refactor: drop empty error messages in history

CHANGELOG.md

@@ -30,6 +30,7 @@
 - Security: per-agent mention patterns and group elevated directives now require explicit mention to avoid cross-agent toggles.
 - Config: support inline env vars in config (`env.*` / `env.vars`) and document env precedence.
 - Agent: enable adaptive context pruning by default for tool-result trimming.
+- Agent: drop empty error assistant messages when sanitizing session history.
 - Doctor: check config/state permissions and offer to tighten them. — thanks @steipete
 - Doctor/Daemon: audit supervisor configs, add --repair/--force flows, surface service config audits in daemon status, and document user vs system services. — thanks @steipete
 - Doctor: repair gateway service entrypoint when switching between npm and git installs; add Docker e2e coverage. — thanks @steipete

src/agents/pi-embedded-helpers.test.ts

@@ -304,6 +304,19 @@ describe("sanitizeSessionMessagesImages", () => {
     expect(out[0]?.role).toBe("user");
   });
 
+  it("drops empty assistant error messages", async () => {
+    const input = [
+      { role: "user", content: "hello" },
+      { role: "assistant", stopReason: "error", content: [] },
+      { role: "assistant", stopReason: "error" },
+    ] satisfies AgentMessage[];
+
+    const out = await sanitizeSessionMessagesImages(input, "test");
+
+    expect(out).toHaveLength(1);
+    expect(out[0]?.role).toBe("user");
+  });
+
   it("leaves non-assistant messages unchanged", async () => {
     const input = [
       { role: "user", content: "hello" },

src/agents/pi-embedded-helpers.ts

@@ -61,6 +61,21 @@ export async function ensureSessionHeader(params: {
 
 type ContentBlock = AgentToolResult<unknown>["content"][number];
 
+function isEmptyAssistantErrorMessage(
+  message: Extract<AgentMessage, { role: "assistant" }>,
+): boolean {
+  if (message.stopReason !== "error") return false;
+  const content = message.content;
+  if (content == null) return true;
+  if (!Array.isArray(content)) return false;
+  return content.every((block) => {
+    if (!block || typeof block !== "object") return true;
+    const rec = block as { type?: unknown; text?: unknown };
+    if (rec.type !== "text") return false;
+    return typeof rec.text !== "string" || rec.text.trim().length === 0;
+  });
+}
+
 export async function sanitizeSessionMessagesImages(
   messages: AgentMessage[],
   label: string,
@@ -101,6 +116,9 @@ export async function sanitizeSessionMessagesImages(
 
     if (role === "assistant") {
       const assistantMsg = msg as Extract<AgentMessage, { role: "assistant" }>;
+      if (isEmptyAssistantErrorMessage(assistantMsg)) {
+        continue;
+      }
       const content = assistantMsg.content;
       if (Array.isArray(content)) {
         const filteredContent = content.filter((block) => {