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[Server][Client] Bridge W3C trace context between HTTP headers and _meta (SEP-414) (#473)

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RequestMeta::parseTraceContext() now falls back to the native traceparent/
tracestate/baggage HTTP headers when _meta carries none, and HeaderFactory
mirrors an application-set _meta trace context onto those same headers on
outbound requests. _meta wins when both are present.
This commit is contained in:
Christopher Hertel 2026-08-24 01:26:22 +02:00 • committed by GitHub
commit d59950f1ac
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1 changed files with 134 additions and 10 deletions
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35
src/Client/Stateless/HeaderFactory.php
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@ -13,6 +13,7 @@ namespace Mcp\Client\Stateless;
use Mcp\Schema\Enum\ProtocolVersion;
use Mcp\Schema\Wire\McpHeader;
use Mcp\Server\Stateless\RequestMeta;
/**
* The HTTP headers a modern-era client puts on every POST (SEP-2243, SEP-2575).
@ -41,18 +42,21 @@ final class HeaderFactory
public function forMessage(array $payload, ProtocolVersion $protocolVersion): array
{
$method = $payload['method'] ?? null;
$params = \is_array($payload['params'] ?? null) ? $payload['params'] : null;
// A response to a server-initiated request carries no method to mirror;
// the version header is unconditional and still applies.
// the version header is unconditional and still applies. Trace context
// still rides along if the application put it in `_meta` — the party
// that started the trace is not necessarily the one with a method to
// report.
if (!\is_string($method)) {
return [McpHeader::PROTOCOL_VERSION => $protocolVersion->value];
return [McpHeader::PROTOCOL_VERSION => $protocolVersion->value, ...$this->traceHeaders($params)];
}
$params = \is_array($payload['params'] ?? null) ? $payload['params'] : null;
$headers = [
McpHeader::PROTOCOL_VERSION => $protocolVersion->value,
McpHeader::METHOD => $method,
...$this->traceHeaders($params),
];
if (null !== $name = McpHeader::nameFor($method, $params)) {
@ -72,4 +76,27 @@ final class HeaderFactory
return $headers;
}
/**
* Mirrors W3C trace context from `_meta` onto its native HTTP headers, so
* an application that only knows how to set `_meta` — the transport-agnostic
* surface — still gets header-based propagation for free on HTTP.
*
* @param array<string, mixed>|null $params
*
* @return array<string, string>
*/
private function traceHeaders(?array $params): array
{
$meta = \is_array($params['_meta'] ?? null) ? $params['_meta'] : [];
$headers = [];
foreach (RequestMeta::TRACE_KEYS as $key) {
if (\is_string($meta[$key] ?? null)) {
$headers[$key] = $meta[$key];
}
}
return $headers;
}
}