### Problem/Motivation Several recent bugs shared one defect class: **the advertised input schema described something other than what the tool actually accepted**. [#3582999](https://git.drupalcode.org/project/tool/-/work_items/3582999) (single property maps unsettable on multi-value map inputs), [#3583000](https://git.drupalcode.org/project/tool/-/work_items/3583000) (datetime coercion emitted an offset the storage format rejects), and tool_belt's [#3616140](https://git.drupalcode.org/project/tool_belt/-/work_items/3616140) (`value` advertised as untyped `any`) were each found by hand, in production use, one at a time. This class of bug is unrecoverable over MCP: the bridge bakes `normalizeInputSchema()` output into `tools/list` and the SDK validates arguments strictly against it (including `additionalProperties: false`) before Drupal is ever reached. A schema-valid value the tool then rejects — or a tool-acceptable value the schema forbids — is a dead input with no correction loop. There was no systematic coverage of the (data type × configuration) matrix: which exact JSON Schema each input shape emits, which LLM-shaped value spellings coerce to the definition's declared format, and — the property that would have caught every bug above — whether a value conforming to the advertised schema is actually accepted by the tool. ### Steps to reproduce The fractional-seconds gap illustrates the class: 1. Give a tool a `datetime_iso8601` input carrying the `DateTime` marker constraint (what `FieldSetValue` refines for datetime fields). Its schema advertises `format: date-time`. 2. Call it with `"2026-11-14T09:32:00.500Z"` — valid RFC 3339 `date-time`, and exactly what JavaScript's `toISOString()` and most LLM clients emit. 3. The value matches none of the coercion subscriber's accepted formats, passes through untouched, and the datetime storage format downstream rejects it. ### Expected behavior - Every value conforming to a tool's advertised input schema is accepted by that tool (zero validation violations). - Unambiguous LLM spellings — numeric strings, boolean words, JSON-encoded structures, bare scalars for lists, offset/Zulu/fractional datetime spellings — coerce to the exact typed shape the definition declares, at every nesting depth. - The emitted schema for every input shape, and the coercion behavior for every accepted and rejected spelling, is pinned by tests so drift is a visible wire-contract change. ### Actual behavior - Fractional-second datetimes pass through uncoerced (step above). - No test asserted schema/acceptance agreement, per-input schema snapshots existed for constraints but not for the full type × configuration matrix, and subscriber composition (invoker slot → coercion → recursive dispatch) was untested. - Building the suite immediately surfaced two more schema-valid-but-rejected bugs (follow-ups below), confirming the gap. ### Proposed Resolution Add a matrix suite under `tests/src/Kernel/Matrix/` (227 tests), a matrix fixture tool, a reusable sample generator, and one coercion format: - **`SchemaMatrixTool`** (tool_test): 26 inputs covering every scalar data type (string, integer, float, boolean, decimal, timestamp, datetime with DateTime/DateOnly/no marker, email, uri, any), constraints (Length, Regex, Range, Choice, AllowedValues), required/optional, defaults, multiple scalars, typed lists, maps (free, typed, multiple), and three-level nesting. - **`InputSchemaMatrixTest`**: exact JSON Schema snapshot per input on both scalar paths (11.1+ core normalizers and the legacy path), plus the root object shape (`additionalProperties: false`, required list). - **`InputCoercionMatrixTest`**: LLM-shaped value in → exact typed value received by `doExecute()`, a fixed-point pass proving coercion is idempotent (replayed values never corrupt), and a rejection provider proving ambiguous values fail naming the input. Carries the regression narratives formerly spread across `InputTypeCoercionTest` (falsy wrapping, malformed JSON, brace-leading strings, never-invented datetimes). - **`AdvertisedSchemaAcceptanceTest`** + **`SchemaSampleGeneratorTrait`** (`Drupal\Tests\tool\Traits`): the round-trip property — for every discovered tool, a value generated from its advertised schema must validate cleanly, in both exact and LLM-lax spellings. The trait is reusable by tool_belt and the MCP bridge against their real tools. - **`TransformSubscriberInteractionTest`**: pins the priority contract (invoker 100 → coercion 50 → recursion −100), that early transforms feed later stages, that `stopPropagation()` visibly disables them, and that recursive dispatch carries the item-level definition. - **`InputTypeCoercionSubscriber`**: add `Y-m-d\TH:i:s.uP` to the accepted datetime formats so fractional-second spellings (1–6 digits, offset or Zulu) convert to the declared storage format. Every accepted format still carries a timezone; sub-second precision is truncated because the storage format cannot hold it. Follow-ups surfaced by the suite, parked behind `@todo` markers in `AdvertisedSchemaAcceptanceTest` and to be filed separately: 1. Any value set on a map input that declares an optional entity property crashes `validateInputs()` with `Call to a member function isNew() on null` when the property is omitted — a schema-valid call fatals. 2. `Count`-constrained multiple inputs reject every possible value: the list-level constraint is copied onto each item, so Symfony's Count validator receives a scalar. 3. Delete `InputTypeCoercionTest` once the previous-major CI job has validated this suite's legacy-path expectations — its 7 unique behaviors have been migrated into the matrix providers and interaction test. ### AI usage (if applicable) - [x] **AI Assisted Issue:** This issue was generated with AI assistance, but was reviewed and refined by the creator. - [ ] **AI Assisted Code:** This code was mainly generated by a human, with AI autocompleting or parts AI generated, but under full human supervision. - [x] **AI Generated Code:** This code was mainly generated by an AI with human guidance, and reviewed, tested, and refined by a human. - [ ] **Vibe Coded:** This code was generated by an AI and has only been functionally tested.