Addresses the third item of #15 ("Chain orchestrator only supports one
MCP server").
> **Stacked on #17** — this PR targets that branch because it builds on
the per-phase `PriorityHint` resolution introduced there. After #17
merges, the base will be switched to `main` (or it can be retargeted now
and rebased).
## What changed
The SEP's chain execution pattern requires: discover via
`interceptors/list` on one or more servers → merge & sort all
interceptors into a single chain by `priorityHint` (ascending,
alphabetical tie-break) → invoke each interceptor on the server that
hosts it (mutations sequential with payload chaining, validations
parallel) → aggregate one result. The SDK previously ran each client's
entire chain sequentially, so a `-1000` mutation on server 2 ran after a
`+1000` mutation on server 1, and each server's validations only saw its
own server's mutations.
- **Orchestrator** (`InterceptorChainOrchestrator`, internal): now
operates on chain entries — descriptor + invoker routing to its host —
mirroring the SEP's `ChainEntry`, with one global sort across all
filtered entries. The stable sort preserves caller-supplied server order
for exact ties.
- **New public API** (`InterceptorChain` / `InterceptorChainEntry`):
`DiscoverAsync` lists from every server in parallel (fail-closed: any
list failure throws rather than silently dropping a server's
interceptors), instance `ExecuteAsync` runs prebuilt entries (a future
caching seam), and a one-shot static `ExecuteAsync(servers, params)`
does discover + execute.
- **`InterceptorChainRunner`** (used by the gateway and
`InterceptingMcpClient`): one merged chain instead of the per-client
sequential loop. The single-client `client.ExecuteChainAsync(...)`
extension keeps its signature and delegates to the same path (still 1
list + N invokes).
- Duplicate interceptor names across servers are not deduplicated — each
entry invokes on its own host (results may share an `InterceptorName`;
documented).
## Behavior changes for multi-client gateways
- Interceptors from all servers form one globally-ordered chain
(previously server 1's full chain ran before server 2's).
- Validations from all servers run as one parallel batch against the
payload after **all** mutations.
- Sinks run once per phase for the merged chain (previously once per
client).
- `TimeoutMs` bounds the whole merged chain, not each per-client
sub-chain.
- Any server's `interceptors/list` failure fails the chain up-front
(fail-closed).
- Cost per phase: M parallel `interceptors/list` calls + N
`interceptor/invoke` calls.
## Testing
`dotnet test` from `csharp/sdk/`: 97 passing (was 87 after #17). New
coverage:
- Orchestrator: global priority ordering across servers, alphabetical
tie-break across servers, payload chaining across servers in one
mutation pass, all-server validations seeing the post-mutation payload,
duplicate names invoking once per host.
- `InterceptorChainTests` (real in-memory servers): discovery merge with
server attribution, merged execution order, fail-closed discovery,
single-client path equivalence.
- Gateway: two interceptor servers where the second hosts the
lower-priority mutation — the backend receives the globally-ordered
payload (fails under the old sequential runner).
Closes#15.
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---------
Co-authored-by: Claude Fable 5 <noreply@anthropic.com>
Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
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