feat: add --reload_agents flag to watch agent files for changes

[nuthalapativarun]

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Link to Issue or Description of Change

• Closes: Support for reloading agents on file change #196

Testing Plan

Unit Tests:

• I have added or updated unit tests for my change.
• All unit tests pass locally.

Test Files  112 passed (112)
Tests  1248 passed (1248)

Manual End-to-End (E2E) Tests:

1. Start the dev server with npx adk web --reload_agents my-agents/
2. Edit an agent file (e.g. change the instruction string)
3. Send a new message — the updated agent is loaded automatically without restarting the server
4. Console shows: [AgentLoader] Detected change in agent.ts, reloading agents...

Without --reload_agents (default false), behaviour is unchanged — agents are loaded once at startup and cached for the process lifetime.

Checklist

• I have read the CONTRIBUTING.md document.
• I have performed a self-review of my own code.
• I have commented my code, particularly in hard-to-understand areas.
• I have added tests that prove my fix is effective or that my feature works.
• New and existing unit tests pass locally with my changes.
• I have manually tested my changes end-to-end.
• Any dependent changes have been merged and published in downstream modules.

Additional context

Adds --reload_agents [boolean] option to both npx adk web and npx adk api_server, matching the --reload_agents flag in adk-python.

Implementation:

• AgentLoader accepts a new watchForChanges constructor parameter (default false).
• After the initial preloadAgents() call, if watchForChanges is true, a fs.watch watcher is started on the agents directory using the Node.js built-in fs module (no new dependencies).
• When a JS/TS file change is detected, invalidateAll() disposes all cached AgentFile instances and resets agentsAlreadyPreloaded = false, so the next request triggers a fresh scan.
• disposeAll() closes the watcher to prevent resource leaks on shutdown.
• AdkApiServer forwards the reloadAgents option from server config to AgentLoader.

[kalenkevich]

Thanks

[nuthalapativarun]

Hi @kalenkevich — I've addressed all the review comments (replaced console.log/console.warn with logger, added --reload_agents to the run command with onAgentFileReloaded callback, updated the description). Could you take another look when you get a chance? Thanks!