MCP Toolset headers are static — impossible to pass per-user JWT in multi-tenant scenarios

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Is your feature request related to a specific problem?

In a multi-user application (e.g. Spring Boot serving multiple authenticated users), each user has their own JWT token that must be forwarded to an MCP server requiring authentication.

Currently, SseServerParameters.headers() returns an ImmutableMap<String, Object> and StreamableHttpServerParameters.headers is a final Map<String, String>. Both are set at construction time and frozen for the lifetime of the toolset.

This means in a typical Spring Boot setup:

// At app startup — no user context available
SseServerParameters params = SseServerParameters.builder()
.url("https://mcp-server.example.com")
.headers(Map.of("Authorization", "Bearer ???")) // No token yet!
.build();
McpToolset toolset = new McpToolset(params);

When User A (with JWT-A) and User B (with JWT-B) both call the agent, the MCP server always receives the same frozen token (or none). There is no way to inject per-request authentication headers.

Describe the Solution You'd Like

• Root cause

  In DefaultMcpTransportBuilder, the asyncHttpRequestCustomizer is called per-request but always reads from the same frozen parameters object:

  .asyncHttpRequestCustomizer(
  (requestBuilder, method, uri, body, context) -> {
  streamableParams.headers() // ← always the same frozen map
  .forEach((key, value) -> requestBuilder.header(key, value));
  return Mono.just(requestBuilder);
  });

  The context parameter is available but never used for dynamic header resolution.

• Proposed solution

  Support a Supplier<Map<String, String>> (or similar functional interface) for headers, evaluated at each request:

  SseServerParameters params = SseServerParameters.builder()
  .url("https://mcp-server.example.com")
  .headersProvider(() -> {
  String jwt = SecurityContextHolder.getContext()
  .getAuthentication().getCredentials().toString();
  return Map.of("Authorization", "Bearer " + jwt);
  })
  .build();

  This would allow frameworks like Spring Security to inject the current user's token on every MCP call, without creating a new toolset per request.

Impact on your work

We are building a multi-tenant Spring Boot application where multiple authenticated users interact with ADK agents concurrently. Each user's request must be forwarded to MCP servers that enforce JWT-based authorization.

Currently, we have to create a new McpToolset instance per request to inject the user's token, which is inefficient (new connection per call) and defeats the purpose of connection pooling and session reuse. This is a blocker for any production multi-user deployment that relies on authenticated MCP servers.

Timeline: We are targeting a production release in Q4 2026. A fix or accepted workaround by then would be ideal.

Willingness to contribute

Are you interested in implementing this feature yourself or submitting a PR?
Yes — we are willing to submit a PR for this. We have already identified the affected files and a viable approach (see Proposed API below).

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🟡 Recommended Information

Describe Alternatives You've Considered

Create a new McpToolset per request — Works but creates a new transport/connection for every call. No session reuse, poor performance under load

Proposed API / Implementation

Add a headersProvider option alongside the existing static headers:

// SseServerParameters / StreamableHttpServerParameters
@Nullable
public abstract Supplier<Map<String, String>> headersProvider();

// DefaultMcpTransportBuilder — evaluate per-request instead of once
.asyncHttpRequestCustomizer(
(requestBuilder, method, uri, body, context) -> {
Supplier<Map<String, String>> provider = params.headersProvider();
if (provider != null) {
provider.get().forEach(requestBuilder::header);
}
return Mono.just(requestBuilder);
});

Usage with Spring Security:
SseServerParameters params = SseServerParameters.builder()
.url("https://mcp-server.example.com")
.headersProvider(() -> {
Authentication auth = SecurityContextHolder.getContext().getAuthentication();
return Map.of("Authorization", "Bearer " + auth.getCredentials());
})
.build();

This is backward-compatible — existing headers(Map) usage continues to work unchanged.

Additional Context

The asyncHttpRequestCustomizer in DefaultMcpTransportBuilder already receives a context parameter per-request but currently ignores it — the infrastructure for dynamic behavior is partially in place.