diff --git a/client/src/components/ToolsTab.tsx b/client/src/components/ToolsTab.tsx
index 3f0261f7..50edaf51 100644
--- a/client/src/components/ToolsTab.tsx
+++ b/client/src/components/ToolsTab.tsx
@@ -160,184 +160,214 @@ const ToolsTab = ({
                     const required = isPropertyRequired(key, inputSchema);
                     return (
                       <div key={key}>
-                        <Label
-                          htmlFor={key}
-                          className="block text-sm font-medium text-gray-700 dark:text-gray-300"
-                        >
-                          {key}
-                          {required && (
-                            <span className="text-red-500 ml-1">*</span>
-                          )}
-                        </Label>
-                        {prop.type === "boolean" ? (
-                          <div className="flex items-center space-x-2 mt-2">
-                            <Checkbox
-                              id={key}
-                              name={key}
-                              checked={!!params[key]}
-                              onCheckedChange={(checked: boolean) =>
-                                setParams({
-                                  ...params,
-                                  [key]: checked,
-                                })
-                              }
-                            />
-                            <label
-                              htmlFor={key}
-                              className="text-sm font-medium text-gray-700 dark:text-gray-300"
-                            >
-                              {prop.description || "Toggle this option"}
-                            </label>
-                          </div>
-                        ) : prop.type === "string" && prop.enum ? (
-                          <Select
-                            value={
-                              params[key] === undefined
-                                ? ""
-                                : String(params[key])
-                            }
-                            onValueChange={(value) => {
-                              if (value === "") {
-                                setParams({
-                                  ...params,
-                                  [key]: undefined,
-                                });
-                              } else {
-                                setParams({
-                                  ...params,
-                                  [key]: value,
-                                });
-                              }
-                            }}
+                        <div className="flex justify-between">
+                          <Label
+                            htmlFor={key}
+                            className="block text-sm font-medium text-gray-700 dark:text-gray-300"
                           >
-                            <SelectTrigger id={key} className="mt-1">
-                              <SelectValue
-                                placeholder={
-                                  prop.description || "Select an option"
-                                }
-                              />
-                            </SelectTrigger>
-                            <SelectContent>
-                              {prop.enum.map((option) => (
-                                <SelectItem key={option} value={option}>
-                                  {option}
-                                </SelectItem>
-                              ))}
-                            </SelectContent>
-                          </Select>
-                        ) : prop.type === "string" ? (
-                          <Textarea
-                            id={key}
-                            name={key}
-                            placeholder={prop.description}
-                            value={
-                              params[key] === undefined
-                                ? ""
-                                : String(params[key])
-                            }
-                            onChange={(e) => {
-                              const value = e.target.value;
-                              if (value === "") {
-                                // Field cleared - set to undefined
-                                setParams({
-                                  ...params,
-                                  [key]: undefined,
-                                });
-                              } else {
-                                // Field has value - keep as string
-                                setParams({
-                                  ...params,
-                                  [key]: value,
-                                });
-                              }
-                            }}
-                            className="mt-1"
-                          />
-                        ) : prop.type === "object" || prop.type === "array" ? (
-                          <div className="mt-1">
-                            <DynamicJsonForm
-                              ref={(ref) => (formRefs.current[key] = ref)}
-                              schema={{
-                                type: prop.type,
-                                properties: prop.properties,
-                                description: prop.description,
-                                items: prop.items,
-                              }}
-                              value={
-                                (params[key] as JsonValue) ??
-                                generateDefaultValue(prop)
-                              }
-                              onChange={(newValue: JsonValue) => {
-                                setParams({
-                                  ...params,
-                                  [key]: newValue,
-                                });
-                                // Check validation after a short delay to allow form to update
-                                setTimeout(checkValidationErrors, 100);
-                              }}
-                            />
-                          </div>
-                        ) : prop.type === "number" ||
-                          prop.type === "integer" ? (
-                          <Input
-                            type="number"
-                            id={key}
-                            name={key}
-                            placeholder={prop.description}
-                            value={
-                              params[key] === undefined
-                                ? ""
-                                : String(params[key])
-                            }
-                            onChange={(e) => {
-                              const value = e.target.value;
-                              if (value === "") {
-                                // Field cleared - set to undefined
-                                setParams({
-                                  ...params,
-                                  [key]: undefined,
-                                });
-                              } else {
-                                // Field has value - try to convert to number, but store input either way
-                                const num = Number(value);
-                                if (!isNaN(num)) {
+                            {key}
+                            {required && (
+                              <span className="text-red-500 ml-1">*</span>
+                            )}
+                          </Label>
+                          {prop.nullable ? (
+                            <div className="flex items-center space-x-2">
+                              <Checkbox
+                                id={key}
+                                name={key}
+                                checked={params[key] === null}
+                                onCheckedChange={(checked: boolean) =>
                                   setParams({
                                     ...params,
-                                    [key]: num,
+                                    [key]: checked ? null : prop.default,
+                                  })
+                                }
+                              />
+                              <label
+                                htmlFor={key}
+                                className="text-sm font-medium text-gray-700 dark:text-gray-300"
+                              >
+                                null
+                              </label>
+                            </div>
+                          ) : null}
+                        </div>
+
+                        <div
+                          role="toolinputwrapper"
+                          className={`${prop.nullable && params[key] === null ? "pointer-events-none opacity-50" : ""}`}
+                        >
+                          {prop.type === "boolean" ? (
+                            <div className="flex items-center space-x-2 mt-2">
+                              <Checkbox
+                                id={key}
+                                name={key}
+                                checked={!!params[key]}
+                                onCheckedChange={(checked: boolean) =>
+                                  setParams({
+                                    ...params,
+                                    [key]: checked,
+                                  })
+                                }
+                              />
+                              <label
+                                htmlFor={key}
+                                className="text-sm font-medium text-gray-700 dark:text-gray-300"
+                              >
+                                {prop.description || "Toggle this option"}
+                              </label>
+                            </div>
+                          ) : prop.type === "string" && prop.enum ? (
+                            <Select
+                              value={
+                                params[key] === undefined
+                                  ? ""
+                                  : String(params[key])
+                              }
+                              onValueChange={(value) => {
+                                if (value === "") {
+                                  setParams({
+                                    ...params,
+                                    [key]: undefined,
                                   });
                                 } else {
-                                  // Store invalid input as string - let server validate
                                   setParams({
                                     ...params,
                                     [key]: value,
                                   });
                                 }
+                              }}
+                            >
+                              <SelectTrigger id={key} className="mt-1">
+                                <SelectValue
+                                  placeholder={
+                                    prop.description || "Select an option"
+                                  }
+                                />
+                              </SelectTrigger>
+                              <SelectContent>
+                                {prop.enum.map((option) => (
+                                  <SelectItem key={option} value={option}>
+                                    {option}
+                                  </SelectItem>
+                                ))}
+                              </SelectContent>
+                            </Select>
+                          ) : prop.type === "string" ? (
+                            <Textarea
+                              id={key}
+                              name={key}
+                              placeholder={prop.description}
+                              value={
+                                params[key] === undefined
+                                  ? ""
+                                  : String(params[key])
                               }
-                            }}
-                            className="mt-1"
-                          />
-                        ) : (
-                          <div className="mt-1">
-                            <DynamicJsonForm
-                              ref={(ref) => (formRefs.current[key] = ref)}
-                              schema={{
-                                type: prop.type,
-                                properties: prop.properties,
-                                description: prop.description,
-                                items: prop.items,
-                              }}
-                              value={params[key] as JsonValue}
-                              onChange={(newValue: JsonValue) => {
-                                setParams({
-                                  ...params,
-                                  [key]: newValue,
-                                });
-                                // Check validation after a short delay to allow form to update
-                                setTimeout(checkValidationErrors, 100);
+                              onChange={(e) => {
+                                const value = e.target.value;
+                                if (value === "") {
+                                  // Field cleared - set to undefined
+                                  setParams({
+                                    ...params,
+                                    [key]: undefined,
+                                  });
+                                } else {
+                                  // Field has value - keep as string
+                                  setParams({
+                                    ...params,
+                                    [key]: value,
+                                  });
+                                }
                               }}
+                              className="mt-1"
                             />
-                          </div>
-                        )}
+                          ) : prop.type === "object" ||
+                            prop.type === "array" ? (
+                            <div className="mt-1">
+                              <DynamicJsonForm
+                                ref={(ref) => (formRefs.current[key] = ref)}
+                                schema={{
+                                  type: prop.type,
+                                  properties: prop.properties,
+                                  description: prop.description,
+                                  items: prop.items,
+                                }}
+                                value={
+                                  (params[key] as JsonValue) ??
+                                  generateDefaultValue(prop)
+                                }
+                                onChange={(newValue: JsonValue) => {
+                                  setParams({
+                                    ...params,
+                                    [key]: newValue,
+                                  });
+                                  // Check validation after a short delay to allow form to update
+                                  setTimeout(checkValidationErrors, 100);
+                                }}
+                              />
+                            </div>
+                          ) : prop.type === "number" ||
+                            prop.type === "integer" ? (
+                            <Input
+                              type="number"
+                              id={key}
+                              name={key}
+                              placeholder={prop.description}
+                              value={
+                                params[key] === undefined
+                                  ? ""
+                                  : String(params[key])
+                              }
+                              onChange={(e) => {
+                                const value = e.target.value;
+                                if (value === "") {
+                                  // Field cleared - set to undefined
+                                  setParams({
+                                    ...params,
+                                    [key]: undefined,
+                                  });
+                                } else {
+                                  // Field has value - try to convert to number, but store input either way
+                                  const num = Number(value);
+                                  if (!isNaN(num)) {
+                                    setParams({
+                                      ...params,
+                                      [key]: num,
+                                    });
+                                  } else {
+                                    // Store invalid input as string - let server validate
+                                    setParams({
+                                      ...params,
+                                      [key]: value,
+                                    });
+                                  }
+                                }
+                              }}
+                              className="mt-1"
+                            />
+                          ) : (
+                            <div className="mt-1">
+                              <DynamicJsonForm
+                                ref={(ref) => (formRefs.current[key] = ref)}
+                                schema={{
+                                  type: prop.type,
+                                  properties: prop.properties,
+                                  description: prop.description,
+                                  items: prop.items,
+                                }}
+                                value={params[key] as JsonValue}
+                                onChange={(newValue: JsonValue) => {
+                                  setParams({
+                                    ...params,
+                                    [key]: newValue,
+                                  });
+                                  // Check validation after a short delay to allow form to update
+                                  setTimeout(checkValidationErrors, 100);
+                                }}
+                              />
+                            </div>
+                          )}
+                        </div>
                       </div>
                     );
                   },
diff --git a/client/src/components/__tests__/ToolsTab.test.tsx b/client/src/components/__tests__/ToolsTab.test.tsx
index ee8cac6d..d9237be2 100644
--- a/client/src/components/__tests__/ToolsTab.test.tsx
+++ b/client/src/components/__tests__/ToolsTab.test.tsx
@@ -44,6 +44,16 @@ describe("ToolsTab", () => {
         },
       },
     },
+    {
+      name: "tool4",
+      description: "Tool with nullable field",
+      inputSchema: {
+        type: "object" as const,
+        properties: {
+          num: { type: ["number", "null"] as const },
+        },
+      },
+    },
   ];
 
   const defaultProps = {
@@ -135,6 +145,38 @@ describe("ToolsTab", () => {
     });
   });
 
+  it("should allow specifying null value", async () => {
+    const mockCallTool = jest.fn();
+    const toolWithNullableField = mockTools[3];
+
+    renderToolsTab({
+      tools: [toolWithNullableField],
+      selectedTool: toolWithNullableField,
+      callTool: mockCallTool,
+    });
+
+    const nullToggleButton = screen.getByRole("checkbox", { name: /null/i });
+    expect(nullToggleButton).toBeInTheDocument();
+
+    await act(async () => {
+      fireEvent.click(nullToggleButton);
+    });
+
+    expect(screen.getByRole("toolinputwrapper").classList).toContain(
+      "pointer-events-none",
+    );
+
+    const runButton = screen.getByRole("button", { name: /run tool/i });
+    await act(async () => {
+      fireEvent.click(runButton);
+    });
+
+    // Tool should have been called with null value
+    expect(mockCallTool).toHaveBeenCalledWith(toolWithNullableField.name, {
+      num: null,
+    });
+  });
+
   it("should disable button and change text while tool is running", async () => {
     // Create a promise that we can resolve later
     let resolvePromise: ((value: unknown) => void) | undefined;
diff --git a/client/src/utils/__tests__/schemaUtils.test.ts b/client/src/utils/__tests__/schemaUtils.test.ts
index 0e832b09..98bfcb0b 100644
--- a/client/src/utils/__tests__/schemaUtils.test.ts
+++ b/client/src/utils/__tests__/schemaUtils.test.ts
@@ -188,6 +188,7 @@ describe("normalizeUnionType", () => {
     expect(normalized.type).toBe("boolean");
     expect(normalized.anyOf).toBeUndefined();
     expect(normalized.description).toBe("Optional boolean parameter");
+    expect(normalized.nullable).toBeTruthy();
   });
 
   test("normalizes array type with string and null to string type", () => {
@@ -200,6 +201,7 @@ describe("normalizeUnionType", () => {
 
     expect(normalized.type).toBe("string");
     expect(normalized.description).toBe("Optional string parameter");
+    expect(normalized.nullable).toBeTruthy();
   });
 
   test("normalizes array type with boolean and null to boolean type", () => {
@@ -212,6 +214,7 @@ describe("normalizeUnionType", () => {
 
     expect(normalized.type).toBe("boolean");
     expect(normalized.description).toBe("Optional boolean parameter");
+    expect(normalized.nullable).toBeTruthy();
   });
 
   test("normalizes anyOf with number and null to number type", () => {
@@ -225,6 +228,7 @@ describe("normalizeUnionType", () => {
     expect(normalized.type).toBe("number");
     expect(normalized.anyOf).toBeUndefined();
     expect(normalized.description).toBe("Optional number parameter");
+    expect(normalized.nullable).toBeTruthy();
   });
 
   test("normalizes anyOf with integer and null to integer type", () => {
@@ -238,6 +242,7 @@ describe("normalizeUnionType", () => {
     expect(normalized.type).toBe("integer");
     expect(normalized.anyOf).toBeUndefined();
     expect(normalized.description).toBe("Optional integer parameter");
+    expect(normalized.nullable).toBeTruthy();
   });
 
   test("normalizes array type with number and null to number type", () => {
@@ -250,6 +255,7 @@ describe("normalizeUnionType", () => {
 
     expect(normalized.type).toBe("number");
     expect(normalized.description).toBe("Optional number parameter");
+    expect(normalized.nullable).toBeTruthy();
   });
 
   test("normalizes array type with integer and null to integer type", () => {
@@ -262,6 +268,7 @@ describe("normalizeUnionType", () => {
 
     expect(normalized.type).toBe("integer");
     expect(normalized.description).toBe("Optional integer parameter");
+    expect(normalized.nullable).toBeTruthy();
   });
 
   test("handles anyOf with reversed order (null first)", () => {
@@ -273,6 +280,7 @@ describe("normalizeUnionType", () => {
 
     expect(normalized.type).toBe("string");
     expect(normalized.anyOf).toBeUndefined();
+    expect(normalized.nullable).toBeTruthy();
   });
 
   test("leaves non-union schemas unchanged", () => {
@@ -294,6 +302,7 @@ describe("normalizeUnionType", () => {
     const normalized = normalizeUnionType(schema);
 
     expect(normalized).toEqual(schema);
+    expect(normalized.nullable).toBeFalsy();
   });
 
   test("leaves anyOf with more than two types unchanged", () => {
@@ -343,6 +352,7 @@ describe("normalizeUnionType", () => {
     expect(normalized.minLength).toBe(1);
     expect(normalized.maxLength).toBe(100);
     expect(normalized.pattern).toBe("^[a-z]+$");
+    expect(normalized.nullable).toBeTruthy();
   });
 });
 
diff --git a/client/src/utils/jsonUtils.ts b/client/src/utils/jsonUtils.ts
index 08de50ba..3a9336f1 100644
--- a/client/src/utils/jsonUtils.ts
+++ b/client/src/utils/jsonUtils.ts
@@ -41,6 +41,7 @@ export type JsonSchemaType = {
   maximum?: number;
   minLength?: number;
   maxLength?: number;
+  nullable?: boolean;
   pattern?: string;
   format?: string;
   enum?: string[];
diff --git a/client/src/utils/schemaUtils.ts b/client/src/utils/schemaUtils.ts
index 4ad8e8ca..73137805 100644
--- a/client/src/utils/schemaUtils.ts
+++ b/client/src/utils/schemaUtils.ts
@@ -158,7 +158,7 @@ export function normalizeUnionType(schema: JsonSchemaType): JsonSchemaType {
     schema.anyOf.some((t) => (t as JsonSchemaType).type === "string") &&
     schema.anyOf.some((t) => (t as JsonSchemaType).type === "null")
   ) {
-    return { ...schema, type: "string", anyOf: undefined };
+    return { ...schema, type: "string", anyOf: undefined, nullable: true };
   }
 
   // Handle anyOf with exactly boolean and null (FastMCP pattern)
@@ -168,7 +168,7 @@ export function normalizeUnionType(schema: JsonSchemaType): JsonSchemaType {
     schema.anyOf.some((t) => (t as JsonSchemaType).type === "boolean") &&
     schema.anyOf.some((t) => (t as JsonSchemaType).type === "null")
   ) {
-    return { ...schema, type: "boolean", anyOf: undefined };
+    return { ...schema, type: "boolean", anyOf: undefined, nullable: true };
   }
 
   // Handle anyOf with exactly number and null (FastMCP pattern)
@@ -178,7 +178,7 @@ export function normalizeUnionType(schema: JsonSchemaType): JsonSchemaType {
     schema.anyOf.some((t) => (t as JsonSchemaType).type === "number") &&
     schema.anyOf.some((t) => (t as JsonSchemaType).type === "null")
   ) {
-    return { ...schema, type: "number", anyOf: undefined };
+    return { ...schema, type: "number", anyOf: undefined, nullable: true };
   }
 
   // Handle anyOf with exactly integer and null (FastMCP pattern)
@@ -188,7 +188,7 @@ export function normalizeUnionType(schema: JsonSchemaType): JsonSchemaType {
     schema.anyOf.some((t) => (t as JsonSchemaType).type === "integer") &&
     schema.anyOf.some((t) => (t as JsonSchemaType).type === "null")
   ) {
-    return { ...schema, type: "integer", anyOf: undefined };
+    return { ...schema, type: "integer", anyOf: undefined, nullable: true };
   }
 
   // Handle array type with exactly string and null
@@ -198,7 +198,7 @@ export function normalizeUnionType(schema: JsonSchemaType): JsonSchemaType {
     schema.type.includes("string") &&
     schema.type.includes("null")
   ) {
-    return { ...schema, type: "string" };
+    return { ...schema, type: "string", nullable: true };
   }
 
   // Handle array type with exactly boolean and null
@@ -208,7 +208,7 @@ export function normalizeUnionType(schema: JsonSchemaType): JsonSchemaType {
     schema.type.includes("boolean") &&
     schema.type.includes("null")
   ) {
-    return { ...schema, type: "boolean" };
+    return { ...schema, type: "boolean", nullable: true };
   }
 
   // Handle array type with exactly number and null
@@ -218,7 +218,7 @@ export function normalizeUnionType(schema: JsonSchemaType): JsonSchemaType {
     schema.type.includes("number") &&
     schema.type.includes("null")
   ) {
-    return { ...schema, type: "number" };
+    return { ...schema, type: "number", nullable: true };
   }
 
   // Handle array type with exactly integer and null
@@ -228,7 +228,7 @@ export function normalizeUnionType(schema: JsonSchemaType): JsonSchemaType {
     schema.type.includes("integer") &&
     schema.type.includes("null")
   ) {
-    return { ...schema, type: "integer" };
+    return { ...schema, type: "integer", nullable: true };
   }
 
   return schema;