Force forked agents to inherit parent model settings by friel-openai · Pull Request #16055 · openai/codex

friel-openai · 2026-03-27T23:29:41Z

Summary

make explicit forked spawns (fork_context = true / fork_turns) ignore child model and reasoning_effort overrides
keep forked agents on the spawning thread's effective model, provider, reasoning, service tier, verbosity, context-window, auto-compact, and profile settings
preserve forked-child prompt-cache behavior by reusing the parent prompt cache key
preserve MCP tool-surface parity for forked children by inheriting the parent MCP connection manager instead of starting a fresh manager
add v1/v2 regression tests for model inheritance, cache-key inheritance, and MCP inheritance

Test

cargo +1.93.1 test -p codex-core spawn_agent_fork_context_ignores_child_model_overrides --lib
cargo +1.93.1 test -p codex-core multi_agent_v2_spawn_fork_turns_ignores_child_model_overrides --lib
cargo +1.93.1 test -p codex-core spawn_agent_can_fork_parent_thread_history_with_sanitized_items --lib

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jif-oai

This is way too large. Forked agent should just inherit the full config of the parent agent. We shouldn't need to touch any other components

Btw profiles will disappear so feel free to ignore them all

friel-openai · 2026-04-09T17:55:15Z

@jif-oai there are a few issues fixed in this PR:

configuration that affects forked agent behavior, such as model, reasoning effort, context window
MCP servers refreshing and generating distinct tool calls, not relying on the parent's MCP
Prompt cache keys

I think your comment is primarily about the first, is that right?

jif-oai

I agree this would be good to share more cache here but I don't think this PR fits at the right level.
I would suggest 3 different small PRs:

Fully re-use the parent config in case of a fork + return a warning or error to the agent if it tries to set anything else
Shared cache key, do a proper wiring that let us "override" the cache key for a given thread. That we fix at the creation of the thread and gets set directly into the API client
Make the MCP tools listing stable. Either by definition or using a snapshot

jif-oai · 2026-04-10T13:47:47Z

-            mcp_connection_manager: Arc::new(RwLock::new(McpConnectionManager::new_uninitialized(
-                &config.permissions.approval_policy,
-            ))),
+            mcp_connection_manager: session_configuration


We can't do this. MCP manager get replaced/modified if we have sandbox updates, elicitation, ...
In general, we can't share the manager without re-designing it
If I get it correctly, the only reason we do this is because we want a stability over built_tools. First, this shouldn't drift most of the time. Second, we should use a snapshot around it to fix the issue then

jif-oai · 2026-04-10T13:48:25Z

        let inherited_exec_policy = self
            .inherited_exec_policy_for_source(&state, Some(&session_source), &config)
            .await;
+        let inherited_prompt_cache_key = self


This resume path should not opportunistically inherit parent cache/MCP state from a live parent. SubAgentSource::ThreadSpawn does not persist whether this child was originally forked, so the same resumed agent will get parent-shared cache/MCP only when the parent is currently loaded, and isolated state otherwise... we can't have such non-determinism

jif-oai · 2026-04-10T13:52:15Z

        inherited_shell_snapshot: None,
        user_shell_override: None,
        inherited_exec_policy: Some(Arc::clone(&parent_session.services.exec_policy)),
+        inherited_prompt_cache_key: Some(parent_session.prompt_cache_key()),


Those are not fork

jif-oai · 2026-04-10T13:53:29Z

@@ -770,6 +773,8 @@ impl ThreadManagerState {
        metrics_service_name: Option<String>,
        inherited_shell_snapshot: Option<Arc<ShellSnapshot>>,


this is a lot of optional inherited things. We should have a small builder pattern for the inherited things

jif-oai · 2026-04-10T13:54:20Z

@@ -225,7 +225,11 @@ fn build_agent_shared_config(turn: &TurnContext) -> Result<Config, FunctionCallE
    let mut config = (*base_config).clone();


All of this should be cleared IMO
We should just re-use the full same config... otherwise this will become brittle over time

jif-oai · 2026-04-10T13:55:20Z

    config.model = Some(turn.model_info.slug.clone());
    config.model_provider = turn.provider.clone();
-    config.model_reasoning_effort = turn.reasoning_effort;
+    // Forked children must preserve the spawning turn's effective model settings, including a


this code path is not only for forked agents

jif-oai · 2026-04-10T13:56:27Z

    pub(crate) inherited_shell_snapshot: Option<Arc<ShellSnapshot>>,
    pub(crate) inherited_exec_policy: Option<Arc<ExecPolicyManager>>,
+    pub(crate) inherited_prompt_cache_key: Option<ThreadId>,
+    pub(crate) inherited_mcp_connection_manager: Option<Arc<RwLock<McpConnectionManager>>>,


Up Option of an Arc of RwLock looks like a code smell in Rust... there are better alternatives for hot swap shared references

jif-oai · 2026-04-10T13:57:30Z

 }

+#[tokio::test]
+async fn spawn_agent_fork_context_ignores_child_model_overrides() {


this test is kind of pointless IMO
A better test would be an integration test that makes sure the full context is the same at the API level

jif-oai · 2026-04-10T13:57:46Z

+
+    let mut config = (*turn.config).clone();
+    let mut role_provider =
+        built_in_model_providers(/* openai_base_url */ /*openai_base_url*/ None)["openai"].clone();


dup in the comment

jif-oai · 2026-04-10T14:00:13Z

-            args.reasoning_effort,
-        )
-        .await?;
+        if !args.fork_context {


we should probably error to the model if it specify fork with a model and reasoning effort. Or at least emit a warning

## Summary When a `spawn_agent` call does a full-history fork, keep the parent's effective agent type and model configuration instead of applying child role/model overrides. This is the minimal config-inheritance slice of #16055. Prompt-cache key inheritance and MCP tool-surface stability are split into follow-up PRs. ## Design - Reject `agent_type`, `model`, and `reasoning_effort` for v1 `fork_context` spawns. - Reject `agent_type`, `model`, and `reasoning_effort` for v2 `fork_turns = "all"` spawns. - Keep v2 partial-history forks (`fork_turns = "N"`) configurable; requested model/reasoning overrides and role config still apply there. - Keep non-forked spawn behavior unchanged. ## Tests - `cargo +1.93.1 test -p codex-core spawn_agent_fork_context --lib` - `cargo +1.93.1 test -p codex-core multi_agent_v2_spawn_fork_turns --lib` - `cargo +1.93.1 test -p codex-core multi_agent_v2_spawn_partial_fork_turns_allows_agent_type_override --lib`

## Summary When a `spawn_agent` call does a full-history fork, keep the parent's effective agent type and model configuration instead of applying child role/model overrides. This is the minimal config-inheritance slice of openai#16055. Prompt-cache key inheritance and MCP tool-surface stability are split into follow-up PRs. ## Design - Reject `agent_type`, `model`, and `reasoning_effort` for v1 `fork_context` spawns. - Reject `agent_type`, `model`, and `reasoning_effort` for v2 `fork_turns = "all"` spawns. - Keep v2 partial-history forks (`fork_turns = "N"`) configurable; requested model/reasoning overrides and role config still apply there. - Keep non-forked spawn behavior unchanged. ## Tests - `cargo +1.93.1 test -p codex-core spawn_agent_fork_context --lib` - `cargo +1.93.1 test -p codex-core multi_agent_v2_spawn_fork_turns --lib` - `cargo +1.93.1 test -p codex-core multi_agent_v2_spawn_partial_fork_turns_allows_agent_type_override --lib`

chatgpt-codex-connector Bot reviewed Mar 27, 2026

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Comment thread codex-rs/core/src/tools/handlers/multi_agents/spawn.rs

Comment thread codex-rs/core/src/tools/handlers/multi_agents_tests.rs

github-actions Bot mentioned this pull request Mar 28, 2026

📊 AI CLI 工具社区动态日报 2026-03-28 gsscsd/big_model_radar#107

Open

friel-openai force-pushed the dev/friel/fork-context-inherits-parent-model branch 4 times, most recently from 8bbc644 to 3379d41 Compare April 1, 2026 00:07

friel-openai mentioned this pull request Apr 1, 2026

Add deferred watchdog namespace tools #16181

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friel-openai force-pushed the dev/friel/fork-context-inherits-parent-model branch from 3379d41 to fc08138 Compare April 1, 2026 23:46

github-actions Bot mentioned this pull request Apr 2, 2026

📊 AI CLI 工具社区动态日报 2026-04-02 gsscsd/big_model_radar#120

Open

friel-openai force-pushed the dev/friel/fork-context-inherits-parent-model branch 3 times, most recently from 7d0d474 to 01c2cff Compare April 7, 2026 17:33

friel-openai mentioned this pull request Apr 7, 2026

Add role-level fork_context defaults #17025

Closed

friel-openai and others added 11 commits April 7, 2026 22:25

feat(core): force forked agents to inherit parent model

1b4579f

Co-authored-by: Codex <noreply@openai.com>

test(core): fix multi-agent v2 fork-context regression

24ef818

Co-authored-by: Codex <noreply@openai.com>

fix(core): preserve effective effort for fork-context children

e2fbe5c

fix(core): restore parent fork spawn config

0994207

Inherit fork cache state from parent thread

9f707a3

Cover forked prompt-cache and MCP inheritance

83f459c

fix(core): allow fork cache plumbing argument count

2bfd6fa

fix(core): restore v1 fork-context config guard

b2496d0

fix(core): resolve slim fork-context rebase drift

7590d28

fix(app-server): update ThreadConfigSnapshot test fixture

e70da7a

fix(core): align spawn config call style after rebase

e351cad

friel-openai force-pushed the dev/friel/fork-context-inherits-parent-model branch from 6b62601 to e351cad Compare April 7, 2026 22:31

jif-oai reviewed Apr 9, 2026

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jif-oai mentioned this pull request Apr 9, 2026

[codex] Keep forked agents on the parent config #17242

Closed

friel-openai mentioned this pull request Apr 9, 2026

Make forked agent spawns keep parent model config #17247

Merged

jif-oai requested changes Apr 10, 2026

View reviewed changes

friel-openai closed this Apr 10, 2026

		@@ -770,6 +773,8 @@ impl ThreadManagerState {
		metrics_service_name: Option<String>,
		inherited_shell_snapshot: Option<Arc<ShellSnapshot>>,

		@@ -225,7 +225,11 @@ fn build_agent_shared_config(turn: &TurnContext) -> Result<Config, FunctionCallE
		let mut config = (*base_config).clone();

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friel-openai commented Mar 27, 2026 •

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