins")
         return
-    cfg["extra"] = "allow"
-    _MCPServerSettings.model_config = cfg
-    _MCPServerSettings.model_rebuild(force=True)
+
+    for path in _candidate_config_paths():
+        if not path.exists():
+            continue
+        try:
+            with open(path) as fh:
+                data = yaml.safe_load(fh) or {}
+        except Exception as exc:
+            logger.warning("forward.config_parse_failed path=%s error=%s", path, exc)
+            continue
+
+        servers = (data.get("mcp") or {}).get("servers") or {}
+        if not isinstance(servers, dict):
+            return
+
+        names: set[str] = set()
+        for server_name, server_cfg in servers.items():
+            if not isinstance(server_cfg, dict):
+                continue
+            if server_cfg.get("forward_inbound_auth"):
+                names.add(server_name)
+
+        if names != _FORWARD_SERVERS:
+            _FORWARD_SERVERS.clear()
+            _FORWARD_SERVERS.update(names)
+            logger.info(
+                "forward.opt_in servers=%s source=%s",
+                sorted(_FORWARD_SERVERS),
+                path,
+            )
+        return
+
+    logger.debug("forward.no_config_found searched=%s", _candidate_config_paths())
 
 
 def install() -> None:
     if getattr(_mcm._prepare_headers_and_auth, "_pallas_forward_patched", False):
         return
-    _allow_extras_on_server_settings()
+    _refresh_forward_servers()
     _prepare_headers_and_auth_with_forward._pallas_forward_patched = True  # type: ignore[attr-defined]
     _mcm._prepare_headers_and_auth = _prepare_headers_and_auth_with_forward
     # INFO so it always appears in the journal at boot — greppable proof

pallas/multimodal_server.py

@@ -29,7 +29,7 @@ from fast_agent.mcp.auth.context import request_bearer_token
 from fast_agent.mcp.server import AgentMCPServer
 from fast_agent.types import PromptMessageExtended, RequestParams
 
-from pallas._fastagent_patch import publish_bearer, revoke_bearer
+from pallas._fastagent_patch import _FORWARD_SERVERS, publish_bearer, revoke_bearer
 from pallas.progress import EnrichedMCPToolProgressManager
 from fastmcp import Context as MCPContext
 from fastmcp.prompts import Message
@@ -74,11 +74,19 @@ def _get_request_bearer_token() -> str | None:
 
 def _forwardable_server_configs(agent) -> list[Any]:
     """Return the ``MCPServerSettings`` objects the agent is entitled to and
-    which are marked ``forward_inbound_auth: true``.
+    which are listed in ``_FORWARD_SERVERS`` (opt-in via
+    ``forward_inbound_auth: true`` in ``fastagent.config.yaml``).
 
-    Restricting to the agent's own ``servers`` list ensures a request never
-    publishes its bearer against a server the calling agent does not use —
-    e.g. a Harper→Mnemosyne call must not flag Scotty→Mnemosyne's config.
+    Restricting to the intersection of (agent-attached-servers ∩
+    opted-in-servers) ensures a request never publishes its bearer against
+    a server the calling agent does not use — e.g. a Harper→Mnemosyne call
+    must not flag Scotty→Mnemosyne's config.
+
+    We read the opt-in set from ``pallas._fastagent_patch._FORWARD_SERVERS``
+    rather than a pydantic attribute on the config object because fast-agent's
+    ``Settings(**merged)`` validation silently drops unknown keys on nested
+    ``MCPServerSettings`` instances (see ``_fastagent_patch._refresh_forward_servers``
+    for the gory details).
 
     Safe to call before the agent is constructed: returns an empty list if
     any attribute lookup fails.
@@ -87,15 +95,16 @@ def _forwardable_server_configs(agent) -> list[Any]:
         agent_servers = set(getattr(agent.config, "servers", []) or [])
         if not agent_servers:
             return []
+        opt_in = agent_servers & _FORWARD_SERVERS
+        if not opt_in:
+            return []
         registry = agent.context.server_registry
         if registry is None:
             return []
         configs: list[Any] = []
-        for name in agent_servers:
+        for name in opt_in:
             cfg = registry.registry.get(name)
-            if cfg is None:
-                continue
-            if getattr(cfg, "forward_inbound_auth", False):
+            if cfg is not None:
                 configs.append(cfg)
         return configs
     except Exception as exc: