|
| 1 | +# FastMCP Progress Notification Investigation |
| 2 | + |
| 3 | +**Date**: July 8, 2025 |
| 4 | +**Issue**: Progress notifications not working when FastMCP proxy uses HTTP-to-Stdio transport |
| 5 | +**Status**: Root cause identified, architectural limitation in FastMCP |
| 6 | + |
| 7 | +## Executive Summary |
| 8 | + |
| 9 | +Progress notifications work correctly when FastMCP proxy communicates with servers via HTTP-to-HTTP transport, but fail when using HTTP-to-Stdio transport. The issue is a context isolation problem in FastMCP's proxy implementation where progress tokens cannot be properly tracked across process boundaries. |
| 10 | + |
| 11 | +## Issue Description |
| 12 | + |
| 13 | +### Observed Behavior |
| 14 | + |
| 15 | +When running a server through MCPM's FastMCP proxy: |
| 16 | + |
| 17 | +**Working Case (HTTP → Proxy → HTTP):** |
| 18 | +- Progress notifications flow correctly from server to client |
| 19 | +- Progress tokens are preserved throughout the request lifecycle |
| 20 | + |
| 21 | +**Failing Case (HTTP → Proxy → Stdio):** |
| 22 | +- Progress notifications are received by proxy but fail to forward to client |
| 23 | +- Error: `[CONTEXT] No progress token available, skipping notification` |
| 24 | + |
| 25 | +### Error Logs |
| 26 | + |
| 27 | +``` |
| 28 | +2025-07-08 20:22:36,003 - timer.main - DEBUG - Sending progress notification: 10000/180000 ms |
| 29 | +[07/08/25 20:22:36] INFO FastMCP.fastmcp.server.proxy: received progress notification: progress=10000.0, total=180000.0, message=None |
| 30 | + DEBUG fastmcp.server.context: [CONTEXT] Reporting progress: progress=10000.0, total=180000.0, message=None |
| 31 | + DEBUG fastmcp.server.context: [CONTEXT] No progress token available, skipping notification |
| 32 | +``` |
| 33 | + |
| 34 | +## Technical Root Cause Analysis |
| 35 | + |
| 36 | +### MCP Protocol Support |
| 37 | + |
| 38 | +The MCP protocol **correctly supports** progress notifications across all transports: |
| 39 | + |
| 40 | +1. **Progress Token Propagation**: MCP passes progress tokens in `_meta.progressToken` field |
| 41 | +2. **Request Metadata**: Found in `/mcp/types.py:43-50`: |
| 42 | + ```python |
| 43 | + class RequestParams(BaseModel): |
| 44 | + class Meta(BaseModel): |
| 45 | + progressToken: ProgressToken | None = None |
| 46 | + """ |
| 47 | + If specified, the caller requests out-of-band progress notifications for |
| 48 | + this request (as represented by notifications/progress). The value of this |
| 49 | + parameter is an opaque token that will be attached to any subsequent |
| 50 | + notifications. |
| 51 | + """ |
| 52 | + ``` |
| 53 | + |
| 54 | +3. **Session Implementation**: In `/mcp/shared/session.py:443+`, `send_request` properly sets progress tokens: |
| 55 | + ```python |
| 56 | + # Set up progress token if progress callback is provided |
| 57 | + if progress_callback is not None: |
| 58 | + # Use request_id as progress token |
| 59 | + if "_meta" not in request_data["params"]: |
| 60 | + request_data["params"]["_meta"] = {} |
| 61 | + request_data["params"]["_meta"]["progressToken"] = request_id |
| 62 | + # Store the callback for this request |
| 63 | + self._progress_callbacks[request_id] = progress_callback |
| 64 | + ``` |
| 65 | + |
| 66 | +### FastMCP Implementation Details |
| 67 | + |
| 68 | +**Progress Handler Location**: `/fastmcp/server/proxy.py:514-539` |
| 69 | +```python |
| 70 | +@classmethod |
| 71 | +async def default_progress_handler( |
| 72 | + cls, |
| 73 | + progress: float, |
| 74 | + total: float | None, |
| 75 | + message: str | None, |
| 76 | +) -> None: |
| 77 | + """ |
| 78 | + A handler that forwards the progress notification from the remote server to the proxy's connected clients. |
| 79 | + """ |
| 80 | + ctx = get_context() |
| 81 | + logger.info("received progress notification: progress=%s, total=%s, message=%s", progress, total, message) |
| 82 | + await ctx.report_progress(progress, total, message) |
| 83 | +``` |
| 84 | + |
| 85 | +**Context Report Progress**: `/fastmcp/server/context.py:125-155` |
| 86 | +```python |
| 87 | +async def report_progress( |
| 88 | + self, progress: float, total: float | None = None, message: str | None = None |
| 89 | +) -> None: |
| 90 | + progress_token = ( |
| 91 | + self.request_context.meta.progressToken |
| 92 | + if self.request_context.meta |
| 93 | + else None |
| 94 | + ) |
| 95 | + |
| 96 | + if progress_token is None: |
| 97 | + logger.debug("[CONTEXT] No progress token available, skipping notification") |
| 98 | + return |
| 99 | + |
| 100 | + await self.session.send_progress_notification( |
| 101 | + progress_token=progress_token, |
| 102 | + progress=progress, |
| 103 | + total=total, |
| 104 | + message=message, |
| 105 | + related_request_id=self.request_id, |
| 106 | + ) |
| 107 | +``` |
| 108 | + |
| 109 | +### Context Isolation Problem |
| 110 | + |
| 111 | +**HTTP-to-HTTP Flow (✅ Working):** |
| 112 | +``` |
| 113 | +Client Request (HTTP) → Proxy → HTTP Server |
| 114 | +├─ Request context with progressToken flows through HTTP session |
| 115 | +├─ Progress handler executes in same HTTP context |
| 116 | +└─ Progress notifications reference original progressToken |
| 117 | +``` |
| 118 | + |
| 119 | +**HTTP-to-Stdio Flow (❌ Broken):** |
| 120 | +``` |
| 121 | +Client Request (HTTP) → Proxy → Stdio Subprocess |
| 122 | +├─ Request context with progressToken sent to subprocess |
| 123 | +├─ Subprocess runs in isolated process with separate context |
| 124 | +├─ Progress handler (`default_progress_handler`) runs in subprocess context |
| 125 | +├─ `get_context()` returns subprocess context (no original progressToken) |
| 126 | +└─ Progress notification dropped due to missing token |
| 127 | +``` |
| 128 | + |
| 129 | +### Key Technical Components |
| 130 | + |
| 131 | +1. **Context Management**: `/fastmcp/server/dependencies.py:27-33` |
| 132 | + ```python |
| 133 | + def get_context() -> Context: |
| 134 | + from fastmcp.server.context import _current_context |
| 135 | + context = _current_context.get() |
| 136 | + if context is None: |
| 137 | + raise RuntimeError("No active context found.") |
| 138 | + return context |
| 139 | + ``` |
| 140 | + |
| 141 | +2. **Context Variables**: Uses Python `ContextVar` which is isolated per async task/process |
| 142 | + |
| 143 | +3. **Stdio Transport**: `/fastmcp/client/transports.py:300+` - Creates subprocess with separate memory space |
| 144 | + |
| 145 | +## Debug Implementation Added |
| 146 | + |
| 147 | +### Debug Middleware |
| 148 | + |
| 149 | +Created `MCPMDebugMiddleware` in `/src/mcpm/fastmcp_integration/middleware.py:19-198`: |
| 150 | + |
| 151 | +- **Limitation Documented**: Middleware only intercepts messages flowing FROM clients TO servers |
| 152 | +- **Progress notifications flow FROM servers TO clients**, bypassing middleware pipeline |
| 153 | +- **Added debug flags** to `mcpm run --debug` and `mcpm profile run --debug` |
| 154 | + |
| 155 | +### Enhanced Logging |
| 156 | + |
| 157 | +Added debug logging in multiple layers: |
| 158 | + |
| 159 | +1. **Proxy Progress Handler**: `/fastmcp/server/proxy.py:524+` |
| 160 | +2. **Context Report Progress**: `/fastmcp/server/context.py:134-147` |
| 161 | +3. **Debug context inspection** in progress handler |
| 162 | + |
| 163 | +## Architectural Limitation |
| 164 | + |
| 165 | +### Why This Is Hard to Fix |
| 166 | + |
| 167 | +FastMCP's proxy architecture has a fundamental limitation: |
| 168 | + |
| 169 | +1. **Process Isolation**: Stdio servers run in separate processes with isolated memory |
| 170 | +2. **Context Variables**: `_current_context` is per-process, not shared |
| 171 | +3. **Missing Mapping**: No mechanism to associate stdio server progress with original client requests |
| 172 | + |
| 173 | +### Required Fix |
| 174 | + |
| 175 | +To properly support HTTP-to-Stdio progress notifications, FastMCP would need: |
| 176 | + |
| 177 | +1. **Request Tracking**: Maintain mapping between client requests and stdio subprocess instances |
| 178 | +2. **Token Preservation**: Store original progress tokens when spawning stdio requests |
| 179 | +3. **Context Injection**: Inject correct progress token when handling progress from stdio servers |
| 180 | + |
| 181 | +Example fix architecture: |
| 182 | +```python |
| 183 | +class ProxyRequestTracker: |
| 184 | + def __init__(self): |
| 185 | + self._request_mapping: Dict[subprocess_id, ProgressToken] = {} |
| 186 | + |
| 187 | + def track_request(self, subprocess_id: str, progress_token: ProgressToken): |
| 188 | + self._request_mapping[subprocess_id] = progress_token |
| 189 | + |
| 190 | + def get_progress_token(self, subprocess_id: str) -> ProgressToken | None: |
| 191 | + return self._request_mapping.get(subprocess_id) |
| 192 | +``` |
| 193 | + |
| 194 | +## Current Workarounds |
| 195 | + |
| 196 | +### For Users |
| 197 | + |
| 198 | +1. **Use HTTP servers** when progress notifications are needed: |
| 199 | + ```bash |
| 200 | + mcpm run server-name --http --port 8001 |
| 201 | + ``` |
| 202 | + |
| 203 | +2. **Debug with HTTP-to-HTTP** setup: |
| 204 | + ```bash |
| 205 | + mcpm run timer --http --debug |
| 206 | + ``` |
| 207 | + |
| 208 | +### For Development |
| 209 | + |
| 210 | +1. **Enable comprehensive debug logging**: |
| 211 | + ```bash |
| 212 | + export PYTHONUNBUFFERED=1 |
| 213 | + export LOG_LEVEL=DEBUG |
| 214 | + mcpm run server-name --debug |
| 215 | + ``` |
| 216 | + |
| 217 | +2. **Use enhanced logging** in FastMCP components (already added) |
| 218 | + |
| 219 | +## Files Modified During Investigation |
| 220 | + |
| 221 | +### MCPM Code Changes |
| 222 | + |
| 223 | +1. **Debug Middleware**: `/src/mcpm/fastmcp_integration/middleware.py` |
| 224 | + - Added `MCPMDebugMiddleware` class with comprehensive logging |
| 225 | + - Handles all MCP operations: tools, resources, prompts, notifications |
| 226 | + - Safe serialization for complex objects |
| 227 | + |
| 228 | +2. **Proxy Factory**: `/src/mcpm/fastmcp_integration/proxy.py` |
| 229 | + - Added `debug: bool` parameter to `MCPMProxyFactory` |
| 230 | + - Debug middleware added first in middleware chain when enabled |
| 231 | + - Updated convenience function `create_mcpm_proxy()` |
| 232 | + |
| 233 | +3. **CLI Commands**: |
| 234 | + - `/src/mcpm/commands/run.py`: Added `--debug` flag |
| 235 | + - `/src/mcpm/commands/profile/run.py`: Added `--debug` flag |
| 236 | + - Debug parameter propagated through all execution paths |
| 237 | + |
| 238 | +### FastMCP Library Changes (for debugging) |
| 239 | + |
| 240 | +1. **Proxy Progress Handler**: `/fastmcp/server/proxy.py:524` |
| 241 | + - Fixed logging syntax error (string formatting) |
| 242 | + - Added comprehensive debug context inspection |
| 243 | + |
| 244 | +2. **Context Report Progress**: `/fastmcp/server/context.py:134-147` |
| 245 | + - Added detailed debug logging for progress reporting |
| 246 | + - Logs progress token availability and notification sending |
| 247 | + |
| 248 | +## Testing and Validation |
| 249 | + |
| 250 | +### Confirmed Working Cases |
| 251 | + |
| 252 | +- **HTTP-to-HTTP**: Progress notifications work correctly |
| 253 | +- **Local servers**: Progress works when not proxied |
| 254 | +- **Debug logging**: All layers now provide detailed debugging information |
| 255 | + |
| 256 | +### Confirmed Failing Cases |
| 257 | + |
| 258 | +- **HTTP-to-Stdio**: Progress notifications fail due to context isolation |
| 259 | +- **Any proxied stdio server**: Same issue across all stdio transport types |
| 260 | + |
| 261 | +### Debug Output Examples |
| 262 | + |
| 263 | +**Working HTTP-to-HTTP**: |
| 264 | +``` |
| 265 | +[PROXY DEBUG] TOOL CALL: timer_tool |
| 266 | +[CONTEXT] Reporting progress: progress=10000.0, total=180000.0, message=None |
| 267 | +[CONTEXT] Sending progress notification with token=12345 |
| 268 | +``` |
| 269 | + |
| 270 | +**Failing HTTP-to-Stdio**: |
| 271 | +``` |
| 272 | +[PROXY DEBUG] TOOL CALL: timer_tool |
| 273 | +INFO: received progress notification: progress=10000.0, total=180000.0, message=None |
| 274 | +[CONTEXT] Reporting progress: progress=10000.0, total=180000.0, message=None |
| 275 | +[CONTEXT] No progress token available, skipping notification |
| 276 | +``` |
| 277 | + |
| 278 | +## Future Work Recommendations |
| 279 | + |
| 280 | +### FastMCP Core Fix |
| 281 | + |
| 282 | +The proper solution requires changes to FastMCP core: |
| 283 | + |
| 284 | +1. **Enhance ProxyClient**: Maintain request-to-subprocess mapping |
| 285 | +2. **Modify default_progress_handler**: Inject correct progress token from mapping |
| 286 | +3. **Add request lifecycle tracking**: Clean up mappings when requests complete |
| 287 | + |
| 288 | +### Alternative Approaches |
| 289 | + |
| 290 | +1. **HTTP Wrapper Pattern**: Create HTTP servers that wrap stdio servers |
| 291 | +2. **Progress Relay Service**: Separate service to bridge context gaps |
| 292 | +3. **Enhanced Middleware**: Middleware that can intercept outgoing notifications |
| 293 | + |
| 294 | +### Testing Strategy |
| 295 | + |
| 296 | +1. **Unit tests** for request tracking components |
| 297 | +2. **Integration tests** for HTTP-to-Stdio progress scenarios |
| 298 | +3. **Performance testing** for overhead of request tracking |
| 299 | + |
| 300 | +## References |
| 301 | + |
| 302 | +### Key Files Investigated |
| 303 | + |
| 304 | +- `/mcp/types.py`: MCP protocol definitions including progress tokens |
| 305 | +- `/mcp/shared/session.py`: Base session with progress token handling |
| 306 | +- `/mcp/client/session.py`: Client session implementation |
| 307 | +- `/fastmcp/server/proxy.py`: Proxy server and progress handlers |
| 308 | +- `/fastmcp/server/context.py`: Request context and progress reporting |
| 309 | +- `/fastmcp/server/dependencies.py`: Context management and retrieval |
| 310 | +- `/fastmcp/client/transports.py`: Transport implementations including stdio |
| 311 | + |
| 312 | +### FastMCP Version |
| 313 | + |
| 314 | +- **Version**: 2.10.2 (from `pyproject.toml`) |
| 315 | +- **Library location**: `.venv/lib/python3.13/site-packages/fastmcp/` |
| 316 | + |
| 317 | +### Related Issues |
| 318 | + |
| 319 | +This issue demonstrates a broader challenge in MCP proxy implementations: maintaining request context across different transport types. Similar issues may exist in other proxy scenarios where context isolation occurs. |
| 320 | + |
| 321 | +--- |
| 322 | + |
| 323 | +**Investigation completed by**: Claude Code |
| 324 | +**Next steps**: Resume work on implementing FastMCP core fix or alternative workarounds |
| 325 | +**Priority**: Medium (affects progress notification functionality, workarounds available) |
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