Skillscript

One-rule fix in src/filters.ts. Phase 6 cold-adopter finding surfaced
that |json double-encodes structured values: a skill body like
emit(text="result: ${R|json}") where R is bound from a $ amp.<tool>
dispatch produced "\"{\\\"id\\\":\\\"abc\\\"}\"" (escaped twice)
instead of '{"id":"abc"}' (cleanly serialized once).

Root cause is structural. applyFilter(value: string, filter: string)
receives all values as strings — substituteRuntime serializes
structured JS values upstream of the filter chain. So |json was
operating on a string that was already a valid JSON representation,
and JSON.stringify re-quoted it.

Fix

|json now tries JSON.parse first. If the input parses cleanly,
pass through (already valid JSON). If parse throws, stringify once
(preserves the v0.3.x behavior for plain-string inputs).

case "json":
  try { JSON.parse(value); return value; }
  catch { return JSON.stringify(value); }

Behavior matrix

| Input | v0.15.6 → | v0.15.7 → |
|---|---|---|
| '{"id":"abc"}' (already JSON) | '"{\\"id\\":\\"abc\\"}"' (double-encoded) | '{"id":"abc"}' (pass through) |
| '[1,2,3]' (already JSON array) | re-encoded | pass through |
| 'hello world' (plain string) | '"hello world"' | '"hello world"' (unchanged) |
| 'he said "hi"' (needs escaping) | '"he said \\"hi\\""' | '"he said \\"hi\\""' (unchanged) |
| 'null' / 'true' / '42' (scalar JSON literals) | '"null"' / '"true"' / '"42"' (re-quoted) | 'null' / 'true' / '42' (pass through) |

The common case — ${OBJ|json} after $ <tool> -> OBJ — is fixed. The
edge case where an author literally wanted to JSON-encode the string
"null" (producing "\"null\"") hits a behavior change; documented
here but probably never the actual intent.

Tests

tests/v0.15.7-json-filter-idempotent.test.ts — 18 cases covering the
matrix: structured-pre-stringified inputs (object/array/nested/empty/
escaped-quotes), plain-string inputs (simple/multi-word/internal-quotes/
newlines/empty), scalar JSON literals (null/true/false/numerics/quoted),
and regression cases ensuring non-JSON-shape strings still stringify.

Phase 6 finding history

The originally-reported "read vs write asymmetry" was retracted by the
warm adopter after deeper investigation — root cause was test
contamination plus this |json double-encoding artifact, not actual
asymmetric binding in the connector or runtime layers. The
HttpMcpConnector path is clean: read and write tool results both
bind as structured objects per unwrapToolResult (runtime.ts:1617);
$ amp.<tool> -> R then ${R.field} is the canonical pattern.

This finding validates that the v0.16.0 bundled HttpMcpConnector
doesn't need response-shape normalization for cross-tool-type
symmetry — the wire layer already handles MCP responses uniformly. The
v0.16 charter scope shrinks slightly; the structural-discipline framing
stays the same (HttpMcpConnector + runtime_capabilities contract-test
fixture).

Files changed

• src/filters.ts — |json case rewritten with try-parse-first.
• tests/v0.15.7-json-filter-idempotent.test.ts (NEW) — 18 cases.
• package.json — version 0.15.6 → 0.15.7.
• tests/dogfood-t7.test.ts — version assertion.
• CHANGELOG.md — this entry.