Bash Output Filter

Overview

CLI tools are built for humans. They're chatty by design. When Claudin runs a command, all of that noise goes straight into the model's context — costing tokens and crowding out the signal that actually matters.

The Bash Output Filter intercepts every Bash tool result and removes progress bars, download lines, verbose headers, and passing test lines, while preserving errors, warnings, and actionable output.

On by default. No configuration required.

Measured across a typical 30-minute session after Phase 12 coverage (JS/Python/Go/Rust package managers, linters, git remote ops). Aggregate measurement across 30 Phase 12 sample fixtures: 75.1% byte reduction (36 170 B → 8 999 B). Reproducible via bun test src/outputFilter/Bash/phase12Report.test.ts.

Commands

The filter ships with built-in rules for the most common CLI tools. Each rule is conservative: on any failure condition, full output is preserved.

bundle install

Strips every Fetching <gem> line. Collapses to a one-liner on clean success. On failure (any error / conflict / could not find line), the full output is preserved.

Before

Fetching gem metadata from https://rubygems.org/..........
Fetching addressable 2.8.7
Fetching bigdecimal 3.1.8
Fetching concurrent-ruby 1.3.4
Fetching connection_pool 2.4.1
Fetching crass 1.0.6
Installing addressable 2.8.7
Installing bigdecimal 3.1.8
Installing concurrent-ruby 1.3.4
Bundle complete! 42 Gemfile dependencies, 187 gems now installed.

After

✓ bundle install completed

pytest

Strips warnings summary, deprecation notices, and PytestUnraisableExceptionWarning blocks. Caps at 100 lines. Collapses on a clean N passed result. On failure (FAILED / ERROR present), full output is preserved including tracebacks.

Before

============================= warnings summary ==============================
tests/test_auth.py::test_login
  /usr/local/lib/python3.11/site-packages/jwt/algorithms.py:393: CryptographyDeprecationWarning: ...
PytestDeprecationWarning: Support for nose-style setup/teardown is deprecated and will be removed.
============================= warnings summary ==============================
PASSED tests/test_auth.py::test_login
PASSED tests/test_api.py::test_health
PASSED tests/test_api.py::test_create_user
================================= 3 passed in 0.84s =================================

After

✓ pytest: all tests passed

rspec

Strips Randomized with seed N footer. Collapses on N examples, 0 failures.

Before

.......

Finished in 0.21394 seconds (files took 1.22 seconds to load)
7 examples, 0 failures

Randomized with seed 54321

After

Finished in 0.21394 seconds (files took 1.22 seconds to load)
7 examples, 0 failures

go test -v

Strips === RUN, === PAUSE, === CONT, --- PASS:, and bare PASS lines. Collapses on ok <pkg> <time>.

Before

=== RUN   TestHandleRequest
=== RUN   TestHandleRequest/valid_payload
=== RUN   TestHandleRequest/missing_field
--- PASS: TestHandleRequest/valid_payload (0.00s)
--- PASS: TestHandleRequest/missing_field (0.00s)
--- PASS: TestHandleRequest (0.00s)
=== RUN   TestParseConfig
--- PASS: TestParseConfig (0.00s)
PASS
ok  	github.com/myorg/myapp	0.015s

After

✓ go test: all tests passed

jest

Strips RUNS ... carousel lines and indented ✓ test name (Nms) per-test lines. Collapses on Tests: N passed, N total.

Before

 RUNS  src/utils/format.test.ts
 RUNS  src/api/client.test.ts
 PASS  src/utils/format.test.ts
  ✓ formats currency correctly (3ms)
  ✓ handles null input (1ms)
  ✓ rounds to 2 decimal places (1ms)
 PASS  src/api/client.test.ts
  ✓ sends correct headers (12ms)
  ✓ retries on 429 (45ms)

Test Suites: 2 passed, 2 total
Tests:       5 passed, 5 total
Snapshots:   0 total
Time:        1.234s

After

✓ jest: all tests passed

vitest

Strips the RUN v... banner and indented ✓ describe > it Nms per-test lines. Collapses on Tests N passed (N).

Before

 RUN  v1.6.0 /home/user/project

 ✓ src/utils/string.test.ts (3)
   ✓ truncate > truncates long strings 2ms
   ✓ truncate > preserves short strings 0ms
   ✓ truncate > handles empty string 0ms

 Test Files  1 passed (1)
 Tests       3 passed (3)
 Start at    14:32:01
 Duration    312ms

After

✓ vitest: all tests passed

bun test

Strips the bun test v... banner and per-test ✓ name [Nms] lines. Collapses on N pass with 0 fail.

Before

bun test v1.1.8 (9f27a12)

src/utils/hash.test.ts:
✓ hashes a string [0.82ms]
✓ returns consistent results [0.11ms]
✓ handles empty input [0.09ms]

 3 pass
 0 fail
Ran 3 tests across 1 files. [48.00ms]

After

✓ bun test: all tests passed

mocha

Strips indented ✓ name (Nms) per-test lines. Collapses on N passing (Xms).

Before

  Authentication
    ✓ rejects invalid token (8ms)
    ✓ accepts valid JWT (3ms)
    ✓ refreshes expired session (14ms)

  Database
    ✓ connects on first call (22ms)
    ✓ pools connections (1ms)


  5 passing (51ms)

After

✓ mocha: all tests passed

playwright test

Strips indented ✓ N [project] › file.spec.ts:L:C › title (Ns) per-test lines. Collapses on N passed (Xm Ys).

Before

Running 4 tests using 2 workers

  ✓  1 [chromium] › tests/auth.spec.ts:12:5 › login flow › shows dashboard (1.2s)
  ✓  2 [chromium] › tests/auth.spec.ts:28:5 › login flow › redirects on logout (0.8s)
  ✓  3 [firefox] › tests/auth.spec.ts:12:5 › login flow › shows dashboard (1.4s)
  ✓  4 [firefox] › tests/auth.spec.ts:28:5 › login flow › redirects on logout (0.9s)

  4 passed (6s)

After

✓ playwright: all tests passed

rubocop

Strips the "new cops available" preamble (cop-entry headers, Enabled: lines, intro paragraphs). Collapses blank-line runs created by the strip.

Before

The following cops were added to RuboCop, but are not configured. Please
set Enabled to either `true` or `false` in your `.rubocop.yml` file.

Please also note that you can opt-in to new cops by default...

Style/RedundantLineContinuation: # new in 1.36
  Enabled: true
Gemspec/AddRuntimeDependency: # new in 1.65
  Enabled: true
For more information: https://docs.rubocop.org/rubocop/versioning.html

Inspecting 42 files
..........................................

42 files inspected, no offenses detected

After

Inspecting 42 files
..........................................

42 files inspected, no offenses detected

ls -la

Replaces each long-format row with [type] name. Preserves the total N header. Symlink targets are preserved.

Before

total 48
drwxr-xr-x 1 viudes viudes  680 May  5 14:20 .
drwxr-xr-x 1 viudes viudes  460 May  4 09:11 ..
-rw-r--r-- 1 viudes viudes 1024 May  5 13:55 .env
drwxr-xr-x 1 viudes viudes  200 May  5 14:20 src
-rw-r--r-- 1 viudes viudes 2048 May  3 10:00 package.json
lrwxrwxrwx 1 viudes viudes   12 May  1 08:00 dist -> build/dist

After

total 48
[d] .
[d] ..
[-] .env
[d] src
[-] package.json
[l] dist -> build/dist

cargo check / cargo build

Strips Compiling <dep> v1.2.3 (transitive deps, no path), Checking <dep> v1.2.3, and download/update lines. Your own crate is always preserved. Collapses on Finished ... in Xs.

Before

    Checking libc v0.2.155
    Checking proc-macro2 v1.0.86
    Checking quote v1.0.36
    Checking syn v2.0.68
    Checking tokio v1.38.0
    Checking serde v1.0.203
    Checking serde_json v1.0.120
    Checking my-project v0.1.0 (/home/viudes/my-project)
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 4.32s

After

    Checking my-project v0.1.0 (/home/viudes/my-project)
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 4.32s

On clean success, collapses further to:

✓ cargo check: ok

tsc / tsc --noEmit

Strips ~~~~~ underline decoration lines and the trailing Errors Files summary table (which duplicates the inline error count). On a clean run, emits nothing.

Before (with errors)

src/api/client.ts(42,5): error TS2322: Type 'string' is not assignable to type 'number'.
    const timeout: number = config.timeout
                   ~~~~~~~

src/utils/parse.ts(17,12): error TS2345: Argument of type 'unknown' is not assignable to parameter of type 'string'.

Found 2 errors in 2 files.

Errors  Files
     1  src/api/client.ts:42
     1  src/utils/parse.ts:17

After

src/api/client.ts(42,5): error TS2322: Type 'string' is not assignable to type 'number'.
    const timeout: number = config.timeout

src/utils/parse.ts(17,12): error TS2345: Argument of type 'unknown' is not assignable to parameter of type 'string'.

Found 2 errors in 2 files.

ps aux

Strips kernel thread rows (VSZ=0, RSS=0, command in brackets). Caps at 50 lines.

Before

USER       PID %CPU %MEM    VSZ   RSS TTY      STAT START   TIME COMMAND
root         1  0.0  0.0 167364 11504 ?        Ss   May04   0:03 /sbin/init
root         2  0.0  0.0      0     0 ?        S    May04   0:00 [kthreadd]
root         3  0.0  0.0      0     0 ?        I<   May04   0:00 [rcu_gp]
root         4  0.0  0.0      0     0 ?        I<   May04   0:00 [rcu_par_gp]
viudes    1234  0.1  0.5 812344 42100 pts/0    Sl+  14:20   0:01 node dist/cli.mjs
viudes    5678  0.0  0.1  12345  8900 pts/1    Ss   14:22   0:00 bash

After

USER       PID %CPU %MEM    VSZ   RSS TTY      STAT START   TIME COMMAND
root         1  0.0  0.0 167364 11504 ?        Ss   May04   0:03 /sbin/init
viudes    1234  0.1  0.5 812344 42100 pts/0    Sl+  14:20   0:01 node dist/cli.mjs
viudes    5678  0.0  0.1  12345  8900 pts/1    Ss   14:22   0:00 bash

git log rewrite

Injects --oneline before the command runs and caps output at 50 lines. The verbose multi-line format is never sent to the model.

Before — git log

commit a1b2c3d4e5f6789012345678901234567890abcd
Author: Viudes <viudes@example.com>
Date:   Mon May 5 14:20:00 2025 +0000

    fix(auth): handle token expiry on refresh

commit b2c3d4e5f6789012345678901234567890abcde
Author: Viudes <viudes@example.com>
Date:   Sun May 4 10:00:00 2025 +0000

    feat(api): add rate limit headers

After — git log --oneline, capped at 50

a1b2c3d fix(auth): handle token expiry on refresh
b2c3d4e feat(api): add rate limit headers

git status rewrite

Replaced with git status --porcelain --branch before the command runs.

Before — git status

On branch main
Your branch is up to date with 'origin/main'.

Changes not staged for commit:
  (use "git add <file>..." to update what will be committed)
  (use "git restore <file>..." to discard changes in working directory)
	modified:   src/api/client.ts

Untracked files:
  (use "git add <file>..." to include in what will be committed)
	src/api/newfeature.ts

no changes added to commit (use "git add" and/or "git commit -a")

After — git status --porcelain --branch

## main...origin/main
 M src/api/client.ts
?? src/api/newfeature.ts

git diff

Strips diff --git a/X b/X header lines, index <hash>..<hash> lines, and \ No newline at end of file markers. Hunks are never touched. Collapses to ✓ git diff: no changes on empty output.

Before

diff --git a/src/api/client.ts b/src/api/client.ts
index a1b2c3d..d4e5f6a 100644
--- a/src/api/client.ts
+++ b/src/api/client.ts
@@ -40,7 +40,7 @@ export class ApiClient {
   async get(path: string) {
-    const timeout = 5000
+    const timeout = 30_000
     return this.request('GET', path, { timeout })
   }
\ No newline at end of file

After

--- a/src/api/client.ts
+++ b/src/api/client.ts
@@ -40,7 +40,7 @@ export class ApiClient {
   async get(path: string) {
-    const timeout = 5000
+    const timeout = 30_000
     return this.request('GET', path, { timeout })
   }

git show

Same diff-body cleanup as git diff, plus collapses Author: Name <email>\nDate: ... into a single Author: Name (date) line.

Before

commit a1b2c3d4e5f6789012345678901234567890abcd
Author: Viudes <viudes@example.com>
Date:   Mon May 5 14:20:00 2025 +0000

    fix(auth): handle token expiry on refresh

diff --git a/src/auth/refresh.ts b/src/auth/refresh.ts
index 0000000..1111111 100644
--- a/src/auth/refresh.ts
+++ b/src/auth/refresh.ts
@@ -10,6 +10,8 @@ export async function refreshToken(token: string) {
+  if (isExpired(token)) throw new TokenExpiredError()

After

commit a1b2c3d4e5f6789012345678901234567890abcd
Author: Viudes (2025-05-05)

    fix(auth): handle token expiry on refresh

--- a/src/auth/refresh.ts
+++ b/src/auth/refresh.ts
@@ -10,6 +10,8 @@ export async function refreshToken(token: string) {
+  if (isExpired(token)) throw new TokenExpiredError()

git commit

Replaces the [branch hash] subject\n N files changed... block with ✓ committed <hash>. Strips create mode / delete mode lines.

Before

[main a1b2c3d] fix(auth): handle token expiry on refresh
 2 files changed, 14 insertions(+), 3 deletions(-)
 create mode 100644 src/auth/errors.ts

After

✓ committed a1b2c3d

git pull

Strips remote progress noise (Enumerating / Counting / Compressing / Writing / Resolving / Unpacking). Preserves From, Updating, fast-forward lines, diff-stat, and conflict markers.

Before

remote: Enumerating objects: 5, done.
remote: Counting objects: 100% (5/5), done.
remote: Compressing objects: 100% (3/3), done.
remote: Total 4 (delta 2), reused 0 (delta 0), pack-reused 0
Unpacking objects: 100% (4/4), done.
From github.com:myorg/myapp
   a1b2c3d..d4e5f6a  main -> origin/main
Updating a1b2c3d..d4e5f6a
Fast-forward
 src/api/client.ts | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

After

From github.com:myorg/myapp
   a1b2c3d..d4e5f6a  main -> origin/main
Updating a1b2c3d..d4e5f6a
Fast-forward
 src/api/client.ts | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

git push

Strips transfer protocol lines (Enumerating / Counting / Delta / Compressing / Writing / Total / Resolving). Preserves remote: lines (server-side hooks, PR URLs) and the To <remote> result.

Before

Enumerating objects: 5, done.
Counting objects: 100% (5/5), done.
Delta compression using up to 8 threads
Compressing objects: 100% (3/3), done.
Writing objects: 100% (4/4), 1.23 KiB | 1.23 MiB/s, done.
Total 4 (delta 2), reused 0 (delta 0), pack-reused 0
remote: Resolving deltas: 100% (2/2), completed with 2 local objects.
remote: Create a pull request: https://github.com/myorg/myapp/pull/new/my-branch
To github.com:myorg/myapp.git
   a1b2c3d..d4e5f6a  my-branch -> my-branch

After

remote: Create a pull request: https://github.com/myorg/myapp/pull/new/my-branch
To github.com:myorg/myapp.git
   a1b2c3d..d4e5f6a  my-branch -> my-branch

git blame

Collapses hash (Author YYYY-MM-DD HH:MM:SS +TZ line) to hash YYYY-MM-DD line). Removes author name, email, time-of-day, and timezone.

Before

a1b2c3d4 (Viudes Ferreira 2025-04-10 11:23:45 +0100 42) export async function fetchUser(id: string) {
b2c3d4e5 (Maria Santos  2025-03-22 09:14:02 +0000 43)   const res = await api.get(`/users/${id}`)

After

a1b2c3d4 2025-04-10 42) export async function fetchUser(id: string) {
b2c3d4e5 2025-03-22 43)   const res = await api.get(`/users/${id}`)

wget

Strips CA / Resolving / Connecting chatter, the "HTTP request sent…" 200/206 success line, Length: / Saving to: headers, and the dot-progress block. Preserves any non-success HTTP status (3xx redirects, 4xx/5xx) and the final saved [bytes/total] summary. -q, --quiet, -O - / -qO- / --output-document=- are passed through unchanged (output is being piped or already silenced).

Before (74 lines, 5 KB — clipped)

--2026-05-12 09:14:02--  https://releases.example.com/dist/big-package-1.2.3.tar.gz
Loaded CA certificate '/etc/ssl/certs/ca-certificates.crt'
Resolving releases.example.com... 151.101.1.91, 151.101.65.91, ...
Connecting to releases.example.com|151.101.1.91|:443... connected.
HTTP request sent, awaiting response... 200 OK
Length: 52428800 (50M) [application/gzip]
Saving to: 'big-package-1.2.3.tar.gz'

     0K .......... .......... .......... .......... ..........  0% 2.34M 21s
    50K .......... .......... .......... .......... ..........  0% 4.12M 17s
... (60+ more dot-progress lines) ...
 51200K .......... ..........                                  100% 14.7M=3.4s

2026-05-12 09:14:05 (14.7 MB/s) - 'big-package-1.2.3.tar.gz' saved [52428800/52428800]

After

--2026-05-12 09:14:02--  https://releases.example.com/dist/big-package-1.2.3.tar.gz
2026-05-12 09:14:05 (14.7 MB/s) - 'big-package-1.2.3.tar.gz' saved [52428800/52428800]

find

Output is already pure signal (one path per line), so nothing is stripped — instead the spec caps long results with a head/tail window (first 50 + last 100, marker in between) and truncates pathological lines beyond 4 KB. Reject list covers flags whose output is custom-formatted and would be unsafe to truncate: -printf, -print0, -exec / -execdir, -ok / -okdir, -ls, -fprint*. Stderr lines like find: '/root': Permission denied survive the cap.

Before (321 lines)

./src/a/b/c/file001.ts
./src/a/b/c/file002.ts
... (300+ more paths) ...
./vendor/pkg/file321.ts

After (151 lines)

./src/a/b/c/file001.ts
... (first 50) ...
./src/a/b/c/file050.ts
[... 170 more lines omitted ...]
./vendor/pkg/file221.ts
... (last 100) ...
./vendor/pkg/file321.ts

Small outputs (< 150 lines) pass through unchanged.

curl (plain, non-verbose)

Collapses CR-overwritten progress bars from the default progress meter to the final state. The verbose form (curl -v) is handled by a separate spec (verbose headers stay; only * debug noise is trimmed).

rsync

Collapses CR-overwritten file-progress lines and strips per-file … speedup is X.XX summaries. Preserves the final sent / received / total block and any rsync: … error lines.

ping

Strips per-packet 64 bytes from … icmp_seq=N time=… ms rows beyond the first one. Preserves the header and the --- ping statistics --- summary block (packet loss, rtt min/avg/max).

tree

Caps the listing at a configurable head/tail window and replaces the rest with [… N entries omitted …]. The final N directories, M files summary is always kept.

ssh -v / -vv / -vvv

Strips debug1: / debug2: / debug3: lines and OpenSSH_… banners. Preserves any line containing Warning, Error, Permission denied, Connection refused, or a fingerprint prompt.

df / du

Drops virtual / pseudo filesystem rows (tmpfs, devtmpfs, overlay, udev, cgroup, proc, sys, efivars) from df. Strips du: cannot access …: Permission denied noise from du. Real mount points and the actual byte counts always survive.

dmesg / journalctl

Strips repeated bracketed-timestamp prefixes from dmesg. For journalctl, removes the noisy -- Boot 0x… -- markers and collapses systemd[1]: Started … / Stopped … / Reached target … chatter, keeping anything tagged error, fail, denied, warning, or kernel:.

stat

No-op when the output is short; only triggers when stat is called over many files and the same field labels (File:, Size:, Blocks:, IO Block:, etc.) repeat — in which case it elides the redundant labels.

jq

Truncates deeply nested pretty-printed JSON beyond a configurable depth/line cap. Top-level keys and the closing brackets are always preserved so the structure stays parseable.

Phase 12 additions

29 new filters covering JS package managers, Python tooling, Git remote ops, Go/Rust extras, and cross-language linters. Same fail-open contract: errors and warnings always survive; only the chatter is stripped.

npm install / npm ci / npm i

Strips added/changed/audited N packages summary tail, npm fund notices, progress bars (CR-collapsed), and the npm http fetch chatter under --verbose. Preserves warnings (npm WARN deprecated, peer-dep warnings) and any npm ERR! block.

Before (~700 bytes for a small package)

added 57 packages, and audited 58 packages in 4s

12 packages are looking for funding
  run `npm fund` for details

found 0 vulnerabilities
npm notice
npm notice New major version of npm available! 10.8.1 -> 11.0.0
npm notice Changelog: https://github.com/npm/cli/releases/tag/v11.0.0
npm notice To update run: npm install -g npm@11.0.0
npm notice

After

added 57 packages in 4s

found 0 vulnerabilities

pnpm install / pnpm add

Strips the boxed "Update available" notice, Progress lines, Lockfile is up to date, Downloading chatter, and the Done in Xs footer. Preserves peer-dep warnings, deprecation notices, ERR_PNPM_* blocks, and the actual dependency tree.

Before (747 bytes for pnpm add express)

   ╭──────────────────────────────────────────────────────────────────╮
   │                                                                  │
   │              Update available! 9.7.0 → 10.0.0.                   │
   │   Changelog: https://github.com/pnpm/pnpm/releases/tag/v10.0.0   │
   │                Run "pnpm add -g pnpm" to update.                 │
   │                                                                  │
   ╰──────────────────────────────────────────────────────────────────╯

Lockfile is up to date, resolution step is skipped
Progress: resolved 70, reused 0, downloaded 67, added 67
Done in 2.1s

After (59 bytes — 92% reduction)

dependencies:
+ express 4.21.2

yarn install / yarn

Strips the 4-phase progress ([1/4] Resolving packages… through [4/4] Building fresh packages…) and the Done in Xs footer. Preserves warnings, errors, and the dependency list.

93% reduction on a typical install. Yarn berry (v2+) output is structurally different but still benefits from progress-line removal.

pip install

Strips Collecting, Downloading, Using cached, Building wheel for … lines, and the multi-line [notice] A new release of pip is available block. Preserves Successfully installed …, version conflicts, and error: subprocess-exited-with-error blocks.

Before (1572 bytes for pip install requests)

Collecting requests
  Downloading requests-2.34.2-py3-none-any.whl.metadata (4.6 kB)
Collecting charset-normalizer<4,>=2 (from requests)
  Downloading charset_normalizer-3.4.4-cp312-cp312-manylinux_2_17_x86_64.whl.metadata (35 kB)
Collecting idna<4,>=2.5 (from requests)
  Downloading idna-3.10-py3-none-any.whl.metadata (10 kB)
...
Downloading requests-2.34.2-py3-none-any.whl (64 kB)
Downloading charset_normalizer-3.4.4-cp312-cp312-manylinux_2_17_x86_64.whl (151 kB)
...
Installing collected packages: certifi, urllib3, idna, charset-normalizer, requests
Successfully installed certifi-2025.4.26 charset-normalizer-3.4.4 idna-3.10 requests-2.34.2 urllib3-2.5.0

[notice] A new release of pip is available: 24.0 -> 24.3.1
[notice] To update, run: pip install --upgrade pip

After (192 bytes — 88% reduction)

Installing collected packages: certifi, urllib3, idna, charset-normalizer, requests
Successfully installed certifi-2025.4.26 charset-normalizer-3.4.4 idna-3.10 requests-2.34.2 urllib3-2.5.0

npm test / npm run / pnpm run

Strips the two-line script header (> pkg@v script\n> actual command) injected by the runner before delegating to the underlying test framework. Combined with the existing jest/vitest/bun-test filters, a typical npm test collapses to a single ✓ <runner>: all tests passed line.

82% reduction for npm test wrapping jest. 80% for pnpm run <task>.

git fetch

Strips the entire remote: Enumerating/Counting/Compressing/Total/Receiving objects/Resolving deltas progress block (which can balloon to tens of KB on large repos when --progress is forced or the terminal is non-TTY). Preserves the From <url> header and every <old>..<new> <ref> -> <remote>/<ref> ref-update line.

Before (21 981 bytes for git fetch origin --progress on a large repo)

remote: Enumerating objects: 4521, done.
remote: Counting objects: 100% (4521/4521), done.
remote: Compressing objects: 100% (1832/1832), done.
remote: Total 4521 (delta 2891), reused 4012 (delta 2521), pack-reused 0
Receiving objects: 100% (4521/4521), 12.4 MiB | 8.2 MiB/s, done.
Resolving deltas: 100% (2891/2891), done.
From github.com:myorg/myapp
   a1b2c3d..d4e5f6a  main       -> origin/main
   * [new branch]    feature/x  -> origin/feature/x

After (133 bytes — 99.4% reduction)

From github.com:myorg/myapp
   a1b2c3d..d4e5f6a  main       -> origin/main
   * [new branch]    feature/x  -> origin/feature/x

--porcelain invocations bypass the filter (machine-readable, already minimal).

go build

Strips go: downloading <mod> v1.2.3, go: finding <mod>, and go: found <mod> lines that appear on cold-cache or go.sum rebuild. Preserves compile errors and warm-cache silent runs (which already emit nothing).

When the output is only download/find/found lines (cold-cache success), the filter short-circuits to a positive marker — ✓ go build: dependencies downloaded, build ok — so the model gets a clear confirmation instead of an empty body.

Before (cold-cache success)

go: finding module for package github.com/spf13/cobra
go: downloading github.com/spf13/cobra v1.10.2
go: found github.com/spf13/cobra in github.com/spf13/cobra v1.10.2
go: downloading github.com/spf13/pflag v1.0.9
go: downloading github.com/inconshreveable/mousetrap v1.1.0

After

✓ go build: dependencies downloaded, build ok

On compile errors (lines matching error: / undefined: / cannot find / build failed / FAIL / typical file.go:L:C: location prefix), the positive marker is suppressed and the error output is preserved verbatim. Warm-cache builds are no-op (zero output → nothing to strip and nothing to confirm).

cargo run

Strips the build prologue (Compiling <dep>, Finished … in Xs, Running `target/debug/<bin>`) so only the program's actual stdout reaches the model. Build errors still surface unchanged.

Before

   Compiling libc v0.2.155
   Compiling proc-macro2 v1.0.86
   ... (many compile lines) ...
   Compiling my-app v0.1.0 (/home/viudes/my-app)
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 4.32s
     Running `target/debug/my-app`
Hello, world!

After (89% reduction)

Hello, world!

cargo fmt --check

Same minimal pass as prettier --check: only diffs/errors survive. ANSI stripped, blank-line runs collapsed. 21% on the diff sample; clean runs emit nothing.

pre-commit run

Strips all <hook id>........................................Passed$ lines and collapses the resulting blank runs. Preserves Failed hooks with their full diagnostic block (- hook id:, - exit code:, captured stdout/stderr).

Before (1396 bytes, 11 hooks with 2 failures)

trim trailing whitespace.................................................Passed
fix end of files.........................................................Passed
check yaml...............................................................Passed
check for added large files..............................................Passed
black....................................................................Failed
- hook id: black
- files were modified by this hook

reformatted src/api/client.py

ruff check...............................................................Passed
...
mypy.....................................................................Failed
- hook id: mypy
- exit code: 1

src/api/client.py:42: error: Incompatible types in assignment

After (58% reduction)

Exactly the Failed hooks plus their diagnostic blocks survive.

prisma generate / prisma migrate

Strips banners (✔ Generated Prisma Client, Loaded the config…, Schema loaded from…) and runtime tips. Preserves drift warnings, schema validation errors, and the actual migration plan.

67% on generate, 44% on migrate.

shellcheck

Strips the trailing For more information: block and the two wiki URLs that follow each diagnostic. Preserves the diagnostic itself (severity, code, caret line, suggestion). ~28% on typical output. Machine-readable formats (-f gcc, -f checkstyle, --format=json, etc.) bypass the filter.

Pass-through / safety-floor filters

These commands ship with a registered filter purely to apply ANSI stripping, collapse blank-line runs, and cap output length. No targeted line-stripping is configured — their output is already mostly signal. Adding them to the registry locks in safety guarantees: ANSI escapes will never leak, and pathological lengths will be capped.

Per-command reduction summary

Measured on the in-repo fixtures via bun test scripts/measure-bash-filter-roi.test.ts. Numbers are byte reduction of the filtered body (the <bash-output-filtered …> wrapper itself adds ~80 bytes, dominant only for very small fixtures).

Pass-throughs (0% but safety-locked — ANSI strip + maxLines cap): cargo clippy, cargo test --no-run, jq, ruff check (clean), stat, find (small), eslint, prettier, mypy, ruff format, markdownlint, yamllint, hadolint, go vet, golangci-lint, git branch, git stash list, git worktree list, glab list, gt log, jj log — output is already pure signal.

The gh pr|issue|run list filters and git status are rewrites, not strippers: they force the underlying command to emit a deterministic format (gh … --json …, git status --porcelain=v2). The byte count on very small fixtures can grow slightly because of the wrapper, but the downstream parser sees a stable schema instead of a fragile pretty-printed table.

Configuration

Activate / deactivate

Open /config inside the Claudin REPL and toggle Bash output filter.

Or edit ~/.claudin/settings.json directly:

{
  "bashOutputFilterEnabled": false
}

Omit the key (or set true) to keep it on. The change takes effect immediately — no restart needed.

User-defined filters

Add your own rules on top of the built-ins. Create ~/.claudin/bash-filters.json:

{
  "filters": [
    {
      "name": "my-tool",
      "matchCommand": "^my-tool\\b",
      "stripAnsi": true,
      "stripLinesMatching": ["^Fetching ", "^Resolving "],
      "maxLines": 50
    }
  ]
}

Patterns are validated against a zod schema on load and guarded against ReDoS before being compiled.