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Google AI Studio rejects any function-declaration enum member that
stringifies to "", e.g.:
GenerateContentRequest.tools[0].function_declarations[N].parameters
.properties[field].enum[0]: cannot be empty
MCP servers commonly model an "unset"/"clear this field" option as a
literal "" enum member (distinct from omitting the argument, which
means "leave unchanged"). That's valid JSON Schema and works fine
against Anthropic/OpenAI, but Gemini validates the entire tools[]
array up front on every generateContent call, so a single tool with
this shape breaks every request on that turn, not just calls to the
offending tool.
Strip "" out of enum arrays in the Gemini tool-schema sanitizer (plain
enums and enums nested inside anyOf/oneOf/allOf branches, since
sanitizeNode already recurses into those). If stripping empties the
enum entirely, drop the constraint and fall back to the bare type.
Reproduced against a real-world MCP server (personas_create /
personas_update / personas_add_detail tools using "" as an explicit
clear-field sentinel) that reliably 400s every Gemini turn while
connected.
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Issue for this PR
Closes #33316
Type of change
What does this PR do?
Gemini's function-calling schema validator rejects any
enummember that's an empty string (enum[0]: cannot be empty). Some MCP servers use""as a sentinel for "unset"/"clear this field" on a string enum, which is valid JSON Schema but breaks every turn on a Gemini model once that server is connected — Gemini validates the wholetools[]array up front, not just the tool being called. That's what #33316 hit (link.any_of[0].enum[0]from a Canva MCP tool), though it got triaged as a Canva/$ref-specific issue at the time.packages/llm/src/protocols/utils/gemini-tool-schema.tsalready sanitizes several Gemini-incompatible schema shapes for MCP tools. This adds one more: strip""out ofenumarrays (including ones nested insideanyOf/oneOf/allOf), and drop the enum constraint entirely if that empties it.How did you verify your code works?
Added a test to
packages/llm/test/tool-schema-projection.test.tscovering a plain enum with a""sentinel, the same sentinel nested in ananyOfnullable branch, and an enum that's only[""]. Ran the fullpackages/llmtest suite (299 pass, 0 fail) andbun run typecheck, both clean.Screenshots / recordings
N/A, no UI change.
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