cachePoint after non-PDF document blocks causes ValidationException on Bedrock ConverseStream

[philmerrell]

Summary

PR #1821 moved cache point injection from the last assistant message to the last user message. This causes a ValidationException when the user message contains non-PDF document content blocks (.md, .docx, .csv, .xlsx, .html, .txt).

PDF documents are unaffected — only non-PDF document formats trigger the error.

Error

botocore.errorfactory.ValidationException: An error occurred (ValidationException) 
when calling the ConverseStream operation: The model returned the following errors: 
messages.0.content.3.type: Field required

Root Cause

When CacheConfig(strategy="auto") is enabled, _inject_cache_point() appends {"cachePoint": {"type": "default"}} to the end of the last user message's content array. When that message contains a document block, the resulting content looks like:

[
    {"text": "Analyze this file\n\n[Attached files: README.md]"},  # index 0
    {"document": {"format": "md", "name": "readme", "source": {"bytes": b"..."}}},  # index 1
    {"cachePoint": {"type": "default"}}  # index 2 — causes the error
]

Bedrock cannot translate the cachePoint block to the Anthropic API format when it follows a non-PDF document block. The Anthropic model validation rejects it because the translated block is missing the required type field.

PDFs work because Anthropic natively supports PDF documents in their API, so the conversion path handles the cache point correctly.

Where the Issue Was Introduced

• PR: fix: place cache point on last user message instead of assistant #1821 — "fix: place cache point on last user message instead of assistant"
• Commit: b0fc796
• Previous behavior: Cache point was appended to the last assistant message, which never contains document blocks, so this issue did not exist.

Reproduction

1. Configure a Strands Agent with CacheConfig(strategy="auto") and a Bedrock model (e.g., us.anthropic.claude-haiku-4-5-20251001-v1:0)
2. Send a message with a non-PDF document attachment (.md, .docx, etc.) as a document content block
3. The converse_stream call fails with the ValidationException above

Suggested Fix

_inject_cache_point() should skip appending a cache point to user messages that contain document blocks (with non-PDF formats), or place the cache point before the document block rather than after all content blocks.

Environment

• strands-agents==1.32.0
• bedrock-agentcore==1.4.7
• Model: us.anthropic.claude-haiku-4-5-20251001-v1:0
• Region: us-west-2

Related

• fix: place cache point on last user message instead of assistant #1821
• [BUG] Invalid Cache Point Injection in BedrockModel #1820

[philmerrell]

@kevmyung appreciate your research and contributions on this front. Been running into this issue when attaching non-PDFs via Bedrock DocumentBlock (reason disclosed above) on the initial message of a new conversation. Seems like a potential edge case.

[kevmyung]

@philmerrell Thanks for reporting this issue.

The problem seems that cachePoint placed directly after a non-PDF document block is not compatible with Bedrock internal API. The fix would be to detect non-PDF document blocks and insert the cachePoint before them instead of appending at the end.

Let me create a PR for the fix.