diff --git a/CHANGELOG.md b/CHANGELOG.md
index a38e52207..286f05f66 100644
--- a/CHANGELOG.md
+++ b/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.
 
+- A `todowrite` call whose output is not JSON no longer leaves the tool call
+  running forever: the completed update still reaches the editor.
+
 - Expand account, privacy, and usage documentation; clarify prepaid Ace access, fixed optional reloads, and current account requirements.
 
 ### Changed
diff --git a/backend/cli/src/acp/agent.ts b/backend/cli/src/acp/agent.ts
index a9b1dccd0..a278c35c2 100644
--- a/backend/cli/src/acp/agent.ts
+++ b/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) => {
@@ -329,14 +345,14 @@ 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 {
@@ -350,8 +366,6 @@ export namespace ACP {
                       .catch((error) => {
                         log.error("failed to send session update for todo", { error })
                       })
-                  } else {
-                    log.error("failed to parse todo output", { error: parsedTodos.error })
                   }
                 }
 
diff --git a/backend/cli/test/acp/event-subscription.test.ts b/backend/cli/test/acp/event-subscription.test.ts
index 6e7a90788..085379f06 100644
--- a/backend/cli/test/acp/event-subscription.test.ts
+++ b/backend/cli/test/acp/event-subscription.test.ts
@@ -368,6 +368,48 @@ describe("acp.agent event subscription", () => {
     })
   })
 
+  test("completes a todowrite tool call whose output is not valid JSON", async () => {
+    await using tmp = await tmpdir()
+    await Instance.provide({
+      directory: tmp.path,
+      fn: async () => {
+        const { agent, controller, updates, stop } = createFakeAgent()
+        const cwd = "/tmp/openscience-acp-test"
+
+        const sessionID = await agent.newSession({ cwd, mcpServers: [] } as any).then((x) => x.sessionId)
+
+        controller.push({
+          directory: cwd,
+          payload: {
+            type: "message.part.updated",
+            properties: {
+              part: {
+                sessionID,
+                messageID: "msg_1",
+                callID: "call_1",
+                type: "tool",
+                tool: "todowrite",
+                state: {
+                  status: "completed",
+                  input: {},
+                  output: "the model narrated instead of emitting json",
+                  title: "todowrite",
+                  metadata: {},
+                },
+              },
+            },
+          } as any,
+        })
+
+        await waitFor(() => (updates.get(sessionID) ?? []).includes("tool_call_update"))
+
+        expect(updates.get(sessionID)).toContain("tool_call_update")
+        expect(updates.get(sessionID)).not.toContain("plan")
+        stop()
+      },
+    })
+  })
+
   test("permission.asked events are handled and replied", async () => {
     await using tmp = await tmpdir()
     await Instance.provide({