MCP: every tool-list failure collapses to "Failed to get tools" — the underlying error is discarded

Summary

When OpenCode's MCP client cannot obtain a server's tool list, the user-visible status collapses to the single string "Failed to get tools", and the error that actually caused it is discarded — not logged anywhere, not attached to the status. This makes an entire class of MCP failures undiagnosable from the API, the CLI, or logs.

Pinned commit: 05ea5073be967c779d326929b2de6228dda4159d (dev).

Where

packages/opencode/src/mcp/catalog.ts:38-40

export function defs(client: Client, timeout?: number) {
  return listTools(client, timeout ?? DEFAULT_TIMEOUT).pipe(Effect.catch(() => Effect.void))
}

Effect.catch(() => Effect.void) turns any failure into undefined, with no log line.

packages/opencode/src/mcp/index.ts:390-394

const listed = mcpClient.getServerCapabilities()?.tools ? yield* McpCatalog.defs(mcpClient, mcp.timeout) : []
if (!listed) {
  return yield* Effect.fail(new Error("Failed to get tools"))
}

The thrown value is a fresh Error with a constant message — the original error is unreachable by then. index.ts:890 (authenticate) reports the same constant. The only thing GET /mcp can ever show is {"status":"failed","error":"Failed to get tools"}.

Two related details in the same area:

• catalog.ts:145-160 retries with TolerantListToolsResultSchema only when isOutputSchemaValidationError(error) matches (catalog.ts:164-166, a regex over the message). Every other shape — transport closed, capability assertion, a server 500, a body that fails to parse — skips the retry and is then swallowed by defs().
• if (!listed) can never fire for the "server advertises no tools" branch, because that branch yields [], and ![] is false. So "server has no tools" is silently treated as success-with-zero-tools, while "tool list threw" is indistinguishable from "server unreachable at list time".

Why it matters (real integration, not hypothetical)

We run OpenCode as an on-device server on Android (Bun for Android, x86_64 emulator + arm64 hardware) with a loopback-only bind, and POST /mcp with {"type":"remote","url":"http://10.0.2.2:4551/mcp"} against a spec-legal StreamableHTTP MCP server (MCP SDK 1.29.0, both response modes) behaves like this:

fixture (the MCP server), same run:
  [p5-mcp] streamable session initialized id=42921813-…     <- OpenCode's session
  [p5-mcp] POST /mcp -> 200 bytes=191   (initialize)
  [p5-mcp] POST /mcp -> 202            (notifications/initialized)
  ...nothing else. No tools/list request ever reaches the server.

OpenCode's API:
  POST /mcp p5-remote-http … -> 200 {"p5-remote-http":{"status":"failed","error":"Failed to get tools"}} [996ms]

Established by measurement, from the same device, same runtime, same route, same server:

• it is not a timeout: the registration returns in ~1 s, not at DEFAULT_TIMEOUT = 30_000;
• it is not response framing: enableJsonResponse (single application/json body) fails with the identical string;
• it is not the network or the server: a raw JSON-RPC client in the same process completes initialize → notifications/initialized → tools/list (681 bytes of tool definitions) against that same listener;
• the server-side handshake succeeded, so connect() resolved; the throw happens inside client.listTools() before a single request byte is written.

What we can say is exactly: upstream's client threw between a completed handshake and issuing tools/list. What we cannot say — because of the two snippets above — is what it threw. No OpenCode log line accompanies it (opencode-server.log for that run contains no entry for this MCP name at all), so from outside the project this is unreproducible-by-report: we can only say OpenCode returned a constant string.

Proposed fix (small, no behaviour change for healthy servers)

Preserve the cause. Either of:

// catalog.ts — keep the fallback semantics, log the reason
return listTools(client, timeout ?? DEFAULT_TIMEOUT).pipe(
  Effect.tapError((error) => Effect.logWarning("mcp tools/list failed", { error: String(error) })),
  Effect.catch(() => Effect.void),
)

// index.ts — keep the message, add the cause
if (!listed) {
  return yield* Effect.fail(new Error(`Failed to get tools (see server log for the underlying MCP error)`))
}

Better still: store the underlying message in the Status ({status:"failed", error} already exists — put the real message there, and keep Failed to get tools as a prefix if some consumer depends on it). Same for index.ts:890. And since Status.error is what every UI, opencode mcp list, and third-party client shows, this is the difference between "our MCP server is broken, here's why" and a string that could mean five different things.

Acceptance

GET /mcp (or the CLI) for a server whose tools/list fails must contain text that distinguishes at least: transport not connected / closed, method not supported by the server, response failed schema validation, server error response, and timeout — and a matching log line must exist with the original error (ideally error.cause too).

Note

This is blocking a downstream integration gate for us (we must not modify OpenCode's server API from our side, so we can only document it as a platform restriction). Happy to test a patch on the Android/Bun-for-Android configuration and report whether the message becomes actionable. If an upstream fix lands, we drop our workaround and link the release here.