mochengqian opened a new issue, #952:
URL: https://github.com/apache/dubbo-go-pixiu/issues/952
### ✅ Verification Checklist
- [x] 🔍 I have searched the existing issues and confirmed this is not a
duplicate
### 📦 Dubbo-go-pixiu Version
`develop` (reproduced on commit `798fcc75`, the parent of #937 — so this is
independent of the MCP tool router work).
### 🚀 Go Version
go 1.21+ (observed on go 1.26, darwin/arm64)
### 📝 Bug Description
When an MCP `tools/call` is forwarded to a backend cluster, the gateway
wraps the
backend response into the MCP JSON-RPC envelope and **overwrites**
`ctx.TargetResp`
in `sendMCPResponse`. The wrapped envelope is larger than the original
backend
body, but the upstream `Content-Length` header left on the response context
is
**not cleared**. The downstream HTTP writer then fails with:
```
Write response failed: http: wrote more than the declared Content-Length
```
As a result the client receives an **empty response body** for an otherwise
successful, authorized tool call. The routing/forwarding itself works — only
the
final response write is broken.
### 🔄 Steps to Reproduce
1. Configure a minimal MCP server filter with one tool mapped to a backend
cluster (no router needed):
```yaml
http_filters:
- name: "dgp.filter.mcp.mcpserver"
config:
server_info: { name: "MCP Baseline", version: "1.0.0" }
tools:
- name: "search_kb"
description: "Search the knowledge base"
cluster: "mock-server"
request: { method: "GET", path: "/api/kb", timeout: "10s" }
args:
- { name: "q", type: "string", in: "query", required: true }
- name: "dgp.filter.http.httpproxy"
clusters:
- name: "mock-server"
type: "STATIC"
lb_policy: "ROUND_ROBIN"
endpoints:
- socket_address: { address: "127.0.0.1", port: 8081 }
```
2. Run any backend on `:8081` that returns 200 + a JSON body for `/api/kb`.
3. `initialize`, then `tools/call`:
```bash
curl -s -X POST http://localhost:8888/mcp \
-H "Content-Type: application/json" -H "Mcp-Session-Id: s1" \
-d
'{"jsonrpc":"2.0","id":1,"method":"initialize","params":{"protocolVersion":"2025-06-18","capabilities":{},"clientInfo":{"name":"demo","version":"1.0"}}}'
curl -s -X POST http://localhost:8888/mcp \
-H "Content-Type: application/json" -H "Mcp-Session-Id: s1" \
-d
'{"jsonrpc":"2.0","id":2,"method":"tools/call","params":{"name":"search_kb","arguments":{"q":"reset"}}}'
```
### ✅ Expected Behavior
The second call returns the backend response wrapped in a standard MCP
`tools/call` result, e.g.
`{"jsonrpc":"2.0","id":2,"result":{"content":[...]}}`.
### ❌ Actual Behavior
The second call returns an **empty body**. The gateway log shows:
```
INFO mcpserver/handlers.go forwarding tool call: search_kb -> GET /api/kb
(cluster: mock-server)
ERROR http/manager.go:139 Write response failed: http: wrote more than
the declared Content-Length
```
This was verified on the clean `develop` baseline (`798fcc75`) which
contains **no
MCP tool router code** at all, confirming the defect is pre-existing and not
introduced by #937.
### 💡 Possible Solution
`sendMCPResponse` (`pkg/filter/mcp/mcpserver/handlers.go`) overwrites
`ctx.TargetResp` with the larger wrapped body but never clears the stale
`Content-Length`. The sibling path `sendJSONResponse`
(`pkg/filter/mcp/mcpserver/filter.go`) already calls
`ctx.ClearContentLengthHeader()` before writing — the tool-call response
path is
missing the equivalent call.
Proposed fix: call `ctx.ClearContentLengthHeader()` in `sendMCPResponse`
after
overwriting `TargetResp` (JSON branch), and add a regression test asserting a
non-empty wrapped body for a forwarded tool call. The SSE branch should be
checked
for the same omission.
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