fix(bridge): recover inline-prefixed metadata in user message body

OpenClaw's canonical convention is to emit metadata envelopes (chat_id,
sender, reply target, …) as SEPARATE user-role messages folded into the
openai-completions request right after the real one — `extractLatestUserMessage`
skips those whole.

Fabric.OpenclawPlugin's dispatch does not split: it passes metadata
blocks and the real user content as ONE merged user-message body,
separated by blank lines. With the prior filter that meant the entire
turn was dropped with "no user message found" (HTTP 400) because the
first line matched a sentinel — the actual prompt sitting after the
metadata blocks never reached the bridge.

When the whole-body check fails for a single-message body, walk past
leading sentinel-prefixed blocks (sentinel header + optional ```json
code fence + blank-line separator) and use whatever non-metadata block
follows. Falls back to the previous "skip entirely" semantics when the
body is metadata-only.

End-user symptom that surfaced this: every contractor agent (Claude /
Gemini) subscribed to a Fabric channel silently failed to reply to
sub-discussion messages during recruitment — fabric dispatch said
"completed" in 1.6s but trajectory had `assistantTexts: []`,
`terminalError: non_deliverable_terminal_turn`,
`errorMessage: "400 \"no user message found\""`. Surfaced
recruiting developer1 on prod-t2 2026-05-31.

plugin/web/input-filter.ts

@@ -61,6 +61,51 @@ function isRuntimeContextMessage(text: string): boolean {
   return INBOUND_META_SENTINELS.includes(firstLine);
 }
 
+/**
+ * Strip *prefixed* metadata blocks from a single user message body.
+ *
+ * Background: OpenClaw's older / canonical convention is to emit metadata
+ * envelopes (chat_id, sender, reply target, …) as SEPARATE user-role
+ * messages folded into the openai-completions request right after the real
+ * one. `extractLatestUserMessage` skips those separate messages whole.
+ *
+ * The Fabric channel plugin (Fabric.OpenclawPlugin) does not split: it
+ * passes the metadata blocks and the actual user content as ONE merged
+ * user-message body, separated by blank lines. Result: a Fabric inbound
+ * turn has a single user message whose first line matches a sentinel, so
+ * `isRuntimeContextMessage` returned true and the whole turn was dropped
+ * with "no user message found" (HTTP 400). Symptom: every contractor agent
+ * subscribed to a Fabric channel silently fails to reply.
+ *
+ * Strip leading sentinel-prefixed blocks (sentinel header + optional code
+ * fence + blank-line separator) until we hit a block whose first line is
+ * NOT a sentinel — that's the real prompt. Returns "" if the entire body
+ * is metadata-only (still a no-op turn, same as before).
+ */
+function stripPrefixedMetadataBlocks(raw: string): string {
+  // Split on blank-line block boundaries. Within a metadata block the JSON
+  // code fence has only single newlines, so it stays in the same chunk as
+  // its sentinel header.
+  const blocks = raw.split(/\n\n+/);
+  const kept: string[] = [];
+  let stillStripping = true;
+  for (const block of blocks) {
+    if (stillStripping) {
+      const trimmed = block.trimStart();
+      const firstLine = trimmed.split("\n", 1)[0].trim();
+      if (
+        INBOUND_META_SENTINELS.includes(firstLine) ||
+        trimmed.startsWith(LEGACY_RUNTIME_CONTEXT_MARKER)
+      ) {
+        continue; // drop this prefix block
+      }
+      stillStripping = false;
+    }
+    kept.push(block);
+  }
+  return kept.join("\n\n").trim();
+}
+
 /**
  * Extract the latest user-authored message from the OpenClaw request.
  *
@@ -85,7 +130,18 @@ export function extractLatestUserMessage(req: BridgeInboundRequest): string {
   for (let i = userMessages.length - 1; i >= 0; i -= 1) {
     const raw = messageText(userMessages[i]);
     if (!raw) continue;
-    if (isRuntimeContextMessage(raw)) continue;
+    // First try the separate-message convention (Discord/Telegram path): a
+    // user message whose entire body is one metadata envelope — skip whole.
+    if (isRuntimeContextMessage(raw)) {
+      // ...but also try inline-prefixed-metadata recovery (Fabric path):
+      // some channels splice metadata + real content into one body. Walk
+      // past leading sentinel blocks; if any non-metadata block follows,
+      // that's the prompt. If nothing follows, this turn really is pure
+      // metadata and we keep skipping.
+      const stripped = stripPrefixedMetadataBlocks(raw);
+      if (stripped) return stripOpenClawTimestampPrefix(stripped);
+      continue;
+    }
     return stripOpenClawTimestampPrefix(raw);
   }
   return "";