mcp: OAuth on *.localhost regressed after the MCP client upgrade in 2.0.4 #54245

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1. github-actions

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   kitlangton

   on Oct 10, 2026
2. opencode-agent commented on Oct 10, 2026

   

   opencode-agentbot

   on Oct 10, 2026 – with OpenCode Agent

   Contributor

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   Thanks for the detailed report. I can confirm this.

   I ran a mock OAuth-protected MCP server at http://mcp.localhost:18259, with its metadata giving http://mcp.localhost:18259/token as the token endpoint, and configured it as a remote MCP server. Then I ran opencode mcp auth local and completed the authorize redirect. On 2.0.26, discovery, client registration and authorization all succeed. The code exchange then fails with the same error you saw:

   Refusing to send credentials to non-https token endpoint 'http://mcp.localhost:18259/token'. OAuth token requests MUST use TLS (localhost / 127.0.0.1 / ::1 are exempt).
   

   It still happens on the latest v2 (55dde88). That branch still pins @modelcontextprotocol/client@2.0.0, and that version's loopback check only allows the exact names localhost, 127.0.0.1 and ::1. The script below reproduces it. It needs mcp.localhost to resolve to 127.0.0.1, for example through /etc/hosts.

   Reproduction script (repro.sh)

   #!/usr/bin/env bash
   # Reproduces anomalyco/opencode#54245: OAuth token exchange refused for http://mcp.localhost.
   # Requires: bun, curl, `opencode` on PATH (or OC_DEV=/path/to/checkout), and
   # mcp.localhost resolving to 127.0.0.1 (e.g. `echo "127.0.0.1 mcp.localhost" >> /etc/hosts`).
   set -u
   DIR="$(cd "$(dirname "$0")" && pwd)"
   # Start a mock OAuth-protected MCP server on 127.0.0.1:18259 that advertises http://mcp.localhost:18259/token
   SRV="$(mktemp -d)/server.ts"
   cat > "$SRV" <<'TS'
   // Minimal OAuth-protected "MCP" server on http://mcp.localhost:18259
   // (needs `127.0.0.1 mcp.localhost` resolvable, e.g. via /etc/hosts).
   const BASE = "http://mcp.localhost:18259"
   const json = (o: unknown, status = 200, headers: Record<string, string> = {}) =>
     new Response(JSON.stringify(o), { status, headers: { "content-type": "application/json", ...headers } })
   
   Bun.serve({
     port: 18259,
     hostname: "127.0.0.1",
     async fetch(req) {
       const url = new URL(req.url)
       console.log(req.method, url.pathname, "Host:", req.headers.get("host"))
       if (url.pathname.startsWith("/.well-known/oauth-protected-resource"))
         return json({ resource: BASE + "/", authorization_servers: [BASE] })
       if (url.pathname.startsWith("/.well-known/oauth-authorization-server") || url.pathname.startsWith("/.well-known/openid-configuration"))
         return json({
           issuer: BASE,
           authorization_endpoint: BASE + "/authorize",
           token_endpoint: BASE + "/token",
           registration_endpoint: BASE + "/register",
           response_types_supported: ["code"],
           grant_types_supported: ["authorization_code", "refresh_token"],
           code_challenge_methods_supported: ["S256"],
           token_endpoint_auth_methods_supported: ["none"],
         })
       if (url.pathname === "/register") {
         const body = (await req.json()) as any
         return json({ ...body, client_id: "test-client", token_endpoint_auth_method: "none" }, 201)
       }
       if (url.pathname === "/authorize") {
         const r = new URL(url.searchParams.get("redirect_uri")!)
         r.searchParams.set("code", "test-code")
         if (url.searchParams.get("state")) r.searchParams.set("state", url.searchParams.get("state")!)
         return Response.redirect(r.toString(), 302)
       }
       if (url.pathname === "/token") {
         console.log("TOKEN ENDPOINT HIT")
         return json({ access_token: "tok", token_type: "Bearer", expires_in: 3600 })
       }
       if (req.headers.get("authorization") !== "Bearer tok")
         return new Response("unauthorized", {
           status: 401,
           headers: { "www-authenticate": `Bearer resource_metadata="${BASE}/.well-known/oauth-protected-resource"` },
         })
       return new Response("ok")
     },
   })
   console.log("listening", BASE)
   TS
   bun "$SRV" > /tmp/oc-54245-server.log 2>&1 &
   SRV_PID=$!
   trap 'kill $SRV_PID 2>/dev/null' EXIT
   sleep 1
   export HOME="${OC_HOME:-/tmp/oc-54245-home}"; mkdir -p "$HOME"
   # stop any opencode background service left over from a previous run
   pkill -f "opencode.*serv" 2>/dev/null; sleep 1
   PROJ="$(mktemp -d)"
   cat > "$PROJ/opencode.json" <<'EOF'
   { "mcp": { "servers": { "local": { "type": "remote", "url": "http://mcp.localhost:18259" } } } }
   EOF
   cd "$PROJ"
   if [ -n "${OC_DEV:-}" ]; then
     # the managed background service is spawned as a child bun process; pass the same flags to it
     export BUN_OPTIONS="--conditions=browser --preload=$OC_DEV/packages/tui/node_modules/@opentui/solid/scripts/preload.js"
     OC=(bun --conditions=browser --preload="$OC_DEV/packages/tui/node_modules/@opentui/solid/scripts/preload.js" "$OC_DEV/packages/cli/src/index.ts")
   else
     OC=(opencode)
   fi
   "${OC[@]}" --version
   OUT="$(mktemp)"
   timeout 60 "${OC[@]}" mcp auth local >"$OUT" 2>&1 &
   PID=$!
   # Act as the browser: wait for the authorize URL, then follow it to the local callback.
   for i in $(seq 1 60); do
     URL="$(grep -o 'http://mcp.localhost:18259/authorize[^ ]*' "$OUT" | head -1)"
     [ -n "$URL" ] && break
     sleep 0.5
   done
   echo "authorize URL: $URL"
   curl -s -L -o /dev/null -w "browser: followed redirect to %{url_effective} (HTTP %{http_code})\n" "$URL"
   wait $PID
   echo "token endpoint requests received by server: $(grep -c "TOKEN ENDPOINT HIT" /tmp/oc-54245-server.log)"
   echo "----- opencode mcp auth output -----"
   sed 's/\x1b\[[0-9;?]*[a-zA-Z]//g' "$OUT"

   Run it: bash repro.sh
3. opencode-agent

   added

   reproduced

   on Oct 10, 2026
4. argszero commented on Oct 10, 2026

   

   argszero

   on Oct 10, 2026

   Contributor

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   I'd like to work on this.

   The pinned @modelcontextprotocol/client@2.0.0 is already patched in this repo for OAuth behavior, so the smallest fix is to extend that patch with the rule typescript-sdk#2597 shipped: treat a hostname ending in .localhost as loopback, while still refusing non-loopback http: token endpoints. I have that change plus a regression test covering both halves running locally.

   Happy to bump the bundled client instead if you would rather take a newer release.
5. argszero commented on Oct 10, 2026

   

   argszero

   on Oct 10, 2026

   Contributor

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   Implemented in #54345.

   The existing @modelcontextprotocol/client@2.0.0 OAuth patch now accepts the whole .localhost namespace as loopback, per RFC 6761 §6.3 — the same predicate upstream added in typescript-sdk#2597 (shipped in client 2.3.1). The patch was regenerated mechanically rather than hand-edited, so the two pre-existing hunks (url.search = issuer.search, prompt=consent) are byte-identical and the header blob hashes remain the real 40-character ones.