feat(mcp): client tool adapters over external MCP servers - #265
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Nexo as an MCP client: external streamable-HTTP MCP servers surface their tools
as namespaced ITool proxies (mcp:{server}:{tool}) that reach agent toolboxes
through a new IToolSource seam.
- src/Nexo.Mcp.Client: McpClientConnectionManager (hosted service) dials
configured servers, pins each tool definition (name/description/raw schema)
for the process lifetime, and faults on drift instead of following remote
redefinitions; unreachable servers degrade to zero tools; remote isError and
transport failures map to error payloads per the repo tool convention;
per-server API keys are env-var named and verified present at startup.
- Nexo.Abstractions.IToolSource: SDK-free seam for post-startup-discovered
tools; RepoFsToolboxFactory gains an extraTools fold-in and
SelfExtendRunnerAdapter snapshots DI-registered sources each cycle.
- src/Nexo.Mcp.Client.Tests: 29 tests, including full protocol round trips -
a real McpClient against the real Nexo.Mcp.Server bridge over in-memory
pipes (list/call/structured-content/protocol-error), plus manager
pin-and-proxy end-to-end.
- Directory.Packages.props: Microsoft.Extensions.Options.ConfigurationExtensions
pin (10.0.11).
Fail-closed: Enabled=false default, ValidateOnStart, AirGapped refusal, empty
allowlists, stdio child processes deliberately excluded from v1.
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>IanFrelingerforce-pushed
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Summary
PR-2 of the MCP/A2A series (stacked on #264 → #263; retarget to
masteras parents merge). Nexo becomes an MCP client: external streamable-HTTP MCP servers contribute their tools to Nexo agent toolboxes as namespacedIToolproxies, with pinned definitions and fail-closed drift handling. Design:docs/architecture/ProtocolIntegration-MCP-A2A.md(Phase 2 section added).Changes
src/Nexo.Mcp.Client—McpClientConnectionManager(hosted service +IToolSource):AllowedToolsnarrowing).mcp:{server}:{tool}— a remote server can never shadowrepo.fs.write(CapabilityRegistry registration is last-wins by id).isError/transport failures →McpToolFailureerror payloads (repo tool convention, model-visible); API keys are env-var-named with presence verified at startup; stdio child processes deliberately out of v1 (command-allowlist design needed first).Nexo.Abstractions.IToolSource— one SDK-free interface for post-startup-discovered tools;RepoFsToolboxFactorygains anextraToolsfold-in andSelfExtendRunnerAdaptersnapshots DI-registered sources each cycle (optional ctor param, no existing call sites broken).src/Nexo.Mcp.Client.Tests— 29 tests, headlined by full protocol round trips: a realMcpClientagainst the real PR-1 server bridge over in-memory pipes (sanitized-name listing, call with structured content, unknown-tool protocol error, manager pin→proxy→invoke end-to-end). Plus 4 fold-in tests inNexo.Tests.BackgroundAgents.Directory.Packages.props:Microsoft.Extensions.Options.ConfigurationExtensions10.0.11 pin.LICENSING.mdOPEN rows,Nexo.slnentries, docs Phase-2 section.Testing
dotnet test src/Nexo.Mcp.Client.Tests— 29/29 locally (roll-forward runtime)dotnet test src/Nexo.Tests.BackgroundAgents --filter RepoFsToolboxFactoryExtraToolsTests— 4/4 on net8.0 + net9.0Testing strategy (blast radius)
Tier-2 adapter + one additive optional-param seam in
BackgroundAgents.HostRunners(outside kernel-gate paths);Nexo.Abstractionsgains one interface (no behavior).make kernel-coverage-gatesrc/Nexo.Core.*/Infrastructurechanges)make kernel-gatemake test-prod-styleChecklist
TODOorNotImplementedExceptionleft unresolvedRepoFsToolboxFactorycall sites unchanged)Release (only when this PR ships a versioned NuGet/GHCR release)
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