[yangyang-cs95]

Purpose

Add a streaming FunctionGemmaToolParser in the Rust frontend for Google's FunctionGemma model family (google/functiongemma-270m-it).

The parser handles the custom function-call format:
<start_function_call>call:func_name{key1:value1,key2:value2}<end_function_call>

Key implementation details:

• Standalone parser — the -delimited argument syntax is a custom format (not JSON), so this lives as a top-level parser under rust/src/parser/src/tool/, not in the
  JSON-marker family
• Arguments converted to JSON — each -delimited value is JSON-parsed to recover its proper type (matching the Python parser's json.loads behavior), then emitted as a
  proper JSON object
• preserve_special_tokens() = true — matches the Python parser's skip_special_tokens = False requirement
• Streaming-safe — argument scan state tracks string boundaries so that <end_function_call> inside a string value is not mistaken for the closing marker
• Pattern routing — auto-resolves functiongemma and function-gemma substrings in model IDs

contributing to "functiongemma tool parser" in the Rust frontend roadmap
(#44280).

Test Plan

• cargo fmt --manifest-path rust/Cargo.toml --all -- --check
• cargo test --manifest-path rust/Cargo.toml -p vllm-parser (375 tests)
• cargo test --manifest-path rust/Cargo.toml -p vllm-chat (factory routing + snapshot tests)
• 18 new unit tests for FunctionGemmaToolParser covering:
  • Complete parse (single/multiple tool calls, empty args, text before tool calls)
  • Streaming with split markers across chunks
  • End marker literal inside string values
  • Numeric and boolean value type recovery
  • Incomplete tool call error reporting
  • preserve_special_tokens() assertion
  • reset() buffer reconstruction
  • Multiple tool call snapshot

Test Result

All tests pass:

• vllm-parser: 375 passed, 0 failed
• vllm-chat (factory + snapshot): 7 + 4 passed, 0 failed
• functiongemma unit tests: 18 passed, 0 failed
• fmt: clean
• clippy: clean

e2e test

start service

VLLM_USE_RUST_FRONTEND=1 vllm serve   /models/functiongemma-270m-it --port 8000 --enforce-eager  --enable-auto-tool-choice --trust-remote-code   --tool-call-parser functiongemma

test cmd

curl http://localhost:8000/v1/chat/completions \
  -H "Content-Type: application/json" \
  -d '{
    "model": "/models/functiongemma-270m-it",
    "messages": [
      {"role": "user", "content": "What'\''s the weather like in Paris?"}
    ],
    "tools": [
      {
        "type": "function",
        "function": {
          "name": "get_weather",
          "description": "Get the current weather in a given location",
          "parameters": {
            "type": "object",
            "properties": {
              "location": {
                "type": "string",
                "description": "The city and state, e.g. San Francisco, CA"
              }
            },
            "required": ["location"]
          }
        }
      }
    ],
    "tool_choice": "auto"
  }'

---

Essential Elements of an Effective PR Description Checklist

• The purpose of the PR, such as "Fix some issue (link existing issues this PR will resolve)".
• The test plan, such as providing test command.
• The test results, such as pasting the results comparison before and after, or e2e results
• (Optional) The necessary documentation update, such as updating supported_models.md and examples for a new model.

[claude]

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[mergify]

This pull request has merge conflicts that must be resolved before it can be
merged. Please rebase the PR, @yangyang-cs95.

https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/working-with-forks/syncing-a-fork

[mergify]

This pull request has merge conflicts that must be resolved before it can be
merged. Please rebase the PR, @yangyang-cs95.

https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/working-with-forks/syncing-a-fork

[mergify]

This pull request has merge conflicts that must be resolved before it can be
merged. Please rebase the PR, @yangyang-cs95.

https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/working-with-forks/syncing-a-fork