diff --git a/agents/autowebcompat-repro/hackbot_agents/autowebcompat_repro/__main__.py b/agents/autowebcompat-repro/hackbot_agents/autowebcompat_repro/__main__.py
index bdce00e69c..f397ee7bb9 100644
--- a/agents/autowebcompat-repro/hackbot_agents/autowebcompat_repro/__main__.py
+++ b/agents/autowebcompat-repro/hackbot_agents/autowebcompat_repro/__main__.py
@@ -1,12 +1,11 @@
-from hackbot_runtime import HackbotContext, run_async
-from pydantic_settings import BaseSettings, SettingsConfigDict
+from hackbot_runtime import BaseAgentInputs, HackbotContext, run_async
 
 from .agent import AutowebcompatReproResult, run_autowebcompat_repro
 from .firefox_install import install_firefox_nightly
 from .setup_profile import setup_profile
 
 
-class AgentInputs(BaseSettings):
+class AgentInputs(BaseAgentInputs):
     bugzilla_mcp_url: str
     bug_data: str | None = None
     bug_id: int | None = None
@@ -14,11 +13,9 @@ class AgentInputs(BaseSettings):
     max_turns: int | None = None
     effort: str | None = None
 
-    model_config = SettingsConfigDict(extra="ignore")
-
 
 async def main(ctx: HackbotContext) -> AutowebcompatReproResult:
-    inputs = AgentInputs()
+    inputs = ctx.load_inputs(AgentInputs)
 
     # Provision a fresh Nightly at startup so each run reproduces against a
     # current build; drive the binary the install reports back.
diff --git a/agents/bug-fix/hackbot_agents/bug_fix/__main__.py b/agents/bug-fix/hackbot_agents/bug_fix/__main__.py
index c1423e3d67..dad341e1d9 100644
--- a/agents/bug-fix/hackbot_agents/bug_fix/__main__.py
+++ b/agents/bug-fix/hackbot_agents/bug_fix/__main__.py
@@ -1,21 +1,18 @@
-from hackbot_runtime import HackbotContext, run_async
-from pydantic_settings import BaseSettings, SettingsConfigDict
+from hackbot_runtime import BaseAgentInputs, HackbotContext, run_async
 
 from .agent import BugFixResult, run_bug_fix
 
 
-class AgentInputs(BaseSettings):
+class AgentInputs(BaseAgentInputs):
     bug_id: int
     bugzilla_mcp_url: str
     model: str | None = None
     max_turns: int | None = None
     effort: str | None = None
 
-    model_config = SettingsConfigDict(extra="ignore")
-
 
 async def main(ctx: HackbotContext) -> BugFixResult:
-    inputs = AgentInputs()
+    inputs = ctx.load_inputs(AgentInputs)
 
     return await run_bug_fix(
         task="Triage and fix the bug, and verify the fix",
diff --git a/agents/build-repair/hackbot_agents/build_repair/__main__.py b/agents/build-repair/hackbot_agents/build_repair/__main__.py
index bef34de4ef..e64194ff51 100644
--- a/agents/build-repair/hackbot_agents/build_repair/__main__.py
+++ b/agents/build-repair/hackbot_agents/build_repair/__main__.py
@@ -1,12 +1,11 @@
 import os
 
-from hackbot_runtime import HackbotContext, run_async
-from pydantic_settings import BaseSettings, SettingsConfigDict
+from hackbot_runtime import BaseAgentInputs, HackbotContext, run_async
 
 from .agent import BuildRepairResult, run_build_repair
 
 
-class AgentInputs(BaseSettings):
+class AgentInputs(BaseAgentInputs):
     bug_id: int | None = None
     git_commit: str
     failure_tasks: dict[str, str]
@@ -15,11 +14,9 @@ class AgentInputs(BaseSettings):
     model: str | None = None
     max_turns: int | None = None
 
-    model_config = SettingsConfigDict(extra="ignore")
-
 
 async def main(ctx: HackbotContext) -> BuildRepairResult:
-    inputs = AgentInputs()
+    inputs = ctx.load_inputs(AgentInputs)
 
     # The build failure lives at this commit; pin the checkout there before the
     # runtime prepares the source tree (consumed in HackbotContext.source_repo).
diff --git a/agents/frontend-triage/hackbot_agents/frontend_triage/__main__.py b/agents/frontend-triage/hackbot_agents/frontend_triage/__main__.py
index aa24b74b05..8254e90149 100644
--- a/agents/frontend-triage/hackbot_agents/frontend_triage/__main__.py
+++ b/agents/frontend-triage/hackbot_agents/frontend_triage/__main__.py
@@ -1,5 +1,4 @@
-from hackbot_runtime import HackbotContext, run_async
-from pydantic_settings import BaseSettings, SettingsConfigDict
+from hackbot_runtime import BaseAgentInputs, HackbotContext, run_async
 
 from .agent import FrontendTriageResult, run_frontend_triage
 
@@ -13,18 +12,16 @@
 )
 
 
-class AgentInputs(BaseSettings):
+class AgentInputs(BaseAgentInputs):
     bug_id: int
     bugzilla_mcp_url: str
     model: str | None = None
     max_turns: int | None = None
     effort: str | None = None
 
-    model_config = SettingsConfigDict(extra="ignore")
-
 
 async def main(ctx: HackbotContext) -> FrontendTriageResult:
-    inputs = AgentInputs()
+    inputs = ctx.load_inputs(AgentInputs)
 
     return await run_frontend_triage(
         task=TRIAGE_TASK,
diff --git a/agents/test-plan-generator/hackbot_agents/test_plan_generator/__main__.py b/agents/test-plan-generator/hackbot_agents/test_plan_generator/__main__.py
index af67eac60b..348e3a76b0 100644
--- a/agents/test-plan-generator/hackbot_agents/test_plan_generator/__main__.py
+++ b/agents/test-plan-generator/hackbot_agents/test_plan_generator/__main__.py
@@ -1,11 +1,10 @@
-from hackbot_runtime import HackbotContext, run_async
-from pydantic_settings import BaseSettings, SettingsConfigDict
+from hackbot_runtime import BaseAgentInputs, HackbotContext, run_async
 
 from .agent import TestPlanGeneratorResult, run_test_plan_generator
 from .firefox_install import install_firefox_nightly
 
 
-class AgentInputs(BaseSettings):
+class AgentInputs(BaseAgentInputs):
     feature_name: str
     feature_description: str
     test_scope: str
@@ -13,11 +12,9 @@ class AgentInputs(BaseSettings):
     max_turns: int | None = None
     effort: str | None = None
 
-    model_config = SettingsConfigDict(extra="ignore")
-
 
 async def main(ctx: HackbotContext) -> TestPlanGeneratorResult:
-    inputs = AgentInputs()
+    inputs = ctx.load_inputs(AgentInputs)
 
     firefox_path = str(install_firefox_nightly())
 
diff --git a/libs/hackbot-runtime/hackbot_runtime/__init__.py b/libs/hackbot-runtime/hackbot_runtime/__init__.py
index 277d2084f0..750a42671e 100644
--- a/libs/hackbot-runtime/hackbot_runtime/__init__.py
+++ b/libs/hackbot-runtime/hackbot_runtime/__init__.py
@@ -1,6 +1,6 @@
 from hackbot_runtime.actions.recorder import ActionsRecorder
 from hackbot_runtime.config import HackbotConfig
-from hackbot_runtime.context import HackbotContext
+from hackbot_runtime.context import BaseAgentInputs, HackbotContext
 from hackbot_runtime.errors import AgentError
 from hackbot_runtime.results import HackbotAgentResult
 from hackbot_runtime.runtime import run, run_async
@@ -13,6 +13,7 @@
     "HackbotAgentResult",
     "HackbotConfig",
     "HackbotContext",
+    "BaseAgentInputs",
     "SignedPolicyUploader",
     "ensure_source_repo",
     "run",
diff --git a/libs/hackbot-runtime/hackbot_runtime/context.py b/libs/hackbot-runtime/hackbot_runtime/context.py
index 8269de8fd1..dc29c2202a 100644
--- a/libs/hackbot-runtime/hackbot_runtime/context.py
+++ b/libs/hackbot-runtime/hackbot_runtime/context.py
@@ -19,15 +19,20 @@
 import uuid
 from functools import cached_property
 from pathlib import Path
-from typing import TYPE_CHECKING
+from typing import TYPE_CHECKING, TypeVar
 
 from pydantic import Field, PrivateAttr
-from pydantic_settings import BaseSettings, SettingsConfigDict
+from pydantic_settings import (
+    BaseSettings,
+    PydanticBaseSettingsSource,
+    SettingsConfigDict,
+)
 
 from hackbot_runtime import artifacts, changes
 from hackbot_runtime.actions.recorder import ActionsRecorder
 from hackbot_runtime.config import HackbotConfig, load_config
 from hackbot_runtime.providers import AnthropicAuth
+from hackbot_runtime.remote_config import load_remote_config
 from hackbot_runtime.source import ensure_source_repo
 from hackbot_runtime.uploader import SignedPolicyUploader
 
@@ -37,6 +42,26 @@
 log = logging.getLogger("hackbot_runtime.context")
 
 
+class BaseAgentInputs(BaseSettings):
+    model_config = SettingsConfigDict(extra="ignore")
+
+    @classmethod
+    def settings_customise_sources(
+        cls,
+        settings_cls: type[BaseSettings],
+        init_settings: PydanticBaseSettingsSource,
+        env_settings: PydanticBaseSettingsSource,
+        dotenv_settings: PydanticBaseSettingsSource,
+        file_secret_settings: PydanticBaseSettingsSource,
+    ) -> tuple[PydanticBaseSettingsSource, ...]:
+        # Environment variables override settings passed through
+        # as init parameters
+        return (env_settings, dotenv_settings, init_settings, file_secret_settings)
+
+
+InputsType = TypeVar("InputsType", bound=BaseAgentInputs)
+
+
 def _default_run_id() -> str:
     """A unique, sortable id for runs that don't get one from the platform.
 
@@ -62,6 +87,7 @@ class HackbotContext(BaseSettings):
     results_prefix: str = ""
     results_policy_url: str | None = None
     results_policy_fields: dict[str, str] = {}
+    run_inputs_url: str | None = None
     # Base for locally-persisted artifacts when no uploader is configured
     # (compose/direct runs). Each run is namespaced under it by run_id (see
     # `run_artifacts_dir`). Overridable via ARTIFACTS_DIR — compose points this
@@ -177,6 +203,14 @@ def log_path(self) -> Path:
     def actions(self) -> ActionsRecorder:
         return ActionsRecorder(self.uploader, artifacts_dir=self.run_artifacts_dir)
 
+    def load_inputs(self, inputs_cls: type[InputsType]) -> InputsType:
+        remote_config = load_remote_config(self.run_inputs_url)
+        if remote_config:
+            kwargs = remote_config
+        else:
+            kwargs = {}
+        return inputs_cls(**kwargs)
+
     def publish_file(
         self, key: str, path: Path, content_type: str | None = None
     ) -> str:
diff --git a/libs/hackbot-runtime/hackbot_runtime/remote_config.py b/libs/hackbot-runtime/hackbot_runtime/remote_config.py
new file mode 100644
index 0000000000..58d24063e2
--- /dev/null
+++ b/libs/hackbot-runtime/hackbot_runtime/remote_config.py
@@ -0,0 +1,25 @@
+from __future__ import annotations
+
+import logging
+from typing import Mapping
+
+import requests
+from pydantic.types import Json
+
+log = logging.getLogger("hackbot_runtime.remote_config")
+
+
+def load_remote_config(config_url: str | None) -> Mapping[str, Json] | None:
+    if config_url is None:
+        return None
+
+    response = requests.get(config_url, timeout=30)
+    response.raise_for_status()
+    config = response.json()
+
+    if not isinstance(config, dict):
+        raise ValueError(
+            f"Config fetched from {config_url} was not a JSON object; got {type(config).__name__}"
+        )
+
+    return config
diff --git a/libs/hackbot-runtime/tests/test_context.py b/libs/hackbot-runtime/tests/test_context.py
index 26ef0b4d1d..29b0c10347 100644
--- a/libs/hackbot-runtime/tests/test_context.py
+++ b/libs/hackbot-runtime/tests/test_context.py
@@ -3,7 +3,7 @@
 from pathlib import Path
 
 import pytest
-from hackbot_runtime import HackbotContext
+from hackbot_runtime import BaseAgentInputs, HackbotContext
 from hackbot_runtime.config import FirefoxConfig, HackbotConfig, SourceConfig
 
 
@@ -13,6 +13,11 @@ def _hb(tmp_path, config: HackbotConfig) -> HackbotContext:
     return hb
 
 
+class _SampleInputs(BaseAgentInputs):
+    bug_id: int
+    model: str | None = None
+
+
 def test_source_repo_without_declaration_raises(tmp_path):
     hb = _hb(tmp_path, HackbotConfig())
     with pytest.raises(RuntimeError, match="\\[source\\]"):
@@ -101,3 +106,51 @@ def test_results_plumbing(tmp_path):
 
     hb.actions.record("bugzilla.update_bug", {"bug_id": 1}, reasoning="r")
     assert hb.actions.actions[0]["type"] == "bugzilla.update_bug"
+
+
+def test_load_inputs_without_url_reads_env(tmp_path, monkeypatch):
+    # Local/docker path: no RUN_INPUTS_URL, so inputs come from the environment.
+    monkeypatch.setenv("BUG_ID", "42")
+    monkeypatch.setenv("MODEL", "claude-opus")
+    hb = _hb(tmp_path, HackbotConfig())
+    assert hb.run_inputs_url is None
+
+    inputs = hb.load_inputs(_SampleInputs)
+
+    assert inputs.bug_id == 42
+    assert inputs.model == "claude-opus"
+
+
+def test_load_inputs_uses_remote_config(tmp_path, monkeypatch):
+    # Production path: the required field is supplied by the fetched file, not env.
+    monkeypatch.delenv("BUG_ID", raising=False)
+    monkeypatch.delenv("MODEL", raising=False)
+    monkeypatch.setattr(
+        "hackbot_runtime.context.load_remote_config",
+        lambda url: {"bug_id": 7, "model": "from-config"},
+    )
+    hb = _hb(tmp_path, HackbotConfig())
+    hb.run_inputs_url = "https://signed.example/inputs.json"
+
+    inputs = hb.load_inputs(_SampleInputs)
+
+    assert inputs.bug_id == 7
+    assert inputs.model == "from-config"
+
+
+def test_load_inputs_env_overrides_remote_config(tmp_path, monkeypatch):
+    # An env var wins over the same key in the config, while keys absent from the
+    # environment still fall through to the config.
+    monkeypatch.setenv("MODEL", "from-env")
+    monkeypatch.delenv("BUG_ID", raising=False)
+    monkeypatch.setattr(
+        "hackbot_runtime.context.load_remote_config",
+        lambda url: {"bug_id": 7, "model": "from-config"},
+    )
+    hb = _hb(tmp_path, HackbotConfig())
+    hb.run_inputs_url = "https://signed.example/inputs.json"
+
+    inputs = hb.load_inputs(_SampleInputs)
+
+    assert inputs.model == "from-env"  # env overrides config
+    assert inputs.bug_id == 7  # config supplies what env doesn't
diff --git a/services/hackbot-api/app/agents.py b/services/hackbot-api/app/agents.py
index 6ad1c5c00d..dfbcfb87a6 100644
--- a/services/hackbot-api/app/agents.py
+++ b/services/hackbot-api/app/agents.py
@@ -1,5 +1,3 @@
-import json
-from collections.abc import Callable
 from dataclasses import dataclass
 
 from pydantic import BaseModel
@@ -19,32 +17,6 @@ class AgentSpec:
     description: str
     job_name: str
     input_schema: type[BaseModel]
-    # Optional override for the rare agent whose env vars don't map 1:1 from
-    # its input schema. Defaults to ``model_to_env`` (field -> UPPER_SNAKE env).
-    build_env: Callable[[BaseModel], dict[str, str]] | None = None
-
-
-def model_to_env(inputs: BaseModel) -> dict[str, str]:
-    """Serialise validated inputs into Cloud Run Job env overrides.
-
-    Each schema field maps to an upper-cased env var (``bug_id`` -> ``BUG_ID``);
-    ``None`` fields are skipped, and the agent reads them back via
-    ``pydantic_settings.BaseSettings`` (which upper-cases field names by
-    default). Lists/dicts are JSON-encoded. Deploy-time constants (e.g. the
-    broker loopback URL) are NOT inputs — they belong in the Job's static env
-    config, not here.
-    """
-    env: dict[str, str] = {}
-    for name, value in inputs.model_dump(mode="json").items():
-        if value is None:
-            continue
-        if isinstance(value, str):
-            env[name.upper()] = value
-        elif isinstance(value, (list, dict)):
-            env[name.upper()] = json.dumps(value)
-        else:
-            env[name.upper()] = str(value)
-    return env
 
 
 AGENT_REGISTRY: dict[str, AgentSpec] = {
diff --git a/services/hackbot-api/app/gcs.py b/services/hackbot-api/app/gcs.py
index 49e720eff8..7ce4c048fc 100644
--- a/services/hackbot-api/app/gcs.py
+++ b/services/hackbot-api/app/gcs.py
@@ -11,6 +11,7 @@
 from google.auth import impersonated_credentials
 from google.auth.transport.requests import Request as AuthRequest
 from google.cloud import storage
+from pydantic import Json
 
 from app.config import settings
 from app.schemas import ArtifactRef, RunSummary
@@ -26,6 +27,10 @@ def summary_blob_name(run_id: str) -> str:
     return f"{run_prefix(run_id)}summary.json"
 
 
+def inputs_blob_name(run_id: str) -> str:
+    return f"{run_prefix(run_id)}inputs.json"
+
+
 @lru_cache(maxsize=1)
 def _signing_credentials() -> impersonated_credentials.Credentials:
     """Impersonate-self credentials so we can `sign_bytes` on Cloud Run.
@@ -126,6 +131,34 @@ async def generate_results_policy(run_id: str) -> dict[str, Any]:
     return await asyncio.to_thread(_generate_post_policy_sync, run_id)
 
 
+def _put_run_inputs_sync(run_id: str, config: dict[str, Json]) -> str:
+    """Upload the run's inputs and return the URL to fetch them.
+
+    This reuses the GCS bucket configured for the agent results.
+    """
+    bucket = _client().bucket(settings.results_bucket)
+    blob = bucket.blob(inputs_blob_name(run_id))
+    blob.upload_from_string(
+        json.dumps(config),
+        content_type="application/json",
+    )
+    # Valid for the whole run: the agent fetches config at startup, but matching
+    # the upload policy's window keeps a retried/slow-starting task covered.
+    expiration_seconds = (
+        settings.job_execution_timeout_seconds + settings.signed_policy_grace_seconds
+    )
+    return blob.generate_signed_url(
+        version="v4",
+        expiration=datetime.timedelta(seconds=expiration_seconds),
+        method="GET",
+        credentials=_signing_credentials(),
+    )
+
+
+async def put_run_inputs(run_id: str, config: dict[str, Json]) -> str:
+    return await asyncio.to_thread(_put_run_inputs_sync, run_id, config)
+
+
 def _read_summary_sync(run_id: str) -> RunSummary | None:
     bucket = _client().bucket(settings.results_bucket)
     blob = bucket.blob(summary_blob_name(run_id))
diff --git a/services/hackbot-api/app/routers/runs.py b/services/hackbot-api/app/routers/runs.py
index ff55d466e8..5d0a2430a0 100644
--- a/services/hackbot-api/app/routers/runs.py
+++ b/services/hackbot-api/app/routers/runs.py
@@ -7,7 +7,7 @@
 from sqlalchemy.ext.asyncio import AsyncSession
 
 from app import gcs, jobs
-from app.agents import AGENT_REGISTRY, AgentSpec, model_to_env
+from app.agents import AGENT_REGISTRY, AgentSpec
 from app.auth import require_api_key
 from app.config import settings
 from app.database.connection import get_db
@@ -77,13 +77,17 @@ async def create_run(
     db.add(run)
     await db.flush()
 
+    run_inputs_url = await gcs.put_run_inputs(
+        str(run_id), inputs.model_dump(mode="json")
+    )
+
     env_overrides: dict[str, str] = {
         "RUN_ID": str(run_id),
         "RESULTS_BUCKET": settings.results_bucket,
         "RESULTS_PREFIX": results_prefix,
         "RESULTS_POLICY_URL": policy["url"],
         "RESULTS_POLICY_FIELDS": json.dumps(policy["fields"]),
-        **(agent.build_env or model_to_env)(inputs),
+        "RUN_INPUTS_URL": run_inputs_url,
     }
 
     try:
diff --git a/services/hackbot-api/tests/test_agents.py b/services/hackbot-api/tests/test_agents.py
index 1e782b9963..0fc86cf9a6 100644
--- a/services/hackbot-api/tests/test_agents.py
+++ b/services/hackbot-api/tests/test_agents.py
@@ -1,9 +1,7 @@
-"""Tests for the agent registry and generic env serialization."""
-
-import json
+"""Tests for the agent registry."""
 
 import pytest
-from app.agents import AGENT_REGISTRY, model_to_env
+from app.agents import AGENT_REGISTRY
 from app.schemas import (
     BugFixInputs,
     BuildRepairInputs,
@@ -14,80 +12,21 @@
 from pydantic import ValidationError
 
 
-def test_model_to_env_uppercases_and_stringifies():
-    env = model_to_env(BugFixInputs(bug_id=12345, model="claude-opus", max_turns=8))
-    assert env["BUG_ID"] == "12345"
-    assert env["MODEL"] == "claude-opus"
-    assert env["MAX_TURNS"] == "8"
-
-
-def test_model_to_env_skips_none_fields():
-    env = model_to_env(BugFixInputs(bug_id=1))
-    assert env == {"BUG_ID": "1"}
-    # Optional fields left unset must not leak as empty/"None" env vars.
-    assert "MODEL" not in env
-    assert "EFFORT" not in env
-
-
-def test_model_to_env_does_not_emit_deploy_constants():
-    # The broker loopback URL is static Job config, not a per-run input.
-    env = model_to_env(BugFixInputs(bug_id=1, model="x", max_turns=2, effort="high"))
-    assert "BUGZILLA_MCP_URL" not in env
-
-
-def test_bug_fix_registry_uses_default_env_serializer():
+def test_bug_fix_registry_entry():
     spec = AGENT_REGISTRY["bug-fix"]
-    # No hand-written build_env: the router falls back to model_to_env.
-    assert spec.build_env is None
     assert spec.input_schema is BugFixInputs
+    assert spec.job_name == "hackbot-agent-bug-fix"
 
 
 def test_build_repair_registry_entry():
     spec = AGENT_REGISTRY["build-repair"]
-    assert spec.build_env is None
     assert spec.input_schema is BuildRepairInputs
     assert spec.job_name == "hackbot-agent-build-repair"
 
 
-def test_model_to_env_json_encodes_failure_tasks_and_bool():
-    tasks = {"build-linux64/opt": "OyF95j0oQ-CF_YuBM1b7vg"}
-    env = model_to_env(
-        BuildRepairInputs(
-            bug_id=1, git_commit="deadbeef", failure_tasks=tasks, run_try_push=True
-        )
-    )
-    assert env["GIT_COMMIT"] == "deadbeef"
-    assert json.loads(env["FAILURE_TASKS"]) == tasks
-    assert env["RUN_TRY_PUSH"] == "True"
-
-
 def test_test_plan_generator_inputs_require_feature_description():
     with pytest.raises(ValidationError):
         PlanGeneratorInputs(
             feature_name="Bookmarks and History",
             test_scope="Bookmarks toolbar behavior.",
         )
-
-
-def test_test_plan_generator_env_serialization():
-    env = model_to_env(
-        PlanGeneratorInputs(
-            feature_name="Bookmarks and History",
-            feature_description="Bookmarks and history controls in Firefox.",
-            test_scope="Bookmarks toolbar behavior.",
-        )
-    )
-
-    assert env == {
-        "FEATURE_NAME": "Bookmarks and History",
-        "FEATURE_DESCRIPTION": "Bookmarks and history controls in Firefox.",
-        "TEST_SCOPE": "Bookmarks toolbar behavior.",
-    }
-
-
-def test_test_plan_generator_registry_uses_default_env_serializer():
-    spec = AGENT_REGISTRY["test-plan-generator"]
-
-    assert spec.build_env is None
-    assert spec.job_name == "hackbot-agent-test-plan-generator"
-    assert spec.input_schema is PlanGeneratorInputs