Comprehension

Comprehension is Spine’s free wedge: eight read-only tools you can use from Claude Code or Codex. They’re deterministic, use no LLM, need no credentials, and write nothing. Ask in plain language — the assistant picks the tool.

The eight tools

Tool	The question it answers
map_repo	“I’ve never seen this repo — what is it?”
blast_radius	“What breaks if I change this?”
explain_symbol	“What is this and who uses it?”
investigate	“Where does this ticket land in the code?”
localize	“Here’s a stack trace — where’s the bug?”
regression_gaps	“What will break silently, with no test to catch it?”
root_cause	“Why is this happening, and what’s the fix shape?”
docs_for	“Which docs describe this — and are they still true?”

A real transcript

you › where would a fix for the "sessions expire early" bug go?

assistant → investigate("sessions expire early")

Likely site:      auth.validate    app/auth.py:60 → reads secret_key
Reachable, untested: commit         db.py:88
Related doc:      docs/auth.md — drifted

Read-only by design

These 8 tools are the read-only half of Spine’s 17 MCP tools. The remaining tools perform SDLC actions — and those write, only after a human gate. See Governance & gates and the MCP tools reference.