diff --git a/docs/principles/README.md b/docs/principles/README.md
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@@ -14,6 +14,7 @@ Read this index before starting a multi-step task. Open a principle file only wh
 | [upstream-is-the-spec](upstream-is-the-spec.md) | OPY or DEL behavior could be "improved" away from the upstream compiler |
 | [independent-authority](independent-authority.md) | the evidence that a change works comes from tests written with the change |
 | [query-dont-encode](query-dont-encode.md) | guidance or docs are about to record counts, versions, or current status |
+| [source-text-is-the-truth](source-text-is-the-truth.md) | a tool changes a project through a structured model instead of through text |
 
 ## Repository-local principles
 
diff --git a/docs/principles/source-text-is-the-truth.md b/docs/principles/source-text-is-the-truth.md
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+# source-text-is-the-truth
+
+**Applies when:** a structured view, an agent operation, or a tool changes a project through a model, such as statements, declarations, settings, or symbols, instead of through text.
+
+**Principle:** Authored source text stays the only truth. Each structured change becomes a local, validated edit of that text. The owner of the language's emission produces the edit, and bytes outside it stay unchanged. No consumer composes source syntax itself.
+
+**Changes the choice between:** building source text in a consumer from catalog spellings, or regenerating a whole file, and asking the owner for a checked local edit.
+
+**Enforced by:** partly: owner source edits reject stale bytes, and edit-transaction revalidation rejects invalid results. Judgment decides who emits and how local an edit is.
+
+**Why:** a second emitter drifts from canonical output in ways validation cannot see. Regenerating a whole file destroys the comments and formatting users wrote.
+
+**Detail:** Wright [ADR-0029](https://github.com/wrightkit/wright/blob/main/docs/adr/0029-structured-statement-edits.md); Wright [tooling architecture, source edits](https://github.com/wrightkit/wright/blob/main/docs/architecture/tooling.md#source-edits); [owner-fixes-it](owner-fixes-it.md)