Danielle Maywood 5480299cef fix: serialize empty MCP tool properties as {} not null for OpenAI (#28380) ...

## Summary

Fixed a bug where MCP tools with an empty `properties` schema had their
tool definitions sent to OpenAI with `"properties": null`, causing
OpenAI to reject the request.

## Problem

When an MCP server reports a tool with an empty input schema, e.g.:

```json
{
  "type": "object",
  "properties": {},
  "required": []
}
```

Coder was forwarding the tool definition to OpenAI as:

```json
{
  "type": "object",
  "properties": null
}
```

OpenAI's function-calling API rejects `properties: null`, making these
tools unusable with OpenAI models.

## Root cause

The empty `properties` object collapsed to Go `nil` at two layers:

1. `agent/x/agentmcp/manager.go` `toolInputSchemaMap` stripped the
present-but-empty `"properties": {}` (guarded by `len(properties) > 0`),
so the wire copy lost it entirely.
2. `coderd/x/chatd/chatloop/chatloop.go` `buildToolDefinitions` embedded
`info.Parameters` directly into the input schema. When nil,
`encoding/json` marshals it as `null`.

## Fix

- `buildToolDefinitions` now substitutes an empty object when
`Parameters` is nil, mirroring the existing `required` guard (the sink,
protects all tool sources).
- `toolInputSchemaMap` now preserves an empty `properties` object
instead of dropping it (the upstream producer).

## Tests

Added regression tests (failing before, passing after) covering both
layers and asserting the schema serializes to
`{"type":"object","properties":{}}`:

- `TestBuildToolDefinitionsNilPropertiesBecomesEmptyObject`
- `TestToolInputSchemaMapPreservesEmptyProperties`
-
`TestWorkspaceMCPToolInfosFromResources/EmptyPropertiesYieldsEmptySchema`

---

*This PR was generated by Coder Agents.*

2026-08-20 15:56:58 +02:00

..

agentdesktop

feat: tag chat-originating agent logs with chat_id (#25019)

2026-05-08 13:25:30 +10:00

agentmcp

fix: serialize empty MCP tool properties as {} not null for OpenAI (#28380)

2026-08-20 15:56:58 +02:00