feat(agent): add MCP server discovery mode for agent tool input by waleedlatif1 · Pull Request #3353 · simstudioai/sim

waleedlatif1 · 2026-02-26T19:58:40Z

Summary

Add mcp-server tool type that lets agents dynamically discover and use all tools from an MCP server at execution time
Extend agent handler to separate mcp-server selections, discover tools via existing discoverMcpToolsForServer, and create tools through buildMcpTool (same pipeline as individual MCP tools)
Add grouped MCP server dropdown in tool picker UI with expandable server folders and "Use all N tools" option
Add keepOpen option to combobox to support expandable dropdown sections
Add isMcpServerAlreadySelected helper and mcp-server guards throughout selected tool rendering

Type of Change

New feature

Testing

Tested manually

Checklist

Code follows project style guidelines
Self-reviewed my changes
Tests added/updated and passing
No new warnings introduced
I confirm that I have read and agree to the terms outlined in the Contributor License Agreement (CLA)

vercel · 2026-02-26T19:58:45Z

The latest updates on your projects. Learn more about Vercel for GitHub.

1 Skipped Deployment

Project	Deployment	Actions	Updated (UTC)
docs	Skipped		Feb 26, 2026 10:57pm

greptile-apps · 2026-02-26T20:03:38Z

Greptile Summary

This PR adds a new "mcp-server" tool type that enables agents to dynamically discover and use all tools from an MCP server at execution time, rather than requiring individual tool selection.

Key Changes:

Added mcp-server tool type with server-level selection in the agent tool input UI
Grouped MCP servers in a folder-based dropdown with expandable sections and "Use all N tools" option
Extended agent handler to separate mcp-server selections and expand them into individual tools via existing discoverMcpToolsForServer and buildMcpTool pipeline
Added keepOpen option to combobox for navigable dropdown sections with ArrowLeft/ArrowRight keyboard support
Added isMcpServerAlreadySelected helper and mcp-server type guards throughout the codebase
Extracted getMcpServerIssue for server-level validation (connectivity, URL changes)

Implementation Quality:

Clean separation between UI (tool-input), execution (agent-handler), and validation (tool-validation)
Reuses existing MCP infrastructure for consistency
Proper state management with drilldown navigation
Comprehensive type guards for the new mcp-server type
Good error handling with appropriate fallbacks

Minor Issues:

Uses Promise.all for tool creation which fails the entire server if any tool fails (should use Promise.allSettled for partial success)

Confidence Score: 4/5

Safe to merge with one minor logic issue in error handling
Well-structured implementation that follows existing patterns and includes proper type guards, validation, and state management. One issue with Promise.all could cause all tools from a server to fail if any single tool fails, but the overall error handling prevents crashes. No security concerns or breaking changes.
apps/sim/executor/handlers/agent/agent-handler.ts requires attention for the Promise.all issue at line 303-312

Important Files Changed

Filename	Overview
apps/sim/executor/handlers/agent/agent-handler.ts	Adds `processMcpServerSelections` to expand mcp-server selections into individual tools via existing `discoverMcpToolsForServer` and `buildMcpTool` functions. One issue: uses `Promise.all` which fails entire server if any tool fails.
apps/sim/app/workspace/[workspaceId]/w/[workflowId]/components/panel/components/editor/components/sub-block/components/tool-input/tool-input.tsx	Implements MCP server drilldown UI with folder navigation, "Use all N tools" option, and proper state management. Correctly handles mcp-server selections, prevents duplicates, and validates server-level issues.
apps/sim/components/emcn/components/combobox/combobox.tsx	Adds `keepOpen` option and ArrowLeft/ArrowRight keyboard navigation for folder-style dropdowns. Properly handles cursor boundary detection in search input to preserve text editing.
apps/sim/executor/handlers/agent/types.ts	Adds documentation for new `mcp-server` tool type that enables server-level selection with dynamic tool discovery at execution time.
apps/sim/app/workspace/[workspaceId]/w/[workflowId]/components/panel/components/editor/components/sub-block/components/tool-input/utils.ts	Adds `isMcpServerAlreadySelected` helper to check if an MCP server is already selected in all-tools mode.
apps/sim/lib/mcp/tool-validation.ts	Extracts `getMcpServerIssue` to validate server-level connectivity (server existence, connection status, URL changes) separately from tool-specific validation.

Flowchart

%%{init: {'theme': 'neutral'}}%%
flowchart TD
    A[User opens tool picker] --> B{Navigates to MCP Servers}
    B --> C[Shows server folders with tool count]
    C --> D{User clicks server folder}
    D --> E[Drilldown: Shows Back + Use all tools + Individual tools]
    E --> F{User selects Use all N tools}
    F --> G[Creates mcp-server StoredTool with serverId]
    G --> H[Removes individual tools from same server]
    H --> I[Tool stored in workflow config]
    
    I --> J[Agent execution starts]
    J --> K[AgentHandler.prepareTools]
    K --> L{Separates tool types}
    L --> M[mcpTools array]
    L --> N[mcpServerSelections array]
    L --> O[otherTools array]
    
    N --> P[processMcpServerSelections]
    P --> Q[For each server: discoverMcpToolsForServer]
    Q --> R[Get all tools from MCP server]
    R --> S[createMcpToolFromDiscoveredServerTool for each]
    S --> T[buildMcpTool - same as individual MCP tools]
    T --> U[Returns array of tool definitions]
    
    M --> V[processMcpToolsBatched]
    O --> W[Process other tools]
    
    U --> X[Combine all tools]
    V --> X
    W --> X
    X --> Y[Agent executes with expanded tool set]

_{Last reviewed commit: 6e2fb7c}

greptile-apps

_{20 files reviewed, 5 comments}

_{Edit Code Review Agent Settings | Greptile}

greptile-apps · 2026-02-26T20:03:57Z

Additional Comments (4)

apps/sim/app/workspace/[workspaceId]/w/[workflowId]/components/panel/components/editor/components/sub-block/components/tool-input/tool-input.tsx
hasMcpTools doesn't include mcp-server type - if only server-level tools are selected, MCP data won't refresh

  const hasMcpTools = selectedTools.some((tool) => tool.type === 'mcp' || tool.type === 'mcp-server')

apps/sim/app/workspace/[workspaceId]/w/[workflowId]/components/panel/components/editor/components/sub-block/components/tool-input/tool-input.tsx
usage control shows for unavailable MCP servers - should also check isMcpServer and unavailability

                  {supportsToolControl && !((isMcpTool || isMcpServer) && isMcpToolUnavailable(tool)) && (

apps/sim/app/workspace/[workspaceId]/w/[workflowId]/components/panel/components/editor/components/sub-block/components/tool-input/tool-input.tsx
MCP servers don't show unavailability badges - add similar badge logic for isMcpServer to indicate when server is disconnected

apps/sim/app/workspace/[workspaceId]/w/[workflowId]/components/panel/components/editor/components/sub-block/components/tool-input/tool-input.tsx
getMcpToolIssue returns null for mcp-server tools - consider adding server-level validation to check connectivity for mcp-server type

_{Note: If this suggestion doesn't match your team's coding style, reply to this and let me know. I'll remember it for next time!}

waleedlatif1 · 2026-02-26T21:19:53Z

@greptile

waleedlatif1 · 2026-02-26T21:19:58Z

@cursor review

apps/sim/executor/handlers/agent/agent-handler.ts

waleedlatif1 · 2026-02-26T21:31:22Z

@greptile

waleedlatif1 · 2026-02-26T21:31:27Z

@cursor review

apps/sim/components/emcn/components/combobox/combobox.tsx

apps/sim/executor/handlers/agent/agent-handler.ts

…vigation

…ver drill-down

…s, and usage control

…l hack

…on drilldown

…t type guards

…ng DB import, route ArrowRight through handleSelect

waleedlatif1 · 2026-02-26T21:58:22Z

@greptile

waleedlatif1 · 2026-02-26T21:58:28Z

@cursor review

...components/panel/components/editor/components/sub-block/components/tool-input/tool-input.tsx

…tale view

waleedlatif1 · 2026-02-26T22:47:55Z

@greptile

waleedlatif1 · 2026-02-26T22:48:00Z

@cursor review

greptile-apps · 2026-02-26T22:54:16Z

Additional Comments (1)

apps/sim/executor/handlers/agent/agent-handler.ts, line 325
Consider parallelizing server processing with Promise.all for better performance when multiple MCP servers are selected

    const results = await Promise.all(
      mcpServerSelections.map(async (serverSelection) => {
        const serverId = serverSelection.params?.serverId
        const serverName = serverSelection.params?.serverName
        const usageControl = serverSelection.usageControl || 'auto'

        if (!serverId) {
          logger.error('MCP server selection missing serverId:', serverSelection)
          return []
        }

        try {
          const discoveredTools = await this.discoverMcpToolsForServer(ctx, serverId)
          const createdTools = await Promise.all(
            discoveredTools.map((mcpTool) =>
              this.createMcpToolFromDiscoveredServerTool(
                mcpTool,
                serverId,
                serverName || serverId,
                usageControl
              )
            )
          )
          logger.info(
            `[AgentHandler] Expanded MCP server ${serverName} into ${discoveredTools.length} tools`
          )
          return createdTools.filter(Boolean)
        } catch (error) {
          logger.error(`[AgentHandler] Failed to process MCP server selection:`, { serverId, error })
          return []
        }
      })
    )

    return results.flat()

_{Note: If this suggestion doesn't match your team's coding style, reply to this and let me know. I'll remember it for next time!}

waleedlatif1 · 2026-02-26T23:00:01Z

@cursor review

waleedlatif1 · 2026-02-26T23:00:05Z

@greptile

cursor

✅ Bugbot reviewed your changes and found no new issues!

Comment @cursor review or bugbot run to trigger another review on this PR

greptile-apps · 2026-02-26T23:11:07Z

Additional Comments (1)

apps/sim/executor/handlers/agent/agent-handler.ts, line 312
Promise.all will reject if any single tool fails, returning no tools for the entire server. Use Promise.allSettled for partial success.

          const createdToolsResults = await Promise.allSettled(
            discoveredTools.map((mcpTool) =>
              this.createMcpToolFromDiscoveredServerTool(
                mcpTool,
                serverId,
                serverName || serverId,
                usageControl
              )
            )
          )
          const createdTools = createdToolsResults
            .filter((result) => result.status === 'fulfilled')
            .map((result) => result.value)
            .filter(Boolean)

cursor

✅ Bugbot reviewed your changes and found no new issues!

Comment @cursor review or bugbot run to trigger another review on this PR

greptile-apps bot reviewed Feb 26, 2026

View reviewed changes

vercel bot temporarily deployed to Preview February 26, 2026 20:10 Inactive

vercel bot temporarily deployed to Preview February 26, 2026 20:23 Inactive

vercel bot temporarily deployed to Preview February 26, 2026 20:35 Inactive

vercel bot temporarily deployed to Preview February 26, 2026 20:46 Inactive

vercel bot temporarily deployed to Preview February 26, 2026 20:51 Inactive

vercel bot temporarily deployed to Preview February 26, 2026 21:16 Inactive

vercel bot temporarily deployed to Preview February 26, 2026 21:18 Inactive

greptile-apps bot reviewed Feb 26, 2026

View reviewed changes

apps/sim/executor/handlers/agent/agent-handler.ts Outdated Show resolved Hide resolved

vercel bot temporarily deployed to Preview February 26, 2026 21:26 Inactive

vercel bot temporarily deployed to Preview February 26, 2026 21:30 Inactive

cursor bot reviewed Feb 26, 2026

View reviewed changes

apps/sim/components/emcn/components/combobox/combobox.tsx Outdated Show resolved Hide resolved

apps/sim/components/emcn/components/combobox/combobox.tsx Show resolved Hide resolved

vercel bot temporarily deployed to Preview February 26, 2026 21:36 Inactive

vercel bot temporarily deployed to Preview February 26, 2026 21:39 Inactive

cursor bot reviewed Feb 26, 2026

View reviewed changes

apps/sim/executor/handlers/agent/agent-handler.ts Outdated Show resolved Hide resolved

vercel bot temporarily deployed to Preview February 26, 2026 21:49 Inactive

waleedlatif1 added 6 commits February 26, 2026 13:52

feat(agent): add MCP server discovery mode for agent tool input

9771489

fix(tool-input): use type variant for MCP server tool count badge

0feb7ab

fix(mcp-dynamic-args): align label styling with standard subblock labels

cad1803

standardized inp format UI

6267ba3

feat(tool-input): replace MCP server inline expand with drill-down na…

1df7b65

…vigation

feat(tool-input): add chevron affordance and keyboard nav for MCP ser…

a83e621

…ver drill-down

waleedlatif1 added 9 commits February 26, 2026 13:52

fix(tool-input): handle mcp-server type in refresh, validation, badge…

e0d7b4c

…s, and usage control

refactor(tool-validation): extract getMcpServerIssue, remove fake too…

70a3e16

…l hack

lint

29bc172

reorder dropdown

7df7942

perf(agent): parallelize MCP server tool creation with Promise.all

55bf764

fix(combobox): preserve cursor movement in search input, reset query …

5c65f55

…on drilldown

fix(combobox): route ArrowRight through handleSelect, remove redundan…

9fdc1d4

…t type guards

fix(agent): rename mcpServers to mcpServerSelections to avoid shadowi…

701d520

…ng DB import, route ArrowRight through handleSelect

docs: update google integration docs

ca14f4d

waleedlatif1 force-pushed the waleedlatif1/mcp-server-tools branch from b8f36cb to ca14f4d Compare February 26, 2026 21:52

vercel bot deployed to Preview February 26, 2026 21:55 View deployment

cursor bot reviewed Feb 26, 2026

View reviewed changes

...components/panel/components/editor/components/sub-block/components/tool-input/tool-input.tsx Show resolved Hide resolved

fix(tool-input): reset drilldown state on tool selection to prevent s…

8fa5dca

…tale view

vercel bot temporarily deployed to Preview February 26, 2026 22:22 Inactive

perf(agent): parallelize MCP server discovery across multiple servers

6e2fb7c

vercel bot temporarily deployed to Preview February 26, 2026 22:57 Inactive

cursor bot reviewed Feb 26, 2026

View reviewed changes

waleedlatif1 merged commit c6e147e into staging Feb 26, 2026
12 checks passed

waleedlatif1 deleted the waleedlatif1/mcp-server-tools branch February 26, 2026 23:17

waleedlatif1 mentioned this pull request Feb 28, 2026

v0.5.102: new integrations, new tools, ci speedups, memory leak instrumentation #3379

Merged

Conversation

waleedlatif1 commented Feb 26, 2026

Summary

Type of Change

Testing

Checklist

Uh oh!

vercel bot commented Feb 26, 2026 • edited Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Uh oh!

greptile-apps bot commented Feb 26, 2026 • edited Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Greptile Summary

Confidence Score: 4/5

Important Files Changed

Flowchart

Uh oh!

greptile-apps bot left a comment • edited Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Choose a reason for hiding this comment

Uh oh!

greptile-apps bot commented Feb 26, 2026

Uh oh!

waleedlatif1 commented Feb 26, 2026

Uh oh!

waleedlatif1 commented Feb 26, 2026

Uh oh!

Uh oh!

waleedlatif1 commented Feb 26, 2026

Uh oh!

waleedlatif1 commented Feb 26, 2026

Uh oh!

Uh oh!

Uh oh!

Uh oh!

waleedlatif1 commented Feb 26, 2026

Uh oh!

waleedlatif1 commented Feb 26, 2026

Uh oh!

Uh oh!

waleedlatif1 commented Feb 26, 2026

Uh oh!

waleedlatif1 commented Feb 26, 2026

Uh oh!

greptile-apps bot commented Feb 26, 2026

Uh oh!

waleedlatif1 commented Feb 26, 2026

Uh oh!

waleedlatif1 commented Feb 26, 2026

Uh oh!

cursor bot left a comment

Choose a reason for hiding this comment

Uh oh!

greptile-apps bot commented Feb 26, 2026

Uh oh!

cursor bot left a comment

Choose a reason for hiding this comment

Uh oh!

Uh oh!

Reviewers

Assignees

Labels

Projects

Milestone

Development

Uh oh!

1 participant

vercel bot commented Feb 26, 2026 •

edited

Loading

greptile-apps bot commented Feb 26, 2026 •

edited

Loading

greptile-apps bot left a comment •

edited

Loading