fix(coderd/x/chatd): prevent nil required field in MCP tool schemas for OpenAI (#23538)

coderd/x/chatd/chatloop/chatloop.go

@@ -968,7 +968,11 @@ func buildToolDefinitions(tools []fantasy.AgentTool, activeTools []string, provi
 		inputSchema := map[string]any{
 			"type":       "object",
 			"properties": info.Parameters,
-			"required":   info.Required,
+		}
+		// Only include "required" when non-empty so that a nil slice
+		// never serializes to null, which OpenAI rejects.
+		if len(info.Required) > 0 {
+			inputSchema["required"] = info.Required
 		}
 		schema.Normalize(inputSchema)
 		prepared = append(prepared, fantasy.FunctionTool{

coderd/x/chatd/mcpclient/mcpclient.go

@@ -429,11 +429,17 @@ func newMCPTool(
 }
 
 func (t *mcpToolWrapper) Info() fantasy.ToolInfo {
+	// Ensure Required is never nil so that it serializes to [] instead
+	// of null. OpenAI rejects null for the JSON Schema "required" field.
+	required := t.required
+	if required == nil {
+		required = []string{}
+	}
 	return fantasy.ToolInfo{
 		Name:        t.prefixedName,
 		Description: t.description,
 		Parameters:  t.parameters,
-		Required:    t.required,
+		Required:    required,
 		Parallel:    true,
 	}
 }

coderd/x/chatd/mcpclient/mcpclient_test.go

@@ -361,6 +361,43 @@ func TestConnectAll_ToolInfoParameters(t *testing.T) {
 	assert.Contains(t, info.Required, "input")
 }
 
+// TestConnectAll_NilRequiredBecomesEmptySlice verifies that a tool
+// whose inputSchema omits "required" produces an empty slice instead
+// of nil.  A nil slice serializes to JSON null, which OpenAI rejects
+// with "None is not of type 'array'".
+func TestConnectAll_NilRequiredBecomesEmptySlice(t *testing.T) {
+	t.Parallel()
+	ctx := context.Background()
+	logger := slogtest.Make(t, &slogtest.Options{IgnoreErrors: true})
+
+	// noRequiredTool defines a tool with no required parameters.
+	noRequiredTool := mcpserver.ServerTool{
+		Tool: mcp.NewTool("optional_only",
+			mcp.WithDescription("A tool with no required fields"),
+			mcp.WithString("note", mcp.Description("An optional note")),
+		),
+		Handler: func(_ context.Context, req mcp.CallToolRequest) (*mcp.CallToolResult, error) {
+			return mcp.NewToolResultText("ok"), nil
+		},
+	}
+
+	ts := newTestMCPServer(t, noRequiredTool)
+	cfg := makeConfig("srv", ts.URL)
+	tools, cleanup := mcpclient.ConnectAll(ctx, logger, []database.MCPServerConfig{cfg}, nil)
+	t.Cleanup(cleanup)
+	require.Len(t, tools, 1)
+
+	info := tools[0].Info()
+	// Required must be a non-nil empty slice, not nil.
+	require.NotNil(t, info.Required, "Required should never be nil")
+	assert.Empty(t, info.Required, "Required should be empty for tools without required fields")
+
+	// Verify it serializes to [] not null.
+	bs, err := json.Marshal(info.Required)
+	require.NoError(t, err)
+	assert.Equal(t, "[]", string(bs))
+}
+
 // TestConnectAll_APIKeyAuth verifies that api_key auth sends the
 // configured header and value on every request.
 func TestConnectAll_APIKeyAuth(t *testing.T) {