rea/docs/web-source-location.md

Captured website source locations

trace_web_source_location maps one selected position in a script retained by export_web_scripts through one explicitly paired local source map. The shared CLI/MCP workflow verifies actual manifest, script and map byte identities and returns the original position and embedded source inline. It acquires no browser, fetches no URL and executes no application code.

rea trace-web-source-location \
  /analysis/scripts/manifest.json 0 \
  /analysis/app.js.map 'https://example.test/maps/app.js.map?v=7#context' \
  1 25 --json

The six positional arguments select the manifest, zero-based script index, absolute map path, absolute map URL context, one-based generated line and zero-based UTF-16 column. The MCP arguments are:

{
  "manifest_path": "/analysis/scripts/manifest.json",
  "script_index": 0,
  "source_map": {
    "path": "/analysis/app.js.map",
    "url": "https://example.test/maps/app.js.map?v=7#context"
  },
  "generated_position": { "line": 1, "column": 25 }
}

The URL supplies the context for resolving sources and sourceRoot; it is not an instruction to fetch. Absolute virtual or file contexts can be selected as well. The map/script association is caller-selected: a declaration or local map does not prove deployment authenticity or execution. Use import tracing to inspect outgoing native module edges.

Meaning of the evidence

Lookup follows the greatest generated position less than or equal to the selected point, across lines, and returns every segment at that position. It includes generated-only mappings. This follows ECMA-426 GetOriginalPositions, with one-based lines in REA's public contract. A point result does not establish coverage of a source range, execution or UI causality.

Mapped entries preserve the raw source name/root, section path, section-local source index and flattened index. Duplicate URLs remain distinct, including entries with different embedded contents. Missing source names retain a null resolved URL. Original coordinates outside embedded text retain the reported coordinates and a null offset.

Embedded content hashes identify the UTF-8 encoding of the decoded sourcesContent string, including its line endings. They do not identify an independently captured original file. Raw map text is retained in Evidence's raw_result, including unknown extensions; these extensions are not interpreted. Capture context and source Evidence links accompany independently verified selected file identities.

Adapter and resource boundaries

The adapter uses the locked upstream @jridgewell/trace-mapping 0.3.31 without modifying its code. It validates map/VLQ structure before allocation, normalizes nested indexed offsets before upstream flattening, checks section boundaries and indexes, and retains raw declaration identity. It supports regular maps and inline indexed sections. External section URLs return an actionable unsupported-profile error. This is a defined decoder profile, not a claim to implement every current source-map extension.

One owned Node subprocess decodes the map. Its private request directory is removed before success is returned. Only this child's inherited NODE_OPTIONS is removed; explicit V8 flags set a 192 MiB old-generation and 8 MiB semi-space budget, one V8 worker and a 20-second deadline. Reported actual V8 heap limits are checked. These are heap limits, not an aggregate process RSS limit. Cleanup may extend the deadline and an unconfirmed cleanup overrides success with cleanup_incomplete and the owning operation/resources.

Complete evidence is bounded by a 4 MiB map, 32 MiB codec reply, 262144 decoded rows/segments and 64 nested section levels. Exceeding a budget returns an explicit failure, without partial results. The existing 32 MiB manifest and 16 MiB selected script boundaries also apply. No global toolchain or browser configuration changes are made.

Verification

Focused tests cover duplicate mappings, indexed offsets/overlap/local indexes, 32-bit VLQ overflow, nullable content, UTF-16/CRLF positions, immutable artifact identity, cancellation, late output closure, cleanup failures, SDK schemas and public CLI behavior. The actual codec process is exercised on the test host.

The optional verify:browser:source-maps lane compiles a source-owned TypeScript fixture with isolated esbuild 0.25.10, serves it to real Chromium, captures and exports the actual response, then checks the original compiler point and exact retained content through CLI and stdio MCP. It can target an installed package. Only this lane requires Chromium and the fixture compiler. Linux x64 is the real workflow covered by the dedicated CI job; other host workflows remain unverified.