Auto-inject SKILL.md on session start when devcontainer is detected

[aniongithub]

Summary

Add a sessionStart hook that detects .devcontainer/devcontainer.json in the working directory and injects the devcontainer-mcp SKILL.md content via additionalContext output. This makes the agent automatically aware of devcontainer-mcp tools without requiring manual skill installation or agent-specific configuration.

Motivation

Currently, agents learn about devcontainer-mcp through SKILL.md files installed in agent-specific skill directories (~/.copilot/skills/, ~/.claude/skills/, etc.). This has several limitations:

• Requires each agent environment to be individually configured
• SKILL.md can go stale if the installed version diverges from the binary
• New agent environments require manual setup
• The agent may not discover the skill at all if the directory structure changes

A sessionStart hook solves all of these — the hook fires on every session, checks for a devcontainer, and injects the current SKILL.md directly into context.

Proposed design

Copilot CLI (~/.copilot/hooks/devcontainer-skill-loader.json)

{
  "version": 1,
  "hooks": {
    "sessionStart": [
      {
        "type": "command",
        "bash": "~/.local/share/devcontainer-mcp/hooks/devcontainer-skill-loader.sh",
        "timeoutSec": 5
      }
    ]
  }
}

Hook script (devcontainer-skill-loader.sh)

#!/usr/bin/env bash
set -euo pipefail

INPUT=$(cat)
CWD=$(echo "$INPUT" | jq -r '.cwd // empty')

if [ -z "$CWD" ] || [ ! -f "${CWD}/.devcontainer/devcontainer.json" ]; then
  exit 0
fi

# Read the SKILL.md content
SKILL_PATH="${HOME}/.local/share/devcontainer-mcp/SKILL.md"
if [ ! -f "$SKILL_PATH" ]; then
  # Fall back to installed skill locations
  for dir in "${HOME}/.copilot/skills/devcontainer-mcp" "${HOME}/.claude/skills/devcontainer-mcp"; do
    if [ -f "${dir}/SKILL.md" ]; then
      SKILL_PATH="${dir}/SKILL.md"
      break
    fi
  done
fi

if [ ! -f "$SKILL_PATH" ]; then
  exit 0
fi

SKILL_CONTENT=$(cat "$SKILL_PATH")

# Output additionalContext — injected into the session
jq -n --arg ctx "$SKILL_CONTENT" '{ "additionalContext": $ctx }'

Claude Code (~/.claude/settings.json)

Claude Code supports SessionStart hooks with the same additionalContext output:

{
  "hooks": {
    "SessionStart": [
      {
        "hooks": [
          {
            "type": "command",
            "command": "~/.local/share/devcontainer-mcp/hooks/devcontainer-skill-loader.sh",
            "timeout": 5
          }
        ]
      }
    ]
  }
}

Benefits

• Zero-config agent awareness — any agent with hook support automatically learns about devcontainer-mcp
• Always current — reads SKILL.md from the installed copy, never stale
• Conditional — only injects when a devcontainer actually exists
• Lightweight — fires once per session, reads one file

References

• Copilot CLI hooks: https://docs.github.com/en/copilot/reference/copilot-cli-reference/cli-hooks-reference#sessionstart--sessionstart
• Copilot SDK session lifecycle: https://docs.github.com/en/copilot/how-tos/copilot-sdk/use-hooks/session-lifecycle#output
• Claude Code hooks: https://code.claude.com/docs/en/hooks

Relationship to existing work

• Builds on Add host-protection hooks for Claude Code and Copilot CLI #16 (host-protection hooks) — same hook infrastructure, same install flow
• Could eventually replace the SKILL.md file-copy step in install.sh/install.ps1