---
title: "Notion API Custom Agent session tools return specific validation and conflict errors: 2026-09-10 | Harbyn"
description: "Notion API Custom Agent session tools return specific validation and conflict errors, announced Sep 10, 2026."
canonical: "https://harbyn.com/changes/notion/2026-09-10-validation-and-conflict-errors-from-custom-agent-session-tools"
last-updated: 2026-10-02
---

# Notion API Custom Agent session tools return specific validation and conflict errors

Markdown version of <https://harbyn.com/changes/notion/2026-09-10-validation-and-conflict-errors-from-custom-agent-session-tools>.

Behavior change - Notion API. low severity. In effect.

Malformed URLs, URLs for a resource other than a session, and full URLs naming another workspace now return HTTP 400 validation\_error. notion-send-message-to-session returns a conflict error if the session already has a run in progress. Valid URLs for missing, inaccessible, or unsupported sessions return HTTP 404 object\_not\_found. These cases previously returned a generic failure.

## Facts

| Fact | Value |
| --- | --- |
| Vendor | [Notion API](https://harbyn.com/vendors/notion.md) |
| Kind | Behavior change |
| Severity | low |
| Status | In effect |
| Announced | Sep 10, 2026 |
| Added to Harbyn | Oct 2, 2026 |
| Lead time | No dated deadline |
| Symbols | `notion-send-message-to-session` |
| Review | Reviewed by a person, extracted by a model |

## Find it in your code

```sh
npx harbyn scan
```

This change names no identifier the scanner can match line by line, so the scan tells you whether your repository uses Notion API, from its packages and API hosts. Nothing leaves your machine.

## Sources

Where this event comes from: the page we read, when we read it, and the hash of that snapshot.

- <https://developers.notion.com/page/changelog.md> (changelog, fetched 2026-10-02 22:20 UTC, sha256 `ec279fb11111f1773751e95d79200a14dbac9e5c21565900e74f4509ca8de3e9`)

## Related changes

### More from Notion API

- [Content queries to notion-search can route to AI search](https://harbyn.com/changes/notion/2026-09-10-ai-search-accepts-filters-and-notion-search-routes-content-queries-to-it-2.md) (Behavior change, announced Sep 10, 2026)
- [MCP search drops unavailable filters instead of failing](https://harbyn.com/changes/notion/2026-09-17-search-drops-unavailable-options-instead-of-failing.md) (Behavior change, announced Sep 17, 2026)
- [notion-search is now keyword-only](https://harbyn.com/changes/notion/2026-09-02-notion-mcp-splits-keyword-search-and-ai-search.md) (Behavior change, announced Sep 2, 2026)
- [Free workspace block limits now apply to the REST API](https://harbyn.com/changes/notion/2026-09-01-free-workspace-block-limits-now-apply-to-the-rest-api.md) (Breaking change, announced Sep 1, 2026)

### Changes at other vendors

- [Amazon Selling Partner API: ORDER\_CHANGE notifications add PreFulfillmentStatusChange for Amazon Autos](https://harbyn.com/changes/amazon-sp-api/2026-10-14-sp-api-updates-new-listings-items-api-ivp-values-in-eu5-jp-and-ca-stores-oct-3.md) (effective Oct 28, 2026)
- [BigCommerce: 301 redirects will apply only to storefront channels](https://harbyn.com/changes/bigcommerce/2026-10-07-301-redirects-now-apply-to-storefront-channels-only.md) (effective Nov 10, 2026)
- [GitHub: Secret scanning audit log business field changes to enterprise slug](https://harbyn.com/changes/github/2026-10-20-updates-to-github-secret-scanning-7.md) (effective Nov 12, 2026)
- [Jira / Atlassian Cloud: New IP address ranges in use from January 1, 2027](https://harbyn.com/changes/atlassian-jira/2026-10-16-atlassian-cloud-adds-new-ip-address-ranges.md) (effective Jan 1, 2027)

[All upcoming deadlines](https://harbyn.com/deadlines.md)