Cap’n Proto workbench

Open Inspect → Cap’n Proto for native Cap’n Proto RPC, implemented in Go. Stable v0.6.1 supports concrete bootstrap calls. Published v0.7.0 also includes the generic bindings and bounded capability plans below. Compiler, endpoint and native-platform acceptance remain separate.

Load a schema, then make a call

1. 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.

2. 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.

1. Select a method in the left list. Loading a schema does not connect to a peer.
2. 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.

3. 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

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. Those fixtures do not establish C++ runtime interoperability for every generic or capability plan. Cap’n Proto is not HTTP or gRPC and does not supply an HTTP-style header editor.

Generic bindings and capability plans · current source

First expand Capability call plan & type bindings, or Generic type bindings & capability plan when the selected method has generic parameters.

Generic type bindings supplies the selected interface/method's type arguments. Use the shown schema parameters and supported type expressions; unresolved or incompatible bindings fail validation instead of guessing a schema.

Expand Capability call plan · up to 8 steps. Add a step, select a method, and target either the bootstrap capability or a typed interface field returned by an earlier step. Returned capability field uses a dot-separated path through named struct fields; list elements and union branches cannot be pipelined. Each step has its own JSON input and applicable generic bindings.

Choose Review plan, inspect the target and steps, then Run reviewed plan to connect. Steps are pipelined on one connection under the selected total deadline and a 4 MiB wire budget in each direction; each JSON input/result remains capped at 64 KiB. The connection closes on completion, cancel or deadline. Plans and capability handles are not persistent sessions.

If a later step fails, earlier results remain visible. Dependent calls may already have been sent; cancellation cannot undo effects at the peer. Local truncation or an incomplete plan is not a successful complete response. Real supported-peer acceptance remains separate from fixture tests.

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.