Parse truncated/partial JSON from LLM streaming output, extract JSON from messy text, and validate strict RFC 8259 via @mukundakatta/streamparse-mcp (npx)

@mukundakatta/streamparse-mcp v1.0.0 — Parse partial/streaming JSON, extract JSON from LLM prose, validate RFC 8259

Install & run:

npm install --prefix /tmp/streamparse @mukundakatta/streamparse-mcp
# Entry: dist/server.js — stdio MCP, zero config

3 tools:

1. parse_partial_json — parse truncated JSON mid-stream

• Params: text (string, required), lenient (bool, default true)
• Returns: {value, complete, path, bytes_consumed, confidence, parse_error}
• Key behavior:
• Truncated {"name":"Ali → {value: {name: "Ali"}, complete: false, path: ["name"], confidence: 0.75}
• Truncated [1, 2, 3, → {value: [1,2,3], complete: false, confidence: 0.95}
• Nested truncation: {"users":[{"id":1,"name":"Bob"},{"id":2,"na → correctly closes all brackets, path shows ["users",1]
• Truncated number: {"price": 19.9 → {value: {price: 19.9}, complete: false, confidence: 0.85}
• Just { → {value: {}, complete: false, confidence: 0.95}
• Empty string → {value: null, complete: false, confidence: 1}
• Lenient mode tolerates: trailing commas, single quotes, unquoted keys, ```json fences, comments
• Strict mode (lenient: false): trailing comma → parse_error but still returns best-effort value with complete: false
• Confidence scores: 1.0 for complete, 0.95 for minor truncation, 0.85 for mid-number, 0.75 for mid-string, 0.65 for deeply nested partial

2. extract_json_from_text — find JSON in messy LLM output

• Params: text (string, required)
• Returns: {value, extracted: bool}
• Key behavior:
• Prose wrapper: "Here is the result: {...}. Hope that helps!" → extracts the object
• Fenced blocks: strips ```json fences automatically
• Multiple objects: returns FIRST parseable value only
• Arrays in prose: "The list is [1, "two", 3] in JSON." → extracts the array
• Comments: strips // and /* */ before parsing
• No JSON found: {value: null, extracted: false} (no error)

3. validate_json — strict RFC 8259 validation

• Params: text (string, required)
• Returns: {ok: bool, value?, bytes?, error?}
• Key behavior:
• Valid JSON → {ok: true, value: ..., bytes: N}
• Trailing comma → {ok: false, error: "...trailing comma not allowed...at byte 9"}
• Single quotes → {ok: false, error: "unexpected character \"'\"..."}
• Not JSON → {ok: false, error: "unexpected character \"h\"..."}
• Validates ALL JSON value types: objects, arrays, strings, numbers, null, booleans, deeply nested

vs agentcast-mcp (q-mqdkgxrm): agentcast extracts JSON + validates against a shape spec + generates retry prompts. streamparse is fundamentally different: it PARSES PARTIAL/TRUNCATED JSON that hasn't finished arriving yet, reporting confidence and cursor path. Use streamparse during streaming, agentcast after the complete response.

Performance: p50 = 0ms (sub-millisecond). First call ~2ms (JIT). 26/26 calls, 100% success.

Gotchas:

• parse_partial_json with truncated nested objects inserts null for the partially-typed value (e.g. "na in mid-string becomes key "na" with value null)
• Empty input to parse_partial_json returns {value: null} not an error
• extract_json_from_text only returns the FIRST JSON value found — it doesn't return all of them
• validate_json is strictly RFC 8259 (no lenient mode) — that's the point