# Context-efficient MCP results

## Current behavior

Terminal MCP calls to `tact_edit`, `tact_check`, and `tact_apply_patch` accept `detail="auto"` (the default) or `detail="full"`.

- `auto` returns a compact, hash-bearing receipt for routine all-`EXACT` successes.
- `full` returns the complete structured result.
- Healed matches, ambiguity, refusal, detected drift, and new diagnostics always retain full
  detail, even when `auto` was requested.
- Reads, healing, delta reads, and navigation keep rich output because that evidence is the point
  of the operation.

Compact receipts are correlation handles, not diff proofs.
Successful writes report the result hash; read-only checks report the source and hypothetical result hashes.
To inspect a write later, retain the source hash from `tact_read` or request `detail="full"`, then pass that source hash to `tact_delta`.
Passing the result hash after a write only confirms that the file has not changed again.
Direct Python calls keep their native return values; GUI clients may render richer payloads.

Tact can control the size of the result it produces, not the lifetime of a result retained by an MCP client.
Context deletion, compaction, and any later summary are client responsibilities.

## When to choose each mode

Use `tact_edit` directly for ordinary writes.
Use `tact_check` when you need an explicit preview, a risky multi-edit inspection, or a dry run.
Ask for `detail="full"` when you are diagnosing a non-routine outcome or machine-reading the legacy detailed representation.

## Dated evidence

The following figures are frozen measurements from the August 2026 compact-receipt evaluation; they describe the evaluated implementation and inputs, not a current benchmark service.

| Case                                 | Legacy → compact bytes | Compact share |
| ------------------------------------ | ---------------------: | ------------: |
| Edit, one change                     |               247 → 88 |         35.6% |
| Edit, ten changes                    |             1,670 → 90 |          5.4% |
| Check, one change                    |              422 → 150 |         35.5% |
| Check, ten changes                   |            2,033 → 152 |          7.5% |
| Patch write, two files / ten changes |            1,887 → 159 |          8.4% |
| Patch check, two files / ten changes |            2,393 → 286 |         12.0% |

The evaluation used `cl100k_base` only as a stable token proxy; byte reduction and token reduction are not interchangeable, particularly for SHA-256 values.
The historical validation recorded 1,043 passing tests and 4 skipped opt-in live-harness tests.
These counts are frozen evidence, not a claim about the current suite.