From 5917dc7b49131b8d30ad03dbf3ac9dc4ea95b6cf Mon Sep 17 00:00:00 2001
From: Kevin O'Connell <17347501+kevoconnell@users.noreply.github.com>
Date: Mon, 21 Sep 2026 13:43:03 -0700
Subject: [PATCH] Log why an MCP server was unavailable, not just that it was

connectRemote and connectLocal both capture the failure reason into
status.error, but the log site only emitted status.status, so a connect
failure reached Loki as `status=failed` with no exception attached.

That is the common failure path: over 15 days of Replo production logs it
accounted for 109 of 111 occurrences, against 2 for the tools/list path
that does log its error. Losing the reason on the frequent one made the
whole class undiagnosable.

Status is a discriminated union and needs_auth carries no error, so the
`in` check narrows to the two variants that do.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
---
 packages/opencode/src/mcp/index.ts           |  7 +++++-
 packages/opencode/test/mcp/lifecycle.test.ts | 26 +++++++++++++++++++-
 2 files changed, 31 insertions(+), 2 deletions(-)

diff --git a/packages/opencode/src/mcp/index.ts b/packages/opencode/src/mcp/index.ts
index 9a750d0b2600..3e08ec5954a9 100644
--- a/packages/opencode/src/mcp/index.ts
+++ b/packages/opencode/src/mcp/index.ts
@@ -399,7 +399,12 @@ const layer = Layer.effect(
 
         if (!mcpClient) {
           if (status.status !== "connected" && status.status !== "disabled") {
-            yield* Effect.logWarning("server unavailable", { key, type: mcp.type, status: status.status })
+            yield* Effect.logWarning("server unavailable", {
+              key,
+              type: mcp.type,
+              status: status.status,
+              error: "error" in status ? status.error : undefined,
+            })
           }
           return { status } satisfies CreateResult
         }
diff --git a/packages/opencode/test/mcp/lifecycle.test.ts b/packages/opencode/test/mcp/lifecycle.test.ts
index 7a3048716aa9..d74edd0220b1 100644
--- a/packages/opencode/test/mcp/lifecycle.test.ts
+++ b/packages/opencode/test/mcp/lifecycle.test.ts
@@ -14,7 +14,7 @@ import {
   type Tool,
 } from "@modelcontextprotocol/sdk/types.js"
 import { LayerNode } from "@opencode-ai/core/effect/layer-node"
-import { Cause, Effect, Exit } from "effect"
+import { Cause, Effect, Exit, Logger } from "effect"
 import type { MCP as MCPNS } from "../../src/mcp/index"
 import { MCP } from "../../src/mcp/index"
 import { McpOAuthCallback } from "../../src/mcp/oauth-callback"
@@ -493,6 +493,30 @@ it.instance("unavailable remote server is marked failed without tools", () =>
   }),
 )
 
+it.instance("unavailable remote server surfaces the underlying error in the log", () =>
+  Effect.gen(function* () {
+    const server = yield* Effect.acquireRelease(
+      Effect.sync(() => Bun.serve({ port: 0, fetch: () => new Response("unavailable", { status: 503 }) })),
+      (http) => Effect.promise(() => http.stop(true)),
+    )
+    const mcp = yield* MCP.Service
+    const captured: Array<unknown> = []
+    const capture = Logger.make((options) => {
+      captured.push(options.message)
+    })
+
+    yield* mcp
+      .add("unlogged", remote(server.url.toString(), 500))
+      .pipe(Effect.provide(Logger.layer([capture], { mergeWithExisting: true })))
+
+    const logged = captured.find((item) => Array.isArray(item) && item[0] === "server unavailable")
+    expect(logged).toBeDefined()
+    if (!Array.isArray(logged)) return
+    const fields: unknown = logged[1]
+    expect(fields).toMatchObject({ status: "failed", error: expect.any(String) })
+  }),
+)
+
 it.instance("tools() prefixes sanitized server and tool names", () =>
   Effect.gen(function* () {
     const server = yield* lifecycleServer({ capabilities: { tools: {} } })