# Validation and conflict errors from Custom Agent session tools

By Steven Van · 2026-09-10

Notion MCP's Custom Agent session tools now return specific HTTP 400 and 404 errors instead of a generic failure.

[Notion](<https://creatorstoolbox.com/tools/notion>) MCP's Custom Agent session tools now [return specific errors](<https://developers.notion.com/page/changelog#september-10-2026>) instead of a generic failure. A malformed session URL, a URL for a resource other than a session, or a full URL naming another workspace all return HTTP 400 validation\_error, and notion-send-message-to-session returns a conflict error when the session already has a run in progress.

A valid URL for a session that's missing, inaccessible, or unsupported still returns HTTP 404 object\_not\_found. Developers should wait for a run to finish with notion-wait-session before sending another message.

[![](<https://notion-demo.mintlify.app/images/og-image.png>)](<https://developers.notion.com/page/changelog#september-10-2026>)

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Sources

[Original source](<https://developers.notion.com/page/changelog#september-10-2026>)

[Read Validation and conflict errors from Custom Agent session tools on Creators Toolbox](<https://creatorstoolbox.com/blog/notion-validation-and-conflict-errors-from-custom-agent-session-tools>)

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Canonical source: https://creatorstoolbox.com/blog/notion-validation-and-conflict-errors-from-custom-agent-session-tools