Map text DataContent to an MCP text resource, not a binary blob

[PratikDhanave]

Problem

When exposing an agent as an MCP tool, agentContentToMCPContent converts a message.DataContent into an mcp.EmbeddedResource. Image and audio media types get dedicated content types, but every other media type — including text/* — fell through to the default case and was written to Resource.Blob as base64 binary:

switch contentValue.TopLevelMediaType() {
case "image": ...
case "audio": ...
default:
    return &mcp.EmbeddedResource{Resource: &mcp.ResourceContents{
        URI: contentValue.Name, MIMEType: contentValue.MediaType, Blob: data, // text ends up here too
    }}
}

mcp.ResourceContents deliberately separates Text (readable) from Blob (base64 binary). The reverse mapping already relies on this — mcpContentToAgentContent branches on Resource.Text != "" to reconstruct a TextContent. So the writer never producing Text is an asymmetry: a text DataContent round-tripped through MCP becomes an unreadable binary blob, and MCP clients reading the resource treat text as binary.

Fix

Add a text case that populates Resource.Text with the decoded string. Binary payloads (e.g. application/octet-stream) still use Blob — unchanged.

Test

TestAddToolReturnsTextDataAsTextResource (black-box) returns a text/plain DataContent from a tool and asserts the resulting EmbeddedResource carries the text in Resource.Text (and not Blob). Fails on main (Text = "", Blob populated), passes with the fix. The existing TestAddToolReturnsBinaryDataAsEmbeddedResource (octet-stream → Blob) is unchanged.

[Copilot]

Pull request overview

This pull request fixes an MCP content-mapping asymmetry when exposing an agent as an MCP tool: message.DataContent with text/* media types is now emitted as an MCP embedded resource with Resource.Text (readable) instead of Resource.Blob (binary), matching how the reverse mapping already interprets text resources.

Changes:

• Add a text media-type case in agentContentToMCPContent that sets mcp.ResourceContents.Text instead of Blob.
• Add a black-box regression test ensuring text/plain DataContent is returned as an embedded text resource (Text set, Blob empty).

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
tool/mcptool/mcp.go	Emits text/* DataContent as an MCP embedded resource using Resource.Text rather than Resource.Blob.
tool/mcptool/mcp_test.go	Adds a regression test covering text DataContent → embedded text resource behavior.

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[karianna]

@PratikDhanave merge conflict