fix: Gate notifications on capabilities #290

david-hamilton-glean · 2025-05-14T19:33:27Z

Description

Servers may report their tools.listChanged capability as false, in which case they indicate that they will not send notifications when available tools change.

Honor the spec by not sending notifications/tools/list_changed notifications when capabilities.tools.listChanged is false.

This PR is downstream of #289.

Type of Change

Bug fix (non-breaking change that fixes an issue)
New feature (non-breaking change that adds functionality)
MCP spec compatibility implementation
Breaking change (fix or feature that would cause existing functionality to not work as expected)
Documentation update
Code refactoring (no functional changes)
Performance improvement
Tests only (no functional changes)
Other (please describe):

Checklist

My code follows the code style of this project
I have performed a self-review of my own code
I have added tests that prove my fix is effective or that my feature works
I have updated the documentation accordingly

MCP Spec Compliance

This PR implements a feature defined in the MCP specification
Link to relevant spec section: MCP spec: server/tools#capabilities
Implementation follows the specification exactly

Additional Information

Summary by CodeRabbit

Bug Fixes
- Improved notification logic so that "tools list changed" notifications are only sent when the server supports this feature and notifications are enabled.
Tests
- Added a test to ensure that when tool capabilities are disabled, tools can still be managed without triggering notifications.

coderabbitai · 2025-05-14T19:33:34Z

Walkthrough

This change updates the logic in the server session management to ensure that "tools list changed" notifications are only sent when the session is initialized and the server's tool capabilities are both present and enabled. Additionally, a new test verifies that disabling tool capabilities suppresses these notifications while still allowing tool management.

Changes

Files/Paths	Change Summary
server/session.go	Added conditional checks for tool capabilities before sending "tools list changed" notifications in tool management.
server/session_test.go	Added a test to verify that notifications are not sent when tool capabilities are disabled, but tool management works.

Possibly related PRs

Add tool capabilities to serverCapabilities #36: Introduced the tool capabilities structure, which this PR extends with additional conditional checks.
fix(session): Don't send tool changed notifications if session not initialized yet #289: Enhanced notification logic by adding capability checks, which this PR further refines.
feat(MCPServer): avoid unnecessary notifications when Resource/Tool not exists #266: Modified notification logic for tools, focusing on different conditions but related to notification sending.

Suggested labels

bug

Suggested reviewers

robert-jackson-glean
pottekkat

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📥 Commits

Reviewing files that changed from the base of the PR and between cf00332 and 1f58f50.

📒 Files selected for processing (2)

server/session.go (2 hunks)
server/session_test.go (1 hunks)

🚧 Files skipped from review as they are similar to previous changes (2)

server/session.go
server/session_test.go

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rwjblue-glean

Overall seems good, I left some inline comments. Dealing with the AddTool/AddSessionTool without having WithToolCapabilities(...) been called is probably fine for a follow-up PR though.

server/session.go

pottekkat

I have been thinking that the SDK should not be opinionated. i.e., even though the spec says:

servers that support tools MUST declare the tools capability.

The SDK allows creating servers with tools WITHOUT mandating declaring this capability. The SDK user still has to manually comply to the spec. I think that is good.

If the SDK is not forcing this, then the SDK should also not prevent sending notifications if this capability is not declared or if it is false.

It should either force users to follow the spec or give proper APIs for people to comply to the spec or not. But it should not be inconsistent.

Maybe checking the official SDKs to see how they do this would help. In either case, we should be consistent IMO.

david-hamilton-glean · 2025-05-14T19:59:17Z

@pottekkat that makes sense.

This PR doesn't actually prevent users from sending notifications -- it just won't do so automatically after adding or deleting tools. I'll take a peek at the official SDKs and see what they do here.

rwjblue-glean · 2025-05-14T20:00:19Z

This PR doesn't actually prevent users from sending notifications

Right, it just makes the notifications that we send spec compliant, if the user wants to do notifications on their own we don't prevent it.

rwjblue-glean · 2025-05-14T20:03:35Z

Maybe checking the official SDKs to see how they do this would help. In either case, we should be consistent IMO.

Ya, this makes sense to me. @david-hamilton-glean can you double check what the SDKs do here (before sending a follow-up PR making it an error if you AddTool without WithToolCapabilities(...))?

Though I think this PR is still good on it's own (because regardless if the user wants to be spec compliant the sdk itself should be internally).

pottekkat · 2025-05-14T20:27:38Z

Right, it just makes the notifications that we send spec compliant, if the user wants to do notifications on their own we don't prevent it.

Got it, I haven't gone through the code changes in detail, so maybe I might have misinterpreted the PR description. I can review it tomorrow if we can hold this until then.

david-hamilton-glean · 2025-05-14T20:39:25Z

@pottekkat sounds good thanks.

I've looked through the typescript SDK. The upshot is I think we should slightly tweak @robert-jackson-glean's original suggestion. When a user adds a tool we should

ensure the tools capability is non-nil
if the tools cap was nil we should set listTools to true
if the tools cap was non-nil we should leave listTools alone

This is not quite what the typescript-sdk does. They don't really have the server/session distinction that exists in mcp-go. Instead a new server is made per connection and is handed an SSETransport object.

When a tool is added via server.tool(), setToolRequestHandlers is called which registers capabilities of

{
      tools: {
        listChanged: true
      }
}

This will deep merge and unconditionally set listChanged to true, even if the user had previously registered capabilities with listChanged: false.

Making tool addition implicitly set s.capabilities.tools will track the official SDKs' behaviours (assuming the ts-sdk is representative).

If the user has explicitly called WithToolCapabilities(false) I don't think mcp-go should implicitly change listChanged though, even though the official ts sdk does this.

pottekkat

Looks good to me. We can merge this and #292

If the user has explicitly called WithToolCapabilities(false) I don't think mcp-go should implicitly change listChanged though, even though the official ts sdk does this.

This makes sense to me as well. Thank you for checking the official SDKs.

server/session.go

pottekkat · 2025-05-15T17:49:57Z

@david-hamilton-glean can you also resolve the conflict?

Servers may report their [tools.listChanged][] capability as false, in which case they indicate that they will not send notifications when available tools change. Honor the spec by not sending notifications/tools/list_changed notifications when capabilities.tools.listChanged is false. [tools.listChanged]: https://modelcontextprotocol.io/specification/2025-03-26/server/tools#capabilities

david-hamilton-glean · 2025-05-15T17:51:27Z

@pottekkat yep was just doing that

rwjblue-glean requested changes May 14, 2025

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david-hamilton-glean force-pushed the hjdivad/honor-listChanged-cap branch from 7c23b66 to 929019a Compare May 14, 2025 19:56

david-hamilton-glean force-pushed the hjdivad/honor-listChanged-cap branch from 929019a to 5a868e2 Compare May 14, 2025 20:26

david-hamilton-glean marked this pull request as ready for review May 14, 2025 20:29

david-hamilton-glean mentioned this pull request May 14, 2025

feat(tools): implicitly register capabilities #292

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pottekkat approved these changes May 15, 2025

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david-hamilton-glean force-pushed the hjdivad/honor-listChanged-cap branch from 5a868e2 to cf00332 Compare May 15, 2025 17:43

david-hamilton-glean force-pushed the hjdivad/honor-listChanged-cap branch from cf00332 to 1f58f50 Compare May 15, 2025 17:51

pottekkat requested a review from rwjblue-glean May 15, 2025 17:54

rwjblue-glean approved these changes May 15, 2025

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rwjblue-glean merged commit eb835b9 into mark3labs:main May 15, 2025
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david-hamilton-glean deleted the hjdivad/honor-listChanged-cap branch May 15, 2025 19:50

fix: Gate notifications on capabilities #290

fix: Gate notifications on capabilities #290

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Description

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MCP Spec Compliance

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