ajayr 7e95bef2e7
feat(mcp): per-user credential injection for shared MCP servers (#4868)
* feat(mcp): per-user credential injection for shared MCP servers

A single HTTP/SSE MCP server entry can now serve several users, each
authenticated to the remote service with their own credential. A server
opts in with a user_auth block mapping user ids to credential header
values ($ENV_VAR references supported):

  "user_auth": {
    "header": "Authorization",
    "users": { "<user-id>": "$SERVICE_TOKEN_ALICE" }
  }

The built-in user-scoped auth interceptor resolves the authenticated
runtime user on every tool call (request runtime -> ambient LangGraph
runtime -> auth config -> request-scoped user ContextVar) and rewrites
the configured header via request.override(), the same per-call
mechanism as the OAuth interceptor. It registers after OAuth in the
shared assembly so its per-user value wins the header when a server
declares both. The entry's static headers are used only for startup
tool discovery.

Fail-closed by default: an unmapped user - including the anonymous
default-user fallback - or a credential whose env reference resolved
empty gets an actionable ToolException instead of another user's
credential; on_missing: "passthrough" opts out per server. Combined
with the existing per-(user, thread) MCP session scoping this gives
credential isolation on shared servers.

Gateway API: user_auth.users values are masked in GET responses, and
PUT round-trips preserve stored credentials for masked values (same
contract as env/headers/oauth secrets); a masked value for a user id
not already stored is rejected.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

* fix(mcp): address review — preserve stored user_auth sub-fields on partial PUT, warn-and-skip stdio, allow extras

Review findings from #4868:

1. A partial user_auth payload (e.g. {"enabled": false}) merged to
   users={} and default on_missing, irreversibly wiping stored
   credentials on PUT. The merge is now sub-field-aware via
   model_fields_set — omitted sub-fields carry the stored values, an
   explicitly sent users map still replaces (so full-round-trip removal
   works), masked values still swap back for stored credentials.

2. user_auth on a stdio server was a silent no-op: rewritten headers go
   to call meta, never a transport header, while deny errors still fired.
   The interceptor builder now warns and skips non-sse/http servers,
   matching the tool_call_timeout transport-mismatch convention.

3. McpUserScopedAuthConfigResponse now allows extra keys like the
   harness-side model, and extras survive masking and merge, matching
   the server-level model_extra handling.

Adds four regression tests (partial-PUT preservation, explicit-map
replacement, extras round-trip, stdio warn-and-skip).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

* fix(mcp): reject blank user_auth.header at the gateway

A blank header passed the gateway response model, was persisted, then
failed the harness-side ExtensionsConfig validator on reload — the PUT
returned 500 after the write and every later config load/startup failed
until the file was hand-edited. Mirror the harness non-blank validator
on McpUserScopedAuthConfigResponse so the PUT fails with 422 before
anything is written.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

* style: ruff format extensions_config.py

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

* fix(mcp): harden the trust chain and masking around user-scoped credential selection

Address review round 4:
- Scrub client-supplied user_id from run context/configurable for
  external callers in inject_authenticated_user_context, before every
  early return, and restamp only from request.state.user. Now that
  user_id selects which user's credential user-scoped MCP auth injects,
  a forged value must not survive any future path that skips the
  restamp. Internal callers (IM channels, scheduler) keep supplying
  end-user identity as before (PR #3294 contract). Regression tests pin
  both the scrub and that a forged body.context.user_id can never
  resolve as another user through merge + inject ordering.
- Include the caller's own resolved user id in the fail-closed deny
  message so operators can copy the exact users key (it differs by
  deployment path), and document the key formats in the mcp.mdx doc.
- Mask sensitive extra keys inside user_auth on GET like server-level
  extras, and swap masked sentinels back for stored values on PUT via
  _merge_extra_value_preserving_masked.
- Extract the interceptor wrap loop into compose_tool_interceptors and
  pin the security property functionally: an OAuth interceptor that
  actually sets Authorization loses the final header to the per-user
  credential through the same composition the session-pool path uses.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

---------

Co-authored-by: Claude Fable 5 <noreply@anthropic.com>
2026-08-23 15:16:04 +08:00
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