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ResourceLink Converted To URL File Block Breaks Bedrock Converse And Custom MCP URIs

A single ResourceLink in a MCP tool result causes LangChain-AWS Bedrock to raise ValueError and terminate the agent run because langchain-mcp-adapters incorrectly maps resource links to URL-sourced file blocks, which Bedrock does not support and which often use non-dereferenceable custom URI schemes.

highConfidence 90%LangChain Affected V<=0.3.1 Fixed V0.3.2

Origin Analysis

The function `_convert_mcp_content_to_lc_block` in `langchain_mcp_adapters/tools.py` maps an MCP `ResourceLink` to a file or image content block with a `url` source. This assumes the URI is a web-fetchable URL and that downstream providers support URL-sourced content. However, MCP resource URIs are often custom schemes (e.g., `demo://resource/dynamic/blob/1`) that are not dereferenceable by model providers, and Bedrock Converse only supports inline base64 document data, not URL sources.

Run the following code without an MCP server or AWS credentials: ```python from langchain_core.messages import ToolMessage from mcp.types import ResourceLink from langchain_mcp_adapters.tools import _convert_mcp_content_to_lc_block from langchain_aws.chat_models.bedrock_converse import _messages_to_bedrock link = ResourceLink( type="resource_link", uri="demo://resource/dynamic/blob/1", name="Resource 1", mimeType="text/plain", ) block = _convert_mcp_content_to_lc_block(link) print("converted block:", block) msg = ToolMessage(content=[block], tool_call_id="call_1") _messages_to_bedrock([msg]) # raises ValueError: File data only supported through in-line base64 format. ```

Fixing Code Block

0x7F 45 4C 46 02 01 01 000x90 12 AB 04 C1 7E 33 08
0xA4 1C 90 3E 77 0B D2 580x11 F6 6A 2C 80 19 DE 44
0x00 00 00 00 03 00 3E 000x01 00 B7 4D 55 90 0C 61
0x6C 69 62 63 2E 73 6F 2E0x36 00 8E 17 42 AA 05 F3
0xDE C0 DE 01 8B 45 FC 890xC7 5D 20 6F 91 03 E4 78
0x13 37 00 4F 9A 2D E1 700x08 B4 5C 22 71 D0 4A 16
0x48 8D 3D 22 01 00 00 E80x91 FF 0A 63 2E 57 B8 04
0x5A 91 C3 0E 44 70 1B 880x02 6F 39 C4 15 80 ED 27
0xFF 15 40 20 60 00 48 890xC3 90 7A 01 4C 28 B1 56
0x0F 1F 44 00 00 55 48 890xE5 C3 18 6D 92 0B 33 A7

Edge Case Audit

This fix changes the content type from file/image to text for ResourceLink, which may reduce the model's ability to directly process image resources if the URI happened to be a valid, fetchable web URL and the provider supported URL sources. If downstream code expects file/image blocks from ResourceLink, it may break. Rollback: keep the previous mapping as an optional branch (e.g., controlled by an environment variable) until all downstream integrations are verified. The change is a pure function with no shared state, so it is safe under concurrency; however, if the function is monkey-patched elsewhere, ensure thread safety.