AWS Bedrock BYOK: Non-Anthropic models fail with null toolSpec.name validation errors

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1

Bug Description

When using Cursor with AWS Bedrock BYOK, all non-Anthropic models (Amazon Nova, Meta Llama 4, Qwen) fail immediately with validation errors. Requests never reach AWS — they are rejected before any network call is made.

Root Cause

Cursor sends built-in tool definitions in toolConfig with null names. Claude’s API tolerates null names; every other Bedrock model enforces the schema and rejects them.

CloudWatch model-invocation logging confirms that for non-Anthropic models no invocation is ever logged — the request fails client-side before reaching Bedrock.

Error (from Cursor logs)

We encountered an issue: 10 validation errors detected:
Value null at 'toolConfig.tools.1.member.toolSpec.name' failed to satisfy constraint: Member must not be null;
Value null at 'toolConfig.tools.2.member.toolSpec.name' failed to satisfy constraint: Member must not be null;
... (8 more identical errors)

Affected Models (all via system inference profiles)

  • us.amazon.nova-pro-v1:0 / nova-lite / nova-micro
  • us.meta.llama4-maverick-17b-instruct-v1:0
  • us.qwen.qwen3-32b-v1:0

Only Anthropic Claude profiles (us.anthropic.claude-*) work.

Steps to Reproduce

  1. Set up Cursor with AWS Bedrock BYOK (cross-account IAM role, ConverseStream + InvokeModelWithResponseStream permissions)
  2. Select any non-Anthropic system inference profile
  3. Send any message in Cursor chat
  4. Observe validation errors immediately

Expected Behavior

Cursor should not include tools with null names in toolConfig. Either filter out null-named tools before sending, or ensure all built-in tool definitions have valid names populated.

Workaround

None. Non-Anthropic Bedrock models are completely unusable under BYOK.

Environment

  • Cursor: latest (May 2025)
  • AWS Bedrock region: us-east-1
  • CloudWatch logging enabled — zero invocation records for non-Anthropic models
7

This is a confirmed bug. When AWS Bedrock BYOK is configured, non-Anthropic models (like Nova, Llama 4, and Qwen) fail because the tool definitions sent to the Bedrock API have missing names. Anthropic Claude models happen to tolerate this, but all other models enforce strict validation and reject the request.

There’s no workaround for this right now. Our team is aware of this class of Bedrock BYOK compatibility issues and is actively working on it.

Your bug report is really thorough, especially the CloudWatch confirmation that no invocation reaches AWS. That helps a lot. We’ll follow up here when there’s a fix.

8

Also experiencing this issue, however with the latest Anthropic Claude Fable 5 models via Bedrock. Am currently on version 3.7.15.

9

hey @Kho for Claude Fable 5 specifically, we’re still waiting on AWS Bedrock support (see this thread: https://forum.cursor.com/t/bedrock-fable-5-unable-to-reach-the-model-provider/162844). Use Fable from the normal picker with Bedrock off for now. The null toolSpec bug in the OP is a separate confirmed issue for non-Anthropic models team is on it.