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>
Addresses the first two items of #15 (the chain-orchestrator
multi-server item follows in a separate PR).
## `mode`: keep `"active"`, add `"audit"` round-trip coverage
> **Update:** an earlier revision of this PR renamed `mode` from
`"active"` to `"enforce"`, following a SEP **draft**. The now-current
revised SEP-1763 ([PR
#2624](https://github.com/modelcontextprotocol/modelcontextprotocol/pull/2624))
defines `mode?: "active" | "audit"` with default `"active"` — so
`"active"` was correct all along. @jeongukjae flagged this; the rename
has been reverted (commit 58edf2c). The C# SDK's wire value `"active"`
matches the SEP and the TypeScript SDK.
`InterceptorMode` keeps members `Active` (wire `"active"`, default) and
`Audit` (wire `"audit"`). Added a deserialization test alongside the
existing serialization test so both wire forms are covered.
## `priorityHint`: support `number | { request?, response? }`
The SDK only supported a plain `int`. This adds a `PriorityHint`
protocol type whose converter accepts both wire forms and preserves them
on round-trip (a number stays a number, an object stays an object).
Chain ordering now resolves the effective priority per phase exactly as
the SEP's `resolvePriority` algorithm (unset → 0, alphabetical tie-break
unchanged).
- `Interceptor.PriorityHint` is now `PriorityHint?`; an implicit
conversion from `int` keeps existing code compiling.
- `[McpServerInterceptor]` gains `RequestPriorityHint` /
`ResponsePriorityHint` (mutually exclusive with the scalar
`PriorityHint`), following the official SDK's backing-field pattern for
optional attribute values.
- Behavior note: attribute-discovered interceptors with no priority set
now omit `priorityHint` from the wire instead of emitting `0` —
semantically identical per the SEP default, and consistent with the
existing default-skipping for `Mode`/`FailOpen`.
## Testing
`dotnet test` from `csharp/sdk/`: all passing, including new coverage
for both `priorityHint` wire forms, partial objects, unknown-key
tolerance, phase-dependent mutation ordering (the SEP's
pii-redactor/compressor example), the attribute conflict error, and both
`mode` wire values.
Part of #15.
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---------
Co-authored-by: Claude Fable 5 <noreply@anthropic.com>
Re-implementation of C# interceptors SDK with clearer focus on
server-side / gateway interceptors.
Fixes a lot of issues with the existing implementation.
Still more boilerplate than I'm happy with, but this is better than what
was there. Will refine further in later PRs.
