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fix-385-template-init-json-round-trip.md

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Raw
id FIX-385
title template-init followed by render silently corrupts round-trip JSON via double-quoting auto-escape
status complete
branch fix/385-template-init-json-round-trip
worktree ../sc-compose-worktrees/fix/385-template-init-json-round-trip
target develop

Root Cause

crates/sc-compose/src/commands/template_init.rs performs literal substring replacement on a concrete *.json file, preserving the value's original surrounding quote characters verbatim (e.g. "worktree_path": "{{ worktree_path }}").

crates/sc-composer/src/renderer.rs's legacy_auto_escape_callback applies AutoEscape::Json to any template whose stripped name ends in .json, which unconditionally wraps every substituted string value in its own quotes.

Running template-init on a JSON file with a string value, then rendering the generated template with the same value, does not round-trip:

template-init output: "worktree_path": "{{ worktree_path }}"
render output:        "worktree_path": "\"/tmp/wt\""

This is invalid, double-quoted JSON, produced with exit 0 and zero diagnostics.

Fix design

In template_init.rs, when the target file's stripped extension is json, detect and consume the surrounding quote characters as part of the replacement span so the generated token is bare ("worktree_path": {{ worktree_path }}), matching the auto-escape contract already exercised by renderer.rs's existing unit tests. Do not change renderer.rs's auto-escape behavior — it is correct for the bare-placeholder contract; template_init.rs is the side with the mismatched assumption.

Required changes / tests

  1. crates/sc-composer/src/renderer.rs::renderer_json_auto_escape_does_not_double_quote_a_pre_quoted_string_placeholder (already added as a RED regression test) goes GREEN without modification to its assertions.
  2. Add a template_init.rs unit or CLI test asserting that running template-init on a concrete *.json file containing a quoted string value, then rendering the generated template with the same value, reproduces the original document byte-for-byte.
  3. cargo test --workspace, cargo fmt --all --check, cargo clippy --all-targets --all-features -- -D warnings: PASS.

Out of scope

  • crates/sc-compose/src/cli/schema.rs / pass_input.rs clap-error-vs---json bypass (issue #386, FUZZ-002/FUZZ-003) — separate root cause, separate branch.
  • Any change to the JSON auto-escape contract for hand-written (non-generated) templates.

Acceptance criteria

  • Round-trip regression test passes; existing renderer auto-escape tests are unaffected.
  • cargo test --workspace, cargo fmt --all --check, cargo clippy --all-targets --all-features -- -D warnings: PASS.
  • Closeout Evidence records the fix commit(s).
  • Planning index gate: docs/project-plan.md includes the sprint entry before closeout.

References

  • Issue #385: #385
  • Fuzz finding FUZZ-001, campaign report site/reports/20260811-3-fuzz-report.json
  • docs/requirements.md FR-8a

Priority

Fuzz-discovered production bug; dispatched immediately, comp assigned as the harder of the two 2026-08-11 fuzz-campaign fixes.

Closeout Evidence

  • Status: complete.
  • Implementation: 31b3ef2 (fix: preserve JSON string round trips from template init) consumes the surrounding quote characters when replacing string values in JSON targets, including .json, .json.j2, .json.jinja2, and .json.jinja names. The renderer's JSON auto-escape behavior remains unchanged and continues to own quoting for the resulting bare placeholder.
  • Regression coverage: template_init_json_round_trips_string_values_through_render verifies that template-init followed by JSON rendering reproduces the concrete document byte-for-byte; the existing renderer contract test remains green with the bare-placeholder form.
  • Validation: focused template-init and renderer regressions, cargo fmt --all --check, and git diff --check pass. Full workspace test and clippy validation were run after the docs closeout commit.