Python: Support Agent Skills

[SergeyMenshykh]

Summary

Adds FileAgentSkillsProvider, a BaseContextProvider that discovers and exposes Agent Skills from filesystem directories following the Agent Skills specification progressive disclosure pattern: advertise → load → read resources.

This is the Python counterpart of #4122 (.NET).

Changes

• FileAgentSkillsProvider — discovers SKILL.md files from configured directories, advertises skills via system prompt injection, and provides load_skill / read_skill_resource tools for on-demand access.
• Internal helpers for skill discovery, YAML frontmatter parsing, and secure resource reading (path traversal / symlink guards).
• Unit tests (61 tests) covering discovery, loading, resource reading, security scenarios (path traversal, symlink escape, similar-prefix attacks), and edge cases.
• Sample (basic_file_skills) demonstrating usage with an expense-report skill.

Key Design Decisions

• Skills are read-only static content; no code execution.
• XML-escaping for prompt injection safety.
• Resource reads are guarded against path traversal and symlink escape.
• Resource link regex matches both markdown links and backtick-quoted paths (aligned with .NET implementation).
• Only skills from trusted sources should be used.

Out of Scope

• Script execution, which will be added in a following PR.

Closes: #3499

[markwallace-microsoft]

Python Test Coverage Report •

File	Stmts	Miss	Cover	Missing
packages/core/agent_framework
_skills.py	225	11	95%	153–154, 167–168, 280–282, 317–318, 398, 582
TOTAL	22033	2775	87%

Python Unit Test Overview

Tests	Skipped	Failures	Errors	Time
4624	244 💤	0 ❌	0 🔥	1m 19s ⏱️

[Copilot]

Pull request overview

This PR adds Python support for the Agent Skills specification by implementing FileAgentSkillsProvider, a context provider that discovers and exposes skills from filesystem directories. This is the Python counterpart to the .NET implementation in PR #4122.

Changes:

• Implements FileAgentSkillsProvider with progressive disclosure pattern (advertise → load → read resources)
• Adds comprehensive security measures including path traversal and symlink guards
• Includes 61 unit tests covering discovery, loading, resource reading, and security scenarios
• Provides a sample demonstrating usage with an expense-report skill

Reviewed changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated 6 comments.

Show a summary per file

File	Description
python/packages/core/agent_framework/_skills.py	Core implementation with skill discovery, YAML parsing, resource extraction, and security guards
python/packages/core/agent_framework/__init__.py	Exports FileAgentSkillsProvider to public API
python/packages/core/tests/core/test_skills.py	Comprehensive test suite with 61 tests covering functionality and security
python/samples/02-agents/context_providers/skills/basic_file_skills.py	Sample demonstrating skill provider usage
python/samples/02-agents/context_providers/skills/skills/expense-report/SKILL.md	Sample skill with expense policy rules
python/samples/02-agents/context_providers/skills/skills/expense-report/references/POLICY_FAQ.md	FAQ resource for the expense-report skill
python/samples/02-agents/context_providers/skills/skills/expense-report/assets/expense-report-template.md	Template asset for the expense-report skill
python/samples/02-agents/context_providers/skills/README.md	Documentation for the skills sample

[eavanvalkenburg]

Looking good, some minor comments left!