diff --git a/index.js b/index.js
index 843b22f..1c1fcdb 100644
--- a/index.js
+++ b/index.js
@@ -34,6 +34,7 @@ import Schema from '@deepseek-ai/schemastery'
 
 import { missingRequired, normalizeConfig, renderSlopoConfig } from './lib/config.js'
 import { probeEmbeddingEndpoint } from './lib/probe.js'
+import { toJsonValue } from './lib/json.js'
 import {
   resolveDshHome,
   workspaceStateDir,
@@ -431,7 +432,18 @@ export function apply(ctx, config) {
           exec.agent === undefined ? {} : { session: exec.agent.session },
         )
 
-        const foreground = (value) => ({ kind: 'foreground', action: args.action, configPath: paths.configPath, logPath: paths.logFile, ...value })
+        // Every return funnels through here. The harness enforces the declared
+        // output schema against this value BEFORE materialising it, and an
+        // explicit `undefined` property is not lossless JSON — which is exactly
+        // how a probe result carrying `error: undefined` broke the status call.
+        const foreground = (value) =>
+          toJsonValue({
+            kind: 'foreground',
+            action: args.action,
+            configPath: paths.configPath,
+            logPath: paths.logFile,
+            ...value,
+          })
 
         if (args.action === 'status') {
           const binary = findSlopo(dshHome)
@@ -698,7 +710,7 @@ export function apply(ctx, config) {
               return { cancel: () => controller.abort(), done }
             },
           })
-          return { kind: 'background', action: args.action, jobId, logPath: paths.logFile, configPath: paths.configPath }
+          return toJsonValue({ kind: 'background', action: args.action, jobId, logPath: paths.logFile, configPath: paths.configPath })
         }
 
         const result = await run(command, { env, signal: exec.signal })
diff --git a/lib/json.js b/lib/json.js
new file mode 100644
index 0000000..51ee180
--- /dev/null
+++ b/lib/json.js
@@ -0,0 +1,65 @@
+/**
+ * Coerce a value into something that is genuinely lossless JSON.
+ *
+ * `undefined` is the trap. It is not a JSON value: `JSON.stringify` silently
+ * drops it from an object, so a result carrying `{ ok: true, error: undefined }`
+ * looks harmless on the way out and is not — the harness enforces the declared
+ * output schema against the canonical value *before* materialising it, and an
+ * `undefined` property is not valid lossless JSON. The tool call fails with a
+ * serialisation error the caller cannot act on.
+ *
+ * Building the value through one chokepoint is what makes that impossible to
+ * reintroduce: any future branch may write `foo: undefined` and it is dropped,
+ * exactly as `JSON.stringify` would have dropped it.
+ *
+ * Non-finite numbers become `null` for the same reason (`JSON.stringify(NaN)`
+ * is `"null"`), and a circular reference throws rather than recursing forever.
+ * Handles are not JSON values at all, so they are converted to `"<handle>"`.
+ */
+
+/**
+ * @param {unknown} value
+ * @param {Set<object>} [seen]
+ * @returns {unknown} a value that survives JSON round-tripping unchanged
+ */
+export function toJsonValue(value, seen = new Set()) {
+  if (value === null) return null
+  switch (typeof value) {
+    case 'string':
+    case 'boolean':
+      return value
+    case 'number':
+      return Number.isFinite(value) ? value : null
+    case 'bigint':
+      return value.toString()
+    case 'undefined':
+    case 'function':
+    case 'symbol':
+      return undefined
+    case 'object':
+      break
+    default:
+      return undefined
+  }
+
+  if (seen.has(value)) throw new TypeError('cannot serialise a circular value')
+  seen.add(value)
+  try {
+    if (Array.isArray(value)) {
+      // An array slot cannot be dropped the way an object key can, so an
+      // unserialisable element becomes null rather than shifting the indices.
+      return value.map((item) => {
+        const converted = toJsonValue(item, seen)
+        return converted === undefined ? null : converted
+      })
+    }
+    const output = {}
+    for (const [key, entry] of Object.entries(value)) {
+      const converted = toJsonValue(entry, seen)
+      if (converted !== undefined) output[key] = converted
+    }
+    return output
+  } finally {
+    seen.delete(value)
+  }
+}
diff --git a/lib/probe.js b/lib/probe.js
index fba3990..886a399 100644
--- a/lib/probe.js
+++ b/lib/probe.js
@@ -143,7 +143,7 @@ export async function probeEmbeddingEndpoint({
     }
     const actual = vector.length
     const expected = Number(dimensions)
-    return {
+    const result = {
       checked: true,
       reachable: true,
       ok: actual === expected,
@@ -151,11 +151,14 @@ export async function probeEmbeddingEndpoint({
       sentModel,
       dimensions: actual,
       expected,
-      error:
-        actual === expected
-          ? undefined
-          : `the model returns ${actual} dimensions but embeddingDimensions is ${expected}`,
     }
+    // Omitted rather than assigned undefined: an explicit `undefined` property
+    // is not lossless JSON and fails the tool's output validation before the
+    // result is ever materialised.
+    if (actual !== expected) {
+      result.error = `the model returns ${actual} dimensions but embeddingDimensions is ${expected}`
+    }
+    return result
   } catch (error) {
     const aborted = error?.name === 'AbortError'
     return {
diff --git a/test/json.test.js b/test/json.test.js
new file mode 100644
index 0000000..5293066
--- /dev/null
+++ b/test/json.test.js
@@ -0,0 +1,59 @@
+import assert from 'node:assert/strict'
+import { test } from 'node:test'
+
+import { toJsonValue } from '../lib/json.js'
+
+test('drops undefined properties, which is the shape that broke status', () => {
+  const value = toJsonValue({ ok: true, error: undefined, url: 'http://x' })
+  assert.deepEqual(value, { ok: true, url: 'http://x' })
+  assert.equal(Object.hasOwn(value, 'error'), false)
+})
+
+test('the result survives a JSON round trip unchanged', () => {
+  const value = toJsonValue({
+    nested: { keep: 1, drop: undefined, deeper: { alsoDrop: undefined } },
+    list: [1, 'two', { three: true, four: undefined }],
+  })
+  assert.deepEqual(value, JSON.parse(JSON.stringify(value)))
+})
+
+test('an undefined array slot becomes null rather than shifting the indices', () => {
+  assert.deepEqual(toJsonValue([1, undefined, 3]), [1, null, 3])
+})
+
+test('non-finite numbers become null, exactly as JSON.stringify would', () => {
+  assert.deepEqual(toJsonValue({ a: Number.NaN, b: Infinity, c: -Infinity, d: 1.5 }), {
+    a: null,
+    b: null,
+    c: null,
+    d: 1.5,
+  })
+})
+
+test('functions, symbols and undefined vanish; bigints become strings', () => {
+  const value = toJsonValue({ fn: () => {}, sym: Symbol('x'), gone: undefined, big: 10n, keep: 'yes' })
+  assert.deepEqual(value, { big: '10', keep: 'yes' })
+})
+
+test('null and primitives pass through', () => {
+  assert.equal(toJsonValue(null), null)
+  assert.equal(toJsonValue('a'), 'a')
+  assert.equal(toJsonValue(0), 0)
+  assert.equal(toJsonValue(false), false)
+})
+
+test('a circular value is refused rather than recursed into', () => {
+  const circular = { name: 'loop' }
+  circular.self = circular
+  assert.throws(() => toJsonValue(circular), /circular/)
+})
+
+test('a repeated but non-circular reference is fine', () => {
+  const shared = { id: 1 }
+  assert.deepEqual(toJsonValue({ a: shared, b: shared }), { a: { id: 1 }, b: { id: 1 } })
+})
+
+test('an already-clean value is returned unchanged in value', () => {
+  const clean = { kind: 'foreground', ok: true, exitCode: 0, report: 'cluster 1:' }
+  assert.deepEqual(toJsonValue(clean), clean)
+})
diff --git a/test/plugin.test.js b/test/plugin.test.js
index 6f2bfdc..a9cd763 100644
--- a/test/plugin.test.js
+++ b/test/plugin.test.js
@@ -76,7 +76,7 @@ const shell = {
  * `jobs` is deliberately absent, which forces the tool down its foreground path
  * so the assertions can see a result rather than a job handle.
  */
-function harness(overrides = {}, { withSettings = true, withJobs = false, sandboxMode, sandboxPolicyResolver } = {}) {
+function harness(overrides = {}, { withSettings = true, withJobs = false, withShell = true, sandboxMode, sandboxPolicyResolver } = {}) {
   const config = { ...FULL_CONFIG, ...overrides }
   // A per-harness shell, so a test can vary sandboxMode and every resolved
   // request is recorded for assertion.
@@ -101,7 +101,7 @@ function harness(overrides = {}, { withSettings = true, withJobs = false, sandbo
   const ctx = {
     logger: () => ({ info() {}, warn() {}, debug() {}, error() {} }),
     get(name) {
-      if (name === 'shell') return scopedShell
+      if (name === 'shell') return withShell ? scopedShell : undefined
       if (name === 'jobs') return withJobs ? state.jobs : undefined
       if (name === 'sandboxPolicy') return sandboxPolicyResolver
       return undefined
@@ -432,3 +432,104 @@ test('configure never touches the shell, and status only runs a read-only versio
     assert.match(request.command, /--version/, 'status may only run a read-only version check')
   }
 })
+
+/**
+ * Walk a returned value and fail on anything that is not lossless JSON.
+ *
+ * `assert.deepStrictEqual` against a JSON round trip catches an `undefined`
+ * property (Node distinguishes `{a:1,b:undefined}` from `{a:1}`), but the walk
+ * names the offending path, which matters when the value is a deep diagnostic.
+ */
+function assertLossless(label, value) {
+  assert.deepEqual(value, JSON.parse(JSON.stringify(value)), `${label}: does not survive a JSON round trip`)
+  const walk = (node, path) => {
+    if (node === null || typeof node !== 'object') return
+    if (Array.isArray(node)) {
+      node.forEach((item, index) => walk(item, `${path}[${index}]`))
+      return
+    }
+    for (const [key, entry] of Object.entries(node)) {
+      assert.notEqual(entry, undefined, `${label}: ${path}.${key} is undefined, which is not lossless JSON`)
+      walk(entry, `${path}.${key}`)
+    }
+  }
+  walk(value, label)
+}
+
+test('status returns lossless JSON when the endpoint probe SUCCEEDS', async () => {
+  // The regression: the probe used to return `error: undefined` on a match, and
+  // an explicit undefined property failed the harness's output validation, so a
+  // perfectly healthy status call could not be serialised at all.
+  const { createServer } = await import('node:http')
+  const server = createServer((request, response) => {
+    response.writeHead(200, { 'content-type': 'application/json' })
+    response.end(JSON.stringify({ data: [{ embedding: new Array(768).fill(0.01) }] }))
+  })
+  await new Promise((resolve) => server.listen(0, '127.0.0.1', resolve))
+  const { port } = server.address()
+  try {
+    const { state } = harness({
+      embeddingModel: 'ollama/example:latest',
+      embeddingDimensions: 768,
+      embeddingApiBase: `http://127.0.0.1:${port}`,
+    })
+    const value = await state.tools.get('slopo').execute({ action: 'status' }, execContext('/tmp'))
+    assertLossless('status', value)
+    assert.equal(value.ok, true)
+    assert.equal(value.details.embeddingEndpoint.ok, true)
+    assert.equal(value.details.embeddingEndpoint.dimensions, 768)
+    assert.equal(Object.hasOwn(value.details.embeddingEndpoint, 'error'), false)
+  } finally {
+    await new Promise((resolve) => server.close(resolve))
+  }
+})
+
+test('status returns lossless JSON when the endpoint probe FAILS', async () => {
+  const { state } = harness({
+    embeddingModel: 'ollama/example:latest',
+    embeddingDimensions: 768,
+    // Nothing is listening on this port, so the fetch rejects.
+    embeddingApiBase: 'http://127.0.0.1:1',
+  })
+  const value = await state.tools.get('slopo').execute({ action: 'status' }, execContext('/tmp'))
+  assertLossless('status (unreachable)', value)
+  assert.equal(value.ok, false)
+  assert.ok(value.details.embeddingEndpoint.error.length > 0)
+})
+
+test('every action that returns without executing anything is lossless JSON', async () => {
+  const cases = [
+    [{ action: 'status' }, 'status (unconfigured)'],
+    [{ action: 'configure', config: { embeddingModel: 'x', nope: 1 } }, 'configure (unknown setting)'],
+    [{ action: 'configure', config: { embeddingModel: 'x' } }, 'configure (ok)'],
+    [{ action: 'configure', config: 'not-an-object' }, 'configure (bad patch)'],
+    [{ action: 'review' }, 'review (unconfigured)'],
+  ]
+  for (const [args, label] of cases) {
+    const { state } = harness()
+    const value = await state.tools.get('slopo').execute(args, execContext('/tmp'))
+    assertLossless(label, value)
+  }
+})
+
+test('an action refused before it runs is lossless JSON', async () => {
+  const configured = {
+    embeddingModel: 'ollama/example:latest',
+    embeddingDimensions: 768,
+    embeddingApiKey: 'secret',
+    embeddingApiBase: 'http://127.0.0.1:11434',
+  }
+
+  const noShell = harness(configured, { withShell: false })
+  const refused = await noShell.state.tools.get('slopo').execute({ action: 'review' }, execContext('/tmp'))
+  assertLossless('review (no shell)', refused)
+  assert.equal(refused.errorCode, 'NO_SHELL')
+
+  const outside = harness(configured, {
+    sandboxMode: 'workspace-write',
+    sandboxPolicyResolver: { resolve: () => ({ mode: 'workspace-write', workspaceRoot: '/some/workspace' }) },
+  })
+  const value = await outside.state.tools.get('slopo').execute({ action: 'review' }, execContext('/some/workspace'))
+  assertLossless('review (state outside sandbox)', value)
+  assert.equal(value.errorCode, 'STATE_OUTSIDE_SANDBOX')
+})