diff --git a/test/helpers/shared-libs-eval-fixture.ts b/test/helpers/shared-libs-eval-fixture.ts
index 284e8a7ea..88e4b76c3 100644
--- a/test/helpers/shared-libs-eval-fixture.ts
+++ b/test/helpers/shared-libs-eval-fixture.ts
@@ -426,6 +426,8 @@ function sharedShellTokens(command: string): SharedShellToken[] {
   return tokens;
 }
 
+const SHARED_OUTPUT_DEVICES = ['/dev/null', '/dev/stdout', '/dev/stderr', '/dev/fd/1', '/dev/fd/2'];
+
 /** Share attempted-write checks across native, semantic and Codex standalone captures. */
 export function sharedReadOnlyViolations(toolCalls: Array<{ tool: string; input: any }>, requests: SourceRequest[] = []): string[] {
   const violations: string[] = [];
@@ -448,10 +450,17 @@ export function sharedReadOnlyViolations(toolCalls: Array<{ tool: string; input:
     };
     for (let i = 0; i < tokens.length; i++) {
       const token = tokens[i].value;
-      if (tokens[i].operator && ['>', '>>', '&>'].includes(token) && !['/dev/null', '/dev/stdout', '/dev/stderr', '/dev/fd/1', '/dev/fd/2'].includes(tokens[i + 1]?.value))
+      if (tokens[i].operator && ['>', '>>', '&>'].includes(token) && !SHARED_OUTPUT_DEVICES.includes(tokens[i + 1]?.value))
         violations.push('shell file output redirection');
       if (tokens[i].operator && token === '>&' && !['1', '2', '-'].includes(tokens[i + 1]?.value)) violations.push('shell file output redirection');
-      if (isCommand(i) && /(?:^|\/)tee$/.test(token) && tokens[i + 1] && !tokens[i + 1].operator) violations.push('tee file output');
+      if (isCommand(i) && /(?:^|\/)tee$/.test(token)) {
+        // Like a redirection, tee may only duplicate to the discard/stdout devices; any other operand is a file.
+        const operands = sharedShellCommandTokens(tokens, i + 1).map(operand => operand.value);
+        const end = operands.indexOf('--');
+        const files = end < 0 ? operands.filter(value => !value.startsWith('-') || value === '-')
+          : [...operands.slice(0, end).filter(value => !value.startsWith('-') || value === '-'), ...operands.slice(end + 1)];
+        if (files.some(file => !SHARED_OUTPUT_DEVICES.includes(file))) violations.push('tee file output');
+      }
       if (isCommand(i) && /(?:^|\/)curl$/.test(token)) {
         // URL variables resolve only in the instrumented process. Its request
         // record supplies endpoint validation; source text still reveals writes.
diff --git a/test/shared-libs-fixture.test.ts b/test/shared-libs-fixture.test.ts
index d62f37840..e847be614 100644
--- a/test/shared-libs-fixture.test.ts
+++ b/test/shared-libs-fixture.test.ts
@@ -904,6 +904,7 @@ describe('shared-code curl source isolation', () => {
       "curl -w '%output{/tmp/report}%{http_code}' https://api.github.com/repos/fixture/shared-libs",
       "curl -X POST https://api.github.com/repos/fixture/shared-libs",
       'printf data > /tmp/report', 'cat README.md >> "/tmp/report"', 'cat README.md | tee /tmp/report',
+      'cat README.md | tee /dev/null report.txt', 'cat README.md | tee -a report.txt', 'cat README.md | tee -- -a',
       "bash <<'SH'\nprintf data > /tmp/report\nSH\n",
       "cat <<'DATA'\njust data\nDATA\ncurl -o /tmp/report https://api.github.com/repos/fixture/shared-libs",
     ]) expect(sharedReadOnlyViolations(bash(command)).length, command).toBeGreaterThan(0);
@@ -913,6 +914,9 @@ describe('shared-code curl source isolation', () => {
       "curl -sS -o - -w 'http=%{http_code}\\n' https://api.github.com/repos/fixture/shared-libs",
       "curl -sS -m 15 -o /dev/null -w 'http=%{http_code}\\n' https://api.github.com/repos/fixture/shared-libs 2>&1",
       'gh auth status 2>&1 | head -5', 'git --no-lazy-fetch log 2>/dev/null',
+      // Paid opportunity-judgment t1 (1213b01): tee to the discard device writes no file.
+      'for p in src/version.ts README.md .gitignore; do echo "===== $p"; gh api --method GET -H "Accept: application/vnd.github.raw" "repos/fixture/shared-libs/contents/$p?ref=987f57ff2613724dd4a5509dda1b0fea155aab8a" 2>&1 | head -c 4000 | tee /dev/null | sha256sum; echo "exit=${PIPESTATUS[0]}"; done',
+      'cat README.md | tee -a /dev/stderr',
       "rg 'a > b' README.md", "rg '>' README.md", "rg '|' README.md",
       'rg tee README.md', 'rg curl README.md',
       "python3 - <<'PY'\nsize = 2\nif size > 1:\n    print(size)\nPY\n",