agent-interlude

agent-interlude

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agent-interlude intercepts the traffic between an AI coding agent (Claude Code, Codex) and its API, persisting the prompt structure (system / tools / messages) of every request/response pair as JSONL. Use it to analyze the fixed skeleton vs. dynamic slots of a prompt, and to compare across agents.

Screenshots

The built-in web UI (agent-interlude-report serve) is a dark monitoring console for browsing captured traffic.

Timeline — per-model token usage and cache-hit rate, stop-reason breakdown, a token-usage chart, and every request/response grouped into sessions and threads.

Conversation graph — one session projected as nodes and edges: continuations, retries, and the tools invoked at each step.

How it works

Both agents let you override the API base URL via an environment variable, so no transparent MITM or certificate forgery is needed. agent-interlude is an explicit reverse proxy:

Claude Code ──(A) plain HTTP──▶ agent-interlude proxy ──(B) normal HTTPS──▶ api.anthropic.com
              localhost:8788                      (proxy re-encrypts as the client)

Segment (A) has no TLS, so the proxy reads the plaintext body straight off the socket — that's the interception point, and it never touches credentials. Responses are copied as they stream through a relay, then the SSE events are reassembled and archived once the stream ends. The agent notices nothing.

Install

Install once with pipx (recommended) or uv tool — both put the agent-interlude command on your PATH in an isolated environment, no project-level setup needed:

pipx install agent-interlude
# or
uv tool install agent-interlude

Requires Python 3.11+ and the agent CLIs you want to capture (claude and/or codex). Zero runtime dependencies — agent-interlude is stdlib-only.

For contributors hacking on agent-interlude itself, see Development setup below.

Quick start

# 1. One command: starts the 3 proxy listeners AND the web UI on :8000
agent-interlude

# 2. In another terminal, point Claude Code at it
ANTHROPIC_BASE_URL=http://localhost:8788 claude

# 3. Open the live browser UI as captures stream in
open http://127.0.0.1:8000/timeline

On startup the bundled launcher prints:

[agent-interlude] claude: http://127.0.0.1:8788 -> https://api.anthropic.com
[agent-interlude] codex:  http://127.0.0.1:8789 -> https://api.openai.com   (Codex + API key)
[agent-interlude] codex:  http://127.0.0.1:8790 -> https://chatgpt.com      (Codex + ChatGPT login)
[agent-interlude] logging to .agent-interlude/log-<timestamp>.jsonl
[agent-interlude] web UI: http://127.0.0.1:8000/timeline (auto-started; disable with --no-ui)
[agent-interlude-report] http://127.0.0.1:8000
[agent-interlude-report] watching .agent-interlude/log-*.jsonl
[agent-interlude-report] auto-reload on (disable with --no-reload)

.agent-interlude/log-<timestamp>.jsonl lands under your current working directory (not next to the installed module), so run agent-interlude from wherever you want the logs collected.

The web UI runs in a child process. Ctrl-C on agent-interlude tears down both proxy and UI cleanly.

Each launch opens a fresh log file; every request prints one line such as [claude] POST /v1/messages.

Variants:

agent-interlude --no-ui            # proxy-only (e.g. CI / headless capture)
agent-interlude --ui-port 9000     # bind the UI on a different port
agent-interlude-report serve       # UI only, against existing logs
agent-interlude-analyze            # text report, no server
python -m agent_interlude          # module-form, equivalent to `agent-interlude`

Development setup

To hack on agent-interlude itself, clone and use the source layout directly:

git clone https://github.com/zondatw/agent-interlude.git
cd agent-interlude
uv sync                              # installs the package in editable mode
uv run agent-interlude                     # runs from src/agent_interlude/

For contributors: install pre-commit and gitleaks (brew install pre-commit gitleaks), then run pre-commit install once. Subsequent git commit will auto-run ruff lint+format, hygiene checks (trailing whitespace, EOF, private keys, yaml/toml syntax), codespell, and gitleaks. Run pre-commit run --all-files to check the whole tree at once.

Release flow (PyPI Trusted Publishing via the beta and release branches) is documented in docs/release.md.

Pointing an agent at the proxy

Claude Code

An environment variable is enough (Claude Code appends /v1/messages to the base URL itself):

ANTHROPIC_BASE_URL=http://localhost:8788 claude
# Non-interactive:
ANTHROPIC_BASE_URL=http://localhost:8788 claude -p "say hi"

Codex

Codex's built-in openai provider does not honor a base-URL override (OPENAI_BASE_URL is ignored), so you must define a custom provider. Pick the route that matches your login method.

A. ChatGPT login (recommended, no API key)

Point the custom provider at the proxy's chatgpt.com listener (port 8790, path /backend-api/codex). Codex sends your ChatGPT token, the proxy forwards to the real https://chatgpt.com/backend-api/codex/responses, and the response is recorded too:

codex exec -s read-only \
  -c model_provider=agent-interlude \
  -c 'model_providers.agent-interlude.base_url="http://localhost:8790/backend-api/codex"' \
  -c 'model_providers.agent-interlude.wire_api="responses"' \
  "say hi"

For a durable setup, write it into ~/.codex/config.toml:

[model_providers.agent-interlude]
name = "agent-interlude"
base_url = "http://localhost:8790/backend-api/codex"
wire_api = "responses"

Then switch to it per-invocation with -c model_provider=agent-interlude (do not set a top-level model_provider, or Codex breaks whenever the proxy is down):

codex -c model_provider=agent-interlude exec -s read-only "say hi"

B. OpenAI API key

If you have an OPENAI_API_KEY with the api.responses.write scope, point at the proxy's api.openai.com listener instead (port 8789, path /v1):

codex exec -s read-only \
  -c model_provider=agent-interlude \
  -c 'model_providers.agent-interlude.base_url="http://localhost:8789/v1"' \
  -c 'model_providers.agent-interlude.wire_api="responses"' \
  "say hi"

Note — Using ChatGPT login but pointing at api.openai.com (route B without a key) returns 401 (the ChatGPT token lacks the api.responses.write scope). The request is still recorded in full; you just get no response back — switch to route A instead.

What gets recorded

Logs live in .agent-interlude/log-<timestamp>.jsonl. Each exchange is two lines paired by id:

// kind="request"
{"id":"ab12…","kind":"request","agent":"claude","wire":"claude-messages",
 "headers_kept":{…},                  // authorization / x-api-key already filtered out
 "request":{…full parsed body…},
 "extract":{"system":…,"tools":…,"messages":…}}

// kind="response" (same id)
{"id":"ab12…","kind":"response","agent":"claude","status":200,
 "stream":true,"event_count":7,"event_types":{…},
 "reconstructed":{"model":"…","text":"…","usage":{…},"tool_uses":[…]}}

A non-streaming response (e.g. Codex's 401) is recorded as "stream":false,"body":{…} instead.

Supported wire formats: claude-messages (/v1/messages), codex-responses (/responses), codex-chat (/chat/completions).

Analysis

agent-interlude-analyze                    # read every log in .agent-interlude
agent-interlude-analyze --agent claude     # one agent only
agent-interlude-analyze --max-slots 30     # print more dynamic slots
agent-interlude-analyze path/to/log.jsonl  # a specific file / glob

The report covers:

• Each agent's system size and fixed skeleton vs. dynamic slots (e.g. the git status and date that Claude injects are flagged as dynamic slots).
• The tools list, count, and schema key (Claude=input_schema / Codex=parameters).
• A cross-agent structure comparison table.

To surface Codex's dynamic slots, run a few sessions with different prompts / at different times (multiple retries of the same prompt share one system prompt and count as just 1 distinct sample).

Web UI

For a browsable view of the same data — with per-request drill-in, skeleton-vs-slot highlighting in context, and a tools schema browser — launch the local web UI:

agent-interlude-report serve                  # http://127.0.0.1:8000 (default)
agent-interlude-report serve --port 9000
agent-interlude-report serve --logs "other/path/log-*.jsonl"

Routes:

Every HTML page has a matching /api/<same path> endpoint that returns the same data as JSON — built in from day one so future features (token usage charts, search/filter, live update) consume a stable backend instead of re-scraping HTML. The page nav surfaces the JSON URL on every view.

Bound to 127.0.0.1 only (the logs hold full prompts; never expose them on LAN). The JSONL loader is mtime-cached, so re-reads stay cheap while the proxy keeps appending — just refresh the page to see new captures.

One-command end-to-end verification

./dogfood.sh

Starts the proxy → fires one Claude and one Codex call → verifies both request and response were recorded with zero credential leakage → tears the proxy down, and finally prints RESULT: PASS.

Manual verification (step by step)

To confirm each link in the chain by hand (rather than just running dogfood.sh):

Terminal 1 — start the proxy:

agent-interlude

Terminal 2 — send one message through Claude Code; it should reply PONG (proving the relay + streaming are intact):

ANTHROPIC_BASE_URL=http://localhost:8788 claude -p "Reply with exactly the word PONG and nothing else."

Back in Terminal 1, the proxy console should show a line: [claude] POST /v1/messages.

Check the log landed (a structural summary that does not dump prompt contents):

LOG=$(ls -t .agent-interlude/log-*.jsonl | head -1)
uv run python - "$LOG" <<'PY'
import json, re, sys
recs = [json.loads(l) for l in open(sys.argv[1], encoding="utf-8")]
for r in recs:
    if r.get("kind", "request") == "request":
        ex = r.get("extract") or {}
        present = [k for k in ("system", "tools", "messages") if ex.get(k) is not None]
        print(f"REQ  {r['agent']:<7} {r['wire']:<16} extract={present}")
    else:
        txt = (r.get("reconstructed") or {}).get("text")
        info = f"text={txt!r}" if r.get("stream") else f"body={type(r.get('body')).__name__}"
        print(f"RESP {r['agent']:<7} status={r['status']:<3} {info[:70]}")
blob = "\n".join(json.dumps(r) for r in recs)
leaks = re.findall(r"Bearer\s+\S{20,}|sk-ant-\S{20,}|eyJ[\w-]{10,}\.eyJ[\w-]{10,}", blob)
print("\ncredential leaks:", len(leaks))
PY

You should see at least one pair:

REQ  claude  claude-messages  extract=['system', 'tools', 'messages']
RESP claude  status=200 text='PONG'

credential leaks: 0

(The first line may be REQ claude unknown → RESP claude status=404; that's Claude Code's connection pre-check HEAD / and can be ignored.)

View the structure analysis:

agent-interlude-analyze

When done, press Ctrl-C in Terminal 1 to shut the proxy down.

Security notes

• .agent-interlude/ contains the full prompt (your code, possibly secrets) → it is gitignored; do not commit or share it.
• Auth headers (authorization / x-api-key / cookie) are forwarded only and never written to the log; headers_kept retains an allowlist of fields only.
• The proxy strips the request's accept-encoding, so the recorded bytes are always plaintext (no gzip/br to deal with).
• To check for rogue connections: lsof -nP -iTCP -sTCP:ESTABLISHED | grep Python, and confirm the proxy only connects to api.anthropic.com / api.openai.com / chatgpt.com.

Adding a new agent

Edit the LISTENERS list at the top of src/agent_interlude/proxy.py and add a row (port, upstream_host, label). Wire detection lives in detect_wire(), and field normalization in extract() (requests) and reconstruct() (responses).

Troubleshooting

Files