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fix(tools): honor parallel_tool_calls=false in the required-tools grammar - #2129
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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 ChangesTool constraint enforcement
Estimated code review effort: 3 (Moderate) | ~20 minutes Merge Risk: 🟡 Moderate · up to Required JSON-schema tool constraints now limit calls when parallel use is disabled, but structurally tagged tool routes may still permit repeated calls, and external consumers of the constraint-generation API may require source changes. These issues should be resolved or explicitly accepted before merge. Sequence Diagram(s)sequenceDiagram
participant Request
participant Preparation
participant ParserRegistry
participant Constraint
Request->>Preparation: Provide parallel tool-call setting
Preparation->>ParserRegistry: generate_tool_constraint(..., parallel_tool_calls)
ParserRegistry->>Constraint: Build required-tool constraint
Constraint-->>ParserRegistry: Add single-call bound 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.
✏️ Learnings added
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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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
🚧 Files skipped from review as they are similar to previous changes (3)
- 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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Reworded in 3a69e5b: the doc now says the registry structural-tag builders do not currently set a repeat bound (stop_after_first), so the setting is unenforced on that path — and that the Harmony stage builds its own tag and does honor it.
…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>
…ool_calls=false The Harmony preparation stage builds its own structural tag and never consulted the request's parallel-tool-calls setting, so tool_choice required with parallel_tool_calls:false still admitted repeated calls on that path. Each Harmony tag is exactly one call, so stop_after_first is the repeat bound: it is now set for a named function and whenever the request disables parallel calls, on both the chat and the responses call sites. Also rewords the constraint doc: structural-tag formats can express a repeat bound; the registry builders just do not set one yet. Signed-off-by: Alex McC <319643551+hello-alexmcc@users.noreply.github.com>
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Thanks — took the Harmony gap here rather than filing it, since the fix is the one line you described. 3a69e5b: |
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👋 The PR description doesn't fully follow PULL_REQUEST_TEMPLATE.md:
Please update the PR description so reviewers have the context they need. |
| let stop_after_first = specific_function.is_some(); | ||
| // Each tag is exactly one call, so `stop_after_first` is the repeat | ||
| // bound: set for a named function and for parallel_tool_calls=false. | ||
| let stop_after_first = specific_function.is_some() || single_tool_call; |
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🟡 Nit: The PR description is now stale with respect to this push. The ## Changes list doesn't mention model_gateway/src/routers/grpc/harmony/stages/preparation.rs (the whole of 3a69e5bc), and the ## Test Plan doesn't cover the new harmony::stages::preparation::tests module or a cargo test -p smg run that exercises it. It also still says "structural-tag grammars carry no repeat bound — documented limitation, unchanged", which the Harmony path now contradicts — worth aligning with the reworded doc comment in factory.rs.
The code change itself looks right: each Harmony tag is exactly one call, so stop_after_first is the correct repeat bound here, and all three build_tool_call_structural_tag call sites plus both generate_tool_call_constraint callers thread the request value through.
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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. The Harmony preparation stage had the same gap on its own structural tag:tool_choice: "required"never set a repeat bound.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).Harmony: each Harmony tag is exactly one call, so
stop_after_firstis the repeat bound.generate_tool_call_constrainttakesparallel_tool_callsat both call sites (chat and responses) and the tag builder setsstop_after_firstfor a named function or whenever the request disables parallel calls.Registry structural-tag builders (kimik2, inkling, mistral, kimi_k3) do not currently set a repeat bound, so the setting is unenforced on that path — documented in the constraint doc comment and tracked as follow-up work.
Changes
crates/tool_parser/src/factory.rs:generate_tool_constraintgainsparallel_tool_calls: Option<bool>;maxItemsstamping inbuild_required_array_schema; doc comment states the structural-tag status preciselymodel_gateway/.../harmony/stages/preparation.rs:generate_tool_call_constraint/build_tool_call_structural_tagtake the parallel setting;stop_after_firstset for named or single-call; 4 unit testsmodel_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 harmony— green, including the 4 new Harmony constraint testscargo test -p smg --test harmony_encoding_test— greencargo clippy -p tool-parser -p smg --all-targets -- -D warnings— cleancargo checkonbindings/golang— cleancargo fmt