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StructuredContent Is Dropped From IsError=True Tool Results In Langchain-Mcp-Adapters

The langchain-mcp-adapters library discards structuredContent when an MCP tool returns isError=True. It raises _MCPToolExecutionError before reading the field and never attaches it to the resulting error ToolMessage, unlike the success path. This causes loss of machine-readable error payloads and leaves only a placeholder text when content is empty.

highConfidence 95%LangChain Affected V0.3.2

Origin Analysis

In _convert_call_tool_result, the isError branch raises _MCPToolExecutionError(tool_content) before the structuredContent field is extracted into MCPToolArtifact. _MCPToolExecutionError stores only tool_content, and _handle_mcp_tool_error builds the ToolMessage without artifact, so structuredContent never reaches the model on error.

1. Set up an in-memory MCP server with a tool that returns CallToolResult(content=[], structuredContent={'code':'RATE_LIMITED','retry_after':30}, isError=True). 2. Load the tool via load_mcp_tools. 3. Invoke the tool with a tool_call input. 4. Observe that msg.status is 'error', msg.artifact is None, and msg.content is the placeholder 'MCP tool returned an error with empty content.' The same structuredContent appears as artifact when isError=False.

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 change may expose artifact on error messages where downstream code previously expected None; update any such assumptions. Rollback by restoring the original three function definitions. Ensure the installed langchain-core version supports the artifact parameter on ToolMessage (the success path already uses it, so likely safe). Large structured error bodies could bloat messages; consider trimming in production if needed.