[Bug]: Slack draft widget’s Send button fails with “This app cannot call that tool” #17829

Activity

1. aromanarguello

   added

   bugSomething is broken or behaving incorrectly.Something is broken or behaving incorrectly.

   needs-triageIssue needs maintainer review and initial categorization.Issue needs maintainer review and initial categorization.

   on Oct 10, 2026
2. juliusmarminge commented on Oct 10, 2026

   

   juliusmarminge

   on Oct 10, 2026

   Member

   More actions

   Note

   Grok responding on behalf of Julius.

   Thanks for the clear repro. Here's where that error comes from (checked against main @ 98beed1a). I haven't reproduced it with a live Slack connector, so the cause below is a hypothesis.

   Where the refusal happens

   • When the card calls a tool, McpAppFrame.tsx (around line 329; mobile has the same check in McpAppWebView.tsx) asks the server for toolInfo first. If callable is false, it throws "This app cannot call that tool." before any approval prompt appears. The server repeats the same check in McpAppRequests.callTool and returns tool-not-callable.
   • McpAppRequests.toolInfo sets callable to tool !== undefined && mcpAppToolCallableByApp(tool._meta). It looks the tool up with findTool, which lists the app's server tools (listTools({ server: app.server })) and matches on an exact tool.name === name.
   • On Codex, listTools calls mcpServerStatus/list with serverName: app.server and detail: "toolsAndAuthOnly", then returns that server entry's tools.

   So callable is false for one of two reasons:

   1. The tool isn't found. The name the widget sends doesn't match any tool Codex lists for app.server. With a Codex app connector, the card's server is likely Codex's built-in codex_apps server, not a server named "slack". Its tools may be listed under a different name than the one the widget calls, or mcpServerStatus/list may not return connector tools at all for that detail level. Either way the lookup comes back undefined. This fits the fact that the agent's own slack_send_message call works, since Codex resolves that name internally.
   2. The tool hides itself from apps. _meta.ui.visibility on Slack's send tool may not include "app". mcpAppToolCallableByApp treats that as not callable, as the spec requires.

   I can't tell which one it is from the code alone. The quickest check is to log what listTools returns for the card's app.server and compare it with the name the widget asks for.

   The stale Send button is a separate issue. The card is a static app render, and nothing tells it that the agent later sent the draft through chat. That part may be expected behavior unless the connector's app polls for state.

   Related: #16992 and #16991 are other MCP App rendering bugs with connector apps. The open PRs #17040, #17026, #17027 and #17235 cover sizing, color scheme, cookies and HTML-render answers. None of them touches the callable check, so none of them should fix this.
3. juliusmarminge

   added

   via-triageFiled through npx t3 triageFiled through npx t3 triage

   and removed

   needs-triageIssue needs maintainer review and initial categorization.Issue needs maintainer review and initial categorization.

   on Oct 10, 2026