diff --git a/apps/extension/src/tools/__tests__/fill.test.ts b/apps/extension/src/tools/__tests__/fill.test.ts
index e256c56..53adbf5 100644
--- a/apps/extension/src/tools/__tests__/fill.test.ts
+++ b/apps/extension/src/tools/__tests__/fill.test.ts
@@ -11,7 +11,6 @@ interface ScriptParams {
 async function setup(markup = '<input value="old">') {
   document.body.innerHTML = markup;
   const element = document.body.firstElementChild as HTMLInputElement;
-  vi.spyOn(document, "hasFocus").mockReturnValue(true);
   const manager = new SessionManager({
     agentWindow: {
       create: async () => 100,
@@ -67,6 +66,13 @@ async function setup(markup = '<input value="old">') {
       case "Input.insertText":
         await insert((params as { text: string }).text);
         return {};
+      case "Input.dispatchKeyEvent":
+        if ((params as { commands?: string[] }).commands?.includes("moveToEndOfDocument")) {
+          if (typeof element.selectionStart === "number") {
+            element.setSelectionRange(element.value.length, element.value.length);
+          }
+        }
+        return {};
       case "Runtime.releaseObject":
         return release();
       default:
@@ -106,7 +112,10 @@ describe("fill result verification", () => {
     "disabled",
   ])("preserves a %s field before rejecting it", async (attribute) => {
     const h = await setup(`<input ${attribute} value="old">`);
-    expect(await h.fill()).toMatchObject({ code: "cdp_failed", message: /not editable/ });
+    expect(await h.fill()).toMatchObject({
+      code: "invalid_params",
+      data: { reason: "target_not_fillable" },
+    });
     expect(h.element.value).toBe("old");
     expect(h.insert).not.toHaveBeenCalled();
     expect(h.release).toHaveBeenCalledOnce();
@@ -121,26 +130,87 @@ describe("fill result verification", () => {
   ])("rejects an unsupported %s input before clearing", async (type) => {
     const h = await setup(`<input type="${type}">`);
     const before = h.element.value;
-    expect(await h.fill()).toMatchObject({ code: "cdp_failed", message: /input type/ });
+    expect(await h.fill()).toMatchObject({
+      code: "invalid_params",
+      data: { reason: "target_not_fillable" },
+    });
     expect(h.element.value).toBe(before);
     expect(h.insert).not.toHaveBeenCalled();
   });
 
   it("rejects an overlong value before clearing", async () => {
     const h = await setup('<input maxlength="3" value="old">');
-    expect(await h.fill()).toMatchObject({ code: "cdp_failed", message: /maxlength/ });
+    expect(await h.fill()).toMatchObject({
+      code: "invalid_params",
+      message: /maxlength/,
+      data: { reason: "fill_value_invalid" },
+    });
     expect(h.element.value).toBe("old");
     expect(h.insert).not.toHaveBeenCalled();
   });
 
-  it("does not type into a field that gained focus during clearing", async () => {
+  it.each([
+    true,
+    false,
+  ])("classifies a DOM.focus rejection on a disabled=%s target", async (disabled) => {
+    const h = await setup(`<input ${disabled ? "disabled" : ""} value="old">`);
+    const send = h.send.getMockImplementation()!;
+    h.send.mockImplementation(async (tabId, method, params) => {
+      if (method === "DOM.focus") throw new Error("Element is not focusable");
+      return send(tabId, method, params);
+    });
+    expect(await h.fill()).toMatchObject({
+      code: disabled ? "invalid_params" : "cdp_failed",
+      data: { reason: disabled ? "target_not_fillable" : "fill_failed" },
+    });
+    expect(h.element.value).toBe("old");
+    expect(h.insert).not.toHaveBeenCalled();
+    expect(h.release).toHaveBeenCalledOnce();
+  });
+
+  it("fills a background document without requiring foreground browser focus", async () => {
+    const h = await setup();
+    vi.spyOn(document, "hasFocus").mockReturnValue(false);
+    expect(await h.fill()).toMatchObject({ value_length: 5 });
+    expect(h.element.value).toBe("hello");
+  });
+
+  it("restores focus once after clearing without typing into another field", async () => {
     const h = await setup();
     const other = document.createElement("input");
     document.body.append(other);
     h.element.addEventListener("input", () => other.focus());
-    expect(await h.fill()).toMatchObject({ code: "cdp_failed", message: /lost focus/ });
+    expect(await h.fill()).toMatchObject({ value_length: 5 });
+    expect(h.element.value).toBe("hello");
+    expect(other.value).toBe("");
+    expect(h.send.mock.calls.filter(([, method]) => method === "DOM.focus")).toHaveLength(2);
+  });
+
+  it.each([
+    "focus",
+    "value",
+  ])("rechecks %s after restoring focus, without retrying again", async (action) => {
+    const h = await setup();
+    const other = document.createElement("input");
+    document.body.append(other);
+    h.element.addEventListener(
+      "input",
+      () => {
+        other.focus();
+        h.element.addEventListener("focus", () => {
+          if (action === "focus") queueMicrotask(() => other.focus());
+          else h.element.value = "changed";
+        });
+      },
+      { once: true },
+    );
+    expect(await h.fill()).toMatchObject({
+      code: "cdp_failed",
+      data: { reason: action === "focus" ? "fill_focus_lost" : "fill_target_changed" },
+    });
     expect(other.value).toBe("");
     expect(h.insert).not.toHaveBeenCalled();
+    expect(h.send.mock.calls.filter(([, method]) => method === "DOM.focus")).toHaveLength(2);
   });
 
   it.each([
@@ -159,14 +229,20 @@ describe("fill result verification", () => {
           h.element.value = "restored";
         });
     });
-    expect(await h.fill()).toMatchObject({ code: "cdp_failed" });
+    expect(await h.fill()).toMatchObject({
+      code: "cdp_failed",
+      data: { reason: "fill_target_changed" },
+    });
     expect(h.insert).not.toHaveBeenCalled();
   });
 
   it("fails when insertText is acknowledged without changing the value", async () => {
     const h = await setup();
     h.insert.mockImplementation(async () => {});
-    expect(await h.fill()).toMatchObject({ code: "cdp_failed", message: /expected value/ });
+    expect(await h.fill()).toMatchObject({
+      code: "cdp_failed",
+      data: { reason: "fill_value_mismatch" },
+    });
     expect(h.release).toHaveBeenCalledOnce();
   });
 
@@ -179,7 +255,10 @@ describe("fill result verification", () => {
       },
       { once: true },
     );
-    expect(await h.fill()).toMatchObject({ code: "cdp_failed", message: /expected value/ });
+    expect(await h.fill()).toMatchObject({
+      code: "cdp_failed",
+      data: { reason: "fill_value_mismatch" },
+    });
     expect(h.element.value).toBe("hel");
   });
 
@@ -212,34 +291,39 @@ describe("fill result verification", () => {
   it("does not report success for a target replaced by a change handler", async () => {
     const h = await setup();
     h.element.addEventListener("change", () => h.element.replaceWith(h.element.cloneNode()));
-    expect(await h.fill()).toMatchObject({ code: "cdp_failed", message: /expected value/ });
+    expect(await h.fill()).toMatchObject({
+      code: "cdp_failed",
+      data: { reason: "fill_target_changed" },
+    });
   });
 
   it.each([
-    1, 2, 3, 4,
+    1, 2, 3, 4, 5,
   ])("rejects script exceptions at phase %s and releases the target", async (phase) => {
     const h = await setup();
+    vi.spyOn(document, "hasFocus").mockReturnValue(false);
     const run = h.script.getMockImplementation()!;
     let count = 0;
     h.script.mockImplementation(async (params) =>
       ++count === phase ? { exceptionDetails: { text: "page secret" } } : run(params),
     );
     const result = await h.fill();
-    expect(result).toMatchObject({ code: "cdp_failed" });
+    expect(result).toMatchObject({ code: "cdp_failed", data: { reason: "fill_failed" } });
     expect(JSON.stringify(result)).not.toContain("page secret");
-    if (phase <= 2) expect(h.insert).not.toHaveBeenCalled();
+    if (phase <= 3) expect(h.insert).not.toHaveBeenCalled();
     expect(h.release).toHaveBeenCalledOnce();
   });
 
-  it.each([1, 2, 4])("rejects missing results at phase %s", async (phase) => {
+  it.each([1, 2, 3, 5])("rejects missing results at phase %s", async (phase) => {
     const h = await setup();
+    vi.spyOn(document, "hasFocus").mockReturnValue(false);
     const run = h.script.getMockImplementation()!;
     let count = 0;
     h.script.mockImplementation(async (params) =>
       ++count === phase ? { result: { type: "undefined" } } : run(params),
     );
     expect(await h.fill()).toMatchObject({ code: "cdp_failed" });
-    if (phase <= 2) expect(h.insert).not.toHaveBeenCalled();
+    if (phase <= 3) expect(h.insert).not.toHaveBeenCalled();
   });
 
   it.each(["input", "change"])("checks state after nested microtasks from %s", async (event) => {
@@ -270,7 +354,7 @@ describe("fill result verification", () => {
     const h = await setup();
     h.element.focus();
     h.element.setSelectionRange(3, 3);
-    expect(await h.fill("!", false)).toMatchObject({ value_length: 1 });
+    expect(await h.fill("!", false)).toMatchObject({ value_length: 4 });
     expect(h.element.value).toBe("old!");
   });
 
@@ -314,18 +398,100 @@ describe("fill result verification", () => {
     expect(await h.fill("one\n")).toMatchObject({ value_length: 4 });
   });
 
-  it("rejects a non-append selection before typing", async () => {
+  it.each([
+    [0, 0],
+    [1, 1],
+    [0, 3],
+  ])("appends after moving selection %s:%s to the end", async (start, end) => {
     const h = await setup();
     h.element.focus();
-    h.element.setSelectionRange(1, 1);
-    expect(await h.fill("!", false)).toMatchObject({ code: "cdp_failed" });
-    expect(h.element.value).toBe("old");
+    h.element.setSelectionRange(start, end);
+    expect(await h.fill("🙂", false)).toMatchObject({ value_length: 5 });
+    expect(h.element.value).toBe("old🙂");
+  });
+
+  it.each([
+    "number",
+    "email",
+  ])("appends to %s without setting an unsupported selection API", async (type) => {
+    const h = await setup(`<input type="${type}" value="12">`);
+    vi.spyOn(h.element, "selectionStart", "get").mockReturnValue(null);
+    vi.spyOn(h.element, "selectionEnd", "get").mockReturnValue(null);
+    const selection = vi.spyOn(h.element, "setSelectionRange").mockImplementation(() => {
+      throw new DOMException("unsupported", "InvalidStateError");
+    });
+    expect(await h.fill("3", false)).toMatchObject({ value_length: 3 });
+    expect(h.element.value).toBe("123");
+    expect(selection).not.toHaveBeenCalled();
+  });
+
+  it("rejects invalid number text before clearing", async () => {
+    const h = await setup('<input type="number" value="12">');
+    expect(await h.fill("abc")).toMatchObject({
+      code: "invalid_params",
+      data: { reason: "fill_value_invalid" },
+    });
+    expect(h.element.value).toBe("12");
     expect(h.insert).not.toHaveBeenCalled();
   });
 
+  it("reports page formatting as unconfirmed without retrying the write", async () => {
+    const h = await setup();
+    h.element.addEventListener("change", () => {
+      h.element.value = h.element.value.toUpperCase();
+    });
+    expect(await h.fill()).toMatchObject({
+      code: "cdp_failed",
+      data: { reason: "fill_value_mismatch" },
+    });
+    expect(h.element.value).toBe("HELLO");
+    expect(h.insert).toHaveBeenCalledOnce();
+  });
+
+  it.each([
+    ["focus", false],
+    ["focus", true],
+    ["value", false],
+    ["value", true],
+    ["cancel", false],
+    ["cancel", true],
+    ["throw", false],
+    ["throw", true],
+  ] as const)("stops when positioning triggers %s (clear=%s) and releases the key", async (action, clearBefore) => {
+    const h = await setup();
+    vi.spyOn(document, "hasFocus").mockReturnValue(false);
+    const controller = new AbortController();
+    const other = document.createElement("input");
+    document.body.append(other);
+    const send = h.send.getMockImplementation()!;
+    h.send.mockImplementation(async (tabId, method, params) => {
+      const reply = await send(tabId, method, params);
+      if (
+        method === "Input.dispatchKeyEvent" &&
+        (params as { type: string }).type === "rawKeyDown"
+      ) {
+        if (action === "focus") other.focus();
+        if (action === "value") h.element.value = "changed";
+        if (action === "cancel") controller.abort();
+        if (action === "throw") throw new Error("dispatch failed");
+      }
+      return reply;
+    });
+    const result = await h.fill("!", clearBefore, controller.signal);
+    expect(result).toMatchObject({ code: action === "cancel" ? "cancelled" : "cdp_failed" });
+    expect(h.insert).not.toHaveBeenCalled();
+    expect(other.value).toBe("");
+    expect(h.send).toHaveBeenCalledWith(
+      4,
+      "Input.dispatchKeyEvent",
+      expect.objectContaining({ type: "keyUp" }),
+    );
+    expect(h.release).toHaveBeenCalledOnce();
+  });
+
   it.each([true, false])("verifies an empty request with clear_before=%s", async (clearBefore) => {
     const h = await setup();
-    expect(await h.fill("", clearBefore)).toMatchObject({ value_length: 0 });
+    expect(await h.fill("", clearBefore)).toMatchObject({ value_length: clearBefore ? 0 : 3 });
     expect(h.element.value).toBe(clearBefore ? "" : "old");
     expect(h.insert).not.toHaveBeenCalled();
   });
diff --git a/apps/extension/src/tools/__tests__/interaction.test.ts b/apps/extension/src/tools/__tests__/interaction.test.ts
index 7d552b2..7cb1d4d 100644
--- a/apps/extension/src/tools/__tests__/interaction.test.ts
+++ b/apps/extension/src/tools/__tests__/interaction.test.ts
@@ -541,13 +541,16 @@ describe("handleHover", () => {
 });
 
 function successfulFillScript(params: unknown) {
-  const args = (params as { arguments?: Array<{ value: unknown }> }).arguments ?? [];
+  const script = params as { arguments?: Array<{ value: unknown }>; functionDeclaration: string };
+  const args = script.arguments ?? [];
   return {
     result: {
       value:
         args.length === 2
-          ? { before: "", expected: args[0].value, valueLength: String(args[0].value).length }
-          : true,
+          ? { before: "", expected: args[0].value }
+          : script.functionDeclaration.startsWith("function(expected)")
+            ? { connected: true, matches: true, valueLength: String(args[0].value).length }
+            : "ready",
     },
   };
 }
@@ -613,7 +616,7 @@ describe("handleFill", () => {
     expect(res.used_ref).toBe("e1");
     const insert = fake.sent.find((c) => c.method === "Input.insertText");
     expect(insert?.params).toEqual({ text: "hello" });
-    // Preparation, readiness, notifications, and final verification.
+    // Foreground replacement needs no extra caret-positioning round trip.
     const callFns = fake.sent.filter((c) => c.method === "Runtime.callFunctionOn");
     expect(callFns).toHaveLength(4);
   });
@@ -634,6 +637,7 @@ describe("handleFill", () => {
         if (args.length === 2) expect(args[1].value).toBe(false);
         return successfulFillScript(p);
       },
+      "Input.dispatchKeyEvent": () => ({}),
       "Runtime.releaseObject": () => ({}),
       "Input.insertText": () => ({}),
     });
diff --git a/apps/extension/src/tools/interaction.ts b/apps/extension/src/tools/interaction.ts
index 5a48349..9e36c2d 100644
--- a/apps/extension/src/tools/interaction.ts
+++ b/apps/extension/src/tools/interaction.ts
@@ -609,11 +609,28 @@ type SelectMutationResult =
 // tool.fill
 // ---------------------------------------------------------------------------
 
-interface FillPreparation {
-  before?: string;
-  expected?: string;
-  valueLength?: number;
-  error?: string;
+type FillFailureReason =
+  | "target_not_fillable"
+  | "fill_value_invalid"
+  | "fill_target_changed"
+  | "fill_focus_lost"
+  | "fill_value_mismatch"
+  | "fill_failed";
+
+type FillPreparation =
+  | { before: string; expected: string }
+  | { reason: FillFailureReason; error: string };
+
+type FillReadiness = "ready" | "background" | "fill_target_changed" | "fill_focus_lost";
+
+function fillError(reason: FillFailureReason, message: string): RpcError {
+  return rpcError(
+    reason === "target_not_fillable" || reason === "fill_value_invalid"
+      ? "invalid_params"
+      : "cdp_failed",
+    reason,
+    message,
+  );
 }
 
 interface FillScriptReply<T> {
@@ -690,12 +707,8 @@ export async function handleFill(
     if (throwIfAborted(deps.signal)) {
       return { code: "cancelled", message: "fill aborted" };
     }
-    await nodeCdp.send(target.tabId, "DOM.focus", { backendNodeId: node.backendNodeId });
   } catch (err) {
-    return {
-      code: "cdp_failed",
-      message: err instanceof Error ? err.message : String(err),
-    };
+    return fillError("fill_failed", err instanceof Error ? err.message : String(err));
   }
 
   if (throwIfAborted(deps.signal)) {
@@ -708,6 +721,31 @@ export async function handleFill(
   const clearBefore = params.clear_before ?? true;
 
   try {
+    if (throwIfAborted(deps.signal)) {
+      return { code: "cancelled", message: "fill aborted" };
+    }
+    try {
+      await nodeCdp.send(target.tabId, "DOM.focus", { backendNodeId: node.backendNodeId });
+    } catch (error) {
+      // Chrome rejects DOM.focus for disabled controls, including inherited
+      // fieldset state. Classify the live target before treating this as a
+      // browser failure; the normal path needs no extra round trip.
+      const editable = await nodeCdp.send<FillScriptReply<boolean>>(
+        target.tabId,
+        "Runtime.callFunctionOn",
+        { objectId, functionDeclaration: FILL_EDITABLE_FUNCTION, returnByValue: true },
+      );
+      if (throwIfAborted(deps.signal)) {
+        return { code: "cancelled", message: "fill aborted" };
+      }
+      if (!editable.exceptionDetails && editable.result?.value === false) {
+        return fillError(
+          "target_not_fillable",
+          "fill target is not editable or its input type is unsupported",
+        );
+      }
+      throw error;
+    }
     if (throwIfAborted(deps.signal)) {
       return { code: "cancelled", message: "fill aborted" };
     }
@@ -721,14 +759,9 @@ export async function handleFill(
         functionDeclaration: `function(value, clearBefore) {
           const tag = this.tagName.toLowerCase();
           const native = tag === 'input' || tag === 'textarea';
-          if (!(${FILL_EDITABLE_FUNCTION}).call(this)) return { error: 'fill target is not editable or its input type is unsupported' };
-          const focused = () => this.getRootNode().activeElement === this && this.ownerDocument.hasFocus();
-          if (!focused()) return { error: 'fill target does not have focus' };
+          if (!(${FILL_EDITABLE_FUNCTION}).call(this)) return { reason: 'target_not_fillable', error: 'fill target is not editable or its input type is unsupported' };
+          if (this.getRootNode().activeElement !== this) return { reason: 'fill_focus_lost', error: 'fill target does not have focus' };
           const before = clearBefore ? '' : (${FILL_VALUE_FUNCTION}).call(this);
-          if (!clearBefore && value !== '' && native && typeof this.selectionStart === 'number' &&
-              (this.selectionStart !== before.length || this.selectionEnd !== before.length)) {
-            return { error: 'fill append requires the caret at the end of the field' };
-          }
           let expected = before + value;
           if (native) {
             // Use the browser's own value sanitization (e.g. textarea
@@ -742,11 +775,11 @@ export async function handleFill(
             if (tag === 'input') expected = expected.replace(/\\r\\n|\\r|\\n/g, ' ');
             normalizer.value = expected;
             if (expected !== '' && normalizer.value === '' && this.type === 'number') {
-              return { error: 'fill value is not valid for a number input' };
+              return { reason: 'fill_value_invalid', error: 'fill value is not valid for a number input' };
             }
             expected = normalizer.value;
             if (this.type !== 'number' && this.maxLength >= 0 && expected.length > this.maxLength) {
-              return { error: 'fill value exceeds the target maxlength' };
+              return { reason: 'fill_value_invalid', error: 'fill value exceeds the target maxlength' };
             }
           } else {
             expected = expected.replace(/\\r\\n?/g, '\\n');
@@ -760,7 +793,7 @@ export async function handleFill(
             }
             this.dispatchEvent(new Event('input', { bubbles: true }));
           }
-          return { before, expected, valueLength: Math.max(0, expected.length - before.length) };
+          return { before, expected };
         }`,
         arguments: [{ value: params.value }, { value: clearBefore }],
         returnByValue: true,
@@ -770,44 +803,95 @@ export async function handleFill(
       return { code: "cancelled", message: "fill aborted" };
     }
     if (prepared.exceptionDetails) {
-      return { code: "cdp_failed", message: "fill preparation script failed" };
+      return fillError("fill_failed", "fill preparation script failed");
     }
     const preparation = prepared.result?.value;
-    if (
-      preparation?.error ||
-      typeof preparation?.before !== "string" ||
-      typeof preparation?.expected !== "string" ||
-      typeof preparation.valueLength !== "number"
-    ) {
-      return {
-        code: "cdp_failed",
-        message: preparation?.error ?? "fill preparation returned an unexpected result",
-      };
+    if (preparation && "error" in preparation) {
+      return fillError(preparation.reason, preparation.error);
+    }
+    if (typeof preparation?.before !== "string" || typeof preparation?.expected !== "string") {
+      return fillError("fill_failed", "fill preparation returned an unexpected result");
     }
     // A separate call runs after the clearing event's microtasks drain.
-    // Confirm the target before sending input to the focused element.
-    const ready = await nodeCdp.send<FillScriptReply<boolean>>(
-      target.tabId,
-      "Runtime.callFunctionOn",
-      {
-        objectId,
-        functionDeclaration: `function(before) {
-          return (${FILL_EDITABLE_FUNCTION}).call(this) &&
-            this.getRootNode().activeElement === this && this.ownerDocument.hasFocus() &&
-            (${FILL_VALUE_FUNCTION}).call(this) === before;
-        }`,
-        arguments: [{ value: preparation.before }],
-        returnByValue: true,
-      },
-    );
+    // DOM focus works in background tabs. Use document.hasFocus() only to
+    // detect deferred focus events, never to reject background input.
+    const checkReady = async (): Promise<FillReadiness | RpcError> => {
+      const reply = await nodeCdp.send<FillScriptReply<FillReadiness>>(
+        target.tabId,
+        "Runtime.callFunctionOn",
+        {
+          objectId,
+          functionDeclaration: `function(before) {
+            if (!(${FILL_EDITABLE_FUNCTION}).call(this) ||
+                (${FILL_VALUE_FUNCTION}).call(this) !== before) return 'fill_target_changed';
+            if (this.getRootNode().activeElement !== this) return 'fill_focus_lost';
+            return this.ownerDocument.hasFocus() ? 'ready' : 'background';
+          }`,
+          arguments: [{ value: preparation.before }],
+          returnByValue: true,
+        },
+      );
+      const state = reply.result?.value;
+      if (
+        reply.exceptionDetails ||
+        (state !== "ready" &&
+          state !== "background" &&
+          state !== "fill_target_changed" &&
+          state !== "fill_focus_lost")
+      ) {
+        return fillError(
+          "fill_failed",
+          "fill readiness script failed or returned an unexpected result",
+        );
+      }
+      return state;
+    };
+    let ready = await checkReady();
     if (throwIfAborted(deps.signal)) {
       return { code: "cancelled", message: "fill aborted" };
     }
-    if (ready.exceptionDetails || ready.result?.value !== true) {
-      return {
-        code: "cdp_failed",
-        message: "fill target changed, lost focus, or could not be checked before typing",
-      };
+    // Restore focus once only if clearing left this same target editable
+    // and unchanged. Recheck after focus handlers have run.
+    if (clearBefore && ready === "fill_focus_lost") {
+      await nodeCdp.send(target.tabId, "DOM.focus", { backendNodeId: node.backendNodeId });
+      if (throwIfAborted(deps.signal)) {
+        return { code: "cancelled", message: "fill aborted" };
+      }
+      ready = await checkReady();
+    }
+    if (throwIfAborted(deps.signal)) {
+      return { code: "cancelled", message: "fill aborted" };
+    }
+    if (typeof ready !== "string") return ready;
+    if (ready !== "ready" && ready !== "background") {
+      return fillError(ready, "fill target changed or lost focus before typing");
+    }
+    if (params.value !== "" && (!clearBefore || ready === "background")) {
+      // Native editing commands also support number/email inputs, whose
+      // selection APIs cannot set a caret, and multiline contenteditables.
+      // Also do this for background replacement: CDP input focuses the
+      // renderer, delivering deferred focus events before it inserts text.
+      // Use a command-only event so this does not invoke an End shortcut.
+      try {
+        await deps.cdp.send(target.tabId, "Input.dispatchKeyEvent", {
+          type: "rawKeyDown",
+          commands: ["moveToEndOfDocument"],
+        });
+      } finally {
+        await deps.cdp.send(target.tabId, "Input.dispatchKeyEvent", { type: "keyUp" });
+      }
+      if (throwIfAborted(deps.signal)) {
+        return { code: "cancelled", message: "fill aborted" };
+      }
+      // Focus and key handlers can change the target or move focus too.
+      ready = await checkReady();
+      if (throwIfAborted(deps.signal)) {
+        return { code: "cancelled", message: "fill aborted" };
+      }
+      if (typeof ready !== "string") return ready;
+      if (ready !== "ready" && ready !== "background") {
+        return fillError(ready, "fill target changed or lost focus while positioning the caret");
+      }
     }
     // CDP `Input.insertText` handles IME / multi-byte input out of the
     // box, much more reliably than per-key `dispatchKeyEvent`.
@@ -835,42 +919,51 @@ export async function handleFill(
       return { code: "cancelled", message: "fill aborted" };
     }
     if (notified.exceptionDetails) {
-      return { code: "cdp_failed", message: "fill notification script failed" };
+      return fillError("fill_failed", "fill notification script failed");
     }
-    const verified = await nodeCdp.send<FillScriptReply<boolean>>(
-      target.tabId,
-      "Runtime.callFunctionOn",
-      {
-        objectId,
-        functionDeclaration: `function(expected) {
-          return this.isConnected && (${FILL_VALUE_FUNCTION}).call(this) === expected;
+    const verified = await nodeCdp.send<
+      FillScriptReply<{ connected: boolean; matches: boolean; valueLength: number }>
+    >(target.tabId, "Runtime.callFunctionOn", {
+      objectId,
+      functionDeclaration: `function(expected) {
+          const value = (${FILL_VALUE_FUNCTION}).call(this);
+          return { connected: this.isConnected, matches: value === expected, valueLength: value.length };
         }`,
-        arguments: [{ value: preparation.expected }],
-        returnByValue: true,
-      },
-    );
+      arguments: [{ value: preparation.expected }],
+      returnByValue: true,
+    });
     if (throwIfAborted(deps.signal)) {
       return { code: "cancelled", message: "fill aborted" };
     }
-    if (verified.exceptionDetails || verified.result?.value !== true) {
-      return {
-        code: "cdp_failed",
-        message: verified.exceptionDetails
-          ? "fill verification script failed"
-          : "fill could not verify the expected value; observe the page before retrying",
-      };
+    const verification = verified.result?.value;
+    if (
+      verified.exceptionDetails ||
+      typeof verification?.connected !== "boolean" ||
+      typeof verification?.matches !== "boolean" ||
+      typeof verification?.valueLength !== "number"
+    ) {
+      return fillError(
+        "fill_failed",
+        "fill verification script failed or returned an unexpected result",
+      );
+    }
+    if (!verification.connected) {
+      return fillError("fill_target_changed", "fill target was removed or replaced during input");
+    }
+    if (!verification.matches) {
+      return fillError(
+        "fill_value_mismatch",
+        "fill could not verify the expected value; observe the page before deciding whether to retry",
+      );
     }
     return attachDialogs(deps.cdp, target.tabId, dialogCursor, {
       tab_id: target.tabId,
       used_ref: node.usedRef,
       used_selector: node.usedSelector,
-      value_length: preparation.valueLength,
+      value_length: verification.valueLength,
     });
   } catch (err) {
-    return {
-      code: "cdp_failed",
-      message: err instanceof Error ? err.message : String(err),
-    };
+    return fillError("fill_failed", err instanceof Error ? err.message : String(err));
   } finally {
     await nodeCdp.send(target.tabId, "Runtime.releaseObject", { objectId }).catch(() => undefined);
   }
diff --git a/apps/extension/src/transport/types.ts b/apps/extension/src/transport/types.ts
index fc9968a..f604993 100644
--- a/apps/extension/src/transport/types.ts
+++ b/apps/extension/src/transport/types.ts
@@ -26,6 +26,11 @@ export type RpcErrorReason =
   | "ref_not_found"
   | "selector_not_found"
   | "target_not_fillable"
+  | "fill_value_invalid"
+  | "fill_target_changed"
+  | "fill_focus_lost"
+  | "fill_value_mismatch"
+  | "fill_failed"
   | "target_not_select"
   | "option_not_found"
   | "single_select_value_count"
diff --git a/crates/bsk-cli/skill/SKILL.md b/crates/bsk-cli/skill/SKILL.md
index 357625c..f27ab2f 100644
--- a/crates/bsk-cli/skill/SKILL.md
+++ b/crates/bsk-cli/skill/SKILL.md
@@ -172,6 +172,10 @@ intended. Read `bsk upload --help` and `bsk download --help` for all flags and e
 - Stale ref: observe again and retry the intended action once.
 - Unknown tab or session: list current tabs/sessions; never guess identifiers.
 - Timeout: inspect current page state before deciding whether one longer purposeful wait is useful.
+- Fill result unconfirmed (`fill_value_mismatch`): observe the field first; the page may have
+  formatted the value. Continue if the visible result satisfies the user's intent. Otherwise correct
+  the remaining difference; do not blindly repeat fill or immediately request human help. For other
+  fill errors, follow the returned hint and inspect current state before retrying.
 - Unsupported command: continue with available capabilities; suggest updating only when the missing
   command is necessary.
 - Unrecoverable failure: report the blocker and stop the session in a finally-style path.
diff --git a/crates/bsk-cli/src/cli/render_error.rs b/crates/bsk-cli/src/cli/render_error.rs
index 01f80dd..ef9312f 100644
--- a/crates/bsk-cli/src/cli/render_error.rs
+++ b/crates/bsk-cli/src/cli/render_error.rs
@@ -43,6 +43,11 @@ pub mod reason {
     pub const REF_NOT_FOUND: &str = "ref_not_found";
     pub const SELECTOR_NOT_FOUND: &str = "selector_not_found";
     pub const TARGET_NOT_FILLABLE: &str = "target_not_fillable";
+    pub const FILL_VALUE_INVALID: &str = "fill_value_invalid";
+    pub const FILL_TARGET_CHANGED: &str = "fill_target_changed";
+    pub const FILL_FOCUS_LOST: &str = "fill_focus_lost";
+    pub const FILL_VALUE_MISMATCH: &str = "fill_value_mismatch";
+    pub const FILL_FAILED: &str = "fill_failed";
     pub const TARGET_NOT_SELECT: &str = "target_not_select";
     pub const OPTION_NOT_FOUND: &str = "option_not_found";
     pub const SINGLE_SELECT_VALUE_COUNT: &str = "single_select_value_count";
@@ -247,7 +252,42 @@ pub fn info_for_error(code: ErrorCode, data: Option<&serde_json::Value>) -> Rend
         (ErrorCode::InvalidParams, reason::TARGET_NOT_FILLABLE) => RenderInfo {
             summary: "target element is not fillable",
             hint: Some(
-                "choose an input, textarea, or contenteditable element from the latest snapshot",
+                "observe the page and choose an enabled, editable text input, textarea, or contenteditable; use the appropriate interaction for other control types",
+            ),
+            exit_code: base.exit_code,
+        },
+        (ErrorCode::InvalidParams, reason::FILL_VALUE_INVALID) => RenderInfo {
+            summary: "the requested text does not fit the field constraints",
+            hint: Some(
+                "check the field's input type and maxlength; use a compatible value without silently truncating or changing the user's intended data",
+            ),
+            exit_code: base.exit_code,
+        },
+        (ErrorCode::CdpFailed, reason::FILL_TARGET_CHANGED) => RenderInfo {
+            summary: "the fill target changed during the action",
+            hint: Some(
+                "observe the current page and field value; if the intended result is missing, use a fresh editable target before retrying",
+            ),
+            exit_code: base.exit_code,
+        },
+        (ErrorCode::CdpFailed, reason::FILL_FOCUS_LOST) => RenderInfo {
+            summary: "the page moved focus away from the fill target",
+            hint: Some(
+                "observe the page for a dialog or rerender and resolve it before retrying; fill manages field focus and does not require bringing the browser to the foreground",
+            ),
+            exit_code: base.exit_code,
+        },
+        (ErrorCode::CdpFailed, reason::FILL_VALUE_MISMATCH) => RenderInfo {
+            summary: "the fill result could not be confirmed",
+            hint: Some(
+                "observe the field before retrying; the page may have formatted the value. Continue if the visible result satisfies the user's intent; otherwise correct the remaining difference. Do not blindly repeat fill or immediately request human help",
+            ),
+            exit_code: base.exit_code,
+        },
+        (ErrorCode::CdpFailed, reason::FILL_FAILED) => RenderInfo {
+            summary: "fill encountered a browser or page-script error",
+            hint: Some(
+                "observe the page before retrying because the field may already have changed; retry only if the intended result is missing and the target is still available",
             ),
             exit_code: base.exit_code,
         },
@@ -442,6 +482,56 @@ mod tests {
         ErrorCode::NoBrowserConnected,
     ];
 
+    #[test]
+    fn fill_reasons_provide_recovery_without_changing_exit_codes() {
+        for (code, reason, hint_fragment) in [
+            (
+                ErrorCode::InvalidParams,
+                reason::TARGET_NOT_FILLABLE,
+                "enabled, editable",
+            ),
+            (
+                ErrorCode::InvalidParams,
+                reason::FILL_VALUE_INVALID,
+                "maxlength",
+            ),
+            (
+                ErrorCode::CdpFailed,
+                reason::FILL_TARGET_CHANGED,
+                "fresh editable target",
+            ),
+            (
+                ErrorCode::CdpFailed,
+                reason::FILL_FOCUS_LOST,
+                "does not require bringing the browser",
+            ),
+            (
+                ErrorCode::CdpFailed,
+                reason::FILL_VALUE_MISMATCH,
+                "Continue if the visible result satisfies",
+            ),
+            (
+                ErrorCode::CdpFailed,
+                reason::FILL_FAILED,
+                "may already have changed",
+            ),
+        ] {
+            let data = serde_json::json!({ "reason": reason });
+            let info = info_for_error(code, Some(&data));
+            assert!(info.hint.unwrap().contains(hint_fragment), "{reason}");
+            assert_eq!(info.exit_code, info_for(code).exit_code);
+        }
+        let mismatch = serde_json::json!({ "reason": reason::FILL_VALUE_MISMATCH });
+        let info = info_for_error(ErrorCode::CdpFailed, Some(&mismatch));
+        assert_eq!(info.summary, "the fill result could not be confirmed");
+        assert!(info.hint.unwrap().contains("Do not blindly repeat fill"));
+        let unknown = serde_json::json!({ "reason": "future_fill_reason" });
+        assert_eq!(
+            info_for_error(ErrorCode::CdpFailed, Some(&unknown)),
+            info_for(ErrorCode::CdpFailed)
+        );
+    }
+
     #[test]
     fn element_not_visible_overrides_permission_denied_copy() {
         let data = serde_json::json!({ "reason": reason::ELEMENT_NOT_VISIBLE });
diff --git a/packages/dsh-plugin-browserskill/skill/SKILL.md b/packages/dsh-plugin-browserskill/skill/SKILL.md
index a2d9c4e..464a24e 100644
--- a/packages/dsh-plugin-browserskill/skill/SKILL.md
+++ b/packages/dsh-plugin-browserskill/skill/SKILL.md
@@ -97,6 +97,10 @@ Continue from returned sequence cursors instead of rereading the same buffer.
 - Unknown tab: list tabs instead of guessing.
 - Unknown session: list owned sessions or start one; never try foreign ids.
 - Timeout: inspect current state before deciding whether one longer purposeful wait is useful.
+- Fill result unconfirmed: observe the field first; the page may have formatted the value. Continue
+  if the visible result satisfies the user's intent. Otherwise correct the remaining difference;
+  do not blindly repeat fill or immediately request human help. For other fill errors, follow the
+  returned hint and inspect current state before retrying.
 - Unrecoverable failure: report the blocker and stop the owned session.
 
 Arbitrary page-script evaluation and interaction recording are intentionally unsupported. Do not
diff --git a/packages/dsh-plugin-browserskill/tests/runner.test.ts b/packages/dsh-plugin-browserskill/tests/runner.test.ts
index 37b2de1..ad6d81a 100644
--- a/packages/dsh-plugin-browserskill/tests/runner.test.ts
+++ b/packages/dsh-plugin-browserskill/tests/runner.test.ts
@@ -124,6 +124,28 @@ describe("parseBskJson", () => {
     expect(() => parseBskJson({ ...base, code: 1, stderr: "boom" }, "x")).toThrow(/boom/);
   });
 
+  it("surfaces fill recovery guidance to the model without retrying", async () => {
+    const hint =
+      "observe the field before retrying; the page may have formatted the value. Continue if the visible result satisfies the user's intent";
+    const reply = {
+      ...base,
+      code: 3,
+      stdout: JSON.stringify({
+        code: "cdp_failed",
+        message: "fill could not verify the expected value",
+        data: { reason: "fill_value_mismatch" },
+        hint,
+      }),
+    };
+    let calls = 0;
+    const result = await runWithSessionBusyRetry(async () => {
+      calls++;
+      return reply;
+    });
+    expect(calls).toBe(1);
+    expect(() => parseBskJson(result, "fill")).toThrow(hint);
+  });
+
   it("reports killed-by-interrupt children (null exit code) as interrupted", () => {
     expect(() => parseBskJson({ ...base, code: null }, "navigate")).toThrow(/interrupted/);
   });
diff --git a/skill/SKILL.md b/skill/SKILL.md
index 357625c..f27ab2f 100644
--- a/skill/SKILL.md
+++ b/skill/SKILL.md
@@ -172,6 +172,10 @@ intended. Read `bsk upload --help` and `bsk download --help` for all flags and e
 - Stale ref: observe again and retry the intended action once.
 - Unknown tab or session: list current tabs/sessions; never guess identifiers.
 - Timeout: inspect current page state before deciding whether one longer purposeful wait is useful.
+- Fill result unconfirmed (`fill_value_mismatch`): observe the field first; the page may have
+  formatted the value. Continue if the visible result satisfies the user's intent. Otherwise correct
+  the remaining difference; do not blindly repeat fill or immediately request human help. For other
+  fill errors, follow the returned hint and inspect current state before retrying.
 - Unsupported command: continue with available capabilities; suggest updating only when the missing
   command is necessary.
 - Unrecoverable failure: report the blocker and stop the session in a finally-style path.