feat: load skills from a directory tree by DaleSeo · Pull Request #1409 · modelcontextprotocol/rust-sdk

DaleSeo · 2026-10-29T19:36:16Z

Motivation and Context

Follows #1392.

It adds SkillCatalog behind a new skills-fs feature. It loads each folder under a root that contains a SKILL.md, then handles skills/list, skills/get, resources/read, and resources/directory/read using an in-memory snapshot.

How Has This Been Tested?

Add tests

Breaking Changes

None.

Types of changes

Bug fix (non-breaking change which fixes an issue)
New feature (non-breaking change which adds functionality)
Breaking change (fix or feature that would cause existing functionality to change)
Documentation update

Checklist

I have read the MCP Documentation
My code follows the repository's style guidelines
New and existing tests pass locally
I have added appropriate error handling
I have added or updated documentation as needed

github-actions · 2026-10-29T19:36:39Z

@gocamille is on triage rotation this week and is the reviewer for this one.

Labels come from the PR labeler. Please review the change.

_{Automated triage. Labels are a first pass and the assigned maintainer has the final say.}

DaleSeo · 2026-10-29T21:35:00Z

+}
+
+#[derive(Debug)]
+struct CatalogFile {


The snapshot stores the full contents of every file, not just the metadata. That uses more memory, but resources/read always returns the exact bytes used to calculate the digests, even if a file changes on disk after it’s loaded.

DaleSeo · 2026-10-29T21:35:00Z

+
+impl Snapshot {
+    fn load(root: &Path) -> Result<Self, SkillCatalogError> {
+        let handle = Dir::open_ambient_dir(root, ambient_authority()).map_err(|source| {


Every listing and read goes through this root handle, cap-std, never through a joined path. Checking file types while walking the directory and then reading files later with std::fs left a window where a file or folder could be replaced with a symlink, which might lead outside the root. cap-std prevents access outside the handle on both Unix and Windows.

chatgpt-codex-connector · 2026-10-29T21:50:09Z

Codex Review Summary

This comment shows the latest Codex review activity on this pull request.

Review	Status	Commit	Review trigger
📝 Code Review	✅ Completed 2026-11-04T14:49:00.508905Z	`58f424a`	New commits
🔒 Security Review	✅ Completed 2026-11-04T14:51:37.518675Z	`58f424a`	New commits

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chatgpt-codex-connector · 2026-10-29T21:51:46Z

+                let bytes = handle
+                    .read(&entry.path)


Enforce skill limits before reading all file contents

When a skill contains a very large file or an excessive number of files, loading reads and retains every file in bytes_of before build_skill checks SKILL_BYTE_LIMIT and SKILL_FILE_LIMIT. A skill containing a multi-gigabyte asset can therefore exhaust the server's memory during startup or reload instead of being skipped at the documented 16 MiB limit; even skills that will subsequently be rejected are fully loaded. Check file counts before loading and enforce the byte budget with bounded reads so oversized skills cannot exhaust memory before validation.

Useful? React with 👍 / 👎.

tsarlandie-oai

LGTM - Left some minor comments.

tsarlandie-oai · 2026-11-03T21:54:56Z

+    uri
+}
+
+/// Whether files of this type are sent as `text` rather than a base64 `blob`.


We read and send 'text' files as utf8 but some of these file formats (xml) have their own encoding definition. It might not be utf8 and this might lead to us breaking encoding.

@tsarlandie-oai We never transcode. A file is sent as text only if its bytes are valid UTF-8. If it’s UTF-16 with a BOM or Latin-1 with non-ASCII characters, we send it as a base64 blob. Either way, the host gets the exact bytes covered by the digest.

tsarlandie-oai · 2026-11-03T21:58:26Z

+        )
+}
+
+fn mime_type_of(path: &str) -> &'static str {


Maybe we could add a dependency on mime_guess?

It also uses a hardcoded list of mime types but it's much longer and we are less likely to make new releases of rmcp just to update the list ;)

Thanks for the suggestion. I looked into it, but mime_guess maps .ts to video/vnd.dlna.mpeg-tts, so TypeScript scripts in a skill would be sent as blobs. The table only decides whether something is text or a blob, and unknown types just omit mimeType, so I'd keep the list short.

tsarlandie-oai · 2026-11-03T22:19:26Z

+        Some("webp") => "image/webp",
+        Some("pdf") => "application/pdf",
+        Some("zip") => "application/zip",
+        _ => "application/octet-stream",


I believe mimeType is optional in the spec so maybe it would be better to return None here and not include it if we are not sure.

Good call! An unknown extension now leaves out mimeType and is still sent as a blob.

tsarlandie-oai · 2026-11-03T22:23:16Z

+        .collect();
+    files.sort_by(|a, b| a.uri.cmp(&b.uri));
+
+    check_limits(files.len(), files.iter().map(|file| file.size).sum())?;


nit: we could check the size limit as we go and before reading the files so that we don't exceed it.

jamadeo · 2026-11-03T22:26:17Z

+                Err(name) => (name.to_string_lossy().into_owned(), false),
+            };
+            if name.starts_with('.') {
+                continue;


does the SEP specify that these should be skipped?

No, @jamadeo, the SEP doesn't cover this. It's a catalog decision to keep .git, .DS_Store, and similar files out of skills. Hidden files aren't served at all, so the manifest stays complete. I've added this to the module docs.

DaleSeo added this pull request to stack #1410 October 29, 2026 19:36

github-actions Bot added T-test Testing related changes T-config Configuration file changes T-core Core library changes T-handler Handler implementation changes labels Oct 29, 2026

github-actions Bot requested a review from gocamille October 29, 2026 19:36

Base automatically changed from DaleSeo/skills-support to main October 29, 2026 20:54

DaleSeo force-pushed the DaleSeo/skills-fs branch 2 times, most recently from 81be0d8 to a17dc19 Compare October 29, 2026 20:56

DaleSeo removed this pull request from stack #1410 October 29, 2026 20:59

DaleSeo linked an issue Oct 29, 2026 that may be closed by this pull request

Support the Skills extension (SEP-2640) #1391

Open

DaleSeo force-pushed the DaleSeo/skills-fs branch from a17dc19 to 31c52d7 Compare October 29, 2026 21:26

github-actions Bot added the T-documentation Documentation improvements label Oct 29, 2026

DaleSeo force-pushed the DaleSeo/skills-fs branch from 31c52d7 to e8b5386 Compare October 29, 2026 21:34

DaleSeo commented Oct 29, 2026

View reviewed changes

DaleSeo marked this pull request as ready for review October 29, 2026 21:49

DaleSeo requested a review from a team as a code owner October 29, 2026 21:50

DaleSeo self-assigned this Oct 29, 2026

chatgpt-codex-connector Bot reviewed Oct 29, 2026

View reviewed changes

tsarlandie-oai previously approved these changes Nov 3, 2026

View reviewed changes

jamadeo previously approved these changes Nov 3, 2026

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tsarlandie-oai mentioned this pull request Nov 3, 2026

fix(skills): validate percent-encoded skill names #1431

Open

DaleSeo dismissed stale reviews from jamadeo and tsarlandie-oai via b9f8c43 November 4, 2026 13:56

DaleSeo force-pushed the DaleSeo/skills-fs branch from 61ebd49 to b9f8c43 Compare November 4, 2026 13:56

DaleSeo added 3 commits November 4, 2026 09:45

feat(server): load skills from a directory tree

314bb59

test: cover the filesystem skill catalog

bcd73af

fix(server): check skill limits before reading files

9a0774e

DaleSeo added 2 commits November 4, 2026 09:45

fix(server): omit mimeType for unknown skill files

0e35626

fix(server): read skill files under one running byte budget

58f424a

DaleSeo force-pushed the DaleSeo/skills-fs branch from 5f7cf89 to 58f424a Compare November 4, 2026 14:47

DaleSeo requested review from jamadeo and tsarlandie-oai November 4, 2026 17:21

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DaleSeo commented Oct 29, 2026 •

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