diff --git a/src/exec/runner.ts b/src/exec/runner.ts
index 233bd8808..214b89cb2 100644
--- a/src/exec/runner.ts
+++ b/src/exec/runner.ts
@@ -846,6 +846,10 @@ export async function runExec(config: Config): Promise<ExecResult> {
       createExecToolCallGate(isAdvertised, {
         isCodex: isCodexProviderName(config.providerName),
       }),
+      // Same promoted-but-unmounted contract as the TUI gate: an activated
+      // name missing from the registry errors toward retry (see run() in
+      // DynamicToolRunner).
+      { isActivated: (name) => activatedToolNames.has(name) },
     );
 
     const { directorHolder, buildAgent } = assembleChatAgent({
diff --git a/src/tui/dynamic-tool-runner.test.ts b/src/tui/dynamic-tool-runner.test.ts
index 42ae36daf..f4612aa8a 100644
--- a/src/tui/dynamic-tool-runner.test.ts
+++ b/src/tui/dynamic-tool-runner.test.ts
@@ -180,6 +180,140 @@ describe("mangled dispatch names", () => {
   });
 });
 
+describe("activated-but-unmounted registry miss", () => {
+  test("a gate-activated name missing from the registry reports reconnecting, not unknown tool", async () => {
+    const runner = createDynamicToolRunner([
+      stringTool("read_file", "core"),
+      stringTool("mcp__acme__do", "blind-result"),
+    ]);
+    const activated = new Set(["mcp__acme__do"]);
+    runner.setCallGate((name) => name === "read_file" || activated.has(name), {
+      isActivated: (name) => activated.has(name),
+    });
+
+    // The tool was promoted (gate open) but its server disconnected before the
+    // call, so removeTools dropped it from the registry mid-window.
+    runner.removeTools(["mcp__acme__do"]);
+    const result = await runner.run(
+      { id: "1", name: "mcp__acme__do", arguments: {} },
+      new AbortController().signal,
+    );
+
+    expect(result.isError).toBe(true);
+    expect(result.content).toContain("mcp__acme__do");
+    expect(result.content).toContain("reconnecting");
+    expect(result.content).toContain("Retry the call shortly");
+    expect(result.content).not.toBe("unknown tool: mcp__acme__do");
+  });
+
+  test("a never-activated miss keeps the exact unknown tool string", async () => {
+    const runner = createDynamicToolRunner([stringTool("read_file", "core")]);
+    runner.setCallGate((name) => name === "read_file", {
+      isActivated: () => false,
+    });
+
+    const result = await runner.run(
+      { id: "1", name: "mcp__gone__tool", arguments: {} },
+      new AbortController().signal,
+    );
+
+    expect(result.isError).toBe(true);
+    expect(result.content).toBe("unknown tool: mcp__gone__tool");
+  });
+});
+
+describe("harness namespace prefix", () => {
+  test("a default.-prefixed call dispatches the registered bare tool", async () => {
+    const runner = createDynamicToolRunner([
+      stringTool("read_file", "core"),
+      stringTool("mcp__acme__do", "blind-result"),
+    ]);
+    const advertised = new Set(["read_file", "mcp__acme__do"]);
+    runner.setCallGate((name) => advertised.has(name));
+
+    const result = await runner.run(
+      { id: "1", name: "default.mcp__acme__do", arguments: {} },
+      new AbortController().signal,
+    );
+
+    expect(result.isError).toBeUndefined();
+    expect(result.content).toBe("blind-result");
+  });
+
+  test("a prefixed miss keeps the exact unknown tool string with the original name", async () => {
+    const runner = createDynamicToolRunner([stringTool("read_file", "core")]);
+    runner.setCallGate((name) => name === "read_file", {
+      isActivated: () => false,
+    });
+
+    const result = await runner.run(
+      { id: "1", name: "default.mcp__gone__tool", arguments: {} },
+      new AbortController().signal,
+    );
+
+    expect(result.isError).toBe(true);
+    expect(result.content).toBe("unknown tool: default.mcp__gone__tool");
+  });
+
+  test("an exact dotted registration wins over the bare suffix (anti-misrouting)", async () => {
+    const runner = createDynamicToolRunner([
+      stringTool("mcp__acme__do", "bare"),
+      stringTool("default.mcp__acme__do", "dotted"),
+    ]);
+    runner.setCallGate(() => true);
+
+    const result = await runner.run(
+      { id: "1", name: "default.mcp__acme__do", arguments: {} },
+      new AbortController().signal,
+    );
+
+    expect(result.isError).toBeUndefined();
+    expect(result.content).toBe("dotted");
+  });
+
+  test("a prefixed miss for an activated-but-unmounted stripped tool reports reconnecting under the original name", async () => {
+    const runner = createDynamicToolRunner([
+      stringTool("read_file", "core"),
+      stringTool("mcp__acme__do", "blind-result"),
+    ]);
+    const activated = new Set(["mcp__acme__do"]);
+    runner.setCallGate((name) => name === "read_file" || activated.has(name), {
+      isActivated: (name) => activated.has(name),
+    });
+
+    // The server dropped between search and call, so the bare tool left the
+    // registry; the model still emits the harness-namespaced form.
+    runner.removeTools(["mcp__acme__do"]);
+    const result = await runner.run(
+      { id: "1", name: "default.mcp__acme__do", arguments: {} },
+      new AbortController().signal,
+    );
+
+    expect(result.isError).toBe(true);
+    expect(result.content).toContain("default.mcp__acme__do");
+    expect(result.content).toContain("reconnecting");
+    expect(result.content).toContain("Retry the call shortly");
+  });
+
+  test("a normalized call rejected by the gate reports the stripped name", async () => {
+    const runner = createDynamicToolRunner([
+      stringTool("read_file", "core"),
+      stringTool("mcp__acme__do", "blind-result"),
+    ]);
+    runner.setCallGate((name) => name === "read_file");
+
+    const result = await runner.run(
+      { id: "1", name: "default.mcp__acme__do", arguments: {} },
+      new AbortController().signal,
+    );
+
+    expect(result.isError).toBe(true);
+    expect(result.content).toContain("mcp__acme__do");
+    expect(result.content).toContain("tool_search");
+    expect(result.content).not.toContain("default.mcp__acme__do");
+  });
+});
+
 describe("terminal control stripping", () => {
   test("strips escape sequences from any tool's result, including MCP", async () => {
     const payload = "before\x1b]52;c;ZXZpbA==\x07\x1b[31mred\x1b[0m\x07after";
diff --git a/src/tui/dynamic-tool-runner.ts b/src/tui/dynamic-tool-runner.ts
index 116a96f28..53160c92e 100644
--- a/src/tui/dynamic-tool-runner.ts
+++ b/src/tui/dynamic-tool-runner.ts
@@ -31,8 +31,18 @@ export type DynamicToolRunner = AgentToolRunner & {
    * the current wire (built-in prefix + pinned + activated) with an error
    * pointing at tool_search, instead of silently dispatching. Without a gate
    * every registered tool stays dispatchable — sub-agent runners never set one.
+   *
+   * `options.isActivated` is the promotion side of the gate: a name the model
+   * was already shown (tool_search activated it) that is absent from the
+   * registry — its server disconnected or the snapshot rebuilt under it —
+   * reports "not currently available, server may be reconnecting, retry
+   * shortly" instead of the bare unknown-tool string. Names never activated
+   * keep the exact unknown-tool string.
    */
-  setCallGate(isCallable: (name: string) => boolean): void;
+  setCallGate(
+    isCallable: (name: string) => boolean,
+    options?: { isActivated?: (name: string) => boolean },
+  ): void;
 };
 
 export function createDynamicToolRunner(
@@ -41,6 +51,7 @@ export function createDynamicToolRunner(
 ): DynamicToolRunner {
   const byName = new Map<string, AgentTool>();
   let callGate: ((name: string) => boolean) | undefined;
+  let isActivated: ((name: string) => boolean) | undefined;
 
   const addTools = (tools: AgentTool[]): void => {
     const incoming = new Set<string>();
@@ -69,14 +80,49 @@ export function createDynamicToolRunner(
     addTools,
     removeTools,
     currentDefinitions,
-    setCallGate(isCallable: (name: string) => boolean): void {
+    setCallGate(
+      isCallable: (name: string) => boolean,
+      options?: { isActivated?: (name: string) => boolean },
+    ): void {
       callGate = isCallable;
+      isActivated = options?.isActivated;
     },
     async run(call: ToolCall, signal: AbortSignal): Promise<ToolResult> {
       const resolved = resolveRegisteredToolName(call.name, (name) =>
         byName.has(name),
       );
       if (resolved === undefined) {
+        // The name was promoted (gate-activated) but the registry no longer
+        // holds it — the server dropped between search and call. Say so: the
+        // model already has the schema from the tool_search card and only
+        // needs to retry, not re-search. A name never activated keeps the
+        // exact unknown-tool string.
+        if (isActivated?.(call.name) === true) {
+          return {
+            callId: call.id,
+            content:
+              `Error: ${call.name} is not currently available - its server may ` +
+              `still be reconnecting. Retry the call shortly.`,
+            isError: true,
+          };
+        }
+        // A harness-namespaced call (`default.<stripped>`) misses the registry
+        // under its original name even though the model was shown `<stripped>`:
+        // consult activation with the stripped form too, but keep the original
+        // name in the message so the transcript matches what the model emitted.
+        const dot = call.name.indexOf(".");
+        if (dot > 0) {
+          const stripped = call.name.slice(dot + 1);
+          if (stripped.length > 0 && isActivated?.(stripped) === true) {
+            return {
+              callId: call.id,
+              content:
+                `Error: ${call.name} is not currently available - its server may ` +
+                `still be reconnecting. Retry the call shortly.`,
+              isError: true,
+            };
+          }
+        }
         return {
           callId: call.id,
           content: `unknown tool: ${call.name}`,
diff --git a/src/tui/runner/session.ts b/src/tui/runner/session.ts
index cf45941c3..fa6aa99c0 100644
--- a/src/tui/runner/session.ts
+++ b/src/tui/runner/session.ts
@@ -498,6 +498,9 @@ export async function assembleTUISession(
   ]);
   toolset.dynamicRunner.setCallGate(
     (name) => unadvertisedCallable.has(name) || isAdvertised(name),
+    // A promoted-but-unmounted name (server dropped between search and call)
+    // reports reconnecting instead of bare unknown-tool at the call gate.
+    { isActivated: (name) => activatedToolNames.has(name) },
   );
 
   // Reload, interrupt, compaction continuation, and proxy deliver share one queue