diff --git a/.agents/prompts/build.txt b/.agents/prompts/build.txt
index 68f6fbe14..77d4462ed 100644
--- a/.agents/prompts/build.txt
+++ b/.agents/prompts/build.txt
@@ -246,6 +246,33 @@ When referencing specific functions or code include the pattern `file_path:line_
   - Immediately warn: "That looks like a credential. Conversation transcripts are stored on disk — treat this value as compromised. Rotate it and store the new value via `aidevops secret set NAME` in your terminal."
   - Do NOT repeat, echo, or reference the pasted credential value in your response
   - Continue helping with the task using a placeholder like `<YOUR_API_KEY>` instead
+#
+# 8.2 Secret as command argument exposure (t4939)
+#    Threat: a secret passed as a command argument (not an env var) can be echoed
+#    back in error messages, appear in `ps` output, and leak into logs — even when
+#    the command's *intent* is safe (e.g., a DB insert). The agent assesses the
+#    command as safe because it's not a `cat` or `echo`, but any program can print
+#    its argv on failure. Error paths are invisible at invocation time.
+#    Incident: qs-agency migration — WEBHOOK_SECRET interpolated into `wp db query`
+#    SQL argument, WP-CLI printed the full argument on parse failure, secret entered
+#    conversation transcript. Required immediate rotation.
+- When a subprocess needs a secret value, ALWAYS pass it as an environment variable, NEVER as a command argument. Command arguments appear in error messages, `ps` output, and logs. Environment variables do not.
+  - UNSAFE: `SECRET=$(gopass show -o name) cmd "INSERT INTO t VALUES ('$SECRET')"` — if `cmd` fails, it may print the argument including the secret
+  - UNSAFE: `curl -H "Authorization: Bearer $TOKEN" ...` — `ps` shows the full command line; error output may echo headers
+  - UNSAFE: `mysql -p"$PASSWORD" dbname` — password visible in process list
+  - SAFE: `SECRET=$(gopass show -o name) MY_SECRET="$SECRET" cmd` — subprocess reads via `getenv("MY_SECRET")`; error handlers never print env vars
+  - SAFE: `aidevops secret NAME -- cmd` — injects as env var with automatic output redaction
+  - SAFE: `SSH_AUTH_SOCK=... ssh ...` — env-based auth, no secret in argv
+  - The subprocess must read the value from its environment (`getenv()` in C/PHP, `process.env` in Node, `os.environ` in Python, `ENV[]` in Ruby), not from `$1`/`argv`.
+  - When the target program only accepts secrets as arguments (no env var support), write the secret to a temporary file (mode 0600), pass the file path as the argument, and delete the file immediately after. This is a last resort — prefer programs that support env var or stdin input.
+  - For SSH/remote commands: `ssh host "ENV_VAR='value' command"` passes the secret in the remote shell's environment, not as an argument to `ssh` itself. Alternatively, use `ssh -o SendEnv=VAR` with server-side `AcceptEnv` configuration.
+#
+# 8.3 Post-execution secret detection (t4939, layer 2)
+#    After any Bash command whose input references a credential variable (gopass,
+#    $*_SECRET, $*_TOKEN, $*_KEY, $*_PASSWORD), verify the output doesn't contain
+#    the secret value before presenting it to the user.
+- After running any command that references a credential variable, assess whether the output could contain the secret value. If the command failed (non-zero exit) and the secret was passed as an argument (violating 8.2), assume the output is contaminated — do not present it to the user. Flag for immediate credential rotation.
+- This is a judgment call, not a regex check. The agent knows which variables contain secrets and can assess whether output looks like it contains credential material (long base64 strings, API key patterns, JSON with auth fields).
 - Confirm destructive operations before execution
 - NEVER create files in `~/` root - use `~/.aidevops/.agent-workspace/work/[project]/`
 - Do not commit files containing secrets (.env, credentials.json, etc.)
diff --git a/.agents/services/hosting/cloudron.md b/.agents/services/hosting/cloudron.md
index 6f930773e..6702cf837 100644
--- a/.agents/services/hosting/cloudron.md
+++ b/.agents/services/hosting/cloudron.md
@@ -290,7 +290,7 @@ docker exec -it mysql mysql -u<username> -p<password> <database>
 docker exec -it mysql mysql -uroot -p"$(cat /home/yellowtent/platformdata/mysql/root_password)"
 ```
 
-> **Security note**: The `docker inspect` command above reveals database credentials. Redact passwords before pasting output into forum posts, tickets, or chat. The `-p$(cat ...)` pattern briefly exposes the password in the process list while the command runs.
+> **Security note**: The `docker inspect` command above reveals database credentials. Redact passwords before pasting output into forum posts, tickets, or chat. The `-p$(cat ...)` pattern briefly exposes the password in the process list while the command runs. Prefer passing credentials via environment variables instead of command arguments where possible (see `prompts/build.txt` section 8.2).
 
 #### **Common Database Fixes**
 
diff --git a/.agents/tools/credentials/gopass.md b/.agents/tools/credentials/gopass.md
index 350c18769..9b7ef2025 100644
--- a/.agents/tools/credentials/gopass.md
+++ b/.agents/tools/credentials/gopass.md
@@ -154,12 +154,15 @@ Agent should start with this warning in chat:
 3. Agent uses secret via: `aidevops secret SECRET_NAME -- command`
 4. Output is automatically redacted
 
+**Env var, not argument**: When passing secrets to subprocesses, ALWAYS use environment variables, never command arguments. Arguments appear in `ps`, error messages, and logs -- even when the command's intent is safe (e.g., a DB insert). Use `aidevops secret NAME -- cmd` which injects as env var automatically, or `MY_SECRET="$value" cmd` where the subprocess reads via `getenv()`. See `prompts/build.txt` section 8.2 for the full rule.
+
 **Prohibited commands** (NEVER run in agent context):
 
 - `gopass show` / `gopass cat` -- prints secret values
 - `cat ~/.config/aidevops/credentials.sh` -- exposes plaintext
 - `echo $SECRET_NAME` -- leaks to agent context
 - `env | grep` -- exposes environment variables
+- `cmd "$SECRET"` -- secret as command argument, visible in `ps` and error output
 
 ## psst Alternative
 
diff --git a/.agents/tools/security/opsec.md b/.agents/tools/security/opsec.md
index 4e8723485..9bd14e76a 100644
--- a/.agents/tools/security/opsec.md
+++ b/.agents/tools/security/opsec.md
@@ -80,6 +80,7 @@ Session safety model for AI-assisted terminals:
 - Prefer key-name checks, masked previews, or fingerprints over raw value display.
 - Avoid writing raw secrets to temporary files (`/tmp/*`) where possible; prefer in-memory handling and immediate cleanup.
 - If a command cannot be made secret-safe, do not run it via AI tools. Instruct the user to run it locally and never ask them to paste the output.
+- **Env var, not argument (t4939)**: When a subprocess needs a secret, pass it as an environment variable, never as a command argument. Arguments appear in `ps`, error messages, and logs. Use `aidevops secret NAME -- cmd` (auto-injects as env var with redaction) or `MY_SECRET="$value" cmd` where the subprocess reads via `getenv()`. See `prompts/build.txt` section 8.2 for the full rule and safe/unsafe patterns.
 
 ## Platform Trust Matrix