Guard whitespace-only old strings in str_replace against empty-match file corruption

[nordicnode]

Problem & Context

str_replace silently corrupted entire files when given a whitespace-only oldString that does not appear in the file. In the last-resort whitespace-insensitive match inside tryMatchOldStr, the search string is stripped of all whitespace: oldStr.replace(/\s+/g, ''). For a whitespace-only oldString (a single space, tabs, newlines, U+00A0, …) the stripped result is '', and indexOf('') vacuously succeeds at position 0. The position-mapping loops then produce an empty match, which the caller applies with content.replaceAll('', newStr) — inserting newString between every character of the file — and the tool returned the success shape, so the corrupted file was written to disk with no error surfaced to the model.

Reproduction (pre-fix, against the real module):

initialContent: "a\tb\nc\td\n"
replacement:    { oldString: " ", newString: "X", allowMultiple: false }
result:         success, content = "XaX\tXbX\nXcX\tXdX\nX"
expected:       error, file untouched

Changes Made

• packages/agent-runtime/src/process-str-replace.ts: guard the whitespace-insensitive fallback so an oldString that strips to '' is reported as not-found (falls through to the existing not-found error return) instead of producing a vacuous empty match. +8/−1 lines, no behavior change on any other path.
• packages/agent-runtime/src/__tests__/process-str-replace.test.ts: six regression/behavior-preservation tests (details below), importing the real production processStrReplace.

Architecture & Conventions Conformance

• Adheres to Dependency Injection (contracts defined in common/src/types/contracts/, no module monkey patching)
• Terminal commands use terminalCommandBroker (no direct spawn or TUI-process bypass) — no process execution touched
• Environment hygiene respected (getCliEnv() for CLI, getSdkEnv() for SDK, no forbidden getProcessEnv() imports) — no env access added
• Freebuff mode compatibility (IS_FREEBUFF preserved, no paid features introduced) — product-agnostic bug fix
• Imports ordered and explicit (import type used for types) — no import changes

Scope Verification

• All modified files are within allowed public directories: packages/agent-runtime/
• NO modifications to web/, freebuff/web/, packages/internal/, packages/billing/, packages/bigquery/, or packages/build-tools/

Testing & Verification

• bun run build:sdk (passed cleanly)
• bun run build:freebuff (passed cleanly)
• bun cli/scripts/smoke-binary.ts cli/bin/freebuff (passed cleanly — with the CI env var set and a writable HOME; the sandboxed checkout has a read-only home directory)
• Unit / integration tests added or updated in affected package
• Anti-flake principles observed (no sleeps, no ports, no filesystem, no timers — pure string-processing tests)

Tests were proven to catch the bug: with the fix stashed, the new tests fail 6/6 against the unfixed source ("XaX\tXbX\nXcX\tXdX\nX" corruption reproduced); with the fix applied, all 30 tests in the file pass. Fix and tests were additionally verified by a differential property fuzz (20,640 cases over whitespace-only old strings, Unicode whitespace classes, CRLF files, allowMultiple, multi-pair sequences, and patch round-trips via applyPatch): 572 violations pre-fix, 0 post-fix.

Verification Output / Log Snippet

$ bun test packages/agent-runtime/src/__tests__/process-str-replace.test.ts
 30 pass
 0 fail
 93 expect() calls
Ran 30 tests across 1 file. [31.00ms]

$ bun run --cwd packages/agent-runtime test   # remaining failures pre-exist on main (MCP schema = #1259, find-files WIP)
 601 pass / 5 fail (identical to baseline without this change)

$ bun run --cwd packages/agent-runtime typecheck
 0 new errors (2 agents-graveyard = #1202; 1 from uncommitted find-files WIP, not this branch's files)

$ bun cli/scripts/smoke-binary.ts cli/bin/freebuff
 smoke-binary: tree-sitter init OK.
 smoke-binary: OK (matched /\x1b\[\?1049h/, exit code null, 9176 bytes captured, attempt 1/3).

New tests (all against the production module, no local reimplementations):

1. does not corrupt the file for a space / tabs / newlines / mixed whitespace — the four collapse-to-empty classes, each must return the not-found error shape
2. reports the not-found error even when the file is entirely whitespace
3. still replaces whitespace-only old strings that exist in the file (exact-match branch preserved)
4. still replaces all whitespace occurrences with allowMultiple: true
5. lets later replacements proceed after a failed whitespace-only one (error-message accumulation intact)
6. keeps whitespace-insensitive matching working for non-empty anchors (guard does not over-correct the fallback)