fix(coder): strip dcp-message-id tags split across stream chunks

The dcp tag (<dcp-message-id>mNNNN</dcp-message-id>) is streamed token-by-token, so it arrives split across SSE deltas. The existing per-chunk stripDcpTags never sees a complete tag in any single fragment, so fragments pass through and the dispatcher reassembles the tag in textChunks (persisted + shown) — and the terminal message.part.updated path that would strip the full text is suppressed by the dedup gate. Add a stateful cross-chunk stripper (dcp-strip.ts: makeDcpStreamStripper) at the dispatcher's opencode frame boundary: it emits text that cannot be part of a forming tag, holds back only a trailing partial-tag prefix (without swallowing legitimate <…> content), and flushes at turn end. Fixes both live delta frames and persisted content. 11 unit tests incl. split-at-every-boundary and the documented per-chunk-fails case. opencode path only; ACP (goose/qwen/claude) untouched.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

apps/coder/src/services/dcp-strip.ts

@@ -0,0 +1,77 @@
+/**
+ * Strip opencode-dcp plugin tags (`<dcp-message-id>mNNNN</dcp-message-id>`) that
+ * the @tarquinen/opencode-dcp plugin appends to assistant text and which
+ * otherwise render as literal text in the UI.
+ *
+ * Why a streaming stripper and not a per-chunk `.replace()`: opencode streams
+ * assistant text token-by-token, so the tag arrives SPLIT across many SSE deltas
+ * (`<dcp`, `-message`, `-id>`, `m0019`, `</dcp`, …). A per-chunk regex never sees
+ * a complete tag in any single fragment, so the fragments pass through and the
+ * dispatcher reassembles the full tag in the persisted/displayed content. The
+ * stripper below buffers across chunks: it emits everything that cannot be part
+ * of a forming tag and holds back only a trailing partial-tag prefix until the
+ * next chunk resolves it — without holding back legitimate `<…>` content.
+ */
+
+const DCP_TAG_RE = /<dcp-message-id>[^<]*<\/dcp-message-id>/g;
+const OPEN = '<dcp-message-id>';
+const CLOSE = '</dcp-message-id>';
+
+/** One-shot strip of COMPLETE tags. Safe for non-streaming / final content. */
+export function stripDcpTags(s: string): string {
+  return s.replace(DCP_TAG_RE, '');
+}
+
+/**
+ * Could `tail` (a substring starting at a `<`) still grow into a complete dcp
+ * tag on a future chunk? If so the caller must hold it back rather than emit it.
+ * Returns false for unrelated `<` content (`<div>`, `<T>`, …) so those stream
+ * normally.
+ */
+function isPartialDcp(tail: string): boolean {
+  // A prefix of the opening marker: '<', '<d', …, '<dcp-message-id'.
+  if (OPEN.startsWith(tail)) return true;
+  // Opening marker fully seen — content (and maybe a forming close) still streaming.
+  if (tail.startsWith(OPEN)) {
+    const rest = tail.slice(OPEN.length);
+    const lt = rest.indexOf('<');
+    if (lt === -1) return true; // still inside the [^<]* content run
+    return CLOSE.startsWith(rest.slice(lt)); // a partial close marker forming
+  }
+  return false;
+}
+
+export interface DcpStreamStripper {
+  /** Feed one text chunk; returns the portion safe to emit now (may be ''). */
+  push(chunk: string): string;
+  /** Stream end: returns whatever was held back, with complete tags stripped. */
+  flush(): string;
+}
+
+/** Stateful, cross-chunk-safe dcp stripper. One instance per turn. */
+export function makeDcpStreamStripper(): DcpStreamStripper {
+  let buf = '';
+  return {
+    push(chunk: string): string {
+      buf += chunk;
+      buf = buf.replace(DCP_TAG_RE, ''); // drop any now-complete tags
+      // Find the earliest `<` whose suffix is a forming dcp tag; hold from there,
+      // emit everything before it (real text, including unrelated `<…>`).
+      for (let i = buf.indexOf('<'); i !== -1; i = buf.indexOf('<', i + 1)) {
+        if (isPartialDcp(buf.slice(i))) {
+          const emit = buf.slice(0, i);
+          buf = buf.slice(i);
+          return emit;
+        }
+      }
+      const emit = buf;
+      buf = '';
+      return emit;
+    },
+    flush(): string {
+      const out = stripDcpTags(buf);
+      buf = '';
+      return out;
+    },
+  };
+}