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If Claude Code returns Unknown skill: probe-api for a skill stored in a package’s .claude/skills/ directory, the skill may not have been discovered yet. In a small test on Claude Code 2.1.285, opening a file in the package with the dedicated Read tool triggered discovery; reading the same file with Bash cat did not. That is a version-specific result, not a guarantee for every file-access method or release.
Why Claude Code can say “Unknown skill”
A skill nested under a subdirectory may not be available by name as soon as a session starts. Anthropic’s Claude Code skills documentation says nested skills load the first time Claude reads or edits a file in that subdirectory and remain available for the rest of the session. Before Claude discovers the skill, calling it directly with the Skill tool can return an unknown-skill error.
The confusing part is that “reads a file” may not mean every operation that displays file contents. In Rulestack’s report, the dedicated Read tool triggered discovery of a package skill, while Bash cat did not.
What the reported Read-versus-cat test found
Rulestack ran its experiment on 2026-09-30 with Claude Code 2.1.285. Its main test used 20 headless runs with Opus 5.5; the author also reports ten further runs repeating five configurations with Sonnet 5.5. The counts below describe that test setup, not general Claude Code behavior.
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| Action or observation | Reported result |
|---|---|
Bash cat on the package source file |
0 of 2 runs loaded the nested skill; a subsequent Skill-tool call returned Unknown skill. |
| Dedicated Read tool on the package file | 6 of 6 runs loaded the nested skill. |
| Skill-tool calls before discovery | The author reports Unknown skill in all ten Opus calls that attempted it and in the pre-discovery Sonnet calls described in the report. |
| Listed skill’s session-start input cost | 60 tokens in the tested configuration, as measured by the report’s author. |
| Later request after a nested skill appeared | About 140 additional tokens in the author’s calculation. |
| Sonnet 5.5 Read-versus-cat rerun | The partial rerun matched the reported Read-versus-cat result; the report also says its skill-listing token costs matched the Opus experiment. |
The sample is small. The results establish what happened in the reported configurations, not how often the behavior occurs across users, projects, tools, or later versions.
Other file tools may behave differently
A separate, open Anthropic GitHub issue report describes Claude Code 2.1.267 on Windows. In that reporter’s setup, nested skills and subdirectory instruction files appeared after Read, but not after Bash cat, sed, or grep, or after the dedicated Grep tool. This is a user report, not an official finding that the behavior is universal or resolved.
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The reports point to a practical tension when a workflow steers Claude toward Bash-first file inspection: if discovery depends on a particular file operation in that setup, a shell command that displays the file may not make the nested skill available. Neither report tests every access or editing tool, so do not assume either that only Read can trigger discovery or that all tools behave the same way.
Ways to make a package skill available earlier
Use the documented /add-dir route
Anthropic documents /add-dir as a way to add a directory early; its documentation says the route requires Claude Code v2.1.257 or later. For a skill needed before Claude begins working in a package, consult the current skills documentation for the command’s usage and add the relevant directory before asking Claude to invoke the skill.
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Start the session in the package
Rulestack reports that starting a session inside the package caused its skill to load at startup in the tested setup. The author cautions that this can change which shared settings load. The test used the --add-dir launch flag, not the interactive /add-dir command, so it does not demonstrate that those two methods have identical effects.
Trigger discovery with Read
If the session is already running and you want to follow the tested path, use Claude Code’s dedicated Read tool on a file inside the package, then try the skill. Rulestack observed discovery in 6 of 6 Read-tool runs on Claude Code 2.1.285, but that sample is not a promise about another version or tool setup.
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What to do when a nested skill shares a name
A package skill can have the same short name as a root-level skill. Anthropic’s skills documentation describes directory-qualified names for nested skills in this situation. After the package skill is discovered, inspect the names Claude Code presents and choose the directory-qualified entry that identifies the intended package; do not assume Claude will always select the right variant automatically.
In Rulestack’s name-collision test, a nested deploy skill appeared with a directory-qualified name after Read, and the model called that variant in both reported runs. That is a two-run observation, not evidence that selection is automatic or reliable in all projects.
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A practical troubleshooting sequence
- Check the skill’s location. Confirm it is under the package’s
.claude/skills/directory and note the package subtree Claude needs to discover. - Check whether discovery has happened. If calling the skill returns
Unknown skill, try reading a file in that package with the dedicated Read tool rather than assuming a Bashcathas made the skill available. - For a skill needed from the start, add the directory early. Use Anthropic’s documented
/add-dirroute where available, or consider starting the session in the package while accounting for possible changes to loaded shared settings. - Inspect the surfaced name. If root and nested skills share a name, use the directory-qualified entry corresponding to the package.
- Keep version and workflow in mind. The Read-versus-
catresult comes from Claude Code 2.1.285; the separate Windows issue report concerns 2.1.267. Neither establishes behavior for every release or access method.
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