feat(chat): add @agent mentions with open_in_agent tool

[rafavalls]

What is this contribution about?

Adds @agent mentions to the chat input for cross-agent collaboration. Users can type @ to see a dropdown of available agents, select one, and the AI will call the new open_in_agent built-in tool to create a task in the target agent and run it in the background. A clickable card renders in the assistant response to navigate to the agent's task.

Also restructures mention triggers: / now shows both prompts and resources in a unified dropdown, while @ is dedicated to agents.

Key changes:

• @ trigger: Agent mention dropdown with icons (AgentAvatar), violet-styled chips
• / trigger: Unified prompts + resources dropdown (replaces separate mention-prompts.tsx and mention-resources.tsx)
• open_in_agent built-in tool (server-side): Validates agent, creates thread, saves user message, runs agent in background via fire-and-forget
• OpenInAgentPart renderer: Clickable card in assistant response that navigates to /$org/$agentId/?taskId=$taskId
• Mention system: Added optional allow prop to Suggestion component for trigger customization

Screenshots/Demonstration

WIP - agent mention dropdown, violet chips in editor, tool call card with agent icon + "Open" arrow

How to Test

1. Start dev server with bun run dev
2. In chat input, type @ — agent dropdown should appear with icons
3. Select an agent, type a message, and send
4. AI should call open_in_agent tool — a card with the agent icon appears
5. Click the card — navigates to the target agent with the created task
6. Type / — should show both prompts and resources in one dropdown

Review Checklist

• PR title is clear and descriptive
• Changes are tested and working
• Documentation is updated (if needed)
• No breaking changes

---

Summary by cubic

Adds @agent mentions to delegate work across agents via the new open_in_agent tool, and rebuilds @ into a two‑level menu (Agents, Resources) with top‑level search. / is unified for prompts + resources, and delegated tasks render as a clickable card that also starts the agent run on the frontend.

• New Features

  • @ trigger: two-level menu with icons and ↵ hint; typing at top level searches agents and resources; selecting an agent inserts a violet chip and delegates via open_in_agent; selecting a resource inserts it inline.
  • open_in_agent tool: validates the agent, creates an empty thread, returns task_id; marked non‑idempotent to avoid duplicate threads on retries.
  • Assistant UI: OpenInAgentPart renders a card that starts the agent run via decopilot/stream (idempotent via a module Set + sessionStorage) and navigates to /$org/$agentId/?taskId=$taskId; collapses 3+ tool calls into one expandable row.
  • / trigger: unified dropdown for prompts and resources; keeps the prompt-args dialog.

• Refactors

  • Tool registration: built-ins now receive userId; open_in_agent is outside the provider gate (works with Claude Code) and excluded from subagent tool sets.
  • Mention system: Suggestion supports allow and onOpenChange; derive-parts distinguishes agent vs resource @ mentions and inserts clear hints for open_in_agent calls.
  • Cleanup: remove unused extractAgentMentions export to satisfy knip.

^{Written for commit 4b78d33. Summary will update on new commits.}

[github-actions]

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Should we run the Virtual MCP strategy benchmark for this PR?

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[github-actions]

Release Options

Suggested: Minor (2.258.0) — based on feat: prefix

React with an emoji to override the release type:

Reaction	Type	Next Version
👍	Prerelease	2.257.1-alpha.1
🎉	Patch	2.257.1
❤️	Minor	2.258.0
🚀	Major	3.0.0

Current version: 2.257.0

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[cubic-dev-ai]

4 issues found across 20 files

Prompt for AI agents (unresolved issues)

Check if these issues are valid — if so, understand the root cause of each and fix them. If appropriate, use sub-agents to investigate and fix each issue separately.


<file name="apps/mesh/src/api/routes/decopilot/built-in-tools/open-in-agent.ts">

<violation number="1" location="apps/mesh/src/api/routes/decopilot/built-in-tools/open-in-agent.ts:61">
P2: `idempotentHint` is incorrectly set to `true` for a state-creating tool; retries can create duplicate tasks.</violation>

<violation number="2" location="apps/mesh/src/api/routes/decopilot/built-in-tools/open-in-agent.ts:221">
P1: `mcpClient` is not closed on error paths; move cleanup to a `finally` so the connection is always released.</violation>
</file>

<file name="apps/mesh/src/web/components/chat/derive-parts.ts">

<violation number="1" location="apps/mesh/src/web/components/chat/derive-parts.ts:294">
P3: `extractAgentMentions` is introduced but not used anywhere in the repository, adding dead code to maintain.</violation>
</file>

<file name="apps/mesh/src/web/components/chat/message/parts/tool-call-part/open-in-agent.tsx">

<violation number="1" location="apps/mesh/src/web/components/chat/message/parts/tool-call-part/open-in-agent.tsx:44">
P2: Remove the client-side debug `console.log` that prints tool output; it can leak task data in production browser consoles.</violation>
</file>

_{Reply with feedback, questions, or to request a fix. Tag @cubic-dev-ai to re-run a review.}

[cubic-dev-ai]

1 issue found across 3 files (changes from recent commits).

Prompt for AI agents (unresolved issues)

Check if these issues are valid — if so, understand the root cause of each and fix them. If appropriate, use sub-agents to investigate and fix each issue separately.


<file name="apps/mesh/src/api/routes/decopilot/built-in-tools/open-in-agent.ts">

<violation number="1" location="apps/mesh/src/api/routes/decopilot/built-in-tools/open-in-agent.ts:63">
P2: The tool still requires `context` but execute ignores it and creates an empty thread. That drops the supplied context, so the delegated agent can start without the intended task details. Either persist the context to the thread or remove/relax the `context` field and update the description to match.</violation>
</file>

_{Reply with feedback, questions, or to request a fix. Tag @cubic-dev-ai to re-run a review.}

[cubic-dev-ai]

P2: The tool still requires context but execute ignores it and creates an empty thread. That drops the supplied context, so the delegated agent can start without the intended task details. Either persist the context to the thread or remove/relax the context field and update the description to match.

Prompt for AI agents

Check if this issue is valid — if so, understand the root cause and fix it. At apps/mesh/src/api/routes/decopilot/built-in-tools/open-in-agent.ts, line 63:

<comment>The tool still requires `context` but execute ignores it and creates an empty thread. That drops the supplied context, so the delegated agent can start without the intended task details. Either persist the context to the thread or remove/relax the `context` field and update the description to match.</comment>

<file context>
@@ -67,16 +54,15 @@ export function createOpenInAgentTool(
     inputSchema: zodSchema(OpenInAgentInputSchema),
     needsApproval,
-    execute: async ({ agent_id, context }, options) => {
+    execute: async ({ agent_id }, options) => {
       const startTime = performance.now();
       try {
</file context>