C# SDK: align mode value and priorityHint with the SEP

[PederHP]

Addresses the first two items of #15 (the chain-orchestrator multi-server item follows in a separate PR).

mode: "active" → "enforce"

The SEP defines mode?: "enforce" | "audit"; the C# SDK serialized "active", making it incompatible with the TypeScript SDK. InterceptorMode.Active is renamed to Enforce with wire value "enforce". Clean break — "active" is no longer accepted on read (experimental extension; the old value was never spec-compliant).

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/: 87 passing (was 66), including new coverage for both wire forms, partial objects, unknown-key tolerance, phase-dependent mutation ordering (the SEP's pii-redactor/compressor example), and the attribute conflict error.

Part of #15.

🤖 Generated with Claude Code

[Copilot]

Pull request overview

Aligns the C# interceptors SDK with SEP-1763 by updating the mode wire value to match the spec and extending priorityHint to support both scalar and per-phase object forms, including phase-specific mutation ordering in the chain orchestrator.

Changes:

• Renames InterceptorMode.Active → InterceptorMode.Enforce and updates default/serialization behavior accordingly.
• Introduces PriorityHint protocol type + JSON converter to round-trip number | { request?, response? } faithfully.
• Updates chain orchestration to sort mutations by phase-effective priority and adds/updates tests for new semantics and attribute behavior.

Reviewed changes

Copilot reviewed 12 out of 12 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
csharp/sdk/src/ModelContextProtocol.Interceptors/Protocol/InterceptorMode.cs	Changes enum member/wire value from active to enforce.
csharp/sdk/src/ModelContextProtocol.Interceptors/Protocol/PriorityHint.cs	Adds new protocol type + converter supporting scalar/object priority hints.
csharp/sdk/src/ModelContextProtocol.Interceptors/Protocol/Interceptor.cs	Changes PriorityHint property type to PriorityHint? and updates docs.
csharp/sdk/src/ModelContextProtocol.Interceptors/InterceptorJsonContext.cs	Registers PriorityHint for source-generated JSON support.
csharp/sdk/src/ModelContextProtocol.Interceptors/Server/McpServerInterceptorAttribute.cs	Adds optional per-phase priority hint attribute properties with backing fields.
csharp/sdk/src/ModelContextProtocol.Interceptors/Server/ReflectionMcpServerInterceptor.cs	Omits default Enforce mode on the wire and resolves scalar vs per-phase priority hints (with conflict error).
csharp/sdk/src/ModelContextProtocol.Interceptors/Client/InterceptorChainOrchestrator.cs	Sorts mutations by phase-effective priority hint.
csharp/sdk/tests/ModelContextProtocol.Interceptors.Tests/ProtocolTypesSerializationTests.cs	Adds serialization/round-trip coverage for PriorityHint and updated InterceptorMode.
csharp/sdk/tests/ModelContextProtocol.Interceptors.Tests/ReflectionMcpServerInterceptorTests.cs	Adds tests for per-phase attribute hints, omission when unset, and conflict detection.
csharp/sdk/tests/ModelContextProtocol.Interceptors.Tests/InterceptorChainOrchestratorTests.cs	Adds tests for phase-dependent ordering, default-zero behavior, and tie-breaking.
csharp/sdk/README.md	Documents per-phase priority hint usage for mutation ordering.
csharp/sdk/CLAUDE.md	Updates test count and documents expanded PriorityHint semantics.

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[PederHP]

Good catch from Copilot — ExecuteChainRequestParams.Phase could indeed be set to the attribute-only InterceptorPhase.Both, which would silently match no hooks while branching as if it were the response phase. Added an explicit guard in 82b552f: the orchestrator now throws ArgumentException for any non-Request/Response phase, with a test covering it.

(Authored by Claude Fable 5)