chat: fix bare backtick flicker in streamed agent host markdown#318498
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roblourens merged 1 commit intoMay 27, 2026
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Streaming the agent host's worktree announcement
("Created isolated worktree for branch `xyz`") briefly rendered a bare
opening backtick because `fillInIncompleteTokens` was not closing the
codespan during progressive rendering. The progressive word renderer in
`chatWordCounter` deliberately excludes backticks from word characters,
so the intermediate state at the end of a streamed chunk is exactly
"…for branch \`xyz" — which is precisely what
`fillInIncompleteTokensOnce` is supposed to patch up.
Two issues conspired:
1. `ChatContentMarkdownRenderer` wraps `supportHtml` markdown in
`<body>...</body>` so dompurify does not strip leading comments.
That makes the lexer emit
`[html('<body>'), paragraph(…`xyz), space, html('</body>')]`, so
`tokens.at(-1)` is `html`, not `paragraph`, and the
codespan / list fix-ups never run.
2. The agent host paths were unconditionally setting
`supportHtml: true` on streamed markdown deltas. The
"supportHtml is load bearing" comment is now stale: PR #318053
replaced the old `AgentHostEditingSession` (which emitted bare
` ```` ` fences as `markdownContent`) with a dedicated
`'externalEdit'` progress part, so model deltas have nothing to
accidentally merge into.
Fixes:
- `fillInIncompleteTokensOnce` now walks past trailing `space` and
`html` tokens to find the last paragraph/list, then preserves those
trailing tokens around the patched-up node. Heading branch is
intentionally left alone.
- Drop `supportHtml: true` (and the stale comment) from both the live
streaming sink in `agentHostSessionHandler` and the history rebuild
in `stateToProgressAdapter`. Drop the now-unused `options` parameter
from `rawMarkdownToString`.
Tests:
- New token-level regression in `markdownRenderer.test.ts` exercises
the `<body>…</body>`-wrapped + bare-backtick scenario.
- New end-to-end test in `chatMarkdownRenderer.test.ts` runs through
the full render pipeline (with `supportHtml: true` and
`fillInIncompleteTokens: true`) and asserts a `<code>` element is
produced with no leftover bare backtick.
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
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Pull request overview
This PR addresses a rendering flicker in streamed agent-host markdown where partial content ending in a bare backtick briefly shows in the DOM before the closing backtick arrives. It does this by making fillInIncompleteTokens resilient to supportHtml-wrapped markdown (which introduces trailing html tokens like </body>), and by removing stale supportHtml: true usage in the agent host markdown emission paths.
Changes:
- Make
fillInIncompleteTokensOnceskip trailingspace/htmltokens when deciding which paragraph/list token to patch, while preserving those trailing tokens in the output token list. - Remove
supportHtml: truefrom agent-host markdown emission (streaming sink + history adapter) and simplifyrawMarkdownToStringaccordingly. - Add regression tests covering codespan completion with
supportHtmlwrapping at both token-level and end-to-end chat rendering levels.
Show a summary per file
| File | Description |
|---|---|
| src/vs/base/browser/markdownRenderer.ts | Adjusts incomplete-token fixup logic to operate correctly even when trailing html/space tokens exist (e.g. </body> wrapper). |
| src/vs/base/test/browser/markdownRenderer.test.ts | Adds a token-level regression test for codespan completion when markdown is wrapped in <body>...</body>. |
| src/vs/workbench/contrib/chat/browser/agentSessions/agentHost/agentHostSessionHandler.ts | Removes supportHtml: true from streamed agent-host markdown delta emission. |
| src/vs/workbench/contrib/chat/browser/agentSessions/agentHost/stateToProgressAdapter.ts | Removes supportHtml: true from markdown history conversion and drops the unused options parameter from rawMarkdownToString. |
| src/vs/workbench/contrib/chat/test/browser/widget/chatMarkdownRenderer.test.ts | Adds an end-to-end regression test ensuring bare codespans are closed when supportHtml wrapping introduces trailing html tokens. |
Copilot's findings
Comments suppressed due to low confidence (1)
src/vs/base/browser/markdownRenderer.ts:1033
fillInIncompleteTokensOncenow skips trailingspace/htmltokens for the list/paragraph fixups, but the heading fixup still usestokens.at(-1). ForsupportHtml-wrapped markdown (where</body>is the literal last token), the existing heading completion (used to avoidhello\n-being parsed as a setext heading) will never run. Consider locating the last heading token using the samelastInterestingIdxlogic (and preserving trailing tokens) so heading-related fixups also work when trailinghtml/spacetokens are present.
const lastToken = tokens.at(-1);
if (lastToken?.type === 'heading') {
const completeTokens = completeHeading(lastToken as marked.Tokens.Heading, mergeRawTokenText(tokens));
if (completeTokens) {
return completeTokens;
}
- Files reviewed: 5/5 changed files
- Comments generated: 0
anthonykim1
approved these changes
May 27, 2026
roblourens
deleted the
agents/vsckb-implement-the-text-created-isolated-worktr-42c3ab69
branch
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Fixes a bare-backtick flicker while streaming agent host responses, plus removes a now-stale
supportHtml: truefrom agent host markdown emission.Symptom
For agent host (AH) sessions, the initial worktree announcement
briefly rendered as
(bare opening backtick, no
<code>) until the closing backtick word arrived. The progressive word renderer (chatWordCounter) deliberately splits at non-backtick boundaries, so the intermediate string ending in a bare`is expected —fillInIncompleteTokensis supposed to handle that.Why
fillInIncompleteTokenswasn't firingChatContentMarkdownRendererwraps anysupportHtmlmarkdown in<body>...</body>(so dompurify keeps leading comments). The lexer then produces:fillInIncompleteTokensOnceonly inspectstokens.at(-1)for its paragraph / list / heading fix-ups. The trailinghtml('</body>')token meant the codespan fix-up never ran.Why AH was opting into
supportHtmlin the first placeThe
// supportHtml is load bearing…comment inagentHostSessionHandler._setupMarkdownPartwas stale. It dated back to whenAgentHostEditingSessionemitted file edits as amarkdownContent('\n````\n')+codeblockUri+textEdit+markdownContent('\n````\n')cluster — thesupportHtml: truetag on model deltas madecanMergeMarkdownStringsreject merging with those bare fence chunks. After #318053 (Connor) replaced that whole thing with a dedicated'externalEdit'progress part, model deltas have nothing to accidentally merge into.Fixes
Both layers are addressed:
Defensive (base layer).
fillInIncompleteTokensOncenow walks past trailingspace/htmltokens to find the last paragraph/list, then preserves those trailing tokens around the patched node. The heading branch is intentionally left alone. This means any future consumer ofsupportHtml: truestreaming markdown also gets correct fix-ups.Root (AH layer). Drop
supportHtml: truefrom:agentHostSessionHandler._setupMarkdownPart(live streaming sink) — and remove the stale "load bearing" comment.stateToProgressAdapter.turnsToHistory(ResponsePartKind.Markdowncase).Also drops the now-unused
optionsparameter fromrawMarkdownToString. This aligns the streaming and history paths with the already-differentactiveTurnToProgresspath (which never set the flag), and with vanilla Copilot chatstream.markdown(...)callsites.Tests
markdownRenderer.test.ts): assertsfillInIncompleteTokenson the<body>…\xyz\n\n` token list matches the lex of the balanced wrapped string.chatMarkdownRenderer.test.ts): renders aMarkdownStringwithsupportHtml: truecontainingCreated isolated worktree for branch \xyzthrough the fullChatContentMarkdownRenderer.render(md, { fillInIncompleteTokens: true })pipeline and asserts axyz` is in the output with no bare backtick in the text content.All 213 tests pass across the affected suites (markdownRenderer + chatMarkdownRenderer + stateToProgressAdapter);
tsgo --noEmitis clean.(Written by Copilot)