AWS Bedrock Error: Bedrock Invocation Fails Due to Missing IAM Role Trust Policy

#aws#bedrock#ai#cloudcomputing

A diagnostic guide to resolving Bedrock invocation failures caused by incorrect IAM role trust relationships.

Problem

AWS Bedrock invocation fails even though:

• IAM permissions include bedrock:InvokeModel
• Model access is enabled
• Region configuration is correct

Typical symptoms:

• Lambda or ECS tasks fail immediately
• No Bedrock request appears to execute
• Errors resemble permission failures but do not reference a specific action

---

Clarifying the Issue

This failure is not caused by missing IAM permissions.

It occurs when the execution role cannot be assumed by the service attempting to invoke Bedrock.

IAM authorization has two layers:

1. Permission policy – what actions the role allows
2. Trust policy – who is allowed to assume the role

If the trust policy is incorrect or missing, Bedrock is never reached.

---

Why It Matters

This issue is common when:

• Creating new Lambda or ECS roles manually
• Copying roles between services
• Migrating workloads to Bedrock-enabled pipelines
• Refactoring execution roles for AI workloads

From the outside, it looks like a Bedrock problem.
In reality, the role assumption fails first.

---

Key Terms

• Trust policy – IAM policy that defines who may assume a role
• Execution role – Role assumed by Lambda, ECS, or EC2 at runtime
• AssumeRole – AWS STS operation used to obtain role credentials
• Service principal – AWS service allowed to assume a role

---

Steps at a Glance

1. Identify the service invoking Bedrock
2. Inspect the role’s trust policy
3. Verify the correct service principal
4. Update the trust relationship
5. Redeploy and retest

---

Detailed Steps

1. Identify the Invoking Service

Confirm which service is invoking Bedrock:

• Lambda
• ECS (task role)
• EC2 (instance profile)
• Step Functions

Each service requires a specific trust policy.

---

2. Open the Role Trust Policy

Navigate to:

IAM → Roles → <execution-role> → Trust relationships

Review the Principal section carefully.

---

3. Verify the Service Principal

Common correct principals:

Lambda

"Principal": {
  "Service": "lambda.amazonaws.com"
}

ECS Task Role

"Principal": {
  "Service": "ecs-tasks.amazonaws.com"
}

EC2 Instance Profile

"Principal": {
  "Service": "ec2.amazonaws.com"
}

If the service principal is missing or incorrect, the role cannot be assumed.

---

4. Update the Trust Relationship

A minimal valid trust policy example (Lambda):

{
  "Version": "2012-10-17",
  "Statement": [
    {
      "Effect": "Allow",
      "Principal": {
        "Service": "lambda.amazonaws.com"
      },
      "Action": "sts:AssumeRole"
    }
  ]
}

Save the policy once updated.

---

5. Redeploy and Retest

• Redeploy the workload (Lambda, ECS task, etc.)
• Ensure no cached credentials remain
• Retry the Bedrock invocation

If the trust policy was the issue, invocation will succeed immediately.

---

Pro Tips

• Permission policies are useless if the role cannot be assumed
• IAM errors caused by trust policies often surface as generic failures
• Lambda and ECS roles are not interchangeable
• Always validate trust policies when reusing roles

---

Conclusion

When Bedrock invocation fails despite correct permissions and model access, the issue may be role assumption, not authorization.

Once the trust policy correctly allows the invoking service to assume the role, Bedrock inference works reliably inside Amazon Web Services.

Fix the trust relationship.
Redeploy the workload.
Retry the call.
Move on.

---

Aaron Rose is a software engineer and technology writer at tech-reader.blog and the author of Think Like a Genius.