imagen_generate tool crashes with validation error: expected string, received object #4951

Environment

• OpenCode Version: 1.0.125
• Platform: macOS (Darwin)
• Installation Method: Homebrew

Bug Description

The imagen_generate tool consistently crashes with a validation error whenever it is invoked, making image generation functionality completely unusable.

Error Message

Tool execution aborted
[
  {
    "code": "invalid_union",
    "errors": [
      [
        {
          "expected": "string",
          "code": "invalid_type",
          "path": ["state", "output"],
          "message": "Invalid input: expected string, received object"
        }
      ],
      [
        {
          "expected": "object",
          "code": "invalid_type",
          "path": ["part"],
          "message": "Invalid input: expected object, received undefined"
        },
        {
          "expected": "string",
          "code": "invalid_type",
          "path": ["delta"],
          "message": "Invalid input: expected string, received undefined"
        }
      ]
    ],
    "path": [],
    "message": "Invalid input"
  }
]

Steps to Reproduce

1. Call the imagen_generate tool with any parameters
2. Example: imagen_generate [prompt=A simple red apple on a white background, quality=flash, num_images=1, auto_preview=false]
3. Observe the validation error and OpenCode crash

Root Cause

The imagen_generate tool is returning a structured object (containing image metadata, cost tracking, etc.) when OpenCode's tool validation layer expects a simple string message in the state.output field.

Evidence That Generation Works

Despite the error, images ARE successfully generated and saved to:

• ~/.config/opencode/nano-banana/images/YYYY-MM-DD/
• Cost tracking is functional and updates config.json
• Image files (PNG/JPG) are created correctly

This indicates the underlying nano-banana functionality works correctly, but the tool wrapper's output format is incompatible with OpenCode's validation schema.

Attempted Workarounds (All Failed)

1. Removed all config files
2. Created minimal/empty config
3. Disabled auto_preview
4. Disabled confirm_before_generate
5. Disabled show_cost_in_summary
6. Disabled save_json_sidecar
7. Set terminal_protocol to "none"
8. Separated cost tracking to external file
9. Made config read-only
10. Removed entire nano-banana directory

None of these configuration changes resolved the issue.

Expected Behavior

The imagen_generate tool should return a simple string message (e.g., "Image generated successfully at /path/to/image.png") instead of a structured object.

Actual Behavior

The tool returns a complex object structure that fails OpenCode's output validation, causing the entire tool execution to abort.

Impact

• The imagen_generate tool is completely unusable in OpenCode v1.0.125
• Users cannot generate images through the OpenCode interface
• This breaks the nano-banana subagent functionality

Suggested Fix

The imagen_generate tool wrapper should either:

1. Convert its output to a simple string message before returning
2. Update OpenCode's tool validation schema to accept object outputs
3. Separate the metadata/tracking from the user-facing output message

Additional Context

The config file at ~/.config/opencode/nano-banana/config.json gets auto-generated and updated with cost tracking data, suggesting the tool is executing but failing at the output validation stage.