diff --git a/packages/agent-runtime/src/runtime.test.ts b/packages/agent-runtime/src/runtime.test.ts
index 6332df0e..d03255d8 100644
--- a/packages/agent-runtime/src/runtime.test.ts
+++ b/packages/agent-runtime/src/runtime.test.ts
@@ -6194,3 +6194,100 @@ describe("DesktopAgentRuntime subagents", () => {
     await runtime.dispose();
   });
 });
+
+describe("DesktopAgentRuntime deferred tool restore (#225)", () => {
+  const now = () => new Date().toISOString();
+  const searchRow = (overrides: Partial<UiMessage> = {}): UiMessage => ({
+    id: "tool-search-1",
+    role: "tool",
+    content: "",
+    createdAt: now(),
+    status: "complete",
+    toolName: "ToolSearch",
+    toolCallId: "call-search-1",
+    toolStatus: "success",
+    toolArgs: { query: "BrowserPreview" },
+    toolResult: {
+      content: [{ type: "text", text: "Activated on-demand tools: BrowserPreview." }],
+      details: { activated: ["BrowserPreview"] },
+      addedToolNames: ["BrowserPreview"],
+    },
+    ...overrides,
+  });
+  const assistantRow: UiMessage = {
+    id: "assistant-1",
+    role: "assistant",
+    content: "Loading the preview tool.",
+    createdAt: now(),
+    status: "complete",
+  };
+  const hasTool = (runtime: DesktopAgentRuntime, name: string) =>
+    (runtime as any).agent.state.tools.some((tool: any) => tool.name === name);
+
+  it("keeps a tool active across prompts while its ToolSearch activation is in context", async () => {
+    const runtime = createRuntime({ history: [assistantRow, searchRow()] });
+
+    (runtime as any).resetDeferredToolsForPrompt();
+
+    expect(hasTool(runtime, "BrowserPreview")).toBe(true);
+    await runtime.dispose();
+  });
+
+  it("restores a tool from its own successful result, not from failed or empty rows", async () => {
+    const runtime = createRuntime({
+      history: [
+        assistantRow,
+        searchRow({ id: "tool-search-failed", toolCallId: "call-failed", toolStatus: "error" }),
+        {
+          id: "tool-preview-empty",
+          role: "tool",
+          content: "",
+          createdAt: now(),
+          status: "complete",
+          toolName: "BrowserPreview",
+          toolCallId: "call-preview-empty",
+          toolStatus: "success",
+          toolArgs: {},
+        },
+      ],
+    });
+
+    (runtime as any).resetDeferredToolsForPrompt();
+    expect(hasTool(runtime, "BrowserPreview")).toBe(false);
+
+    (runtime as any).fullEntries.push(
+      ...(createRuntime({
+        history: [
+          assistantRow,
+          {
+            id: "tool-preview-ok",
+            role: "tool",
+            content: "",
+            createdAt: now(),
+            status: "complete",
+            toolName: "BrowserPreview",
+            toolCallId: "call-preview-ok",
+            toolStatus: "success",
+            toolArgs: {},
+            toolResult: { content: [{ type: "text", text: "opened" }] },
+          },
+        ],
+      }) as any).fullEntries,
+    );
+    (runtime as any).resetDeferredToolsForPrompt();
+    expect(hasTool(runtime, "BrowserPreview")).toBe(true);
+    await runtime.dispose();
+  });
+
+  it("restores the activation again after a mode round trip", async () => {
+    const runtime = createRuntime({ history: [assistantRow, searchRow()] });
+    (runtime as any).resetDeferredToolsForPrompt();
+    expect(hasTool(runtime, "BrowserPreview")).toBe(true);
+
+    runtime.setMode("plan");
+    runtime.setMode("agent");
+
+    expect(hasTool(runtime, "BrowserPreview")).toBe(true);
+    await runtime.dispose();
+  });
+});
diff --git a/packages/agent-runtime/src/runtime.ts b/packages/agent-runtime/src/runtime.ts
index f676f9b2..e9eeccc0 100644
--- a/packages/agent-runtime/src/runtime.ts
+++ b/packages/agent-runtime/src/runtime.ts
@@ -268,6 +268,18 @@ function mutationTerminationAdvice(
   return "Re-read the live file, regenerate a narrower Edit, and avoid repeating the same payload.";
 }
 export const TOOL_SEARCH_NAME = "ToolSearch";
+/** Stands in for a persisted tool row that never recorded a result. */
+const MISSING_TOOL_RESULT_PLACEHOLDER = "[no tool result recorded]";
+
+function isMissingToolResultPlaceholder(
+  content: ToolResultMessage["content"],
+): boolean {
+  return (
+    content.length === 1 &&
+    content[0].type === "text" &&
+    content[0].text === MISSING_TOOL_RESULT_PLACEHOLDER
+  );
+}
 export const ASK_TOOL_NAME = "asktool";
 
 /**
@@ -1232,7 +1244,7 @@ function toolResultFromUi(
       type: "text",
       text: interrupted
         ? "[tool call was interrupted before a result was recorded]"
-        : "[no tool result recorded]",
+        : MISSING_TOOL_RESULT_PLACEHOLDER,
     });
   }
   return {
@@ -1693,6 +1705,7 @@ Delegation rules:
     this.mode = mode;
     this.activeDeferredToolNames.clear();
     this.rebuildToolCatalog();
+    this.restoreDeferredToolsFromContext();
     this.agent.state.systemPrompt = this.composeSystemPrompt();
     this.agent.state.tools = this.activeTools();
     this.setPlanningState(planningState, details);
@@ -4062,9 +4075,37 @@ Delegation rules:
 
   private resetDeferredToolsForPrompt(): void {
     this.activeDeferredToolNames.clear();
+    this.restoreDeferredToolsFromContext();
     this.agent.state.tools = this.activeTools();
   }
 
+  /**
+   * Re-activates the on-demand tools whose successful activation the model
+   * can still see. The context keeps every ToolSearch result that announced
+   * "Activated on-demand tools: X" and every result X itself produced, so
+   * starting a turn with an empty set while those rows remain leaves the
+   * model calling tools that are missing from the schema (#225). Only
+   * successful results count, and only for names still in the deferred
+   * catalog, which `rebuildToolCatalog` already limits to the current mode.
+   */
+  private restoreDeferredToolsFromContext(): void {
+    if (this.deferredToolNames.size === 0) return;
+    const { messages } = buildSessionContext(this.entriesWithCompaction());
+    for (const message of messages) {
+      if (message.role !== "toolResult" || message.isError) continue;
+      if (isMissingToolResultPlaceholder(message.content)) continue;
+      const names =
+        message.toolName === TOOL_SEARCH_NAME
+          ? (message.addedToolNames ?? [])
+          : [message.toolName];
+      for (const name of names) {
+        if (this.deferredToolNames.has(name)) {
+          this.activeDeferredToolNames.add(name);
+        }
+      }
+    }
+  }
+
   private buildSubmitTool(kind: ProposalKind): AgentTool {
     const name = SUBMIT_TOOL_NAMES[kind];
     return {