diff --git a/src/features/tmux-subagent/pane-state-parser.test.ts b/src/features/tmux-subagent/pane-state-parser.test.ts
new file mode 100644
index 000000000..991c3fd95
--- /dev/null
+++ b/src/features/tmux-subagent/pane-state-parser.test.ts
@@ -0,0 +1,72 @@
+/// <reference path="../../../bun-test.d.ts" />
+
+import { describe, expect, it } from "bun:test"
+import { parsePaneStateOutput } from "./pane-state-parser"
+
+describe("parsePaneStateOutput", () => {
+  it("rejects malformed integer fields", () => {
+    // given
+    const stdout = "%0\t120oops\t40\t0\t0\t1\t120\t40\n"
+
+    // when
+    const result = parsePaneStateOutput(stdout)
+
+    // then
+    expect(result).toBe(null)
+  })
+
+  it("rejects negative integer fields", () => {
+    // given
+    const stdout = "%0\t-1\t40\t0\t0\t1\t120\t40\n"
+
+    // when
+    const result = parsePaneStateOutput(stdout)
+
+    // then
+    expect(result).toBe(null)
+  })
+
+  it("rejects empty integer fields", () => {
+    // given
+    const stdout = "%0\t\t40\t0\t0\t1\t120\t40\n"
+
+    // when
+    const result = parsePaneStateOutput(stdout)
+
+    // then
+    expect(result).toBe(null)
+  })
+
+  it("rejects non-binary active flags", () => {
+    // given
+    const stdout = "%0\t120\t40\t0\t0\tx\t120\t40\n"
+
+    // when
+    const result = parsePaneStateOutput(stdout)
+
+    // then
+    expect(result).toBe(null)
+  })
+
+  it("rejects numeric active flags other than zero or one", () => {
+    // given
+    const stdout = "%0\t120\t40\t0\t0\t2\t120\t40\n"
+
+    // when
+    const result = parsePaneStateOutput(stdout)
+
+    // then
+    expect(result).toBe(null)
+  })
+
+  it("rejects empty active flags", () => {
+    // given
+    const stdout = "%0\t120\t40\t0\t0\t\t120\t40\n"
+
+    // when
+    const result = parsePaneStateOutput(stdout)
+
+    // then
+    expect(result).toBe(null)
+  })
+})
diff --git a/src/features/tmux-subagent/pane-state-parser.ts b/src/features/tmux-subagent/pane-state-parser.ts
index c9aa1ecc9..3ae6579d8 100644
--- a/src/features/tmux-subagent/pane-state-parser.ts
+++ b/src/features/tmux-subagent/pane-state-parser.ts
@@ -60,6 +60,7 @@ function parsePaneLine(line: string): ParsedPaneLine | null {
   const height = parseInteger(heightString)
   const left = parseInteger(leftString)
   const top = parseInteger(topString)
+  const isActive = parseActiveValue(activeString)
   const windowWidth = parseInteger(windowWidthString)
   const windowHeight = parseInteger(windowHeightString)
 
@@ -68,6 +69,7 @@ function parsePaneLine(line: string): ParsedPaneLine | null {
     height === null ||
     left === null ||
     top === null ||
+    isActive === null ||
     windowWidth === null ||
     windowHeight === null
   ) {
@@ -82,7 +84,7 @@ function parsePaneLine(line: string): ParsedPaneLine | null {
       left,
       top,
       title: fields.slice(MANDATORY_PANE_FIELD_COUNT).join("\t"),
-      isActive: activeString === "1",
+      isActive,
     },
     windowWidth,
     windowHeight,
@@ -120,6 +122,14 @@ function getMandatoryPaneFields(fields: string[]): MandatoryPaneFields | null {
 }
 
 function parseInteger(value: string): number | null {
+  if (!/^\d+$/.test(value)) return null
+
   const parsedValue = Number.parseInt(value, 10)
   return Number.isNaN(parsedValue) ? null : parsedValue
 }
+
+function parseActiveValue(value: string): boolean | null {
+  if (value === "1") return true
+  if (value === "0") return false
+  return null
+}