diff --git a/packages/code-runtime/code-runtime-worker/src/index.ts b/packages/code-runtime/code-runtime-worker/src/index.ts
index 0fa66dbd71..198bec694b 100644
--- a/packages/code-runtime/code-runtime-worker/src/index.ts
+++ b/packages/code-runtime/code-runtime-worker/src/index.ts
@@ -15,7 +15,7 @@ import { CodeRuntime } from '@deepseek-ai/dsh-code-runtime'
 import type { CodeBindingFunction, CodeJsonValue, CodeRunFailure, CodeRunRequest, CodeRunResult } from '@deepseek-ai/dsh-code-runtime'
 import { snapshotJsonValue } from '@deepseek-ai/dsh-session'
 import type { ReplyMessage, WorkerBootData, WorkerToHost } from './protocol.ts'
-import { truncateJsonStringBytes } from './output-json.ts'
+import { jsonStringBytesUpTo, jsonValueBytesUpTo, truncateJsonStringBytes } from './output-json.ts'
 
 /** Plugin config: every execution cap, changeable from `cordis.yml` (no hardcoded tunables). */
 export interface Config {
@@ -141,11 +141,6 @@ function parseWorkerMessage(raw: unknown): WorkerToHost | undefined {
 }
 
 
-/** Serialized byte size of one lossless JSON value. */
-function jsonBytes(value: CodeJsonValue): number {
-  return Buffer.byteLength(JSON.stringify(value), 'utf8')
-}
-
 /** One run's combined outer-output ledger; binding values never enter it. */
 class OutputLedger {
   private bytes = 2 // JSON serialization of the empty logs array: []
@@ -155,9 +150,10 @@ class OutputLedger {
 
   /** Admit one exact log entry, or report that the hard cap was crossed. */
   admit(text: string, sink: string[]): boolean {
-    const cost = Buffer.byteLength(JSON.stringify(text), 'utf8') + (this.entries > 0 ? 1 : 0)
-    if (this.bytes + cost > this.maxBytes) return false
-    this.bytes += cost
+    const separatorBytes = this.entries > 0 ? 1 : 0
+    const stringBytes = jsonStringBytesUpTo(text, this.maxBytes - this.bytes - separatorBytes)
+    if (stringBytes === undefined) return false
+    this.bytes += stringBytes + separatorBytes
     this.entries += 1
     sink.push(text)
     return true
@@ -165,46 +161,45 @@ class OutputLedger {
 
   /** Finalize a successful absent-or-JSON completion against the combined cap. */
   success(logs: string[], value?: CodeJsonValue): CodeRunResult {
-    if (value !== undefined && this.bytes + jsonBytes(value) > this.maxBytes) return this.limit(logs)
+    if (value !== undefined && jsonValueBytesUpTo(value, this.maxBytes - this.bytes) === undefined) return this.limit(logs)
     return { logs, ...value !== undefined ? { value } : {} }
   }
 
   /** Finalize a failure diagnostic, with output-limit taking precedence when combined bytes exceed the cap. */
   failure(logs: string[], error: CodeRunFailure): CodeRunResult {
-    if (this.bytes + Buffer.byteLength(JSON.stringify(error.message), 'utf8') > this.maxBytes) return this.limit(logs)
+    if (jsonStringBytesUpTo(error.message, this.maxBytes - this.bytes) === undefined) return this.limit(logs)
     return { logs, error }
   }
 
   /** Build the explicit output-limit failure while retaining a fitting prefix of the final log. */
   limit(logs: string[]): CodeRunResult {
     const fullMessage = `outer output exceeded ${this.maxBytes} bytes`
-    const messageBytes = Buffer.byteLength(JSON.stringify(fullMessage), 'utf8')
-    const retained = [...logs]
-    let retainedBytes = jsonBytes(retained)
+    // The fixed diagnostic is ASCII, so every character is one byte plus the quotes.
+    const messageBytes = fullMessage.length + 2
+    const retained: string[] = []
+    let retainedBytes = 2
     const logBudget = this.maxBytes - messageBytes
-    while (retained.length > 0 && retainedBytes > logBudget) {
-      const removed = retained.pop()
-      /* v8 ignore next -- the while guard proves pop cannot return undefined. */
-      if (removed === undefined) throw new Error('output ledger lost its final log entry')
+    for (const text of logs) {
       const separatorBytes = retained.length > 0 ? 1 : 0
-      retainedBytes -= Buffer.byteLength(JSON.stringify(removed), 'utf8') + separatorBytes
-      const prefix = truncateJsonStringBytes(removed, logBudget - retainedBytes - separatorBytes)
-      if (prefix.length > 0) {
-        retained.push(prefix)
-        retainedBytes += Buffer.byteLength(JSON.stringify(prefix), 'utf8') + separatorBytes
-        break
+      const availableBytes = logBudget - retainedBytes - separatorBytes
+      const stringBytes = jsonStringBytesUpTo(text, availableBytes)
+      if (stringBytes !== undefined) {
+        retained.push(text)
+        retainedBytes += stringBytes + separatorBytes
+        continue
       }
-    }
-    if (logBudget < 2) {
-      retained.length = 0
-      retainedBytes = 2
+      const prefix = truncateJsonStringBytes(text, availableBytes)
+      if (prefix.length > 0) {
+        const prefixBytes = jsonStringBytesUpTo(prefix, availableBytes)
+        /* v8 ignore next -- truncateJsonStringBytes guarantees its returned prefix fits the same budget. */
+        if (prefixBytes === undefined) throw new Error('output ledger produced an oversized log prefix')
+        retained.push(prefix)
+        retainedBytes += prefixBytes + separatorBytes
+      }
+      break
     }
     const availableMessageBytes = this.maxBytes - retainedBytes
-    // This fixed diagnostic is ASCII with no JSON escapes, so two bytes are
-    // the surrounding quotes and every retained character costs one byte.
-    const message = messageBytes <= availableMessageBytes
-      ? fullMessage
-      : fullMessage.slice(0, availableMessageBytes - 2)
+    const message = truncateJsonStringBytes(fullMessage, availableMessageBytes)
     return { logs: retained, error: { kind: 'output-limit', message } }
   }
 }
@@ -410,12 +405,9 @@ export class WorkerCodeRuntime extends CodeRuntime {
           reply({ type: 'reply', id: message.id, ok: false, message: `unknown binding ${JSON.stringify(`${message.global}.${message.name}`)}` })
           return
         }
-        let args: CodeJsonValue | undefined
-        try {
-          args = snapshotJsonValue(message.args) as CodeJsonValue | undefined
-        } catch {
-          args = undefined
-        }
+        // Structured clone has already removed accessors and proxies, so the
+        // host can repeat the lossless snapshot without a reflective throw.
+        const args = snapshotJsonValue(message.args) as CodeJsonValue | undefined
         if (args === undefined) {
           reply({ type: 'reply', id: message.id, ok: false, message: 'binding arguments must be lossless JSON' })
           return
diff --git a/packages/code-runtime/code-runtime-worker/src/output-json.ts b/packages/code-runtime/code-runtime-worker/src/output-json.ts
index dd3c3529f5..2110a4ab27 100644
--- a/packages/code-runtime/code-runtime-worker/src/output-json.ts
+++ b/packages/code-runtime/code-runtime-worker/src/output-json.ts
@@ -1,5 +1,7 @@
 /** JSON string-prefix accounting for the outer-output ledger. @module @deepseek-ai/dsh-code-runtime-worker/output-json */
 
+import type { CodeJsonValue } from '@deepseek-ai/dsh-code-runtime'
+
 /** Control characters with a two-byte short JSON escape instead of `\u00XX`. */
 const SHORT_ESCAPE_CODES = new Set([0x08, 0x09, 0x0a, 0x0c, 0x0d])
 
@@ -13,6 +15,69 @@ function serializedCharacterBytes(character: string): number {
   return Buffer.byteLength(character, 'utf8')
 }
 
+/**
+ * Measure one JSON string without materializing its complete escaped form.
+ * @param text - the candidate string.
+ * @param maxBytes - largest serialized size the caller can admit.
+ * @returns Exact serialized bytes, or `undefined` as soon as the cap is crossed.
+ */
+export function jsonStringBytesUpTo(text: string, maxBytes: number): number | undefined {
+  if (maxBytes < 2) return undefined
+  let bytes = 2
+  for (const character of text) {
+    bytes += serializedCharacterBytes(character)
+    if (bytes > maxBytes) return undefined
+  }
+  return bytes
+}
+
+/**
+ * Measure one lossless JSON value without allocating its serialized form.
+ * @param value - already validated lossless JSON.
+ * @param maxBytes - largest serialized size the caller can admit.
+ * @returns Exact serialized bytes, or `undefined` as soon as the cap is crossed.
+ */
+export function jsonValueBytesUpTo(value: CodeJsonValue, maxBytes: number): number | undefined {
+  if (value === null) return maxBytes >= 4 ? 4 : undefined
+  if (typeof value === 'string') return jsonStringBytesUpTo(value, maxBytes)
+  if (typeof value === 'number') {
+    const bytes = Buffer.byteLength(String(value), 'utf8')
+    return bytes <= maxBytes ? bytes : undefined
+  }
+  if (typeof value === 'boolean') {
+    const bytes = value ? 4 : 5
+    return bytes <= maxBytes ? bytes : undefined
+  }
+
+  let bytes = 2
+  if (bytes > maxBytes) return undefined
+  if (Array.isArray(value)) {
+    for (let index = 0; index < value.length; index++) {
+      if (index > 0 && ++bytes > maxBytes) return undefined
+      const item = value[index]
+      if (item === undefined) return undefined
+      const itemBytes = jsonValueBytesUpTo(item, maxBytes - bytes)
+      if (itemBytes === undefined) return undefined
+      bytes += itemBytes
+    }
+    return bytes
+  }
+
+  let entries = 0
+  for (const [key, item] of Object.entries(value)) {
+    if (entries > 0 && ++bytes > maxBytes) return undefined
+    const keyBytes = jsonStringBytesUpTo(key, maxBytes - bytes)
+    if (keyBytes === undefined) return undefined
+    bytes += keyBytes + 1
+    if (bytes > maxBytes) return undefined
+    const itemBytes = jsonValueBytesUpTo(item, maxBytes - bytes)
+    if (itemBytes === undefined) return undefined
+    bytes += itemBytes
+    entries += 1
+  }
+  return bytes
+}
+
 /**
  * Return the longest code-point-aligned prefix whose JSON string encoding,
  * including its surrounding quotes, fits `maxBytes`.
@@ -23,7 +88,6 @@ function serializedCharacterBytes(character: string): number {
  */
 export function truncateJsonStringBytes(text: string, maxBytes: number): string {
   if (maxBytes < 2) return ''
-  if (Buffer.byteLength(JSON.stringify(text), 'utf8') <= maxBytes) return text
   let bytes = 2
   let end = 0
   for (const character of text) {
@@ -32,5 +96,5 @@ export function truncateJsonStringBytes(text: string, maxBytes: number): string
     bytes += cost
     end += character.length
   }
-  return text.slice(0, end)
+  return end === text.length ? text : text.slice(0, end)
 }
diff --git a/packages/code-runtime/code-runtime-worker/tests/output-json.spec.ts b/packages/code-runtime/code-runtime-worker/tests/output-json.spec.ts
index 8340d6e74c..36f6ebcfad 100644
--- a/packages/code-runtime/code-runtime-worker/tests/output-json.spec.ts
+++ b/packages/code-runtime/code-runtime-worker/tests/output-json.spec.ts
@@ -1,10 +1,12 @@
-import { describe, expect, it } from 'vitest'
-import { truncateJsonStringBytes } from '../src/output-json.ts'
+import { describe, expect, it, vi } from 'vitest'
+import { jsonStringBytesUpTo, jsonValueBytesUpTo, truncateJsonStringBytes } from '../src/output-json.ts'
 
 describe('truncateJsonStringBytes', () => {
   it('returns a fitting string whole and rejects budgets without JSON quotes', () => {
     expect(truncateJsonStringBytes('fits', 6)).toBe('fits')
     expect(truncateJsonStringBytes('x', 1)).toBe('')
+    expect(jsonStringBytesUpTo('fits', 6)).toBe(6)
+    expect(jsonStringBytesUpTo('fits', 5)).toBeUndefined()
   })
 
   it('accounts every JSON escape and cuts only between complete code points', () => {
@@ -15,4 +17,43 @@ describe('truncateJsonStringBytes', () => {
     expect(truncateJsonStringBytes(text, budget)).toBe(prefix)
     expect(Buffer.byteLength(JSON.stringify(truncateJsonStringBytes(text, budget)), 'utf8')).toBe(budget)
   })
+
+  it('bounds hostile strings without materializing their complete escaped form', () => {
+    const stringify = vi.spyOn(JSON, 'stringify').mockImplementation(() => { throw new Error('must not stringify') })
+    try {
+      expect(jsonStringBytesUpTo('"'.repeat(10_000), 32)).toBeUndefined()
+      expect(truncateJsonStringBytes('"'.repeat(10_000), 32)).toBe('"'.repeat(15))
+    } finally {
+      stringify.mockRestore()
+    }
+  })
+})
+
+describe('jsonValueBytesUpTo', () => {
+  it('matches JSON serialization for every lossless value branch and stops at the cap', () => {
+    const value = {
+      empty: {},
+      nil: null,
+      yes: true,
+      no: false,
+      number: 1.5,
+      text: '"\n😀',
+      array: [1, 'x'],
+    }
+    const bytes = Buffer.byteLength(JSON.stringify(value), 'utf8')
+
+    expect(jsonValueBytesUpTo(value, bytes)).toBe(bytes)
+    expect(jsonValueBytesUpTo(value, bytes - 1)).toBeUndefined()
+    expect(jsonValueBytesUpTo({}, 1)).toBeUndefined()
+    expect(jsonValueBytesUpTo(null, 3)).toBeUndefined()
+    expect(jsonValueBytesUpTo(10, 1)).toBeUndefined()
+    expect(jsonValueBytesUpTo(false, 4)).toBeUndefined()
+    expect(jsonValueBytesUpTo(new Array<never>(1), 10)).toBeUndefined()
+    expect(jsonValueBytesUpTo([null], 5)).toBeUndefined()
+    expect(jsonValueBytesUpTo([0, 0], 3)).toBeUndefined()
+    expect(jsonValueBytesUpTo({ a: null, b: null }, 10)).toBeUndefined()
+    expect(jsonValueBytesUpTo({ long: null }, 2)).toBeUndefined()
+    expect(jsonValueBytesUpTo({ '': null }, 4)).toBeUndefined()
+    expect(jsonValueBytesUpTo({ a: null }, 9)).toBeUndefined()
+  })
 })
diff --git a/packages/code-runtime/code-runtime-worker/tests/runtime.spec.ts b/packages/code-runtime/code-runtime-worker/tests/runtime.spec.ts
index 8bcee8389b..4d4bb4c3c1 100644
--- a/packages/code-runtime/code-runtime-worker/tests/runtime.spec.ts
+++ b/packages/code-runtime/code-runtime-worker/tests/runtime.spec.ts
@@ -401,6 +401,22 @@ describe('WorkerCodeRuntime — hostile programs (real workers)', () => {
     expect(Buffer.byteLength(JSON.stringify(result.logs), 'utf8')).toBeLessThan(200)
   })
 
+  it('bounds one oversized forged log while retaining its fitting escaped prefix', async () => {
+    const { runtime } = await setup({ maxOutputBytes: 96 })
+    const result = await runtime.run({
+      program: `
+        const { parentPort } = await import('node:worker_threads');
+        parentPort.postMessage({ type: 'log', text: '"'.repeat(1_000_000) });
+        for (;;) {}
+      `,
+      bindings: [],
+    })
+    expect(result.error).toEqual({ kind: 'output-limit', message: 'outer output exceeded 96 bytes' })
+    expect(result.logs).toHaveLength(1)
+    expect(result.logs[0]).toMatch(/^"+$/)
+    expect(Buffer.byteLength(JSON.stringify(result.logs), 'utf8') + Buffer.byteLength(JSON.stringify('outer output exceeded 96 bytes'), 'utf8')).toBeLessThanOrEqual(96)
+  })
+
   it('drops a malformed forged done carrying both value and error', async () => {
     const { runtime } = await setup()
     const result = await runtime.run({