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fix: add UnsupportedCapabilityError to handle capabilities custom exception#1329

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gotamm wants to merge 8 commits intomodelcontextprotocol:mainfrom
gotamm:supported-capabilities-custom-exception
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fix: add UnsupportedCapabilityError to handle capabilities custom exception#1329
gotamm wants to merge 8 commits intomodelcontextprotocol:mainfrom
gotamm:supported-capabilities-custom-exception

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@gotamm gotamm commented Dec 20, 2025 •
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Replace generic Error with UnsupportedCapabilityError when capabilities are not supported, allowing applications to distinguish capability errors from actual failures.

Motivation and Context

It fixes the issue #430

How Has This Been Tested?

Breaking Changes

There are no breaking changes.

Types of changes

  • Bug fix (non-breaking change which fixes an issue)
  • New feature (non-breaking change which adds functionality)
  • Breaking change (fix or feature that would cause existing functionality to change)
  • Documentation update

Checklist

  • I have read the MCP Documentation
  • My code follows the repository's style guidelines
  • New and existing tests pass locally
  • I have added appropriate error handling
  • I have added or updated documentation as needed

Additional context

Added UnsupportedCapabilityError class in packages/core/src/types/types.ts.
I assigned to it an error code of -32043 following what's been done before, but it should be checked if it's correct.

Open Source Saturday

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changeset-bot bot commented Dec 20, 2025 •
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⚠️ No Changeset found

Latest commit: 3086e54

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@modelcontextprotocol/client

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@modelcontextprotocol/express

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@modelcontextprotocol/hono

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@modelcontextprotocol/node

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commit: 3086e54

@gotamm gotamm marked this pull request as ready for review December 20, 2025 14:49
@gotamm gotamm requested a review from a team as a code owner December 20, 2025 14:49
KKonstantinov
KKonstantinov approved these changes Dec 23, 2025
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@KKonstantinov KKonstantinov left a comment •
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Hi, thank you for this. The idea if differentiating on various Errors is something we should apply in other places too so users can distinguish them easier.

I've left two comments to be addressed please

KKonstantinov
KKonstantinov requested changes Dec 23, 2025
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@codewithkenzo codewithkenzo mentioned this pull request Feb 2, 2026
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gotamm commented Feb 21, 2026

Hi @KKonstantinov, I think this PR is no longer needed and should be closed due to #1454.
Am I right?

felixweinberger added a commit that referenced this pull request Mar 25, 2026
@felixweinberger @gotamm
fix: convert remaining capability throws to SdkError
9153c8f 
Follow-up to #1454. Converts the 6 capability-assertion call sites that
were missed to throw SdkError(SdkErrorCode.CapabilityNotSupported, ...)
instead of plain Error:

- Client.assertCapability() in packages/client/src/client/client.ts
- assertToolsCallTaskCapability() in experimental/tasks/helpers.ts (2 throws)
- assertClientRequestTaskCapability() in experimental/tasks/helpers.ts (3 throws)

Fixes #430
Closes #1329

Co-authored-by: Matteo Gobbo <info@matteogobbo.it>
@felixweinberger felixweinberger mentioned this pull request Mar 25, 2026
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felixweinberger commented Mar 25, 2026 •
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Thanks @gotamm for following up - opened #1761 to finish the SdkError conversion that #1454 started, covering the 6 call sites your audit identified (including Client.assertCapability() and the task helpers). Added you as co-author. Closing this in favor of that PR since it uses the now-standard SdkError + enum pattern.

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@gotamm @felixweinberger @KKonstantinov