diag(engine): log transcript tail when pi state error looks like null access

pi-coding-agent swallows its own exceptions onto session.state.errorMessage
without preserving a stack, so fusion sees only "Cannot read properties
of undefined (reading 'filter')" with no indication which message is
malformed. When promptSessionAndCheck rethrows an error that matches the
generic TypeError shape, dump the last few state.messages (role, content
type, toolName, stopReason) so the offending message can be identified
next run.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

packages/engine/src/pi.ts

@@ -113,6 +113,33 @@ async function promptSessionAndCheck(session: AgentSession, prompt: string, opti
 
   const stateError = getSessionStateError(session);
   if (stateError) {
+    // pi-coding-agent swallows its own exceptions into state.errorMessage
+    // without preserving a stack. When the message looks like a generic
+    // TypeError (undefined/null property access), dump the session transcript
+    // shape so the malformed message can be identified next time.
+    if (/Cannot read propert(y|ies) of (undefined|null)/i.test(stateError)) {
+      try {
+        const messages = (session as any).agent?.state?.messages ?? (session as any).state?.messages;
+        if (Array.isArray(messages)) {
+          const recent = messages.slice(-6).map((m: Record<string, unknown>, idx: number) => {
+            const i = messages.length - 6 + idx;
+            const content = m?.content;
+            return {
+              index: i < 0 ? idx : i,
+              role: m?.role,
+              contentType: Array.isArray(content) ? `array(len=${content.length})` : typeof content,
+              toolName: (m as { toolName?: unknown }).toolName,
+              stopReason: (m as { stopReason?: unknown }).stopReason,
+            };
+          });
+          piLog.error(`pi state error — transcript tail (${messages.length} msgs total): ${JSON.stringify(recent)}`);
+        } else {
+          piLog.error(`pi state error — state.messages is not an array: ${typeof messages}`);
+        }
+      } catch (inspectErr) {
+        piLog.warn(`pi state error — failed to inspect transcript: ${inspectErr instanceof Error ? inspectErr.message : String(inspectErr)}`);
+      }
+    }
     throw new Error(stateError);
   }
 }