fix(bedrock-mantle): honor api_base for VPC endpoint routing on bedro…

[shivamrawat1]

Resolves LIT-3962

Summary
This PR fixes Mantle routing for bedrock/mantle/... so LiteLLM honors custom endpoint overrides (api_base, aws_bedrock_runtime_endpoint) instead of always using the public regional Mantle host.

Root cause
The bedrock/mantle/... integration overrides URL construction in both the chat and messages configs (AmazonMantleConfig and AmazonMantleMessagesConfig). Each override hardcoded:

https://bedrock-mantle.{region}.api.aws/anthropic/v1/messages

and returned it directly from get_complete_url, even though api_base was passed in. Standard Bedrock invoke paths do not have this problem because they call get_runtime_endpoint(...), which correctly prefers api_base and aws_bedrock_runtime_endpoint. Mantle was added with a separate hardcoded template and never wired into that endpoint resolution path, so VPC/VPCE/GovCloud customers could not redirect traffic even when they configured the endpoint correctly.

Fix
Added a shared helper, build_mantle_messages_url(...), in litellm/llms/bedrock/common_utils.py and used it from both Mantle configs. The helper resolves the endpoint in this order: api_base, then aws_bedrock_runtime_endpoint, then the existing public regional fallback. If the override is a host only, LiteLLM appends /anthropic/v1/messages; if the override already includes that path, it is used as-is (no duplication). Behavior without an override is unchanged.

Regression tests were added in tests/test_litellm/llms/bedrock/test_mantle.py for host override, full-path override, trailing slash, runtime-endpoint override, and an end-to-end litellm.anthropic_messages call asserting the request URL is the VPC host.

model_list:
  # Primary: host-only api_base (VPCE DNS); LiteLLM appends /anthropic/v1/messages
  - model_name: mantle-claude-vpce
    litellm_params:
      model: bedrock/mantle/anthropic.claude-mythos-preview
      api_base: https://vpce-REPLACE_ME.bedrock-mantle.us-gov-west-1.vpce.amazonaws.com
      aws_region_name: us-gov-west-1

Before:

After (can see it connect to vpc url):

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.

---

Shivam Rawat seems not to be a GitHub user. You need a GitHub account to be able to sign the CLA. If you have already a GitHub account, please add the email address used for this commit to your account.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

📢 Thoughts on this report? Let us know!

[greptile-apps]

Greptile Summary

This PR fixes the Bedrock Mantle integration so that api_base and aws_bedrock_runtime_endpoint are honoured when constructing the request URL, enabling VPC/VPCE/GovCloud routing. Previously both AmazonMantleConfig and AmazonMantleMessagesConfig hardcoded the public regional URL and ignored any override.

• Introduces build_mantle_messages_url in common_utils.py as a single, shared endpoint-resolution helper that prefers api_base, then aws_bedrock_runtime_endpoint, then the public regional fallback; handles trailing slashes and full-path overrides without duplication.
• Both Mantle configs (chat/mantle/transformation.py and messages/mantle_transformation.py) now delegate to this helper instead of using the deleted MANTLE_ENDPOINT_TEMPLATE constant.
• Six new mock-based tests cover host-only override, full-path override, trailing slash normalisation, aws_bedrock_runtime_endpoint override, and an end-to-end URL assertion through the litellm.anthropic_messages call path.

Confidence Score: 5/5

The change is safe to merge — it is a targeted, backward-compatible fix with no impact on callers that do not supply an endpoint override.

The fix is minimal and well-contained: a single new helper function with clear priority ordering, two one-line call-site changes, and thorough mock tests covering the new code paths. The fallback behaviour (no override provided) is identical to before, so existing deployments are unaffected. No auth logic, no database access, and no breaking changes are involved.

No files require special attention.

Important Files Changed

Filename	Overview
litellm/llms/bedrock/common_utils.py	Adds build_mantle_messages_url helper that resolves endpoint from api_base → aws_bedrock_runtime_endpoint → public regional fallback; logic is correct and handles trailing slashes and full-path overrides cleanly.
litellm/llms/bedrock/chat/mantle/transformation.py	Replaces hardcoded MANTLE_ENDPOINT_TEMPLATE with a call to the new build_mantle_messages_url helper; straightforward and correct.
litellm/llms/bedrock/messages/mantle_transformation.py	Same hardcoded-template replacement as the chat config; both configs now share the same endpoint-resolution logic via the helper.
tests/test_litellm/llms/bedrock/test_mantle.py	Adds six new mock-only tests covering host override, full-path override, trailing slash, runtime-endpoint override, and an end-to-end URL assertion; no real network calls.

_{Reviews (2): Last reviewed commit: "fix(bedrock-mantle): honor api_base for ..." | Re-trigger Greptile}

[veria-ai]

PR overview

All previously flagged issues have been addressed. No open security concerns remain on this pull request.

Security review

No open security issues remain on this pull request.

Fixed/addressed: 1 · PR risk: 0/10

[mateo-berri]

is this a legit concern?

[shivamrawat1]

@greptile-apps isn't the P2 issue behavior something not introduced by this PR. So isn't that test out of scope?

[greptile-apps]

You're right. The priority between api_base and aws_bedrock_runtime_endpoint isn't new behavior introduced by this PR — it's a trivial or in the new helper, and each branch is already covered independently by the existing tests. The combined precedence test is out of scope. Happy to drop this.

[shivamrawat1]

@greptile-apps yes. update the score based on your last comment.

[mateo-berri]

LGTM; thanks!