[Mehedi Hasan (Mahdi-Hasan)]

Motivation & Context

BedrockChatClient and BedrockEmbeddingClient read BEDROCK_REGION before the region of a caller-supplied boto3_session. A session set to eu-west-1 was sent to us-east-1 when the env var said so, with no error. An injected runtime client also reported the ambient region in client.region and service_url().

Description & Review Guide

• What are the major changes?
  • Region now resolves in this order: injected client's meta.region_name, explicit region=, boto3_session.region_name, BEDROCK_REGION, then us-east-1.
  • Explicit region= still overrides the session, as the issue asks.
  • Embedding client: renamed a walrus variable that shadowed the region argument.
  • Docstrings updated to the new order.
• What is the impact of these changes?
  • Callers who pass a session and also set BEDROCK_REGION to a different value now get the session's region. This is the fix the issue asks for. Callers who want the old result can pass region=.
• What do you want reviewers to focus on?
  • The existing env-region-wins test cases encoded the old order. I replaced them with session-region-beats-env and added an env-region case for a session with no region.

Related Issue

Fixes #9962

AI Assistance

• No material AI assistance was used.
• This is an AI-assisted contribution. I reviewed, understood, and verified all submitted content and accept responsibility for it.

AI assistance details: implementation, tests, and this description. The issue's repro was re-run against a real boto3.Session before and after the change.

Contribution Checklist

• The code builds clean without any errors or warnings
• All unit tests pass, and I have added new tests where possible
• The PR follows the Contribution Guidelines
• This PR links to an agreed issue with no competing open PR, or the Related Issue section documents a trivial-change or repository-automation exception.
• This is not a breaking change. If it is a breaking change, add the breaking change label (or add "[BREAKING]" to the title prefix, before or after any language prefix) — a workflow keeps the label and title prefix in sync automatically.

🤖 Generated with Claude Code

[Copilot]

Copilot was unable to review this pull request because the user who requested the review has insufficient remaining quota. Try requesting a review at a lower effort level.

[Mehedi Hasan (Mahdi-Hasan)]

@microsoft-github-policy-service agree

[Copilot]

🟢 Approval recommended

No active review issues remain, and the region-precedence changes, documentation updates, and relevant client tests are addressed across the affected Bedrock clients.

No open issues 🎉

_{Review effort: Lite}