diff --git a/ui-tui-opentui-v2/src/boundary/log.ts b/ui-tui-opentui-v2/src/boundary/log.ts
index 5506291a54b..d57014f34ed 100644
--- a/ui-tui-opentui-v2/src/boundary/log.ts
+++ b/ui-tui-opentui-v2/src/boundary/log.ts
@@ -20,10 +20,46 @@ import { appendFileSync, mkdirSync } from 'node:fs'
 import { homedir } from 'node:os'
 import { dirname, join } from 'node:path'
 
-export type LogLevel = 'debug' | 'info' | 'warn' | 'error'
+import { Schema } from 'effect'
+
+// LogLevel is modeled schema-first (the schema-inferred-types idiom, mirroring
+// `boundary/schema/GatewayEvent.ts`): declare the literal union once and INFER
+// the TS type from it, so the two can never drift.
+export const LogLevelSchema = Schema.Literals(['debug', 'info', 'warn', 'error'])
+export type LogLevel = typeof LogLevelSchema.Type
 
 const PRIORITY: Record<LogLevel, number> = { debug: 0, info: 1, warn: 2, error: 3 }
 
+/**
+ * Serialize a value to JSON that NEVER throws. A caller-supplied `data` can hold
+ * a circular reference or a BigInt — plain `JSON.stringify` throws on both, which
+ * (in the file-write `catch` below) would flip `fileBroken` and kill ALL file
+ * logging for the session. Instead we degrade a bad payload to a placeholder:
+ *   - circular refs (tracked via a per-call `WeakSet` of seen objects) → '[Circular]'
+ *   - BigInt → `\`${n}n\`` (JSON has no bigint; keep it readable + reversible-ish)
+ * and wrap the whole thing so any other throw (e.g. a hostile `toJSON`) falls back
+ * to `String(value)`, then to '[unserializable]' if even that throws.
+ */
+export function safeStringify(value: unknown): string {
+  try {
+    const seen = new WeakSet<object>()
+    return JSON.stringify(value, (_key, val: unknown) => {
+      if (typeof val === 'bigint') return `${val}n`
+      if (typeof val === 'object' && val !== null) {
+        if (seen.has(val)) return '[Circular]'
+        seen.add(val)
+      }
+      return val
+    })
+  } catch {
+    try {
+      return String(value)
+    } catch {
+      return '[unserializable]'
+    }
+  }
+}
+
 export interface LogEntry {
   readonly t: number // epoch ms
   readonly level: LogLevel
@@ -82,7 +118,7 @@ export class Log {
 
     if (this.file && !this.fileBroken) {
       try {
-        appendFileSync(this.file, JSON.stringify(entry) + '\n')
+        appendFileSync(this.file, safeStringify(entry) + '\n')
       } catch {
         this.fileBroken = true // stop hammering a broken path; the ring keeps working
       }
diff --git a/ui-tui-opentui-v2/src/test/log.test.ts b/ui-tui-opentui-v2/src/test/log.test.ts
new file mode 100644
index 00000000000..7b258bc56ad
--- /dev/null
+++ b/ui-tui-opentui-v2/src/test/log.test.ts
@@ -0,0 +1,81 @@
+/**
+ * Log hardening (boundary/log.ts): safeStringify never throws on a circular ref /
+ * BigInt / hostile toJSON, so one bad `data` payload can't flip `fileBroken` and
+ * kill file logging for the session.
+ */
+import { describe, expect, test } from 'bun:test'
+import { mkdtempSync, readFileSync, rmSync } from 'node:fs'
+import { tmpdir } from 'node:os'
+import { join } from 'node:path'
+
+import { Log, safeStringify } from '../boundary/log.ts'
+
+describe('safeStringify', () => {
+  test('handles a circular object without throwing', () => {
+    const a: Record<string, unknown> = { name: 'a' }
+    a.self = a
+    const out = safeStringify(a)
+    expect(typeof out).toBe('string')
+    expect(out).toContain('[Circular]')
+    expect(out).toContain('"name":"a"')
+  })
+
+  test('handles a BigInt without throwing', () => {
+    const out = safeStringify({ big: 10n, nested: { x: 9007199254740993n } })
+    expect(typeof out).toBe('string')
+    expect(out).toContain('"10n"')
+    expect(out).toContain('"9007199254740993n"')
+  })
+
+  test('handles a mixed circular + BigInt payload', () => {
+    const node: Record<string, unknown> = { id: 1n }
+    node.parent = node
+    expect(() => safeStringify({ node, list: [1n, 2n] })).not.toThrow()
+  })
+
+  test('degrades a hostile toJSON to a placeholder instead of throwing', () => {
+    const hostile = {
+      toJSON() {
+        throw new Error('boom')
+      }
+    }
+    let out = ''
+    expect(() => {
+      out = safeStringify(hostile)
+    }).not.toThrow()
+    expect(typeof out).toBe('string')
+  })
+
+  test('round-trips a plain object identically to JSON.stringify', () => {
+    const v = { a: 1, b: 'two', c: [3, 4], d: null }
+    expect(safeStringify(v)).toBe(JSON.stringify(v))
+  })
+})
+
+describe('Log file logging survives a poison payload', () => {
+  test('a circular/BigInt data field still writes a line and keeps filePath', () => {
+    const dir = mkdtempSync(join(tmpdir(), 'hermes-log-poison-'))
+    const file = join(dir, 'opentui-v2.log')
+    try {
+      const log = new Log(file, 'debug')
+      const circular: Record<string, unknown> = {}
+      circular.self = circular
+      log.info('test', 'with circular', circular)
+      log.info('test', 'with bigint', { n: 42n })
+      // file logging must NOT be broken by the poison payloads
+      expect(log.filePath).toBe(file)
+      const lines = readFileLines(file)
+      expect(lines.length).toBe(2)
+      expect(lines[0]).toContain('[Circular]')
+      expect(lines[1]).toContain('42n')
+    } finally {
+      rmSync(dir, { recursive: true, force: true })
+    }
+  })
+})
+
+function readFileLines(path: string): string[] {
+  // trailing newline produces an empty tail we drop
+  const text = readFileSync(path, 'utf8')
+  return text.split('\n').filter(line => line.length > 0)
+}