diff --git a/src/shared/pty-process-inspection-evidence.test.ts b/src/shared/pty-process-inspection-evidence.test.ts
index e99511f947f..dd05d806018 100644
--- a/src/shared/pty-process-inspection-evidence.test.ts
+++ b/src/shared/pty-process-inspection-evidence.test.ts
@@ -52,6 +52,37 @@ describe('readPtyProcessInspectionEvidence normalization', () => {
     expect(evidence.children).toEqual({ verdict: 'unverifiable', reason: 'unspecified' })
   })
 
+  it('coerces an observed verdict with a non-string processName to unverifiable', () => {
+    // A number here would flow into recognizeAgentProcess and count toward the
+    // "foreground is not a recognized agent" leg — malformed foreign data must
+    // never feed the positive-exited path.
+    const evidence = readPtyProcessInspectionEvidence({
+      foregroundProcess: null,
+      hasChildProcesses: false,
+      processEvidence: {
+        foreground: { verdict: 'observed', processName: 42 } as never,
+        children: { verdict: 'exited' }
+      }
+    })
+    expect(evidence.foreground).toEqual({
+      verdict: 'unverifiable',
+      reason: 'malformed foreground inspection evidence'
+    })
+  })
+
+  it('replaces a non-string unverifiable reason instead of forwarding it', () => {
+    const evidence = readPtyProcessInspectionEvidence({
+      foregroundProcess: null,
+      hasChildProcesses: false,
+      processEvidence: {
+        foreground: { verdict: 'unverifiable', reason: 42 } as never,
+        children: { verdict: 'unverifiable', reason: { deep: true } } as never
+      }
+    })
+    expect(evidence.foreground).toEqual({ verdict: 'unverifiable', reason: 'unspecified' })
+    expect(evidence.children).toEqual({ verdict: 'unverifiable', reason: 'unspecified' })
+  })
+
   it('synthesizes the legacy reading when the host predates evidence', () => {
     expect(
       readPtyProcessInspectionEvidence({ foregroundProcess: 'codex', hasChildProcesses: true })
diff --git a/src/shared/pty-process-inspection-evidence.ts b/src/shared/pty-process-inspection-evidence.ts
index e501738aa85..d2736617417 100644
--- a/src/shared/pty-process-inspection-evidence.ts
+++ b/src/shared/pty-process-inspection-evidence.ts
@@ -78,14 +78,24 @@ export function readPtyProcessInspectionEvidence(result: {
   }
 }
 
+// Field types are validated, not just verdicts: a foreign host can put any
+// JSON in these slots, and an out-of-type payload must degrade to
+// `unverifiable` — never ride an `observed` verdict into the exit gate.
+function normalizeReason(reason: unknown): string {
+  return typeof reason === 'string' ? reason : 'unspecified'
+}
+
 function normalizeForegroundEvidence(
   evidence: PtyForegroundProcessEvidence | undefined
 ): PtyForegroundProcessEvidence {
   if (evidence?.verdict === 'observed') {
-    return { verdict: 'observed', processName: evidence.processName ?? null }
+    const processName = evidence.processName ?? null
+    if (processName === null || typeof processName === 'string') {
+      return { verdict: 'observed', processName }
+    }
   }
   if (evidence?.verdict === 'unverifiable') {
-    return { verdict: 'unverifiable', reason: evidence.reason ?? 'unspecified' }
+    return { verdict: 'unverifiable', reason: normalizeReason(evidence.reason) }
   }
   return { verdict: 'unverifiable', reason: 'malformed foreground inspection evidence' }
 }
@@ -97,7 +107,7 @@ function normalizeChildrenEvidence(
     return { verdict: evidence.verdict }
   }
   if (evidence?.verdict === 'unverifiable') {
-    return { verdict: 'unverifiable', reason: evidence.reason ?? 'unspecified' }
+    return { verdict: 'unverifiable', reason: normalizeReason(evidence.reason) }
   }
   return { verdict: 'unverifiable', reason: 'malformed child-process inspection evidence' }
 }