Add the F# Interactive window on the JSON-RPC protocol by xperiandri · Pull Request #20565 · dotnet/fsharp

xperiandri · 2026-09-16T13:46:10Z

Replaces the Visual Studio F# Interactive window with one hosted on Microsoft.VisualStudio.InteractiveWindow — the same REPL engine C# Interactive and Python Interactive use — driving fsi over the JSON-RPC protocol fsi gained in #20396 instead of parsing SERVER-PROMPT> text on stdin.

github-actions · 2026-09-16T13:46:27Z

✅ No release notes required

T-Gro · 2026-09-16T19:52:31Z

+
+  <ItemGroup>
+    <ProjectReference Include="$(FSharpSourcesRoot)\FSharp.Core\FSharp.Core.fsproj" />
+    <ProjectReference Include="$(FSharpSourcesRoot)\Compiler\FSharp.Compiler.Service.fsproj" />


How would the architecture need to change for the window to locate the SDK, and start the JSON-RPC server from the SDK? i.e. the Window NOT having a reference to FSharp.Compiler.Service.fsproj

That way, we decouple VS versions and SDK versions (steered via global.json).
We would also gain parity of CLI fsi, CLI build VS FSI - guaranteed to use same compiler bits.

And more importantly, the VS FSI could then be powered by net11 F# compiler, benefiting from any perf improvements on the .NET side.

Done in the last commit: the session starts in the solution folder with the dotnet a shell there would run, so the host resolves the SDK from global.json exactly as dotnet fsi does — nothing in the window re-implements that, and the fsi it evaluates on is the SDK's. The FCS reference that remains serves the tokenizer only (Enter-vs-newline in SubmissionAnalysis, lexical colouring) — the same FCS FSharp.Editor already loads; it plays no part in evaluation.

🤖🕵️ @xperiandri This is still occurring.

<ProjectReference Include="$(FSharpSourcesRoot)\Compiler\FSharp.Compiler.Service.fsproj" />

SDK selection is fixed; the requested no-FCS window boundary is not.

You are right, and my earlier answer only covered evaluation. The window no longer references FSharp.Compiler.Service.fsproj, and cannot compile against it: DisableTransitiveProjectReferences stops FSharp.VS.FSI (which keeps the compiler for the old window) from handing it on. What the window used the compiler for was its lexer, to colour input and output and to decide when Enter submits. That is now ILexicalScannerFactory in LexicalScanning.fs, and FSharp.Editor, which already runs on the compiler Visual Studio ships, exports the implementation. The session's own compiler stays the SDK's, in the dotnet fsi process. The submission rule is still tested, with the editor's scanner, in SubmissionAnalysisTests.

T-Gro · 2026-09-16T20:02:14Z

+        UICultureLcid: int
+    }
+
+module internal FsiLocator =


This should use the SDK locator as if you launched dotnet fsi from the solution folder (i.e. following global.json).

Also as long as we keep the legacy FSI in current shape, the new window via JSON-RPC could be netcoreapp only and not offer any of the .NET Framework options.

Both done: the platform choice, the desktop executables and the shadow-copy switch are gone from this window (they stay with the old one), and the session starts in the solution folder so global.json picks the SDK. When a session comes up the window prints which fsi answered, on what runtime and where.

xperiandri · 2026-09-20T20:28:04Z

Re-stacked on xperiandri/fsharp@c5501123 (#20396 after its second pass: the server is .NET-only, the pipe is CurrentUserOnly, and EvalInteraction evaluates a whole selection, so the window no longer relies on the server splitting on ;;). The only change here is that the window passes --fsi-server-client-pid, so a Visual Studio that dies between launching fsi and connecting no longer leaves a session waiting on the pipe.

Two things from the review are still open and are the next steps for this PR rather than for #20396:

Locating fsi through the SDK that global.json resolves to, instead of FSHARP_INTERACTIVE_PATH or dotnet from PATH — @T-Gro's thread on InteractiveHost.fs.
Dropping the .NET Framework platform choices from this window. The server now exists only in the .NET fsi, so picking a desktop platform ends with fsi's own "The JSON-RPC server mode is not available in this build of F# Interactive." on the error stream.

One limit worth knowing: the client side cannot ask for PipeOptions.CurrentUserOnly on net472, so the GUID pipe name is what stands between the window and a pipe squatter until the window itself targets .NET.

T-Gro · 2026-09-21T13:59:53Z

@xperiandri Current head 0e32fd1 is conflicting. Build MacOS Batch2, Build CheckCodeFormatting, fsharp-ci, and check_release_notes are failing.

T-Gro

🤖🕵️ Review follow-up.

T-Gro · 2026-09-21T13:59:53Z

+        addSwitch $"{CommandLine.ServerOption}{pipeName}"
+        // How the host names itself: a session whose host dies, even before the handshake, exits instead of
+        // waiting on the pipe forever.
+        addSwitch $"--fsi-server-client-pid:{clientProcessId}"


🤖🕵️ @xperiandri user arguments can override the trusted owner PID; closing VS leaves FSI alive.

trusted: --fsi-server-client-pid:<VS pid> UserArguments: --fsi-server-client-pid:0 owner exits -> server_alive=true

// Reject duplicate ownership switches, or append the trusted PID after UserArguments.

Fixed. The window's own switches now come last, after the user's arguments, and for --fsi-server-jsonrpc and --fsi-server-client-pid the last occurrence wins, so --fsi-server-client-pid:0 in the arguments no longer names the owner. the last owner named on the command line is the one that counts starts a session with two owners and closes the first; the session keeps serving.

The window package records itself in the properties of the buffers it creates for input but not for output, so asking a buffer which window it belongs to could never answer for output. It also replaces the output buffer on reset, which rules out stamping one. The windows evaluating F# now keep a register, and an output buffer is ours when one of them currently owns it. Ownership is also settled later than before. The buffer reaches the provider before the window has finished claiming it, so deciding at that moment decided once and for all against us; it is now asked on each request until known. Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

The session used to learn its host from the handshake alone, so a Visual Studio that died between launching fsi and connecting left it waiting on the pipe for good. `--fsi-server-client-pid` names the process before the pipe opens; the handshake still carries it for a session started some other way. Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

The window picked its own fsi: `dotnet.exe` from Program Files, or one of the desktop executables the extension ships, chosen through the options page the old window reads. The desktop ones cannot serve the protocol at all — the server exists in the .NET fsi alone — and the machine-wide `dotnet` ignored the SDK a solution pins. The session now starts in the open solution's folder with the `dotnet` a shell there would run, so the host resolves the SDK from that folder's `global.json` exactly as `dotnet fsi` typed at a prompt does, and the window runs the same compiler bits as `dotnet build`. The platform choice, the desktop executables and the shadow-copy switch stay with the old window. When a session comes up the window prints which fsi answered, on what runtime and where, so a pinned SDK is seen rather than guessed at. Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

For --fsi-server-jsonrpc and --fsi-server-client-pid the last occurrence wins, so the user's own arguments could name another pipe or another process as the session's owner, and the session would outlive the window. The window's switches now come last; a test pins the last-wins rule they rely on. Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

…the F# compiler The window used the compiler for one thing, its lexer, to colour input and output and to decide when Enter submits. It now asks an ILexicalScannerFactory, which FSharp.Editor, already running on the compiler Visual Studio ships, exports. The window drops its reference to FSharp.Compiler.Service, and DisableTransitiveProjectReferences keeps FSharp.VS.FSI from handing it back; the session's own compiler remains the SDK's, in the dotnet fsi process. The submission rule is tested with the editor's scanner. Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

The JSON-RPC server the interactive window drives first ships in SDK 11.0.200, and only the .NET build of fsi has it. Until that SDK is the floor, the Microsoft.FSharp.Compiler setup package also installs that build, in FSharp\Tools\Interactive: the assemblies of fsi's output folder and its runtime configuration, run with `dotnet exec`. The debug VSIX lays it out the same way, and the F5 profile points FSHARP_INTERACTIVE_PATH at the .NET fsi.dll instead of the desktop fsi.exe, which has no server. Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

The window tries `dotnet fsi` first, so the SDK that global.json names still supplies the compiler. An SDK that predates the server rejects the option and exits before the handshake; the window then starts the copy installed with Visual Studio. What the rejected attempt printed is held back and shown only if nothing else starts. The fsi that worked is remembered per folder, so a reset does not try the old SDK again, and the session banner says where the fsi came from. The exit handler is now attached before the process starts, so an early exit cannot leave the connect waiting. Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

The filter sits on every F# editor view, so answering S_OK for the whole VsStd11 group hid the status of every other command in it from the targets behind. Commands it does not handle now go to the next target. Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

The exit handler reports an exit only once the session is current, so one that exited between the handshake and the install was dropped and the dead session installed. The install now checks the session under the same lock and reports the exit as a failed start. Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

The origin was cleared only after the session started, so a send while fsi could not start left it for the next text typed into the window. Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Closing disposed the evaluator but kept the window and evaluator fields, so the next open submitted through the disposed host. Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Registration, the VSIX entry and the editor commands have landed; only the debugger commands are left. Also drop a stray prefix from the title. Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

…fewer comments Round one of the NoBloat and code-compaction pass asked for in review. Removed from the pull request: - docs/ide/FSI-Modern-Interactive-Window-Plan.md, the plan and status of the work; - .claude/rules/CcrOptout.md, an agent rule that is no part of this feature; - members nothing calls: the host client's IsRunning, EvaluatingProcessId, Initialization, ExecuteFileAsync and SetPathsAsync, the session's EvaluatingProcessId and Process, the evaluator's EvaluatingProcessId and Host, the provider's Window and Evaluator, the package's Provider and its InitializeAsync override that only called the base; - TolerateUnusedBindings: the project compiles with unused bindings as errors; - comments that restated the code, told its history or defended the design. Reused instead of repeated: - PredefinedInteractiveContentTypes and PredefinedClassificationTypeNames for the names the classifier spelled as literals; - TextWriter.Null for a window that is not there, in place of four null-checking writers; - Result.bind, Result.map and List.sortBy where the host matched and filtered by hand. Extracted: - HeldOutput, the buffer that keeps a starting session's output back until its handshake; - SessionProcess, for the command line, the output pumps and the quiet teardown; - one way to build a candidate that runs on the dotnet host, one to describe an early exit, and one to install a started session; - the evaluator's subscriptions as one list, and one function that reports a failed start; - LexicalToken.IsWithin, the bounds check the classifier and the submission rule both made. The send-to-interactive active pattern yields the kind, so Exec has one arm for both commands, and the continuation tokens are three lines of text instead of fifty-five of list. FsiCommandService.fs keeps the byte-order mark it has on main. Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

…onstructor Round two of the NoBloat and code-compaction pass asked for in review. - SubmissionAnalysisTests: eleven facts that each asserted a list of inputs become one theory over named scenarios, and the terminator test a second one. The 32 pairs of input and expectation are the same. - FsiLocator builds its three candidates through one function, and the dotnet host is dotnet.exe without asking the platform: the assembly only loads in Visual Studio. - The output pump is a loop under the quiet wrapper the teardown uses, not a recursive function in a try inside a thread's lambda. - The execute request takes its two optional fields through ValueOption.toObj and toNullable. - The exit code is read in one place, the session's own member. - The project no longer references PresentationCore, PresentationFramework and WindowsBase, which nothing in it uses. - What an SDK too old for the server does is said once, where the candidates are ordered. Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

xperiandri · 2026-10-11T09:50:47Z

@T-Gro Two rounds are pushed, 9b95f7a and 512effa. The diff against main went from 2357 added lines to 1644, the comment lines in it from 210 to 105 and the let/member definitions from 258 to 214; the server tests pass (79 cases, the submission rule's eleven facts now being one theory over the same 29 inputs), and FSharp.Editor and FSharp.Interactive.Window build with -p:Optimize=true. The process artifact, docs/ide/FSI-Modern-Interactive-Window-Plan.md, is removed, together with a stray .claude/rules/CcrOptout.md and the members nothing called yet (SetPathsAsync, ExecuteFileAsync, EvaluatingProcessId, IsRunning and their like). Extracted or reused: HeldOutput and SessionProcess came out of the host client, the fsi candidates share one constructor, the evaluator's four null-checking writers became TextWriter.Null and its subscriptions one list, the classifier takes its names from PredefinedClassificationTypeNames and PredefinedInteractiveContentTypes, and the send-to-interactive active pattern yields the kind, so Exec has one arm for both commands.

T-Gro

The diff is still way too big - apply the NoBloat instructions and CodeCompaction skill.

Especially the massive code comments would benefit not just with drastic reduction, but ideally with well named functions/arguments/bindings/types making the intention clear.

If a well named piece of code just tells what the comment says - delete the comment.

This diff adds 88 new code comment lines and 7116 new code comment chars.
Comments bypass the type system and become stale maintenance state. Keep only durable public contracts; express implementation intent through names, functions, active patterns, discriminated unions, and types.

T-Gro

🤖🕵️ AI review — verify independently.

T-Gro · 2026-10-17T07:45:57Z

+        let toolWindow = this.Open(0, focus = false)
+
+        evaluator |> ValueOption.iter _.SetNextSubmissionOrigin(sourcePath, startLine)


🤖🕵️ Queued submissions overwrite the single source-origin slot.

A from A.fsx:10 B from B.fsx:20 A runs with B's origin; B runs without one

…fewer comments Round one of the NoBloat and code-compaction pass asked for in review. Removed from the pull request: - docs/ide/FSI-Modern-Interactive-Window-Plan.md, the plan and status of the work; - .claude/rules/CcrOptout.md, an agent rule that is no part of this feature; - members nothing calls: the host client's IsRunning, EvaluatingProcessId, Initialization, ExecuteFileAsync and SetPathsAsync, the session's EvaluatingProcessId and Process, the evaluator's EvaluatingProcessId and Host, the provider's Window and Evaluator, the package's Provider and its InitializeAsync override that only called the base; - TolerateUnusedBindings: the project compiles with unused bindings as errors; - comments that restated the code, told its history or defended the design. Reused instead of repeated: - PredefinedInteractiveContentTypes and PredefinedClassificationTypeNames for the names the classifier spelled as literals; - TextWriter.Null for a window that is not there, in place of four null-checking writers; - Result.bind, Result.map and List.sortBy where the host matched and filtered by hand. Extracted: - HeldOutput, the buffer that keeps a starting session's output back until its handshake; - SessionProcess, for the command line, the output pumps and the quiet teardown; - one way to build a candidate that runs on the dotnet host, one to describe an early exit, and one to install a started session; - the evaluator's subscriptions as one list, and one function that reports a failed start; - LexicalToken.IsWithin, the bounds check the classifier and the submission rule both made. The send-to-interactive active pattern yields the kind, so Exec has one arm for both commands, and the continuation tokens are three lines of text instead of fifty-five of list. FsiCommandService.fs keeps the byte-order mark it has on main. This is 9b95f7a of dotnet#20565 without its two deletions: docs/ide/FSI-Modern-Interactive-Window-Plan.md and .claude/rules/CcrOptout.md leave the pull request only and stay on this branch. Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

T-Gro

🤖🕵️ AI review — verify independently.

Keep the existing 18.12.772 pin and remove the older duplicate that causes NU1506 during restore. Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com> Copilot-Session: 1750b0a0-85af-4cd4-b6a1-c4b5cc8240fb

Align Shell.15.0 with the newer VS dependencies, handle nullable editor API results, and update the integration-test generator and UI-thread checks. Replace the build-completion semaphore with a cancellable asynchronous signal. Verified clean bootstrap and full Debug VisualFSharp solution build; 53 focused editor classification tests passed. Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com> Copilot-Session: 1750b0a0-85af-4cd4-b6a1-c4b5cc8240fb

abonie · 2026-10-22T15:20:06Z

/azp run

azure-pipelines · 2026-10-22T15:20:21Z

Azure Pipelines: Successfully started running 1 pipeline(s).

T-Gro

🤖 🕵️ Re-reviewed all recent changes as of 09f9fa9; LGTM

…fewer comments Round one of the NoBloat and code-compaction pass asked for in review. Removed from the pull request: - docs/ide/FSI-Modern-Interactive-Window-Plan.md, the plan and status of the work; - .claude/rules/CcrOptout.md, an agent rule that is no part of this feature; - members nothing calls: the host client's IsRunning, EvaluatingProcessId, Initialization, ExecuteFileAsync and SetPathsAsync, the session's EvaluatingProcessId and Process, the evaluator's EvaluatingProcessId and Host, the provider's Window and Evaluator, the package's Provider and its InitializeAsync override that only called the base; - TolerateUnusedBindings: the project compiles with unused bindings as errors; - comments that restated the code, told its history or defended the design. Reused instead of repeated: - PredefinedInteractiveContentTypes and PredefinedClassificationTypeNames for the names the classifier spelled as literals; - TextWriter.Null for a window that is not there, in place of four null-checking writers; - Result.bind, Result.map and List.sortBy where the host matched and filtered by hand. Extracted: - HeldOutput, the buffer that keeps a starting session's output back until its handshake; - SessionProcess, for the command line, the output pumps and the quiet teardown; - one way to build a candidate that runs on the dotnet host, one to describe an early exit, and one to install a started session; - the evaluator's subscriptions as one list, and one function that reports a failed start; - LexicalToken.IsWithin, the bounds check the classifier and the submission rule both made. The send-to-interactive active pattern yields the kind, so Exec has one arm for both commands, and the continuation tokens are three lines of text instead of fifty-five of list. FsiCommandService.fs keeps the byte-order mark it has on main. This is 9b95f7a of dotnet#20565 without its two deletions: docs/ide/FSI-Modern-Interactive-Window-Plan.md and .claude/rules/CcrOptout.md leave the pull request only and stay on this branch. Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

github-project-automation Bot added this to F# Compiler and Tooling Sep 16, 2026

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

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F# Interactive window: IntelliSense and semantic colouring in the input buffer #20608

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xperiandri and others added 12 commits October 10, 2026 23:40

Forget an editor submission's origin when its execution fails

c9b1932

The origin was cleared only after the session started, so a send while fsi could not start left it for the next text typed into the window. Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Open a new window after the old one is closed

b543537

Closing disposed the evaluator but kept the window and evaluator fields, so the next open submitted through the disposed host. Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Bring the interactive window plan's status up to date

d74dfcc

Registration, the VSIX entry and the editor commands have landed; only the debugger commands are left. Also drop a stray prefix from the title. Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

xperiandri force-pushed the modern-repl branch from 09338b1 to d74dfcc Compare October 10, 2026 22:28

xperiandri and others added 2 commits October 11, 2026 11:32

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Merge branch 'main' into modern-repl

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Comment thread eng/Packages.props Outdated

abonie and others added 2 commits October 22, 2026 11:01

Remove duplicate Shell.Design central package version

aa8e698

Keep the existing 18.12.772 pin and remove the older duplicate that causes NU1506 during restore. Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com> Copilot-Session: 1750b0a0-85af-4cd4-b6a1-c4b5cc8240fb

T-Gro reviewed Oct 23, 2026

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		let toolWindow = this.Open(0, focus = false)

		evaluator \|> ValueOption.iter _.SetNextSubmissionOrigin(sourcePath, startLine)

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