reverse-engineering-anything/docs/guides/web/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 using the ECMA-426 URL parsing rule; it is not an instruction to fetch. Repeated path separators remain part of the resolved URL identity. A non-empty sourceRoot prefixes each source, adding / only when the root does not already end with one. An empty, null or omitted root adds no prefix. webpack 5 and tsc maps ("sourceRoot": "") therefore resolve against the map URL, as Chrome, Firefox and the pinned codec do. 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. Numeric version: 3 is required on the root and every embedded map, including indexed ancestors. Invalid embedded headers report their section path and cannot produce descendant mapping facts. This uses the rejection option for version errors allowed by ECMA-426. Equal section starts are allowed when earlier sections contribute no mappings; actual decoded mappings must stay before the next section boundary. 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 decode_timeout_ms (default 20000). These are the defaults: select source_map_limits.codec_heap_mib for a different heap request. Reported actual V8 heap limits are checked. These are heap limits, not an aggregate process RSS limit. When REA runs on Bun, the subprocess is Bun, which ignores these V8 flags: runtime records id: "bun", the Bun version, its Node.js compatibility version and a null v8_heap_limit_bytes. On Windows, the codec's owned Job Object then limits its committed memory to the selected codec heap request plus a 512 MiB reserve for Bun itself (704 MiB with the default request); process_memory_limit_bytes records that limit, the limitations state it, and reaching it returns resource_constraint with codec_process_memory_limit_bytes. Elsewhere Bun has no enforced codec limit: process_memory_limit_bytes is null and the limitations state that no codec heap limit is established. Cleanup may extend the deadline and an unconfirmed cleanup overrides success with cleanup_incomplete and the owning operation/resources.

The decoder releases its owned codec process before removing the private request directory. A cleanup failure reports only the owners still pending: the process run token while process release is unconfirmed, and the directory until removal succeeds. A later successful retry settles every error bound to that command; historical failure diagnostics remain available.

Complete evidence defaults to an 8 MiB map, 32 MiB codec reply and a shared 262144-record decoder budget. Generated rows, mapping segments, leaf maps, source and name declarations, sourcesContent entries and indexed sections all consume this budget. Generated rows and mapping segments also each have a 262144 limit; indexed maps default to 64 nested section levels. Select per-call source_map_limits to change these admissions. 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.

Compact point delivery and retained source text

Summary delivery is useful for large point answers. Complete output can answer a small-map task with fewer calls. The delivery qualification records measured task costs and first-use discovery tradeoffs.

Complete delivery remains the default and includes raw map text and embedded source content. Select detail: "summary" in MCP, or --detail summary with --evidence_output /analysis/source-point.json in CLI, to retain the same complete Evidence and return body-free point metadata. Summary includes source/map/manifest identity, requested and matched generated positions, original source positions, content digests and sizes, match count, runtime qualifications and limitations. MCP returns the summary Evidence directly; its normalized_result.parent_evidence_id identifies the complete parent. CLI returns { summary: <Evidence>, evidence_bundle: { path, bytes } }; read summary.normalized_result.parent_evidence_id and use the published bundle path for later views. The receipt byte count describes the written complete Evidence bundle. Delivery selection does not enter that parent's analysis parameters or change its ID.

Summary returns the first metadata page (up to 64 matches and 1 MiB), with explicit coverage.total, coverage.next_offset and coverage.exhausted. Every equal-position alternative remains in the parent, including duplicate reported names, distinct indexed-section/source indexes and generated-only segments. Page exhaustion describes the retained lookup alternatives; execution and deployment authenticity remain unknown. For another page, call inspect_analysis_view with the same retained parent:

{
  "source": {
    "kind": "retained-evidence",
    "evidence_id": "<parent_evidence_id>"
  },
  "view": {
    "kind": "source-location",
    "collection": "matches",
    "offset": 64,
    "limit": 64
  }
}

Source text is explicit. Select collection: "source-text", an original match_index from the metadata page, and UTF-16 offset/limit (default 0/1024). For a snippet around the mapped point, use its original_position.offset when known. The result preserves content digest/size and total code units, returns only the selected text, and reports text-page coverage. Unretained or generated-only content returns item: null and unavailable coverage. An offset splitting a surrogate pair is rejected; a page never splits a complete Unicode character.

Both collections accept max_bytes (default 1048576), independently of their row/code-unit limit and MCP response limits. A byte refusal names the allowance and retained parent: select a smaller page or increase max_bytes, then retry without decoding again. A large individual identity may require a larger view allowance or complete export. Complete map text and diagnostics remain in the parent. An optional view can accompany detail: "summary" on the initial trace, returning that selected page immediately. view with complete delivery is rejected before artifact acquisition. Summary also refuses before acquisition when session retention or the CLI Evidence output is unavailable.

These views read retained facts even after the original map is removed. CLI can read the saved parent with inspect-analysis-view and an evidence-file source pointing to the written bundle; select its evidence_id when the bundle has multiple records. MCP can export the complete session with export_evidence_bundle. A complete MCP response that exceeds transport admission retains its parent and supplies a source-location matches continuation.

Selecting budgets and recovering the same map

Both trace_web_source_location and analyze_web_bundle accept an optional source_map_limits object. Omitted fields retain the defaults above and stay omitted in Evidence parameters. An update applies on the next call without a REA restart. Failure text names the exact field, effective value, supported range and next action; increasing a deadline does not change any of these budgets.

Field	Default	Scope
map_bytes	8388608	Local selected map admission, enforced before reading and again by the codec
output_bytes	33554432	Complete local request/reply or aggregate fetched-map representation
decoded_rows	262144	Generated rows before upstream allocation
decoded_segments	262144	Mapping segments before upstream allocation
decoded_records	262144	Shared structural work across leaves, declarations, rows, segments and sections
section_depth	64	Indexed-section nesting
codec_heap_mib	192	Local codec old-generation request; process-memory enforcement remains host dependent
fetch_bytes	67108864	Aggregate response bytes and, independently, resolved-identity bytes

The local 8 MiB admission covers the pinned 4,284,329-byte main map and 5,529,840-byte DateSelect map from Actual Budget without an admission retry. This is an artifact compatibility allowance, not a context or latency optimization: complete Evidence retains raw map text, and those two measured replies estimate 1.59 and 2.23 million tokens. A smaller admission refuses the map; it does not compact successful Evidence. MCP retains complete Evidence before encoding; an oversized response returns its retained reference for export_evidence_bundle, or callers can use complete CLI JSON. Select opt-in compact delivery above to keep complete Evidence accessible while returning only point metadata or an explicitly selected text page. Browser collection keeps its separate conservative defaults; see the calibration record.

Local tracing accepts all listed fields except fetch_bytes. Bundle fetching accepts all except map_bytes and codec_heap_mib: it uses its existing inspection process rather than launching the local codec. Unsupported fields are rejected rather than silently ignored. Byte fields are bounded below Node's single-string representation ceiling, record fields by the signed 32-bit decoder coordinate range, section depth by the same supported count range and shared structural admission, and heap requests by the existing 128..16384 MiB worker envelope. These bounds remain independent; a larger map can require a larger encoded request budget and codec heap, and the outer MCP client still owns its receive buffer/deadline.

For example, a valid 10 MiB selected local map can be retried with:

{
  "source_map_limits": { "map_bytes": 16777216 }
}

Keep the original manifest/map/point selection in that retry. The CLI spelling is --source-map-bytes 8388608; the other local flags are --source-map-output-bytes, --source-map-decoded-rows, --source-map-decoded-segments, --source-map-decoded-records, --source-map-section-depth and --source-map-codec-heap-mib. analyze-web-bundle instead offers --source-map-fetch-bytes and the shared output/record/depth flags. Raising only decoded_records does not raise the independent row or segment budgets. Failed local decoding returns no partial point evidence; completed browser map observations retain their existing partial/unknown semantics. Wait for retained cleanup to settle before retrying.

Verification

Browser bundle collection uses absolute section offsets for nested inline maps. If caller cancellation stops browser map fetching after some items complete, the failed bundle request retains those items with the captured page in partial Evidence and marks bundle analysis not_performed. This is distinct from the local source-map decoder's failure semantics above. An enclosing column offset contributes only when the nested section starts on that enclosing section's first generated line. Parent boundaries still clip preceding browser mappings when a nested section starts with a gap or has no child leaves. Raw map text, original source identities and the dedicated decoder profile are preserved.

Browser original_module_edges carry a declaration_index joining each edge to its original_sources declaration, including repeated source URLs across indexed sections. Imports are deduplicated within each declaration. Older observations may omit the index; their edge declaration identity remains unknown.

Browser collection defaults to 64 MiB of map responses per inspection and preflights indexed sections, sources, names, generated rows and mapping segments against a 262144-record decode budget before upstream flattening. Indexed maps default to 64 nested levels. These structural limits bound decoded object growth while preserving large embedded sourcesContent strings within the response-byte budget. It also bounds the expanded inline evidence to 32 MiB across the inspection, checking resolved source identities and mapping rows before publishing them, and bounds raw resolved-source identities to 64 MiB before upstream materializes their arrays. Exceeding either decoder budget returns a fetch_failed item while retaining other maps. Decoded coordinates and source/name indexes are validated per map; a malformed map is reported as invalid without discarding earlier or later maps. Collection reserves each requested map's failure context before fetching it. When the aggregate inline-result budget cannot reserve another item, remaining requests are left unprocessed and reported as unknown through the top-level limitation; requested retains the full count and processed counts returned items. Malformed UTF-8 map responses are reported as invalid rather than repaired with replacement characters. For browser fetching, an invalid map or structural or decoder capacity failure has artifact: null, empty decoded arrays, and an explicit limitation. When its bounded response body was read in full, optional response_identity records that body's SHA-256 digest and byte count after HTTP content decoding and before UTF-8 decoding. It is not a digest of partial reads or a claim that the map was valid; historical items may omit it. Malformed text is omitted so it does not consume the 32 MiB inline budget needed by later valid maps. A response whose text cannot fit that budget is refused before JSON parsing and source-map decoding, and retains the same completed-body identity. Successful and partial maps keep their full original text, including BOM and XSSI framing; only their parsing view removes it.

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.