Sentinel Blocks

Get structured data out of LLM text — reliably, in any language.

Teach the model to wrap each payload between unmistakable markers, then extract it with one pass that never re-parses content as code. Survives the quotes, newlines, backticks, and braces that shatter a naive JSON.parse.

🌐 Website & guide → eth-interchained.github.io/sentinel-blocks

<<<JSON>>>
{ "summary": "one object, taken verbatim", "ok": true }
<<<END>>>

<<<FILE src/app.ts>>>
export const greet = (n: string) => `hi ${n}`;   // code lives OUTSIDE the JSON
<<<END>>>

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The problem

You ask a model for JSON. It mostly complies — until the day a value contains a quote, a newline, a stray brace, or a code snippet:

{ "code": "const re = /\{.*\}/; // oops — unescaped braces & slashes" }
//                                  ^ JSON.parse throws: "Expected ',' or '}'…"

Now your pipeline falls back to a mock, retries, or crashes. The root cause is almost always code or free text stuffed into a JSON string.

The idea (the "lingo")

Stop fighting the escaping. Tell the model to put each payload between sentinels:

<<<TAG>>>
...payload, taken VERBATIM...
<<<END>>>

and to give code its own named block instead of burying it in JSON:

<<<FILE path/to/file>>>
...file contents, verbatim...
<<<END>>>

Extraction is a single regex (or a tiny scanner). The bytes between the markers are returned untouched — so quotes, braces, and newlines inside them are harmless. For JSON specifically: keep the JSON to data only, and the #1 cause of parse failures disappears.

Distilled from the surgical-edit parser in KeyStone-Lite, generalized into a tiny format with a formal spec and ports in 10 languages.

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Install

npm install sentinel-blocks      # Node / TypeScript / Bun / Deno
pip install sentinel-blocks      # Python 3.8+

Every other language is a single dependency-free file — vendor it:

Language	Copy this into your project
C	implementations/c/sentinel_blocks.{h,c}
C++ (header-only)	implementations/cpp/sentinel_blocks.hpp
Go	implementations/go/sentinelblocks.go
Rust	implementations/rust (cargo add from git)
Java	implementations/java/SentinelBlocks.java
Ruby	implementations/ruby/sentinel_blocks.rb
PHP	implementations/php/sentinel_blocks.php
JS (no build)	implementations/javascript/sentinel-blocks.{mjs,cjs}

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Quickstart

TypeScript / JavaScript

import { jsonFromResponse, extractTaggedBlocks } from "sentinel-blocks";

const reply = await llm(prompt);                 // raw completion text

const meta = jsonFromResponse(reply);            // robust: never breaks on code-in-string
for (const { arg, content } of extractTaggedBlocks(reply, "FILE")) {
  writeFileSync(arg, content);                   // arg = path, content = verbatim file
}

Python

from sentinel_blocks import json_from_response, extract_tagged_blocks

meta = json_from_response(reply)                 # dict; raises only if nothing is parseable
for arg, content in extract_tagged_blocks(reply, "FILE"):
    open(arg, "w").write(content)

Go, Rust, C, C++, Java, Ruby, PHP

import sb "sentinelblocks"
meta, err := sb.JSONFromResponse(reply, "")          // map[string]interface{}
files := sb.ExtractTaggedBlocks(reply, "FILE")        // []sb.Block{Arg, Content}

use sentinel_blocks as sb;
let json_text = sb::json_text_from_response(&reply, None);   // Option<String> → serde_json
let files = sb::extract_tagged_blocks(&reply, "FILE");        // Vec<Block { arg, content }>

char *json = sb_json_text_from_response(reply, NULL);   // feed to cJSON/jansson; sb_free(json)
sb_blocks files = sb_extract_blocks(reply, "FILE");     // sb_blocks_free(&files)

auto json  = sentinel::jsonTextFromResponse(reply);     // std::optional<std::string>
auto files = sentinel::extractTaggedBlocks(reply, "FILE");

var json  = SentinelBlocks.jsonTextFromResponse(reply, null); // Optional<String>
var files = SentinelBlocks.extractTaggedBlocks(reply, "FILE");

meta  = SentinelBlocks.json_from_response(reply)        # Hash
files = SentinelBlocks.extract_tagged_blocks(reply, "FILE")

$meta  = SentinelBlocks\json_from_response($reply);     // associative array
$files = SentinelBlocks\extract_tagged_blocks($reply, 'FILE');

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API at a glance

Same behavior everywhere; names follow each language's conventions.

Purpose	TS / JS / C++ / Java	Python / Ruby / PHP / Rust / C
First block's content	extractBlock	extract_block
All blocks' content	extractBlocks	extract_blocks
Blocks with their arg	extractTaggedBlocks	extract_tagged_blocks
Build a block	wrap / wrapNamed	wrap / wrap_named
Strip fences + trailing commas	repairJson	repair_json
First balanced {…}	firstJsonObject	first_json_object
Robust JSON entry point	jsonFromResponse¹	json_from_response¹

¹ Languages with a stdlib JSON parser (JS, TS, Python, Go, Ruby, PHP) return a parsed value. Languages without one (C, C++, Rust, Java) expose jsonTextFromResponse / json_text_from_response returning the cleaned JSON text to hand to your JSON library. See the spec §4.

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The strategy: prompt the model to use it

The format only pays off if the model emits it. Two ready-to-paste prompts:

• examples/prompts/json-extraction.md — a single JSON object.
• examples/prompts/codegen-files.md — metadata + multiple files.

The golden rules (full version in docs/PROMPTING.md):

1. Reply only in sentinel blocks — nothing before the first or after the last.
2. JSON blocks carry data only — never code, never markdown fences.
3. Code and long text go in their own <<<FILE …>>> blocks, verbatim.

Runnable end-to-end examples: examples/openai-node.mjs, examples/anthropic_python.py, and an EJS prompt-templating demo in examples/ejs/.

Why it works

• Content is never re-parsed as code. Extraction returns the bytes between the markers untouched, so quotes/braces/newlines inside them can't corrupt anything.
• Code leaves the JSON. The most common JSON.parse failure — unescaped code in a string — is designed out, not patched up.
• <<<…>>> is rare in natural text and survives markdown, diffs, and streaming.
• Graceful JSON fallback. jsonFromResponse tries the block, then a light repair, then a balanced-brace slice, and only fails loudly — it never fabricates.

More detail and failure-mode analysis: docs/WHY-IT-WORKS.md.

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Supported languages

All ports pass the same 8-invariant conformance suite (spec §7).

Language	Source	Tests
TypeScript	src/index.ts	node tests/test_core.ts
JavaScript (ESM + CJS)	implementations/javascript	node tests/test_core.mjs · node tests/test_core.cjs
Python	python/sentinel_blocks	python3 tests/test_core.py
Go	implementations/go	go test ./...
Rust	implementations/rust	cargo test
C	implementations/c	cc test_sentinel_blocks.c sentinel_blocks.c -o t && ./t
C++	implementations/cpp	c++ -std=c++17 test_sentinel_blocks.cpp -o t && ./t
Java	implementations/java	javac *.java && java SentinelBlocksTest
Ruby	implementations/ruby	ruby test_sentinel_blocks.rb
PHP	implementations/php	php test_sentinel_blocks.php

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Repo layout

sentinel-blocks/
├── SPEC.md                     formal format specification (v1.0)
├── src/index.ts                canonical TypeScript (npm package source)
├── python/sentinel_blocks/     canonical Python (PyPI package source)
├── implementations/            ports: javascript, c, cpp, go, rust, java, ruby, php
├── tests/                      shared invariant suites (TS, JS, Python)
├── examples/                   prompts, OpenAI/Python runners, EJS templating
└── docs/                       PROMPTING.md, WHY-IT-WORKS.md

Contributing

New language ports are very welcome — porting is mechanical and the conformance suite tells you when you're done. See CONTRIBUTING.md for the "add a language in 6 steps" guide.

Credits & License

The technique originates in KeyStone-Lite's surgical-edit parser, generalized here into a documented, multi-language format.

Licensed under GPL-3.0-or-later — see LICENSE. Part of the Interchained ecosystem.

_{Curious where the Sentinels come from? There's a bit of lore. 🛡️}

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Authors

Built by Mark Allen Evans Jr. (INTERCHAINED, LLC) with Claude Sonnet 4.6 on Hyperagent.

"Take one idea, turn it into an LP, then an app, then a system, then a platform, then infrastructure that is irreplaceable."