[tdakhran]

Overview

Fixes two lfm2 tool-parser bugs that together make LFM2.5 agents loop on the same tool call under tool_choice="required":

1. Forced tool_choice (required/named) was decoded as a JSON tool-call list instead of LFM2's trained <|tool_call_start|>[name(k=v)]<|tool_call_end|> block. An lfm2 structural tag now constrains it to the native block.
2. Tool names with hyphens (sub-task-ending) came back as content. They are now parsed.

Related: #57989 (the same hyphen fix for FunctionGemma).

Claims

• On a ToolSandbox turn where the search already succeeded, LiquidAI/LFM2.5-2.6B repeats it in 28/30 samples on main and ends the subtask in 30/30 with this PR.
• required/named calls are decoded in the native block and parsed by Lfm2ToolParser, like auto. auto is constrained only when a structural tag applies anyway (a strict tool, --tool-strict-level, parallel_tool_calls=false), and plain text stays valid.
• Hyphenated names parse in streaming and non-streaming, including parallel blocks that mix them with plain names.

Validation

Reproducer. ToolSandbox's contacts task with tool_choice="required", at the turn after a successful search_contacts(relationship="friend"). The harness adds a sub-task-ending tool, which is the correct next call.

vllm serve LiquidAI/LFM2.5-2.6B --enable-auto-tool-choice --tool-call-parser lfm2 --reasoning-parser qwen3
curl -s localhost:8000/v1/chat/completions -H 'Content-Type: application/json' -d @toolsandbox_repro.json \
  | jq -r '.choices[].message.tool_calls | map(.function.name) | join(",")' | sort | uniq -c

toolsandbox_repro.json (the model's default sampling, n=30)

{
  "model": "LiquidAI/LFM2.5-2.6B",
  "messages": [
    {
      "role": "system",
      "content": "Execute only the current subtask in the final user message. Earlier user/assistant pairs are completed prerequisites: the user message is the prerequisite objective and the assistant message is its final result. Use them only as context. Never attempt later subtasks.\n\nRules:\n- Use only exposed tools through API tool calls; never output plain text or textual tool-call JSON.\n- Call task tools as needed, including repeated calls, and wait for each response. Continue until the complete objective is satisfied.\n- Never fabricate, guess, or assume data. Use only user-message or tool-response values, even when another value seems obvious.\n- Include only needed optional arguments known from a user message or tool response. Omit unknown optional arguments; never use placeholders such as `0`, `\"\"`, or `null`.\n- Do not ask the user for credentials or task data; retrieve them with available tools.\n- Call `sub-task-ending` exactly once after task tools, or immediately if none are needed.\n\n`sub-task-ending` arguments:\n- Required top-level `status`: `Completed`, `UnknownError`, or `MissingRequiredInputOrTool`.\n- Required `data`: a non-empty string.\n- `Completed`: ONLY if every objective requirement was achieved. Failed, partial, or unverified outcomes are NEVER `Completed`, even when `data` explains them. Include all requested result values.\n- `MissingRequiredInputOrTool`: a required capability or information cannot be obtained; state what is missing in `data`.\n- `UnknownError`: an unexpected failure that may be corrected with available information and tools; describe it in `data`."
    },
    {
      "role": "user",
      "content": "Search for all contacts to identify friends"
    },
    {
      "role": "assistant",
      "content": "",
      "tool_calls": [
        {
          "id": "call_search",
          "type": "function",
          "function": {
            "name": "search_contacts",
            "arguments": "{\"relationship\": \"friend\"}"
          }
        }
      ]
    },
    {
      "role": "tool",
      "tool_call_id": "call_search",
      "content": "{\"status\":\"ok\",\"tool\":\"search_contacts\",\"http_status\":200,\"response\":{\"tool_name\":\"search_contacts\",\"arguments\":{\"relationship\":\"friend\"},\"result\":[{\"person_id\":\"9e137f06-916a-5310-8174-cf0b7e9f7054\",\"name\":\"Fredrik Thordendal\",\"phone_number\":\"+12453344098\",\"relationship\":\"friend\",\"is_self\":false},{\"person_id\":\"a22e1984-6c6c-530c-8831-c3ea3b5138e7\",\"name\":\"John Petrucci\",\"phone_number\":\"+1234560987\",\"relationship\":\"friend\",\"is_self\":false}],\"tool_error\":null,\"tool_trace\":[]}}"
    }
  ],
  "tools": [
    {
      "type": "function",
      "function": {
        "name": "search_contacts",
        "description": "Search for a contact person based on provided arguments Each field has a search criteria of either\n1. Exact value matching\n2. Fuzzy string matching with a predefined threshold\nSearch results contains all contact entries that matched all criteria",
        "parameters": {
          "type": "object",
          "properties": {
            "person_id": {
              "type": "string",
              "description": "String format unique identifier for the contact person, will be exact matched"
            },
            "name": {
              "type": "string",
              "description": "Name of contact person, will be fuzzy matched"
            },
            "phone_number": {
              "type": "string",
              "description": "Phone number of contact person, will be exact matched"
            },
            "relationship": {
              "type": "string",
              "description": "Relationship between user and this contact, will be fuzzy matched"
            },
            "is_self": {
              "type": "boolean",
              "description": "If the requested contact corresponds to the current user. Will be exact matched"
            }
          },
          "required": []
        }
      }
    },
    {
      "type": "function",
      "function": {
        "name": "sub-task-ending",
        "description": "End the current executor subtask. Call this exactly once after the subtask has completed, failed unexpectedly, or cannot continue because a required capability or required information is unavailable.",
        "parameters": {
          "type": "object",
          "properties": {
            "status": {
              "type": "string",
              "enum": [
                "Completed",
                "UnknownError",
                "MissingRequiredInputOrTool"
              ],
              "description": "Required top-level subtask status. Use exactly one of Completed, UnknownError, or MissingRequiredInputOrTool."
            },
            "data": {
              "type": "string",
              "description": "Required non-empty text. For Completed include the complete result and every value requested by the objective. For failures describe the problem precisely."
            }
          },
          "required": [
            "status",
            "data"
          ]
        }
      }
    }
  ],
  "tool_choice": "required",
  "temperature": 0.1,
  "top_k": 50,
  "repetition_penalty": 1.1,
  "n": 30
}

	main	this PR
curl above, 30 samples	search_contacts repeated in 28	sub-task-ending in 30
full agent loop, 10 runs × up to 8 turns	repeat loop in 7	0

Tests. vllm/vllm-openai:latest (v0.32.0) on an RTX 4070 (8 GB). E2E: main's or this branch's two changed files copied over the installed ones. Unit: this branch's Python tree with the image's compiled extensions.

# before (main)
$ pytest tests/entrypoints/openai/chat_completion/test_lfm2_tool_calls.py
E   AssertionError: assert [] == ['get-weather']
E   AssertionError: assert 124905 in [67, 266, 229, 206, 2718, 463, ...]   # no <|tool_call_start|>
4 failed

# after (this branch)
$ pytest tests/entrypoints/openai/chat_completion/test_lfm2_tool_calls.py
4 passed
$ pytest tests/tool_parsers/test_lfm2_tool_parser.py tests/tool_parsers/test_structural_tag_registry.py
348 passed

New tests cover hyphenated names (parser), the lfm2 grammar's accept/reject cases, the real token stream and a DelegatingParser round trip (registry), and an e2e test on LFM2.5-2.6B that runs in the existing entrypoints/openai/chat_completion CI step.

Details

Forced tool_choice. Lfm2ToolParser had no structural_tag_model, so forced calls got the generic JSON schema and bypassed the parser. Outside its trained format the model copies its previous call: after </think>, the JSON prefix [{"name": " gives search 0.53 / sub 0.32 at the first token, while <|tool_call_start|>[ gives sub 1.00. The fix registers get_lfm2_structural_tag (like hermes, longcat and plamo3; nothing shared changes): the start token, then [ + an allowed name + (, then free text up to the end token. The sentinels are strings because request validation compiles the tag without a tokenizer, and the arguments use AnyTextFormat because LFM2 tokens such as (city span the forced prefix. VLLM_ENFORCE_STRICT_TOOL_CALLING=0 restores the old behavior.

Hyphenated names. ast.parse reads sub-task-ending(...) as a subtraction. Requested names that aren't identifiers are swapped for _lfm2_tool_alias_N at call positions before parsing and restored afterwards. Names not in request.tools are left alone.

Limitations. Only the first call's name is constrained. Further calls and all arguments are free text, so arguments aren't checked against the schema, even for strict tools. parallel_tool_calls=false is enforced after parsing, not by the grammar.

Docs. New LFM2.5 section in docs/features/tool_calling.md. The LFM rows in docs/models/supported_models.md now list LFM2.5 checkpoints; the deprecated LFM2 and LFM2-VL examples are dropped.

Not a duplicate. No open upstream PR covers LFM2 tool calling or an lfm2 structural tag. #52900 (Rust-frontend LFM2 parser) is unaffected.

---

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[mergify]

Documentation preview: https://vllm--64750.org.readthedocs.build/en/64750/

[sfeng33]

Without excludes, the free text can contain <|tool_call_start|> again, e.g. <|tool_call_start|>[get_weather(city='x')<|tool_call_start|>]<|tool_call_end|>. The parser slices from the first start sentinel, ast.parse fails, and the whole output falls back to content.

Can we use AnyTextFormat(excludes=[_LFM2_TOOL_CALL_START, _LFM2_TOOL_CALL_END]) here, like the kimi_k3 builder does?

[tdakhran]

Done in cda3af3

[sfeng33]

_lfm2_tool_alias_1 is a prefix of _lfm2_tool_alias_10, and this replaces in insertion order, so with 11 or more hyphenated tools in one block an alias inside a string argument gets corrupted. I hit {"note": "then, tool-b0(y)"} for an argument that should have been then, tool-k(y).

A terminated placeholder like _lfm2_tool_alias_{n}_, or replacing longest alias first, would fix it.

[tdakhran]

great catch! I didn't test with more than 11 tool calls, will fix

[tdakhran]

Fixed in cda3af3

[smiyamoto3]

Thanks for the update. I think a named tool_choice can now return extra calls, since the grammar pins only the first [name(. With end-task named, <|tool_call_start|>[end-task(), made_up(x=1)]<|tool_call_end|> is accepted and parses to both calls. On main it is always one end-task call. Could a named tool_choice keep only the first call after parsing, like parallel_tool_calls=false does?

[smiyamoto3]

nit: names that are Python keywords (import, class, lambda) skip the alias and still fail ast.parse. Adding and not keyword.iskeyword(part) here fixes them, and the lfm2 tests still pass.

[tdakhran]

good catch, thanks, will address

[tdakhran]

@smiyamoto3 , thank you for the review, addressed and rebased

[smiyamoto3]

LGTM. Named now streams one call too.