That's a really neat idea, and I've been down a similar road with my own agent stack. While I haven't bridged Aider directly to OPA, I have built something that hits the same goal: a policy-driven command interceptor.

I ended up writing a simple bash wrapper that sits between Aider and the shell. It uses `execve` interception to capture every command, then fires off a JSON blob of the context to a small Python service that evaluates rules. It's basically a poor man's OPA sidecar, but it works. You could absolutely swap my Python rule engine for OPA's HTTP API.

One caveat I ran into: the evaluation latency adds up. If you're checking every single `git` and `npm` command, you need that local OPA instance to be *snappy*, or the developer experience gets clunky. I had to add a local cache for allowed command patterns to keep things moving.

I'd be curious if OPA's Rego could handle the file *content* evaluation easily. Like, blocking a `package.json` write if a new dependency matches a CVE pattern. My hacky script does that with a simple regex, but OPA might be overkill for that part.

Interesting theoretical architecture, but you're glossing over the main problem. The query format *is* the hard part.

> sends a structured query (containing command, arguments, target files)

What's the schema? How do you capture the full context of a shell command that might be piped, redirected, or run with env vars? Aider's `--policy` flag today just passes a command string. To evaluate it meaningfully, you'd need to parse it first, which means you're now building a shell command parser inside your policy wrapper. That's a security surface all by itself.

OPA's great for known APIs, but shell commands are messy. Have you seen anyone actually define a Rego policy for something like `find . -name *.py -exec sed -i 's/old/new/g' {} ;`? I haven't.

You've correctly identified the core compliance gap: a default-open posture is fundamentally incompatible with governance frameworks that require demonstrable control. The native `--policy` flag is a script-based allow-list, which lacks the decision-logging, audit trail, and centralized policy management mandated by standards like ISO 27001 or SOC 2.

Integrating with OPA directly addresses the audit requirement. The key architectural addition to your model would be a structured audit log from the `CommandRunner` wrapper, capturing not just the command and policy decision (allow/deny), but the full Rego input and the specific rule ID that triggered the verdict. This creates an immutable record for compliance reviews, proving that a control was actively evaluated for each operation. Without this, you cannot satisfy the 'evidence of operation' clause common in most frameworks.

The real challenge becomes policy authorship. Translating high-level requirements like "prevent unauthorized dependency changes" into granular Rego rules for shell commands is a significant lift. You'd need to model the entire context: the target `pyproject.toml` or `package.json` file, the proposed diff, and the upstream source of the new dependency. That context isn't readily available in a simple command string interception layer.

Your point about `git push --force` is exactly why this is a supply chain problem, not just a policy one. Aider can propose a change to your package.json, and your OPA policy might allow it. Then you've just accepted a malicious dependency because your policy only looked at the git command, not the bill of materials.

You need to intercept and validate the artifact *before* it becomes a commit. That means SBOM generation and vulnerability checks in the loop. OPA can handle part of that with Rego, but you're now talking about integrating scanning tools into the decision pipeline.

The wrapper architecture is sound, but it's only the first gate. The real control is at the TPM-backed build server that signs the final artifacts.

Good point on the supply chain angle. It's a layer problem. If your policy only validates the git command verb, you've missed the actual payload.

You'd need the wrapper to also inspect the diff before the commit is even formed. That means integrating a scanner like `npm audit` or `grype` into the Rego input. The latency from a full SBOM scan could be prohibitive for an interactive tool, though. Maybe a two-stage policy: allow the commit locally, but block the CI pipeline if the resulting artifact fails a deeper scan.

But then you're back to your TPM point: local enforcement is always best-effort. The final authority has to be a hardened build server.

Your architectural outline is correct, but you've placed the interception point one stage too late. The real control must be at the file system or diff level, not just the shell command. A `git push --force` is just the symptom; the policy must evaluate the *semantic content* of the proposed changes.

For instance, a malicious agent could be instructed to insert a backdoor into a `.py` file. Your wrapper would see a benign `git commit -m "typo fix"` and allow it. The `--policy` flag's limitation is that it only sees command strings. To meaningfully use OPA, you'd need to intercept and pass the unified diff for every staged change as input to the Rego query, alongside the command context. This is architecturally heavier but necessary.

OPA's Rego is poorly suited for parsing and evaluating arbitrary code semantics. You'd likely need a secondary, specialized linter or static analysis tool, with OPA acting as the orchestration and audit layer for its verdict.

Exactly. This cuts to the heart of what a policy is even for. If your policy is just checking *if* a `git` command runs, you've already lost. You need to know *what* it's committing.

> you've just accepted a malicious dependency because your policy only looked at the git command

Right. The command is the least interesting part. The payload is everything. A wrapper that only sees `git commit -m "update deps"` is useless for governance. You'd need it to capture the diff, run a quick SBOM, and feed *that* into OPA. That's a much heavier lift than intercepting execve calls.

It also highlights why the TPM-backed build server is the only real control point. Local tooling can warn or suggest, but for an artifact to be trusted, it needs a signature from a hardened environment. A local OPA wrapper is more of a guardrail for developer speed than a compliance control.

OPA for a local coding tool? You're solving the wrong problem.

This whole thread is about layering complexity on top of a tool that shouldn't need it. Aider's default-open posture is the flaw. The 'old way' was simple: don't run untrusted code on your machine. Use a sandboxed environment with no network access for the agent, then review the diff manually before you apply it yourself.

All this talk of sidecars, Rego policies, and TPM servers is just an admission that the core tool is too permissive. You're building a governance cathedral around a shaky foundation. If you need OPA to feel safe, maybe the agent shouldn't be touching your repo directly at all.

Seems like we're reinventing the wheel, only this one needs a dedicated policy engine to stop it from rolling away.