feat(engine)!: the descriptor carries the config type's derived JSON Schema by tato123 · Pull Request #2224 · tatolab/streamlib

tato123 · 2026-09-11T00:10:48Z

Summary

The processor descriptor's config slot held a type-name string. An agent reading /api/registry learned the name of a Rust type it had no way to look up, so a wrong config key was discovered after the node was already in the graph. The slot now holds the config type's JSON Schema, with each field's type, its doc-comment description, its serde default, and a required list.

The #[processor] macro derives that document at the site that emitted the name. It goes through a bound-carrying trait rather than schemars directly, so a config type without the derive fails to compile on a message naming the fix:

error[E0277]: `ProbeConfigWithoutTheDerive` is a processor `config =` type but does not derive `JsonSchema`
  --> ...
13 |     config = crate::ProbeConfigWithoutTheDerive,
   |              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ unsatisfied trait bound
   |
   = note: add `#[derive(streamlib::sdk::schemars::JsonSchema)]` and `#[schemars(crate = "streamlib::sdk::schemars")]` to `ProbeConfigWithoutTheDerive`
   = note: the SDK re-exports `schemars` at `streamlib::sdk::schemars`, so the crate needs no new dependency

Every in-tree config type takes that same re-export path, so the route a third-party crate walks is the route the tree itself walks. EmptyConfig gains an empty-object schema and stops discarding a configuration it cannot act on: it names the key with nowhere to go.

One dialect leaves the seam: draft 2020-12 with no $schema key. schemars 0.8 emits draft-07, and the conversion is three things. The meta-schema key and the pointer prefix are generator settings, so the references come out pointed at $defs with nothing to rewrite. The root keyword is renamed, because the root schema type hard-codes it. A tuple field's positional schemas are moved to prefixItems, by a typed visitor rather than a walk over the serialized document, so a config type whose own default data holds a key named items is never touched.

Closes

Closes #2221

Exit criteria

GET /api/registry serves every registered Rust processor's config_schema as a 2020-12 document with types, descriptions, defaults and a required list. The test pattern source shows width and height at 1280 and 720.
A processor declaring no config shows an empty-object schema and refuses a non-empty configuration, naming the key.
A config = type without the derive fails to compile, naming the derive and the re-export path.
The dead per-field machinery and the synthesized id are gone, and the grammar tests that pinned the id are replaced by tests of the emitted schema.

Test plan

gate	what it covers
`cargo test -p streamlib-processor-schema --lib`	the document's fields, the 2020-12 rewrite, the missing-derive note
`cargo test -p streamlib-macros --lib`	the emitted bound, the no-config case, the retired attribute key
`cargo test -p streamlib-engine --test attribute_macro_test`	a real expansion's document; the empty config's refusal and schema
`cargo test -p streamlib-engine --test compile_fail_config_without_json_schema`	the real compiler refusal for a config type missing the derive
`cargo test -p streamlib-engine --lib core::json_schema::config_schema_rendering_tests`	the descriptor to output hop
`cargo test -p streamlib-media-builtins --lib test_pattern_source`	the exit criterion on a real built-in
`cargo test -p streamlib-api-server --lib`	`/api/registry` rendering, and served-equals-generated OpenAPI

Both new engine-lib tests are named in test.yml's slice and the xtask mirror.

Review

Both reviewers ran. The scope-and-plan review returned APPROVE, having run every gate itself and reproduced the missing-derive diagnostic out of tree. The craftsmanship review returned two should-fix items, both taken:

The draft-07 to 2020-12 rewrite was a hand-rolled walk over the serialized document. The generator exposes the pointer prefix and the meta-schema key as settings, so both are now configured rather than patched afterwards, and the recursive $ref walker is gone.
The conversion was incomplete and the module doc claimed otherwise. A tuple-typed config field emits draft-07 positional items, which a 2020-12 validator reads as a schema for every element. No config type in the tree has one today, but the trait is a public blanket impl, so the first third-party crop: (u32, u32, u32, u32) would have published a document labelled 2020-12 that reads wrong with nothing red. It is handled now, with tests for the tuple case, the plain sequence case that must not change, and the bounded tail.

Four smaller items also taken: the empty config moved to its own file beside its siblings, an unreachable visitor method deleted, the registry test's global-registry mutation documented, and the diagnostic test made robust to the attribute being re-wrapped.

Three declined, with reasons: the library expect is genuinely unreachable and has a sibling precedent in the macro-emitted descriptor; memoizing the document per processor type buys microseconds at add and costs codegen complexity; and a ConfigSchemaDocument newtype would contradict the approved change file, which names Option<serde_json::Value>.

Notes for owner

Two ticket claims I corrected against the tree. Both are edited into #2221's body with the original struck through.

The config-type enumeration was short by three groups the tree does not compile without: the wheel's test-harness config, the ten fixture configs in packages/test-fixtures (a workspace member and a contract source, not a consumer), and the two config types in the engine's codec round-trip rig example, which CI compiles. All three are migrated here.
The ticket and the change file both say the committed OpenAPI artifact is regenerated in this PR. That artifact has never been tracked: dist/ is gitignored at .gitignore:65. I re-ran generate_openapi to confirm the spec still builds and renders config_schema as an untyped object, and the served-equals-generated test remains the gate. The change file carries the same wrong claim, so it wants a line at /ship-change.

One consumer breaks, by design. examples/tokio-integration is a converted consumer whose config type does not derive JsonSchema, so it will stop compiling. Per §Consumers a converted consumer's breakage is backlog filed at ship, never work in this stream. The three held consumers with config types — packages/clap, packages/jpeg, packages/screen-capture — lag by design and are owed nothing.

How the compile-failure message is gated. On the owner's call, trybuild is now a dev-dependency of the engine and a compile-fail case hands the compiler a config type without the derive, diffing the real diagnostic against a checked-in snapshot. The source-reading proxy test went with it — two gates on one claim, and the weaker one broke on reformatting rather than on meaning. The gate is named in test.yml and the xtask mirror, and runs in about 36 seconds warm. A compiler upgrade that reflows a diagnostic reddens it; TRYBUILD=overwrite refreshes the snapshot, and the test file carries that instruction.

An empty configuration still passes. ProcessorSpec serializes the empty config to {}, and a legacy nil is still accepted, so only a configuration carrying keys is refused. That refusal is the one behaviour change a running graph can notice.

…Schema The descriptor's config slot held a type-name string an agent could do nothing with: the fields, defaults and descriptions existed only as Rust source, so a wrong key was learned after the node was already in the graph. It now holds the config type's JSON Schema, and `/api/registry` serves it. The `#[processor]` macro emits the document through a bound-carrying trait, so a config type missing the derive fails to compile with a note naming the derive and the SDK's `schemars` re-export rather than a bare trait-bound error. Every in-tree config type takes that same re-export path — the ten built-in configs and their three enums, the codec enum one of them carries, the control plane's config, the wheel's harness config, the ten fixture configs and the rig example's two — so the route a third-party crate takes is the route the tree itself takes. One dialect leaves the seam: draft 2020-12 with no `$schema` key. schemars 0.8 emits draft-07, whose only difference is `definitions` and the references into it, so one normalizing function is the whole conversion. `EmptyConfig` gains a schema of its own — an object with no properties — and stops discarding a configuration it cannot act on: it names the key with nowhere to go. The dead per-field machinery goes with this: `ConfigField`, the `ConfigDescriptor` trait and its derive, `ConfigFieldOutput`, and the synthesized `config_schema_id` with its attribute key. BREAKING CHANGE: `ProcessorDescriptor::config_schema` and `with_config_schema` take a JSON Schema document rather than a name; the `#[processor(config_schema = "…")]` attribute key and the `ConfigDescriptor` derive are deleted; a processor declaring no config refuses a configuration instead of discarding it.

coderabbitai · 2026-09-11T00:11:39Z

📝 Walkthrough

Walkthrough

The change replaces processor configuration schema identifiers with embedded JSON Schema documents. It adds schema generation and draft conversion, updates processor macros and descriptors, derives schemas for in-tree configurations, rejects populated no-config input, and validates registry rendering.

Changes

Processor configuration schema flow

Layer / File(s)	Summary
Schema document contract `sdk/streamlib-processor-schema/...`	Adds `ProcessorConfigJsonSchema`, converts generated draft-07 documents to draft 2020-12, and stores complete schema documents in `ProcessorDescriptor`.
Processor macro integration `sdk/streamlib-macros/...`	Makes `#[processor]` derive schemas from the declared config type. Removes `config_schema`, `config_schema_id`, and the `ConfigDescriptor` derive macro.
Engine rendering and no-config behavior `runtime/streamlib-engine/src/core/...`, `runtime/streamlib-engine/src/lib.rs`, `sdk/streamlib-sdk/src/lib.rs`	Exposes schema documents in descriptor output, omits absent schemas, adds SDK `schemars` re-exports, and makes `EmptyConfig` reject populated maps while publishing an empty-object schema.
Schema adoption and validation `runtime/streamlib-media-builtins/src/...`, `packages/test-fixtures/...`, `runtime/streamlib-api-server/...`, `sdk/streamlib-python-wheel/...`, `.github/workflows/test.yml`, `xtask/src/main.rs`	Adds `JsonSchema` derives to in-tree configuration types. Tests validate field metadata, defaults, registry responses, and CI execution.

Priority: ➖ Normal

Estimated code review effort: 4 (Complex) | ~45 minutes

Severity of issue fixed: Medium

Sequence Diagram(s)

sequenceDiagram
  participant ProcessorDefinition
  participant ProcessorMacro
  participant SchemaGenerator
  participant ProcessorRegistry
  participant RegistryAPI
  ProcessorDefinition->>ProcessorMacro: declare config type
  ProcessorMacro->>SchemaGenerator: generate config schema
  SchemaGenerator-->>ProcessorMacro: return normalized JSON Schema
  ProcessorMacro->>ProcessorRegistry: register descriptor with schema
  ProcessorRegistry->>RegistryAPI: provide descriptor
  RegistryAPI-->>ProcessorDefinition: serve config_schema

Merge Risk: 🟡 Moderate · up to 44a6a

This can expose unsupported schema data to Python consumers and publish registry schemas rejected by strict JSON Schema 2020-12 tooling. Resolve both compatibility issues before merge.

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 62.07% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 58 functions across 29 files. (1 skipped:…	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Linked Issues check	✅ Passed	The changes implement the linked issue objectives [`#2221`]. They replace the schema ID with a normalized JSON Schema document, expose it through the registry, add JsonSchema requirements and diagnostic…
Out of Scope Changes check	✅ Passed	The changes are aligned with the linked issue [`#2221`]. The workflow, fixture updates, schema derives, macro changes, normalization logic, registry rendering, and tests all support the descriptor JSON …
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly and concisely describes the primary breaking change: processor descriptors now carry the configuration type's derived JSON Schema.

Full details: Docstring Coverage

Explanation

Docstring coverage is 62.07% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 58 functions across 29 files. (1 skipped: 1 unsupported.)

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Inline comments:
In `@sdk/streamlib-processor-schema/src/config_schema_document.rs`:
- Line 35: Update rewrite_draft_07_document_as_2020_12 to convert tuple schemas
with array-valued draft-07 items into 2020-12 prefixItems, while preserving
single-schema items for fixed-size Rust arrays; add a regression test validating
the converted tuple document, or retain the draft-07 dialect if complete
conversion is not supported.

In `@sdk/streamlib-python-wheel/src/python_test_harness_endpoints.rs`:
- Around line 35-36: Update TestHarnessChannelConfig’s processor/descriptor
setup to prevent config_schema from being generated or serialized for the Python
descriptors, including removing the unconditional with_config_schema behavior
for this type while preserving the TestBagFeeder and TestBagCollector
configuration.

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ℹ️ Review info

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Plan: Advanced

Run ID: 87d39b52-a7f1-418e-ab82-b71318424e74

📥 Commits

Reviewing files that changed from the base of the PR and between d7aab5f and 44a6a89.

📒 Files selected for processing (31)

.github/workflows/test.yml
packages/test-fixtures/src/test_fixture_processor_configs.rs
runtime/streamlib-api-server/src/api_server_config.rs
runtime/streamlib-api-server/src/handlers.rs
runtime/streamlib-engine/examples/codec_roundtrip_rig.rs
runtime/streamlib-engine/src/core/descriptors.rs
runtime/streamlib-engine/src/core/json_schema.rs
runtime/streamlib-engine/src/core/processors/mod.rs
runtime/streamlib-engine/src/lib.rs
runtime/streamlib-engine/tests/attribute_macro_test.rs
runtime/streamlib-media-builtins/src/audio_window_to_encoded_packet_encoder.rs
runtime/streamlib-media-builtins/src/camera_source.rs
runtime/streamlib-media-builtins/src/display_window.rs
runtime/streamlib-media-builtins/src/encoded_frame_to_published_surface_decoder.rs
runtime/streamlib-media-builtins/src/encoded_video_frame.rs
runtime/streamlib-media-builtins/src/microphone_source.rs
runtime/streamlib-media-builtins/src/mp4_sink.rs
runtime/streamlib-media-builtins/src/published_surface_to_encoded_frame_encoder.rs
runtime/streamlib-media-builtins/src/speaker_sink.rs
runtime/streamlib-media-builtins/src/test_pattern_source.rs
runtime/streamlib-media-builtins/src/virtual_camera_sink.rs
sdk/streamlib-macros/src/codegen.rs
sdk/streamlib-macros/src/config_descriptor.rs
sdk/streamlib-macros/src/grammar.rs
sdk/streamlib-macros/src/lib.rs
sdk/streamlib-processor-schema/src/config_schema_document.rs
sdk/streamlib-processor-schema/src/descriptors.rs
sdk/streamlib-processor-schema/src/lib.rs
sdk/streamlib-python-wheel/src/python_test_harness_endpoints.rs
sdk/streamlib-sdk/src/lib.rs
xtask/src/main.rs

💤 Files with no reviewable changes (1)

sdk/streamlib-macros/src/config_descriptor.rs

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… patching its output Two review findings on the new seam. The draft-07 to 2020-12 rewrite walked the serialized document to repoint every `$ref` and strip `$schema`. Both are generator settings, so they are configured now and the references arrive already pointed at `$defs`. Only the root keyword is renamed by hand, because `RootSchema` hard-codes it. A generic walk over serialized JSON also rewrote any string under a key named `$ref`, including a config type's own data. The conversion was incomplete and the module doc claimed otherwise. A tuple field emits draft-07 positional `items`, which 2020-12 reads as a schema for every element. Nothing in the tree has one, but the trait is a public blanket impl, so the first third-party tuple field would publish a document labelled 2020-12 that a validator reads wrong. A typed visitor moves them to `prefixItems`, so a config type whose own default data holds a key named `items` is never touched. Beside those: `EmptyConfig` moves to its own file rather than dominating a module index, its unreachable `visit_none` goes, the registry test says why it mutates a process-global with no teardown, and the diagnostic test survives the attribute being re-wrapped.

…iler, not by a proxy The ticket's contract is that an author whose `config =` type lacks `JsonSchema` gets a message naming the derive and the re-export path. Nothing in CI handed the compiler such a type: the macro's emitted bound was gated by a codegen test and the note's wording by a test that read its own source, so both halves could stay green while they stopped composing. `trybuild` compiles the real thing and diffs the real diagnostic. The source-reading test goes with it — two gates on one claim, and the weaker one broke on reformatting rather than on meaning. A compiler upgrade that reflows a diagnostic reddens this; `TRYBUILD=overwrite` refreshes the snapshot, and the test file says so.

tato123 · 2026-09-11T14:38:33Z

Review comments, dispositioned

Tuple schemas must be prefixItems — FIXED (d4332e6). Landed before this comment arrived, from the same finding raised independently by the craftsmanship review. A typed visitor moves a tuple's positional schemas to prefixItems and its additionalItems to items, with tests for the tuple case, the plain-sequence case that must not change, and the bounded tail. Typed rather than a walk over serialized JSON, so a config type whose own default data holds a key named items is never touched.

config_schema on the wheel's harness processors — NOT A DEFECT, no change. Answered inline. The two processors named are Rust, not Python; the Python-class descriptor path never sets the slot and is untouched here.

Docstring coverage 62% vs an 80% threshold — IGNORED. This repo's comment rules (.claude/rules/comments.md) set the opposite policy: default to none, one line for a public item, and no narration. Most of the 58 functions counted are tests, which carry a doc only where it says something the name cannot. Writing docstrings to clear a coverage number would violate the rule the repo actually enforces.

One related call worth stating: with_config_schema has no doc comment. All ten sibling builders on ProcessorDescriptor have none either, and the field it sets is documented. A doc on one of ten is noise, so it stays as it is.

…rchive it (#2243) Folds the three [agent-readable-processor-catalog] entries in §Processor model as built (#2224, #2226, #2228) and adds the MCP resources and prompts sentence §Control plane owed #2215 (#2232). Both section headings drop the change arrow and stay IN-FLIGHT for their remaining OPEN entries. The change file moves to archive/ under the last ticket's merge date. Co-authored-by: Claude Opus 5 <noreply@anthropic.com>

tato123 added 2 commits September 10, 2026 19:57

docs(macros): the config grammar names the JsonSchema requirement

44a6a89

coderabbitai Bot reviewed Sep 11, 2026

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Comment thread sdk/streamlib-processor-schema/src/config_schema_document.rs Outdated

Comment thread sdk/streamlib-python-wheel/src/python_test_harness_endpoints.rs

tato123 added 2 commits September 10, 2026 20:27

tato123 merged commit 51fafdd into main Sep 11, 2026
11 checks passed

tato123 deleted the feat/2221-descriptor-carries-config-schema branch September 11, 2026 14:38

github-actions Bot mentioned this pull request Sep 11, 2026

chore: release main #2225

Merged

tato123 mentioned this pull request Sep 13, 2026

docs(plan): fold agent-readable-processor-catalog into the plan and archive it #2243

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feat(engine)!: the descriptor carries the config type's derived JSON Schema - #2224

feat(engine)!: the descriptor carries the config type's derived JSON Schema#2224
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