test: introduce api conformance test

this test runs on each PR and uses a new conformance workflow to compare the base (main) branch openapi spec to the one on this PR
if one of our "stable" APIs change, the test will fail.

this workflow uses `oasdiff` to identify breaking changes for paths we want to ensure comptability for.

specifically this is using `oasdiff breaking` with `--match-path` which only checks breaking changes for the specified paths.

As a follow up to this, we can add an optional way to make it so that it is OK to make these change if properly documented or in a changelog or something.

related to #3237

Signed-off-by: Charlie Doern <cdoern@redhat.com>

.github/workflows/README.md

@@ -5,6 +5,7 @@ Llama Stack uses GitHub Actions for Continuous Integration (CI). Below is a tabl
 | Name | File | Purpose |
 | ---- | ---- | ------- |
 | Update Changelog | [changelog.yml](changelog.yml) | Creates PR for updating the CHANGELOG.md |
+| API Conformance Tests | [conformance.yml](conformance.yml) | Run the API Conformance test suite on the changes. |
 | Installer CI | [install-script-ci.yml](install-script-ci.yml) | Test the installation script |
 | Integration Auth Tests | [integration-auth-tests.yml](integration-auth-tests.yml) | Run the integration test suite with Kubernetes authentication |
 | SqlStore Integration Tests | [integration-sql-store-tests.yml](integration-sql-store-tests.yml) | Run the integration test suite with SqlStore |

.github/workflows/conformance.yml

@@ -0,0 +1,57 @@
+# API Conformance Tests
+# This workflow ensures that API changes maintain backward compatibility and don't break existing integrations
+# It runs schema validation and OpenAPI diff checks to catch breaking changes early
+
+name: API Conformance Tests
+
+run-name: Run the API Conformance test suite on the changes.
+
+on:
+  push:
+    branches: [ main ]
+  pull_request:
+    branches: [ main ]
+    types: [opened, synchronize, reopened]
+    paths:
+      - 'llama_stack/**'
+      - '!llama_stack/ui/**'
+      - 'tests/**'
+      - 'uv.lock'
+      - 'pyproject.toml'
+      - '.github/workflows/conformance.yml' # This workflow itself
+
+concurrency:
+  group: ${{ github.workflow }}-${{ github.ref == 'refs/heads/main' && github.run_id || github.ref }}
+  # Cancel in-progress runs when new commits are pushed to avoid wasting CI resources
+  cancel-in-progress: true
+
+jobs:
+  # Job to check if API schema changes maintain backward compatibility
+  check-schema-compatibility:
+    runs-on: ubuntu-latest
+    steps:
+      # Using specific version 4.1.7 because 5.0.0 fails when trying to run this locally using `act`
+      # This ensures consistent behavior between local testing and CI
+      - name: Checkout PR Code
+        uses: actions/checkout@692973e3d937129bcbf40652eb9f2f61becf3332 # v4.1.7
+
+      # Checkout the base branch to compare against (usually main)
+      # This allows us to diff the current changes against the previous state
+      - name: Checkout Base Branch
+        uses: actions/checkout@692973e3d937129bcbf40652eb9f2f61becf3332 # v4.1.7
+        with:
+          ref: ${{ github.event.pull_request.base.ref }}
+          path: 'base'
+
+      # Install oasdiff: https://github.com/oasdiff/oasdiff, a tool for detecting breaking changes in OpenAPI specs.
+      - name: Install oasdiff
+        run: |
+          curl -fsSL https://raw.githubusercontent.com/oasdiff/oasdiff/main/install.sh | sh
+
+      # Run oasdiff to detect breaking changes in the API specification
+      # This step will fail if incompatible changes are detected, preventing breaking changes from being merged
+      - name: Run OpenAPI Breaking Change Diff
+        run: |
+          oasdiff breaking --fail-on ERR base/docs/_static/llama-stack-spec.yaml docs/_static/llama-stack-spec.yaml --match-path '^/v1/openai/v1' \
+          --match-path '^/v1/vector-io' \
+          --match-path '^/v1/vector-dbs'