diff --git a/packages/opencode/test/config/config.test.ts b/packages/opencode/test/config/config.test.ts
index f3c72c7584..ccc63d807d 100644
--- a/packages/opencode/test/config/config.test.ts
+++ b/packages/opencode/test/config/config.test.ts
@@ -1446,159 +1446,131 @@ test("config parser preserves permission order while rejecting unknown top-level
 
 // MCP config merging tests
 
-test("project config can override MCP server enabled status", async () => {
-  await using tmp = await tmpdir({
-    init: async (dir) => {
-      // Simulates a base config (like from remote .well-known) with disabled MCP
-      await Filesystem.write(
-        path.join(dir, "opencode.json"),
-        JSON.stringify({
-          $schema: "https://opencode.ai/config.json",
-          mcp: {
-            jira: {
-              type: "remote",
-              url: "https://jira.example.com/mcp",
-              enabled: false,
-            },
-            wiki: {
-              type: "remote",
-              url: "https://wiki.example.com/mcp",
-              enabled: false,
-            },
-          },
-        }),
-      )
-      // Project config enables just jira
-      await Filesystem.write(
-        path.join(dir, "opencode.jsonc"),
-        JSON.stringify({
-          $schema: "https://opencode.ai/config.json",
-          mcp: {
-            jira: {
-              type: "remote",
-              url: "https://jira.example.com/mcp",
-              enabled: true,
-            },
-          },
-        }),
-      )
-    },
-  })
-  await withTestInstance({
-    directory: tmp.path,
-    fn: async (ctx) => {
-      const config = await load(ctx)
-      // jira should be enabled (overridden by project config)
-      expect(config.mcp?.jira).toEqual({
-        type: "remote",
-        url: "https://jira.example.com/mcp",
-        enabled: true,
-      })
-      // wiki should still be disabled (not overridden)
-      expect(config.mcp?.wiki).toEqual({
-        type: "remote",
-        url: "https://wiki.example.com/mcp",
-        enabled: false,
-      })
-    },
-  })
-})
-
-test("MCP config deep merges preserving base config properties", async () => {
-  await using tmp = await tmpdir({
-    init: async (dir) => {
-      // Base config with full MCP definition
-      await Filesystem.write(
-        path.join(dir, "opencode.json"),
-        JSON.stringify({
-          $schema: "https://opencode.ai/config.json",
-          mcp: {
-            myserver: {
-              type: "remote",
-              url: "https://myserver.example.com/mcp",
-              enabled: false,
-              headers: {
-                "X-Custom-Header": "value",
-              },
-            },
-          },
-        }),
-      )
-      // Override just enables it, should preserve other properties
-      await Filesystem.write(
-        path.join(dir, "opencode.jsonc"),
-        JSON.stringify({
-          $schema: "https://opencode.ai/config.json",
-          mcp: {
-            myserver: {
-              type: "remote",
-              url: "https://myserver.example.com/mcp",
-              enabled: true,
-            },
-          },
-        }),
-      )
-    },
-  })
-  await withTestInstance({
-    directory: tmp.path,
-    fn: async (ctx) => {
-      const config = await load(ctx)
-      expect(config.mcp?.myserver).toEqual({
-        type: "remote",
-        url: "https://myserver.example.com/mcp",
-        enabled: true,
-        headers: {
-          "X-Custom-Header": "value",
+it.instance("project config can override MCP server enabled status", () =>
+  Effect.gen(function* () {
+    const test = yield* TestInstance
+    // Simulates a base config (like from remote .well-known) with disabled MCP.
+    yield* writeConfigEffect(test.directory, {
+      $schema: "https://opencode.ai/config.json",
+      mcp: {
+        jira: {
+          type: "remote",
+          url: "https://jira.example.com/mcp",
+          enabled: false,
         },
-      })
-    },
-  })
-})
+        wiki: {
+          type: "remote",
+          url: "https://wiki.example.com/mcp",
+          enabled: false,
+        },
+      },
+    })
+    // Project config enables just jira.
+    yield* writeConfigEffect(
+      test.directory,
+      {
+        $schema: "https://opencode.ai/config.json",
+        mcp: {
+          jira: {
+            type: "remote",
+            url: "https://jira.example.com/mcp",
+            enabled: true,
+          },
+        },
+      },
+      "opencode.jsonc",
+    )
 
-test("local .opencode config can override MCP from project config", async () => {
-  await using tmp = await tmpdir({
-    init: async (dir) => {
-      // Project config with disabled MCP
-      await Filesystem.write(
-        path.join(dir, "opencode.json"),
-        JSON.stringify({
-          $schema: "https://opencode.ai/config.json",
-          mcp: {
-            docs: {
-              type: "remote",
-              url: "https://docs.example.com/mcp",
-              enabled: false,
-            },
+    const config = yield* Config.Service.use((svc) => svc.get())
+    expect(config.mcp?.jira).toEqual({
+      type: "remote",
+      url: "https://jira.example.com/mcp",
+      enabled: true,
+    })
+    expect(config.mcp?.wiki).toEqual({
+      type: "remote",
+      url: "https://wiki.example.com/mcp",
+      enabled: false,
+    })
+  }),
+)
+
+it.instance("MCP config deep merges preserving base config properties", () =>
+  Effect.gen(function* () {
+    const test = yield* TestInstance
+    yield* writeConfigEffect(test.directory, {
+      $schema: "https://opencode.ai/config.json",
+      mcp: {
+        myserver: {
+          type: "remote",
+          url: "https://myserver.example.com/mcp",
+          enabled: false,
+          headers: {
+            "X-Custom-Header": "value",
           },
-        }),
-      )
-      // Local .opencode directory config enables it
-      const opencodeDir = path.join(dir, ".opencode")
-      await fs.mkdir(opencodeDir, { recursive: true })
-      await Filesystem.write(
-        path.join(opencodeDir, "opencode.json"),
-        JSON.stringify({
-          $schema: "https://opencode.ai/config.json",
-          mcp: {
-            docs: {
-              type: "remote",
-              url: "https://docs.example.com/mcp",
-              enabled: true,
-            },
+        },
+      },
+    })
+    yield* writeConfigEffect(
+      test.directory,
+      {
+        $schema: "https://opencode.ai/config.json",
+        mcp: {
+          myserver: {
+            type: "remote",
+            url: "https://myserver.example.com/mcp",
+            enabled: true,
           },
-        }),
-      )
-    },
-  })
-  await withTestInstance({
-    directory: tmp.path,
-    fn: async (ctx) => {
-      const config = await load(ctx)
-      expect(config.mcp?.docs?.enabled).toBe(true)
-    },
-  })
-})
+        },
+      },
+      "opencode.jsonc",
+    )
+
+    const config = yield* Config.Service.use((svc) => svc.get())
+    expect(config.mcp?.myserver).toEqual({
+      type: "remote",
+      url: "https://myserver.example.com/mcp",
+      enabled: true,
+      headers: {
+        "X-Custom-Header": "value",
+      },
+    })
+  }),
+)
+
+it.instance("local .opencode config can override MCP from project config", () =>
+  Effect.gen(function* () {
+    const test = yield* TestInstance
+    yield* writeConfigEffect(test.directory, {
+      $schema: "https://opencode.ai/config.json",
+      mcp: {
+        docs: {
+          type: "remote",
+          url: "https://docs.example.com/mcp",
+          enabled: false,
+        },
+      },
+    })
+    yield* mkdirEffect(path.join(test.directory, ".opencode"))
+    yield* writeConfigEffect(
+      path.join(test.directory, ".opencode"),
+      {
+        $schema: "https://opencode.ai/config.json",
+        mcp: {
+          docs: {
+            type: "remote",
+            url: "https://docs.example.com/mcp",
+            enabled: true,
+          },
+        },
+      },
+      "opencode.json",
+    )
+
+    const config = yield* Config.Service.use((svc) => svc.get())
+    expect(config.mcp?.docs?.enabled).toBe(true)
+  }),
+)
 
 test("project config overrides remote well-known config", async () => {
   const originalFetch = globalThis.fetch
diff --git a/perf/test-suite.md b/perf/test-suite.md
index 2516bdc409..b37b868bbd 100644
--- a/perf/test-suite.md
+++ b/perf/test-suite.md
@@ -80,6 +80,7 @@ Repeated setup work, long sleeps/timeouts, serial integration tests, filesystem/
 | Remaining simple config load cases can use Effect-aware instance fixtures                                 | Migrated default config load and legacy TUI-key cases to `it.instance`                         | 7.78s     | 6.39s   | keep     | Single baseline before edit; after median from three sequential reruns (5.76, 6.39, 6.53). Keep as cleanup with cautious timing. |
 | Managed settings config cases can use Effect-aware instance fixtures                                      | Migrated managed override and missing-managed-file cases to `it.instance`                      | 2.40s     | 1.76s   | keep     | Single baseline before edit; after median from three sequential reruns (1.75, 1.76, 1.80).                                       |
 | Local plugin and subagent config fixtures can use Effect-aware instance fixtures                          | Migrated scoped npm plugin and custom subagent markdown cases to `it.instance`                 | 2.37s     | 1.67s   | keep     | Single baseline before edit; after median from three sequential reruns (1.66, 1.67, 1.67).                                       |
+| MCP merge config cases can use Effect-aware instance fixtures                                             | Migrated three MCP merge/override cases to `it.instance`                                       | 1.98s     | 1.95s   | keep     | Neutral timing within noise; removes manual `tmpdir` + `withTestInstance` setup from isolated filesystem-only config cases.      |
 
 ## Profiling Results
 
@@ -135,3 +136,4 @@ Full-suite sanity checks:
 | Socket reset retry test can shorten its idle-timeout path              | Reduced Bun server idle timeout and tried forced server close                            | 16.46s | failed | discard  | Shorter idle timeout changed the error shape; forced close hung. Keep the real socket reset.  |
 | `tool/webfetch` can avoid per-test instance setup                      | Switched local HTTP tests from `it.instance` to `it.live`                                | 1.219s | failed | discard  | Tool execution reads instance-local agent state, so the temp instance is required.            |
 | LSP client interop tests can shorten coarse request-handling sleeps    | Reduced fixed post-notification waits from 100ms to 10ms                                 | 4.270s | 4.740s | discard  | First run improved to 3.870s but verification was slower than baseline; not a clear win.      |
+| Config content env cases can use Effect-aware instance fixtures        | Migrated two `OPENCODE_CONFIG_CONTENT` token substitution cases to `it.instance`         |  1.95s |  2.06s | discard  | Passing but not neutral-or-better in focused reruns; keep existing explicit env cleanup.      |