pi-mono/packages/coding-agent
Petr Houška 9e870f6f00
fix(coding-agent): do not split surrogate pairs when truncating text (#11757)
Several places truncate text by UTF-16 code units with `slice()`. When
the cut lands between the two halves of a surrogate pair (an emoji or
other character outside the Basic Multilingual Plane), the result keeps
a lone high or low surrogate. That is invalid UTF-16.

Example: a grep match line with 499 ASCII characters followed by `😀` is
cut at 500 units, so the output ends in `\uD83D... [truncated]`.

The bundled providers strip lone surrogates before sending
(`sanitizeSurrogates` in pi-ai), so the usual effect is a dropped
character. But the broken text still reaches the session file, RPC and
JSON clients (strict UTF-8 encoders such as Rust's serde_json reject
lone surrogates), extensions, and custom providers.

This adds `sliceHead(text, n)` and `sliceTail(text, n)` to
`core/tools/truncate.ts`. They behave like `slice(0, n)` and `slice(-n)`
but return one unit fewer when the cut would split a pair, and are used
where truncated text reaches the model:
- `truncateLine` (grep match lines), the case reported in #7121
- codemode `truncateOutput` head and tail, and its argument and error
previews
- compaction `truncateForSummary` (tool results sent to the summarizer)
- MCP server summaries in the system prompt and stderr tails in
connection errors

The cost is two `charCodeAt` checks per truncation, and nothing on the
untruncated path.

The surrogate check follows the fix in #7122 by @IKEASven69, which
@cristinaponcela reviewed in a follow-up branch. That PR was closed
before merging.

Not changed: TUI-only previews (renderers, visual truncation) and
line-based slicing. pi-private's pi-durable codemode `output.ts` has the
same head/tail slicing and needs a separate change there.

**Testing**
- New `test/truncate-surrogates.test.ts`: an emoji at the cut in
`truncateLine` is dropped whole, an emoji ending exactly at the limit is
kept, `sliceHead`/`sliceTail` never leave a lone surrogate for any
length, and they match `slice` on text without surrogates.
- `test/tools.test.ts`, compaction, MCP extension, and codemode tests
pass.
- `npm run check` passes.

fixes #7121

This PR is AI-generated.
2026-11-04 20:59:46 +01:00
..
docs fix(ai): allow prompt cache keys for compatible proxies (#9442) 2026-11-04 11:36:35 +01:00
examples fix(coding-agent): fix CI type check and image-only steering test 2026-11-04 10:34:45 +00:00
install-lock feat(coding-agent): upgrade Mermaid terminal rendering (#8158) 2026-11-04 11:06:51 +01:00
schemas fix(ai): allow prompt cache keys for compatible proxies (#9442) 2026-11-04 11:36:35 +01:00
scripts docs(ai,coding-agent): generate models.json reference from schemas (#11038) 2026-10-16 21:53:00 +01:00
src fix(coding-agent): do not split surrogate pairs when truncating text (#11757) 2026-11-04 20:59:46 +01:00
test fix(coding-agent): do not split surrogate pairs when truncating text (#11757) 2026-11-04 20:59:46 +01:00
.gitignore Fix tool call ID normalization for cross-provider switches to Anthropic/GitHub Copilot 2026-01-13 04:07:10 +01:00
CHANGELOG.md fix(coding-agent): do not split surrogate pairs when truncating text (#11757) 2026-11-04 20:59:46 +01:00
package.json feat(coding-agent): upgrade Mermaid terminal rendering (#8158) 2026-11-04 11:06:51 +01:00
README.md feat(coding-agent): add Nix flake (#9137) 2026-10-02 18:30:51 +02:00
tsconfig.build.json feat(coding-agent): remove the experimental server stack and the client, protocol, and server packages 2026-10-12 10:41:59 +00:00
tsconfig.examples.json chore: migrate pi packages to earendil works scope 2026-05-07 15:59:42 +02:00
vitest.config.ts fix: use bounded runtime paths for Unix sockets 2026-08-14 17:02:03 +03:00

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