Quick Answer
The answer is the IAM role is missing the s3:ListBucket permission. This error occurs because SageMaker’s training job IAM permissions for S3 checkpoint access require more than just s3:PutObject on the checkpoint prefix; the SDK first performs a ListObjects call against the bucket to verify its existence and check the prefix state before writing, and without s3:ListBucket on the bucket resource, that initial verification fails and aborts the write. On the AWS Certified Machine Learning Specialty MLS-C01 exam, this tests your understanding that SageMaker training jobs implicitly need both read and list permissions on the bucket itself, even when the action is a write—a common trap is assuming PutObject alone suffices. Remember the mnemonic: “To put a file in a folder, you must first see the shelf”—ListBucket is the shelf-check that enables the PutObject.
This MLS-C01 practice question tests your understanding of modeling. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
A data scientist is using Amazon SageMaker to train a natural language processing model using a custom Docker container. The training script reads data from an S3 bucket and writes checkpoints to an S3 bucket. The training job is failing with the error 'Unable to write to checkpoint path: s3://my-bucket/checkpoints/'. The IAM role associated with the training job has the following policy: {'Effect': 'Allow', 'Action': 's3:PutObject', 'Resource': 'arn:aws:s3:::my-bucket/checkpoints/*'}. The bucket 'my-bucket' exists and the prefix 'checkpoints/' is empty. What is the most likely cause of the failure?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "most likely"
Why it matters: Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
The IAM role is missing the s3:ListBucket permission
SageMaker needs ListBucket to access the bucket.
The IAM role does not have s3:PutObject permission
Why wrong: PutObject is allowed.
The S3 bucket does not exist
Why wrong: The bucket exists.
The checkpoint prefix already contains objects
Why wrong: The prefix is empty.
Answer choices
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
The IAM role is missing the s3:ListBucket permission
The error 'Unable to write to checkpoint path' occurs because the SageMaker training job's IAM role lacks the `s3:ListBucket` permission. Even though the role has `s3:PutObject` on the checkpoint prefix, SageMaker's S3 client first performs a `ListObjects` (or `HeadObject`) call to verify the bucket exists and to check the prefix state before writing. Without `s3:ListBucket` on the bucket itself, the API call fails, causing the write operation to abort.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
For each option: why learners choose it and why it is or isn't the right answer here.
The IAM role is missing the s3:ListBucket permission
Why this is correct
SageMaker needs ListBucket to access the bucket.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
The IAM role does not have s3:PutObject permission
Why it's wrong here
PutObject is allowed.
The S3 bucket does not exist
Why it's wrong here
The bucket exists.
The checkpoint prefix already contains objects
Why it's wrong here
The prefix is empty.
Common exam traps
The trap here is that candidates assume `s3:PutObject` alone is sufficient for writing to S3, but AWS requires `s3:ListBucket` on the bucket to verify the path before writing, a nuance frequently tested in MLS-C01 and SAA exams.
Detailed technical explanation
Under the hood, the AWS SDK for Python (boto3) used by SageMaker's training container performs a `list_objects_v2` or `head_object` call to validate the S3 path before any `put_object` call. The `s3:ListBucket` permission is required at the bucket level (Resource: `arn:aws:s3:::my-bucket`) to allow this validation. In real-world scenarios, this is a common oversight when granting write-only access to a prefix, as the initial directory check is often forgotten.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
A media company stores terabytes of video archives that are accessed once a year for audit purposes. Moving these objects to a cold storage tier (Azure Archive, S3 Glacier, or Google Nearline) costs a fraction of hot storage. Questions like this test whether you understand storage tiers, access frequency tradeoffs, and retrieval latency requirements.
What to study next
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FAQ
Modeling — This question tests Modeling — Read the scenario before looking for a memorised answer..
The correct answer is: The IAM role is missing the s3:ListBucket permission — The error 'Unable to write to checkpoint path' occurs because the SageMaker training job's IAM role lacks the `s3:ListBucket` permission. Even though the role has `s3:PutObject` on the checkpoint prefix, SageMaker's S3 client first performs a `ListObjects` (or `HeadObject`) call to verify the bucket exists and to check the prefix state before writing. Without `s3:ListBucket` on the bucket itself, the API call fails, causing the write operation to abort.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
Read the scenario before looking for a memorised answer.
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Last reviewed: Jun 24, 2026
This MLS-C01 practice question is part of Courseiva's free Amazon Web Services certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the MLS-C01 exam.
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