diff --git a/.changeset/fix-fillform-value-hallucination.md b/.changeset/fix-fillform-value-hallucination.md
new file mode 100644
index 000000000..6e7b1c794
--- /dev/null
+++ b/.changeset/fix-fillform-value-hallucination.md
@@ -0,0 +1,5 @@
+---
+"@browserbasehq/stagehand": patch
+---
+
+Fix fillForm tool using LLM-hallucinated values instead of caller-provided values
diff --git a/packages/core/lib/v3/agent/tools/fillform.ts b/packages/core/lib/v3/agent/tools/fillform.ts
index b7e0c9198..a81bc5ac9 100644
--- a/packages/core/lib/v3/agent/tools/fillform.ts
+++ b/packages/core/lib/v3/agent/tools/fillform.ts
@@ -56,7 +56,22 @@ export const fillFormTool = (
 
         const completed = [] as unknown[];
         const replayableActions: Action[] = [];
+        let fillIndex = 0;
         for (const res of observeResults) {
+          if (res.method === "fill") {
+            if (fillIndex < fields.length) {
+              res.arguments = [fields[fillIndex].value];
+              fillIndex++;
+            } else {
+              v3.logger({
+                category: "agent",
+                message: `fillForm: observe returned more fill actions than provided fields (${fields.length}); skipping extra fill`,
+                level: 1,
+              });
+              continue;
+            }
+          }
+
           const actOptions = variables
             ? { variables, timeout: toolTimeout }
             : { timeout: toolTimeout };
@@ -66,6 +81,13 @@ export const fillFormTool = (
             replayableActions.push(...(actResult.actions as Action[]));
           }
         }
+        if (fillIndex < fields.length) {
+          v3.logger({
+            category: "agent",
+            message: `fillForm: observe returned fewer fill actions (${fillIndex}) than provided fields (${fields.length}); ${fields.length - fillIndex} field value(s) were not applied`,
+            level: 1,
+          });
+        }
         v3.recordAgentReplayStep({
           type: "fillForm",
           fields,
diff --git a/packages/core/tests/unit/fillform-value-override.test.ts b/packages/core/tests/unit/fillform-value-override.test.ts
new file mode 100644
index 000000000..b43888b26
--- /dev/null
+++ b/packages/core/tests/unit/fillform-value-override.test.ts
@@ -0,0 +1,191 @@
+import { describe, expect, it, vi } from "vitest";
+import { fillFormTool } from "../../lib/v3/agent/tools/fillform.js";
+import type { V3 } from "../../lib/v3/v3.js";
+
+/**
+ * Minimal mock of V3 that captures arguments passed to act().
+ */
+function createMockV3(
+  observeResults: Array<{
+    method: string;
+    arguments: string[];
+    elementId?: string;
+    description?: string;
+  }>,
+) {
+  const actCalls: Array<{
+    method: string;
+    arguments: string[];
+  }> = [];
+
+  const mock = {
+    logger: vi.fn(),
+    recordAgentReplayStep: vi.fn(),
+    act: vi.fn(async (res: { method: string; arguments: string[] }) => {
+      actCalls.push({ method: res.method, arguments: [...res.arguments] });
+      return {
+        success: true,
+        message: "ok",
+        actionDescription: "done",
+        actions: [],
+      };
+    }),
+    observe: vi.fn(async () => observeResults),
+    actCalls,
+  };
+
+  return mock as unknown as V3 & { actCalls: typeof actCalls };
+}
+
+const toolCtx = {
+  toolCallId: "t1",
+  messages: [] as never[],
+  abortSignal: new AbortController().signal,
+};
+
+describe("fillForm value override", () => {
+  it("uses caller-provided values instead of LLM-hallucinated values", async () => {
+    // observe() returns hallucinated placeholder values
+    const v3 = createMockV3([
+      {
+        method: "fill",
+        arguments: ["test@example.com"], // hallucinated!
+        elementId: "0-395",
+        description: "email field",
+      },
+      {
+        method: "fill",
+        arguments: ["password123"], // hallucinated!
+        elementId: "0-396",
+        description: "password field",
+      },
+    ]);
+
+    const tool = fillFormTool(v3);
+    await tool.execute!(
+      {
+        fields: [
+          {
+            action: "type email into email field",
+            value: "user@example.org",
+          },
+          {
+            action: "type password into password field",
+            value: "s3cret!",
+          },
+        ],
+      },
+      toolCtx,
+    );
+
+    // With the fix applied, act() should receive the REAL values
+    expect(v3.actCalls[0].arguments).toEqual(["user@example.org"]);
+    expect(v3.actCalls[1].arguments).toEqual(["s3cret!"]);
+  });
+
+  it("does not override non-fill methods (e.g. click)", async () => {
+    const v3 = createMockV3([
+      {
+        method: "click",
+        arguments: [],
+        elementId: "0-100",
+        description: "click submit",
+      },
+    ]);
+
+    const tool = fillFormTool(v3);
+    await tool.execute!(
+      {
+        fields: [{ action: "click submit button", value: "ignored" }],
+      },
+      toolCtx,
+    );
+
+    // click method should NOT have its arguments overridden
+    expect(v3.actCalls[0].arguments).toEqual([]);
+  });
+
+  it("handles interleaved non-fill actions (fillIndex alignment)", async () => {
+    // observe() returns a click between two fills
+    const v3 = createMockV3([
+      { method: "click", arguments: [], description: "focus email" },
+      { method: "fill", arguments: ["placeholder1"], description: "email" },
+      { method: "fill", arguments: ["placeholder2"], description: "password" },
+    ]);
+
+    const tool = fillFormTool(v3);
+    await tool.execute!(
+      {
+        fields: [
+          { action: "type email", value: "real@email.com" },
+          { action: "type password", value: "realPass" },
+        ],
+      },
+      toolCtx,
+    );
+
+    // The fix should use a fillIndex counter so:
+    // - click at i=0 is skipped
+    // - first fill maps to fields[0].value
+    // - second fill maps to fields[1].value
+    expect(v3.actCalls[0].arguments).toEqual([]); // click unchanged
+    expect(v3.actCalls[1].arguments).toEqual(["real@email.com"]);
+    expect(v3.actCalls[2].arguments).toEqual(["realPass"]);
+  });
+
+  it("handles empty string value (clearing a field)", async () => {
+    const v3 = createMockV3([
+      {
+        method: "fill",
+        arguments: ["hallucinated"],
+        description: "search box",
+      },
+    ]);
+
+    const tool = fillFormTool(v3);
+    await tool.execute!(
+      {
+        fields: [{ action: "clear the search box", value: "" }],
+      },
+      toolCtx,
+    );
+
+    // Empty string is a valid value (clearing a field) —
+    // it should NOT fall back to the hallucinated value
+    expect(v3.actCalls[0].arguments).toEqual([""]);
+  });
+
+  it("skips extra fills when observe returns more fills than fields", async () => {
+    const v3 = createMockV3([
+      { method: "fill", arguments: ["hal1"], description: "email" },
+      { method: "fill", arguments: ["hal2"], description: "password" },
+      { method: "fill", arguments: ["hal3"], description: "confirm password" },
+    ]);
+
+    const tool = fillFormTool(v3);
+    await tool.execute!(
+      {
+        fields: [
+          { action: "type email", value: "real@email.com" },
+          { action: "type password", value: "realPass" },
+        ],
+      },
+      toolCtx,
+    );
+
+    // Only the two matched fills should be acted on
+    expect(v3.actCalls).toHaveLength(2);
+    expect(v3.actCalls[0].arguments).toEqual(["real@email.com"]);
+    expect(v3.actCalls[1].arguments).toEqual(["realPass"]);
+
+    // Warning should be logged for the skipped fill
+    expect(v3.logger).toHaveBeenCalledWith(
+      expect.objectContaining({
+        category: "agent",
+        message: expect.stringContaining(
+          "more fill actions than provided fields",
+        ),
+      }),
+    );
+  });
+});