Merge pull request #881 from jlowin/dev-20250619-173841

src/fastmcp/server/context.py

@@ -178,6 +178,37 @@ class Context:
         """Get the unique ID for this request."""
         return str(self.request_context.request_id)
 
+    @property
+    def session_id(self) -> str | None:
+        """Get the MCP session ID for HTTP transports.
+
+        Returns the session ID that can be used as a key for session-based
+        data storage (e.g., Redis) to share data between tool calls within
+        the same client session.
+
+        Returns:
+            The session ID for HTTP transports (SSE, StreamableHTTP), or None
+            for stdio and in-memory transports which don't use session IDs.
+
+        Example:
+            ```python
+            @server.tool
+            def store_data(data: dict, ctx: Context) -> str:
+                if session_id := ctx.session_id:
+                    redis_client.set(f"session:{session_id}:data", json.dumps(data))
+                    return f"Data stored for session {session_id}"
+                return "No session ID available (stdio/memory transport)"
+            ```
+        """
+        try:
+            from fastmcp.server.dependencies import get_http_headers
+
+            headers = get_http_headers(include_all=True)
+            return headers.get("mcp-session-id")
+        except RuntimeError:
+            # No HTTP context available (stdio/in-memory transport)
+            return None
+
     @property
     def session(self):
         """Access to the underlying session for advanced usage."""

tests/server/test_context.py

@@ -86,3 +86,40 @@ class TestParseModelPreferences:
     def test_parse_model_preferences_invalid_type(self, context):
         with pytest.raises(ValueError):
             context._parse_model_preferences(123)
+
+
+class TestSessionId:
+    def test_session_id_with_http_headers(self, context):
+        """Test that session_id returns the value from mcp-session-id header."""
+        mock_headers = {"mcp-session-id": "test-session-123"}
+
+        with patch(
+            "fastmcp.server.dependencies.get_http_headers", return_value=mock_headers
+        ):
+            assert context.session_id == "test-session-123"
+
+    def test_session_id_without_http_headers(self, context):
+        """Test that session_id returns None when no HTTP headers are available."""
+        with patch(
+            "fastmcp.server.dependencies.get_http_headers",
+            side_effect=RuntimeError("No active HTTP request found."),
+        ):
+            assert context.session_id is None
+
+    def test_session_id_with_missing_header(self, context):
+        """Test that session_id returns None when mcp-session-id header is missing."""
+        mock_headers = {"other-header": "value"}
+
+        with patch(
+            "fastmcp.server.dependencies.get_http_headers", return_value=mock_headers
+        ):
+            assert context.session_id is None
+
+    def test_session_id_with_empty_header(self, context):
+        """Test that session_id returns None when mcp-session-id header is empty."""
+        mock_headers = {"mcp-session-id": ""}
+
+        with patch(
+            "fastmcp.server.dependencies.get_http_headers", return_value=mock_headers
+        ):
+            assert context.session_id == ""  # Empty string is still returned as-is