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fix(server): send skill text with NUL bytes as a blob #1439

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fix(server): send skill text with NUL bytes as a blob #1439

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3 changes: 2 additions & 1 deletion crates/rmcp/src/handler/server/skill_catalog.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -215,7 +215,8 @@ impl SkillCatalog {
.mime_type
.is_some_and(is_textual)
.then(|| std::str::from_utf8(&file.bytes).ok())
.flatten();
.flatten()
.filter(|text| !text.contains('\0'));

@DaleSeo DaleSeo Nov 8, 2026

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I went with a NUL check instead of trying to detect the charset from a BOM or XML declaration. We still don't transcode, and readable text shouldn't contain \0, so treating it as a blob is the safer fallback.

let contents = match text {
Some(text) => ResourceContents::text(text, uri),
None => ResourceContents::blob(BASE64_STANDARD.encode(&file.bytes), uri),
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21 changes: 21 additions & 0 deletions crates/rmcp/tests/test_skill_catalog.rs
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Expand Up @@ -332,6 +332,27 @@ mod serving {
));
}

#[test]
fn should_serve_a_text_type_containing_nul_as_a_blob() {
let dir = skills_tree();
let utf16le: Vec<u8> = "<a/>".bytes().flat_map(|b| [b, 0]).collect();
write(dir.path(), "pdf-processing/form.xml", &utf16le);
let catalog = load(&dir);

let result = catalog
.read_resource("skill://pdf-processing/form.xml")
.expect("served");

assert!(matches!(
&result.contents[0],
ResourceContents::BlobResourceContents { mime_type: Some(mime), .. } if mime == "application/xml"
));
assert_eq!(
read_bytes(&catalog, "skill://pdf-processing/form.xml"),
utf16le
);
}

#[test]
fn should_omit_the_mime_type_of_an_unknown_extension() {
let dir = skills_tree();
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