Add Mattermost incoming webhooks integration

Receive flag change notifications in Mattermost channels via Mattermost's
incoming webhook feature. Uses the auditLogEventsHook capability with
three templates (default, project, environment) targeting Mattermost's
Slack-compatible attachment payload schema.

All 2,387 tests in npm test pass, including JSON-validity checks
across the project/environment/flag sample contexts.

Co-authored-by: Cursor <cursoragent@cursor.com>

Author: 2 people

integrations/mattermost/LICENSE.md

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+Copyright 2026 Catamorphic Co.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.

integrations/mattermost/README.md

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+# Mattermost
+
+User documentation for this integration is available on the LaunchDarkly documentation site: [Mattermost](https://launchdarkly.com/docs/integrations/mattermost)
+
+API documentation for this integration is available on Mattermost's documentation site: [Incoming webhooks for Mattermost](https://docs.mattermost.com/developer/webhooks-incoming.html)
+
+## Getting started with the Mattermost integration
+
+Run `npm run curl mattermost` in the root repository directory to generate a `curl` command to send data to Mattermost.

integrations/mattermost/assets/images/horizontal.svg

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+{
+  "name": "Mattermost incoming webhooks",
+  "version": "1.0.0",
+  "overview": "Receive flag change notifications in your Mattermost channels.",
+  "description": "Receive LaunchDarkly notifications in your Mattermost channels. Subscribe to changes in flags, projects, environments, and other resources via Mattermost's incoming webhook integration.",
+  "author": "LaunchDarkly",
+  "supportEmail": "support@launchdarkly.com",
+  "links": {
+    "site": "https://mattermost.com",
+    "launchdarklyDocs": "https://launchdarkly.com/docs/integrations/mattermost",
+    "privacyPolicy": "https://mattermost.com/privacy-policy/"
+  },
+  "categories": ["messaging"],
+  "icons": {
+    "square": "assets/images/square.svg",
+    "horizontal": "assets/images/horizontal.svg"
+  },
+  "formVariables": [
+    {
+      "key": "url",
+      "name": "Incoming webhook URL",
+      "type": "uri",
+      "description": "Enter your Mattermost [incoming webhook URL](https://docs.mattermost.com/developer/webhooks-incoming.html).",
+      "isSecret": false
+    }
+  ],
+  "capabilities": {
+    "auditLogEventsHook": {
+      "endpoint": {
+        "url": "{{{url}}}",
+        "method": "POST",
+        "headers": [
+          {
+            "name": "Content-Type",
+            "value": "application/json"
+          }
+        ]
+      },
+      "templates": {
+        "project": "templates/project.json.hbs",
+        "environment": "templates/environment.json.hbs",
+        "default": "templates/default.json.hbs",
+        "validation": "templates/default.json.hbs"
+      },
+      "defaultPolicy": [
+        {
+          "effect": "allow",
+          "resources": ["proj/*:env/production:flag/*"],
+          "actions": ["*"]
+        }
+      ]
+    }
+  }
+}

integrations/mattermost/assets/images/square.svg

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+{
+  "username": "LaunchDarkly",
+  "attachments": [
+    {
+      "fallback": "{{{title.plainText}}}",
+      "color": "#405BFF",
+      "title": "{{{name}}}",
+      "title_link": "{{{_links.site.href}}}",
+      "text": "{{{title.markdown}}}{{#if details.markdown}}\n\n{{{details.markdown}}}{{/if}}{{#if comment}}\n\n**Comment:** {{{comment}}}{{/if}}",
+      "fields": [
+        {{#if project.name}}{ "title": "Project", "value": "{{{project.name}}}", "short": true },
+        {{/if}}{{#if project.environment.name}}{ "title": "Environment", "value": "{{{project.environment.name}}}", "short": true },
+        {{/if}}{ "title": "Action", "value": "{{{verbKind}}}", "short": true }
+      ],
+      "footer": "LaunchDarkly",
+      "ts": {{timestamp.seconds}}
+    }
+  ]
+}

integrations/mattermost/manifest.json

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+{
+  "username": "LaunchDarkly",
+  "attachments": [
+    {
+      "fallback": "{{{title.plainText}}}",
+      "color": "#405BFF",
+      "title": "{{{name}}}",
+      "title_link": "{{{_links.site.href}}}",
+      "text": "{{{title.markdown}}}{{#if details.markdown}}\n\n{{{details.markdown}}}{{/if}}{{#if comment}}\n\n**Comment:** {{{comment}}}{{/if}}",
+      "fields": [
+        { "title": "Project", "value": "{{{project.name}}}", "short": true }
+      ],
+      "footer": "LaunchDarkly",
+      "ts": {{timestamp.seconds}}
+    }
+  ]
+}

integrations/mattermost/templates/default.json.hbs

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+{
+  "username": "LaunchDarkly",
+  "attachments": [
+    {
+      "fallback": "{{{title.plainText}}}",
+      "color": "#405BFF",
+      "title": "{{{name}}}",
+      "title_link": "{{{_links.site.href}}}",
+      "text": "{{{title.markdown}}}{{#if details.markdown}}\n\n{{{details.markdown}}}{{/if}}{{#if comment}}\n\n**Comment:** {{{comment}}}{{/if}}",
+      "footer": "LaunchDarkly",
+      "ts": {{timestamp.seconds}}
+    }
+  ]
+}