Gemini API rejects tool requests: propertyNames not stripped from JSON Schema

[Dilidonka]

Summary

All agents using the Gemini provider fail with INVALID_ARGUMENT errors because the propertyNames JSON Schema keyword is not stripped from tool schemas before API calls.

Error

API error (400): INVALID_ARGUMENT: Invalid JSON payload received.
Unknown name "propertyNames" at 'tools[0].function_declarations[214].parameters.properties[14].value.properties[0].value': Cannot find field.

Root Cause

The normalize_schema_recursive function in crates/openfang-types/src/tool.rs (lines 84-97) strips unsupported JSON Schema fields for non-Anthropic providers:

if matches!(
    key.as_str(),
    "$schema"
        | "$defs"
        | "$ref"
        | "additionalProperties"
        | "default"
        | "$id"
        | "$comment"
        | "examples"
        | "title"
        | "const"
        | "format"
) {
    continue;
}

"propertyNames" is missing from this list. The Gemini API does not support this JSON Schema keyword, so any tool schema containing propertyNames causes the entire request to be rejected.

Impact

• Severity: Critical — affects all agents using the Gemini provider
• Agents enter a crash → recovery → re-crash loop
• Agents hang for 300+ seconds before auto-recovery kicks in
• Effectively makes the service unavailable when Gemini is the primary model

Proposed Fix

Add "propertyNames" to the matches! block:

if matches!(
    key.as_str(),
    "$schema"
        | "$defs"
        | "$ref"
        | "additionalProperties"
        | "default"
        | "$id"
        | "$comment"
        | "examples"
        | "title"
        | "const"
        | "propertyNames"
        | "format"
) {
    continue;
}

This is a one-line addition. The propertyNames keyword is a standard JSON Schema keyword that constrains property name patterns, but is not supported by the Gemini Function Calling API.

Workaround

Switch agents to a provider that accepts full JSON Schema (e.g., anthropic, openrouter), or route through OpenRouter with google/gemini-2.5-pro-preview-03-25 which handles schema normalization on their side.

Environment

• OpenFang version: 0.5.5
• Provider: Gemini (native driver)
• Affected: All tool-bearing agents

[Dilidonka]

Additional finding: required: {} (empty object) instead of required: []

While debugging further with request tracing, found a second schema normalization issue in the same code path.

Problem

4 tool declarations in a 222-tool agent had "required": {} (empty object) instead of "required": [] (empty array) or omitting the field entirely. Gemini rejects this with:

INVALID_ARGUMENT: Invalid value at 'tools[0].function_declarations[58].parameters' (required), Starting an object on a scalar field

Affected tools (in our deployment)

Index	Tool	Source
58	system_time	Built-in
201	mcp_notion_api_get_users	Notion MCP
202	mcp_notion_api_get_self	Notion MCP
203	mcp_notion_api_post_search	Notion MCP

Root cause

When a tool has no required parameters, the required field is serialized as an empty JSON object {} instead of an empty array []. This likely happens when required is typed as a Map or HashMap somewhere in the serialization path rather than a Vec<String>.

The fix in normalize_schema_recursive should either:

1. Convert required: {} to required: []
2. Or strip empty required fields entirely (Gemini infers all-optional when required is absent)

Workaround

We're stripping empty required objects in our OpenResty Lua proxy (same proxy as the propertyNames workaround).

[Dilidonka]

Third finding: Array properties with missing items field

Problem

7 tool declarations from MCP integrations (Sentry, NotebookLM) have properties with "type": "array" but no items field. Gemini requires items on every array-typed property and rejects with:

GenerateContentRequest.tools[0].function_declarations[77].parameters.properties[fields].items: missing field.

Affected tools

Tool	Property	MCP Source
mcp_sentry_list_events	fields	Sentry
mcp_notebooklm_notebook_query	source_ids	NotebookLM
mcp_notebooklm_notebook_query_start	source_ids	NotebookLM
mcp_notebooklm_research_import	source_indices	NotebookLM
mcp_notebooklm_source_add	urls	NotebookLM
mcp_notebooklm_source_delete	source_ids	NotebookLM
mcp_notebooklm_studio_create	source_ids	NotebookLM

Root cause

These MCP tools declare "type": "array" without specifying "items": {"type": "string"}. This is technically valid JSON Schema (items defaults to {} which accepts anything), but Gemini's API requires the field explicitly.

The fix in normalize_schema_recursive should inject "items": {"type": "string"} as a default when type is array and items is absent.

Workaround

Our OpenResty proxy now injects "items": {"type": "string"} for any array property missing it.

Summary of all schema normalization gaps found

Issue	Field	Fix needed in tool.rs
propertyNames not stripped	Any level	Add to matches! exclusion list
required: {} (empty object)	parameters.required	Convert {} to [] or strip
Array without items	parameters.properties[*]	Inject default items for arrays