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Add a JSON-RPC server mode to F# Interactive - #20396

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T-Gro merged 30 commits into
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xperiandri:fsi-json-rpc
Sep 21, 2026
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Add a JSON-RPC server mode to F# Interactive#20396
T-Gro merged 30 commits into
dotnet:mainfrom
xperiandri:fsi-json-rpc

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@xperiandri

@xperiandri xperiandri commented Aug 29, 2026 •
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F# Interactive gets a server mode, --fsi-server-jsonrpc:<pipe name>. A host submits code over a named pipe and receives structured diagnostics, exceptions, evaluated values, and process information; program output remains on stdout/stderr. This removes prompt parsing and gives editors deterministic request boundaries and source positions. The existing Visual Studio window still uses its current transport; this PR provides the protocol and an end-to-end reference client.

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xperiandri requested a review from a team as a code owner August 29, 2026 22:48
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✅ No release notes required

@github-actions github-actions Bot added ⚠️ Affects-Compiler-Output Tooling check: PR touches IL emission or codegen ⚠️ Affects-Build-Infra Tooling check: PR touches build infrastructure ⚠️ Affects-Restore Tooling check: PR touches NuGet packages or feeds labels Aug 29, 2026
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@majocha

majocha commented Aug 30, 2026

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An idea: we could initially make it an out of process part of VS, a fsi session wrapper alternative to fsmain.

@github-actions github-actions Bot added the ⚠️ Affects-Test-Tooling Tooling check: PR touches test framework infrastructure label Aug 31, 2026
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T-Gro
T-Gro reviewed Sep 3, 2026
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I do like the idea of a server mode.

That being said, I think the protocol must be validated by usage.

Is there any benefit in dotnet fsi using it (maybe some bonus arguments referring to a single shared session?)

Could the VS-FSI be ported to use it? (would be a great dogfooding test for feature completeness and API UX)

(would also love if @TheAngryByrd, @baronfel or @auduchinok could list here any wishes from an FSI JSON-RPC server API and protocol wise)

@github-project-automation github-project-automation Bot moved this from New to In Progress in F# Compiler and Tooling Sep 7, 2026
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xperiandri commented Sep 10, 2026
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xperiandri requested a review from T-Gro September 10, 2026 09:43
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xperiandri force-pushed the fsi-json-rpc branch 3 times, most recently from 4c8c5fd to b7e2742 Compare September 11, 2026 14:44
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🤖🕵️ Please shorten the description using this guidance. Focus on the problem and why the change is needed, in simplified technical English. Leave the implementation inventory to the Files tab and retain necessary caveats.

xperiandri and others added 18 commits September 21, 2026 18:23
@xperiandri @claude
Interpolate strings and mark up the doc comments in the new code
f2c92ec 
Every `sprintf`/`failwithf` in the server, the protocol and their tests
becomes an interpolated string, keeping the format specifiers so the types
are still checked.

Doc comments that carry more than one paragraph get `<summary>` and
`<remarks><para>`, with code references in `<c>` rather than backticks;
`<pipe name>` inside the option's description is escaped, since it now sits
in XML.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
@xperiandri @claude
Use .NET format specifiers for the generated names in the tests
1532d0e 
`{Guid.NewGuid():N}` reads better than `%s{Guid.NewGuid().ToString "N"}`
and drops the triple quotes the nested literal forced. The two remaining
concatenations in the harness — the pipe name and the server switch — go
the same way.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
@xperiandri @claude
Drop the % specifiers from the interpolated strings
11e9159 
Plain `{expr}` throughout, with a .NET format string where the text needs
one — `FS{d.errorNumber:D4}` keeps the four-digit error number.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
@xperiandri @claude
Shorten the test pipe name to fit the macOS socket path limit
27d7ce3 
On Unix a named pipe is a socket under $TMPDIR, and macOS caps socket
paths at 104 characters; with its long $TMPDIR every session failed to
start. A failed connection now also reports the session's stderr.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
@xperiandri @claude
Strip the macOS /private prefix before comparing working directories
8d26619 
Path.GetTempPath() returns /var/folders/..., but the session reports
/private/var/folders/... after chdir — the kernel resolves /var through
/private when it canonicalises a path, and Path.GetFullPath does not.
setPaths changes the working directory failed on every macOS CI run.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
@xperiandri
Address FSI JSON-RPC review findings
ddea77a 
Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>
@xperiandri
Return evaluated FSI values over JSON-RPC
c22be51 
Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>
@xperiandri
Fix remaining FSI JSON-RPC review issues
57625b8 
Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>
@xperiandri
Format FSI server fixup
f8f30bf 
Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>
@xperiandri
Watch FSI host before pipe connection
70fce7d 
Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>
@xperiandri
Register FSI RPC handlers explicitly
6447908 
Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>
@xperiandri @claude
Evaluate every interaction EvalInteraction is given and expose the se…
7292816 
…rver options on the session

EvalInteraction parsed one interaction and discarded the text after its first `;;`, although its documentation promises one or more. It now commits each completed interaction and parses the next from the same lexer, as the standard-input loop does, so a host can hand over a whole selection and a line directive at its top holds for all of it.

The JSON-RPC pipe name and the host process id become members of FsiEvaluationSession, read from the options the session has already parsed — response files included — instead of a second scan of argv in the entry point. The console reader is gated on the server options rather than asserted absent.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
@xperiandri @claude
Admit only the current user to the fsi server pipe and let the sessio…
6689126 
…n split and print

The pipe accepted any local client, and whoever connected first ran code as the user. It now opens with PipeOptions.CurrentUserOnly. The process the server watches is the one named on the command line, so a host that dies before connecting takes the session with it.

The hand-written `;;` splitter is gone: EvalInteractionNonThrowing evaluates the whole text, so a verbatim string ending in a backslash or a `(*)` operator no longer misleads the server, and a line directive covers the entire selection. Values are formatted by the session's own printer under its print settings instead of `%A`, and a ToString that throws becomes the value's text rather than a failed interaction. Paths spliced into #load, #I and #silentCd are verbatim literals, so a quote or a trailing backslash in a path survives. Requests carry their parameters as one object, the shape the protocol documents.

The server is compiled into the .NET fsi only: the .NET Framework fsi does not carry StreamJsonRpc and a source-only build cannot provide it. A session asked for the mode without it exits with a message instead of idling.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
@xperiandri @claude
Ship the server's assembly closure in Microsoft.FSharp.Compiler, not …
0a63a81 
…in the VS setup

The SDK lays out `dotnet fsi` from this package's lib folder, and the Visual Studio installer entries added before reached no SDK. The project file lists the closure once and feeds the nuspec through a token that is empty under a source-only build. A test checks the list against what fsi's build restores and starts a session from the staged files alone.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
@xperiandri @claude
Compare the reported framework and file name ordinally
cf9482c 
xUnit's StartsWith and EndsWith default to the current culture.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
perf-bundle @xperiandri
Harden FSI JSON-RPC protocol handling
c3057e6 
Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>
perf-bundle @xperiandri
Compact FSI JSON-RPC remediation
737f671 
Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>
@xperiandri @claude
Keep the last value EvalInteraction produced, and let the harness los…
68e6f0a 
…e the pipe race

The remediation passed the two values to Option.orElse the wrong way round, so a later value was replaced by an earlier one: `41;; 42;;` gave 41. The interaction's own status already carries the merged value. Closing a corrupted control channel can find the pipe disposed already, when the session hangs up and the client's JsonRpc disposes it first.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
auto-merge was automatically disabled September 21, 2026 16:33

Head branch was pushed to by a user without write access

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T-Gro enabled auto-merge (squash) September 21, 2026 17:45
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T-Gro enabled auto-merge (squash) September 21, 2026 19:18
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@T-Gro
T-Gro merged commit bd4e77f into dotnet:main Sep 21, 2026
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@github-project-automation github-project-automation Bot moved this from In Progress to Done in F# Compiler and Tooling Sep 21, 2026
@xperiandri
xperiandri deleted the fsi-json-rpc branch September 21, 2026 21:03
@xperiandri xperiandri mentioned this pull request Sep 21, 2026
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