Gemma-4 tool calling: constrained (xgrammar) vs unconstrained

model: google/gemma-4-E2B-it via HF transformers
constraint: xgr.get_model_structural_tag("gemma_4", tools=..., tool_choice="required") compiled and applied with xgr.contrib.hf.LogitsProcessor
4 samples per scenario per mode; identical sampling settings for both modes (per-scenario values below)
outputs validated with a pure-Python port of sglang's Gemma4Detector parser plus jsonschema

Results (passing samples / total)

A. complex-schema

temperature=0.9, top_p=0.95, max_new_tokens=384, reasoning=False

metric	constrained	unconstrained
well_formed	4/4 ✅	4/4 ✅
valid_name	4/4 ✅	4/4 ✅
schema_valid	4/4 ✅	4/4 ✅
stops_at_boundary	4/4 ✅	0/4 ❌

B. multiturn-thinking

temperature=0.9, top_p=0.95, max_new_tokens=384, reasoning=True

metric	constrained	unconstrained
well_formed	4/4 ✅	4/4 ✅
valid_name	4/4 ✅	4/4 ✅
schema_valid	4/4 ✅	4/4 ✅
stops_at_boundary	4/4 ✅	0/4 ❌
thought_ok	4/4 ✅	0/4 ❌

C. adversarial-payload

temperature=1.5, top_p=1.0, max_new_tokens=640, reasoning=False

metric	constrained	unconstrained
well_formed	4/4 ✅	4/4 ✅
valid_name	4/4 ✅	4/4 ✅
schema_valid	4/4 ✅	4/4 ✅
stops_at_boundary	4/4 ✅	0/4 ❌

Metric definitions

well_formed — every tool call block is complete and parseable (sglang parser port)
valid_name — every called tool exists
schema_valid — parsed arguments validate against the tool's JSON schema
stops_at_boundary — generation did not run past <tool_call|> into <|tool_response>
thought_ok — <|channel>thought opened and closed before the first tool call

Failure examples

A. complex-schema / unconstrained #0 — failed stops_at_boundary: ran past <tool_call|> into <|tool_response> after the call

<|tool_call>call:create_calendar_event{attendees:[{email:<|"|>alice.kim@example.com<|"|>,name:<|"|>Alice Kim<|"|>},{email:<|"|>bob.lee@example.com<|"|>,name:<|"|>Bob Lee<|"|>},{email:<|"|>chris.park@example.com<|"|>,name:<|"|>Chris Park<|"|>}],duration_minutes:90,location:{floor:7,room:<|"|>Jupiter<|"|>},priority:<|"|>high<|"|>,send_invites:true,start:<|"|>2026-07-10T14:00:00+09:00<|"|>,title:<|"|>Q3 Roadmap Review ("final" draft)<|"|>}<tool_call|><|tool_response>

A. complex-schema / unconstrained #1 — failed stops_at_boundary: ran past <tool_call|> into <|tool_response> after the call

<|tool_call>call:create_calendar_event{attendees:[{email:<|"|>alice.kim@example.com<|"|>,name:<|"|>Alice Kim<|"|>},{email:<|"|>bob.lee@example.com<|"|>,name:<|"|>Bob Lee<|"|>},{email:<|"|>chris.park@example.com<|"|>,name:<|"|>Chris Park<|"|>}],duration_minutes:90,location:{floor:7,room:<|"|>Jupiter<|"|>},priority:<|"|>high<|"|>,send_invites:true,start:<|"|>2026-07-10T14:00:00+09:00<|"|>,title:<|"|>Q3 Roadmap Review ("final" draft)<|"|>}<tool_call|><|tool_response>

A. complex-schema / unconstrained #2 — failed stops_at_boundary: ran past <tool_call|> into <|tool_response> after the call

<|tool_call>call:create_calendar_event{attendees:[{email:<|"|>alice.kim@example.com<|"|>,name:<|"|>Alice Kim<|"|>},{email:<|"|>bob.lee@example.com<|"|>,name:<|"|>Bob Lee<|"|>},{email:<|"|>chris.park@example.com<|"|>,name:<|"|>Chris Park<|"|>}],duration_minutes:90,location:{floor:7,room:<|"|>Jupiter<|"|>},priority:<|"|>high<|"|>,send_invites:true,start:<|"|>2026-07-10T14:00:00+09:00<|"|>,title:<|"|>Q3 Roadmap Review ("final" draft)<|"|>}<tool_call|><|tool_response>

A. complex-schema / unconstrained #3 — failed stops_at_boundary: ran past <tool_call|> into <|tool_response> after the call

<|tool_call>call:create_calendar_event{attendees:[{email:<|"|>alice.kim@example.com<|"|>,name:<|"|>Alice Kim<|"|>},{email:<|"|>bob.lee@example.com<|"|>,name:<|"|>Bob Lee<|"|>},{email:<|"|>chris.park@example.com<|"|>,name:<|"|>Chris Park<|"|>}],duration_minutes:90,location:{floor:7,room:<|"|>Jupiter<|"|>},priority:<|"|>high<|"|>,send_invites:true,start:<|"|>2026-07-10T14:00:00+09:00<|"|>,title:<|"|>Q3 Roadmap Review ("final" draft)<|"|>}<tool_call|><|tool_response>

B. multiturn-thinking / unconstrained #0 — failed stops_at_boundary, thought_ok: ran past <tool_call|> into <|tool_response> after the call; skipped or never closed the <|channel>thought section

<|tool_call>call:convert_temperature{from_unit:<|"|>celsius<|"|>,to_unit:<|"|>fahrenheit<|"|>,value:3}<tool_call|><|tool_response>

B. multiturn-thinking / unconstrained #1 — failed stops_at_boundary, thought_ok: ran past <tool_call|> into <|tool_response> after the call; skipped or never closed the <|channel>thought section

<|tool_call>call:convert_temperature{from_unit:<|"|>celsius<|"|>,to_unit:<|"|>fahrenheit<|"|>,value:3}<tool_call|><|tool_response>

B. multiturn-thinking / unconstrained #2 — failed stops_at_boundary, thought_ok: ran past <tool_call|> into <|tool_response> after the call; skipped or never closed the <|channel>thought section

<|tool_call>call:convert_temperature{from_unit:<|"|>celsius<|"|>,to_unit:<|"|>fahrenheit<|"|>,value:3}<tool_call|><|tool_response>

B. multiturn-thinking / unconstrained #3 — failed stops_at_boundary, thought_ok: ran past <tool_call|> into <|tool_response> after the call; skipped or never closed the <|channel>thought section

<|tool_call>call:convert_temperature{from_unit:<|"|>celsius<|"|>,to_unit:<|"|>fahrenheit<|"|>,value:3}<tool_call|><|tool_response>

C. adversarial-payload / unconstrained #0 — failed stops_at_boundary: ran past <tool_call|> into <|tool_response> after the call

<|tool_call>call:create_ticket{affected:{regions:[<|"|>ap-northeast-2<|"|>,<|"|>us-east-1<|"|>],service:<|"|>payments<|"|>,version:<|"|>2.14.1<|"|>},body:<|"|>Deployed config: {"retry": {"max": 3, "backoff_ms": [100, 200], "jitter": true}}
Error line: TypeError: cannot destructure {id: undefined} at applyRetry (retry.js:42).<|"|>,cc_emails:[<|"|>dev-alerts@example.com<|"|>],estimated_minutes:45,labels:[<|"|>regression<|"|>,<|"|>backend<|"|>],severity:<|"|>blocker<|"|>,title:<|"|>Payments retry storm after config rollout<|"|>,urgent_escalation:true}<tool_call|><|tool_response>

C. adversarial-payload / unconstrained #1 — failed stops_at_boundary: ran past <tool_call|> into <|tool_response> after the call

<|tool_call>call:create_ticket{affected:{regions:[<|"|>ap-northeast-2<|"|>,<|"|>us-east-1<|"|>],service:<|"|>payments<|"|>,version:<|"|>2.14.1<|"|>},body:<|"|>Deployed config: {"retry": {"max": 3, "backoff_ms": [100, 200], "jitter": true}}
Error line: TypeError: cannot destructure {id: undefined} at applyRetry (retry.js:42).<|"|>,cc_emails:[<|"|>dev-alerts@example.com<|"|>],estimated_minutes:45,labels:[<|"|>bug<|"|>,<|"|>regression<|"|>,<|"|>backend<|"|>],severity:<|"|>blocker<|"|>,title:<|"|>Payments retry storm after config rollout<|"|>,urgent_escalation:true}<tool_call|><|tool_response>

C. adversarial-payload / unconstrained #2 — failed stops_at_boundary: ran past <tool_call|> into <|tool_response> after the call

<|tool_call>call:create_ticket{affected:{regions:[<|"|>ap-northeast-2<|"|>,<|"|>us-east-1<|"|>],service:<|"|>payments<|"|>,version:<|"|>2.14.1<|"|>},body:<|"|>Deployed config: {"retry": {"max": 3, "backoff_ms": [100, 200], "jitter": true}}
Error line: TypeError: cannot destructure {id: undefined} at applyRetry (retry.js:42)<|"|>,cc_emails:[<|"|>dev-alerts@example.com<|"|>],estimated_minutes:45,labels:[<|"|>bug<|"|>,<|"|>regression<|"|>,<|"|>backend<|"|>],severity:<|"|>blocker<|"|>,title:<|"|>Payments retry storm after config rollout<|"|>,urgent_escalation:true}<tool_call|><|tool_response>

C. adversarial-payload / unconstrained #3 — failed stops_at_boundary: ran past <tool_call|> into <|tool_response> after the call

<|tool_call>call:create_ticket{affected:{regions:[<|"|>ap-northeast-2<|"|>,<|"|>us-east-1<|"|>],service:<|"|>payments<|"|>,version:<|"|>2.14.1<|"|>},body:<|"|>Deployed config: {"retry": {"max": 3, "backoff_ms": [100, 200], "jitter": true}}
Error line: TypeError: cannot destructure {id: undefined} at applyRetry (retry.js:42).<|"|>,cc_emails:[<|"|>dev-alerts@example.com<|"|>],estimated_minutes:45,labels:[<|"|>bug<|"|>,<|"|>regression<|"|>,<|"|>backend<|"|>],severity:<|"|>blocker<|"|>,title:<|"|>Payments retry storm after config rollout<|"|>,urgent_escalation:true}<tool_call|><|tool_response>

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