fix(acp): do not fail a session load on one bad tool output

CHANGELOG.md

@@ -13,6 +13,9 @@ tagged release also ships native binaries for Linux, macOS, and Windows.
 
 - Fix duration day/minute boundaries, file directory and extension labels, and payload-size rounding. Unknown payload sizes now display clearly. Background queues process undefined items, reject invalid concurrency, and release timeout timers when an operation fails.
 
+- Loading or forking a session no longer fails outright when one `todowrite`
+  entry in its history is not JSON; the rest of the history replays.
+
 - Expand account, privacy, and usage documentation; clarify prepaid Ace access, fixed optional reloads, and current account requirements.
 
 ### Changed

backend/cli/src/acp/agent.ts

@@ -91,6 +91,22 @@ export namespace ACP {
     return getNewContent(content, diff)
   }
 
+  function parseTodoOutput(output: string) {
+    let raw: unknown
+    try {
+      raw = JSON.parse(output)
+    } catch (error) {
+      log.error("could not parse todowrite output as JSON", { error })
+      return undefined
+    }
+    const parsed = z.array(Todo.Info).safeParse(raw)
+    if (!parsed.success) {
+      log.error("failed to parse todo output", { error: parsed.error })
+      return undefined
+    }
+    return parsed.data
+  }
+
   export async function init({ sdk: _sdk }: { sdk: OpenScienceClient }) {
     return {
       create: (connection: AgentSideConnection, fullConfig: ACPConfig) => {
@@ -800,14 +816,14 @@ export namespace ACP {
               }
 
               if (part.tool === "todowrite") {
-                const parsedTodos = z.array(Todo.Info).safeParse(JSON.parse(part.state.output))
-                if (parsedTodos.success) {
+                const todos = parseTodoOutput(part.state.output)
+                if (todos) {
                   await this.connection
                     .sessionUpdate({
                       sessionId,
                       update: {
                         sessionUpdate: "plan",
-                        entries: parsedTodos.data.map((todo) => {
+                        entries: todos.map((todo) => {
                           const status: PlanEntry["status"] =
                             todo.status === "cancelled" ? "completed" : (todo.status as PlanEntry["status"])
                           return {
@@ -821,8 +837,6 @@ export namespace ACP {
                     .catch((err) => {
                       log.error("failed to send session update for todo", { error: err })
                     })
-                } else {
-                  log.error("failed to parse todo output", { error: parsedTodos.error })
                 }
               }
 

backend/cli/test/acp/event-subscription.test.ts

@@ -368,6 +368,46 @@ describe("acp.agent event subscription", () => {
     })
   })
 
+  test("replays a session whose todowrite history is not valid JSON", async () => {
+    await using tmp = await tmpdir()
+    await Instance.provide({
+      directory: tmp.path,
+      fn: async () => {
+        const { agent, updates, stop, sdk } = createFakeAgent()
+        const cwd = "/tmp/openscience-acp-test"
+
+        const sessionID = await agent.newSession({ cwd, mcpServers: [] } as any).then((x) => x.sessionId)
+
+        sdk.session.messages = async () => ({
+          data: [
+            {
+              info: { role: "assistant", sessionID },
+              parts: [
+                {
+                  type: "tool",
+                  callID: "call_replay",
+                  tool: "todowrite",
+                  state: {
+                    status: "completed",
+                    input: {},
+                    output: "not json",
+                    title: "todowrite",
+                    metadata: {},
+                  },
+                },
+              ],
+            },
+          ],
+        })
+
+        await agent.loadSession({ sessionId: sessionID, cwd, mcpServers: [] } as any)
+
+        expect(updates.get(sessionID)).toContain("tool_call_update")
+        stop()
+      },
+    })
+  })
+
   test("permission.asked events are handled and replied", async () => {
     await using tmp = await tmpdir()
     await Instance.provide({