@@ -42,7 +42,7 @@ Everything else on this page is identical across all four. Headers, subprocesses
 Four read-only properties, populated the moment you enter the block:
 
 * `client.server_info`: the server's identity, or `None` for a 2026-era server that does not report one (python-sdk servers do by default). `server_info.name` here is `"Bookshop"`, `server_info.version` is whatever the server reports.
-* `client.server_capabilities`: what the server can do (`tools`, `resources`, `prompts`, `completions`, ...). A capability the server doesn't have is `None`.
+* `client.server_capabilities`: what the server can do (`tools`, `resources`, `prompts`, `completions`, ...). A capability the server doesn't have is `None`. Capabilities the SDK doesn't model, such as MCP Events' `events`, are in `client.server_capabilities.model_extra`.
 * `client.protocol_version`: the protocol version the two sides agreed on. Here it is `"2026-07-28"`.
 * `client.instructions`: the server's `instructions=` string, or `None` if it didn't set one.
 
 
@@ -365,6 +365,8 @@ params = CallToolRequestParams(
 
 If you relied on extra fields round-tripping through MCP types, move that data into `_meta`.
 
+`ClientCapabilities` and `ServerCapabilities` are exceptions: the spec defines them as an open set, so unknown top-level keys are kept in `model_extra` and sent on the wire.
+
 ### Resource URI type changed from `AnyUrl` to `str`
 
 The `uri` field on resource-related types now uses `str` instead of Pydantic's `AnyUrl`. This aligns with the [MCP specification schema](https://github.com/modelcontextprotocol/modelcontextprotocol/blob/main/schema/2025-11-25/schema.ts) which defines URIs as plain strings (`uri: string`) without strict URL validation. This change allows relative paths like `users/me` that were previously rejected.
@@ -2653,7 +2655,7 @@ Validation runs when the result is serialized onto the wire, not when the model
 
 ### Client validates inbound traffic against the protocol schema
 
-`ClientSession` now validates server requests, notifications, and results against the negotiated protocol version's schema before parsing them into `mcp.types` models. Spec-invalid server output that the previous monolith parse tolerated may now raise `pydantic.ValidationError` from `list_tools()`, `call_tool()`, and similar calls. `_meta` remains the sanctioned place for result extras (and `experimental` for capability extras).
+`ClientSession` now validates server requests, notifications, and results against the negotiated protocol version's schema before parsing them into `mcp.types` models. Spec-invalid server output that the previous monolith parse tolerated may now raise `pydantic.ValidationError` from `list_tools()`, `call_tool()`, and similar calls. `_meta` remains the sanctioned place for result extras. Capabilities are an open set, so unknown capability keys are kept.
 
 ### Unknown request methods now return `-32601` (Method not found)
 
 
@@ -106,12 +106,24 @@
 }
 
 # Classes the spec defines as open key-value bags: `_meta` content, the
-# JSON-Schema-document fields on `Tool`, and the schemas with explicit
-# `additionalProperties: {}`. These keep `extra="allow"` so the sieve preserves
-# arbitrary keys; every other class ignores extras. Per-version because codegen
-# reuses class names across versions for unrelated schemas (e.g. `Data`).
+# JSON-Schema-document fields on `Tool`, the capability objects (documented as
+# "not a closed set"), and the schemas with explicit `additionalProperties: {}`.
+# These keep `extra="allow"` so the sieve preserves arbitrary keys; every other
+# class ignores extras. Per-version because codegen reuses class names across
+# versions for unrelated schemas (e.g. `Data`).
 OPEN_CLASSES: dict[str, frozenset[str]] = {
-    "2025-11-25": frozenset({"Meta", "InputSchema", "OutputSchema", "Result", "GetTaskPayloadResult", "Data"}),
+    "2025-11-25": frozenset(
+        {
+            "Meta",
+            "InputSchema",
+            "OutputSchema",
+            "Result",
+            "GetTaskPayloadResult",
+            "Data",
+            "ClientCapabilities",
+            "ServerCapabilities",
+        }
+    ),
     "2026-07-28": frozenset(
         {
             "MetaObject",
@@ -122,6 +134,8 @@
             "InputSchema",
             "OutputSchema",
             "Result",
+            "ClientCapabilities",
+            "ServerCapabilities",
         }
     ),
 }
@@ -236,7 +250,7 @@ def patch(match: re.Match[str]) -> str:
         return match.group(0).replace('extra="ignore"', 'extra="allow"')
 
     source = re.sub(
-        r'^class (\w+)\(WireModel\):\n(?: {4}.*\n|\n)*? {4}model_config = ConfigDict\(\n {8}extra="ignore",\n {4}\)\n',
+        r'^class (\w+)\(WireModel\):\n(?: {4}.*\n|\n)*? {4}model_config = ConfigDict\(extra="ignore"\)\n',
         patch,
         source,
         flags=re.MULTILINE,
@@ -296,6 +310,9 @@ def build(entry: dict[str, str]) -> str:
     # them to full URLs so they resolve from the rendered API docs and pass the
     # strict mkdocs link validation.
     source = source.replace("](/", "](https://modelcontextprotocol.io/")
+    source = re.sub(
+        r'model_config = ConfigDict\(\n\s+(extra="ignore"),\n\s+\)', r"model_config = ConfigDict(\1)", source
+    )
     source = allow_open_class_extras(source, OPEN_CLASSES[version])
     if epilogue := EPILOGUES.get(version, ""):
         # Resolve aliases before rebuilding models with forward references.
 
@@ -393,6 +393,8 @@ class ClientCapabilities(MCPModel):
     `initialize` through 2025-11-25; per-request in `_meta` on 2026-07-28.
     """
 
+    model_config = ConfigDict(extra="allow")
+
     experimental: dict[str, dict[str, Any]] | None = None
     """Experimental, non-standard capabilities that the client supports."""
     sampling: SamplingCapability | None = None
@@ -485,6 +487,8 @@ class ServerTasksCapability(MCPModel):
 class ServerCapabilities(MCPModel):
     """Capabilities that a server may support. Not a closed set."""
 
+    model_config = ConfigDict(extra="allow")
+
     experimental: dict[str, dict[str, Any]] | None = None
     """Experimental, non-standard capabilities that the server supports."""