diff --git a/packages/web/server/index.js b/packages/web/server/index.js
index 7c174659..7fffdc21 100644
--- a/packages/web/server/index.js
+++ b/packages/web/server/index.js
@@ -76,6 +76,7 @@ import { configureOpenCodeRuntimeProviders, resetOpenCodeRuntimeProviders } from
 import { createOpenCodeWatcherRuntime } from './lib/opencode/watcher.js';
 import { createSessionAssistRuntime } from './lib/session-assist/runtime.js';
 import { createSessionGoalRuntime } from './lib/session-goal/runtime.js';
+import { applySmallModelOverrideToOpenCodeConfig } from './lib/small-model/config-injection.js';
 import { createContextObligatoryRuntime } from './lib/context-obligatory/runtime.js';
 import { createSessionKnowledgeRuntime } from './lib/session-knowledge/runtime.js';
 import { createScheduledTasksRuntime } from './lib/scheduled-tasks/runtime.js';
@@ -1204,11 +1205,25 @@ const openCodeLifecycleRuntime = createOpenCodeLifecycleRuntime({
     const managedEnv = includeControl || includeWeb || includeMemory
       ? await (agentToolRuntime?.prepareManagedOpenCodeEnv({ includeControl, includeWeb, includeMemory }) || {})
       : {};
-    if (settings?.optimizeSystemPrompt !== true) return managedEnv;
+    const envWithSystemPrompt = settings?.optimizeSystemPrompt === true
+      ? {
+          ...managedEnv,
+          ...(await systemPromptRuntime.prepareManagedOpenCodeEnv(
+            managedEnv.OPENCODE_CONFIG_CONTENT ?? process.env.OPENCODE_CONFIG_CONTENT,
+          )),
+        }
+      : managedEnv;
 
-    const configContent = managedEnv.OPENCODE_CONFIG_CONTENT ?? process.env.OPENCODE_CONFIG_CONTENT;
-    const systemPromptEnv = await systemPromptRuntime.prepareManagedOpenCodeEnv(configContent);
-    return { ...managedEnv, ...systemPromptEnv };
+    // Apply the explicit Small Model override to the managed OpenCode config
+    // so OpenCode's own title/summary generation uses the user's chosen model.
+    const configContent = envWithSystemPrompt.OPENCODE_CONFIG_CONTENT ?? process.env.OPENCODE_CONFIG_CONTENT;
+    const withSmallModel = applySmallModelOverrideToOpenCodeConfig({
+      configContent,
+      smallModelUseDefault: settings?.smallModelUseDefault,
+      smallModelOverride: settings?.smallModelOverride,
+    });
+    if (withSmallModel === configContent) return envWithSystemPrompt;
+    return { ...envWithSystemPrompt, OPENCODE_CONFIG_CONTENT: withSmallModel };
   },
 });
 
diff --git a/packages/web/server/lib/small-model/DOCUMENTATION.md b/packages/web/server/lib/small-model/DOCUMENTATION.md
index 51bfe98f..209cb749 100644
--- a/packages/web/server/lib/small-model/DOCUMENTATION.md
+++ b/packages/web/server/lib/small-model/DOCUMENTATION.md
@@ -135,6 +135,17 @@ other runtime API.
 - `routes.js` — `GET /api/small-model` (resolution preview) and
   `POST /api/small-model/generate` (`{ prompt, system?, maxOutputTokens?,
   model?, directory? }` → `{ text, providerID, modelID, source }`).
+- `config-injection.js` — applies the Settings → Chat → Small Model override
+  to the config injected into the **managed OpenCode process**
+  (`OPENCODE_CONFIG_CONTENT`), so OpenCode's own internal `small_model`
+  consumers — session title and summary generation — use the user's explicit
+  choice instead of OpenCode's fallback chain. Only an explicit override
+  (`smallModelUseDefault === false` with a non-empty `smallModelOverride`) is
+  injected; "use default" leaves the config untouched so OpenCode's own
+  resolution stays authoritative. Wired into `getManagedOpenCodeEnv` in
+  `server/index.js`; the pure helper is unit-tested in
+  `config-injection.test.js`. External OpenCode servers are unaffected (they
+  are not launched with this env).
 
 ## Which providers the pickers may offer
 
diff --git a/packages/web/server/lib/small-model/config-injection.js b/packages/web/server/lib/small-model/config-injection.js
new file mode 100644
index 00000000..3738ec8e
--- /dev/null
+++ b/packages/web/server/lib/small-model/config-injection.js
@@ -0,0 +1,51 @@
+/**
+ * Applies the user's explicit Small Model override (Settings → Chat → Small
+ * Model) to the configuration injected into the managed OpenCode process.
+ *
+ * OpenCode's own session-title and summary generation reads `small_model`
+ * from its config layers. Previously the OpenChamber settings override only
+ * fed OpenChamber's own `/api/small-model/generate` utility service, so a
+ * configured Small Model never reached OpenCode's title generation and
+ * sessions kept their fallback/untitled state. Injecting the override as
+ * `small_model` in the managed `OPENCODE_CONFIG_CONTENT` closes that gap for
+ * the managed server.
+ *
+ * Only an explicit override applies (`smallModelUseDefault === false` with a
+ * non-empty `smallModelOverride`). "Use default" leaves the config untouched,
+ * so OpenCode's own resolution chain (config `small_model`, then its family
+ * scan) stays authoritative — this mirrors the precedence documented in
+ * `packages/web/server/lib/small-model/DOCUMENTATION.md`.
+ *
+ * Malformed user config is left untouched rather than rewritten: OpenCode's
+ * own loader is the right place to surface it, and silently rewriting it
+ * would hide the error.
+ */
+export const applySmallModelOverrideToOpenCodeConfig = ({
+  configContent,
+  smallModelUseDefault,
+  smallModelOverride,
+}) => {
+  if (smallModelUseDefault !== false) {
+    return configContent;
+  }
+  const override = typeof smallModelOverride === 'string' ? smallModelOverride.trim() : '';
+  if (!override) {
+    return configContent;
+  }
+
+  const current = (() => {
+    if (typeof configContent !== 'string' || configContent.trim().length === 0) {
+      return {};
+    }
+    try {
+      return JSON.parse(configContent);
+    } catch {
+      return null;
+    }
+  })();
+  if (current === null || typeof current !== 'object' || Array.isArray(current)) {
+    return configContent;
+  }
+
+  return JSON.stringify({ ...current, small_model: override });
+};
diff --git a/packages/web/server/lib/small-model/config-injection.test.js b/packages/web/server/lib/small-model/config-injection.test.js
new file mode 100644
index 00000000..22342038
--- /dev/null
+++ b/packages/web/server/lib/small-model/config-injection.test.js
@@ -0,0 +1,90 @@
+import { describe, expect, it } from 'vitest';
+import { applySmallModelOverrideToOpenCodeConfig } from './config-injection.js';
+
+describe('applySmallModelOverrideToOpenCodeConfig', () => {
+  it('leaves config unchanged when use-default is not explicitly disabled', () => {
+    const config = '{"model":"anthropic/claude-sonnet-4-5"}';
+    expect(
+      applySmallModelOverrideToOpenCodeConfig({
+        configContent: config,
+        smallModelUseDefault: true,
+        smallModelOverride: 'anthropic/claude-haiku-4-5',
+      }),
+    ).toBe(config);
+    expect(
+      applySmallModelOverrideToOpenCodeConfig({
+        configContent: config,
+        smallModelUseDefault: undefined,
+        smallModelOverride: 'anthropic/claude-haiku-4-5',
+      }),
+    ).toBe(config);
+  });
+
+  it('leaves config unchanged when the override is empty or whitespace', () => {
+    const config = '{"model":"anthropic/claude-sonnet-4-5"}';
+    expect(
+      applySmallModelOverrideToOpenCodeConfig({
+        configContent: config,
+        smallModelUseDefault: false,
+        smallModelOverride: '   ',
+      }),
+    ).toBe(config);
+    expect(
+      applySmallModelOverrideToOpenCodeConfig({
+        configContent: config,
+        smallModelUseDefault: false,
+        smallModelOverride: undefined,
+      }),
+    ).toBe(config);
+  });
+
+  it('injects small_model into an empty config', () => {
+    const result = applySmallModelOverrideToOpenCodeConfig({
+      configContent: undefined,
+      smallModelUseDefault: false,
+      smallModelOverride: 'anthropic/claude-haiku-4-5',
+    });
+    expect(JSON.parse(result)).toEqual({ small_model: 'anthropic/claude-haiku-4-5' });
+  });
+
+  it('injects small_model while preserving existing config keys and plugins', () => {
+    const result = applySmallModelOverrideToOpenCodeConfig({
+      configContent: '{"model":"anthropic/claude-sonnet-4-5","plugin":["file:///tool.js"]}',
+      smallModelUseDefault: false,
+      smallModelOverride: 'google/gemini-2.5-flash',
+    });
+    expect(JSON.parse(result)).toEqual({
+      model: 'anthropic/claude-sonnet-4-5',
+      plugin: ['file:///tool.js'],
+      small_model: 'google/gemini-2.5-flash',
+    });
+  });
+
+  it('replaces an existing small_model with the override', () => {
+    const result = applySmallModelOverrideToOpenCodeConfig({
+      configContent: '{"small_model":"anthropic/claude-haiku-4-5"}',
+      smallModelUseDefault: false,
+      smallModelOverride: 'google/gemini-2.5-flash',
+    });
+    expect(JSON.parse(result)).toEqual({ small_model: 'google/gemini-2.5-flash' });
+  });
+
+  it('leaves malformed config untouched instead of rewriting it', () => {
+    const config = '{not-valid-json';
+    expect(
+      applySmallModelOverrideToOpenCodeConfig({
+        configContent: config,
+        smallModelUseDefault: false,
+        smallModelOverride: 'anthropic/claude-haiku-4-5',
+      }),
+    ).toBe(config);
+    const arrayConfig = '["not","an","object"]';
+    expect(
+      applySmallModelOverrideToOpenCodeConfig({
+        configContent: arrayConfig,
+        smallModelUseDefault: false,
+        smallModelOverride: 'anthropic/claude-haiku-4-5',
+      }),
+    ).toBe(arrayConfig);
+  });
+});