PR Decorator

Automatically analyses and decorates pull requests using Amazon Bedrock Nova Pro. Triggered on every PR open, update, or reopen- no manual input needed.

What it does

When a PR is opened or updated, the workflow:

1. Authenticates with AWS using OIDC (no stored access keys)
2. Calls Amazon Bedrock Nova Pro to analyse the PR diff
3. Updates the PR body with a structured summary
4. Posts a completion comment

Sequence Diagram

sequenceDiagram
    autonumber

    actor Dev as 👨‍💻 Developer
    participant Repo as GitHub Repo
    participant PR as Pull Request
    participant GHA as GitHub Actions
    participant GOIDC as GitHub OIDC Provider<br/>(token.actions.githubusercontent.com)
    participant STS as AWS STS<br/>(sts.amazonaws.com)
    participant IAM as IAM Role + Policy
    participant AOIDC as AWS OpenID<br/>Connector
    participant BR as Amazon Bedrock<br/>Nova Pro (APAC)

    rect rgb(230, 240, 255)
        Note over Dev,PR: ── Trigger ──
        Dev->>Repo: Push branch / open PR
        Repo->>PR: Create Pull Request
        PR->>GHA: Trigger workflow<br/>(on: pull_request)
    end

    rect rgb(255, 248, 230)
        Note over GHA,AOIDC: ── OIDC Authentication ──
        GHA->>GOIDC: 1a. Request JWT token
        GOIDC-->>GHA: 1b. Return signed JWT

        GHA->>STS: 2. AssumeRoleWithWebIdentity(JWT)
        STS->>AOIDC: 4. Validate JWT against<br/>registered OIDC provider
        AOIDC-->>STS: JWT valid ✓
        STS->>IAM: 3. Validates trust policy<br/>& attached permissions
        IAM-->>STS: Policy allows bedrock:InvokeModel ✓
        STS-->>GHA: 5. Return temp credentials<br/>(AccessKeyId + SessionToken, 15 min)
    end

    rect rgb(230, 255, 240)
        Note over GHA,BR: ── Bedrock Invocation ──
        GHA->>BR: 6. bedrock-runtime invoke-model<br/>model: amazon.nova-pro-v1:0<br/>body: PR diff + context<br/>auth: temp credentials
        Note over BR: inference profile/Foundation Model<br/>routes cross-region to<br/>optimal Nova Pro endpoint
        BR-->>GHA: 7. PR analysis response<br/>(structured summary)
    end

    rect rgb(230, 240, 255)
        Note over GHA,PR: ── PR Decoration ──
        GHA->>PR: 8a. PATCH /pulls/{number}<br/>Update PR body with analysis
        GHA->>PR: 8b. POST /issues/{number}/comments<br/>Add completion comment
        PR-->>Dev: 🤖 PR decorated with AI summary
    end

Architecture

How authentication works

This setup uses OpenID Connect (OIDC). GitHub generates a short-lived token per workflow run. AWS trusts that token and returns temporary credentials- no AWS_ACCESS_KEY_ID or AWS_SECRET_ACCESS_KEY stored anywhere.

Prerequisites

• AWS account with Bedrock access enabled for Amazon Nova Pro
• Admin access to your GitHub repository
• AWS CLI installed locally (for setup commands)

Step 1: AWS setup

1a. Register GitHub as an identity provider

Run this once per AWS account:

aws iam create-open-id-connect-provider \
  --url https://token.actions.githubusercontent.com \
  --client-id-list sts.amazonaws.com

Note: If you get EntityAlreadyExists, the provider is already registered — skip this step.

1b. Create the IAM trust policy

Create trust-policy.json:

{
  "Version": "2012-10-17",
  "Statement": [{
    "Effect": "Allow",
    "Principal": {
      "Federated": "arn:aws:iam::<YOUR_ACCOUNT_ID>:oidc-provider/token.actions.githubusercontent.com"
    },
    "Action": "sts:AssumeRoleWithWebIdentity",
    "Condition": {
      "StringEquals": {
        "token.actions.githubusercontent.com:aud": "sts.amazonaws.com"
      },
      "StringLike": {
        "token.actions.githubusercontent.com:sub": "repo:{YOUR_GITHUB_USERNAME||YOUR_GITHUB_ORG}/YOUR_REPO_NAME:*"
      }
    }
  }]
}

Replace:

1c. Create the IAM role

aws iam create-role \
  --role-name pr-decorator-bedrock-role \
  --assume-role-policy-document file://trust-policy.json \
  --description "Role for GitHub Actions PR Decorator"

1d. Create and attach the Bedrock access policy

Create bedrock-policy.json:

{
    "Version": "2012-10-17",
    "Statement": [
        {
            "Sid": "AllowNovaModelsApSouth1Only",
            "Effect": "Allow",
            "Action": [
                "bedrock:InvokeModel"
            ],
            "Resource": [
                "arn:aws:bedrock:::foundation-model/amazon.nova-lite-v1:0",
                "arn:aws:bedrock:::foundation-model/amazon.nova-pro-v1:0",
                "arn:aws:bedrock::<YOUR_ACCOUNT_ID>:inference-profile/apac.amazon.nova-pro-v1:0",
                "arn:aws:bedrock::<YOUR_ACCOUNT_ID>:inference-profile/apac.amazon.nova-lite-v1:0"
            ]
        }
    ]
}

Attach it:

aws iam create-policy \
  --policy-name BedrockProfilePolicy \
  --policy-document file://bedrock-policy.json

aws iam attach-role-policy \
  --role-name pr-decorator-bedrock-role \
  --policy-arn arn:aws:iam::<YOUR_ACCOUNT_ID>:policy/BedrockProfilePolicy

1e. Enable Nova Pro model access

1. Go to the Amazon Bedrock console
2. Click Model access in the left navigation
3. Find Amazon Nova Pro and click Request access
4. Wait for approval (usually instant)

Step 2: GitHub setup

2a. Add repository secret

Go to Settings → Secrets and variables → Actions → New repository secret:

2b. Add repository variables

On the same page, click the Variables tab:

i.e apac preferred region set

2c. Add the workflow file

Create .github/workflows/pr-decorator.yml:

name: PR Decorator (Automatic)

on:
  pull_request:
    types: [opened, synchronize, reopened]
    branches:
      - main

permissions:
  contents: read
  pull-requests: write
  id-token: write   # required for OIDC

jobs:
  decorate-pr:
    runs-on: ubuntu-latest
    # Block fork PRs — they cannot access secrets
    if: github.event.pull_request.head.repo.full_name == github.repository

    steps:
      - name: Checkout code
        uses: actions/checkout@v6
        with:
          fetch-depth: 0

      - name: Configure AWS credentials
        uses: aws-actions/configure-aws-credentials@v6
        with:
          role-to-assume: ${{ secrets.AWS_IAM_ROLE_ARN }}
          role-session-name: GitHubActions-AutoPRDecorator
          aws-region: ${{ vars.AWS_REGION }}

      # ubuntu-latest ships with CLI v1 which lacks bedrock-runtime
      - name: Upgrade AWS CLI
        run: |
          curl -s https://awscli.amazonaws.com/awscli-exe-linux-x86_64.zip -o awscliv2.zip
          unzip -q awscliv2.zip
          sudo ./aws/install --update

      - name: Test Bedrock access
        run: |
          aws bedrock-runtime invoke-model \
            --model-id ${{ vars.AWS_BEDROCK_MODEL_ID }} \
            --body '{"messages":[{"role":"user","content":[{"text":"ping"}]}]}' \
            --region ${{ vars.AWS_REGION }} \
            --cli-binary-format raw-in-base64-out \
            test-response.json
          echo "Bedrock access confirmed"

      - name: Run PR decorator
        uses: ./
        with:
          pr-decorator-version: source
          pr-number: ${{ github.event.pull_request.number }}
          base-sha: ${{ github.event.pull_request.base.sha }}
          head-sha: ${{ github.event.pull_request.head.sha }}
          head-ref: ${{ github.event.pull_request.head.ref }}
          aws-region: ${{ vars.AWS_REGION }}
          region: ${{ vars.AWS_REGION }}
          mode: "body"
          overwrite: "false"

      - name: Add completion comment (optional)
        uses: actions/github-script@v9
        with:
          script: |
            github.rest.issues.createComment({
              issue_number: ${{ github.event.pull_request.number }},
              owner: context.repo.owner,
              repo: context.repo.repo,
              body: '🤖 **PR Decorator Complete**\n\nAnalysed using AWS Bedrock Nova Pro (APAC).\n\n_Triggered by: ${{ github.event_name }} on ${{ github.event.pull_request.head.ref }}_'
            })

Step 3: Verify

Create a test PR to trigger the workflow:

git checkout -b test/bedrock-setup
echo "test" >> README.md
git add README.md
git commit -m "test: trigger PR decorator"
git push origin test/bedrock-setup

Open a PR targeting main. In the Actions tab, a successful run shows all steps green, the PR body updated, and a completion comment posted.

Troubleshooting

Verify your setup

# Check OIDC provider exists
aws iam list-open-id-connect-providers

# Check trust policy
aws iam get-role \
  --role-name pr-decorator-bedrock-role \
  --query Role.AssumeRolePolicyDocument

# Check attached policies
aws iam list-attached-role-policies \
  --role-name pr-decorator-bedrock-role

# Test Bedrock directly
aws bedrock-runtime invoke-model \
  --model-id apac.amazon.nova-pro-v1:0 \
  --body '{"messages":[{"role":"user","content":[{"text":"hello"}]}]}' \
  --region ap-south-1 \
  --cli-binary-format raw-in-base64-out \
  response.json && cat response.json

Security notes

• Fork PRs are blocked — the if: condition prevents forks from accessing secrets
• OIDC tokens are short-lived (15 min) and scoped to a single workflow run
• The IAM role is limited to bedrock:InvokeModel only- no broader AWS access
• The trust policy sub condition locks the role to this specific repository