fix: resolve model-emitted short path prefixes in tool calls

[Archelunch]

Problem

My OS: MacOS

The FastContext model consistently uses the last directory component of the workspace path as a prefix in its tool calls. For example, when the workspace is /home/user/projects/myrepo, the model emits paths like /myrepo/backend/src/worker.py instead of /home/user/projects/myrepo/backend/src/worker.py.

This causes every Grep, Glob, and Read call to fail with a permission error:

Permission error: `/myrepo/backend` is not within the working directory `/home/user/projects/myrepo`

The agent receives zero useful search results, hallucinates file paths in its final answer, and the citation validator filters them all out, producing an empty <final_answer> block. This affects any workspace whose absolute path has more than one component.

Reproduction

git clone https://github.com/some/repo ~/projects/myrepo
cd ~/projects/myrepo

BASE_URL=http://your-endpoint/v1 \
MODEL=microsoft/FastContext-1.0-4B-RL \
API_KEY=dummy \
fastcontext -q "Where is the main entry point?" --max-turns 4 --citation --verbose

All tool calls fail with permission errors; the final answer is empty.

Changes

• resolve_path() helper (agent/tool/utils.py): detects the /{cwd_name}/... short-prefix pattern and rewrites it to the full absolute path before the is_relative_to(cwd) check, with a longest-existing-suffix fallback. Returns the original path when nothing resolves, preserving the existing error behavior.
• Applied to Grep, Glob, and Read, and to citation validation in format_citations() / get_final_answer() so short paths in the final answer pass the existence check.
• Read: return a clear error when the path is a directory instead of attempting to open it as a file.
• Glob: strip a path prefix from the pattern argument when the model passes a full path (e.g. /myrepo/backend/**/*.py -> backend/**/*.py).

Tests

Added tests/test_path_resolution.py covering the resolver: passthrough for in-cwd paths, short-prefix rewrite, suffix-match fallback, and unresolvable-path passthrough. All pass; existing tests are unaffected. ruff check is clean on all changed files.

Verified against

• Models: microsoft/FastContext-1.0-4B-RL and microsoft/FastContext-1.0-4B-SFT, served via SGLang v0.5.13 (FP8, 262K context)
• Before: empty <final_answer>, all tool calls fail with permission errors
• After: the model finds the correct files and returns valid citations with line ranges

[Archelunch]

@microsoft-github-policy-service agree