fix(cli): forward user input to MCP prompts with no declared arguments (#6571)

* fix(cli): forward user input to MCP prompts with no declared arguments

When a prompt declares no arguments, parseArgs() silently discarded
all user input. Forward named args as-is and positional input under
the "input" key, matching Claude Code's behavior.

Fixes #6563

* fix(cli): strip quotes and guard input key in MCP prompt arg forwarding

- Use positionalArgs.join(' ') instead of positionalArgsString to
  properly strip quotes from positional input, consistent with the
  existing single-arg path.
- Guard against overwriting a user-provided --input named arg with
  positional text.
- Add comment explaining the input key convention.

* fix(cli): update help text for no-argument MCP prompts

The help text previously said the prompt 'has no arguments', which is
now misleading — user input is forwarded as-is. Updated to explain
that free-form text is accepted and how it maps to the input key.

packages/cli/src/services/McpPromptLoader.test.ts

@@ -174,6 +174,51 @@ describe('McpPromptLoader', () => {
         pos3: 'p3 "with quotes"',
       });
     });
+
+    it('should forward positional input as "input" when promptArgs is undefined', () => {
+      const loader = new McpPromptLoader(mockConfig);
+      const result = loader.parseArgs('hello world', undefined);
+      expect(result).toEqual({ input: 'hello world' });
+    });
+
+    it('should forward positional input as "input" when promptArgs is empty', () => {
+      const loader = new McpPromptLoader(mockConfig);
+      const result = loader.parseArgs('hello world', []);
+      expect(result).toEqual({ input: 'hello world' });
+    });
+
+    it('should return empty object when promptArgs is undefined and no input', () => {
+      const loader = new McpPromptLoader(mockConfig);
+      const result = loader.parseArgs('', undefined);
+      expect(result).toEqual({});
+    });
+
+    it('should forward named args when promptArgs is undefined', () => {
+      const loader = new McpPromptLoader(mockConfig);
+      const result = loader.parseArgs('--key="value"', undefined);
+      expect(result).toEqual({ key: 'value' });
+    });
+
+    it('should forward both named and positional args when promptArgs is undefined', () => {
+      const loader = new McpPromptLoader(mockConfig);
+      const result = loader.parseArgs('--key="value" some text', undefined);
+      expect(result).toEqual({ key: 'value', input: 'some text' });
+    });
+
+    it('should strip quotes from positional input when promptArgs is undefined', () => {
+      const loader = new McpPromptLoader(mockConfig);
+      const result = loader.parseArgs('"hello world"', undefined);
+      expect(result).toEqual({ input: 'hello world' });
+    });
+
+    it('should not overwrite a named "input" arg with positional text', () => {
+      const loader = new McpPromptLoader(mockConfig);
+      const result = loader.parseArgs(
+        '--input="named value" some text',
+        undefined,
+      );
+      expect(result).toEqual({ input: 'named value' });
+    });
   });
 
   describe('loadCommands', () => {
@@ -222,6 +267,24 @@ describe('McpPromptLoader', () => {
       });
     });
 
+    it('should forward user input when prompt has no declared arguments', async () => {
+      vi.spyOn(cliCore, 'getMCPServerPrompts').mockReturnValue([
+        { ...mockPrompt, arguments: undefined },
+      ]);
+      const loader = new McpPromptLoader(mockConfigWithPrompts);
+      const commands = await loader.loadCommands(new AbortController().signal);
+      const action = commands[0].action!;
+      const context = {} as CommandContext;
+      const result = await action(context, 'some user input');
+      expect(mockPrompt.invoke).toHaveBeenCalledWith({
+        input: 'some user input',
+      });
+      expect(result).toEqual({
+        type: 'submit_prompt',
+        content: JSON.stringify('Hello, world!'),
+      });
+    });
+
     it('should return an error message if prompt invocation fails', async () => {
       vi.spyOn(mockPrompt, 'invoke').mockRejectedValue(
         new Error('Invocation failed!'),

packages/cli/src/services/McpPromptLoader.ts

@@ -63,7 +63,7 @@ export class McpPromptLoader implements ICommandLoader {
                   return {
                     type: 'message',
                     messageType: 'info',
-                    content: `Prompt "${prompt.name}" has no arguments.`,
+                    content: `Prompt "${prompt.name}" has no declared arguments. Any text you provide will be forwarded as-is (e.g., /${prompt.name} some text sends { input: "some text" }).`,
                   };
                 }
 
@@ -273,7 +273,16 @@ export class McpPromptLoader implements ICommandLoader {
       positionalArgs.push((match[1] ?? match[2]).replace(/\\(.)/g, '$1'));
     }
 
-    if (!promptArgs) {
+    if (!promptArgs || promptArgs.length === 0) {
+      Object.assign(promptInputs, argValues);
+      // Forward positional text as a default "input" argument when the prompt
+      // declares no arguments, matching Claude Code's behavior. This key is a
+      // client-side convention, not part of the MCP spec. A user-provided
+      // --input named arg takes precedence over positional text.
+      const positionalInput = positionalArgs.join(' ');
+      if (positionalInput && !Object.hasOwn(argValues, 'input')) {
+        promptInputs['input'] = positionalInput;
+      }
       return promptInputs;
     }
     for (const arg of promptArgs) {