diff --git a/packages/web/server/lib/opencode/shared.js b/packages/web/server/lib/opencode/shared.js
index 6df1faea..062d3d0f 100644
--- a/packages/web/server/lib/opencode/shared.js
+++ b/packages/web/server/lib/opencode/shared.js
@@ -49,9 +49,36 @@ function ensureDirs() {
 
 // ============== MARKDOWN FILE OPERATIONS ==============
 
+// Mirror of OpenCode's markdown frontmatter sanitizer (packages/opencode/src/
+// config/markdown.ts): other coding agents accept unquoted colons in YAML
+// values (e.g. `description: Build agent: creates builds`), which strict YAML
+// rejects. Rewrite those values as block scalars and retry the parse, so files
+// OpenCode accepts are parsed identically here.
+function sanitizeFrontmatter(frontmatter) {
+  return frontmatter
+    .split(/\r?\n/)
+    .flatMap((line) => {
+      if (line.trim().startsWith('#') || line.trim() === '' || /^\s+/.test(line)) return [line];
+      const entry = line.match(/^([a-zA-Z_][a-zA-Z0-9_]*)\s*:\s*(.*)$/);
+      if (!entry) return [line];
+      const value = entry[2].trim();
+      if (value === '' || value === '>' || value === '|' || value.startsWith('"') || value.startsWith("'")) return [line];
+      if (!value.includes(':')) return [line];
+      return [`${entry[1]}: |-`, `  ${value}`];
+    })
+    .join('\n');
+}
+
 function parseMdFile(filePath) {
-  const content = fs.readFileSync(filePath, 'utf8');
-  const match = content.match(/^---\r?\n([\s\S]*?)\r?\n---\r?\n([\s\S]*)$/);
+  const rawContent = fs.readFileSync(filePath, 'utf8');
+  // Strip a UTF-8 BOM so frontmatter is recognized regardless of the editor
+  // that saved the file.
+  const content = rawContent.charCodeAt(0) === 0xfeff ? rawContent.slice(1) : rawContent;
+  // The closing `---` may sit at end-of-file without a trailing newline.
+  // gray-matter (used by OpenCode) accepts that, so we must too: otherwise the
+  // whole file is treated as the prompt body and a later save rewrites the
+  // existing YAML block into the body, duplicating the frontmatter.
+  const match = content.match(/^---\r?\n([\s\S]*?)\r?\n---(?:\r?\n|$)([\s\S]*)$/);
 
   if (!match) {
     return { frontmatter: {}, body: content.trim() };
@@ -61,8 +88,14 @@ function parseMdFile(filePath) {
   try {
     frontmatter = yaml.parse(match[1]) || {};
   } catch (error) {
-    console.warn(`Failed to parse markdown frontmatter ${filePath}, treating as empty:`, error);
-    frontmatter = {};
+    // Lenient fallback for frontmatter that strict YAML rejects but OpenCode
+    // still accepts (unquoted colons in scalar values).
+    try {
+      frontmatter = yaml.parse(sanitizeFrontmatter(match[1])) || {};
+    } catch {
+      console.warn(`Failed to parse markdown frontmatter ${filePath}, treating as empty:`, error);
+      frontmatter = {};
+    }
   }
 
   const body = match[2].trim();
diff --git a/packages/web/server/lib/opencode/shared.test.js b/packages/web/server/lib/opencode/shared.test.js
new file mode 100644
index 00000000..e131aa0c
--- /dev/null
+++ b/packages/web/server/lib/opencode/shared.test.js
@@ -0,0 +1,202 @@
+import { afterEach, beforeEach, describe, expect, it } from 'vitest';
+import fs from 'fs';
+import os from 'os';
+import path from 'path';
+
+import { parseMdFile, writeMdFile } from './shared.js';
+import { updateAgent } from './agents.js';
+
+const FIXTURE_DIR = path.join(os.tmpdir(), `openchamber-shared-test-${process.pid}`);
+
+const STANDARD_MD = [
+  '---',
+  'description: My build agent',
+  'model: anthropic/claude-sonnet-4',
+  'mode: primary',
+  '---',
+  '',
+  'This is the prompt body.',
+  '',
+].join('\n');
+
+const writeFixture = (name, content) => {
+  const filePath = path.join(FIXTURE_DIR, name);
+  fs.mkdirSync(path.dirname(filePath), { recursive: true });
+  fs.writeFileSync(filePath, content, 'utf8');
+  return filePath;
+};
+
+describe('parseMdFile', () => {
+  beforeEach(() => {
+    fs.rmSync(FIXTURE_DIR, { recursive: true, force: true });
+    fs.mkdirSync(FIXTURE_DIR, { recursive: true });
+  });
+
+  afterEach(() => {
+    fs.rmSync(FIXTURE_DIR, { recursive: true, force: true });
+  });
+
+  it('parses standard YAML frontmatter', () => {
+    const file = writeFixture('standard.md', STANDARD_MD);
+    const { frontmatter, body } = parseMdFile(file);
+    expect(frontmatter).toEqual({
+      description: 'My build agent',
+      model: 'anthropic/claude-sonnet-4',
+      mode: 'primary',
+    });
+    expect(body).toBe('This is the prompt body.');
+  });
+
+  it('parses frontmatter whose closing --- is at end-of-file without a trailing newline', () => {
+    // gray-matter (used by OpenCode) accepts this shape; OpenChamber must too,
+    // otherwise a later save duplicates the YAML block.
+    const file = writeFixture('eof-close.md', [
+      '---',
+      'description: My build agent',
+      'model: anthropic/claude-sonnet-4',
+      '---',
+    ].join('\n'));
+    const { frontmatter, body } = parseMdFile(file);
+    expect(frontmatter).toEqual({
+      description: 'My build agent',
+      model: 'anthropic/claude-sonnet-4',
+    });
+    expect(body).toBe('');
+  });
+
+  it('parses frontmatter with CRLF line endings', () => {
+    const file = writeFixture('crlf.md', STANDARD_MD.replace(/\n/g, '\r\n'));
+    const { frontmatter, body } = parseMdFile(file);
+    expect(frontmatter.model).toBe('anthropic/claude-sonnet-4');
+    expect(body).toBe('This is the prompt body.');
+  });
+
+  it('parses frontmatter preceded by a UTF-8 BOM', () => {
+    const file = writeFixture('bom.md', `\uFEFF${STANDARD_MD}`);
+    const { frontmatter, body } = parseMdFile(file);
+    expect(frontmatter.description).toBe('My build agent');
+    expect(body).toBe('This is the prompt body.');
+  });
+
+  it('falls back to lenient YAML for unquoted colons in values, matching OpenCode', () => {
+    const file = writeFixture('colon.md', [
+      '---',
+      'description: Build agent: creates builds',
+      'model: anthropic/claude-sonnet-4',
+      '---',
+      '',
+      'Body',
+      '',
+    ].join('\n'));
+    const { frontmatter, body } = parseMdFile(file);
+    expect(frontmatter).toEqual({
+      description: 'Build agent: creates builds',
+      model: 'anthropic/claude-sonnet-4',
+    });
+    expect(body).toBe('Body');
+  });
+
+  it('treats files without frontmatter as a plain body', () => {
+    const file = writeFixture('plain.md', 'Just a prompt body.');
+    const { frontmatter, body } = parseMdFile(file);
+    expect(frontmatter).toEqual({});
+    expect(body).toBe('Just a prompt body.');
+  });
+});
+
+describe('writeMdFile', () => {
+  beforeEach(() => {
+    fs.rmSync(FIXTURE_DIR, { recursive: true, force: true });
+    fs.mkdirSync(FIXTURE_DIR, { recursive: true });
+  });
+
+  afterEach(() => {
+    fs.rmSync(FIXTURE_DIR, { recursive: true, force: true });
+  });
+
+  it('round-trips a canonical single frontmatter block', () => {
+    const file = writeFixture('roundtrip.md', STANDARD_MD);
+    const parsed = parseMdFile(file);
+    parsed.frontmatter.model = 'openai/gpt-5';
+    writeMdFile(file, parsed.frontmatter, parsed.body);
+
+    const content = fs.readFileSync(file, 'utf8');
+    // Exactly one frontmatter block.
+    expect(content.match(/^---\r?\n/g)).toHaveLength(1);
+
+    const reparsed = parseMdFile(file);
+    expect(reparsed.frontmatter).toEqual({
+      description: 'My build agent',
+      model: 'openai/gpt-5',
+      mode: 'primary',
+    });
+    expect(reparsed.body).toBe('This is the prompt body.');
+  });
+});
+
+describe('updateAgent frontmatter preservation', () => {
+  beforeEach(() => {
+    fs.rmSync(FIXTURE_DIR, { recursive: true, force: true });
+    fs.mkdirSync(FIXTURE_DIR, { recursive: true });
+  });
+
+  afterEach(() => {
+    fs.rmSync(FIXTURE_DIR, { recursive: true, force: true });
+  });
+
+  it('updates the model in place without duplicating YAML for a file with EOF-closed frontmatter', () => {
+    // Repro of OPE-178: the file's closing --- sits at EOF (no trailing
+    // newline). OpenCode parses it; OpenChamber previously treated the whole
+    // file as the prompt body and prepended a second frontmatter block on save.
+    const projectDir = path.join(FIXTURE_DIR, 'project');
+    const agentPath = path.join(projectDir, '.opencode', 'agents', 'strateg.md');
+    writeFixture(path.join('project', '.opencode', 'agents', 'strateg.md'), [
+      '---',
+      'description: Strategy agent',
+      'model: anthropic/claude-sonnet-4',
+      'temperature: 0.7',
+      '---',
+    ].join('\n'));
+
+    updateAgent('strateg', { model: 'openai/gpt-5' }, projectDir);
+
+    const content = fs.readFileSync(agentPath, 'utf8');
+    expect(content.match(/^---\r?\n/g)).toHaveLength(1);
+
+    const parsed = parseMdFile(agentPath);
+    expect(parsed.frontmatter).toEqual({
+      description: 'Strategy agent',
+      model: 'openai/gpt-5',
+      temperature: 0.7,
+    });
+    expect(parsed.body).toBe('');
+  });
+
+  it('preserves unrelated frontmatter fields when saving one field', () => {
+    const projectDir = path.join(FIXTURE_DIR, 'project');
+    const agentPath = path.join(projectDir, '.opencode', 'agents', 'strateg.md');
+    writeFixture(path.join('project', '.opencode', 'agents', 'strateg.md'), [
+      '---',
+      'description: Strategy agent',
+      'mode: primary',
+      'temperature: 0.7',
+      '---',
+      '',
+      'Body of strateg.',
+      '',
+    ].join('\n'));
+
+    updateAgent('strateg', { description: 'Updated strategy agent' }, projectDir);
+
+    const content = fs.readFileSync(agentPath, 'utf8');
+    expect(content.match(/^---\r?\n/g)).toHaveLength(1);
+
+    const parsed = parseMdFile(agentPath);
+    expect(parsed.frontmatter).toEqual({
+      description: 'Updated strategy agent',
+      mode: 'primary',
+      temperature: 0.7,
+    });
+    expect(parsed.body).toBe('Body of strateg.');
+  });
+});