diff --git a/packages/vscode/src/DOCUMENTATION.md b/packages/vscode/src/DOCUMENTATION.md
index f6345cae..e3447e21 100644
--- a/packages/vscode/src/DOCUMENTATION.md
+++ b/packages/vscode/src/DOCUMENTATION.md
@@ -58,7 +58,7 @@ The webview CSP permits `blob:` only for `worker-src` so shared UI parsers can r
 - `bridge-config-runtime.ts`
   - Config and skills message handlers (`api:config/*`).
   - Includes OpenCode resolution diagnostics parity handler used by shared UI (`/api/config/opencode-resolution`).
-  - OpenCode JSONC reads in `opencodeConfig.ts` fail closed on a partial or non-object `jsonc-parser` tree (`INVALID_JSONC`) so mutations cannot rewrite a `$schema`-only stub over an existing config. Comment-only files read as empty. A broken layer is omitted from the merge and recorded on `layerErrors`; valid sibling layers still load, including plugin list/read via `getPluginConfigSources`. Writes still refuse to overwrite the broken file.
+  - OpenCode JSONC reads in `opencodeConfig.ts` fail closed on a partial or non-object `jsonc-parser` tree (`INVALID_JSONC`) so mutations cannot rewrite a `$schema`-only stub over an existing config. Comment-only files read as empty, while other content that yields no JSON value (YAML, plain text) fails closed. A broken layer is omitted from the merge and recorded on `layerErrors`; valid sibling layers still load, including plugin list/read via `getPluginConfigSources`. Writes still refuse to overwrite the broken file.
 
 - `bridge-settings-runtime.ts`
   - Settings read/write and OpenCode skills discovery via API for bridge consumers.
diff --git a/packages/vscode/src/opencodeConfig.config-parse.test.ts b/packages/vscode/src/opencodeConfig.config-parse.test.ts
index 7c696696..74d6d737 100644
--- a/packages/vscode/src/opencodeConfig.config-parse.test.ts
+++ b/packages/vscode/src/opencodeConfig.config-parse.test.ts
@@ -48,6 +48,14 @@ const VALID_CONFIG = [
   '',
 ].join('\n');
 
+const isInvalidJsoncError = (error: unknown): boolean => {
+  if (!(error instanceof Error) || !/cannot be loaded safely/.test(error.message)) {
+    return false;
+  }
+  // SAFETY: the config layer throws Error instances carrying the coded `code` field.
+  return (error as Error & { code?: string }).code === 'INVALID_JSONC';
+};
+
 describe('opencodeConfig JSONC parse safety (issue #2923)', () => {
   let tempDir: string;
   let previousOpenCodeConfig: string | undefined;
@@ -68,14 +76,7 @@ describe('opencodeConfig JSONC parse safety (issue #2923)', () => {
     fs.writeFileSync(configPath, PARTIAL_PARSE_CONFIG, 'utf8');
     process.env.OPENCODE_CONFIG = configPath;
 
-    assert.throws(
-      () => updateMcpConfig('openproject', { enabled: true }),
-      (error: unknown) => (
-        error instanceof Error
-        && /cannot be loaded safely/.test(error.message)
-        && (error as Error & { code?: string }).code === 'INVALID_JSONC'
-      ),
-    );
+    assert.throws(() => updateMcpConfig('openproject', { enabled: true }), isInvalidJsoncError);
     assert.equal(fs.readFileSync(configPath, 'utf8'), PARTIAL_PARSE_CONFIG);
     assert.equal(fs.existsSync(`${configPath}.openchamber.backup`), false);
   });
@@ -102,6 +103,17 @@ describe('opencodeConfig JSONC parse safety (issue #2923)', () => {
     assert.deepEqual(listPluginEntries(), []);
   });
 
+  test('refuses MCP updates against content that yields no JSON value at all', () => {
+    const configPath = path.join(tempDir, 'yamlish.jsonc');
+    const contents = 'mcp:\n  openproject:\n    type: remote\n';
+    fs.writeFileSync(configPath, contents, 'utf8');
+    process.env.OPENCODE_CONFIG = configPath;
+
+    assert.throws(() => updateMcpConfig('openproject', { enabled: true }), isInvalidJsoncError);
+    assert.equal(fs.readFileSync(configPath, 'utf8'), contents);
+    assert.equal(fs.existsSync(`${configPath}.openchamber.backup`), false);
+  });
+
   test('lists custom-layer plugins when a project layer is unparseable', () => {
     const customPath = path.join(tempDir, 'custom.jsonc');
     const projectDir = path.join(tempDir, 'project');
diff --git a/packages/vscode/src/opencodeConfig.ts b/packages/vscode/src/opencodeConfig.ts
index 66142c36..78297fc6 100644
--- a/packages/vscode/src/opencodeConfig.ts
+++ b/packages/vscode/src/opencodeConfig.ts
@@ -567,12 +567,17 @@ const formatJsoncParseError = (filePath: string, errors: ParseError[]): string =
 const isInvalidJsoncError = (error: unknown): error is Error & { code: string } =>
   Boolean(error && typeof error === 'object' && 'code' in error && error.code === INVALID_JSONC);
 
+// Comment-only / whitespace-only files parse to undefined with nothing but
+// ValueExpected. Any other error means real content we failed to understand
+// (YAML, plain text, a stray leading token), which must not read as empty.
+const isCommentOnlyParse = (parsed: unknown, errors: ParseError[]): boolean =>
+  parsed === undefined
+  && errors.every((entry) => printParseErrorCode(entry.error) === 'ValueExpected');
+
 const parseConfigObject = (content: string, filePath: string): Record<string, unknown> => {
   const errors: ParseError[] = [];
   const parsed = parseJsonc(content, errors, { allowTrailingComma: true });
-  // Comment-only / no JSON value: jsonc-parser returns undefined plus ValueExpected.
-  // That is empty config, not a partial tree. The data-loss bug is errors + object.
-  if (parsed === undefined) {
+  if (isCommentOnlyParse(parsed, errors)) {
     return {};
   }
   if (errors.length > 0 || !parsed || typeof parsed !== 'object' || Array.isArray(parsed)) {
diff --git a/packages/web/server/lib/opencode/DOCUMENTATION.md b/packages/web/server/lib/opencode/DOCUMENTATION.md
index e783bb36..de3afd8d 100644
--- a/packages/web/server/lib/opencode/DOCUMENTATION.md
+++ b/packages/web/server/lib/opencode/DOCUMENTATION.md
@@ -71,7 +71,7 @@ This module provides OpenCode server integration utilities for the web server ru
 - `ensureDirs()`: Creates required OpenCode directories.
 - `parseMdFile(filePath)`, `writeMdFile(filePath, frontmatter, body)`: Markdown file operations with YAML frontmatter.
 - `getConfigPaths(workingDirectory)`, `readConfigLayers(workingDirectory)`, `readConfig(workingDirectory)`: Config file operations with layer merging (user, project, custom). `readConfigLayers` isolates `INVALID_JSONC` per layer: a broken file is omitted from the merge (`{}` for that layer only), recorded on `layerErrors`, and does not block valid sibling layers. Writes still refuse to overwrite the broken file.
-- `readConfigFile(filePath)`: Reads one config file. Missing, whitespace-only, and comment-only files (no JSON value) return `{}`. A `jsonc-parser` error that produces a partial or non-object tree throws `INVALID_JSONC` — partial parse trees must never be treated as authoritative (avoids rewriting a `$schema`-only stub over a full config).
+- `readConfigFile(filePath)`: Reads one config file. Missing, whitespace-only, and comment-only files return `{}`; a comment-only file is recognized by `ValueExpected` being the only parse error. A `jsonc-parser` error that produces a partial or non-object tree throws `INVALID_JSONC` — partial parse trees must never be treated as authoritative (avoids rewriting a `$schema`-only stub over a full config). Content that yields no JSON value for any other reason (YAML, plain text) also throws instead of reading as empty.
 - `readConfigLayer(filePath)`: Same parse as `readConfigFile`, but isolates `INVALID_JSONC` to `{ config: {}, error }` so plugin/MCP/agent readers can skip one broken layer without aborting valid siblings. Writes still refuse to overwrite the broken file.
 - `writeConfig(config, filePath)`: Writes config with automatic backup. Refuses to overwrite an existing non-empty file that fails the same JSONC parse check.
 - `getJsonEntrySource(layers, sectionKey, entryName)`: Resolves which config layer provides an entry. A failed custom or user layer throws `INVALID_JSONC` instead of treating that file as empty. A failed project layer is skipped so a valid user/custom entry can still be found.
diff --git a/packages/web/server/lib/opencode/shared.js b/packages/web/server/lib/opencode/shared.js
index bc90ff31..d01168f3 100644
--- a/packages/web/server/lib/opencode/shared.js
+++ b/packages/web/server/lib/opencode/shared.js
@@ -182,12 +182,18 @@ function formatJsoncParseError(filePath, errors) {
   return `OpenCode configuration at ${filePath} contains invalid JSONC and cannot be loaded safely${location}`;
 }
 
+function isCommentOnlyParse(parsed, errors) {
+  // Comment-only / whitespace-only files parse to undefined with nothing but
+  // ValueExpected. Any other error means real content we failed to understand
+  // (YAML, plain text, a stray leading token), which must not read as empty.
+  return parsed === undefined
+    && errors.every((entry) => printParseErrorCode(entry.error) === 'ValueExpected');
+}
+
 function parseConfigObject(content, filePath) {
   const errors = [];
   const parsed = parseJsonc(content, errors, { allowTrailingComma: true });
-  // Comment-only / no JSON value: jsonc-parser returns undefined plus ValueExpected.
-  // That is empty config, not a partial tree. The data-loss bug is errors + object.
-  if (parsed === undefined) {
+  if (isCommentOnlyParse(parsed, errors)) {
     return {};
   }
   if (errors.length > 0 || !parsed || typeof parsed !== 'object' || Array.isArray(parsed)) {
diff --git a/packages/web/server/lib/opencode/shared.test.js b/packages/web/server/lib/opencode/shared.test.js
index 88384ae9..2c644b19 100644
--- a/packages/web/server/lib/opencode/shared.test.js
+++ b/packages/web/server/lib/opencode/shared.test.js
@@ -343,6 +343,16 @@ describe('readConfigFile / writeConfig JSONC safety (issue #2923)', () => {
     expect(readConfigFile(file)).toEqual({});
   });
 
+  it('throws INVALID_JSONC for content that yields no JSON value at all', () => {
+    const yamlish = writeFixture('yamlish.jsonc', 'mcp:\n  openproject:\n    type: remote\n');
+    expect(() => readConfigFile(yamlish)).toThrow(/cannot be loaded safely/);
+    expect(() => writeConfig({ $schema: 'https://opencode.ai/config.json' }, yamlish)).toThrow(
+      /cannot be loaded safely/,
+    );
+    expect(fs.readFileSync(yamlish, 'utf8')).toBe('mcp:\n  openproject:\n    type: remote\n');
+    expect(fs.existsSync(`${yamlish}.openchamber.backup`)).toBe(false);
+  });
+
   it('keeps a valid custom layer readable when a project layer is unparseable', () => {
     const custom = writeFixture('custom.jsonc', VALID_CONFIG);
     const projectDir = path.join(FIXTURE_DIR, 'project');