PromptCapsule

Pass the prompt, not the payload. Lossless prompt capsules for agent-to-agent handoff: inline when small, vault-backed when large, fail-closed integrity by default.

pip install promptcapsule            # core: zero dependencies
pip install "promptcapsule[vault]"   # adds S3 and GitHub Gist backends

Python 3.8–3.13 · MIT · 130+ tests on Linux, macOS and Windows · Changelog

What it does

A capsule is a short string one agent hands to another. The receiver gets the exact original prompt back, or an exception — never silently corrupted text.

Inline capsules are about packaging, not shrinking: very short prompts get longer (a 42-byte prompt becomes a 78-character capsule). The size win is vault mode, where a 2,480-byte prompt becomes a 41-character capsule.

Quick start

from promptcapsule import PromptCapsule, IntegrityError
from promptcapsule.backends import SQLiteBackend

pc = PromptCapsule()

capsule = pc.compress("You are a helpful Python coding assistant.")
# cap_i_45b72418_c-o81FI7k^N>xZy$Vkm8NGr`z2&gQ{$j?(q&QHnAOIJuNF3v12Nz5zJ0{}*34}|

vault = SQLiteBackend("prompts.db")
long_prompt = "You are a senior software architect reviewing a pull request. " * 40
long_capsule = pc.compress(long_prompt, vault_backend=vault)
# cap_v_66152d9b_sql_JME_tuuntLjuEVRztHpH4A  (the key part is random)

try:
    text = pc.decompress(long_capsule, vault_backend=vault).text
except IntegrityError:
    ...  # tampered or corrupted: do not use

decompress is strict by default: a checksum mismatch raises IntegrityError.

Signed capsules (authenticity)

The 8-hex checksum detects corruption, but anyone can compute it. To know a capsule came from someone holding a shared secret, sign it with HMAC-SHA256:

from promptcapsule import SignatureError

capsule = pc.compress("Summarise the Q3 report", sign="shared-secret")

try:
    text = pc.decompress(capsule, verify_signature="shared-secret").text
except SignatureError:
    ...  # wrong key, tampered, or unsigned capsule: do not use

• Receivers must pass the key. When verify_signature is a key (or True), unsigned capsules — including ones with the _sig_… suffix stripped — are rejected before anything is decompressed or fetched from a vault. With the default verify_signature=None, unsigned capsules are accepted.
• sign=True / verify_signature=True read the key from PROMPT_CAPSULE_HMAC_KEY. Empty keys are rejected.
• The signature is a 128-bit truncated HMAC-SHA256 over the prompt text, compared in constant time.
• Signing proves who created the prompt, not when: a valid capsule can be replayed.

Command line

echo "You are a helpful assistant" | promptcapsule pack --file -
promptcapsule pack --file long_prompt.txt --vault prompts.db
promptcapsule unpack --file capsule.txt --vault prompts.db
promptcapsule verify --file capsule.txt           # check without printing the prompt
promptcapsule inspect --file capsule.txt --json   # mode, checksum, signed?

export PROMPT_CAPSULE_HMAC_KEY=...                 # or use --key-file PATH
promptcapsule pack --file prompt.txt --sign
promptcapsule unpack --file capsule.txt --require-signature

Keys are never accepted as command-line arguments, so they stay out of shell history and process listings.

Backends

All live in promptcapsule.backends. Both agents must reach the same backend. Vault keys are random (secrets.token_urlsafe), and the capsule checksum is bound to the stored content, so swapping keys between capsules fails verification.

Custom backend: subclass promptcapsule.core.VaultBackend and implement store(text, checksum) -> key, retrieve(key), and retrieve_with_checksum(key) -> (text, checksum).

Limits

Oversized input raises SizeLimitError.

Errors

All exceptions derive from PromptCapsuleError:

IntegrityError, FormatError and SizeLimitError also subclass ValueError.

Security model

Provides: exact reconstruction; tamper and corruption detection; optional authenticity with a shared secret; bounded decompression.

Does not provide:

• Encryption. Anyone holding an inline capsule, or with access to the vault, can read the prompt.
• Identity or access control. Protect your vault with its own ACLs (IAM, private gists, file permissions).
• Replay protection. Add your own nonce or expiry if you need it.

See TRUST.md for the full threat model.

How it compares

Development

pip install -e ".[dev]"
pytest

See CONTRIBUTING.md. Report security issues privately via GitHub Security Advisories or data.pycap@gmail.com.

License

MIT — see LICENSE.