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.