diff --git a/src/providers/devin-provider-merge-migration.ts b/src/providers/devin-provider-merge-migration.ts
index 746e346143..647763e657 100644
--- a/src/providers/devin-provider-merge-migration.ts
+++ b/src/providers/devin-provider-merge-migration.ts
@@ -195,33 +195,54 @@ const DEFAULT_DEPS: DevinProviderMergeStartupDeps = {
  * the snapshot is taken strictly before the save, and a backup failure throws
  * rather than writing without a rollback point.
  *
- * The auth half is deliberately detached. `startServer` is synchronous — an
- * `await` in the boot window would suspend the composition root — and
+ * Both destination slots are inspected before either account-bound file is
+ * changed. The auth write itself is deliberately detached. `startServer` is
+ * synchronous — an `await` in the boot window would suspend the composition root — and
  * `mutateStore` is async-only, so the rekey is fired after its snapshot and
- * its outcome is logged when it lands. That is safe here: the credential is
- * valid under either slot name while the `devin-cli` alias exists, a conflict
- * refuses by design, and a failed rekey simply retries on the next boot.
+ * its outcome is logged when it lands. A late concurrent conflict refuses by
+ * design, and a failed rekey simply retries on the next boot.
  */
 export function runDevinProviderMergeStartupMigration(
   config: OcxConfig,
   deps: DevinProviderMergeStartupDeps = DEFAULT_DEPS,
 ): OcxConfig {
   const projection = deps.project(config);
-  // Warnings are emitted even on a no-op: the collision case IS the warning.
+  const hasSourceConfig = config.providers?.[FROM_ID] !== undefined;
+  const hasSourceAuth = deps.hasAuthSlot(FROM_ID);
+  const hasDestinationAuth = deps.hasAuthSlot(TO_ID);
+
+  // A configured provider and its credentials are one account-bound unit. Do
+  // not move either half if the config projection refused, or if the target
+  // credential slot could belong to another account.
+  if (hasSourceConfig && (!projection.changed || hasDestinationAuth)) {
+    // Projection warnings still matter on a no-op: a config collision is the warning.
+    for (const warning of projection.warnings) console.warn(`[devin-provider-merge] ${warning}`);
+    if (projection.changed && hasDestinationAuth) {
+      console.warn(
+        `[devin-provider-merge] auth.json already has a "${TO_ID}" credential slot; `
+        + `provider "${FROM_ID}" and both credential slots were left untouched. Remove the `
+        + "unused destination credential manually, then restart.",
+      );
+    }
+    return config;
+  }
+
   for (const warning of projection.warnings) console.warn(`[devin-provider-merge] ${warning}`);
+
   let result = config;
   if (projection.changed) {
+    // Snapshot both account-bound files before changing either one.
+    if (hasSourceAuth) deps.backupAuth();
     deps.backupConfig();
     deps.save(projection.config);
     result = projection.config;
   }
 
-  // The auth rekey runs even when the config half refused or had nothing to
-  // do: a `devin-cli` credential slot is orphaned state regardless of whether
-  // a provider row still points at it, and the conflict check inside the
-  // rekey is the same refuse-on-occupied rule the config half applies.
-  if (!deps.hasAuthSlot(FROM_ID)) return result;
-  deps.backupAuth();
+  // With no legacy config row, a `devin-cli` credential slot is orphaned and
+  // can still be rekeyed under the helper's refuse-on-occupied rule. A refused
+  // config migration returned above so its account-bound slot stays put.
+  if (!hasSourceAuth) return result;
+  if (!projection.changed) deps.backupAuth();
   void deps.rekey(FROM_ID, TO_ID).then(outcome => {
     if (outcome === "conflict") {
       console.warn(
diff --git a/structure/providers/xai-grok.md b/structure/providers/xai-grok.md
index 6213ce1449..30742c36b4 100644
--- a/structure/providers/xai-grok.md
+++ b/structure/providers/xai-grok.md
@@ -136,3 +136,5 @@ Pool quota producers and account commands follow the [bounded raw-observation co
 Account quota surfaces use [safe probe diagnostics](../transports/inventory.md#account-quota-failure-diagnostics) separately from quota validity, credential health and routing authority.
 
 Live sideband admission and its bounded upstream handshake follow the [runtime contract](../runtime.md#live-sideband-handshake); the ordinary Responses WebSocket exchange remains separate.
+
+Shared startup provider-id migration preserves the account binding between configuration and OAuth credentials; see the [runtime contract](../runtime.md).
diff --git a/structure/runtime.md b/structure/runtime.md
index a09d007d7b..4daa0ba59d 100644
--- a/structure/runtime.md
+++ b/structure/runtime.md
@@ -332,6 +332,8 @@ The lightweight top-level CLI help counts Cline CLI among the fifteen registered
 
 Devin CLI credential path composition in `src/oauth/devin/cli-import.ts` follows the selected platform: Windows uses Win32 APPDATA paths, other platforms use POSIX XDG-data paths. The explicit absolute override remains verbatim; credential parsing and login behavior are unchanged.
 
+The `src/providers/devin-provider-merge-migration.ts` startup migration treats the legacy provider row and OAuth slot as one account-bound unit: an occupied destination or a refused config projection leaves both unchanged, and both backups complete before either file changes.
+
 Native Chat applies qualifying effort ceilings independently of model pins; pin selection precedes the cap and only pins or cap rewrites enter wire mapping. The [catalog effort contract](catalog.md#ultra-reasoning-level) records the V1/compaction exemptions and caller-preservation boundary.
 Pool quota producers and account commands follow the [bounded raw-observation contract](providers/openai-tiers.md#bounded-pool-quota-observations), separate from the latest display snapshot and capacity estimates.
 
diff --git a/structure/subagents.md b/structure/subagents.md
index 69047b076b..5aa8295601 100644
--- a/structure/subagents.md
+++ b/structure/subagents.md
@@ -371,3 +371,5 @@ Exact [model input declarations](config.md#explicit-per-model-capability-declara
 Provider-scoped approval reviewer settings are projected by the [catalog owner](catalog.md#provider-scoped-approval-reviewer); this surface retains its existing routing, transport and account-selection behavior.
 
 Renamed fixed-key providers receive [missing reasoning metadata](catalog.md#renamed-destination-reasoning-metadata) during derivation; explicit per-model entries and provider defaults retain precedence.
+
+Startup provider-id migration preserves the account binding between configuration and OAuth credentials; see the [runtime contract](runtime.md).
diff --git a/structure/transports/inventory.md b/structure/transports/inventory.md
index 10b5be6a6b..f649dd6646 100644
--- a/structure/transports/inventory.md
+++ b/structure/transports/inventory.md
@@ -142,3 +142,5 @@ The [explicit model-capability contract](../config.md#explicit-per-model-capabil
 Provider-scoped approval reviewer settings are projected by the [catalog owner](../catalog.md#provider-scoped-approval-reviewer); this surface retains its existing routing, transport and account-selection behavior.
 
 Renamed fixed-key providers receive [missing reasoning metadata](../catalog.md#renamed-destination-reasoning-metadata) during derivation; explicit per-model entries and provider defaults retain precedence.
+
+Startup provider-id migration preserves the account binding between configuration and OAuth credentials; see the [runtime contract](../runtime.md).
diff --git a/tests/providers/devin-provider-merge-migration.test.ts b/tests/providers/devin-provider-merge-migration.test.ts
index 9f89f2405a..a9caf50470 100644
--- a/tests/providers/devin-provider-merge-migration.test.ts
+++ b/tests/providers/devin-provider-merge-migration.test.ts
@@ -129,7 +129,7 @@ describe("devin provider merge startup runner", () => {
       backupConfig: () => { order.push("backupConfig"); },
       backupAuth: () => { order.push("backupAuth"); },
       save: () => { order.push("save"); },
-      hasAuthSlot: () => opts.hasAuthSlot ?? false,
+      hasAuthSlot: (provider: string) => provider === "devin-cli" && (opts.hasAuthSlot ?? false),
       rekey: async (from: string, to: string) => { order.push(`rekey:${from}->${to}`); return opts.rekey ? opts.rekey() : "moved" as const; },
     };
   }
@@ -137,7 +137,7 @@ describe("devin provider merge startup runner", () => {
   test("snapshots config strictly before saving, and rekeys the auth slot", async () => {
     const order: string[] = [];
     const result = runDevinProviderMergeStartupMigration(migratableConfig(), depsWith(order, { hasAuthSlot: true }));
-    expect(order.slice(0, 2)).toEqual(["backupConfig", "save"]);
+    expect(order.slice(0, 3)).toEqual(["backupAuth", "backupConfig", "save"]);
     expect(order).toContain("backupAuth");
     expect(order).toContain("rekey:devin-cli->devin");
     expect(result.providers!['devin']).toBeDefined();
@@ -178,13 +178,41 @@ describe("devin provider merge startup runner", () => {
     expect(warnings.join(" ")).toContain("[devin-provider-merge]");
   });
 
-  test("a rekey conflict warns rather than throwing out of startup", async () => {
+  test("an auth destination collision refuses both halves of the migration", () => {
     const order: string[] = [];
     const warnings: string[] = [];
     const originalWarn = console.warn;
     console.warn = (...args: unknown[]) => { warnings.push(args.map(String).join(" ")); };
     try {
-      runDevinProviderMergeStartupMigration(migratableConfig(), depsWith(order, { hasAuthSlot: true, rekey: async () => "conflict" }));
+      const deps = depsWith(order, { hasAuthSlot: true });
+      deps.hasAuthSlot = provider => provider === "devin-cli" || provider === "devin";
+      const config = migratableConfig();
+      const result = runDevinProviderMergeStartupMigration(config, deps);
+      expect(result).toBe(config);
+    } finally {
+      console.warn = originalWarn;
+    }
+    expect(order).toEqual([]);
+    expect(warnings.join(" ")).toContain('auth.json already has a "devin" credential slot');
+  });
+
+  test("a config collision never independently rekeys credentials", () => {
+    const order: string[] = [];
+    const config = migratableConfig();
+    config.providers!["devin"] = { adapter: "devin" } as never;
+    runDevinProviderMergeStartupMigration(config, depsWith(order, { hasAuthSlot: true }));
+    expect(order).toEqual([]);
+  });
+
+  test("a late rekey conflict warns rather than throwing out of startup", async () => {
+    const order: string[] = [];
+    const warnings: string[] = [];
+    const originalWarn = console.warn;
+    console.warn = (...args: unknown[]) => { warnings.push(args.map(String).join(" ")); };
+    try {
+      const deps = depsWith(order, { hasAuthSlot: true, rekey: async () => "conflict" });
+      deps.hasAuthSlot = provider => provider === "devin-cli";
+      runDevinProviderMergeStartupMigration(migratableConfig(), deps);
       // The detached promise needs a real tick, not one microtask.
       await new Promise(resolve => setTimeout(resolve, 0));
     } finally {