Migrate AI CHANGELOG entry generation to Copilot CLI

GitHub Models (the models.github.ai inference API backing
actions/ai-inference@v1) was fully retired on July 30, 2026:

  Error: 410 GitHub Models is temporarily unavailable as part of a
  scheduled retirement brownout.

actions/ai-inference@v1 has since been rewritten (v3) to exclusively
shell out to the GitHub Copilot CLI instead of calling GitHub Models
directly. Update the workflow to match:

- Install and authenticate the Copilot CLI (npm install -g
  @github/copilot) before invoking actions/ai-inference@v1, using a
  COPILOT_GITHUB_TOKEN sourced from a new COPILOT_PAT repo secret
  (requires a Copilot-enabled account/subscription; free for
  individual/OSS maintainer tiers, but no longer works with the
  default GITHUB_TOKEN).
- Drop the models: read permission, no longer needed.
- The v3 action also dropped structured JSON schema output
  (responseFormat/jsonSchema); the prompt now asks the model to
  return a bare JSON object with no markdown fences, and the
  "Build changelog entries" step strips any ``` fences defensively
  before parsing with jq.

NOTE: requires adding a COPILOT_PAT secret (a PAT for an account
with Copilot CLI access) to this repository's Actions secrets for
the workflow to actually run.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: dblock

.github/prompts/changelog-entries.prompt.yml

@@ -6,7 +6,8 @@ messages:
       are code-generated from JSON method definitions vendored from
       https://github.com/slack-ruby/slack-api-ref (via a git submodule and rake task).
 
-      Given a diffstat and a diff of the regenerated files, respond with a JSON object:
+      Given a diffstat and a diff of the regenerated files, respond with ONLY a single JSON
+      object (no markdown code fences, no other text) in exactly this shape:
       {"entries": [string, ...]}
 
       Rules for entries:
@@ -29,24 +30,3 @@ messages:
 
       Diff (may be truncated):
       {{diff}}
-model: openai/gpt-4o-mini
-responseFormat: json_schema
-jsonSchema: |-
-  {
-    "name": "changelog_entries",
-    "strict": true,
-    "schema": {
-      "type": "object",
-      "properties": {
-        "entries": {
-          "type": "array",
-          "items": {
-            "type": "string"
-          },
-          "description": "1-8 short, imperative-mood CHANGELOG entry descriptions, no leading bullet or trailing period"
-        }
-      },
-      "additionalProperties": false,
-      "required": ["entries"]
-    }
-  }

.github/workflows/update_api.yml

@@ -10,7 +10,6 @@ jobs:
     permissions:
       contents: write
       pull-requests: write
-      models: read
     steps:
       - uses: actions/checkout@v7
         with:
@@ -46,21 +45,30 @@ jobs:
         run: |
           git diff --stat | sed 's/^/      /' > /tmp/diff_stat.txt
           git diff | head -c 20000 | sed 's/^/      /' > /tmp/diff.txt
+      - name: Set up Node
+        if: steps.changes.outputs.changed == 'true'
+        uses: actions/setup-node@v6
+      - name: Install Copilot CLI
+        if: steps.changes.outputs.changed == 'true'
+        run: npm install -g @github/copilot
       - name: Generate changelog entries with AI
         if: steps.changes.outputs.changed == 'true'
         id: ai
-        uses: actions/ai-inference@v1
+        uses: actions/ai-inference@v3
         with:
           prompt-file: ./.github/prompts/changelog-entries.prompt.yml
+          model: ''
           file_input: |
             diff_stat: /tmp/diff_stat.txt
             diff: /tmp/diff.txt
+        env:
+          COPILOT_GITHUB_TOKEN: ${{ secrets.COPILOT_PAT }}
       - name: Build changelog entries
         id: entries
         run: |
           entries=""
           if [ "${{ steps.changes.outputs.changed }}" = "true" ]; then
-            entries="$(jq -r '.entries[]? // empty' "${{ steps.ai.outputs.response-file }}" 2>/dev/null)"
+            entries="$(grep -v '^```' "${{ steps.ai.outputs.response-file }}" 2>/dev/null | jq -r '.entries[]? // empty' 2>/dev/null)"
           fi
           if [ -z "$entries" ]; then
             entries="Update API from slack-api-ref@${{ steps.api-ref.outputs.api-ref }}"

CHANGELOG.md

@@ -2,6 +2,7 @@
 
 * [#591](https://github.com/slack-ruby/slack-ruby-client/pull/591): Generate AI CHANGELOG entries and PR summaries for automated API update PRs, lock simplecov below 1.1.0 to avoid breaking Coveralls - [@dblock](https://github.com/dblock).
 * [#592](https://github.com/slack-ruby/slack-ruby-client/pull/592): Fix a YAML indentation bug in AI CHANGELOG entry generation that broke the automated API update workflow - [@dblock](https://github.com/dblock).
+* [#593](https://github.com/slack-ruby/slack-ruby-client/pull/593): Migrate AI CHANGELOG entry generation to Copilot CLI after GitHub Models retirement - [@dblock](https://github.com/dblock).
 * Your contribution here.
 
 ### 3.2.0 (2026/07/05)