diff --git a/src/__integration__/import-format-roundtrip.test.ts b/src/__integration__/import-format-roundtrip.test.ts
new file mode 100644
index 0000000..26427d8
--- /dev/null
+++ b/src/__integration__/import-format-roundtrip.test.ts
@@ -0,0 +1,262 @@
+/**
+ * Import format — save/reload integration (#324).
+ *
+ * The unit tests in `src/utils/importFormat.test.ts` prove the codec carries
+ * every field. This file proves the SQL on either side of it does too: a format
+ * written by `importSourceService` and read back is the same format, field by
+ * field. That is the test that would have caught the root bug — before v17 the
+ * INSERT simply had no `amount_mode` / `sign_convention` column to write to, so
+ * a positive-expense source silently came back inverted on its second import.
+ *
+ * Like `balance-flow.test.ts`, real `tauri-plugin-sql` cannot be started outside
+ * the Tauri WebView, so the services run against an in-memory FakeDb that
+ * interprets the handful of statements they actually issue. It stores what it
+ * is given, so `has_header` lands as the 0/1 integer SQLite stores and comes
+ * back as one — the exact shape the codec has to absorb.
+ */
+
+import { describe, it, expect, beforeEach, vi } from "vitest";
+
+vi.mock("../services/db", () => {
+  const getDb = vi.fn();
+  return {
+    getDb,
+    withTransaction: vi.fn(async (fn: (db: unknown) => unknown) => fn(await getDb())),
+  };
+});
+
+import { getDb } from "../services/db";
+import {
+  createSource,
+  getSourceByName,
+  updateSource,
+} from "../services/importSourceService";
+import {
+  createTemplate,
+  getAllTemplates,
+  updateTemplate,
+} from "../services/importConfigTemplateService";
+import { formatFromRow, formatToRow, FORMAT_FIELD_PAIRS } from "../utils/importFormat";
+import type { ImportFormat, SourceConfig } from "../shared/types";
+
+// ---------------------------------------------------------------------------
+// FakeDb — a tiny interpreter for the statements these two services issue.
+// ---------------------------------------------------------------------------
+
+type Row = Record<string, unknown>;
+
+function makeFakeDb() {
+  const tables: Record<string, Row[]> = {
+    import_sources: [],
+    import_config_templates: [],
+  };
+  let nextId = 1;
+
+  const insert = (sql: string, params: unknown[]) => {
+    const [, table, cols] =
+      /INSERT INTO (\w+) \(([^)]+)\)/.exec(sql) ?? [];
+    const columns = cols.split(",").map((c) => c.trim());
+    const row: Row = { id: nextId++ };
+    columns.forEach((c, i) => (row[c] = params[i]));
+
+    if (/ON CONFLICT\(name\) DO UPDATE/.test(sql)) {
+      const clash = tables[table].find((r) => r.name === row.name);
+      if (clash) {
+        // Mimic `excluded.*`: overwrite every listed column, keep the id.
+        columns.forEach((c, i) => (clash[c] = params[i]));
+        nextId--;
+        return { lastInsertId: 0, rowsAffected: 1 };
+      }
+    }
+    tables[table].push(row);
+    return { lastInsertId: row.id as number, rowsAffected: 1 };
+  };
+
+  const update = (sql: string, params: unknown[]) => {
+    const [, table, assignments] =
+      /UPDATE (\w+)\s+SET ([\s\S]+?)\s+WHERE id\s*=\s*\$(\d+)/.exec(sql) ?? [];
+    const id = params[params.length - 1];
+    const row = tables[table].find((r) => r.id === id);
+    if (!row) return { rowsAffected: 0 };
+    for (const [, column, index] of assignments.matchAll(
+      /(\w+)\s*=\s*\$(\d+)/g
+    )) {
+      row[column] = params[Number(index) - 1];
+    }
+    return { rowsAffected: 1 };
+  };
+
+  return {
+    tables,
+    execute: vi.fn(async (sql: string, params: unknown[] = []) => {
+      if (sql.trimStart().startsWith("INSERT")) return insert(sql, params);
+      if (sql.trimStart().startsWith("UPDATE")) return update(sql, params);
+      throw new Error(`FakeDb: unsupported statement ${sql.slice(0, 40)}`);
+    }),
+    select: vi.fn(async (sql: string, params: unknown[] = []) => {
+      const [, table] = /FROM (\w+)/.exec(sql) ?? [];
+      const rows = tables[table];
+      if (/WHERE name = \$1/.test(sql)) return rows.filter((r) => r.name === params[0]);
+      if (/WHERE id = \$1/.test(sql)) return rows.filter((r) => r.id === params[0]);
+      return [...rows].sort((a, b) => String(a.name).localeCompare(String(b.name)));
+    }),
+  };
+}
+
+let db: ReturnType<typeof makeFakeDb>;
+
+beforeEach(() => {
+  db = makeFakeDb();
+  vi.mocked(getDb).mockResolvedValue(db as never);
+});
+
+// ---------------------------------------------------------------------------
+// Fixtures — a credit-card statement: expenses are POSITIVE. This is the
+// configuration the old code could not keep.
+// ---------------------------------------------------------------------------
+
+const CREDIT_CARD: SourceConfig = {
+  name: "Visa Desjardins",
+  delimiter: ",",
+  encoding: "windows-1252",
+  dateFormat: "YYYY-MM-DD",
+  skipLines: 2,
+  hasHeader: false,
+  columnMapping: { date: 1, description: 3, amount: 4 },
+  amountMode: "single",
+  signConvention: "positive_expense",
+};
+
+/** What `useImportWizard.selectSource` does on a stored source. */
+async function reload(name: string): Promise<SourceConfig> {
+  const stored = await getSourceByName(name);
+  if (!stored) throw new Error(`source ${name} not found`);
+  return { name: stored.name, ...formatFromRow(stored) };
+}
+
+function expectSameFormat(actual: ImportFormat, expected: ImportFormat) {
+  for (const field of Object.keys(FORMAT_FIELD_PAIRS) as Array<keyof ImportFormat>) {
+    expect(actual[field], `field ${field}`).toEqual(expected[field]);
+  }
+}
+
+// ---------------------------------------------------------------------------
+
+describe("configure -> save -> reload (#324)", () => {
+  it("returns the saved format field by field", async () => {
+    await createSource({ name: CREDIT_CARD.name, ...formatToRow(CREDIT_CARD) });
+    expectSameFormat(await reload(CREDIT_CARD.name), CREDIT_CARD);
+  });
+
+  it("keeps positive_expense on the second import and every one after", async () => {
+    await createSource({ name: CREDIT_CARD.name, ...formatToRow(CREDIT_CARD) });
+
+    // Three consecutive imports re-save what was reloaded, as the wizard does.
+    let current = await reload(CREDIT_CARD.name);
+    for (let i = 0; i < 3; i++) {
+      const id = (await getSourceByName(CREDIT_CARD.name))!.id;
+      await updateSource(id, { name: current.name, ...formatToRow(current) });
+      current = await reload(CREDIT_CARD.name);
+    }
+
+    expect(current.signConvention).toBe("positive_expense");
+    expectSameFormat(current, CREDIT_CARD);
+  });
+
+  it("stores has_header as the integer SQLite holds, not a JS boolean", async () => {
+    await createSource({ name: CREDIT_CARD.name, ...formatToRow(CREDIT_CARD) });
+    expect(db.tables.import_sources[0].has_header).toBe(0);
+    expect((await reload(CREDIT_CARD.name)).hasHeader).toBe(false);
+  });
+
+  it("keeps the chosen mode when the mapping does not betray it", async () => {
+    const debitCredit: SourceConfig = {
+      ...CREDIT_CARD,
+      amountMode: "debit_credit",
+      // No debitAmount: the old restore re-inferred "single" from this shape.
+      columnMapping: { date: 1, description: 3, creditAmount: 5 },
+    };
+    await createSource({ name: debitCredit.name, ...formatToRow(debitCredit) });
+    expect((await reload(debitCredit.name)).amountMode).toBe("debit_credit");
+  });
+
+  it("updates the format in place when the source is re-configured", async () => {
+    const id = await createSource({
+      name: CREDIT_CARD.name,
+      ...formatToRow(CREDIT_CARD),
+    });
+
+    const flipped: SourceConfig = {
+      ...CREDIT_CARD,
+      signConvention: "negative_expense",
+      amountMode: "debit_credit",
+      columnMapping: { date: 1, description: 3, debitAmount: 4, creditAmount: 5 },
+    };
+    await updateSource(id, { name: flipped.name, ...formatToRow(flipped) });
+
+    expectSameFormat(await reload(CREDIT_CARD.name), flipped);
+    expect(db.tables.import_sources).toHaveLength(1);
+  });
+});
+
+describe("template_id is provenance, never format (#324)", () => {
+  async function seedLinkedPair() {
+    const templateId = await createTemplate({
+      name: "Desjardins carte",
+      ...formatToRow(CREDIT_CARD),
+    });
+    await createSource({
+      name: CREDIT_CARD.name,
+      ...formatToRow(CREDIT_CARD),
+      template_id: templateId,
+    });
+    return templateId;
+  }
+
+  it("records the template the source was configured from", async () => {
+    const templateId = await seedLinkedPair();
+    expect((await getSourceByName(CREDIT_CARD.name))!.template_id).toBe(templateId);
+  });
+
+  // The acceptance criterion: the eight source columns are authoritative, so a
+  // template edit cannot reach through the link and change how a source reads.
+  it("editing the template alters no linked source", async () => {
+    const templateId = await seedLinkedPair();
+
+    await updateTemplate(templateId, {
+      name: "Desjardins carte",
+      ...formatToRow({
+        ...CREDIT_CARD,
+        delimiter: "\t",
+        encoding: "utf-8",
+        dateFormat: "DD/MM/YYYY",
+        skipLines: 0,
+        hasHeader: true,
+        columnMapping: { date: 0, description: 1, debitAmount: 2, creditAmount: 3 },
+        amountMode: "debit_credit",
+        signConvention: "negative_expense",
+      }),
+    });
+
+    expectSameFormat(await reload(CREDIT_CARD.name), CREDIT_CARD);
+
+    // …and the template really did change, so the test is not vacuous.
+    const [stored] = await getAllTemplates();
+    expect(formatFromRow(stored).signConvention).toBe("negative_expense");
+    expect(formatFromRow(stored).amountMode).toBe("debit_credit");
+  });
+
+  it("survives a re-import that re-saves the format", async () => {
+    const templateId = await seedLinkedPair();
+    const id = (await getSourceByName(CREDIT_CARD.name))!.id;
+    const current = await reload(CREDIT_CARD.name);
+
+    await updateSource(id, {
+      name: current.name,
+      ...formatToRow(current),
+      template_id: templateId,
+    });
+
+    expect((await getSourceByName(CREDIT_CARD.name))!.template_id).toBe(templateId);
+  });
+});
diff --git a/src/components/import/ColumnMappingEditor.tsx b/src/components/import/ColumnMappingEditor.tsx
index 6bba264..970b507 100644
--- a/src/components/import/ColumnMappingEditor.tsx
+++ b/src/components/import/ColumnMappingEditor.tsx
@@ -1,12 +1,19 @@
 import { useTranslation } from "react-i18next";
 import type { ColumnMapping, AmountMode } from "../../shared/types";
+import { clearMappingForMode } from "../../utils/importFormat";
 
 interface ColumnMappingEditorProps {
   headers: string[];
   mapping: ColumnMapping;
   amountMode: AmountMode;
   onMappingChange: (mapping: ColumnMapping) => void;
-  onAmountModeChange: (mode: AmountMode) => void;
+  /**
+   * The mode carries the pruned mapping with it. Both have to land in a single
+   * state update: the parent's handlers each spread the same `config` prop, so
+   * two consecutive calls would see the same stale value and the second would
+   * overwrite the first.
+   */
+  onAmountModeChange: (mode: AmountMode, mapping: ColumnMapping) => void;
 }
 
 export default function ColumnMappingEditor({
@@ -18,6 +25,12 @@ export default function ColumnMappingEditor({
 }: ColumnMappingEditorProps) {
   const { t } = useTranslation();
 
+  // The mode is the source of truth for the mapping, not the reverse: leaving a
+  // mode drops its columns so a stale key cannot keep deciding how amounts are
+  // read (#324).
+  const selectMode = (mode: AmountMode) =>
+    onAmountModeChange(mode, clearMappingForMode(mapping, mode));
+
   const columnOptions = headers.map((h, i) => (
     <option key={i} value={i}>
       {i}: {h}
@@ -79,7 +92,7 @@ export default function ColumnMappingEditor({
               name="amountMode"
               value="single"
               checked={amountMode === "single"}
-              onChange={() => onAmountModeChange("single")}
+              onChange={() => selectMode("single")}
               className="accent-[var(--primary)]"
             />
             {t("import.config.singleAmount")}
@@ -90,7 +103,7 @@ export default function ColumnMappingEditor({
               name="amountMode"
               value="debit_credit"
               checked={amountMode === "debit_credit"}
-              onChange={() => onAmountModeChange("debit_credit")}
+              onChange={() => selectMode("debit_credit")}
               className="accent-[var(--primary)]"
             />
             {t("import.config.debitCredit")}
diff --git a/src/components/import/SourceConfigPanel.tsx b/src/components/import/SourceConfigPanel.tsx
index 23995c1..47a4409 100644
--- a/src/components/import/SourceConfigPanel.tsx
+++ b/src/components/import/SourceConfigPanel.tsx
@@ -314,8 +314,8 @@ export default function SourceConfigPanel({
           onMappingChange={(mapping: ColumnMapping) =>
             onConfigChange({ ...config, columnMapping: mapping })
           }
-          onAmountModeChange={(mode: AmountMode) =>
-            onConfigChange({ ...config, amountMode: mode })
+          onAmountModeChange={(mode: AmountMode, mapping: ColumnMapping) =>
+            onConfigChange({ ...config, amountMode: mode, columnMapping: mapping })
           }
         />
       )}
diff --git a/src/components/reports/FilterPanel.test.tsx b/src/components/reports/FilterPanel.test.tsx
index 2cb00bc..d4f18a9 100644
--- a/src/components/reports/FilterPanel.test.tsx
+++ b/src/components/reports/FilterPanel.test.tsx
@@ -69,6 +69,8 @@ const SOURCE_CHEQUING: ImportSource = {
   column_mapping: "{}",
   skip_lines: 0,
   has_header: true,
+  amount_mode: "single",
+  sign_convention: "negative_expense",
   created_at: "2026-01-01",
   updated_at: "2026-01-01",
 };
diff --git a/src/hooks/useImportWizard.ts b/src/hooks/useImportWizard.ts
index 7bbbf2f..15e8059 100644
--- a/src/hooks/useImportWizard.ts
+++ b/src/hooks/useImportWizard.ts
@@ -11,7 +11,6 @@ import type {
   ImportReport,
   ImportSource,
   ImportConfigTemplate,
-  ColumnMapping,
 } from "../shared/types";
 import {
   getImportFolder,
@@ -46,6 +45,17 @@ import {
   preprocessQuotedCSV,
   autoDetectConfig as runAutoDetect,
 } from "../utils/csvAutoDetect";
+import {
+  formatFromRow,
+  formatToRow,
+  ImportFormatError,
+} from "../utils/importFormat";
+
+/** Error text for the banner: an i18n key when we have one, the raw message otherwise. */
+function errorMessage(e: unknown): string {
+  if (e instanceof ImportFormatError) return e.i18nKey;
+  return e instanceof Error ? e.message : String(e);
+}
 
 interface WizardState {
   step: ImportWizardStep;
@@ -297,71 +307,85 @@ export function useImportWizard() {
 
       dispatch({ type: "SET_SELECTED_SOURCE", payload: sortedSource });
       dispatch({ type: "SET_SELECTED_FILES", payload: newFiles });
-      dispatch({ type: "SET_SELECTED_TEMPLATE_ID", payload: null });
 
-      // Check if this source already has config in DB
-      const existing = await getSourceByName(source.folder_name);
-      dispatch({ type: "SET_EXISTING_SOURCE", payload: existing });
+      try {
+        // Check if this source already has config in DB
+        const existing = await getSourceByName(source.folder_name);
+        dispatch({ type: "SET_EXISTING_SOURCE", payload: existing });
 
-      let activeDelimiter = defaultConfig.delimiter;
-      let activeEncoding = "utf-8";
-      let activeSkipLines = 0;
-      let activeHasHeader = true;
+        // Provenance of the stored format, not the format itself: the template
+        // is never re-read, it is only shown as the source's origin. Blindly
+        // resetting it to null used to make an already-linked source look
+        // unconfigured.
+        dispatch({
+          type: "SET_SELECTED_TEMPLATE_ID",
+          payload: existing?.template_id ?? null,
+        });
 
-      if (existing) {
-        // Restore config from DB
-        const mapping = JSON.parse(existing.column_mapping) as ColumnMapping;
-        const config: SourceConfig = {
-          name: existing.name,
-          delimiter: existing.delimiter,
-          encoding: existing.encoding,
-          dateFormat: existing.date_format,
-          skipLines: existing.skip_lines,
-          columnMapping: mapping,
-          amountMode:
-            mapping.debitAmount !== undefined ? "debit_credit" : "single",
-          signConvention: "negative_expense",
-          hasHeader: !!existing.has_header,
-        };
-        dispatch({ type: "SET_SOURCE_CONFIG", payload: config });
-        activeDelimiter = existing.delimiter;
-        activeEncoding = existing.encoding;
-        activeSkipLines = existing.skip_lines;
-        activeHasHeader = !!existing.has_header;
-      } else {
-        // Auto-detect encoding for first file
-        if (source.files.length > 0) {
+        let activeDelimiter = defaultConfig.delimiter;
+        let activeEncoding = "utf-8";
+        let activeSkipLines = 0;
+        let activeHasHeader = true;
+
+        // Restore the format as it was SAVED. Nothing is re-inferred and
+        // nothing is defaulted here — the amount mode and the sign convention
+        // are columns now (v17), and reading them is the whole point of #324.
+        let restored: SourceConfig | null = null;
+        if (existing) {
           try {
-            activeEncoding = await invoke<string>("detect_encoding", {
-              filePath: source.files[0].file_path,
-            });
-          } catch {
-            // fallback to utf-8
+            restored = { name: existing.name, ...formatFromRow(existing) };
+          } catch (e) {
+            // A stored format we cannot decode is REPORTED, not quietly swapped
+            // for a plausible default — that substitution is how amounts got
+            // flipped. The wizard then opens on a fresh configuration, because
+            // "reconfigure this source" has to be an action the user can take.
+            dispatch({ type: "SET_ERROR", payload: errorMessage(e) });
           }
         }
 
-        dispatch({
-          type: "SET_SOURCE_CONFIG",
-          payload: {
-            ...defaultConfig,
-            name: source.folder_name,
-            encoding: activeEncoding,
-          },
-        });
-      }
+        if (restored) {
+          dispatch({ type: "SET_SOURCE_CONFIG", payload: restored });
+          activeDelimiter = restored.delimiter;
+          activeEncoding = restored.encoding;
+          activeSkipLines = restored.skipLines;
+          activeHasHeader = restored.hasHeader;
+        } else {
+          // Auto-detect encoding for first file
+          if (source.files.length > 0) {
+            try {
+              activeEncoding = await invoke<string>("detect_encoding", {
+                filePath: source.files[0].file_path,
+              });
+            } catch {
+              // fallback to utf-8
+            }
+          }
 
-      // Load preview headers from first file
-      if (source.files.length > 0) {
-        await loadHeadersWithConfig(
-          source.files[0].file_path,
-          activeDelimiter,
-          activeEncoding,
-          activeSkipLines,
-          activeHasHeader
-        );
-      }
+          dispatch({
+            type: "SET_SOURCE_CONFIG",
+            payload: {
+              ...defaultConfig,
+              name: source.folder_name,
+              encoding: activeEncoding,
+            },
+          });
+        }
 
-      dispatch({ type: "SET_STEP", payload: "source-config" });
+        // Load preview headers from first file
+        if (source.files.length > 0) {
+          await loadHeadersWithConfig(
+            source.files[0].file_path,
+            activeDelimiter,
+            activeEncoding,
+            activeSkipLines,
+            activeHasHeader
+          );
+        }
+
+        dispatch({ type: "SET_STEP", payload: "source-config" });
+      } catch (e) {
+        dispatch({ type: "SET_ERROR", payload: errorMessage(e) });
+      }
     },
     [state.importedFilesBySource] // eslint-disable-line react-hooks/exhaustive-deps
   );
@@ -576,36 +600,12 @@ export function useImportWizard() {
     }
   }, [state.selectedFiles, parseFilesInternal]);
 
-  // Internal helper: runs duplicate checking against parsed rows
+  // Internal helper: runs duplicate checking against parsed rows.
+  //
+  // Deliberately writes NOTHING. The source config used to be saved here, so an
+  // import abandoned at the duplicate step still left a — possibly wrong —
+  // configuration behind. It is written by `executeImport` now (#324).
   const checkDuplicatesInternal = useCallback(async (parsedRows: ParsedRow[]) => {
-    // Save/update source config in DB
-    const config = state.sourceConfig;
-    const mappingJson = JSON.stringify(config.columnMapping);
-
-    let sourceId: number;
-    if (state.existingSource) {
-      sourceId = state.existingSource.id;
-      await updateSource(sourceId, {
-        name: config.name,
-        delimiter: config.delimiter,
-        encoding: config.encoding,
-        date_format: config.dateFormat,
-        column_mapping: mappingJson,
-        skip_lines: config.skipLines,
-        has_header: config.hasHeader,
-      });
-    } else {
-      sourceId = await createSource({
-        name: config.name,
-        delimiter: config.delimiter,
-        encoding: config.encoding,
-        date_format: config.dateFormat,
-        column_mapping: mappingJson,
-        skip_lines: config.skipLines,
-        has_header: config.hasHeader,
-      });
-    }
-
     // Check file-level duplicates (check ALL selected files, not just the first)
     let fileAlreadyImported = false;
     let existingFileId: number | undefined;
@@ -679,7 +679,7 @@ export function useImportWizard() {
       },
     });
     dispatch({ type: "SET_STEP", payload: "duplicate-check" });
-  }, [state.sourceConfig, state.existingSource, state.selectedFiles]);
+  }, [state.selectedFiles]);
 
   // Check duplicates using already-parsed preview data
   const checkDuplicates = useCallback(async () => {
@@ -727,10 +727,26 @@ export function useImportWizard() {
     try {
       const config = state.sourceConfig;
 
-      // Get or create source ID
-      const dbSource = await getSourceByName(config.name);
-      if (!dbSource) throw new Error("Source not found in database");
-      const sourceId = dbSource.id;
+      // Persist the format — the ONLY write point. It happens here rather than
+      // at the duplicate step so an abandoned import leaves no configuration
+      // behind, and it goes through `formatToRow` so no field can be dropped.
+      // It has to precede the file records, which carry a `source_id` FK.
+      const formatRow = formatToRow(config);
+      let sourceId: number;
+      if (state.existingSource) {
+        sourceId = state.existingSource.id;
+        await updateSource(sourceId, {
+          name: config.name,
+          ...formatRow,
+          template_id: state.selectedTemplateId,
+        });
+      } else {
+        sourceId = await createSource({
+          name: config.name,
+          ...formatRow,
+          template_id: state.selectedTemplateId,
+        });
+      }
 
       // Determine rows to import: new rows + non-excluded duplicates
       const includedDuplicates = state.duplicateResult.duplicateRows
@@ -854,6 +870,8 @@ export function useImportWizard() {
   }, [
     state.duplicateResult,
     state.sourceConfig,
+    state.existingSource,
+    state.selectedTemplateId,
     state.excludedDuplicateIndices,
     state.parsedPreview,
     state.selectedFiles,
@@ -919,18 +937,7 @@ export function useImportWizard() {
   }, [state.selectedFiles, state.sourceConfig, loadHeadersWithConfig]);
 
   const saveConfigAsTemplate = useCallback(async (name: string) => {
-    const config = state.sourceConfig;
-    await createTemplate({
-      name,
-      delimiter: config.delimiter,
-      encoding: config.encoding,
-      date_format: config.dateFormat,
-      skip_lines: config.skipLines,
-      has_header: config.hasHeader ? 1 : 0,
-      column_mapping: JSON.stringify(config.columnMapping),
-      amount_mode: config.amountMode,
-      sign_convention: config.signConvention,
-    });
+    await createTemplate({ name, ...formatToRow(state.sourceConfig) });
     const templates = await getAllTemplates();
     dispatch({ type: "SET_CONFIG_TEMPLATES", payload: templates });
   }, [state.sourceConfig]);
@@ -938,19 +945,21 @@ export function useImportWizard() {
   const applyConfigTemplate = useCallback((templateId: number) => {
     const template = state.configTemplates.find((t) => t.id === templateId);
     if (!template) return;
-    const mapping = JSON.parse(template.column_mapping) as ColumnMapping;
-    const newConfig: SourceConfig = {
-      name: state.sourceConfig.name,
-      delimiter: template.delimiter,
-      encoding: template.encoding,
-      dateFormat: template.date_format,
-      skipLines: template.skip_lines,
-      columnMapping: mapping,
-      amountMode: template.amount_mode,
-      signConvention: template.sign_convention,
-      hasHeader: !!template.has_header,
-    };
+
+    let newConfig: SourceConfig;
+    try {
+      newConfig = {
+        name: state.sourceConfig.name,
+        ...formatFromRow(template),
+      };
+    } catch (e) {
+      dispatch({ type: "SET_ERROR", payload: errorMessage(e) });
+      return;
+    }
+
     dispatch({ type: "SET_SOURCE_CONFIG", payload: newConfig });
+    // Applying a template COPIES its format onto the source. The id recorded
+    // here is provenance only — the copy is what the next import reads.
     dispatch({ type: "SET_SELECTED_TEMPLATE_ID", payload: templateId });
 
     // Reload headers with new config
@@ -969,17 +978,11 @@ export function useImportWizard() {
     if (!state.selectedTemplateId) return;
     const template = state.configTemplates.find((t) => t.id === state.selectedTemplateId);
     if (!template) return;
-    const config = state.sourceConfig;
+    // Writes to `import_config_templates` only: a source configured from this
+    // template keeps its own eight columns and is untouched.
     await updateTemplate(state.selectedTemplateId, {
       name: template.name,
-      delimiter: config.delimiter,
-      encoding: config.encoding,
-      date_format: config.dateFormat,
-      skip_lines: config.skipLines,
-      has_header: config.hasHeader ? 1 : 0,
-      column_mapping: JSON.stringify(config.columnMapping),
-      amount_mode: config.amountMode,
-      sign_convention: config.signConvention,
+      ...formatToRow(state.sourceConfig),
     });
     const templates = await getAllTemplates();
     dispatch({ type: "SET_CONFIG_TEMPLATES", payload: templates });
diff --git a/src/i18n/locales/en.json b/src/i18n/locales/en.json
index b8db2b1..fe4e08c 100644
--- a/src/i18n/locales/en.json
+++ b/src/i18n/locales/en.json
@@ -186,6 +186,11 @@
       "confirm": "Confirm",
       "import": "Import"
     },
+    "errors": {
+      "unsupportedAmountMode": "The amount mode saved for this source is not recognized. Reconfigure the source before importing.",
+      "unsupportedSignConvention": "The sign convention saved for this source is not recognized. Reconfigure the source before importing.",
+      "invalidColumnMapping": "The column mapping saved for this source cannot be read. Reconfigure the source before importing."
+    },
     "help": {
       "title": "How to import bank statements",
       "tips": [
diff --git a/src/i18n/locales/fr.json b/src/i18n/locales/fr.json
index 4a72f2d..2e097d0 100644
--- a/src/i18n/locales/fr.json
+++ b/src/i18n/locales/fr.json
@@ -186,6 +186,11 @@
       "confirm": "Confirmer",
       "import": "Importer"
     },
+    "errors": {
+      "unsupportedAmountMode": "Le mode de montant enregistré pour cette source n'est pas reconnu. Reconfigurez la source avant d'importer.",
+      "unsupportedSignConvention": "La convention de signe enregistrée pour cette source n'est pas reconnue. Reconfigurez la source avant d'importer.",
+      "invalidColumnMapping": "Le mapping de colonnes enregistré pour cette source est illisible. Reconfigurez la source avant d'importer."
+    },
     "help": {
       "title": "Comment importer des relevés bancaires",
       "tips": [
diff --git a/src/pages/ImportPage.tsx b/src/pages/ImportPage.tsx
index 4ce7744..211526a 100644
--- a/src/pages/ImportPage.tsx
+++ b/src/pages/ImportPage.tsx
@@ -58,8 +58,13 @@ export default function ImportPage() {
       {state.error && (
         <div className="mb-4 p-3 rounded-xl bg-[var(--card)] border-2 border-[var(--negative)] flex items-center gap-2">
           <AlertCircle size={16} className="text-[var(--negative)] shrink-0" />
+          {/*
+            The wizard reports a translation key when it has one (a stored
+            format it cannot decode) and a raw message otherwise; `defaultValue`
+            renders the latter unchanged.
+          */}
           <p className="text-sm text-[var(--foreground)]">
-            {state.error}
+            {t(state.error, { defaultValue: state.error })}
           </p>
         </div>
       )}
diff --git a/src/services/importConfigTemplateService.ts b/src/services/importConfigTemplateService.ts
index 29d86b7..e0df7c6 100644
--- a/src/services/importConfigTemplateService.ts
+++ b/src/services/importConfigTemplateService.ts
@@ -1,5 +1,15 @@
 import { getDb } from "./db";
-import type { ImportConfigTemplate } from "../shared/types";
+import type { ImportConfigTemplate, ImportFormatRow } from "../shared/types";
+
+/**
+ * A template is a named format, nothing more. Taking `ImportFormatRow` here —
+ * the same shape `importSourceService` takes — is what keeps both writers on
+ * the codec: a field added to the format cannot reach one table and miss the
+ * other.
+ */
+export interface ImportTemplateInput extends ImportFormatRow {
+  name: string;
+}
 
 export async function getAllTemplates(): Promise<ImportConfigTemplate[]> {
   const db = await getDb();
@@ -9,7 +19,7 @@ export async function getAllTemplates(): Promise<ImportConfigTemplate[]> {
 }
 
 export async function createTemplate(
-  template: Omit<ImportConfigTemplate, "id" | "created_at">
+  template: ImportTemplateInput
 ): Promise<number> {
   const db = await getDb();
   const result = await db.execute(
@@ -30,9 +40,14 @@ export async function createTemplate(
   return result.lastInsertId as number;
 }
 
+/**
+ * Editing a template touches `import_config_templates` and nothing else. A
+ * source that was configured from this template keeps its own eight columns —
+ * `import_sources.template_id` is a provenance tag, never re-read as format.
+ */
 export async function updateTemplate(
   id: number,
-  template: Omit<ImportConfigTemplate, "id" | "created_at">
+  template: ImportTemplateInput
 ): Promise<void> {
   const db = await getDb();
   await db.execute(
diff --git a/src/services/importSourceService.ts b/src/services/importSourceService.ts
index a5f5353..dfeeb7b 100644
--- a/src/services/importSourceService.ts
+++ b/src/services/importSourceService.ts
@@ -1,5 +1,24 @@
 import { getDb } from "./db";
-import type { ImportSource } from "../shared/types";
+import type { ImportFormatRow, ImportSource } from "../shared/types";
+
+/**
+ * What a writer hands to this service: the eight format fields in their
+ * persisted shape (produced by `formatToRow` — never assembled by hand) plus
+ * the source's own columns.
+ *
+ * Note it is NOT `Omit<ImportSource, "id" | ...>`: `ImportSource.has_header`
+ * is declared boolean while the codec emits the 0/1 SQLite actually stores.
+ * Taking `ImportFormatRow` here is what makes the codec the only place that
+ * normalization happens.
+ */
+export interface ImportSourceInput extends ImportFormatRow {
+  name: string;
+  description?: string | null;
+  /** Drift-detection metadata, written by #330. */
+  header_signature?: string | null;
+  /** Provenance tag — recorded and displayed, never re-read as format. */
+  template_id?: number | null;
+}
 
 export async function getAllSources(): Promise<ImportSource[]> {
   const db = await getDb();
@@ -29,12 +48,12 @@ export async function getSourceById(
 }
 
 export async function createSource(
-  source: Omit<ImportSource, "id" | "created_at" | "updated_at">
+  source: ImportSourceInput
 ): Promise<number> {
   const db = await getDb();
   const result = await db.execute(
-    `INSERT INTO import_sources (name, description, date_format, delimiter, encoding, column_mapping, skip_lines, has_header)
-     VALUES ($1, $2, $3, $4, $5, $6, $7, $8)
+    `INSERT INTO import_sources (name, description, date_format, delimiter, encoding, column_mapping, skip_lines, has_header, amount_mode, sign_convention, template_id)
+     VALUES ($1, $2, $3, $4, $5, $6, $7, $8, $9, $10, $11)
      ON CONFLICT(name) DO UPDATE SET
        description = excluded.description,
        date_format = excluded.date_format,
@@ -43,16 +62,22 @@ export async function createSource(
        column_mapping = excluded.column_mapping,
        skip_lines = excluded.skip_lines,
        has_header = excluded.has_header,
+       amount_mode = excluded.amount_mode,
+       sign_convention = excluded.sign_convention,
+       template_id = excluded.template_id,
        updated_at = CURRENT_TIMESTAMP`,
     [
       source.name,
-      source.description || null,
+      source.description ?? null,
       source.date_format,
       source.delimiter,
       source.encoding,
       source.column_mapping,
       source.skip_lines,
-      source.has_header ? 1 : 0,
+      source.has_header,
+      source.amount_mode,
+      source.sign_convention,
+      source.template_id ?? null,
     ]
   );
   // On conflict, lastInsertId may be 0 — look up the existing row
@@ -63,45 +88,30 @@ export async function createSource(
 
 export async function updateSource(
   id: number,
-  source: Partial<Omit<ImportSource, "id" | "created_at" | "updated_at">>
+  source: Partial<ImportSourceInput>
 ): Promise<void> {
   const db = await getDb();
   const fields: string[] = [];
   const values: unknown[] = [];
   let paramIndex = 1;
 
-  if (source.name !== undefined) {
-    fields.push(`name = $${paramIndex++}`);
-    values.push(source.name);
-  }
-  if (source.description !== undefined) {
-    fields.push(`description = $${paramIndex++}`);
-    values.push(source.description);
-  }
-  if (source.date_format !== undefined) {
-    fields.push(`date_format = $${paramIndex++}`);
-    values.push(source.date_format);
-  }
-  if (source.delimiter !== undefined) {
-    fields.push(`delimiter = $${paramIndex++}`);
-    values.push(source.delimiter);
-  }
-  if (source.encoding !== undefined) {
-    fields.push(`encoding = $${paramIndex++}`);
-    values.push(source.encoding);
-  }
-  if (source.column_mapping !== undefined) {
-    fields.push(`column_mapping = $${paramIndex++}`);
-    values.push(source.column_mapping);
-  }
-  if (source.skip_lines !== undefined) {
-    fields.push(`skip_lines = $${paramIndex++}`);
-    values.push(source.skip_lines);
-  }
-  if (source.has_header !== undefined) {
-    fields.push(`has_header = $${paramIndex++}`);
-    values.push(source.has_header ? 1 : 0);
-  }
+  const setColumn = (column: string, value: unknown) => {
+    fields.push(`${column} = $${paramIndex++}`);
+    values.push(value);
+  };
+
+  if (source.name !== undefined) setColumn("name", source.name);
+  if (source.description !== undefined) setColumn("description", source.description);
+  if (source.date_format !== undefined) setColumn("date_format", source.date_format);
+  if (source.delimiter !== undefined) setColumn("delimiter", source.delimiter);
+  if (source.encoding !== undefined) setColumn("encoding", source.encoding);
+  if (source.column_mapping !== undefined) setColumn("column_mapping", source.column_mapping);
+  if (source.skip_lines !== undefined) setColumn("skip_lines", source.skip_lines);
+  if (source.has_header !== undefined) setColumn("has_header", source.has_header);
+  if (source.amount_mode !== undefined) setColumn("amount_mode", source.amount_mode);
+  if (source.sign_convention !== undefined) setColumn("sign_convention", source.sign_convention);
+  if (source.header_signature !== undefined) setColumn("header_signature", source.header_signature);
+  if (source.template_id !== undefined) setColumn("template_id", source.template_id);
 
   if (fields.length === 0) return;
 
diff --git a/src/shared/types/index.ts b/src/shared/types/index.ts
index d2c36e4..d33d9c1 100644
--- a/src/shared/types/index.ts
+++ b/src/shared/types/index.ts
@@ -10,6 +10,28 @@ export interface ImportSource {
   column_mapping: string;
   skip_lines: number;
   has_header: boolean;
+  /**
+   * The two fields that decide how an amount is READ, added by migration v17.
+   * Before v17 they lived only on `import_config_templates`, so restoring a
+   * source re-inferred the mode and hardcoded the convention (#324).
+   *
+   * They are never read directly: `formatFromRow` is the single conversion
+   * point and validates them, because the column `CHECK` admits
+   * `absolute_indicator` — a value the app cannot map today.
+   */
+  amount_mode: AmountMode;
+  sign_convention: SignConvention;
+  /**
+   * Normalized header labels seen at the last successful import, for drift
+   * detection. NOT a format field: written by #330, not by the codec.
+   */
+  header_signature?: string | null;
+  /**
+   * Provenance tag only — records which template this source was configured
+   * from, and is NEVER re-read as format. The eight format fields above are
+   * authoritative, so editing a template alters no linked source.
+   */
+  template_id?: number | null;
   created_at: string;
   updated_at: string;
 }
@@ -154,17 +176,14 @@ export interface BudgetYearRow {
   previousYearTotal: number; // actual (transactions) total from the previous year
 }
 
-export interface ImportConfigTemplate {
+/**
+ * A named, reusable format. It carries exactly the eight persisted format
+ * fields (via `ImportFormatRow`) plus its identity — the same eight an
+ * `import_sources` row carries since v17.
+ */
+export interface ImportConfigTemplate extends ImportFormatRow {
   id: number;
   name: string;
-  delimiter: string;
-  encoding: string;
-  date_format: string;
-  skip_lines: number;
-  has_header: number;
-  column_mapping: string;
-  amount_mode: AmountMode;
-  sign_convention: SignConvention;
   created_at: string;
 }
 
@@ -213,16 +232,57 @@ export interface ColumnMapping {
 export type AmountMode = "single" | "debit_credit";
 export type SignConvention = "negative_expense" | "positive_expense";
 
-export interface SourceConfig {
-  name: string;
+/**
+ * --- The import format ------------------------------------------------------
+ *
+ * The eight fields that fully decide how a CSV file is read. They exist in two
+ * shapes that CANNOT be a single composed type: the persisted rows are
+ * snake_case with the mapping serialized to JSON and no boolean type in SQLite,
+ * while the wizard works in camelCase on a parsed mapping.
+ *
+ * The guarantee that no field is ever half-persisted therefore does not come
+ * from the type structure but from `src/utils/importFormat.ts`, the SINGLE
+ * conversion point between the two shapes, and from its completeness test.
+ */
+
+/** Persisted shape — snake_case, mapping as JSON, `has_header` normalized to 0/1. */
+export interface ImportFormatRow {
+  delimiter: string;
+  encoding: string;
+  date_format: string;
+  skip_lines: number;
+  /** SQLite has no boolean: written as 0/1. Reads tolerate both, see `ImportFormatRowInput`. */
+  has_header: number;
+  column_mapping: string;
+  amount_mode: AmountMode;
+  sign_convention: SignConvention;
+}
+
+/**
+ * What `formatFromRow` accepts. `import_sources` rows declare `has_header` as a
+ * boolean and `import_config_templates` rows as a number; both are the same 0/1
+ * integer at runtime, and the codec normalizes it. Widening the field here is
+ * what lets a row from either table be decoded without a cast.
+ */
+export type ImportFormatRowInput = Omit<ImportFormatRow, "has_header"> & {
+  has_header: number | boolean;
+};
+
+/** Domain shape — camelCase, mapping parsed. What the wizard manipulates. */
+export interface ImportFormat {
   delimiter: string;
   encoding: string;
   dateFormat: string;
   skipLines: number;
+  hasHeader: boolean;
   columnMapping: ColumnMapping;
   amountMode: AmountMode;
   signConvention: SignConvention;
-  hasHeader: boolean;
+}
+
+/** A format plus the source it belongs to. */
+export interface SourceConfig extends ImportFormat {
+  name: string;
 }
 
 export interface ParsedRow {
diff --git a/src/utils/importFormat.test.ts b/src/utils/importFormat.test.ts
new file mode 100644
index 0000000..8f40519
--- /dev/null
+++ b/src/utils/importFormat.test.ts
@@ -0,0 +1,388 @@
+// importFormat — the format codec (#324).
+//
+// The chantier's root bug was a format field that never reached the database
+// and a restore that invented a value for it. These tests hold the two halves
+// of the repair: the codec carries EVERY field in both directions, and the
+// wizard restores what was saved instead of re-deriving it.
+
+import { describe, it, expect } from "vitest";
+import { readFileSync } from "node:fs";
+import { resolve } from "node:path";
+import type {
+  ImportFormat,
+  ImportFormatRow,
+  ImportFormatRowInput,
+} from "../shared/types";
+import {
+  AMOUNT_MODES,
+  FORMAT_FIELD_PAIRS,
+  ImportFormatError,
+  SIGN_CONVENTIONS,
+  clearMappingForMode,
+  formatFromRow,
+  formatToRow,
+} from "./importFormat";
+
+// ---------------------------------------------------------------------------
+// Samples. Every field deliberately differs from the wizard's default config,
+// so a dropped field surfaces as a mismatch rather than as an accidental pass.
+// ---------------------------------------------------------------------------
+
+const SAMPLE_FORMAT: ImportFormat = {
+  delimiter: ",",
+  encoding: "windows-1252",
+  dateFormat: "YYYY-MM-DD",
+  skipLines: 3,
+  hasHeader: false,
+  columnMapping: { date: 2, description: 5, amount: 7 },
+  amountMode: "single",
+  signConvention: "positive_expense",
+};
+
+const SAMPLE_ROW: ImportFormatRow = {
+  delimiter: ",",
+  encoding: "windows-1252",
+  date_format: "YYYY-MM-DD",
+  skip_lines: 3,
+  has_header: 0,
+  column_mapping: '{"date":2,"description":5,"amount":7}',
+  amount_mode: "single",
+  sign_convention: "positive_expense",
+};
+
+// ---------------------------------------------------------------------------
+// Completeness — the property the codec exists to hold
+// ---------------------------------------------------------------------------
+
+describe("codec completeness (#324)", () => {
+  // `FORMAT_FIELD_PAIRS` is typed `Record<keyof ImportFormat, keyof
+  // ImportFormatRow>`, so a field added to `ImportFormat` fails to BUILD until
+  // it is listed. These two assertions close the other half: a field listed but
+  // not wired through a codec body fails the TEST.
+  it("formatToRow emits exactly the persisted fields of the table", () => {
+    expect(Object.keys(formatToRow(SAMPLE_FORMAT)).sort()).toEqual(
+      Object.values(FORMAT_FIELD_PAIRS).sort()
+    );
+  });
+
+  it("formatFromRow emits exactly the domain fields of the table", () => {
+    expect(Object.keys(formatFromRow(SAMPLE_ROW)).sort()).toEqual(
+      Object.keys(FORMAT_FIELD_PAIRS).sort()
+    );
+  });
+
+  it("covers the eight format fields, no more and no fewer", () => {
+    expect(Object.keys(FORMAT_FIELD_PAIRS)).toHaveLength(8);
+  });
+
+  it("maps every domain field to a distinct persisted field", () => {
+    const persisted = Object.values(FORMAT_FIELD_PAIRS);
+    expect(new Set(persisted).size).toBe(persisted.length);
+  });
+});
+
+// ---------------------------------------------------------------------------
+// Round trip — configure -> save -> reload
+// ---------------------------------------------------------------------------
+
+describe("round trip (#324)", () => {
+  it("survives domain -> row -> domain field by field", () => {
+    const restored = formatFromRow(formatToRow(SAMPLE_FORMAT));
+    for (const field of Object.keys(FORMAT_FIELD_PAIRS) as Array<
+      keyof ImportFormat
+    >) {
+      expect(restored[field]).toEqual(SAMPLE_FORMAT[field]);
+    }
+    expect(restored).toEqual(SAMPLE_FORMAT);
+  });
+
+  it("survives row -> domain -> row field by field", () => {
+    expect(formatToRow(formatFromRow(SAMPLE_ROW))).toEqual(SAMPLE_ROW);
+  });
+
+  // THE regression: the credit-card case. Before v17 the convention was not a
+  // column, so restoring one wrote "negative_expense" and every expense of the
+  // next import landed as income.
+  it("keeps positive_expense across a save and a reload", () => {
+    const positive: ImportFormat = {
+      ...SAMPLE_FORMAT,
+      signConvention: "positive_expense",
+    };
+    expect(formatFromRow(formatToRow(positive)).signConvention).toBe(
+      "positive_expense"
+    );
+  });
+
+  it("keeps negative_expense across a save and a reload", () => {
+    const negative: ImportFormat = {
+      ...SAMPLE_FORMAT,
+      signConvention: "negative_expense",
+    };
+    expect(formatFromRow(formatToRow(negative)).signConvention).toBe(
+      "negative_expense"
+    );
+  });
+
+  // The mode used to be re-derived from `mapping.debitAmount !== undefined`. A
+  // debit/credit source whose debit column is not mapped yet came back as
+  // "single" — this is that shape.
+  it("keeps debit_credit even when the mapping carries no debit column", () => {
+    const partial: ImportFormat = {
+      ...SAMPLE_FORMAT,
+      amountMode: "debit_credit",
+      columnMapping: { date: 0, description: 1, creditAmount: 3 },
+    };
+    const restored = formatFromRow(formatToRow(partial));
+    expect(restored.amountMode).toBe("debit_credit");
+    expect(restored.columnMapping).toEqual({
+      date: 0,
+      description: 1,
+      creditAmount: 3,
+    });
+  });
+
+  it("keeps single even when the mapping still carries debit/credit keys", () => {
+    const stale: ImportFormat = {
+      ...SAMPLE_FORMAT,
+      amountMode: "single",
+      columnMapping: { date: 0, description: 1, amount: 2, debitAmount: 3 },
+    };
+    expect(formatFromRow(formatToRow(stale)).amountMode).toBe("single");
+  });
+});
+
+// ---------------------------------------------------------------------------
+// has_header normalization
+// ---------------------------------------------------------------------------
+
+describe("has_header normalization", () => {
+  it("writes 0/1, never a boolean — SQLite has no boolean type", () => {
+    expect(formatToRow({ ...SAMPLE_FORMAT, hasHeader: true }).has_header).toBe(1);
+    expect(formatToRow({ ...SAMPLE_FORMAT, hasHeader: false }).has_header).toBe(0);
+  });
+
+  it("reads the integer an import_config_templates row carries", () => {
+    expect(formatFromRow({ ...SAMPLE_ROW, has_header: 1 }).hasHeader).toBe(true);
+    expect(formatFromRow({ ...SAMPLE_ROW, has_header: 0 }).hasHeader).toBe(false);
+  });
+
+  it("reads the boolean an import_sources row is declared with", () => {
+    const asBoolean: ImportFormatRowInput = { ...SAMPLE_ROW, has_header: true };
+    expect(formatFromRow(asBoolean).hasHeader).toBe(true);
+    expect(formatFromRow({ ...asBoolean, has_header: false }).hasHeader).toBe(false);
+  });
+});
+
+// ---------------------------------------------------------------------------
+// Validation — a value the app cannot map must raise, never fall back
+// ---------------------------------------------------------------------------
+
+describe("formatFromRow validation", () => {
+  it("accepts every mode and convention the app implements", () => {
+    for (const amount_mode of AMOUNT_MODES) {
+      expect(formatFromRow({ ...SAMPLE_ROW, amount_mode }).amountMode).toBe(
+        amount_mode
+      );
+    }
+    for (const sign_convention of SIGN_CONVENTIONS) {
+      expect(
+        formatFromRow({ ...SAMPLE_ROW, sign_convention }).signConvention
+      ).toBe(sign_convention);
+    }
+  });
+
+  // The v17 CHECK admits 'absolute_indicator' so the third mode ships without a
+  // migration. Until it is implemented, reading one must raise — a fall-back to
+  // the `single` branch would read the wrong column for every row.
+  it("rejects an amount mode the app cannot map", () => {
+    const row = {
+      ...SAMPLE_ROW,
+      amount_mode: "absolute_indicator",
+    } as unknown as ImportFormatRowInput;
+    expect(() => formatFromRow(row)).toThrow(ImportFormatError);
+    try {
+      formatFromRow(row);
+    } catch (e) {
+      expect((e as ImportFormatError).i18nKey).toBe(
+        "import.errors.unsupportedAmountMode"
+      );
+    }
+  });
+
+  it("rejects an unknown sign convention rather than defaulting", () => {
+    const row = {
+      ...SAMPLE_ROW,
+      sign_convention: "whatever",
+    } as unknown as ImportFormatRowInput;
+    expect(() => formatFromRow(row)).toThrow(ImportFormatError);
+    try {
+      formatFromRow(row);
+    } catch (e) {
+      expect((e as ImportFormatError).i18nKey).toBe(
+        "import.errors.unsupportedSignConvention"
+      );
+    }
+  });
+
+  it("rejects a column mapping that is not valid JSON", () => {
+    expect(() =>
+      formatFromRow({ ...SAMPLE_ROW, column_mapping: "{not json" })
+    ).toThrow(ImportFormatError);
+  });
+
+  it("rejects a column mapping without usable date/description columns", () => {
+    for (const column_mapping of ["null", "[]", '{"date":"2"}', "{}"]) {
+      expect(() => formatFromRow({ ...SAMPLE_ROW, column_mapping })).toThrow(
+        ImportFormatError
+      );
+    }
+  });
+});
+
+// ---------------------------------------------------------------------------
+// clearMappingForMode — the mode owns the mapping
+// ---------------------------------------------------------------------------
+
+describe("clearMappingForMode", () => {
+  const full = {
+    date: 0,
+    description: 1,
+    amount: 2,
+    debitAmount: 3,
+    creditAmount: 4,
+  };
+
+  it("drops the debit/credit columns when switching to single", () => {
+    expect(clearMappingForMode(full, "single")).toEqual({
+      date: 0,
+      description: 1,
+      amount: 2,
+    });
+  });
+
+  it("drops the amount column when switching to debit/credit", () => {
+    expect(clearMappingForMode(full, "debit_credit")).toEqual({
+      date: 0,
+      description: 1,
+      debitAmount: 3,
+      creditAmount: 4,
+    });
+  });
+
+  it("keeps date and description untouched in both directions", () => {
+    expect(clearMappingForMode({ date: 4, description: 6 }, "single")).toEqual({
+      date: 4,
+      description: 6,
+    });
+    expect(
+      clearMappingForMode({ date: 4, description: 6 }, "debit_credit")
+    ).toEqual({ date: 4, description: 6 });
+  });
+
+  // #325 replaces the `?? 0` fallbacks with an explicit "amount column not
+  // mapped" row error. Materializing the column the <select> merely DISPLAYS
+  // would make that error unreachable, so an absent column stays absent.
+  it("does not invent the column the select displays by default", () => {
+    const cleared = clearMappingForMode({ date: 0, description: 1 }, "single");
+    expect("amount" in cleared).toBe(false);
+    const pair = clearMappingForMode({ date: 0, description: 1 }, "debit_credit");
+    expect("debitAmount" in pair).toBe(false);
+    expect("creditAmount" in pair).toBe(false);
+  });
+
+  it("emits no undefined-valued keys, so the persisted JSON stays clean", () => {
+    const row = formatToRow({
+      ...SAMPLE_FORMAT,
+      columnMapping: clearMappingForMode(full, "single"),
+    });
+    expect(row.column_mapping).toBe('{"date":0,"description":1,"amount":2}');
+  });
+
+  it("is idempotent", () => {
+    const once = clearMappingForMode(full, "debit_credit");
+    expect(clearMappingForMode(once, "debit_credit")).toEqual(once);
+  });
+});
+
+// ---------------------------------------------------------------------------
+// Static guards on useImportWizard.
+//
+// The restore path and the config write point live inside `useCallback`s of a
+// React hook and the repository has no jsdom (see FilterPanel.test.tsx), so the
+// only way to assert on them is to read the source. Same technique as the guard
+// link 1 left on `parseFilesInternal` in csvAutoDetect.test.ts.
+// ---------------------------------------------------------------------------
+
+const WIZARD_SRC = readFileSync(
+  resolve(import.meta.dirname, "..", "hooks", "useImportWizard.ts"),
+  "utf-8"
+);
+
+/** The body of a `const <name> = useCallback(...)` block, up to the next one. */
+function callbackBody(name: string, nextName: string): string {
+  const start = WIZARD_SRC.indexOf(`const ${name} =`);
+  const end = WIZARD_SRC.indexOf(`const ${nextName} =`, start);
+  expect(start).toBeGreaterThan(-1);
+  expect(end).toBeGreaterThan(start);
+  return WIZARD_SRC.slice(start, end);
+}
+
+/**
+ * The block of `selectSource` that rebuilds a stored format. Scoped tightly on
+ * purpose: the fresh-source arm below it legitimately names `encoding` while
+ * auto-detecting one.
+ */
+function restoreBranch(): string {
+  const body = callbackBody("selectSource", "loadHeadersWithConfig");
+  const start = body.indexOf("let restored: SourceConfig | null = null;");
+  const end = body.indexOf("if (restored) {", start);
+  expect(start).toBeGreaterThan(-1);
+  expect(end).toBeGreaterThan(start);
+  return body.slice(start, end);
+}
+
+describe("useImportWizard — restore reads, never re-derives (#324)", () => {
+  it("routes the restore through the codec", () => {
+    expect(restoreBranch()).toContain("...formatFromRow(existing)");
+  });
+
+  it("no longer re-infers the amount mode from the mapping shape", () => {
+    expect(WIZARD_SRC).not.toContain("mapping.debitAmount !== undefined");
+  });
+
+  // The spread alone is not enough: a literal AFTER it silently wins, which is
+  // how `signConvention: "negative_expense"` overrode the stored value. The
+  // restore must name no format field of its own.
+  it("names no format field of its own in the restore", () => {
+    const branch = restoreBranch();
+    for (const field of Object.keys(FORMAT_FIELD_PAIRS)) {
+      expect(branch, `restore hardcodes ${field}`).not.toContain(`${field}:`);
+    }
+  });
+
+  it("restores the template provenance instead of blanking it", () => {
+    expect(callbackBody("selectSource", "loadHeadersWithConfig")).toContain(
+      "existing?.template_id ?? null"
+    );
+  });
+});
+
+describe("useImportWizard — the config write point (#324)", () => {
+  it("writes nothing at the duplicate step", () => {
+    const body = callbackBody("checkDuplicatesInternal", "checkDuplicates");
+    expect(body).not.toContain("createSource(");
+    expect(body).not.toContain("updateSource(");
+  });
+
+  it("writes the format at import time, through the codec", () => {
+    const body = callbackBody("executeImport", "goToStep");
+    expect(body).toContain("formatToRow(config)");
+    expect(body).toContain("createSource(");
+    expect(body).toContain("updateSource(");
+  });
+
+  it("keeps a single write point in the whole hook", () => {
+    expect(WIZARD_SRC.match(/await createSource\(/g)).toHaveLength(1);
+    expect(WIZARD_SRC.match(/await updateSource\(/g)).toHaveLength(1);
+  });
+});
diff --git a/src/utils/importFormat.ts b/src/utils/importFormat.ts
new file mode 100644
index 0000000..7fe86e9
--- /dev/null
+++ b/src/utils/importFormat.ts
@@ -0,0 +1,187 @@
+/**
+ * The import format codec — the SINGLE conversion point between the persisted
+ * shape of a CSV format (`ImportFormatRow`, shared by `import_sources` and
+ * `import_config_templates`) and its domain shape (`ImportFormat`).
+ *
+ * Why a codec and not a shared type: the two shapes cannot be unified. The
+ * persisted rows are snake_case, store the column mapping as JSON, and have no
+ * boolean type; the wizard works in camelCase on a parsed mapping. See the
+ * comment block on `ImportFormatRow` in `shared/types`.
+ *
+ * The property this file exists to hold: a field cannot be added to the format
+ * and then silently skipped on the way to or from the database. That is
+ * enforced on two levels — `FORMAT_FIELD_PAIRS` is typed against
+ * `keyof ImportFormat`, so the build breaks until a new field is listed, and
+ * `importFormat.test.ts` compares the codec's actual output keys against that
+ * table, so listing a field without wiring it fails the test. Losing
+ * `sign_convention` on the way out is what this chantier is repairing (#324).
+ */
+
+import type {
+  AmountMode,
+  ColumnMapping,
+  ImportFormat,
+  ImportFormatRow,
+  ImportFormatRowInput,
+  SignConvention,
+} from "../shared/types";
+
+/**
+ * Values the application can actually map. The database `CHECK` on
+ * `import_sources.amount_mode` deliberately admits `absolute_indicator` so the
+ * third mode ships without another migration — but until it is implemented,
+ * reading one must be a visible error, never a silent fall-through to the
+ * `single` branch (which would read the wrong column for every row).
+ */
+export const AMOUNT_MODES: readonly AmountMode[] = ["single", "debit_credit"];
+export const SIGN_CONVENTIONS: readonly SignConvention[] = [
+  "negative_expense",
+  "positive_expense",
+];
+
+/**
+ * The field correspondence table, domain key -> persisted key.
+ *
+ * Typed as `Record<keyof ImportFormat, keyof ImportFormatRow>`: adding a field
+ * to `ImportFormat` makes this literal stop satisfying the mapped type and the
+ * build fails until the field is listed here. The test then verifies that both
+ * codec directions actually emit every listed field.
+ */
+export const FORMAT_FIELD_PAIRS: Record<keyof ImportFormat, keyof ImportFormatRow> = {
+  delimiter: "delimiter",
+  encoding: "encoding",
+  dateFormat: "date_format",
+  skipLines: "skip_lines",
+  hasHeader: "has_header",
+  columnMapping: "column_mapping",
+  amountMode: "amount_mode",
+  signConvention: "sign_convention",
+};
+
+/** i18n keys for the ways a stored format can be unreadable. */
+export type ImportFormatErrorKey =
+  | "import.errors.unsupportedAmountMode"
+  | "import.errors.unsupportedSignConvention"
+  | "import.errors.invalidColumnMapping";
+
+/**
+ * Thrown when a persisted format cannot be decoded. It carries an i18n key so
+ * the wizard can surface it verbatim instead of falling back to a default
+ * format — a wrong default is precisely how the original bug wrote reversed
+ * amounts without an error.
+ */
+export class ImportFormatError extends Error {
+  readonly i18nKey: ImportFormatErrorKey;
+
+  constructor(i18nKey: ImportFormatErrorKey, detail: string) {
+    super(`${i18nKey}: ${detail}`);
+    this.name = "ImportFormatError";
+    this.i18nKey = i18nKey;
+  }
+}
+
+/** Domain format -> persisted row. `has_header` is normalized to 0/1 here. */
+export function formatToRow(format: ImportFormat): ImportFormatRow {
+  return {
+    delimiter: format.delimiter,
+    encoding: format.encoding,
+    date_format: format.dateFormat,
+    skip_lines: format.skipLines,
+    has_header: format.hasHeader ? 1 : 0,
+    column_mapping: JSON.stringify(format.columnMapping),
+    amount_mode: format.amountMode,
+    sign_convention: format.signConvention,
+  };
+}
+
+/**
+ * Persisted row -> domain format.
+ *
+ * Every enumerated field is validated against the application's whitelist and
+ * an unknown value raises rather than falls back, because both fall-backs are
+ * silent data corruption: an unmapped `amount_mode` reads the wrong column, and
+ * anything other than `positive_expense` would mean `negative_expense` and flip
+ * every sign.
+ */
+export function formatFromRow(row: ImportFormatRowInput): ImportFormat {
+  if (!AMOUNT_MODES.includes(row.amount_mode)) {
+    throw new ImportFormatError(
+      "import.errors.unsupportedAmountMode",
+      String(row.amount_mode)
+    );
+  }
+  if (!SIGN_CONVENTIONS.includes(row.sign_convention)) {
+    throw new ImportFormatError(
+      "import.errors.unsupportedSignConvention",
+      String(row.sign_convention)
+    );
+  }
+
+  let columnMapping: ColumnMapping;
+  try {
+    columnMapping = JSON.parse(row.column_mapping) as ColumnMapping;
+  } catch {
+    throw new ImportFormatError(
+      "import.errors.invalidColumnMapping",
+      row.column_mapping
+    );
+  }
+  if (
+    columnMapping === null ||
+    typeof columnMapping !== "object" ||
+    typeof columnMapping.date !== "number" ||
+    typeof columnMapping.description !== "number"
+  ) {
+    throw new ImportFormatError(
+      "import.errors.invalidColumnMapping",
+      row.column_mapping
+    );
+  }
+
+  return {
+    delimiter: row.delimiter,
+    encoding: row.encoding,
+    dateFormat: row.date_format,
+    skipLines: row.skip_lines,
+    hasHeader: !!row.has_header,
+    columnMapping,
+    amountMode: row.amount_mode,
+    signConvention: row.sign_convention,
+  };
+}
+
+/**
+ * Drop the columns belonging to the mode being left behind, so the amount mode
+ * is the source of truth for the mapping rather than the reverse.
+ *
+ * Before v17 the mode was re-inferred from `mapping.debitAmount !== undefined`,
+ * which made a stale key from an abandoned mode silently decide how amounts
+ * were read. The mode is persisted now, but a mapping still carrying both
+ * shapes would keep the two disagreeing — so switching mode prunes.
+ *
+ * The column the `<select>` merely DISPLAYS by default (`mapping.amount ?? 0`)
+ * is deliberately not materialized: #325 turns an unmapped amount column into
+ * an explicit row error, and writing a 0 here would make that error
+ * unreachable.
+ */
+export function clearMappingForMode(
+  mapping: ColumnMapping,
+  mode: AmountMode
+): ColumnMapping {
+  const base = { date: mapping.date, description: mapping.description };
+  if (mode === "debit_credit") {
+    return {
+      ...base,
+      ...(mapping.debitAmount !== undefined
+        ? { debitAmount: mapping.debitAmount }
+        : {}),
+      ...(mapping.creditAmount !== undefined
+        ? { creditAmount: mapping.creditAmount }
+        : {}),
+    };
+  }
+  return {
+    ...base,
+    ...(mapping.amount !== undefined ? { amount: mapping.amount } : {}),
+  };
+}