Zero-argument MCP tools whose input schema omits properties reach the OpenAI function schema as "properties": null, which OpenAI rejects with Invalid schema for function '<tool>': None is not of type 'object' (HTTP 400) before the tool executes. Anthropic-backed chats are unaffected because they do not strict-validate the schema, so the same tool works on Sonnet but fails on GPT.

This is the object-valued counterpart of the already-merged required fix (#23538): an absent or nil parameter map is passed straight through and serializes to null instead of {}.

Fix

Coerce a nil parameter map to an explicit empty object in two places:

• buildToolDefinitions (coderd/x/chatd/chatloop/chatloop.go) — the provider boundary that every AgentTool (MCP, built-in, and dynamic tools) passes through.
• mcpToolWrapper.Info (coderd/x/chatd/mcpclient/mcpclient.go) — normalizes at the source and additionally covers the nested schema built when model_intent wrapping is enabled, which the boundary fix cannot reach.

This mirrors the guard already merged for the sibling required field.

Refs #27252

Verification

Added regression tests in both packages using a zero-argument tool whose schema omits properties (plain and model_intent variants). Each test fails without the guard (parameters come back nil, serializing to null) and passes with it.

$ go test ./coderd/x/chatd/mcpclient/ -run TestConnectAll_NilParametersBecomesEmptyObject -v
--- PASS: TestConnectAll_NilParametersBecomesEmptyObject_ModelIntent (0.00s)
--- PASS: TestConnectAll_NilParametersBecomesEmptyObject (0.00s)
ok  	github.com/coder/coder/v2/coderd/x/chatd/mcpclient

$ go test ./coderd/x/chatd/chatloop/ -run TestBuildToolDefinitions_NilParametersBecomesEmptyObject -v
--- PASS: TestBuildToolDefinitions_NilParametersBecomesEmptyObject (0.00s)
ok  	github.com/coder/coder/v2/coderd/x/chatd/chatloop

Confirmed the same two tests fail on the unpatched tree (Parameters should never be nil / properties should never be nil). golangci-lint run, paralleltestctx, and go vet are clean on both packages; full mcpclient and chatloop package suites pass, including under -race.

AI disclosure

This change was authored with AI assistance (Claude Code) per the repository's AI contribution guidelines. The diff, test design, and verification above were reviewed manually by the submitter.

🤖 Generated with Claude Code