[FEATURE]: Full MCP client capabilities

[Arcadi4]

Feature hasn't been suggested before.

• I have verified this feature I'm about to request hasn't been suggested before.

Describe the enhancement you want to request

OpenCode's MCP client features are way behind the latest MCP standard. The gaps below were touched across many issues and PRs. I myself also encountered many difficulties due to the incomplete capability while developing MCP extensions for OpenCode. A one-time implementation is probably the cleanest fix to all of them and leaves no technical debt. This should also bring architectural advantages by flattening skills and tasks into the MCP layer.

I know this proposal for a project of this size might be a bit unreal. Rewriting the MCP client would definitely break loads of plugins and dependents, and the migration wouldn't be easy. Even so, the count of issues on this is large enough that I think the discussion is at least worth having.

Behavioral gaps

Missing entirely:

• sampling/createMessage and the sampling capability¹
• elicitation/create (form + URL modes), notifications/elicitation/complete, URLElicitationRequiredError -32042, and the elicitation capability²
• roots/list, notifications/roots/list_changed, and the roots capability³
• resources/subscribe / resources/unsubscribe / notifications/resources/updated / notifications/resources/list_changed⁴
• resources/templates/list⁵
• completion/complete⁶
• logging/setLevel and notifications/message⁷
• ping⁸
• The entire experimental tasks/* system from 2025-11-25⁹
• No capabilities object passed to new Client(...) — opencode advertises nothing at initialize¹⁰

Partial or incorrect:

• Progress: resetTimeoutOnProgress: true is passed to callTool, but onprogress is never wired through, so no progress token is ever sent and the timeout never resets in practice¹¹
• Cancellation: notifications/cancelled is never sent when a user aborts a tool call¹²
• Pagination: tools/list, resources/list, prompts/list are called without cursors; large lists silently truncate¹³
• Tool results: structuredContent is ignored¹⁴; outputSchema is stripped via TolerantListToolsResultSchema rather than validated¹⁵; resource_link¹⁶ and audio¹⁷ content types are dropped
• Tool annotations (readOnlyHint, destructiveHint, idempotentHint, openWorldHint)¹⁸ and icons[]¹⁹ are not consumed
• tools/list_changed is the only notification handled²⁰
• The server's InitializeResult.instructions field is fetched but never threaded into the LLM context, so the server's own usage hints are lost²¹

Authorization:

• Token refresh: refreshToken is stored but never executed — tokens silently expire²²
• RFC 8707 resource indicators: not sent²³
• RFC 9728 protected resource metadata: no explicit WWW-Authenticate resource_metadata parsing²⁴
• OpenID Connect Discovery 1.0 (added in 2025-11-25 as a second discovery option): not implemented²⁵
• Client ID Metadata Documents: not implemented²⁶
• Step-up authorization (insufficient_scope re-auth): not implemented²⁷
• Configured oauth.scope is not threaded into client metadata²⁸
• OAuth callback server (port 19876) is not stopped after auth completes
• OAuth callback page has unescaped error query parameter (XSS / CWE-79)
• headers config is not sent during mcp auth / mcp debug

Networking:

• Streamable HTTP Accept header doesn't always list both application/json and text/event-stream²⁹
• MCP-Protocol-Version header is not explicitly set; opencode mcp debug hardcodes protocolVersion: "2024-11-05" (three revisions behind)³⁰
• Mcp-Session-Id is not handled; session loss permanently breaks the connection³¹
• Last-Event-ID resumption is not implemented³²
• No transport-level auto-reconnect on socket errors²⁹
• Local stdio servers leak processes on disconnect / replace / rollback³³
• Origin header validation, SSRF protection, and session-hijacking mitigations from the spec's Security Best Practices are not implemented³⁴

Related issues and PRs

Sampling:

• [FEATURE]: Add MCP sampling support (createMessage) #11948

Elicitation:

• [FEATURE]: MCP elicitation support #23066
• [Feature Request] Native Elicitation support for MCP tools (Commit Message Refinement) #21231
• Add MCP elicitation support #8251
• feat(mcp): add elicitation request handling #8243
• Resource content type in MCP tool output is not supported #7878
• [FEATURE]: MCP Server instructions exposure #7373
• feat: Implement MCP server instruction fetching and integration #7334
• Support MCP Roots #2308

Resources (subscriptions, templates, model-invoked):

• [FEATURE]: Support MCP Resources (resources/read) in addition to MCP Tools #15535
• [FEATURE]:Resource subscription #12092
• [FEATURE]: MCP resource support for LLM #19102
• feat: add MCP resource subscription support #20682
• MCP Ressources with custom scheme #14753
• Bug: MCP resource @mention dispatches local file read instead of resources/read RPC #17543
• [FEATURE]:model-invoked MCP resources #8928
• MCP server not loaded when it only exposes a resource #11841
• MCP resources mimeType errors #8920
• Resource content type in MCP tool output is not supported #7878
• MCP Resources and Prompts #806
• feat(opencode): add MCP resource subscription support #20672
• feat(opencode): add MCP resource subscriptions #14569
• fix(opencode): ignore MCP resource file downloads #28466

Prompts:

• MCP prompts/list polling storm causes 100% CPU and renderer crash (TypeError: Failed to fetch) #27477
• MCP compatibility: don't error when prompts/list is unsupported (-32601) #26048
• MCP hidden from UI panel when server doesn't implement prompts/list (-32601) #20174
• Feature: Support MCP Prompt content injection as system message (hidden from user) #23480
• MCP prompts/list -32601 causes infinite npx process spawning and UI freeze #26012
• MCP client fails to initialize servers that don't implement optional prompts/list and resources/list methods #24985
• FastMCP compatibility with prompts #14613
• Issue: MCP Client disconnects immediately if Server returns a Prompt with an empty name {"name":""} #11816
• fix(mcp): cache unsupported prompt lists #28301

Progress:

• MCP client doesn't pass onprogress to callTool, so progress notifications can't extend the request timeout #28186
• [BUG]: opencode doesn't receive progress notifications from MCP servers #24965
• BUG-001: MCP tool call missing progressToken injection causing timeout on long reviews (-32001) #14499
• [FEATURE]: Progress indicator for MCP calls #26328
• Implement MCP progress notifications #2101
• [FEATURE]: MCP progress events via server/sdk #11821
• fix: pass onprogress to callTool so progressToken is set and long-run… #28246
• fix(mcp): pass onprogress so resetTimeoutOnProgress actually works #24964
• feat: add support for progress and cancel notifications #26161

Cancellation:

• [FEATURE]: Notify MCP servers when users cancel tool calls #24312
• feat: add support for progress and cancel notifications #26161

Structured content / outputSchema / resource_link:

• [FEATURE]: support structuredContent in responses of MCP #27263
• [FEATURE]: outputSchema for mcp tools should be exposed to LLM #27577
• MCP servers with outputSchema in tools/list break tool discovery #21591
• "Failed to get tools" when MCP server returns outputSchema in tools/list response #21373

OAuth / auth:

• MCP OAuth scope config is not applied to authorization flow #26301
• opencode mcp auth fails to open browser for OAuth flow (Google Drive MCP) #26195
• [FEATURE]: Support for MCP Support Client ID Metadata Document (CIMD) #25961
• Agents have no awareness of MCPs that need auth #27724
• opencode mcp auth does not print authorize URL to terminal, making it unusable over SSH #23951
• [Bug]: OAuth callback server (port 19876) not stopped after authentication, causing cross-instance CSRF failures #23568
• [Bug]: OAuth callback server (port 19876) not stopped after authentication completes #23563
• MCP OAuth: opencode mcp auth doesn't refresh token in active sessions #21702
• The mcp auth and mcp debug don't send configured headers during OAuth flow #20286
• Remote MCP OAuth browser flow never triggered - Datadog MCP silently fails to authenticate #18130
• fix(mcp): OAuth callback error page XSS via unescaped error parameter (CWE-79) #17364
• Remote MCP OAuth: browser never opens for re-authentication after session loss #16893
• Entra Authentication for MCP doesn't work #12308
• OAuth: client_credentials configuration results in authorization_code flow #7306
• Oath MCP always using root level even though my rul is .../xmcp/mcp #7135
• Reauthentication with Jira MCP via Oauth doesn't work #5953
• MCP with oauth doesn't work #5444
• GitHub MCP OAuth authentication fails with HTTP 422 (Dynamic client registration) #6067
• MCP OAuth: No way to configure callback port, breaking Slack MCP and other services with fixed redirect URIs #18955
• Remote MCP OAuth: token immediately overwritten after successful browser callback #17822
• Remote MCP servers with OAuth fail with 'No OAuth state saved for MCP server' #15546
• MCP server auth cache corruption causes JSON parse errors and requires manual file deletion to recover #25008
• Allow configurable redirect_uri for MCP OAuth clients #23787
• fix(mcp): handle oauth flows that connect without tokens #27785
• fix(mcp): support OAuth for servers without RFC 8414 discovery #27068
• fix(opencode): pass MCP OAuth scope to client metadata #26854
• feat(mcp): add OAuth redirect URI configuration for MCP servers #21385
• feat(mcp): add OAuth authentication support for remote MCP servers #5014

Transport / Streamable HTTP / SSE:

• MCP remote type: Accept header missing for Streamable HTTP servers (HTTP 400) #25650
• MCP streamable-HTTP transport: 'Session not found' from server permanently breaks tool calls (no auto-reconnect) #25137
• Remote MCP servers with Bearer headers fail on Linux (SSE 400, no Streamable HTTP fallback) #24946
• MCP client sends deprecated protocol version causing GitLab MCP server to fail (401) #20239
• Unauthenticated streamable HTTP MCP servers are reported as SSE errors instead of missing auth #16247
• MCP client doesn't send Accept header required by Context7 (causes 406 Not Acceptable error) #6972
• Feature Request: Add HTTP Streamable transport support for remote MCP servers #8058
• Remote MCP should negotiate HTTP/Streamable before deprecated SSE #6242
• Remote MCP: StreamableHTTPClientTransport missing Accept: text/event-stream header (406 from SSE-over-POST servers) #15103
• fix(mcp): ensure Accept header includes both required values for Streamable HTTP #25654
• fix(mcp): show auth-required status for streamable HTTP 401 #16315

Connection lifecycle / reconnect / stdio leaks:

• Local stdio MCP servers leak processes on disconnect / replace / rollback #26714
• MCP remote client has no transport-level retry on socket/connection errors #25287
• MCP client connection status goes stale after initial connect #25682
• fix(mcp): MCP status shows connected green after server reconnect even though server is down #24855
• fix(mcp): MCP server loses connection mid-session causing intermittent failures #23997
• MCP stdio transport sends batched requests without proper flush, breaking local servers #21516
• Bug: MCP tools permanently lost mid-session after single transient listTools() failure — no retry, no reconnect #17099
• fix: terminate local stdio mcp processes on disconnect / replace / rollback #26715
• fix(mcp): auto-reconnect on transport errors (#25287) #25670
• fix(opencode): reconnect MCP transport on session-expiration error and retry once #25135

Custom / dynamic headers:

• [FEATURE]: add mcp.call.before plugin hook for per-call MCP request headers (e.g. sessionID forwarding) #28225
• [FEATURE]: add dynamic MCP headersHelper support for remote authentication #27161
• Remote MCP headers: {env:...} interpolation not working #23664
• The mcp auth and mcp debug don't send configured headers during OAuth flow #20286
• MCP config should allow headers to be sent for remote mcp clients #1095
• feat(mcp): add mcp.call.before plugin hook for per-call MCP request headers #28319

Server notifications / _meta / logging:

• [FEATURE]: Support _meta when calling downstream MCP tools #17084
• [FEATURE]: propagate W3C trace context on outbound MCP HTTP/SSE requests #28295
• feat: handle MCP notifications/message from servers #19928
• feat(mcp): forward session ID and plugin _meta to MCP tool calls #21624

TLS / mTLS / cert validation:

• [FEATURE]: Mutual TLS / client-side certificate support for remote MCP #26862
• [FEATURE]: Add MCP client support for skipping certificate validation #23506

Windows-specific

• Python FastMCP server (STDIO) exits immediately on Windows — Not connected #26128
• fix: local MCP servers fail on Windows without native shell context #25904
• Local MCP server always times out on Windows #22310
• MCP Subsystem entirely unresponsive on Windows Desktop App (/config loads, but /mcp times out) #16449

Schema:

• [MCP] Schema validation fails for tools with multiple discriminated-union/nested-object parameters (e.g. obsidian-mcp-server patch_note, append_to_note) #26277
• MCP tool wrappers send extra _placeholder param causing ZodError in Supabase MCP + empty response in PythonAnywhere MCP #15041
• OpenCode warns on nonstandard MCP schema formats before jsonSchema() #20749
• MCP tool schema resolution: RecursionError on circular schema references #16899

Performance:

• [FEATURE]: Load MCP servers asynchronously to not block startup #20755
• [FEATURE]: Let LLM enable MCP on demand, default all MCPs off #7466
• [FEATURE]:Add mcp search tool, reduce mcp tool occupying a lot of context #8625

Footnotes

1. https://modelcontextprotocol.io/specification/2025-11-25/client/sampling ↩

2. https://modelcontextprotocol.io/specification/2025-11-25/client/elicitation ↩

3. https://modelcontextprotocol.io/specification/2025-11-25/client/roots ↩

4. https://modelcontextprotocol.io/specification/2025-11-25/server/resources#subscriptions ↩

5. https://modelcontextprotocol.io/specification/2025-11-25/server/resources#resource-templates ↩

6. https://modelcontextprotocol.io/specification/2025-11-25/server/utilities/completion ↩

7. https://modelcontextprotocol.io/specification/2025-11-25/server/utilities/logging ↩

8. https://modelcontextprotocol.io/specification/2025-11-25/basic/utilities/ping ↩

9. https://modelcontextprotocol.io/specification/2025-11-25/basic/utilities/tasks ↩

10. https://modelcontextprotocol.io/specification/2025-11-25/basic/lifecycle#capability-negotiation ↩

11. https://modelcontextprotocol.io/specification/2025-11-25/basic/utilities/progress ↩

12. https://modelcontextprotocol.io/specification/2025-11-25/basic/utilities/cancellation ↩

13. https://modelcontextprotocol.io/specification/2025-11-25/server/utilities/pagination ↩

14. https://modelcontextprotocol.io/specification/2025-11-25/server/tools#structured-content ↩

15. https://modelcontextprotocol.io/specification/2025-11-25/server/tools#output-schema ↩

16. https://modelcontextprotocol.io/specification/2025-11-25/server/tools#resource-links ↩

17. https://modelcontextprotocol.io/specification/2025-11-25/server/tools#tool-result ↩

18. https://modelcontextprotocol.io/specification/2025-11-25/server/tools#tool ↩

19. https://modelcontextprotocol.io/specification/2025-11-25/basic#icons ↩

20. https://modelcontextprotocol.io/specification/2025-11-25/server/tools#list-changed-notification ↩

21. https://modelcontextprotocol.io/specification/2025-11-25/basic/lifecycle#initialization ↩

22. https://modelcontextprotocol.io/specification/2025-11-25/basic/authorization ↩

23. https://modelcontextprotocol.io/specification/2025-11-25/basic/authorization#resource-parameter-implementation ↩

24. https://modelcontextprotocol.io/specification/2025-11-25/basic/authorization#authorization-server-discovery ↩

25. https://modelcontextprotocol.io/specification/2025-11-25/basic/authorization#authorization-server-metadata-discovery ↩

26. https://modelcontextprotocol.io/specification/2025-11-25/basic/authorization#client-id-metadata-documents ↩

27. https://modelcontextprotocol.io/specification/2025-11-25/basic/authorization#scope-challenge-handling ↩

28. https://modelcontextprotocol.io/specification/2025-11-25/basic/authorization#scope-selection-strategy ↩

29. https://modelcontextprotocol.io/specification/2025-11-25/basic/transports#streamable-http ↩ ↩²

30. https://modelcontextprotocol.io/specification/2025-11-25/basic/transports#protocol-version-header ↩

31. https://modelcontextprotocol.io/specification/2025-11-25/basic/transports#session-management ↩

32. https://modelcontextprotocol.io/specification/2025-11-25/basic/transports#resumability-and-redelivery ↩

33. https://modelcontextprotocol.io/specification/2025-11-25/basic/transports#stdio ↩

34. https://modelcontextprotocol.io/specification/2025-11-25/basic/security_best_practices ↩