Cap’n Proto RPC Client: Schemas, TLS & JSON

Available in current source after v0.5.0 at Inspect → Cap’n Proto. This is a native Cap’n Proto RPC client for a connection's bootstrap capability, implemented in Go.

Load a schema, then make a call

Choose your .capnp files together and select the root file, or paste a single source schema.
Source loading uses capnp from the ProtoPeek host's PATH. The compiler is optional and is installed separately using the official setup guide.
Without a compiler on the ProtoPeek host, load an unpacked binary CodeGeneratorRequest (.bin)
generated on another machine. For example, in a shell with binary-safe redirection:

   capnp compile -o- service.capnp > service.bin

Windows PowerShell 5 redirection converts native stdout to text: use cmd.exe or PowerShell 7.4+ for this binary output, as described in Microsoft's redirection guide. Compiling before upload also handles nested imports conveniently.

Select a method in the left list. Loading a schema does not connect to a peer.
Enter host:port (or [IPv6]:port), choose TCP or verified TLS, and enter request JSON.
Omitted fields use schema defaults. Expand Parameter fields to see names and types.
Click Call method. Inspect JSON, elapsed time, the connected address and TLS version.
Copy the response or save a JSON report. Cancel stops waiting and closes the connection; a request already delivered to the service may still have taken effect.

TLS verifies both the certificate chain and server identity. Advanced controls accept a server name override and additional CA certificates. Verification cannot be disabled. Each call opens one connection and has a configurable 0.1–30 second deadline; calls are never automatically retried.

JSON representation

Schema value	Request and response representation
Struct or group	Object using schema field names; unknown names are errors
Int64 / UInt64	Exact decimal strings recommended; unquoted request integers also preserve all digits; responses always use strings
Other integers	JSON numbers within the schema type's range
Float	JSON number, or strings `NaN`, `Infinity`, `-Infinity`
Enum	Enumerant name or numeric value
Text / Data	UTF-8 string / base64 string
List	JSON array, including nested lists and structs
Union	At most one member in the request object
Capability / AnyPointer	Null input only; non-null responses are labeled opaque, not followed

Schemas, TLS CA input, requests and responses stay in memory for the open page. They are not saved as profiles. Reloading clears them. The local server receives schema and request bytes; only an explicit call sends the encoded request to the chosen endpoint.

Bounds and compatibility

Source upload accepts at most 32 files / 512 KiB, with imports restricted to supplied files; embedded files and access to other host files are rejected. Compiled schemas are at most 2 MiB, 1,024 nodes and 256 methods. JSON requests and results are limited to 64 KiB. Wire messages, allocation, traversal depth, list size, compiler runtime and simultaneous work also have bounds.

Concrete bootstrap methods, defaults, unions, groups, nested structs and lists have fixture coverage. Encoding is checked against output from the official C++ compiler. TCP and TLS RPC tests use a real Go peer; a C++ RPC peer has not yet been part of this acceptance pass. Generic interface/method bindings, returned-capability workflows and pipelining are not supported. Cap’n Proto is not HTTP or gRPC and does not supply an HTTP-style header editor.

The Go RPC runtime is embedded. The C++ source compiler is not bundled. Dependency license texts are included in release archives; see THIRD_PARTY_NOTICES.md in the repository.

Cap’n Proto workbench

Choose a schema

Select a bootstrap method

Read the native response

Load a schema, then make a call

JSON representation

Bounds and compatibility