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fix: write custom bindings as x- extensions in AsyncAPI 3 documents - #67
In AsyncAPI 3 (3.0 and 3.1), the bindings objects only accept the specification's binding names and x- extensions (additionalProperties: false). AsyncApiBindings<TBinding>.SerializeV3 wrote every binding under its BindingKey, so:
a custom binding (BindingKey => "signalr") produced bindings.signalr, and the document failed validation against the AsyncAPI JSON Schema
the reader accepted these documents without diagnostics, so validating through ByteBard didn't catch it
Changes
AsyncApiBindingKeys (new, public, in ByteBard.AsyncAPI.Bindings): IsSpecificationKey(key) checks against every binding name the AsyncAPI 3.0/3.1 specification text lists. That includes mqtt5 and mercure, which the JSON Schemas leave out; the prose is normative, so they keep their own keys. ToV3Key(key) returns the key as is for a specification name or an x- key, and x-<key> otherwise.
Writer:SerializeV3 writes ToV3Key(binding.Key). If that name clashes with an extension or another binding, it throws an AsyncApiWriterException instead of writing a duplicate property. SerializeV2 is unchanged, because AsyncAPI 2's bindingsObject allows any key.
Reader (3.x only):
x-<key> is loaded as the binding when a parser for <key> is registered; otherwise it stays an extension, as before
a registered custom binding under its bare key is still loaded (and written back as x-<key>), but the reader adds an error: ChannelBinding signalr is not an AsyncAPI 3 binding name; write a custom binding as x-signalr
pattern fields are now parsed per property in the same loop, so x-<key> isn't also added as an extension
a key with a registered parser is read as that binding even when it starts with x- (in 2.x and 3.x), so a binding whose BindingKey is already x-signalr reads back as a typed binding rather than a generic extension
The model key doesn't change: Bindings["signalr"] and TryGetValue<T> work as before.
Tests
New CustomBindingKey_Should covers:
3.x writes x-my, 2.x writes my, and specification bindings keep their own names
reading and round-tripping x-my in 3.x
the error for a bare my in 3.x, and no diagnostics for it in 2.x
an unregistered x- key staying an extension
IsSpecificationKey and ToV3Key
a binding registered under an x- key reads and writes as that binding in 2.x and 3.x
AsyncAPI 3 now formats custom binding keys with the required x- prefix when writing documents, while preserving keys already prefixed.
Registered custom bindings can be read from prefixed keys; unregistered extensions remain extensions. Nonconforming binding keys are retained and reported.
Documentation
Clarified how custom binding keys are handled across AsyncAPI 2 and 3.
No actionable comments were generated in the recent review.
Walkthrough
AsyncAPI 3 serialization now writes non-specification binding keys as extension keys. Reading resolves registered custom bindings under extension keys and reports an error for registered custom bindings under bare keys. AsyncAPI 2 binding-key behavior remains unchanged.
Changes: Binding Key Rules
Layer / File(s)
Summary
Key conversion and serialization src/ByteBard.AsyncAPI/Models/AsyncApiBindingKeys.cs, src/ByteBard.AsyncAPI/Models/AsyncApiBindings{TBinding}.cs
The new helper identifies specification binding keys and converts other keys for AsyncAPI 3 serialization. AsyncAPI 2 serialization remains unchanged.
The reader uses the document version to parse registered bindings, including custom bindings under registered extension keys. A registered binding under a bare key is retained and produces an error in AsyncAPI 3. Tests and the guide cover these rules.
Unusual binding combinations can produce invalid AsyncAPI 3 documents. The MQTT5 examples are correct. Merge is reasonable with explicit ownership of the two remaining serialization fixes.
Pre-merge checks: 3 · 2
Failed checks: 2 warnings
Check name
Status
Explanation
Resolution
Linked Issues check
Directly linked issue #66 has one active coding objective group with three requirements.
1. Writer (AsyncAPI 3 only), non-specification keys written as x-: Met for custom keys. AsyncApiBindings<…
In src/ByteBard.AsyncAPI/Models/AsyncApiBindingKeys.cs, remove "mqtt5" from SpecificationKeys so that the 3.x writer emits x-mqtt5. Confirm whether "mercure" belongs in the set, using the AsyncAPI 3.0 and 3.1 specification text, and remove …
Docstring Coverage
Docstring coverage is 22.22% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff with cyclomatic complexity of at least 2. Documenta…
Write docstrings for the functions missing them to satisfy the coverage threshold.
Passed checks: 3 passed
Check name
Status
Explanation
Title check
The title names the main change: custom bindings are written as x- extensions in AsyncAPI 3 documents. The changeset matches this. The change adds AsyncApiBindingKeys.ToV3Key, updates SerializeV3, and…
Out of Scope Changes check
All changed areas connect to #66 or support its key-handling requirements.
- AsyncApiBindingKeys (new, public): implements the specification-name check and the x- key mapping. The name list is wider …
Description check
The description is detailed and matches the repository template in spirit. It explains the problem, the changes to the new AsyncApiBindingKeys helper, the writer and reader behavior, and the tests. It…
Full details: Title check
Explanation
The title names the main change: custom bindings are written as x- extensions in AsyncAPI 3 documents. The changeset matches this. The change adds AsyncApiBindingKeys.ToV3Key, updates SerializeV3, and updates the reader for AsyncAPI 3. The title uses the conventional 'fix:' prefix and is concise, so it passes.
Full details: Out of Scope Changes check
Explanation
All changed areas connect to #66 or support its key-handling requirements.
AsyncApiBindingKeys (new, public): implements the specification-name check and the x- key mapping. The name list is wider than the list in the issue because it adds mercure and mqtt5. This difference is covered by the linked-issue assessment, not by this scope check.
AsyncApiBindings<TBinding>.SerializeV3: implements the writer requirement. SerializeV2 is unchanged.
BindingDeserializer.LoadBindings (new AsyncApiVersion parameter) and the four 3.x binding deserializers that pass AsyncApiVersion.AsyncApi3_0: implement the reader requirements. The per-property pattern-field handling prevents x-<key> from also being added as an extension.
Registered parsing of x--prefixed keys in 2.x and 3.x: this changes 2.x reading, which the issue does not request. It supports round-tripping a binding whose BindingKey already starts with x-. It is connected to the same key-handling rules, so it is not unrelated work.
CustomBindingKey_Should tests: cover the requirements and the 2.x behavior.
MQTT5 test updates: follow from the reader error and writer output. They depend on the mqtt5 decision in the linked-issue assessment.
Documentation (custom-bindings guide and MQTT 5 page): describes the 3.x key behavior.
No unrelated refactoring or feature work was identified in the described changes.
Full details: Description check
Explanation
The description is detailed and matches the repository template in spirit. It explains the problem, the changes to the new AsyncApiBindingKeys helper, the writer and reader behavior, and the tests. It links the issue with 'Closes #66'. It does not follow the template headings exactly: it has no '## About the PR' heading and no '### Changelog' section with Add/Remove entries. Those are structural gaps, but the required information is present, so the check passes.
Full details: Linked Issues check
Explanation
Directly linked issue #66 has one active coding objective group with three requirements.
Writer (AsyncAPI 3 only), non-specification keys written as x-<key>: Met for custom keys. AsyncApiBindings<TBinding>.SerializeV3 writes AsyncApiBindingKeys.ToV3Key(binding.Key). ToV3Key adds the x- prefix to keys that are not in SpecificationKeys. SerializeV2 is unchanged. The model key is unchanged.
Reader (AsyncAPI 3 only), x-<key> loaded when a parser for <key> is registered, otherwise kept as an extension: Met by the reviewed code and PR description. Unregistered x- keys stay extensions.
Reader (AsyncAPI 3 only), registered binding under bare key still loaded with an error: Met for custom keys. The error text is ChannelBinding signalr is not an AsyncAPI 3 binding name; write a custom binding as x-signalr.
Unmet requirement, mqtt5: Issue #66 states that the built-in mqtt5 binding has the same problem in 3.x. It lists the AsyncAPI 3 binding names without mqtt5, and it states that AsyncAPI 3 has no mqtt5 binding. The reviewed head adds "mqtt5" (and "mercure") to AsyncApiBindingKeys.SpecificationKeys. As a result, SerializeV3 writes mqtt5 as a bare key, not x-mqtt5. The PR description states that the JSON Schemas omit mqtt5. The issue reports the failure as schema validation failure. The bare mqtt5 key therefore still fails the official AsyncAPI 3 JSON Schema, which leaves the reported problem for mqtt5 unresolved. The previous PASS assumed mqtt5 was written as x-mqtt5. That assumption no longer holds at the current head. The specification-prose argument is not verified from repository evidence, and it contradicts the issue requirement.
Uncertainty, mercure: The issue does not list mercure as a specification binding name. The code treats it as one. This is not the basis for the verdict, but the specification text should confirm it.
Not verified directly: test file contents. The reported test results come from the PR description.
Resolution
In src/ByteBard.AsyncAPI/Models/AsyncApiBindingKeys.cs, remove "mqtt5" from SpecificationKeys so that the 3.x writer emits x-mqtt5. Confirm whether "mercure" belongs in the set, using the AsyncAPI 3.0 and 3.1 specification text, and remove it if it does not. Update the 3.x MQTT5 expectations in MQTT5Bindings_Should, BindingsCollection_Should, and the MQTT 5 documentation to use x-mqtt5. Keep the 2.x expectations as mqtt5. If the author instead maintains that mqtt5 is a specification binding name, issue #66 must be revised first, because its stated requirement conflicts with that position.
Full details: Docstring Coverage
Explanation
Docstring coverage is 22.22% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff with cyclomatic complexity of at least 2. Documentation-only edits and deletions within surviving functions are included; unchanged diff context and fully deleted functions are excluded. Analyzed 9 functions across 8 files. (1 skipped: 1 unsupported.)
Resolution
Write docstrings for the functions missing them to satisfy the coverage threshold.
Finishing Touches
Generate unit tests (beta)
Create a new PR
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Actionable comments posted: 3
Fix comments: Fix CodeRabbit comments on this PR
Prompt to fix all issues
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Inline comments:
Review comments at @src/ByteBard.AsyncAPI/Models/AsyncApiBindingKeys.cs:
- Around line 20-23: Update ToV3Key to determine whether a binding name is
specification-defined for its specific bindings object, rather than using a flat
set across all scopes. Ensure custom ros2 and pulsar bindings in
AsyncApiBindings<IMessageBinding> are written with the x- prefix while
preserving unprefixed names in scopes where they are specification-defined.
Review comments at @src/ByteBard.AsyncAPI/Models/AsyncApiBindings{TBinding}.cs:
- Around line 49-50: Update SerializeV3 to detect when distinct bindings map to
the same V3 property name through AsyncApiBindingKeys.ToV3Key, including a
custom binding and an existing x- extension such as x-my. Reject the collision
with an error before writing either conflicting property, while preserving
serialization of non-colliding bindings.
Review comments at @website/docs/bindings/mqtt5.md:
- Line 90: Update both AsyncAPI 3 reader examples in the MQTT 5 binding
documentation to use `x-mqtt5` instead of the bare `mqtt5` key, preserving the
existing binding values and references.
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AsyncAPI 3 bindings objects only allow the specification's binding names
and x- extensions, but SerializeV3 wrote every binding under its
BindingKey, so a document with a custom binding (or the deprecated
mqtt5 binding) failed validation against the AsyncAPI JSON Schema, while
the reader accepted it without diagnostics.
When serializing AsyncAPI 3, write a binding whose key is not a
specification binding name as x-<key>; AsyncApiBindingKeys exposes the
name list and the mapping. When reading AsyncAPI 3, load x-<key> as the
binding when a parser for <key> is registered, and report an error for
a registered custom binding under its bare key (it is still read).
AsyncAPI 2 allows any key and is unchanged.
ClosesByteBardOrg#66
A custom binding may already use an x- key (BindingKey => "x-signalr") so
its documents validate with any reader. Such a key matched the extension
pattern first and was read as a generic extension. Look up the registered
parsers before treating an x- key as an extension, in both versions.
The AsyncAPI 3.0 and 3.1 specification text lists mqtt5 and mercure as fixed
fields of the bindings objects; only the JSON schemas leave them out. Follow the
normative text, so mqtt5 is written and read under its own key in 3.x instead of
x-mqtt5.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
A custom binding "my" is written as "x-my" in AsyncAPI 3, which can clash with
an extension or another binding of that name. Throw an AsyncApiWriterException
instead of writing a duplicate property.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Yes. In 3.0/3.1 the Server/Channel/Operation/Message Bindings Objects have a fixed list of protocol fields and "MAY be extended with Specification Extensions". The JSON Schema enforces this with additionalProperties: false plus the ^x- pattern, so bindings.signalr is invalid in 3.x and bindings.x-signalr is valid. 2.x's bindingsObject allows any key, so 2.x is unchanged. The spec doesn't give x- keys any binding meaning. Reading x-<key> back as a typed binding when a parser for <key> is registered is our convention, and other tools will see it as a plain extension.
If the BindingKey is already x-whatever, ToV3Key leaves it unchanged (there's no x-x-whatever). The reader also matches a registered key exactly before it checks for the x- prefix, so that binding round-trips as typed in both 2.x and 3.x. CustomBindingKey_Should covers this. If a custom whatever binding and an x-whatever extension end up in the same 3.x bindings object, the writer now throws instead of writing a duplicate property.
While checking this I found that the spec text lists mqtt5 and mercure as binding fields, but the JSON Schemas leave them out. I went with the spec text, so mqtt5 keeps its own name in 3.x. I've pushed that change.
I honestly think this is an issue with the spec (and someone not thinking things through), as in 2.6 this was supported.
Ive created an RFC here asyncapi/spec#1251 for this to be changed.
Additionally, this change changes semantics around extensions.
Registering a signalr parser now causes x-signalr to be consumed as a typed binding rather than retained in Extensions. Parser registration alone does not establish that an existing extension follows that binding’s format. Exact registered x- keys also change reading behavior in v2, despite the claim that v2 is unchanged.
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Closes #66.
Problem
In AsyncAPI 3 (3.0 and 3.1), the bindings objects only accept the specification's binding names and
x-extensions (additionalProperties: false).AsyncApiBindings<TBinding>.SerializeV3wrote every binding under itsBindingKey, so:BindingKey => "signalr") producedbindings.signalr, and the document failed validation against the AsyncAPI JSON SchemaChanges
AsyncApiBindingKeys(new, public, inByteBard.AsyncAPI.Bindings):IsSpecificationKey(key)checks against every binding name the AsyncAPI 3.0/3.1 specification text lists. That includesmqtt5andmercure, which the JSON Schemas leave out; the prose is normative, so they keep their own keys.ToV3Key(key)returns the key as is for a specification name or anx-key, andx-<key>otherwise.SerializeV3writesToV3Key(binding.Key). If that name clashes with an extension or another binding, it throws anAsyncApiWriterExceptioninstead of writing a duplicate property.SerializeV2is unchanged, because AsyncAPI 2'sbindingsObjectallows any key.x-<key>is loaded as the binding when a parser for<key>is registered; otherwise it stays an extension, as beforex-<key>), but the reader adds an error:ChannelBinding signalr is not an AsyncAPI 3 binding name; write a custom binding as x-signalrx-<key>isn't also added as an extensionx-(in 2.x and 3.x), so a binding whoseBindingKeyis alreadyx-signalrreads back as a typed binding rather than a generic extensionBindings["signalr"]andTryGetValue<T>work as before.Tests
CustomBindingKey_Shouldcovers:x-my, 2.x writesmy, and specification bindings keep their own namesx-myin 3.xmyin 3.x, and no diagnostics for it in 2.xx-key staying an extensionIsSpecificationKeyandToV3Keyx-key reads and writes as that binding in 2.x and 3.xThe custom-bindings guide describes the 3.x key.
🤖 Generated with Claude Code
Summary by CodeRabbit
x-prefix when writing documents, while preserving keys already prefixed.