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[Schema] Add resource_link content block (#400)

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* Add resource_link content block

The spec's `resource_link` content type (protocol revision 2025-06-18,
still present in 2025-11-25) had no corresponding schema class: it lets a
tool result or prompt message reference a resource by URI/name without
embedding its full contents, the way `EmbeddedResource` does. Without it,
tools returning many or large resource references (e.g. search results
over a DAM) are stuck smuggling URIs through text/structured content.

Add `Mcp\Schema\Content\ResourceLink`, mirroring the spec's `Resource`
shape (uri, name, title, description, mimeType, annotations, size, icons,
_meta) with a fixed `type: "resource_link"`, following the conventions of
the sibling `EmbeddedResource`/`ResourceDefinition` classes.

Register it everywhere `resource` content blocks are (de)serialized by
type per the spec's `ContentBlock` union:
- `CallToolResult::fromArray()`
- `PromptMessage` (constructor type, `fromArray()`, phpstan types)
- `PromptResultFormatter` (typed-array and direct-instance paths)

`SamplingMessage`/`CreateSamplingMessageResult` are intentionally left
unchanged: the spec's `SamplingMessageContentBlock` does not include
`ResourceLink`.

Fixes #399

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

* Add resource_link example coverage to discovery-userprofile

Add a lookup_user tool to the discovery-userprofile example: given a
userId, it returns a short text summary plus a resource_link pointing
at the existing user://{userId}/profile resource template, instead of
embedding the profile. This mirrors the PR's motivating use case (a
tool referencing many/large resources by URI rather than inlining
them) and exercises resource_link through the example's own inspector
snapshot tests (tools_call-lookup_user[_not_found], tools_list).

Wiring this up surfaced a real bug: ToolReference::extractStructuredContent()
passed any raw array return value straight through as structuredContent,
even when it held Content instances (e.g. the TextContent/ResourceLink
pair above). That serializes to a JSON list, which fails the spec's
requirement that structuredContent be an object - the inspector's MCP
client rejected the tool call outright until fixed. Skip structuredContent
extraction whenever the raw result contains a Content item, since those
are already represented in `content` and were never meant to double as
structured data.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

---------

Co-authored-by: Alex Rothberg <51059+cancan101@users.noreply.github.com>
Co-authored-by: Claude Fable 5 <noreply@anthropic.com>
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Alex Rothberg 2026-08-14 17:20:56 -04:00 • committed by GitHub
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13
docs/mcp-elements.md
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@ -107,7 +107,7 @@ public function returnVoid(): void { /* no return */ } // Empty conten
For fine control over output formatting:
```php
use Mcp\Schema\Content\{TextContent, ImageContent, AudioContent, EmbeddedResource};
use Mcp\Schema\Content\{TextContent, ImageContent, AudioContent, ResourceLink, EmbeddedResource};
public function getFormattedCode(): TextContent
{
@ -142,6 +142,17 @@ public function getEmbeddedResource(): EmbeddedResource
resource: ['uri' => 'file://data.json', 'text' => 'File content']
);
}
public function getResourceLink(): ResourceLink
{
// Reference a resource by URI without embedding its contents, e.g. when
// a tool result would otherwise need to inline many or large resources.
return new ResourceLink(
uri: 'file://data.json',
name: 'data.json',
mimeType: 'application/json'
);
}
```
#### Multiple Content Items