The narrow passage your
tool outputs squeeze through.

Measurement before rewriting

Verbose tool outputs are a large share of context on some workflows and a rounding error on others. Shadow mode (ISTHMOS_SHADOW=1) logs what rules would save on your real traffic before a single byte is rewritten.

Rule-based pruning

Tool names are glob-matched; listed keys are dropped recursively. max_items caps arrays, max_str caps strings, min_bytes leaves small payloads alone. When rules overlap, the strictest positive limit wins.

Text that keeps its errors

max_lines keeps head and tail, dedup collapses repeated lines into a labelled count, and error-looking lines (error, panic, traceback) are never dropped. That is what an agent is usually looking for.

Reversible truncation

Cut payloads are encrypted (AES-256-GCM) into a local store and the marker carries the recovery command: isthmos reveal <id>. If the store cannot be written, isthmos does not truncate at all.

Fail-open, out of the credential path

No proxy in front of your API keys. One static binary with a fast cold start, and any error means untouched passthrough. A replacement is only ever emitted when the result is strictly smaller.

Adapters, not lock-in

A native Claude Code PostToolUse hook, a generic filter mode for anything that can pipe through a command, and an importable Go package. The core is agent-agnostic.

Claude Code (native hook)

{
  "hooks": {
    "PostToolUse": [
      {
        "matcher": "mcp__.*",
        "hooks": [
          {"type": "command",
           "command": "$HOME/.local/bin/isthmos hook",
           "timeout": 5}
        ]
      }
    ]
  }
}

Rewrites tool_response in place via updatedToolOutput. The example points at $HOME/.local/bin; adjust it to wherever your install landed (which isthmos, Homebrew uses its own prefix).

Any other agent

# stdin in, pruned stdout out
some-tool --json | isthmos filter -tool mcp__github__search_repos

# verify the wiring
isthmos doctor

Wire it into any wrapper, shell function, or orchestrator that can interpose a pipe. doctor exits non-zero only when something is actually broken.