fix(tools): honor parallel_tool_calls=false in the required-tools grammar - #2129
fix(tools): honor parallel_tool_calls=false in the required-tools grammar#2129pallasathena92 wants to merge 1 commit into
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📝 WalkthroughSummary by CodeRabbit
WalkthroughThe parser adds an optional single-call limit to required JSON-schema constraints. Gateway and Go binding paths forward parallel tool-call settings. Messages preparation maps Anthropic’s disable setting to the parser option. Tests cover bounded, unbounded, delegated, and named-tool behavior. ChangesTool constraint enforcement
Estimated code review effort: 3 (Moderate) | ~20 minutes Mergeability Score: 🟡 Moderate · up to Requests that disable parallel tool use are bounded for generic required-tool schemas, but configured structural grammars can still produce multiple tool calls, potentially allowing multiple actions in one response. The public parser API also requires coordinated caller updates, so the PR is not merge-ready until the enforcement gap and compatibility impact are addressed or explicitly accepted. Sequence Diagram(s)sequenceDiagram
participant Request
participant Preparation
participant ParserRegistry
participant JSONSchema
Request->>Preparation: Provide parallel tool-call setting
Preparation->>ParserRegistry: generate_tool_constraint(..., parallel_tool_calls)
ParserRegistry->>JSONSchema: Build required-tool schema
JSONSchema-->>ParserRegistry: Add maxItems: 1 when disabled
ParserRegistry-->>Preparation: Return tool constraint
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Code looks good. The approach is clean: maxItems: 1 is stamped only on the array-schema path (required tool choice), named-function constraints are single-call by construction, and the old generate_tool_constraint entry point delegates unchanged for backward compatibility. All three MessagesToolChoice variants carrying disable_parallel_tool_use are covered, and the tests pin the important invariants.
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Inline comments:
In `@crates/tool_parser/src/factory.rs`:
- Around line 205-206: Update generate_tool_constraint_with_limit and every
structural-tag builder to propagate single_tool_call and enforce a single
generated tool call, preferably by encoding the builder’s stop-after-first
equivalent; preserve unrestricted structural-tag behavior when the flag is false
and add a regression test for the bounded structural-tag path.
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crates/tool_parser/src/factory.rscrates/tool_parser/src/tests.rsmodel_gateway/src/routers/grpc/regular/stages/chat/preparation.rsmodel_gateway/src/routers/grpc/regular/stages/messages/preparation.rs
| /// Named-function constraints are single-call by construction; structural | ||
| /// tags are unaffected (tag grammars carry no repeat bound). |
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🎯 Functional Correctness | 🟠 Major | 🏗️ Heavy lift
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Structural-tag path ignores single-call limit from request.
When generate_tool_constraint_with_limit receives single_tool_call=true and the configured parser supports structural tags, the function calls build_fn(tools, at_least_one) at lines 229–230 and returns immediately. The single_tool_call parameter is never passed to the builder or used to constrain the tag. All structural tag builders (Mistral, KimiK2, KimiK3, Inkling) accept only (tools: &[Tool], at_least_one: bool) and do not encode repeat bounds. The JSON-schema fallback (line 246) respects single_tool_call by adding maxItems: 1, but this logic is unreachable when a structural tag parser is configured.
A request with parallel_tool_calls: false can therefore still receive an unbounded grammar. Downstream (zmq_client.rs line 1278) passes the constraint directly to the backend without enforcing repeat limits. Add single_tool_call parameter to the structural tag builder signatures, propagate it through generate_tool_constraint_with_limit, and update all builders to encode stop_after_first: true or equivalent when a single call is required. Alternatively, fall back to the bounded JSON-schema path when both a structural tag parser is configured and single_tool_call=true. Add a regression test covering this scenario.
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In `@crates/tool_parser/src/factory.rs` around lines 205 - 206, Update
generate_tool_constraint_with_limit and every structural-tag builder to
propagate single_tool_call and enforce a single generated tool call, preferably
by encoding the builder’s stop-after-first equivalent; preserve unrestricted
structural-tag behavior when the flag is false and add a regression test for the
bounded structural-tag path.
Source: Coding guidelines
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Acknowledged as a real limitation, deliberately taken and documented in the PR body and doc comment: structural-tag grammars keep their existing semantics because (a) the four native-format builders each need their own verified stop-after-first encoding (e.g. dropping subsequent-call tag variants), which is per-builder grammar work, and (b) the alternative — falling back to the JSON-schema path for a configured native-format parser — would silently switch the model to an output format it wasn't prompted for, a worse failure than an unbounded repeat. The flag is inert for structural-tag parsers today (behavior unchanged from main); per-builder bounding is tracked as follow-up work. The JSON-schema path — the default for every parser without a tag builder — is bounded by this PR.
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@pallasathena92, thanks for the clarification.
I agree that a JSON-schema fallback would break the native-format contract for structural-tag parsers. The current behavior is therefore the safer scope boundary for this PR.
The structural-tag limitation remains valid until each builder has a verified single-call grammar. Is there an existing GitHub issue for that follow-up? If not, I can create one with the affected builders and acceptance criteria.
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Learnt from: pallasathena92
Repo: smg-project/smg PR: 2129
File: crates/tool_parser/src/factory.rs:0-0
Timestamp: 2026-08-20T15:36:05.795Z
Learning: In `crates/tool_parser/src/factory.rs`, structural-tag parsers intentionally do not enforce `parallel_tool_calls: Some(false)` yet. Falling back from a configured structural-tag parser to the JSON-schema constraint is unsafe because the model was prompted to emit its native tool-call format. Per-builder structural-tag single-call bounds require verified grammar work and should be handled as a follow-up.
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…mmar The JSON-schema fallback constraint for tool_choice=required emits an array schema with minItems 1 and no upper bound, and nothing in the constraint path consults the request's parallel-tool-calls setting - so parallel_tool_calls:false (or Anthropic disable_parallel_tool_use:true) still lets guided decoding produce several calls. The constraint generator now takes the request's parallel-tool-calls setting directly and stamps maxItems 1 on the required-tools array schema when parallel calls are disabled. All five call sites - both gRPC preparation stages and the three Go-binding entry points - pass their request's real value, which fixes the same gap on the Go SDK surface. Named-function constraints are already single-call by construction; structural-tag grammars carry no repeat bound (documented limitation). Signed-off-by: yifeng liu <31553858+pallasathena92@users.noreply.github.com>
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Inline comments:
In `@crates/tool_parser/src/factory.rs`:
- Around line 197-204: Restore the existing three-argument
generate_tool_constraint API as a wrapper that delegates with no limit, move the
current logic into a new generate_tool_constraint_with_limit method accepting
the additional parameter, and update request-aware callers to invoke the new
method.
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bindings/golang/src/client.rsbindings/golang/src/policy.rsbindings/golang/src/preprocessor.rscrates/tool_parser/src/factory.rscrates/tool_parser/src/tests.rsmodel_gateway/src/routers/grpc/regular/stages/chat/preparation.rsmodel_gateway/src/routers/grpc/regular/stages/messages/preparation.rs
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- model_gateway/src/routers/grpc/regular/stages/chat/preparation.rs
- model_gateway/src/routers/grpc/regular/stages/messages/preparation.rs
- crates/tool_parser/src/tests.rs
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The JSON-schema fallback fix looks right — the changed arm covers exactly Required and AllowedTools{mode: "required"}, the Anthropic inverse mapping is correct, the named-choice no-op is right, and the tests pin both directions.
One gap worth tracking: the Harmony path has the same bug and isn't covered here. generate_tool_call_constraint in model_gateway/src/routers/grpc/harmony/stages/preparation.rs builds its own structural tag and never consults parallel_tool_calls (both call sites — chat and responses — ignore it), so tool_choice: required + parallel_tool_calls: false on Harmony models still permits multiple tool calls. Harmony's tags are one call per tag and the builder already sets stop_after_first for named-function choice, so bounding required-mode looks small: set stop_after_first: true when parallel calls are disabled. Fine as a follow-up if you'd rather keep this PR scoped to the JSON-schema path — but in that case let's file it.
| /// by construction; structural tags are unaffected (tag grammars carry no | ||
| /// repeat bound). |
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Nit: "tag grammars carry no repeat bound" isn't quite accurate — the structural-tag format can express one via stop_after_first (the Harmony builder sets it for named-function choice). The actual limitation is that the registry builders never receive the setting: for kimik2/inkling (one call per tag) stop_after_first alone would bound it, while mistral/kimi_k3 pack multiple calls into one tag and would also need their tag content restructured. Suggest rewording to something like: "structural-tag builders don't currently set a repeat bound (stop_after_first), so the setting is unenforced on that path."
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Description
Bounds the guided-decoding grammar to a single tool call when the request disables parallel tool calls.
Problem
The JSON-schema fallback constraint for
tool_choice: "required"emits an array schema withminItems: 1and no upper bound, and nothing in the constraint path consults the request's parallel-tool-calls setting — soparallel_tool_calls: false(OpenAI semantics: at most one call) or Anthropic'sdisable_parallel_tool_use: truecan still get several tool calls out of constrained decoding.Solution
generate_tool_constraintnow takes the request's parallel-tool-calls setting directly as a fourth argument and stampsmaxItems: 1on the required-tools array schema when parallel calls are disabled. One canonical signature — no delegating variant — and all five call sites pass their request's real value: both gRPC preparation stages (the Messages stage maps Anthropic's inversedisable_parallel_tool_use) and the three Go-binding entry points, which fixes the same gap on the Go SDK surface. Named-function constraints are already single-call by construction (pinned by test); structural-tag grammars carry no repeat bound — documented limitation, unchanged.Changes
crates/tool_parser/src/factory.rs:generate_tool_constraintgainsparallel_tool_calls: Option<bool>;maxItemsstamping inbuild_required_array_schemamodel_gateway/.../stages/chat/preparation.rs: passrequest.parallel_tool_callsmodel_gateway/.../stages/messages/preparation.rs: derive from Anthropicdisable_parallel_tool_use(inverse mapping)bindings/golang/src/{preprocessor,policy,client}.rs: passchat_request.parallel_tool_callscrates/tool_parser/src/tests.rs: bounded / unbounded (None and Some(true) identical) / named-choice-no-op pinsTest Plan
cargo test -p tool-parser— all green including new constraint testscargo test -p smg --lib— 1486 passedcargo clippy -p tool-parser -p smg --all-targets -- -D warnings— cleancargo checkonbindings/golang— cleancargo fmt