terse

The lossless-first MCP compression proxy: it makes tool output smaller without ever changing what your agent reads — byte-faithful by default, lossy only where you explicitly opt in.

terse reduces tokens two ways: one that carries the day-to-day value, and one that is harder for a competitor to copy. Keeping those straight is the whole positioning.

1. The lossless codec — the value. This is the lever that does the work. terse removes only structural overhead: pretty-print whitespace, keys repeated once per record, repeated values, repeated nested schema. The transformed bytes are the model's input — a denser but still legible representation, not an offload. There is no decode step, no ML model in the loop, and every transform has an exact inverse (a round-trip gate asserts decompress(compress(x)) == x over the whole corpus). This is the guarantee most tools in this space decline to make: headroom's JSON path is lossless on uniform arrays but falls back to dropping rows on larger/irregular record sets, recoverable only via a retrieve round-trip against a cache that expires (verified, v0.32.0; default 30-min TTL); Anthropic/OpenAI context-editing drops old tool results server-side. terse never silently mutates what the model sees — and "lossless" is the category, not the token count. In terse's own production ledger this codec is where essentially all the savings come from (see Status). Its one honest caveat: the tabularization primitive is public (formats like TOON publish it standalone, MIT-licensed, ~40% on flat arrays), so a motivated competitor could clone the codec in a weekend.

2. The stateful cross-call diff — the defensible axis. When the same tool is called again — poll a list, re-read a file — terse emits a lossless delta against the prior result instead of the whole payload (~73% smaller on the repeated call in the model below). This is the one axis a stateless encoder architecturally cannot reach: TOON, headroom's stateless per-call compressor, and server-side history-pruning all pay the full column every call because none of them remember the last result — terse can only do it because it lives in the session as a transparent proxy. That makes it the harder half to copy. But it is a bonus tier, not the headline: it only pays off when a workload actually repeats a call with a similar-enough payload. That measured ~0.4% of results in terse's own 7-day traffic — but most of that was structural, not workload: results arriving as N content blocks were excluded from diffing outright, which was 71% of tokens. The cross-block join (below) removed that exclusion, and across every third-party server benchmarked in BENCHMARKS §6 a repeated call now produces a delta. How often your loop repeats a call is still yours to measure (terse stats). When your loop does re-fetch mostly-unchanged results it compounds hard; when it doesn't, it costs nothing (lossless, and emitted only when smaller). Default-on since its validation program completed (see Status).

Around those two sits the bundle that turns a byte filter into a control plane you don't want to rip out: MCP-native proxy packaging (transparent to any downstream server, no client-side reformatting), a live savings ledger (terse stats), a fluency-gated lossy escape hatch, and self-installing ops tooling (install-mcp, mcp-status) — each diff/lossy tier validated by a behavioral eval before it was ever turned on by default.

It is selective by design. Measurement on real tool output showed the win is strongly per-tool (0–30%): large on record/symbol-shaped verbose output, near-zero on already-minified or already-projected tools. So terse applies per-tool policy rather than compressing everything blindly.

How It Works

terse transforms a tool's JSON output through a tiered, fully-lossless pipeline, then (optionally) serves it through a per-tool policy that decides which tiers run.

• Tier 0 — minify: strip insignificant whitespace.

• Tier 0 — tabularize: a list of uniform records becomes one header + value rows (keys written once, not once per record), recursively hoisting nested uniform-dict columns into a shared header.

• Tier 0.5 — dictionary code: repeated string values and repeated whole subtrees are folded into an inline legend (~0, ~1, …) proven disjoint from every literal in the payload. Committed only when it actually saves tokens, so it never regresses.

• Tier 0.7 — cross-call diff (stateful, OPT-IN — "diff": true): when the same tool is called repeatedly, the proxy emits a lossless delta against the prior result instead of the full payload (the 91%-overlap headroom). Self-describing, verified to reconstruct exactly, and emitted only when smaller — falls back to the full form otherwise. Default-on since its validation program completed (fluency, nested-record coverage, and the drift soak — see Status); opt out with proxy --no-diff / install-mcp --no-diff or a policy-file "diff": false. Record-shaped JSON gets a row/key diff; non-JSON results (file reads, source excerpts, log tails) get a separate content-defined-chunking (CDC) diff — a rolling hash cuts chunk boundaries by content, not position, so an edit anywhere only perturbs the chunk(s) it overlaps and the rest is sent as references to the prior result. Each shape keeps its own diff base per tool.

• Cross-block join (ON by default): some MCP servers return one record per content block, so each block is a lone object the codec above can barely fold and the diff tier skips entirely (it reasons about one logical payload). When every text block of a result is a JSON object, the proxy joins them into one record array before compressing — so tabularize/dictionary fold across records and the whole result becomes diff-eligible. This changes the number of content blocks the client sees (N → 1), which the MCP spec permits (block count carries no meaning). Opt out with proxy --no-join-blocks / install-mcp --no-join-blocks or a policy-file "join_blocks": false; lossy field rules still resolve per block, before the join.

• Tier 1 — lossy (opt-in, per field): truncate caps and annotates a field marked {"lossy":"truncate","max":N}, gated by an acceptable-loss check (only marked, non-critical fields may differ, each only as a valid truncation). drop-to-retrieve replaces a marked field with a handle, stores the original per session, and serves it back via a synthetic terse.retrieve tool the proxy injects — gated so a drop is accepted only if the handle resolves to the exact original. summarize (needs a model) is still parsed but deferred — warned and left lossless. Off everywhere by default.

Every transform has an exact inverse, and a round-trip gate asserts decompress(compress(x)) == x over the whole corpus. The transformed bytes are the model's input — a denser but still-readable representation, not an offload.

The proxy also keeps a live savings ledger (on by default; --no-stats to opt out): one payload-free JSONL record per result — sizes, tokens, and the decision taken, never content — so terse stats can answer "how much did terse actually save me this week?" from real sessions, not just the synthetic corpus.

Related MCP server: Refract

Install

Needs Python 3.11+.

uv tool install terse-mcp   # global `terse` CLI  (or: pipx install terse-mcp)

Or pip install terse-mcp into a virtualenv for library/embedded use.

Docker

docker build -t terse .
docker run -i --rm terse                      # proxies the bundled demo server
docker run -i --rm terse uvx some-mcp-server  # proxies a real one

-i is required — MCP stdio is stdin/stdout. Everything after the image name is the downstream server command; terse has no tools of its own, so with no downstream there is nothing to compress. The default is examples/demo_mcp_server.py, a stdlib-only server whose demo_orders tool returns a 40-record order book — you get back terse's compressed form of it (9,019 → 3,187 chars), which is the fastest way to see what the proxy does.

The image ships uv/uvx so uvx-launched servers work out of the box. Node is not installed — for an npx-launched server, build FROM this image and add it.

The build reads the version from git describe, so build from a normal git clone. Any context without a resolvable .git — a source tarball, or a git worktree whose .git is a pointer file — needs --build-arg TERSE_VERSION=0.3.1 instead.

Quick Start (under a minute)

terse sits between your MCP client and a server and shrinks the server's tool results in flight. No config needed — the proxy is lossless-everywhere by default:

# 1. Wrap ANY stdio MCP server. Your agent talks to it exactly as before;
#    terse compresses the results it returns, losslessly.
terse proxy -- uvx some-mcp-server --flags

# 2. See what it saved (the payload-free ledger is on by default):
terse stats

Want to eyeball the codec first, no server involved?

echo '[{"id":1,"state":"open","repo":"acme/widgets"},{"id":2,"state":"open","repo":"acme/widgets"},{"id":3,"state":"open","repo":"acme/widgets"},{"id":4,"state":"open","repo":"acme/widgets"},{"id":5,"state":"open","repo":"acme/widgets"},{"id":6,"state":"open","repo":"acme/widgets"}]' | terse gate -
# → round-trip lossless: PASS ; ~36% fewer cl100k tokens

(Savings grow with record count and repetition; on a single tiny object terse correctly declines and passes it through unchanged — it never inflates what it can't shrink.)

Does terse help my server?

The win is per-tool and terse only keeps what pays, so it never hurts — but it helps a lot more on some shapes than others. Point it at a server and run terse stats to see for real; as a rule of thumb:

Wire it into your MCP client (permanent)

install-mcp rewrites your MCP config to launch a server through terse — reversible, and transparent to the client. It needs a per-tool policy; the smallest useful one is:

echo '{"version":1,"defaults":{"tiers":["minify","tabularize","dictionary"]}}' > terse-policy.json

# Claude Code, user scope (~/.claude.json) — wrap a server you've already registered by name:
terse install-mcp --policy terse-policy.json <server-name>

# Project scope (a committed .mcp.json instead):
terse install-mcp --policy terse-policy.json --scope project --file .mcp.json <server-name>

terse mcp-status                       # confirm what's wrapped
terse uninstall-mcp <server-name>      # cleanly restore the original entry

Other MCP clients (Cursor, etc.) read the same config shape — wherever a server is launched as cmd --flags, launch it as terse proxy -- cmd --flags to get the same effect. See USAGE.md for tuning a policy (terse tune) and reading terse stats.

From source (contributors): uv sync then uv run terse ...; uv run pytest is the lossless gate.

Project Structure

src/terse/
  transforms.py  lossless tiers (minify, tabularize, dict coding) + round-trip gate
  policy.py      selective per-tool policy: load, match, apply
  proxy.py       MCP stdio middleware: compress a downstream server's tool results
  stats.py       live savings ledger (payload-free) + the `terse stats` aggregation
  capture.py     corpus capture (shape-tagged envelopes) + shape classifier
  measure.py     per-payload + cross-tokenizer token measurement
  probes.py      value-redundancy + cross-call-overlap ceiling probes
  fluency/       does a model read the compressed form as accurately as raw JSON?
                 (questions / scoring / answerers / harnesses / pack behind one facade)
  tokenize.py    cl100k / o200k token counting
  report.py      markdown reports (savings, per-tool, probes, tokenizer, fluency)
  html_report.py charted HTML companion (inline SVG, no JS/CDN) for measure/verify
  cli.py         entrypoint: gate / policy / compress / capture / measure / probe / validate /
                 proxy / stats / fluency / tune / install-mcp / uninstall-mcp / mcp-status / verify
scripts/
  gen_stress_corpus.py  synthetic stress corpus for the fluency eval
  bench/                terse-vs-TOON token benchmark on a real GitHub-API corpus
                        (fetch_corpus.sh, benchmark.py, diff_demo.py, toon_encode.mjs)
  bench/mcp_servers/    what terse does, zero-config, to popular third-party MCP servers
                        (mcp_probe.py harness + pinned repo/web fixtures; BENCHMARKS §6)
examples/
  demo_mcp_server.py    stdlib-only stdio MCP server; the container's default downstream,
                        so `docker run` demonstrates the proxy without a real server
tests/           round-trip, measurement, probe, policy, and fluency tests
Dockerfile       terse + the demo downstream, for registries and one-command trials
policy.example.json   selective policy encoding the measured per-tool insight
corpus/          captured tool outputs (gitignored; may contain real data)

Verify it yourself

terse sits in your agent's critical path, so it earns trust by inspection. See VERIFY.md for the full walkthrough — or generate a self-contained report (lossless gate + per-tool token savings) in one command:

terse verify --out reports/verify-report.md          # bundled sample, zero setup
terse verify --corpus corpus --out report.md         # your own captured traffic
terse verify --html --out reports/verify-report.md   # + a charted HTML report alongside it
terse verify --corpus corpus --json                  # machine-readable gate + savings (CI-checkable)

Benchmarks: terse vs alternatives

A head-to-head token-reduction benchmark on a corpus of real, public GitHub API payloads (scripts/bench/) — the nested, record-shaped output that dominates real MCP tool traffic. Everything below is lossless and verified per payload (a row is dropped from the total if either encoder fails its round-trip), counted in cl100k_base (the same tokenizer terse uses). Reproduce end-to-end:

cd scripts/bench && npm install          # pins the official @toon-format/toon encoder
./fetch_corpus.sh                        # OR use the committed corpus snapshot as-is
uv run scripts/bench/benchmark.py        # terse vs TOON vs baselines
uv run scripts/bench/diff_demo.py        # terse's cross-call diff (its own axis)

The only directly-comparable public tool is TOON — a lossless encoding that shares terse's tabularization primitive. The corpus is compact JSON (no pretty-print whitespace), so minify saves ~0% and every number below is pure structural gain, the hardest honest case:

(% = fewer cl100k tokens than raw; higher is better; bold = winner.)

Honest reading of this:

• On real nested API records, terse wins decisively and TOON regresses (−7.1% overall — worse than raw). TOON is built for flat, uniform arrays; GitHub records are deeply nested and non-uniform (a PR embeds repeated user/head/base/repo subtrees), which terse's dictionary tier folds and TOON's tabular layout cannot — it adds key-path overhead instead. terse's headline 76% on gh_pulls is exactly this: 60 repeated copies of the same repo object collapsed to one legend entry.

• TOON is not beaten everywhere — and the boundary is value repetition, not column width. On gh_labels (9 records × 7 columns — TOON's designed sweet spot) TOON leads, +19.0% vs terse's +15.2%. terse's decisive corpus win comes from nested repeated subtrees and long repeated string values — its dictionary and subtree-aliasing tiers fold them, TOON's flat tabular layout cannot — which is exactly what real GitHub records carry. On stripped-flat synthetic tables with none of that redundancy, the two converge: a seeded column-width sweep (uv run scripts/bench/width_sweep.py) shows them within a few points at every width, trading the lead by parity, with no clean column-count crossover (an earlier claim of a ≤3/≥4 boundary did not reproduce — see BENCHMARKS.md). So the honest frame is: terse wins where records repeat or nest (real tool output); TOON stays competitive on flat, low-redundancy uniform tables — "different niche," not "terse strictly dominant."

• Neither tool helps much when free text dominates (gh_commits_flat: long commit messages, ~2% either way) or on tiny single objects — matching terse's own "selective, 0–30%, per-tool" claim rather than contradicting it. Cross-call diff — the axis no stateless encoding has. When the same tool is called again (poll a list, re-read a file), terse emits a lossless delta against the prior result instead of the whole payload. TOON, minify, and terse's own single-shot codec all pay the full column every call. Modeling one repeated call per payload (two records changed, one appended — the poll-again pattern), the second call costs (uv run scripts/bench/diff_demo.py):

The diff cost scales with what changed, not with payload size — so its win compounds exactly where token cost otherwise does: a long agent loop re-fetching mostly-unchanged results. (gh_workflow_runs is lower here only because its records are large and this model changed a big nested field; a status/timestamp churn would diff far smaller.) This is on top of the single-shot reduction above, and stacks with it. These figures are the diff's win when it fires on a repeated call; how often that actually happens is a separate question, and workload-dependent — measure yours with terse stats. Treat this as a defensible bonus tier, not the headline lever (that's the codec — see the positioning note at the top and Status).

Tools not benchmarked head-to-head, and why (no invented numbers):

Adoption honesty: terse is new (just published to PyPI, few/no stars); TOON (24.9k★) and headroom (widely adopted, star figures cited vary ~29–49k) are far more established. terse's defensible wedge is narrow and specific — unconditionally lossless (no expiring retrieve-cache), no ML dependency, MCP-transparent, plus cross-call diffing — not breadth of adoption.

Related Documentation

• Benchmarks — dated, reproducible numbers: terse-vs-TOON (§1–2), the cross-call diff axis (§3), competitors (§4), the live production ledger (§5), and popular third-party MCP servers + a repo-size sweep (§6)

• Verify it yourself — prove losslessness, savings, and no-egress locally

• Technical Reference — architecture, pipeline, policy schema, limitations

• Usage Guide — running the CLI day-to-day and reading its output

• Changelog — notable changes per release

Status

A working, measured, selective lossless library, CLI, and MCP stdio proxy. The proxy's open question — does a model read the compressed form as well as raw JSON? — now has a measured answer: on a stress corpus, Claude Haiku 4.5 and Gemini 2.5 Flash match raw-JSON accuracy on the compressed form (100% paired) at a 37% token saving (terse fluency; see TECHNICAL.md). Whole-subtree aliasing (folding repeated objects, not just strings) is built. Cross-call diffing is a lossless tier that is off by default (#170) — not for lack of confidence, but on cost: its primer paragraph is 190 of 402 cl100k tokens, re-read every turn per wrapped server, against a measured 0.38% hit rate, so the explanation cost ~900–2,700x what the tier saved. Its full validation program did pass: pair fluency (fluency --diff, 4-model panel 100%), the nested-record surface (structure: diff 100% vs full-terse 94%), and long-chain drift soaked from both sides — mechanically (tests/test_diff_soak.py — exact reconstruction hundreds of chained hops deep) and behaviorally (fluency --diff-soak — no depth-correlated accuracy loss up to the keyframe bound). Opt IN per proxy (--diff) or per policy ("diff": true) — worth it for a workload that really does re-call the same tool with the same arguments. Cross-block joining (N content blocks folded into one record array before compressing) is built and on by default — it removed the structural exclusion that kept 71% of real traffic out of the diff tier entirely. The Tier 1 lossy modes truncate and drop-to-retrieve are built (opt-in, off by default); summarize remains designed but not yet built — see TECHNICAL.md "Known Limitations".

Evidence now spans three kinds: a fixed public corpus (BENCHMARKS §1–4), terse's own live production ledger (§5), and popular third-party MCP servers measured zero-config with pinned fixtures (§6) — filesystem, git, memory, fetch, plus serena and playwright-mcp. The §6 headline: the codec pays on JSON output (18–58%, depending on whether the server pretty-prints) while every text-shaped tool is 0% one-shot yet still wins on a repeat — so the codec is the JSON-specific lever and the diff is the broad, shape-independent one.