diff --git a/packages/logger/src/index.test.ts b/packages/logger/src/index.test.ts
index 076fd74c293..78d88313267 100644
--- a/packages/logger/src/index.test.ts
+++ b/packages/logger/src/index.test.ts
@@ -216,4 +216,131 @@ describe('Logger', () => {
       )
     })
   })
+
+  describe('structured serialization safety', () => {
+    const createEnabledLogger = () =>
+      new Logger('Test', { enabled: true, colorize: false, logLevel: LogLevel.DEBUG })
+
+    test('should emit a line instead of throwing on cyclic metadata', () => {
+      const cyclic: Record<string, unknown> = { id: 'x' }
+      cyclic.self = cyclic
+
+      expect(() => createEnabledLogger().info('hello', cyclic)).not.toThrow()
+      expect(consoleLogSpy).toHaveBeenCalledTimes(1)
+      const parsed = JSON.parse(consoleLogSpy.mock.calls[0][0] as string)
+      expect(parsed.message).toBe('hello')
+      expect(parsed.id).toBe('x')
+      expect(parsed.self.self).toBe('[Circular]')
+    })
+
+    test('should emit a line instead of throwing on BigInt metadata', () => {
+      expect(() => createEnabledLogger().error('boom', { size: 10n })).not.toThrow()
+      expect(consoleErrorSpy).toHaveBeenCalledTimes(1)
+      const parsed = JSON.parse(consoleErrorSpy.mock.calls[0][0] as string)
+      expect(parsed.message).toBe('boom')
+      expect(parsed.size).toBe('10')
+    })
+
+    test('should fall back to a minimal entry when a value cannot be serialized at all', () => {
+      const hostile = {
+        get boom() {
+          throw new Error('getter exploded')
+        },
+      }
+
+      expect(() => createEnabledLogger().info('hello', hostile)).not.toThrow()
+      const parsed = JSON.parse(consoleLogSpy.mock.calls[0][0] as string)
+      expect(parsed.message).toBe('hello')
+      expect(parsed.module).toBe('Test')
+      expect(parsed.serializationError).toBe(true)
+    })
+
+    test('should not throw when withMetadata receives a throwing getter', () => {
+      const hostile = {
+        safe: 'kept',
+        get boom() {
+          throw new Error('getter exploded')
+        },
+      } as unknown as Parameters<Logger['withMetadata']>[0]
+
+      let child: Logger | undefined
+      expect(() => {
+        child = createEnabledLogger().withMetadata(hostile)
+      }).not.toThrow()
+
+      expect(() => child?.info('hello')).not.toThrow()
+      const parsed = JSON.parse(consoleLogSpy.mock.calls[0][0] as string)
+      expect(parsed.message).toBe('hello')
+      expect(parsed.safe).toBe('kept')
+      expect(parsed.boom).toBe('[Unreadable]')
+    })
+
+    test('should keep repeated references that are not cycles', () => {
+      const shared = { s: 'REAL_DATA', n: 42 }
+      const payload = { p: shared, q: shared, arr: [shared, shared], big: 1n } as unknown as object
+
+      createEnabledLogger().info('hello', payload)
+
+      const parsed = JSON.parse(consoleLogSpy.mock.calls[0][0] as string)
+      expect(parsed.p).toEqual({ s: 'REAL_DATA', n: 42 })
+      expect(parsed.q).toEqual({ s: 'REAL_DATA', n: 42 })
+      expect(parsed.arr).toEqual([
+        { s: 'REAL_DATA', n: 42 },
+        { s: 'REAL_DATA', n: 42 },
+      ])
+      expect(parsed.big).toBe('1')
+    })
+
+    test('should still mark a genuine cycle as circular', () => {
+      const cyclic: Record<string, unknown> = { name: 'root' }
+      cyclic.self = cyclic
+      const payload = { cyclic, big: 1n } as unknown as object
+
+      createEnabledLogger().info('hello', payload)
+
+      const parsed = JSON.parse(consoleLogSpy.mock.calls[0][0] as string)
+      expect(parsed.cyclic.name).toBe('root')
+      expect(parsed.cyclic.self).toBe('[Circular]')
+    })
+
+    test('should not throw when retained metadata has a throwing toJSON', () => {
+      const hostile = {
+        evil: {
+          toJSON() {
+            throw new Error('toJSON exploded')
+          },
+        },
+      } as unknown as Parameters<Logger['withMetadata']>[0]
+
+      const child = createEnabledLogger().withMetadata(hostile)
+
+      expect(() => child.info('hello')).not.toThrow()
+      const parsed = JSON.parse(consoleLogSpy.mock.calls[0][0] as string)
+      expect(parsed.message).toBe('hello')
+      expect(parsed.module).toBe('Test')
+      expect(parsed.serializationError).toBe(true)
+      expect(parsed.evil).toBeUndefined()
+    })
+
+    test('should not throw when withMetadata receives a hostile proxy', () => {
+      const hostile = new Proxy(
+        {},
+        {
+          ownKeys() {
+            throw new Error('ownKeys exploded')
+          },
+        }
+      ) as Parameters<Logger['withMetadata']>[0]
+
+      let child: Logger | undefined
+      expect(() => {
+        child = createEnabledLogger().withMetadata(hostile)
+      }).not.toThrow()
+
+      expect(() => child?.info('hello')).not.toThrow()
+      const parsed = JSON.parse(consoleLogSpy.mock.calls[0][0] as string)
+      expect(parsed.message).toBe('hello')
+      expect(parsed.metadataError).toBe(true)
+    })
+  })
 })
diff --git a/packages/logger/src/index.ts b/packages/logger/src/index.ts
index 25e6e3f9ae3..47da5e03f0c 100644
--- a/packages/logger/src/index.ts
+++ b/packages/logger/src/index.ts
@@ -139,6 +139,110 @@ const formatObject = (obj: unknown, isDev: boolean): string => {
   }
 }
 
+/** Merges caller-supplied log arguments into the structured entry. */
+const mergeArgs = (entry: Record<string, unknown>, args: unknown[]): Record<string, unknown> => {
+  for (const arg of args) {
+    if (arg === null || arg === undefined) continue
+    if (arg instanceof Error) {
+      entry.error = arg.message
+      entry.stack = arg.stack
+    } else if (typeof arg === 'object') {
+      Object.assign(entry, arg)
+    } else {
+      entry.extra = arg
+    }
+  }
+  return entry
+}
+
+/** JSON replacer that tolerates cyclic references and BigInt values. */
+const tolerantReplacer = () => {
+  const ancestors: object[] = []
+  return function (this: unknown, _key: string, value: unknown): unknown {
+    if (typeof value === 'bigint') return value.toString()
+    if (value === null || typeof value !== 'object') return value
+    /**
+     * Track the ancestor path, not every object ever visited. `this` is the
+     * object holding the current key, so unwinding to it drops the siblings we
+     * have finished descending. A set of everything seen would label the second
+     * appearance of a merely repeated reference `[Circular]` and discard real
+     * data, since a payload that references one object twice has no cycle.
+     */
+    while (ancestors.length > 0 && ancestors[ancestors.length - 1] !== this) ancestors.pop()
+    if (ancestors.includes(value)) return '[Circular]'
+    ancestors.push(value)
+    return value
+  }
+}
+
+/**
+ * Builds and serializes a production log entry without ever throwing.
+ *
+ * Caller-supplied arguments are merged in verbatim, so a cyclic reference, a
+ * BigInt, or a throwing getter would otherwise raise inside the caller's code
+ * path — losing the line and aborting whatever was being logged about. A
+ * logger must never be able to break its caller.
+ */
+const serializeEntry = (base: Record<string, unknown>, args: unknown[]): string => {
+  try {
+    return JSON.stringify(mergeArgs({ ...base }, args))
+  } catch {}
+
+  try {
+    return JSON.stringify(mergeArgs({ ...base }, args), tolerantReplacer())
+  } catch {}
+
+  return minimalEntry(base)
+}
+
+/**
+ * Last-resort entry built only from fields this module controls.
+ *
+ * A replacer cannot rescue a throwing `toJSON`, because `JSON.stringify` invokes
+ * it before the replacer ever sees the value. So the final fallback drops every
+ * caller-supplied value instead of re-serializing it, and passes strings through
+ * only when they are already strings — coercing would re-enter hostile
+ * `toString`. What remains cannot throw.
+ */
+const minimalEntry = (base: Record<string, unknown>): string => {
+  const asString = (value: unknown) => (typeof value === 'string' ? value : '[Unserializable]')
+  return JSON.stringify({
+    timestamp: asString(base.timestamp),
+    level: asString(base.level),
+    module: asString(base.module),
+    message: asString(base.message),
+    serializationError: true,
+  })
+}
+
+/**
+ * Copies caller-supplied metadata into a plain object without ever throwing.
+ *
+ * `LoggerMetadata` is structurally typed, so nothing stops a caller from handing
+ * over an object carrying a throwing getter or a hostile proxy. A spread invokes
+ * those traps, so the copy degrades key-by-key and finally to a marker rather
+ * than raising inside the caller's code path.
+ */
+const materializeMetadata = (metadata: LoggerMetadata): LoggerMetadata => {
+  try {
+    return { ...metadata }
+  } catch {}
+
+  const safe: LoggerMetadata = {}
+  try {
+    for (const key of Object.keys(metadata)) {
+      try {
+        safe[key] = metadata[key]
+      } catch {
+        safe[key] = '[Unreadable]'
+      }
+    }
+    return safe
+  } catch {}
+
+  return { metadataError: true }
+}
+
 /**
  * Logger class for standardized console logging
  *
@@ -191,7 +295,7 @@ export class Logger {
     child.module = this.module
     child.config = this.config
     child.isDev = this.isDev
-    child.metadata = { ...this.metadata, ...metadata }
+    child.metadata = { ...this.metadata, ...materializeMetadata(metadata) }
     return child
   }
 
@@ -280,33 +384,17 @@ export class Logger {
       }
     } else {
       // Structured JSON for production — CloudWatch Log Insights auto-parses JSON lines
-      const entry: Record<string, unknown> = {
+      const base: Record<string, unknown> = {
         timestamp,
         level,
         module: this.module,
         message,
       }
       for (const [k, v] of metadataEntries) {
-        entry[k] = v
-      }
-      // Merge extra args into the entry
-      for (const arg of args) {
-        if (
-          arg !== null &&
-          arg !== undefined &&
-          typeof arg === 'object' &&
-          !(arg instanceof Error)
-        ) {
-          Object.assign(entry, arg)
-        } else if (arg instanceof Error) {
-          entry.error = arg.message
-          entry.stack = arg.stack
-        } else if (arg !== null && arg !== undefined) {
-          entry.extra = arg
-        }
+        base[k] = v
       }
 
-      const line = JSON.stringify(entry)
+      const line = serializeEntry(base, args)
       if (level === LogLevel.ERROR) {
         console.error(line)
       } else {