Include mermaid tool output when copying chat responses

[Copilot]

Description

Copying chat responses omitted rendered mermaid diagrams when the output came from renderMermaidDiagram tool invocations. This update ensures mermaid tool output is serialized into the copied response text as a proper fenced mermaid block.

• Tool output serialization
  • Include tool result output details in response stringification.
  • Special-case text/vnd.mermaid to emit fenced mermaid markdown for copy.
• Test coverage
  • Add a response string unit test for mermaid tool output.

const fence = getFenceForContent(mermaidSource);
const copied = `${fence}mermaid\n${mermaidSource}\n${fence}`;

Original prompt

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This section details on the original issue you should resolve

<issue_title>Copy Chat doesn't copy diagrams when using renderMermaidDiagram </issue_title>
<issue_description>
Type: Bug

Copy Chat doesn't copy diagrams when using renderMermaidDiagram

1- Ask to create diagrams using renderMermaidDiagrams
2- Right click and copy response
3- paste in a markdown file - Note no mermaid diagrams <-

Extension version: 0.37.2026020404
VS Code version: Code - Insiders 1.109.0-insider (Universal) (bdd88df, 2026-02-04T02:00:40.203Z)
OS version: Darwin arm64 25.2.0
Modes:

Logs

Trace: choice {"delta":{"content":""},"finish_reason":null}
Trace: [NES][NextEditProvider][f842][_getNextEdit][fetchNextEdit][_executeNewNextEditRequest][XtabProvider][doGetNextEditWithSelection][streamEditsWithFiltering][streamEdits] starting to diff stream against edit window lines with latency 281.1888750000071 ms
Trace: choice {"delta":{"content":"\n\n"},"finish_reason":null}
Trace: [NES][NextEditProvider][f842][_getNextEdit][fetchNextEdit][_executeNewNextEditRequest][XtabProvider][doGetNextEditWithSelection][streamEditsWithFiltering][streamEdits] Line 0 emitted with latency 281.65779199999815 ms
Trace: choice {"delta":{"content":"Here"},"finish_reason":null}
Trace: [NES][NextEditProvider][f842][_getNextEdit][fetchNextEdit][_executeNewNextEditRequest][XtabProvider][doGetNextEditWithSelection][streamEditsWithFiltering][streamEdits] Line 1 emitted with latency 281.68758300000627 ms
Trace: choice {"delta":{"content":"’s"},"finish_reason":null}
Trace: choice {"delta":{"content":" an"},"finish_reason":null}
Trace: choice {"delta":{"content":" example"},"finish_reason":null}
Trace: choice {"delta":{"content":" Mermaid"},"finish_reason":null}
Trace: choice {"delta":{"content":" diagram"},"finish_reason":null}
Trace: choice {"delta":{"content":" rendered"},"finish_reason":null}
Trace: choice {"delta":{"content":"."},"finish_reason":null}
Trace: choice {"delta":{"content":" Want"},"finish_reason":null}
Trace: choice {"delta":{"content":" a"},"finish_reason":null}
Trace: choice {"delta":{"content":" different"},"finish_reason":null}
Trace: choice {"delta":{"content":" style"},"finish_reason":null}
Trace: choice {"delta":{"content":" ("},"finish_reason":null}
Trace: choice {"delta":{"content":"sequence"},"finish_reason":null}
Trace: choice {"delta":{"content":","},"finish_reason":null}
Trace: choice {"delta":{"content":" class"},"finish_reason":null}
Trace: choice {"delta":{"content":","},"finish_reason":null}
Trace: choice {"delta":{"content":" G"},"finish_reason":null}
Trace: choice {"delta":{"content":"antt"},"finish_reason":null}
Trace: choice {"delta":{"content":")"},"finish_reason":null}
Trace: choice {"delta":{"content":" or"},"finish_reason":null}
Trace: choice {"delta":{"content":" theme"},"finish_reason":null}
Trace: choice {"delta":{"content":"?"},"finish_reason":null}
Trace: choice {"delta":{"content":""},"finish_reason":"stop"}
Info: message 0 returned. finish reason: [stop]
Debug: message 0 details: finishOffset: [undefined] completionId: [{}] created: [{1770184114}]
Info: request done: requestId: [17d96803-ebf1-4c9e-99df-f663f5cae2d7] model deployment ID: []
Info: ccreq:2b187fe6.copilotmd | success | copilot-nes-oct | 490ms | [XtabProvider]
Trace: [NES][NextEditProvider][f842][_getNextEdit][fetchNextEdit][_executeNewNextEditRequest][XtabProvider][doGetNextEditWithSelection][streamEditsWithFiltering][streamEdits] Line 2 emitted with latency 492.47849999999744 ms
Trace: [NES][NextEditProvider][f842][_getNextEdit][fetchNextEdit][_executeNewNextEditRequest][pushEdit] processing edit microsoft/vscode#0 (starts at 0)
Trace: [NES][NextEditProvider][f842][_getNextEdit][fetchNextEdit][_executeNewNextEditRequest][pushEdit] resetting shouldExpandEditWindow to false due to NoSuggestions
Trace: [NES][NextEditProvider][f842][_getNextEdit][fetchNextEdit][_executeNewNextEditRequest][pushEdit] no edit, reason: noSuggestions
Trace: [NES][NextEditProvider][f842][_getNextEdit] resolved firstEdit promise
Trace: [NES][NextEditProvider][f842][_getNextEdit] First edit latency: 2601 ms
Trace: [NES][NextEditProvider][f842][_getNextEdit] failed to fetch next edit noSuggestions
Trace: [NES][NextEditProvider][f842][_getNextEdit] had no edit
Trace: [NES][Provider][provideInlineCompletionItems][f842] giving some more time to diagnostics provider
Trace: [NES][DiagnosticsNextEditProvider] DiagnosticsInlineCompletionProvider runUntilNextEdit complete (cancelled)
Trace: [NES][Provider][provideInlineCompletionItems][f842] no next edit suggestion
Info: ccreq:2c450338.copilotmd | markdown
Trace: [NES][Provider][handleListEndOfLifetime][f842] List icr-f842b294-24ff-447b-99e1-22dc724a79e9 disposed, reason: Empty
Error: Error: Failed to fetch custom instructi...

• Fixes Copy Chat doesn't copy diagrams when using renderMermaidDiagram  #292771

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