M5: rmcp stdio client and tool adapters

Adds an rmcp-based stdio MCP client in harness-mcp plus adapters exposing a configured server's tools as namespaced harness-tools, with an echo-server fixture and integration test, and wiring through harness-app.
2026-07-10 16:20:22 +02:00
parent f71a347061
commit fe6d00ce6d
8 changed files with 770 additions and 55 deletions
@@ -0,0 +1,86 @@
+#!/usr/bin/env python3
+"""Minimal MCP stdio server fixture for harness-mcp integration tests.
+
+Speaks newline-delimited JSON-RPC (the framing rmcp's child-process transport uses) and
+implements just enough of the protocol to be discovered and called: `initialize`,
+`notifications/initialized`, `tools/list`, and `tools/call`. Exposes one tool, `echo`,
+which returns its `text` argument, plus `boom`, which returns an error result.
+"""
+import json
+import sys
+
+PROTOCOL_VERSION = "2024-11-05"
+
+TOOLS = [
+    {
+        "name": "echo",
+        "description": "Returns the text it is given.",
+        "inputSchema": {
+            "type": "object",
+            "properties": {"text": {"type": "string"}},
+            "required": ["text"],
+        },
+    },
+    {
+        "name": "boom",
+        "description": "Always fails.",
+        "inputSchema": {"type": "object", "properties": {}},
+    },
+]
+
+
+def reply(msg_id, result):
+    sys.stdout.write(json.dumps({"jsonrpc": "2.0", "id": msg_id, "result": result}) + "\n")
+    sys.stdout.flush()
+
+
+def main():
+    # readline() rather than `for line in sys.stdin`: the latter's read-ahead buffer blocks
+    # until it fills, which would stall the JSON-RPC handshake line-by-line.
+    while True:
+        line = sys.stdin.readline()
+        if line == "":  # EOF: parent closed stdin
+            break
+        line = line.strip()
+        if not line:
+            continue
+        msg = json.loads(line)
+        method = msg.get("method")
+        msg_id = msg.get("id")
+
+        if method == "initialize":
+            reply(msg_id, {
+                "protocolVersion": PROTOCOL_VERSION,
+                "capabilities": {"tools": {}},
+                "serverInfo": {"name": "echo-fixture", "version": "0.1.0"},
+            })
+        elif method == "notifications/initialized":
+            pass  # notification: no response
+        elif method == "tools/list":
+            reply(msg_id, {"tools": TOOLS})
+        elif method == "tools/call":
+            params = msg.get("params") or {}
+            name = params.get("name")
+            args = params.get("arguments") or {}
+            if name == "echo":
+                reply(msg_id, {
+                    "content": [{"type": "text", "text": args.get("text", "")}],
+                    "isError": False,
+                })
+            elif name == "boom":
+                reply(msg_id, {
+                    "content": [{"type": "text", "text": "kaboom"}],
+                    "isError": True,
+                })
+            else:
+                reply(msg_id, {
+                    "content": [{"type": "text", "text": f"unknown tool {name}"}],
+                    "isError": True,
+                })
+        elif msg_id is not None:
+            # Unknown request: empty result keeps the client happy.
+            reply(msg_id, {})
+
+
+if __name__ == "__main__":
+    main()