diff --git a/apps/server/src/orchestration-v2/Adapters/AcpAdapterV2.test.ts b/apps/server/src/orchestration-v2/Adapters/AcpAdapterV2.test.ts
index b7808ef897ee..88505e9749d6 100644
--- a/apps/server/src/orchestration-v2/Adapters/AcpAdapterV2.test.ts
+++ b/apps/server/src/orchestration-v2/Adapters/AcpAdapterV2.test.ts
@@ -2456,12 +2456,7 @@ describe("AcpAdapterV2", () => {
           },
           stopReason === "error",
         );
-        const expectedStatus =
-          stopReason === "error"
-            ? "failed"
-            : stopReason === "cancelled"
-              ? "cancelled"
-              : "completed";
+        const expectedStatus = stopReason === "cancelled" ? "cancelled" : "failed";
         yield* reportNextPromptStopped(stopReason);
         assertReinjectedInstructions(yield* runTurn("Start work.", expectedStatus), "Start work.");
         assertReinjectedInstructions(
diff --git a/apps/server/src/orchestration-v2/Adapters/AcpRegistryAdapterV2.test.ts b/apps/server/src/orchestration-v2/Adapters/AcpRegistryAdapterV2.test.ts
index 70f45bc8a46c..d0014aa6574d 100644
--- a/apps/server/src/orchestration-v2/Adapters/AcpRegistryAdapterV2.test.ts
+++ b/apps/server/src/orchestration-v2/Adapters/AcpRegistryAdapterV2.test.ts
@@ -2755,6 +2755,65 @@ describe("AcpRegistryAdapterV2", () => {
     }).pipe(Effect.provide(layerTest)),
   );
 
+  it.effect.each([
+    { stopReason: "refusal", childUpdate: "state_update" },
+    { stopReason: "max_tokens", childUpdate: "subagent_update" },
+  ])(
+    "fails Claude's turn and child and closes its running tool when they stop with $stopReason via $childUpdate",
+    ({ stopReason, childUpdate }) =>
+      Effect.gen(function* () {
+        const { update, idle, prompt } = claudeFrames;
+        const session = yield* openClaudeReplaySession([
+          ...claudeFrames.open,
+          ...prompt(1),
+          ...claudeChild,
+          update(CLAUDE_ROOT_SESSION, {
+            sessionUpdate: "tool_call_update",
+            toolCallId: "root-tool",
+            name: "Bash",
+            title: "Read package",
+            kind: "execute",
+            status: "in_progress",
+            rawInput: { command: "cat package.json" },
+          }),
+          update("child-1", { sessionUpdate: "state_update", state: "running" }),
+          childUpdate === "state_update"
+            ? update("child-1", { sessionUpdate: "state_update", state: "idle", stopReason })
+            : update(CLAUDE_ROOT_SESSION, {
+                sessionUpdate: "subagent_update",
+                sessionId: "child-1",
+                state: { state: "idle", stopReason },
+              }),
+          idle(stopReason),
+        ]);
+        yield* session.runtime.startTurn(session.turnInput(1));
+        const events = yield* session.until((event) => event.type === "turn.terminal");
+        const terminal = events.at(-1)!;
+        if (terminal.type !== "turn.terminal") throw new Error("Missing root terminal");
+        assert.equal(terminal.status, "failed");
+        assert.equal(terminal.failure?.code, stopReason);
+        const child = events.findLast((event) => event.type === "subagent.updated");
+        if (child?.type !== "subagent.updated") throw new Error("Missing child update");
+        assert.equal(child.subagent.status, "failed");
+        assert.isNotNull(child.subagent.result, "the child shows why it stopped");
+        assert.isFalse(
+          events.some(
+            (event) =>
+              event.type === "message.updated" &&
+              event.message.threadId === child.subagent.childThreadId &&
+              event.message.role === "assistant",
+          ),
+          "the child's conversation has no reply it didn't send",
+        );
+        const tool = events.findLast(
+          (event) =>
+            event.type === "turn_item.updated" && event.turnItem.type === "command_execution",
+        );
+        if (tool?.type !== "turn_item.updated") throw new Error("Missing tool update");
+        assert.equal(tool.turnItem.status, "failed");
+      }).pipe(Effect.provide(layerTest), Effect.scoped),
+  );
+
   it.effect("keeps Claude activity after a refused Stop and closes it on retry", () =>
     Effect.gen(function* () {
       const gate = makeProviderReplayGate(["close-response", "queued-close-response"]);
diff --git a/packages/provider-acp-registry/src/server/claudeAcp.ts b/packages/provider-acp-registry/src/server/claudeAcp.ts
index 4c875ce597f5..7804a323accc 100644
--- a/packages/provider-acp-registry/src/server/claudeAcp.ts
+++ b/packages/provider-acp-registry/src/server/claudeAcp.ts
@@ -2,9 +2,10 @@ import * as Option from "effect/Option";
 import * as Schema from "effect/Schema";
 import type * as AcpSchema from "effect-acp/compat";
 import type { AcpToolCallState } from "@t3tools/provider-acp/server/runtimeModel";
-import type {
-  AcpAdapterV2SubagentUpdate,
-  AcpSubagentSessionEvent,
+import {
+  acpUnfinishedStopMessage,
+  type AcpAdapterV2SubagentUpdate,
+  type AcpSubagentSessionEvent,
 } from "@t3tools/provider-acp/server/adapter";
 
 const decodeAgentInput = Schema.decodeUnknownOption(
@@ -86,6 +87,7 @@ export function extractClaudeSubagentSessionEvent(
     const child = decodeChildState(update.raw);
     if (Option.isNone(child)) return;
     const { sessionId, state } = child.value;
+    const unfinishedMessage = acpUnfinishedStopMessage(state.stopReason);
     return {
       type: "state",
       childSessionId: sessionId,
@@ -93,12 +95,12 @@ export function extractClaudeSubagentSessionEvent(
       status:
         state.state === "running" || state.state === "requires_action"
           ? "running"
-          : state.stopReason === "error"
+          : state.stopReason === "error" || unfinishedMessage !== undefined
             ? "failed"
             : state.stopReason === "cancelled" || state.state === "unknown"
               ? "cancelled"
               : "completed",
-      result: state.error?.message ?? null,
+      result: state.error?.message ?? unfinishedMessage ?? null,
     };
   }
   if (update.originalSessionUpdate === "session_message") {
diff --git a/packages/provider-acp/src/server/adapter.ts b/packages/provider-acp/src/server/adapter.ts
index 332c4bee95ec..77c68669eea0 100644
--- a/packages/provider-acp/src/server/adapter.ts
+++ b/packages/provider-acp/src/server/adapter.ts
@@ -1021,6 +1021,22 @@ function makeProviderThread(input: {
   };
 }
 
+/** Stop reasons where the agent ended without finishing. ACP asks clients to show refusals. */
+export function acpUnfinishedStopMessage(
+  stopReason: EffectAcpSchema.StopReason | null | undefined,
+): string | undefined {
+  switch (stopReason) {
+    case "refusal":
+      return "The agent refused this request. The agent's next turn won't include this message or its reply.";
+    case "max_tokens":
+      return "The agent stopped because its reply reached the output token limit.";
+    case "max_turn_requests":
+      return "The agent stopped because it reached its request limit for this turn.";
+    default:
+      return undefined;
+  }
+}
+
 function appendToolContentText(content: unknown, text: string): unknown[] {
   const next: unknown[] = Array.isArray(content) ? [...content] : [];
   const last = unknownRecord(next.at(-1));
@@ -3441,8 +3457,10 @@ export const makeAcpAdapterV2 = Effect.fn("makeAcpAdapterV2")(function* (
             );
           }
 
+          // A failed child's result explains the failure; it is not the child's reply.
           if (
             taskStatus !== "running" &&
+            reported.status !== "failed" &&
             subagent.assistantText.length === 0 &&
             update.result !== null
           ) {
@@ -3812,16 +3830,18 @@ export const makeAcpAdapterV2 = Effect.fn("makeAcpAdapterV2")(function* (
           const update = notification.update;
           if (update.sessionUpdate === "state_update") {
             const child = nativeChild(notification.sessionId);
+            const unfinishedMessage =
+              update.state === "idle" ? acpUnfinishedStopMessage(update.stopReason) : undefined;
             setNativeChildState(
               child,
               update.state !== "idle"
                 ? "running"
-                : update.stopReason === "error"
+                : update.stopReason === "error" || unfinishedMessage !== undefined
                   ? "failed"
                   : update.stopReason === "cancelled"
                     ? "cancelled"
                     : "completed",
-              update.state === "idle" ? (update.error?.message ?? null) : null,
+              update.state === "idle" ? (update.error?.message ?? unfinishedMessage ?? null) : null,
             );
           }
           if (context !== null) yield* syncNativeChildren(context);
@@ -5027,6 +5047,17 @@ export const makeAcpAdapterV2 = Effect.fn("makeAcpAdapterV2")(function* (
             class: "provider_error",
           });
 
+        const stopReasonFailure = (
+          stopReason: EffectAcpSchema.StopReason | null | undefined,
+          error: EffectAcpSchema.PromptResponse["error"],
+        ) => {
+          if (stopReason === "error") return idleErrorFailure(error);
+          const message = acpUnfinishedStopMessage(stopReason);
+          return message === undefined
+            ? undefined
+            : makeProviderFailure({ message, code: stopReason, class: "provider_error" });
+        };
+
         const settleIdleWork = Effect.fnUntraced(function* (
           context: ActiveAcpTurn,
         ): Effect.fn.Return<void> {
@@ -5043,7 +5074,10 @@ export const makeAcpAdapterV2 = Effect.fn("makeAcpAdapterV2")(function* (
           if (work.stopReason === "cancelled" && pendingStopCloseTurn === context) {
             context.interrupted = true;
           }
-          const failure = work.stopReason === "error" ? idleErrorFailure(work.error) : undefined;
+          const failure = context.interrupted
+            ? undefined
+            : stopReasonFailure(work.stopReason, work.error);
+          if (failure !== undefined) yield* terminalizeOpenForegroundTools(context, "failed");
           yield* finalizeTurn(
             context,
             failure !== undefined
@@ -8671,8 +8705,7 @@ export const makeAcpAdapterV2 = Effect.fn("makeAcpAdapterV2")(function* (
                       });
                     }
                     if (context.finalized || followsAgentWorkState) return;
-                    // ACP v2 reports a failed prompt as an idle `error`, not an RPC error.
-                    const idleError = result.stopReason === "error" && !context.interrupted;
+                    // ACP v2 reports a failed prompt through its idle stop reason, not an RPC error.
                     const hiddenFailure =
                       checkPromptOutcome === undefined ||
                       context.interrupted ||
@@ -8684,7 +8717,10 @@ export const makeAcpAdapterV2 = Effect.fn("makeAcpAdapterV2")(function* (
                       yield* Ref.set(runtimeRestartRequired, true);
                     }
                     const failure =
-                      hiddenFailure ?? (idleError ? idleErrorFailure(result.error) : undefined);
+                      hiddenFailure ??
+                      (context.interrupted
+                        ? undefined
+                        : stopReasonFailure(result.stopReason, result.error));
                     const status =
                       failure !== undefined
                         ? "failed"