Add SEP-2640 Skills extension support by girishkvs · Pull Request #1856 · modelcontextprotocol/csharp-sdk

girishkvs · 2026-09-05T02:39:32Z

Summary

Adds ModelContextProtocol.Extensions.Skills, implementing the skills/list and skills/get methods of SEP-2640 (io.modelcontextprotocol/skills, accepted 2026-09-01), alongside the existing Tasks and Apps extension packages. The capability is declared inline and both methods are registered through McpServerOptions.RequestHandlers, so the extension consumes only public Core API.

The Core API changes are intentional, and I am happy to split them into their own PR. PaginatedResult and PaginatedRequestParams had private protected constructors, so a package outside Core cannot implement a paginated method. skills/list is paginated. Widening those to protected is source and binary compatible, and package validation passes against the 2.0.0 baseline. CacheScopeConverter is also public so extensions can reuse Core's read-side leniency for unrecognized scope values.

A skill's manifest uses a closed SkillResources type admitting only an enumerated file list or the "dynamic" marker, and "resources": null is rejected on read. Entries receive structural validation, and pagination uses keyset cursors over URI order. The built-in catalog owns validated snapshots; custom catalogs remain checked on each request.

ttlMs, cacheScope and resultType are emitted on both list/get results for protocol revision 2026-07-28 and omitted on older revisions. Cache values are configurable and default to immediately stale/private. Client-side wrappers, byte-consistent file authoring, a bounded YAML reader and both client/server samples are included.

Directory loading excludes dot-prefixed, hidden and system entries by default, with explicit opt-in. Aggregating client listing defaults to 100 pages and 1,000 entries, with configurable positive limits and cursor-cycle errors rather than silent truncation. ListSkillsWithDiagnosticsAsync is an explicit alternative that returns valid entries and indexed failures while ordinary list helpers remain strict.

The latest review follow-up adds optional entry-level _meta with owned snapshots, Unicode-scalar limits for description and compatibility, and authorization-safe pagination in the shared-policy example. Encoded Unicode names and native absolute resource URIs are supported without rewriting resource identity. The frontmatter reader rejects unsupported nested flow values rather than publishing different data, octal conversion avoids growing a BigInteger for every digit, and null resource-read contents fail with SkillVerificationException.

The timeout remarks now distinguish discovery/session initialization from initial explicit-SSE transport authorization. This wording change does not restrict which OAuth work pauses an active probe or change SSE runtime behavior.

I would still like maintainer input on the authorization boundary and public Core API shape:

Skills served through RequestHandlers do not automatically pass through AuthorizationFilterSetup. The current implementation keeps one shared application policy for catalog visibility and resource access, with HTTP allow/deny and cursor-disclosure coverage. It does not derive authorization from resources/list, since that listing may be partial while direct reads remain valid. Would you prefer a general filter hook for custom request handlers in Core?
Making CacheScopeConverter public is deliberate, alongside the pagination constructors. I can split these API changes if that is the preferred package boundary.

Following CONTRIBUTING.md I would normally open an issue first. I am opening this as a PR because the working group asked for a reference implementation in a Tier 1 SDK and Go already has one in flight. Happy to convert this to an issue, split it, or reshape the API if you would prefer.

Changes

File	What
`Extensions.Skills/SkillsProtocol.cs`	New. Method names, extension id, limits, sentinels
`Extensions.Skills/Protocol/Skill.cs`, `SkillResource.cs`	New. Entry and manifest file shapes, including optional entry `_meta`
`Extensions.Skills/Protocol/SkillResources.cs`, `SkillResourcesConverter.cs`	New. Array-or-`"dynamic"` union and its converter
`Extensions.Skills/Protocol/ListSkills.cs`, `GetSkill.cs`	New. Required request/result shapes and protocol-gated cache hints
`Extensions.Skills/McpSkillsJsonContext.cs`	New. Source-generated serialization for AOT
`Extensions.Skills/Server/IMcpSkillCatalog.cs`, `McpSkillPage.cs`, `McpSkillRequestContext.cs`	New. Request-aware catalog abstraction and caller context
`Extensions.Skills/Server/InMemoryMcpSkillCatalog.cs`	New. Keyset cursors, strict UTF-8 decoding, validated owned snapshots and a narrow built-in fast path
`Extensions.Skills/Server/McpSkillsBuilderExtensions.cs`, `McpSkillsOptions.cs`	New. `WithSkills()`, directory discovery, safe selection defaults and options
`Extensions.Skills/Server/McpServerSkill*.cs`, `SkillFrontmatter.cs`	New. Manifests computed from served bytes; selected directory files, encoded path names, link rejection and bounded-subset frontmatter parsing
`Extensions.Skills/SkillValidation.cs`	New. Structural validation, Unicode-scalar bounds, string-valued known metadata and native/encoded URI checks without losing traversal guards
`Extensions.Skills/Client`	New. Capability/list/get helpers, bounded aggregation, diagnostic pages and manifest-bound resource verification, including explicit null-content errors
`Core/Protocol/PaginatedResult.cs`, `PaginatedRequest.cs`, `CacheScopeConverter.cs`	Widen pagination constructors to `protected`; expose the existing cache-scope converter
Core discovery / OAuth / tool and prompt binding	Pause the active probe during OAuth while preserving caller/session-initialization cancellation, HTTP-timeout fallback and configured limits; keep injected parameters out of wire schemas without relying on reflection-object identity
`ConformanceServer/Skills/ConformanceSkills.cs`	New. Skill fixture; digests computed from the same bytes the resources serve
`ConformanceServer/Program.cs`, conformance fixture/tests	Register skills/resources, await owned host readiness, use assigned loopback ports and retain bounded startup/cleanup ownership
Skills protocol, server and client tests	New. Serialization, hostile responses, file/catalog ownership, pagination, policy alignment, Unicode/native URI wire behavior and parser regressions
Conformance package / runner tests	Pin alpha.12, select all three Skills scenarios explicitly for both revisions, require successful native process exits and a single positive, complete summary
`samples/SkillsServer`, `samples/SkillsClient`, guide and link workflow	New. Serving and verified-consumption samples; preserve `main` links and validate same-repository paths against the proposed checkout

Validation

Local Windows results for committed source 393c495d, tree b9175a66:

Builds clean across net10.0, net9.0, net8.0 and netstandard2.0, with warnings treated as errors
Skills pack/APICompat passed; the public Core API surface is unchanged from the previously validated 2.0.0-baseline check
Complete Skills-selected Core and HTTP suites, zero failures/skips:

Target	Core Skills cases	HTTP Skills/summary cases
.NET 10	637 passed	22 passed
.NET 9	637 passed	22 passed
.NET 8	637 passed	22 passed
.NET Framework 4.7.2	633 passed	Not an HTTP project target

The .NET 9 reflection-disabled setting remains unchanged. These are the current commit's focused results; earlier broader-suite counts are not presented as new full-suite runs on this commit.

Native AOT win-x64 publish and execution passed; verified native PE, app returned Success!
DocFX 2.78.5 warnings-as-errors passed with zero warnings/errors
SEP-2640 conformance scenarios in pinned 0.2.0-alpha.12, run explicitly against the correct ConformanceServer endpoints:

Scenario	2025-11-25	2026-07-28
`sep-2640-skills-enumeration`	30/30	32/32
`sep-2640-skills-manifest`	6/6	6/6
`sep-2640-skills-directory`	1/1, 6 not applicable (`directoryRead` not declared)	1/1, 6 not applicable

All six scenario/version combinations passed on .NET 10/9/8 with Node 22.23.3 and strict process exits. A printed success summary cannot override a failed process. The summary assertion also rejects zero checks, unequal passed/total counts and duplicate summaries. Earlier Node 24 aborts remain recorded, not retroactively green.

New-head hosted CI, including Linux/macOS, must be checked separately after publication.

resources/directory/read is not implemented in this PR, so directoryRead is not declared. Client wrappers are included; the results above are server-side conformance, not a claim to implement every host policy. Hosts must still parse fetched SKILL.md and compare frontmatter before loading. For YAML outside the bounded reader, explicit metadata remains the caller's full-validation responsibility. A maintained YAML library/provider, full host approval binding and origin isolation remain separate scope.

Alumniminium · 2026-09-08T07:56:05Z

Lets get this merged asap please

PederHP · 2026-09-08T19:42:03Z

At least one sample should be included imo.

Make CacheScopeConverter public in Core instead of compiling its source into the Skills package, which was the only package doing so. Adding a public type passes package validation against the 2.0.0 baseline. Validate only what the Agent Skills specification states as requirements beyond name and description: compatibility is 1 to 500 characters if provided, and metadata is a mapping. license, allowed-tools, and the values inside metadata pass through verbatim. Hosts compare frontmatter against the file and ignore allowed-tools for MCP-origin skills, so rejecting a whole skill over the shape of such a field would help nobody. Repeat the authorization caveat on WithSkills(IEnumerable<McpServerSkill>) and WithSkillsFromDirectory, whose in-memory catalog serves every caller, and explain why a directory without SKILL.md is skipped while an invalid skill is an error. Guard the public verifier and client read path against a Skill whose required Resources property was assigned null, and document SkillVerificationException on the remaining GetSkillAsync overload. Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com> Claude-Session: https://claude.ai/code/session_01P6omaTvQiryHtzFHRqUE3b

tarekgh · 2026-10-08T18:11:24Z

@PederHP I see that you’ve opened PR #1864. Do you think we should keep both PRs, or would it make sense to close one in favor of the other?

PederHP · 2026-10-09T17:35:26Z

@PederHP I see that you’ve opened PR #1864: #1864. Do you think we should keep both PRs, or would it make sense to close one in favor of the other?

I think we should pick one. I was a little impatient when I made the other, as it represented what I'd like to have merged before the MCPCon in September. From what I remember the one I opened was more comprehensive, building on this one, and I put the author of this one as co-author of the foundational elements that I reused.

I would suggest having one or more agents compare the two and pick based on that.

…ected Both base classes had private protected constructors, so a package outside Core could not implement a paginated method. The Skills extension's skills/list is paginated. Widening to protected is source and binary compatible and passes package validation against the 2.0.0 baseline. Carried over from modelcontextprotocol#1856. Co-authored-by: Girish Konda <girish.sai1@gmail.com> Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com> Claude-Session: https://claude.ai/code/session_01P6omaTvQiryHtzFHRqUE3b

Implements the MCP Skills extension (io.modelcontextprotocol/skills) as a separate package alongside Extensions.Apps and Extensions.Tasks. Protocol: Skill, SkillResource, SkillResources (a closed array-or-"dynamic" union whose converter rejects null and every other shape), and the skills/list and skills/get request and result types, source-generated for AOT. ListSkillsResult links Core's internal CacheScopeConverter so unknown cacheScope values do not break deserialization of a listing. Server: McpServerSkill.Create and CreateFromDirectory build a skill's entry and its file resources together, computing each digest and size from the bytes the resource serves. Text is served as TextResourceContents only when the bytes round-trip through UTF-8, since hosts hash the UTF-8 of the text they receive; everything else is a blob. WithSkills(IEnumerable<McpServerSkill>) registers the methods, the extension and resources capabilities, and every file resource; WithSkills(IMcpSkillCatalog) backs the methods with a custom catalog. InMemoryMcpSkillCatalog uses keyset cursors. Every entry is validated at construction against the specification's structural MUSTs. Client: SupportsSkills, ListSkillsAsync (all pages or one), GetSkillAsync, and ReadSkillResourceAsync, which reads through resources/read and verifies size and digest against the held entry, refusing unlisted URIs before any request is sent. SkillVerifier exposes the same checks. resultType, ttlMs, and cacheScope are emitted only on requests negotiated under 2026-07-28 or later, where the latter two default to 0 and private when unset, matching Core's handling of the built-in list methods (modelcontextprotocol#1721). YAML frontmatter parsing and resources/directory/read are deliberately not included; see the docs page and PR description. The SkillResources union and converter and the keyset-cursor catalog design are carried over from modelcontextprotocol#1856. Co-authored-by: Girish Konda <girish.sai1@gmail.com> Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com> Claude-Session: https://claude.ai/code/session_01P6omaTvQiryHtzFHRqUE3b

Unit tests cover serialization (including every invalid manifest shape and frontmatter pass-through), the in-memory catalog (ordering, keyset pagination, cursor validation, and each structural validation rule), McpServerSkill (manifest computation, path normalization, resource metadata, directory loading), and SkillVerifier (known digest vectors, size and digest mismatches, text versus blob, unlisted files). End-to-end tests drive the client extensions against an in-process server for both WithSkills overloads: listing and pagination, skills served but not listed, dynamic skills, verified reads including a tampered resource, the -32602 error contract, and the wire shape of resultType, ttlMs, and cacheScope on 2025-11-25 and 2026-07-28 sessions, including the defaults applied when no options are set. The conformance server gains the SEP-2640 fixture: three static skills built with McpServerSkill plus a dynamic one, paged two at a time. The SEP-2640 scenarios from modelcontextprotocol/conformance#330 pass 30/30, 6/6, 1/1 stateless and 29/29, 6/6, 1/1 on a 2025-11-25 session. The AOT compatibility app now also lists, gets, and verifies a skill. The serialization tests and the fixture's skill set are carried over from modelcontextprotocol#1856. Co-authored-by: Girish Konda <girish.sai1@gmail.com> Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com> Claude-Session: https://claude.ai/code/session_01P6omaTvQiryHtzFHRqUE3b

Make CacheScopeConverter public in Core instead of compiling its source into the Skills package, which was the only package doing so. Adding a public type passes package validation against the 2.0.0 baseline. Validate only what the Agent Skills specification states as requirements beyond name and description: compatibility is 1 to 500 characters if provided, and metadata is a mapping. license, allowed-tools, and the values inside metadata pass through verbatim. Hosts compare frontmatter against the file and ignore allowed-tools for MCP-origin skills, so rejecting a whole skill over the shape of such a field would help nobody. Repeat the authorization caveat on WithSkills(IEnumerable<McpServerSkill>) and WithSkillsFromDirectory, whose in-memory catalog serves every caller, and explain why a directory without SKILL.md is skipped while an invalid skill is an error. Guard the public verifier and client read path against a Skill whose required Resources property was assigned null, and document SkillVerificationException on the remaining GetSkillAsync overload. Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com> Claude-Session: https://claude.ai/code/session_01P6omaTvQiryHtzFHRqUE3b

Import the net changes from PR modelcontextprotocol#1864 on top of the rebased modelcontextprotocol#1856 foundation, including the client, request-aware catalogs, validation, byte-snapshot authoring, YAML reader, samples, docs and regression tests. The shared foundation is retained once, not replayed. Source PR: modelcontextprotocol#1864 Source head: ecb11e6 Same-main source tree: ba9234d Source lineage (including already-shared foundation): 4a7d3f5 Widen PaginatedRequestParams and PaginatedResult constructors to protected 9569706 Add ModelContextProtocol.Extensions.Skills (SEP-2640) 50f7cd7 Add tests and a conformance fixture for the Skills extension 44d80ae Add Skills samples and docs 53e5425 Harden skill authoring and verified reads b2e92aa Read skill frontmatter from SKILL.md 9dc02bb Align the frontmatter reader with reference YAML parsers 72a0406 Point SkillVerifier remarks at SkillFrontmatter for frontmatter checks 932c318 Link the Skills docs page to the samples by relative path f18d7dc Address the Copilot review of modelcontextprotocol#1864 8c39436 Address the second Copilot review of modelcontextprotocol#1864 8593f0d Bind skills/get answers to the requested URI 82f0943 Enforce skill limits while reading a directory; match Core's URI equivalence fe89998 Address review feedback from the author of modelcontextprotocol#1856 4813b72 Complete earlier fixes and align the reader further with reference parsers ecb11e6 Close the neighbouring cases a differential YAML run and a second review pass found This local squash preserves human authorship, not source signatures. No consolidation repairs are included in this import. Co-authored-by: Girish Konda <girish.sai1@gmail.com>

girishkvs · 2026-10-11T20:10:38Z

@PederHP @tarekgh @Alumniminium

I compared #1856 and #1864 and found a few remaining gaps in #1864. I pulled Peder's changes into this #1856, preserving his authorship and crediting him as co-author, and then added my fixes on top of it.

I kept his client helpers, entry validation, file authoring, samples and docs. I also fixed the skills/get cache hints missing from both PRs.

Comparison and fixes

Compared sources

Original Add SEP-2640 Skills extension support #1856: cbd8a588.
Peder's Add the Skills extension (SEP-2640) as ModelContextProtocol.Extensions.Skills #1864: ecb11e62.
Common comparison base: 3338e88e.
Consolidated update: 6b0870cb.

Both PRs were compared on the same main. Both original heads had the old SourceLink NU1902 failure. The update already on main clears it; that isn't a new Skills fix.

Implementation comparison

Area	Original #1856	Peder's #1864	This update
Protocol foundation	Request/result types, `SkillResources` union, source-generated serialization, catalog and conformance fixtures	Reuses those foundations, with additional APIs and validation	Kept once, with the existing credit
Capabilities and version handling	Bare catalog registration lacks the `resources` declaration; current defaults and legacy `resultType` behavior have gaps	Declares `resources`, supplies list defaults and gates current-only fields	Peder's behavior retained, including older-protocol compatibility
`skills/get` cache hints	Missing	Missing	Added required `ttlMs` and `cacheScope` on current requests, including defaults and configured values; omitted on legacy requests
Request handling	Incorrectly typed `uri`/`cursor` can return `InternalError`	Typed parameter handling returns `InvalidParams`	Peder's handling retained
Catalog ownership	Caller-owned mutable entries	Defensive snapshots and keyset pagination	Peder's implementation retained
Caller context	Catalog interface has no request context	Catalog receives request context and caller identity	Retained; applications still need to apply their authorization policy consistently to entries and resources
Client APIs	No dedicated client helpers	Capability checks, list/get helpers and manifest-bound resource reads	Retained, including rejection of unlisted files and binding reads to the requested URI
Byte/file authoring	Application supplies metadata and resources	Manifest and served resources use the same copied bytes; directory helpers enforce file/URI boundaries	Retained
YAML convenience	Application supplies a `JsonObject`	Optional hand-written subset reader and automatic authoring	Reader retained; confirmed parsing errors fixed; unsupported-format and caller responsibilities stated explicitly
Samples and docs	No standalone Skills client/server samples	Both samples and a guide	Retained and updated; the client sample now parses fetched frontmatter and compares it before displaying the file
AOT and packaging	Generated metadata and pagination constructor changes	Adds Skills-specific AOT coverage and package integration	Retained, with additional assertions for our cache/parser changes

The stable spec added the skills/get cache requirement after both implementations. Both needed this fix; it doesn't invalidate their earlier conformance results.

Additional corrections

skills/get cache hints. Added ICacheableResult and set the fields in the raw handler. Defaults are 0 / private; configured values are honored. The interface alone wouldn't set them.
Next-line YAML values. Valid plain, quoted and nested values no longer fail because they start on the next line.
Folded TAB-led content. Kept the line breaks instead of silently replacing them with spaces.
Trailing comments. Require whitespace after a quoted or flow value before its comment. Plain values containing # still work.
Flow-collection keys. Kept the existing unsupported-YAML path so explicit metadata still works. Added regression cases alongside ordinary flow values and quoted keys.
Client sample verification. Kept the manifest, URI, digest and size checks. The sample now also parses fetched SKILL.md and compares its frontmatter before displaying it.
Docs and links. Clarified what the application, authoring helper and host each validate. Fixed the two sample links using a published source snapshot.

The trailing-comment check matches the yaml reference parser. Some other YAML libraries accept those forms, so this isn't a failure in every host.

Parser responsibility

I kept Peder's parser as an optional authoring helper. There's no new YAML dependency.

Automatic authoring requires the bounded reader to succeed.
With explicit metadata, the application must validate the complete file and supply matching JSON. Existing checks remain, but an unsupported-parser exception doesn't prove the rest of the YAML is valid.
Hosts must independently parse fetched frontmatter and compare it field by field before loading. Digest verification alone is not enough.

This keeps the separation used in the TypeScript and Python proposals without dropping the authoring helpers. Go also separates digest checks from frontmatter verification. Those peer PRs were open when inspected, not released implementations.

The explicit-metadata fallback still can't validate the rest of a file after unsupported syntax. I haven't claimed that as a parser fix. The docs now state the caller's responsibility, and hosts must still verify the fetched file. A maintained YAML-library/provider can be a separate follow-up.

History and credit

3338e88e  Current main
  47b9db7d  Original implementation - Girish
    f3d028b7  Peder's additional implementation - Peder
      6b0870cb  Added fixes and responsibility alignment - Girish

Peder is the author of the import commit, which records #1864 and the source commits. Before our fixes, its full tree is ba9234da7a7a3ff5ad2a4e78ca9308a72ad004d9, identical to his selected head. Our fixes are separately authored.

Validation of the consolidated commit

These results are for 6b0870cb on Windows x64.

Check	Result
Complete Skills suite, .NET 10 / 9 / 8	470 passed on each target; zero failures or skips
Complete Skills suite, .NET Framework 4.7.2	466 passed; four modern-.NET-only symlink cases are not compiled on this target
Full non-manual .NET 10 suite	2,860 passed, zero failures, four declared skips
Pack and API compatibility	Passed; Core checked against the existing 2.0.0 baseline
NativeAOT, Windows x64	Published and executed the Skills list/get/read/verify and parser/cache assertions successfully
Source/artifact identity	Package metadata, informational versions and production SourceLink identify `6b0870cb`; 31 PE/PDB pairs passed source/binary inspection
Client/server samples and documentation links	Samples passed; normal related and full documentation link checks reported zero errors
Skills conformance, `2026-07-28`	39 applicable checks passed; all three scenario processes exited zero. Six optional `directoryRead` checks were not applicable
Skills conformance, `2025-11-25`	Enumeration exited zero with 30 checks passed and two version-inapplicable cache checks. Manifest and directory processes aborted with `UV_HANDLE_CLOSING` after printing successful checks; neither process is counted green

The full-suite filter excludes two declared manual Docker cases. The four skips are two API-key-dependent cases, one unsupported-platform case and one optional-default-extraction limitation. No failing assertion or audit check was disabled.

The Windows legacy runner shutdown failure is still open. This isn't an all-green conformance matrix, and local results don't replace hosted CI.

Remaining scope and review decisions

No new general YAML dependency, directoryRead support, full host implementation or general Core authorization hook is included.
Package placement, the public reader and Core API changes still need maintainer review.
Hosted CI hasn't run on this head. The earlier Ubuntu Debug OAuth failure isn't fixed by these Skills changes.
Add the Skills extension (SEP-2640) as ModelContextProtocol.Extensions.Skills #1864 remains open; this update doesn't assume agreement to close it.

— girishkvs's Copilot 🤖

PederHP · 2026-10-12T16:28:57Z

Looks good. I made a PR (girishkvs#1) with some suggestions mostly related to docs and description. I was going to close #1864 so we can focus on getting this one over the finish line, but I was beat to it by a few minutes. Very much agree on joining forces and thank you for taking in what you did from the other one.

Copilot

🟡 Changes recommended

Protocol conformance, authorization exposure, validation gaps, and inaccurate PR metadata remain unresolved.

7 open findings

Apply authorization to skill catalog handlers · New
Require skills property to reject omitted response members · New
Reject invalid UTF-8 in cursors · New
Validate Unicode lowercase letters by scalar value · New
Enforce string-only metadata values · New
Document or revert unintended public converter API · New
Document newly added public client wrappers · New

What changed in this PR

Adds SEP-2640 Skills support, including protocol models, server/client APIs, validation, samples, conformance fixtures, and AOT coverage.

Changes:

Adds Skills discovery, retrieval, authoring, and integrity verification.
Adds directory-backed and custom catalogs with pagination.
Expands Core pagination and cache-converter APIs.

File	Description
`README.md`	Lists the Skills package.
`ModelContextProtocol.slnx`	Adds Skills projects and samples.
`docs/concepts/index.md`	Links Skills documentation.
`docs/concepts/toc.yml`	Adds Skills navigation.
`docs/concepts/skills/skills.md`	Documents serving, consuming, and security.
`src/Directory.Build.targets`	Includes Skills in package dependency versioning.
`src/PACKAGE.md`	Lists the Skills package.
`src/ModelContextProtocol.Core/Protocol/CacheScopeConverter.cs`	Makes the converter public.
`src/ModelContextProtocol.Core/Protocol/PaginatedRequest.cs`	Allows external derivation.
`src/ModelContextProtocol.Core/Protocol/PaginatedResult.cs`	Allows external derivation.
`src/ModelContextProtocol.Extensions.Skills/ModelContextProtocol.Extensions.Skills.csproj`	Defines the new package.
`src/ModelContextProtocol.Extensions.Skills/SkillsProtocol.cs`	Defines protocol constants and limits.
`src/ModelContextProtocol.Extensions.Skills/McpSkillsJsonContext.cs`	Adds AOT serialization metadata.
`src/ModelContextProtocol.Extensions.Skills/SkillFrontmatter.cs`	Implements bounded YAML frontmatter parsing.
`src/ModelContextProtocol.Extensions.Skills/SkillValidation.cs`	Validates skill entries and manifests.
`src/ModelContextProtocol.Extensions.Skills/Protocol/Skill.cs`	Defines skill entries.
`src/ModelContextProtocol.Extensions.Skills/Protocol/SkillResource.cs`	Defines manifest files.
`src/ModelContextProtocol.Extensions.Skills/Protocol/SkillResources.cs`	Models static or dynamic manifests.
`src/ModelContextProtocol.Extensions.Skills/Protocol/SkillResourcesConverter.cs`	Serializes the manifest union.
`src/ModelContextProtocol.Extensions.Skills/Protocol/ListSkillsRequestParams.cs`	Defines list parameters.
`src/ModelContextProtocol.Extensions.Skills/Protocol/ListSkillsResult.cs`	Defines paginated list results.
`src/ModelContextProtocol.Extensions.Skills/Protocol/GetSkillRequestParams.cs`	Defines get parameters.
`src/ModelContextProtocol.Extensions.Skills/Protocol/GetSkillResult.cs`	Defines get results.
`src/ModelContextProtocol.Extensions.Skills/Server/IMcpSkillCatalog.cs`	Defines the catalog contract.
`src/ModelContextProtocol.Extensions.Skills/Server/InMemoryMcpSkillCatalog.cs`	Implements keyset pagination.
`src/ModelContextProtocol.Extensions.Skills/Server/McpSkillPage.cs`	Models catalog pages.
`src/ModelContextProtocol.Extensions.Skills/Server/McpSkillRequestContext.cs`	Exposes caller context to catalogs.
`src/ModelContextProtocol.Extensions.Skills/Server/McpServerSkill.cs`	Authors entries and file resources.
`src/ModelContextProtocol.Extensions.Skills/Server/McpServerSkillFile.cs`	Models authored files.
`src/ModelContextProtocol.Extensions.Skills/Server/McpSkillsBuilderExtensions.cs`	Registers Skills handlers and resources.
`src/ModelContextProtocol.Extensions.Skills/Server/McpSkillsOptions.cs`	Configures caching hints.
`src/ModelContextProtocol.Extensions.Skills/Client/McpSkillsClientExtensions.cs`	Adds client Skills operations.
`src/ModelContextProtocol.Extensions.Skills/Client/SkillVerifier.cs`	Verifies resource integrity.
`src/ModelContextProtocol.Extensions.Skills/Client/SkillVerificationException.cs`	Defines verification failures.
`samples/SkillsClient/SkillsClient.csproj`	Defines the client sample.
`samples/SkillsClient/Program.cs`	Demonstrates discovery and verification.
`samples/SkillsClient/README.md`	Documents the client sample.
`samples/SkillsServer/SkillsServer.csproj`	Defines the server sample.
`samples/SkillsServer/Program.cs`	Configures directory-backed skills.
`samples/SkillsServer/README.md`	Documents the server sample.
`samples/SkillsServer/Properties/launchSettings.json`	Configures the sample endpoint.
`samples/SkillsServer/Skills/git-workflow/SKILL.md`	Adds a Git workflow skill.
`samples/SkillsServer/Skills/git-workflow/references/COMMIT_STYLE.md`	Adds commit guidance.
`samples/SkillsServer/Skills/git-workflow/templates/PULL_REQUEST.md`	Adds a pull-request template.
`samples/SkillsServer/Skills/refunds/SKILL.md`	Adds a refunds skill.
`samples/SkillsServer/Skills/refunds/policy/REFUND_POLICY.md`	Adds refund policy data.
`samples/SkillsServer/Skills/refunds/examples/approved.md`	Adds an approval response.
`samples/SkillsServer/Skills/refunds/examples/declined.md`	Adds a decline response.
`tests/ModelContextProtocol.AotCompatibility.TestApp/ModelContextProtocol.AotCompatibility.TestApp.csproj`	Roots the Skills assembly for AOT.
`tests/ModelContextProtocol.AotCompatibility.TestApp/Program.cs`	Exercises Skills under AOT.
`tests/ModelContextProtocol.ConformanceServer/ModelContextProtocol.ConformanceServer.csproj`	References the Skills package.
`tests/ModelContextProtocol.ConformanceServer/Program.cs`	Registers conformance skills.
`tests/ModelContextProtocol.ConformanceServer/Skills/ConformanceSkills.cs`	Defines conformance fixtures.
`tests/ModelContextProtocol.Tests/ModelContextProtocol.Tests.csproj`	References the Skills package.
`tests/ModelContextProtocol.Tests/ClientServerTestBase.cs`	Enables derived async cleanup.
`tests/ModelContextProtocol.Tests/Protocol/SkillSerializationTests.cs`	Tests protocol serialization.
`tests/ModelContextProtocol.Tests/Client/McpSkillsClientValidationTests.cs`	Tests hostile server responses.
`tests/ModelContextProtocol.Tests/Client/SkillVerifierTests.cs`	Tests digest verification.
`tests/ModelContextProtocol.Tests/Server/InMemoryMcpSkillCatalogTests.cs`	Tests catalog behavior.
`tests/ModelContextProtocol.Tests/Server/McpServerSkillAuthoringBoundaryTests.cs`	Tests authoring boundaries.
`tests/ModelContextProtocol.Tests/Server/McpServerSkillTests.cs`	Tests skill authoring.
`tests/ModelContextProtocol.Tests/Server/McpServerSkillsCatalogTests.cs`	Tests custom catalogs.
`tests/ModelContextProtocol.Tests/Server/McpServerSkillsDefaultsTests.cs`	Tests default cache hints.
`tests/ModelContextProtocol.Tests/Server/McpServerSkillsFromDirectoryTests.cs`	Tests directory loading.
`tests/ModelContextProtocol.Tests/Server/McpServerSkillsIntegrityTests.cs`	Tests content integrity.
`tests/ModelContextProtocol.Tests/Server/McpServerSkillsOwnershipTests.cs`	Tests metadata and byte ownership.
`tests/ModelContextProtocol.Tests/Server/McpServerSkillsTests.cs`	Tests end-to-end Skills behavior.
`tests/ModelContextProtocol.Tests/Server/SkillFrontmatterCorpusTests.cs`	Tests parser compatibility corpus.
`tests/ModelContextProtocol.Tests/Server/SkillFrontmatterNodeClassificationTests.cs`	Tests YAML node classification.
`tests/ModelContextProtocol.Tests/Server/SkillFrontmatterRegressionTests.cs`	Adds parser regression coverage.
`tests/ModelContextProtocol.Tests/Server/SkillFrontmatterTests.cs`	Tests frontmatter parsing.

🧠 Review effort: Balanced

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tarekgh · 2026-10-12T18:12:16Z

Should we upgrade the conformance test package to 0.2.0-alpha.12 to ensure the added skills tests are covered? By the way, I already ran these tests against the PR, and all passed.

girishkvs · 2026-10-13T22:05:07Z

@tarekgh Re your alpha.12 question:

Pinned alpha.12 and added explicit Skills scenarios for both protocol revisions; a nonzero runner exit now fails even if stdout reports success, and the tests require actual executed checks.

— girishkvs's Copilot 🤖

Adds ModelContextProtocol.Extensions.Skills implementing the skills/list and skills/get methods of SEP-2640, alongside the existing Tasks and Apps extension packages. Declares the io.modelcontextprotocol/skills capability inline and registers both methods through McpServerOptions.RequestHandlers, so Core is unchanged. Skill manifests are modelled as SkillResources, a closed type admitting only an enumerated file list or the dynamic marker, so an invalid manifest cannot be constructed. Pagination uses keyset cursors over URI order. skills/get sets RoutingNameParameter to uri, matching how Core routes resources/read for the Mcp-Name header. ttlMs and cacheScope are gated on the negotiated protocol version by default, which SEP-2640 currently requires, but the gate is configurable while the working group settles whether that condition is removed.

Import the net changes from PR modelcontextprotocol#1864 on top of the rebased modelcontextprotocol#1856 foundation, including the client, request-aware catalogs, validation, byte-snapshot authoring, YAML reader, samples, docs and regression tests. The shared foundation is retained once, not replayed. Source PR: modelcontextprotocol#1864 Source head: ecb11e6 Same-main source tree: ba9234d Source lineage (including already-shared foundation): 4a7d3f5 Widen PaginatedRequestParams and PaginatedResult constructors to protected 9569706 Add ModelContextProtocol.Extensions.Skills (SEP-2640) 50f7cd7 Add tests and a conformance fixture for the Skills extension 44d80ae Add Skills samples and docs 53e5425 Harden skill authoring and verified reads b2e92aa Read skill frontmatter from SKILL.md 9dc02bb Align the frontmatter reader with reference YAML parsers 72a0406 Point SkillVerifier remarks at SkillFrontmatter for frontmatter checks 932c318 Link the Skills docs page to the samples by relative path f18d7dc Address the Copilot review of modelcontextprotocol#1864 8c39436 Address the second Copilot review of modelcontextprotocol#1864 8593f0d Bind skills/get answers to the requested URI 82f0943 Enforce skill limits while reading a directory; match Core's URI equivalence fe89998 Address review feedback from the author of modelcontextprotocol#1856 4813b72 Complete earlier fixes and align the reader further with reference parsers ecb11e6 Close the neighbouring cases a differential YAML run and a second review pass found This local squash preserves human authorship, not source signatures. No consolidation repairs are included in this import. Co-authored-by: Girish Konda <girish.sai1@gmail.com>

Add ICacheableResult and source-generated cache-hint serialization to skills/get. Apply the configured hints or zero/private defaults on 2026-07-28 requests, and keep all versioned fields off 2025-11-25 replies. Read scalar values starting on the next line, preserve folded line breaks around TAB-led content, and reject comments without whitespace after a quoted scalar or flow collection. Keep plain hashes, existing chomping rules and non-JSON number rejection intact. Add parser, authoring, wire and serialization regressions. Exercise the new cache fields and authoring cases in the native AOT app. Update the cache option docs and pin both sample links to their published source.

Consolidate frontmatter verification responsibilities and use main-branch sample links. Include both commits from PederHP's documentation pull request, without our follow-up changes.

Address Skills review feedback with strict entry validation, safe directory selection, bounded aggregation, explicit diagnostic pages, and validated catalog snapshots. Exercise Skills conformance with strict process exits. Count Unicode scalars consistently, retain entry metadata in owned snapshots, and authorize catalog entries before pagination. Support encoded names and native absolute resource URIs without rewriting resource identity. Reject unsupported flow mappings instead of publishing incorrect frontmatter. Pack octal integers once, preserve arbitrary precision, and report null read contents as verification failures. Strengthen conformance summary assertions and cover nested Unicode and native URI resource reads. Keep discovery probe time separate from OAuth work while preserving caller cancellation and configured budgets. Keep injected parameters out of wire schemas without relying on reflection-object identity. Use owned conformance-host readiness, bounded startup and cleanup, and deterministic cancellation-observation tests. Validate repository links against the proposed checkout.

PederHP

Did a review with Claude. Found a few issues. I have a preference for not hard-failing during validation on anything that isn't a "MUST NOT" (skills can be all over the place and it'll just lead to instant issue reports).

PederHP · 2026-10-17T18:49:18Z

+        var options = new McpSkillsOptions();
+        configure?.Invoke(options);
+
+        builder.Services.AddSingleton<IConfigureOptions<McpServerOptions>>(


Calling WithSkills/WithSkillsFromDirectory twice (for example two skill roots, or a library plus the app) registers the handlers twice, and the server fails at startup with Core's A custom request handler ... uses the method 'skills/list', which is already handled by the server (reproduced with two WithSkillsFromDirectory calls). Could registrations accumulate into one catalog with the handlers registered once, or at least fail with a Skills-specific error at registration time?

— PederHP's Claude 🤖

PederHP · 2026-10-17T18:49:18Z

+                }
+
+                registeredFiles.Add(key, (entry.Uri, entry.Digest));
+                builder.Services.AddSingleton(skill.Resources[i]);


Shared files are checked across skills, but not against resources the app registered itself. With .WithResources([...UriTemplate = "skill://git-workflow/SKILL.md"...]).WithSkillsFromDirectory(...) the app's resource is served and every verified read fails (is 11 bytes, but its manifest entry declares 724 bytes), with no startup error. Could this be detected once the server's resource collection is populated?

— PederHP's Claude 🤖

PederHP · 2026-10-17T18:49:18Z

+            // Hexadecimal and octal integers take no sign in the core schema, and may exceed 64 bits.
+            if (text.StartsWith("0x", StringComparison.Ordinal) && text.Length > 2 && IsAll(text, 2, IsHexDigit))
+            {
+                var value = BigInteger.Parse("0" + text.Substring(2), NumberStyles.AllowHexSpecifier, CultureInfo.InvariantCulture);


Hex and octal integers go through BigInteger → decimal text, which is quadratic in digit count: v: 0x followed by 50k/100k/200k Fs takes 0.6 s / 1.5 s / 6.3 s on net10.0. Hosts are told to parse fetched SKILL.md from untrusted servers with this (the sample does), and Parse has no cancellation, so a server can pin a host CPU with a few hundred KB (16 MiB is allowed per skill). Suggest capping the literal length, or rejecting or stringifying values wider than 64 bits, and adding an overall frontmatter size cap. Same at line 1254.

— PederHP's Claude 🤖

PederHP · 2026-10-17T18:49:18Z

+/// <see cref="FormatException"/> from it is a failed verification unless the host parses the file another way.
+/// </para>
+/// </remarks>
+public static class SkillFrontmatter


If the package ships stable, this makes a deliberately partial YAML reader public API, so later changes to what it accepts or how it types scalars become breaking. Worth deciding explicitly whether it should be public now, or internal until there's a fuller parser story. Related: UnsupportedYamlException is internal, so callers can't tell "unsupported, try a full parser" from "malformed", which the remarks ask hosts to do.

— PederHP's Claude 🤖

PederHP · 2026-10-17T18:49:18Z

+            // The application must already have validated the entire frontmatter and supplied matching JSON.
+            // Reaching an unsupported construct establishes neither whole-document validity nor equality.
+        }
+        catch (FormatException e)


The explicit-frontmatter overload tolerates only UnsupportedYamlException; any other FormatException still throws even when the caller supplies correct JSON. For example, valid YAML tags: [? a] with matching frontmatter {"tags":[{"a":null}]} fails with a plain scalar cannot start with '?'.... Since this overload is the escape hatch for YAML the reader can't handle, should reader rejections be tolerated when the caller supplied the frontmatter?

— PederHP's Claude 🤖

PederHP · 2026-10-17T18:49:18Z

+                // Schema generation can enumerate distinct ParameterInfo instances, including
+                // when the runtime's cold parameter cache is initialized concurrently.
+                IncludeParameter = parameter =>
+                    !ConfigureParameterBinding(parameter).ExcludeFromSchema &&


This works around MEAI matching parameters by ParameterInfo identity, which was fixed upstream in dotnet/extensions#7677 (first shipped in 10.9.0; we pin 10.8.3). I'd rather bump MEAI in a separate PR and keep InjectedParameterSchemaTests as the regression test. As written, ConfigureParameterBinding also now runs twice per parameter. The same applies to AIFunctionMcpServerPrompt.

— PederHP's Claude 🤖

PederHP · 2026-10-17T18:49:18Z

-                        probeCts.CancelAfter(probeTimeout);
-                    }
+                    bool initializationBoundsProbe =
+                        _options.InitializationTimeout != Timeout.InfiniteTimeSpan &&


Behavior change: with InitializationTimeout = Timeout.InfiniteTimeSpan, main applied no probe timeout (-1 ms > probeTimeout is false), whereas now the 5 s probe applies. Arguably that is what the docs promise, but it should be called out, which is another reason to land the DiscoverProbe/OAuth work as its own PR with a changelog entry.

— PederHP's Claude 🤖

PederHP · 2026-10-17T18:49:18Z

+
+// 2. Enumerate. A listing may be empty or partial; a skill can always be fetched by URI.
+Console.WriteLine("=== skills/list ===");
+ListSkillsResult firstPage = await client.ListSkillsAsync(new ListSkillsRequestParams());


Nit: this sends a separate skills/list only to print the cache hints, then lists everything again on line 40. Defaulting requestParams isn't an option: it would be ambiguous with the parameterless overload (CS0121), and Core's ListToolsAsync(ListToolsRequestParams) has the same shape. I'd fix the sample instead: drop the first-page call, or page with the paged overload and print hints and entries from the same responses.

— PederHP's Claude 🤖

PederHP · 2026-10-17T18:49:18Z

+    /// <param name="includeHiddenAndSystemFiles">Whether to include dot-prefixed, hidden, and system entries.</param>
+    /// <returns>The skill with its manifest computed from the selected files.</returns>
+    /// <remarks>Included symbolic links remain errors. See <see cref="CreateFromDirectory(string)"/>.</remarks>
+    public static McpServerSkill CreateFromDirectory(string directoryPath, bool includeHiddenAndSystemFiles)


Nit (API shape, worth settling before release): the includeHiddenAndSystemFiles bool doubles the CreateFromDirectory/WithSkillsFromDirectory overloads. An options type (IncludeHiddenAndSystemFiles, UriPrefix, maybe a file filter) would stay extensible without more overloads.

— PederHP's Claude 🤖

PederHP · 2026-10-17T18:49:18Z

+    /// </summary>
+    /// <param name="jsonRpcRequest">The JSON-RPC request being answered.</param>
+    /// <exception cref="ArgumentNullException"><paramref name="jsonRpcRequest"/> is <see langword="null"/>.</exception>
+    public McpSkillRequestContext(JsonRpcRequest jsonRpcRequest)


Nit: a custom catalog gets no IServiceProvider or McpServer here, and WithSkills(IMcpSkillCatalog) only takes an instance, so a database-backed catalog can't reach scoped services. Consider a WithSkills<TCatalog>() or factory overload and exposing services on the context. That may need Core plumbing for raw handlers, so it's fine as a follow-up.

— PederHP's Claude 🤖

panyam mentioned this pull request Sep 7, 2026

feat(sep-2640): skills server conformance against the Accepted SEP, enumeration + manifest + directory modelcontextprotocol/conformance#330

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7 of 9 tasks

sambhav mentioned this pull request Sep 8, 2026

docs: add Skills extension overview and support tracking modelcontextprotocol/modelcontextprotocol#3353

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This was referenced Sep 9, 2026

Add support for the Skills extension (SEP-2640) #1863

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Add the Skills extension (SEP-2640) as ModelContextProtocol.Extensions.Skills #1864

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pja-ant mentioned this pull request Sep 11, 2026

SEP-2640: Skills Extension modelcontextprotocol/modelcontextprotocol#2640

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This was referenced Sep 22, 2026

SEP-2640: Skills Extension modelcontextprotocol/java-sdk#1140

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feat: Add support for SEP-2640 Skills extension modelcontextprotocol/java-sdk#1141

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girishkvs force-pushed the skills_sep2640 branch from cbd8a58 to 6b0870c Compare October 11, 2026 20:10

PederHP mentioned this pull request Oct 12, 2026

Unpin the Skills sample links and consolidate the frontmatter-verification docs girishkvs/csharp-sdk#1

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tarekgh requested review from halter73 and a balanced review from Copilot October 12, 2026 16:24

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RickWinter mentioned this pull request Oct 16, 2026

Support publishing and retrieving Agent Skills over MCP microsoft/mcp#3655

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girishkvs force-pushed the skills_sep2640 branch from 4837a3f to 393c495 Compare October 16, 2026 23:43

girishkvs and others added 5 commits October 16, 2026 18:25

docs: consolidate Skills verification guidance and sample links

c70b619

Consolidate frontmatter verification responsibilities and use main-branch sample links. Include both commits from PederHP's documentation pull request, without our follow-up changes.

girishkvs force-pushed the skills_sep2640 branch from 9cbffe7 to 262354c Compare October 17, 2026 01:40

girishkvs requested review from PederHP and tarekgh October 17, 2026 01:41

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girishkvs commented Oct 11, 2026

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