diff --git a/src/browser/server.ts b/src/browser/server.ts
index 62ad1872a..77a099c14 100644
--- a/src/browser/server.ts
+++ b/src/browser/server.ts
@@ -26,6 +26,7 @@ import {
   DEFAULT_BROWSER_SCREENSHOT_MAX_SIDE,
   normalizeBrowserScreenshot,
 } from "./screenshot.js";
+import { resolveTargetIdFromTabs } from "./target-id.js";
 
 export type BrowserTab = {
   targetId: string;
@@ -270,7 +271,15 @@ export async function startBrowserControlServerFromConfig(
     const reachable = await isChromeReachable(state.cdpPort, 300);
     if (!reachable) return jsonError(res, 409, "browser not running");
     try {
-      await activateTab(state.cdpPort, targetId);
+      const tabs = await listTabs(state.cdpPort);
+      const resolved = resolveTargetIdFromTabs(targetId, tabs);
+      if (!resolved.ok) {
+        if (resolved.reason === "ambiguous") {
+          return jsonError(res, 409, "ambiguous target id prefix");
+        }
+        return jsonError(res, 404, "tab not found");
+      }
+      await activateTab(state.cdpPort, resolved.targetId);
       res.json({ ok: true });
     } catch (err) {
       jsonError(res, 500, String(err));
@@ -284,7 +293,15 @@ export async function startBrowserControlServerFromConfig(
     const reachable = await isChromeReachable(state.cdpPort, 300);
     if (!reachable) return jsonError(res, 409, "browser not running");
     try {
-      await closeTab(state.cdpPort, targetId);
+      const tabs = await listTabs(state.cdpPort);
+      const resolved = resolveTargetIdFromTabs(targetId, tabs);
+      if (!resolved.ok) {
+        if (resolved.reason === "ambiguous") {
+          return jsonError(res, 409, "ambiguous target id prefix");
+        }
+        return jsonError(res, 404, "tab not found");
+      }
+      await closeTab(state.cdpPort, resolved.targetId);
       res.json({ ok: true });
     } catch (err) {
       jsonError(res, 500, String(err));
@@ -304,8 +321,20 @@ export async function startBrowserControlServerFromConfig(
     try {
       const tabs = await listTabs(state.cdpPort);
       const chosen = targetId
-        ? tabs.find((t) => t.targetId === targetId)
-        : tabs.at(0);
+        ? (() => {
+            const resolved = resolveTargetIdFromTabs(targetId, tabs);
+            if (!resolved.ok) {
+              if (resolved.reason === "ambiguous") {
+                return "AMBIGUOUS" as const;
+              }
+              return null;
+            }
+            return tabs.find((t) => t.targetId === resolved.targetId) ?? null;
+          })()
+        : (tabs.at(0) ?? null);
+      if (chosen === "AMBIGUOUS") {
+        return jsonError(res, 409, "ambiguous target id prefix");
+      }
       if (!chosen?.wsUrl) return jsonError(res, 404, "tab not found");
 
       let shot: Buffer<ArrayBufferLike> = Buffer.alloc(0);
diff --git a/src/browser/target-id.test.ts b/src/browser/target-id.test.ts
new file mode 100644
index 000000000..d00036532
--- /dev/null
+++ b/src/browser/target-id.test.ts
@@ -0,0 +1,40 @@
+import { describe, expect, it } from "vitest";
+
+import { resolveTargetIdFromTabs } from "./target-id.js";
+
+describe("browser target id resolution", () => {
+  it("resolves exact ids", () => {
+    const res = resolveTargetIdFromTabs("FULL", [
+      { targetId: "AAA" },
+      { targetId: "FULL" },
+    ]);
+    expect(res).toEqual({ ok: true, targetId: "FULL" });
+  });
+
+  it("resolves unique prefixes (case-insensitive)", () => {
+    const res = resolveTargetIdFromTabs("57a01309", [
+      { targetId: "57A01309E14B5DEE0FB41F908515A2FC" },
+    ]);
+    expect(res).toEqual({
+      ok: true,
+      targetId: "57A01309E14B5DEE0FB41F908515A2FC",
+    });
+  });
+
+  it("fails on ambiguous prefixes", () => {
+    const res = resolveTargetIdFromTabs("57A0", [
+      { targetId: "57A01309E14B5DEE0FB41F908515A2FC" },
+      { targetId: "57A0BEEF000000000000000000000000" },
+    ]);
+    expect(res.ok).toBe(false);
+    if (!res.ok) {
+      expect(res.reason).toBe("ambiguous");
+      expect(res.matches?.length).toBe(2);
+    }
+  });
+
+  it("fails when no tab matches", () => {
+    const res = resolveTargetIdFromTabs("NOPE", [{ targetId: "AAA" }]);
+    expect(res).toEqual({ ok: false, reason: "not_found" });
+  });
+});
diff --git a/src/browser/target-id.ts b/src/browser/target-id.ts
new file mode 100644
index 000000000..4e53ea300
--- /dev/null
+++ b/src/browser/target-id.ts
@@ -0,0 +1,24 @@
+export type TargetIdResolution =
+  | { ok: true; targetId: string }
+  | { ok: false; reason: "not_found" | "ambiguous"; matches?: string[] };
+
+export function resolveTargetIdFromTabs(
+  input: string,
+  tabs: Array<{ targetId: string }>,
+): TargetIdResolution {
+  const needle = input.trim();
+  if (!needle) return { ok: false, reason: "not_found" };
+
+  const exact = tabs.find((t) => t.targetId === needle);
+  if (exact) return { ok: true, targetId: exact.targetId };
+
+  const lower = needle.toLowerCase();
+  const matches = tabs
+    .map((t) => t.targetId)
+    .filter((id) => id.toLowerCase().startsWith(lower));
+
+  const only = matches.length === 1 ? matches[0] : undefined;
+  if (only) return { ok: true, targetId: only };
+  if (matches.length === 0) return { ok: false, reason: "not_found" };
+  return { ok: false, reason: "ambiguous", matches };
+}