feat(symphony): type a tagged parameter's value by the tool's schema

[slin1237]

Description

Problem

Symphony reads one tool-call syntax today, a JSON object between <tool_call> markers (Qwen3-8B and its kin). Qwen 3.5, 3.6 and 3.8, Qwen3-Coder and several other families write a call as tags instead:

<tool_call>
<function=get_weather>
<parameter=city>
Paris
</parameter>
</function>
</tool_call>

The model writes no arguments object, so the parser has to write one, and a value's text does not say what type it is: 150 is the number 150 for an integer parameter and the string "150" for a string one. The old crate decides with the tool's declared parameter types, in safe_val and coerce_value (crates/tool_parser/src/parsers/qwen_xml.rs) and coerce_by_schema_type and param_types_for_function (helpers.rs).

This is the first step of porting that parser: the decision alone, before the assembler that turns a call's text into events and before the format.

Solution

symphony::tagged::value:

• Declared holds the parameter types a request's tools declare, by function and parameter name (Declared::of(&tools), declared.kind(function, parameter)).
• Kind is the declared type when it is string or integer, the two that change how text is read. Any other name, a list of types and a missing type declare nothing.
• json(text, kind) gives the JSON for one value's text. It always returns one JSON value that serde_json reads back.

The rules:

• A declared string is its text, every byte of it.
• A declared integer is that integer when the text is a sign and digits, ignoring the whitespace around it. It may be spelled +5 or 007, and its digits are copied, not passed through a machine integer.
• Anything else is inferred, as before: text that is JSON is that JSON; True, False and None are true, false and null; the rest is a string. That includes a declared number, boolean, array or object: the templates write those as JSON, which is what inference reads.
• JSON means what serde_json reads into a value from inside the arguments object, because that is how the adapters read a call's arguments back. A lone surrogate escape, or nesting that reaches the depth limit once the object is around it, makes the text a string, and so does a run of digits too long to be read as a number, so one odd value never costs a call its other arguments.

Three things differ from the old functions. The first two are how the templates write values and what bellwether's references record:

• A string keeps its text exactly. The old functions trimmed it, and unquoted a value declared string that happened to be a JSON string literal. A search for "exact phrase" lost its quotes, and a file's contents lost their final newline.
• JSON keeps the model's bytes. The old functions parsed the value and wrote it again, so {"a": 1} reached the client as {"a":1} and 1.50 as 1.5. Now [1, 2.5] stays spaced as written, 1.50 stays 1.50, and an integer wider than 64 bits keeps every digit.
• Four rules are gone. The old functions had one each for number, boolean, array and object, and each gave exactly what inference gives. (Found in review: the first commit carried them over.)

Not in this step: the assembler (function and parameter tags to events, with string values streamed as they arrive) and the Qwen format that uses it. They follow as their own pull requests.

Changes

• crates/symphony/src/tagged/value.rs: Kind, Declared, json, and eleven tests.
• crates/symphony/src/tagged/mod.rs: the module, with what the tagged formats share.
• crates/symphony/src/lib.rs: pub mod tagged.

Nothing outside crates/symphony/ changes; Cargo.lock is unchanged. The commit credits the authors of both old files.

Test Plan

Run on the head, exit codes checked:

• cargo +nightly fmt -p smg-symphony: 0, no change.
• cargo +1.98.0 clippy -p smg-symphony --all-targets --all-features -- -D warnings: 0.
• cargo +1.98.0 test -p smg-symphony: 0, on the branch rebased onto fb4e88d. 147 unit tests (13 new), the two fixture tests on their skip paths, 7 contract tests. The same five gates at bc985d1 (the review commit for unions, enums and the newline contract): all 0.
• RUSTDOCFLAGS="-D warnings" cargo +1.98.0 doc -p smg-symphony --no-deps: 0.
• git diff --stat origin/main...HEAD: the three files above, 607 lines added.

The old parser's value tests are carried over as cases of the new ones: test_safe_val_json, test_safe_val_python_literals and test_safe_val_string_fallback in any_other_value_is_inferred; test_safe_val_preserves_html_entities and test_arg_values_with_entities_roundtrip_unchanged there and in a_declared_string_is_its_text_exactly; test_schema_aware_coercion_keeps_strings in the tests of the two declared kinds. The one old expectation that changes is safe_val(" spaces "), which now keeps its spaces.

New here: a declared string that is a quoted literal keeps its quotes; whitespace in strings is kept; a number wider than a float keeps its digits; text the adapters could not read back is a string (a lone surrogate escape, and nesting walked across the depth limit with the value inside an object); a declared integer walked across what a machine integer and a float can hold; and, for 40,000 generated texts under each kind, surrogate escapes among them, the result always reads back as one JSON value and a declared string always decodes to its text.

Preview, not parity. The recordings below are not on bellwether main, so this is a measurement and no claim. With a hand-written split of each recorded output into names and values, I built every call's arguments with json and compared them byte for byte with the reference message (run again after the review commits, same numbers), over BFCL's parse cases recorded for the 18 cached models that write this syntax (Qwen 3.5, 3.6 and 3.8, Qwen3-Coder, and others that share it):

• 41,293 cases, 54,961 calls: 53,006 byte-identical.
• The other 1,955 are the same 115 calls in each of 17 models, and all have one cause: the reference's value is not of the type its tool declares. 96 are null, 17 a boolean and 2 a number, each under a parameter declared string, so the output text is null or false and a reader that trusts the schema gives the string. Whether such a case can be a reference at all is bellwether's to settle (smg-project/bellwether#24).
• No call differs in bytes while agreeing in value, so the separators, escapes and pass-through hold on every real call measured.

Checklist

• cargo +nightly fmt passes
• cargo clippy --all-targets --all-features -- -D warnings passes for smg-symphony (the workspace-wide run needs OpenCV for llm-multimodal on this machine; CI runs it)
• Documentation updated (module docs)
• (Optional) Please join us on Slack #sig-smg to discuss, review, and merge PRs

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📝 Summary

Summary by CodeRabbit

• New Features
  • Added support for interpreting tagged tool-call parameters according to declared string, nullable string, and integer types. String values are preserved, nullable strings can represent null and None, and valid signed integers are converted to JSON numbers.
  • When no usable type is declared, valid JSON is retained, Python literals (True, False, and None) are converted to JSON equivalents, and other text is represented as a JSON string. Invalid declared integer values use this same fallback.

Walkthrough

The crate now exposes a tagged module. Its value module reads parameter declarations from tool schemas and converts tagged parameter text to JSON using declared types or inference.

Changes

Tagged parameter values

Layer / File(s)	Summary
Expose tagged types and tool declarations crates/symphony/src/lib.rs, crates/symphony/src/tagged/mod.rs, crates/symphony/src/tagged/value.rs	The crate exposes the tagged module and re-exports Declared, Kind, and json. Declared::of reads recognized parameter kinds from tool schemas and keeps the first declaration for each function name. Tests cover schema recognition and parameter lookup.
Convert tagged text to JSON crates/symphony/src/tagged/value.rs	The json function handles declared strings and integers, then infers values for other inputs. Tests cover conversion behavior, preservation of JSON text, parse limits, and generated cases.

Priority: ➖ Normal

Estimated code review effort: 3 (Moderate) | ~20 minutes

Change: Feature

Merge Risk: ⚪ Minimal · up to e8e75

The tagged-value changes are ready to merge based on the reviewed evidence; positive integer spellings above i64::MAX remain numeric when readable as arguments, and no actionable defect was identified.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly summarizes the main change: interpreting tagged parameter values using the tool schema.
Description check	✅ Passed	The description explains the problem, the new schema-based value interpretation, its scope, and the reported tests. It is directly related to the changeset.
Docstring Coverage	✅ Passed	Docstring coverage is 88.46% which is sufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 26 functions across 3 files.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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Inline comments:
Review comments at @crates/symphony/src/tagged/value.rs:
- Line 107: Update the integer parsing branch in json to fall back to parsing as
u64 when i64 parsing fails, so leading-plus values above i64::MAX produce JSON
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• crates/symphony/src/lib.rs
• crates/symphony/src/tagged/mod.rs
• crates/symphony/src/tagged/value.rs

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[slin1237]

Rebased onto fb4e88d after #2799 merged: one conflict, the module list in lib.rs, resolved by keeping both lines. The three commits are otherwise unchanged (70342b4, cea1951, 1183fc1), and the gates were run again on the rebased head: fmt 0, clippy 1.98.0 0, 144 unit tests, rustdoc 0.

[hello-alexmcc]

Review at 6d19288: requesting changes for one defect and a contract the docs leave out. The depth problem I found at 7fb677b is fixed. Below is the vLLM table you asked for before touching the rules for undeclared parameters, aliases and type lists.

What I ran

• Gates at 7fb677b. cargo +nightly fmt 0. cargo +1.98.0 clippy -p smg-symphony --all-targets --all-features -- -D warnings 0. cargo +1.98.0 test -p smg-symphony 0: 132 unit tests, the fixture test on its skip path, 6 contract tests. RUSTDOCFLAGS="-D warnings" cargo doc 0.
• At 6d19288. The 11 tagged tests pass. I re-ran the differential below against this head.
• The old functions. I ported main's coerce_value/safe_val (qwen_xml.rs:70-98) and coerce_by_schema_type (helpers.rs:41-64) and ran 52 texts under number, boolean, array and object against inferred. All 208 agree, so "each gave exactly what inference gives" holds.
• vLLM, differential. I compiled vLLM v0.31.0's rust/src/parser/src/tool/parameters.rs (db9527a4) verbatim in a scratch crate; its own 14 tests pass on the copy. I ran it against Declared::of plus json at 6d19288: 29 schema shapes × 44 texts, 1,276 rows. 577 differ in value and 51 only in bytes.
• vLLM, Python. vllm serve --tool-call-parser qwen3_coder (and qwen3_xml, mimo) runs the Python parser engine, not this Rust file (vllm/tool_parsers/__init__.py:145-148, 185-192). I read that path but did not run it. It agrees with Rust on the causes below that matter most (A and D).

Changes requested

Fixed since 7fb677b: the depth check. At 7fb677b, a value nested 127 deep passed serde_json::from_str on its own but not inside {"p": ...}, so the Messages adapter's input() (adapt/messages.rs:367-369) fell back to {}. Measured at 6d19288, 1183fc1's reads_back_as_an_argument (value.rs:149-155) closes it:

Depth	Passed through as JSON	{"p": json(..)} reads back
125, 126	yes	yes
127, 128, 129	no, a string	yes

6d19288 applies the same check to a declared integer, and nesting_is_json_only_as_deep_as_the_arguments_object_can_hold_it walks every depth from 1 to 140 through read_back. That pins it.

1. A union or enum that admits a string is inferred (Kind::of, value.rs:57-66).

   • These all declare nothing, so their values are inferred:
     • anyOf: [{"type": "string"}, {"type": "null"}], which is what Pydantic writes for Optional[str];
     • type: ["string", "null"];
     • an enum of strings.
   • An Optional[str] zip code 12345 therefore comes back as the number 12345, and true as a boolean.
   • The template rendered a string, and both of vLLM's readers give the string back:
     • Python: extract_types_from_schema (vllm/tool_parsers/utils.py:1008-1040), then coerce_to_schema_type, which tries null first and returns the text for string;
     • Rust: parameters.rs:160-168 and :192-209, then :343-345.
   • So this breaks the round trip against the Hugging Face reference as well as parity with vLLM. Both sources agree, so it is not a policy choice.
   • Fix: give any union or enum that admits a string the string kind. The text null should still give null when the union admits null, as both vLLM paths do.

2. The wrapping newlines.

   • The templates write '<parameter=' + name + '>\n', then the value, then '\n</parameter>\n' (Qwen3-Coder-30B chat_template.jinja:88-91, Qwen3.5-27B :121-124).
   • Both vLLM paths strip exactly one \n on each side before typing: Python vllm/parser/qwen3.py:60-66, Rust qwen_coder.rs:278-284 and :297-301. MiMo's wrapper does not.
   • json keeps every byte of a declared string, so it is right only if the caller has already stripped them. Otherwise \nParis\n gives "\nParis\n" where vLLM gives "Paris".
   • Nothing says so: not the module doc, not json's doc (value.rs:104-105), not tagged/mod.rs. Please state the contract on json, so the assembler's pull request knows to strip.
   • Two tests feed framed text and pass only because integers and JSON are trimmed: value.rs:268 ("\n150\n") and :321. Unframed, they would state the contract too.

For Simo: where this pull request and vLLM differ

Grouped by cause. The vLLM line that decides each is given.

• A. Nothing to go on. This covers an undeclared parameter, an unknown function, a schema without type, and type: "null".
  • vLLM keeps the text as a string: Rust parameters.rs:311-313; Python parser_engine.py:288-291 skips any key not in properties. This pull request infers JSON.
  • Examples: 1 → 1 vs "1"; True → true vs "True"; {"a": 1} → an object vs a string.
  • This affects 26 of 44 texts per shape.
  • The Hugging Face references hold non-strings there, so for tagged formats this is the Hugging Face reference against vLLM. bellwether#24's record-time rule will refuse a case whose output cannot carry the type.
• B. Null spellings. vLLM reads null (Rust also none, in any case) as null for every parameter not typed exactly string. This pull request reads only null and None, so NULL and none stay strings.
• C. Type names. vLLM normalizes aliases, case and padding:
  • aliases str, int, float, bool, dict, list, and prefixes such as int32, uint8, float64 and long;
  • case and padding: String, " integer ".
  • Sources: Rust parameters.rs:212-235; Python _TYPE_ALIASES in utils.py.
  • This pull request declares nothing for any of them. For example, int with +5 gives "+5" vs 5.
• D. Unions and enums, as in change 1. This is the one I would not leave to a decision.
• E–H. Spellings the templates never write:
  • booleans TRUE, 1, 0;
  • numbers +5, 007, 5.;
  • a declared integer with whitespace around it, which vLLM does not trim and so keeps as a string;
  • empty text under array or object, which vLLM reads as [] or {} and this pull request as "".
• I. Duplicate tool names. vLLM keeps the last; this pull request keeps the first (value.rs:76-83).
• J. Whitespace. Rust's trim strips Unicode whitespace (U+00A0, U+3000, VT, FF). JSON's whitespace is four characters.
• Bytes. vLLM rewrites the value: Rust writes compact {"a":1}, Python uses json.dumps defaults. So 1.50 becomes 1.5, 1e5 becomes 100000.0, and integers wider than u64 lose precision. vLLM's serde_json has preserve_order and deliberately not arbitrary_precision (parameters.rs:526-536). This pull request keeps the model's bytes.

The two agree on these: NaN, inf, 1e400, a lone surrogate, and Python-literal arrays and objects such as ['a', 'b'] all become strings on both sides.

Smaller

• value.rs:20-21 says the templates write numbers and booleans as JSON. Qwen3-Coder-30B and Qwen3.5-27B write any value that is not a mapping or sequence with | string (lines 89 and 122), so booleans and null arrive as True, False and None. The Python-literal rule handles them; only the sentence is wrong.
• tagged/mod.rs:14 re-exports Declared and Kind but not json.
• Allocation is per value, not per token, so it is fine for now:
  • inferred builds and drops a whole Value;
  • string() copies the text before serializing it (value.rs:157-160);
  • Declared::of clones every function and property name per request.
• Missing tests:
  • 1e400 staying a string, which would flip silently if a dependency turned on arbitrary_precision;
  • aliases and case variants declaring nothing, if that stays the rule.

Rule 17

• Four commits on main at fb4e88d, all signed off, with no AI trailer. The co-author credits cover every direct author of both old files.
• Three files, Cargo.lock unchanged.
• The fixture test takes its skip path here ("BELLWETHER_FIXTURES is not set"), so it shows nothing until the assembler and the format land.

[slin1237]

Both changes are in bc985d1, with the smaller points.

1. A schema that admits a string declares one. Kind::of now reads every type name a schema admits: type as one name or a list, the members of anyOf and oneOf (recursively), and string for an enum whose values are all strings. A schema that admits a string declares Kind::String, or Kind::NullableString when it admits null too; an integer is declared only alone, so ["integer", "null"] stays inferred, where inference already gives the integer and the null. Under NullableString, the text null is null and so is None, because that is what the template writes for a null argument (| string on Python's None); every other text is the string. On None the two vLLM paths part ways (Rust reads none in any case as null, Python keeps "None"), so that spelling stays on the list for Simo under your cause B; null is what both agree on and what you asked for.

2. The contract on json. Its doc and the module doc now say that text is the value's own: the caller has removed the one newline the template writes after the opening tag and the one before the closing tag, and nothing else. The two tests that fed "\n150\n" and "\n[60,30]\n" feed " 150 " and " [60,30] ", so they state what they test (whitespace around an integer or JSON is ignored) and not the frame.

Smaller. The module doc now says the templates write a boolean or null as True, False and None. json is re-exported from tagged. Two tests added: 1e400 stays a string (and reads back inside the object), and aliases, case and padding of a type name declare nothing today, named as Simo's decision to change.

Gates at bc985d1: fmt 0, clippy 0, test 0 (147 unit tests, 13 new), rustdoc 0, width 0, spelling 0; branch current with main; git diff --stat origin/main...HEAD touches crates/symphony/src/{lib.rs,tagged/mod.rs,tagged/value.rs} only.

For Simo, from your table, as I will put them to him: A (an undeclared or untyped parameter: vLLM keeps the text as a string, this crate infers JSON; the Hugging Face references hold non-strings there), B (null spellings), C (type aliases, case and padding), G (whitespace around a declared integer), H (empty text under array or object), I (duplicate tool names: vLLM keeps the last, this crate the first) and J (Unicode whitespace in trim). None of them is changed here.

[hello-alexmcc]

Re-review at bc985d1: approve. Both changes I asked for are in, and the gates and the vLLM differential bear them out.

What I ran

• cargo +nightly fmt -p smg-symphony -- --check: 0.
• cargo +1.98.0 clippy -p smg-symphony --all-targets --all-features -- -D warnings: 0.
• cargo +1.98.0 test -p smg-symphony: 0 (147 unit tests, the fixture test on its skip path, 7 contract tests).
• RUSTDOCFLAGS="-D warnings" cargo +1.98.0 doc -p smg-symphony --no-deps: 0.
• The same differential as last time (29 schema shapes × 44 texts, vLLM v0.31.0's parameters.rs verbatim). Value disagreements are down from 577 at 6d19288 to 471, and no result or assembled arguments object fails to read back.

The two changes

1. A schema that admits a string now declares one.
   • type: ["string", "null"], anyOf string/null (Pydantic's Optional[str]), oneOf, nested anyOf and an enum of strings all read as String or NullableString.
   • The enum-of-strings shape now agrees with vLLM on all 44 texts.
   • Optional[str] differs only on NULL and none: vLLM's Rust frontend treats any case of null and none as null, while this keeps them as strings.
   • An Optional[str] value 12345 or true now comes back as the string, as the template wrote it.
2. The newline contract is stated in the module doc and on json: the caller removes exactly the one newline after the opening tag and the one before the closing tag. The framed test inputs are gone.

For Simo's list (not blocking)

Two places where vLLM's own two readers disagree with each other, so no single answer matches "vLLM":

• None under a nullable string. Here json gives null.
  • vLLM's Rust frontend (parameters.rs:287-303) gives null.
  • vLLM's Python parser, which vllm serve --tool-call-parser qwen3_coder runs (vllm/tool_parsers/utils.py, coerce_to_schema_type), tries only value.lower() == "null" and returns the string "None".
  • Qwen3-Coder's template writes a null argument as None, so with that template the Hugging Face reference (null) and vllm serve disagree.
  • bellwether's #24 rule refuses exactly those cases, so no fixture pins either answer.
• Digits under anyOf string/integer. Here json gives the string.
  • The Rust frontend tries the members in order, string first, and also gives the string.
  • The Python parser tries integer before string and gives the number.

One small follow-up

{"enum": ["a", null]} declares nothing, because not every value is a string, so its values are inferred: true comes back as a boolean. vLLM reads it as string or null. An enum whose non-null values are all strings could be NullableString.

Rule 17

• Commits: five on main at fb4e88d, each signed off, with no AI trailer.
• Credits: the co-author credits cover the old files' authors.
• Scope: only crates/symphony/ changes; Cargo.lock is unchanged.